7.0 Chapter Introduction
7.0.1 Chapter IntroductionPage 1:
To support our communication, the OSI model divides the functions of a data network into layers.
To recap:
- The Application layer provides the interface to the user.
- The Transport layer is responsible for dividing and managing communications between the processes running in the two end systems.
- The Network layer protocols organize our communication data so that it can travel across internetworks from the originating host to a destination host.
It is the role of the OSI Data Link layer to prepare Network layer packets for transmission and to control access to the physical media.
This chapter introduces the general functions of the Data Link layer and the protocols associated with it.
Learning Objectives
Upon completion of this chapter, you will be able to:
- Explain the role of Data Link layer protocols in data transmission.
- Describe how the Data Link layer prepares data for transmission on network media.
- Describe the different types of media access control methods.
- Identify several common logical network topologies and describe how the logical topology determines the media access control method for that network.
- Explain the purpose of encapsulating packets into frames to facilitate media access.
- Describe the Layer 2 frame structure and identify generic fields.
- Explain the role of key frame header and trailer fields, including addressing, QoS, type of protocol, and Frame Check Sequence.
7.0.1 - Chapter Introduction
The diagram depicts the seven layers of the O S I Model with the Data Link Layer highlighted. A router is attached to the Physical Layer, and a user at a PC is attached to the Application Layer. The router has an arrow pointing to a cloud labeled Network. The Data Link Layer prepares network data for the physical network.
The diagram depicts the seven layers of the O S I Model with the Data Link Layer highlighted. A router is attached to the Physical Layer, and a user at a PC is attached to the Application Layer. The router has an arrow pointing to a cloud labeled Network. The Data Link Layer prepares network data for the physical network.
7.1 Data Link Layer - Accessing the Media
7.1.1 Data Link Layer - Supporting & Connecting to Upper Layer ServicesPage 1:
The Data Link layer provides a means for exchanging data over a common local media.
The Data Link layer performs two basic services:
- Allows the upper layers to access the media using techniques such as framing
- Controls how data is placed onto the media and is received from the media using techniques such as media access control and error detection
Frame - The Data Link layer PDU
Node - The Layer 2 notation for network devices connected to a common medium
Media/medium (physical)* - The physical means for the transfer of information between two nodes
Network (physical)** - Two or more nodes connected to a common medium
The Data Link layer is responsible for the exchange of frames between nodes over the media of a physical network.
* It is important to understand the meaning of the words medium and media within the context of this chapter. Here, these words refer to the material that actually carries the signals representing the transmitted data. Media is the physical copper cable, optical fiber, or atmosphere through which the signals travel. In this chapter media does not refer to content programming such as audio, animation, television, and video as used when referring to digital content and multimedia.
** A physical network is different from a logical network. Logical networks are defined at the Network layer by the arrangement of the hierarchical addressing scheme. Physical networks represent the interconnection of devices on a common media. Sometimes, a physical network is also referred to as a network segment.
7.1.1 - Data Link Layer - Supporting and Connecting to Upper Layer Services
The diagram depicts Data Link Layer terms. These include frame, node, media, and network.
Frame: A PDU at the Data Link Layer is called a frame.
Node: A node is a device on a network. Examples shown include a PC, PDA, and IP phone.
Media: The physical means used to carry data signals. A satellite dish is shown communicating wirelessly with a satellite.
Network: A network is two or more devices connected to a common medium. A PC has a wired connection to router R1, and a laptop has a wired connection to wireless router R2. A wireless laptop has a wireless connection to router R2. Routers R1 and R2 are connected.
The diagram depicts Data Link Layer terms. These include frame, node, media, and network.
Frame: A PDU at the Data Link Layer is called a frame.
Node: A node is a device on a network. Examples shown include a PC, PDA, and IP phone.
Media: The physical means used to carry data signals. A satellite dish is shown communicating wirelessly with a satellite.
Network: A network is two or more devices connected to a common medium. A PC has a wired connection to router R1, and a laptop has a wired connection to wireless router R2. A wireless laptop has a wireless connection to router R2. Routers R1 and R2 are connected.
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Upper Layer Access to Media
As we have discussed, a network model allows each layer to function with minimal concern for the roles of the other layers. The Data Link layer relieves the upper layers from the responsibility of putting data on the network and receiving data from the network. This layer provides services to support the communication processes for each medium over which data is to be transmitted.
In any given exchange of Network layer packets, there may be numerous Data Link layer and media transitions. At each hop along the path, an intermediary device - usually a router - accepts frames from a medium, decapsulates the frame, and then forwards the packet in a new frame appropriate to the medium of that segment of the physical network.
Imagine a data conversation between two distant hosts, such as a PC in Paris with an Internet server in Japan. Although the two hosts may be communicating with their peer Network layer protocols (IP for example), it is likely that numerous Data Link layer protocols are being used to transport the IP packets over various types of LANs and WANs. This packet exchange between two hosts requires a diversity of protocols that must exist at the Data Link layer. Each transition at a router could require a different Data Link layer protocol for transport on a new medium.
Notice in the figure that each link between devices uses a different medium. Between the PC and the router may be an Ethernet link. The routers are connected through a satellite link, and the laptop is connected through a wireless link to the last router. In this example, as an IP packet travels from the PC to the laptop, it will be encapsulated into Ethernet frame, decapsulated, processed, and then encapsulated into a new data link frame to cross the satellite link. For the final link, the packet will use a wireless data link frame from the router to the laptop.
The Data Link layer effectively insulates the communication processes at the higher layers from the media transitions that may occur end-to-end. A packet is received from and directed to an upper layer protocol, in this case IPv4 or IPv6, that does not need to be aware of which media the communication will use.
Without the Data Link layer, a Network layer protocol, such as IP, would have to make provisions for connecting to every type of media that could exist along a delivery path. Moreover, IP would have to adapt every time a new network technology or medium was developed. This process would hamper protocol and network media innovation and development. This is a key reason for using a layered approach to networking.
The range of Data Link layer services has to include all of the currently used types of media and the methods for accessing them. Because of the number of communication services provided by the Data Link layer, it is difficult to generalize their role and provide examples of a generic set of services. For that reason, please note that any given protocol may or may not support all these Data Link layer services.
Internetworking Basics - http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html
MTU - http://www.tcpipguide.com/free/t_IPDatagramSizeMaximumTransmissionUnitMTUFragmentat.htm
7.1.1 - Data Link Layer - Supporting and Connecting to Upper Layer Services
The animation depicts the movement of a frame and the protocols of the Data Link Layer as a frame travels over various link types.
Data Link Layer protocols govern how to format a frame for different media.
As the animation progresses, a frame travels over the following links:
1. From PC to router R1 over a copper wired link.
2. From router R1 to satellite Dish1 over a fiber-optic link.
3. From satellite Dish1 to a satellite over a wireless link.
4. From satellite to satellite Dish2 over a wireless link.
5. From satellite Dish2 to wireless router R2 over a fiber-optic link.
6. From wireless router R2 to a wireless laptop over a wireless link.
At each hop along the path, an intermediary device accepts frames from one medium, decapsulates the frame, and then forwards the packets in a new frame. The headers of each frame are formatted for the specific medium that it crosses.
A More Information popup displays the following text:
It is important to understand the meaning of the words medium and media within the context of this chapter. Here, these words refer to material that actually carries the signals representing the transmitted data. Media is the physical copper cable, optical fiber, or atmosphere through which the signals travel. In this chapter, media does not refer to content programming, such as audio, animation, television, and video, as used when referring to digital content and multimedia.
A physical network has a separate connotation than a logical network. Logical networks are structures defined at the Network Layer by the arrangement of the hierarchical addressing scheme. Physical networks represent the interconnection of devices on a common media. Sometimes, a physical network is also referred to as a network segment.
The animation depicts the movement of a frame and the protocols of the Data Link Layer as a frame travels over various link types.
Data Link Layer protocols govern how to format a frame for different media.
As the animation progresses, a frame travels over the following links:
1. From PC to router R1 over a copper wired link.
2. From router R1 to satellite Dish1 over a fiber-optic link.
3. From satellite Dish1 to a satellite over a wireless link.
4. From satellite to satellite Dish2 over a wireless link.
5. From satellite Dish2 to wireless router R2 over a fiber-optic link.
6. From wireless router R2 to a wireless laptop over a wireless link.
At each hop along the path, an intermediary device accepts frames from one medium, decapsulates the frame, and then forwards the packets in a new frame. The headers of each frame are formatted for the specific medium that it crosses.
A More Information popup displays the following text:
It is important to understand the meaning of the words medium and media within the context of this chapter. Here, these words refer to material that actually carries the signals representing the transmitted data. Media is the physical copper cable, optical fiber, or atmosphere through which the signals travel. In this chapter, media does not refer to content programming, such as audio, animation, television, and video, as used when referring to digital content and multimedia.
A physical network has a separate connotation than a logical network. Logical networks are structures defined at the Network Layer by the arrangement of the hierarchical addressing scheme. Physical networks represent the interconnection of devices on a common media. Sometimes, a physical network is also referred to as a network segment.
7.1.2 Data Link Layer - Controlling Transfer across Local Media
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Layer 2 protocols specify the encapsulation of a packet into a frame and the techniques for getting the encapsulated packet on and off each medium. The technique used for getting the frame on and off media is called the media access control method. For the data to be transferred across a number of different media, different media access control methods may be required during the course of a single communication.
Each network environment that packets encounter as they travel from a local host to a remote host can have different characteristics. For example, one network environment may consist of many hosts contending to access the network medium on an ad hoc basis. Another environment may consist of a direct connection between only two devices over which data flows sequentially as bits in an orderly way.
The media access control methods described by the Data Link layer protocols define the processes by which network devices can access the network media and transmit frames in diverse network environments.
A node that is an end device uses an adapter to make the connection to the network. For example, to connect to a LAN, the device would use the appropriate Network Interface Card (NIC) to connect to the LAN media. The adapter manages the framing and media access control.
At intermediary devices such as a router, where the media type could change for each connected network, different physical interfaces on the router are used to encapsulate the packet into the appropriate frame, and a suitable media access control method is used to access each link. The router in the figure has an Ethernet interface to connect to the LAN and a serial interface to connect to the WAN. As the router processes frames, it will use Data Link layer services to receive the frame from one medium, decapsulate it to the Layer 3 PDU, re-encapsulate the PDU into a new frame, and place the frame on the medium of the next link of the network.
7.1.2 - Data Link Layer - Controlling Transfer Across Local Media
The animation depicts transferring frames from an Ethernet LAN to serial WAN link. Two user PC's are connected to an Ethernet switch, which is connected to router R1's LAN interface. Router R1 is connected to router R2 via a serial WAN link.
As the animation progresses, a packet is sent from user PC1 through the switch to the router R1 LAN interface. During this time, it is encapsulated in a LAN header and trailer. When the LAN encapsulated packet is forwarded by router R1 to router R2 over the WAN link, the LAN header and trailer are replaced with a WAN header and trailer.
The Data Link Layer is responsible for controlling the transfer of frames across the media.
The animation depicts transferring frames from an Ethernet LAN to serial WAN link. Two user PC's are connected to an Ethernet switch, which is connected to router R1's LAN interface. Router R1 is connected to router R2 via a serial WAN link.
As the animation progresses, a packet is sent from user PC1 through the switch to the router R1 LAN interface. During this time, it is encapsulated in a LAN header and trailer. When the LAN encapsulated packet is forwarded by router R1 to router R2 over the WAN link, the LAN header and trailer are replaced with a WAN header and trailer.
The Data Link Layer is responsible for controlling the transfer of frames across the media.
7.1.3 Data Link Layer - Creating a Frame
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The description of a frame is a key element of each Data Link layer protocol. Data Link layer protocols require control information to enable the protocols to function. Control information may tell:
- Which nodes are in communication with each other
- When communication between individual nodes begins and when it ends
- Which errors occurred while the nodes communicated
- Which nodes will communicate next
Unlike the other PDUs that have been discussed in this course, the Data Link layer frame includes:
- Data - The packet from the Network layer
- Header - Contains control information, such as addressing, and is located at the beginning of the PDU
- Trailer - Contains control information added to the end of the PDU
7.1.3 - Data Link Layer - Creating a Frame
The diagram depicts a frame at Layer 2 with a header and trailer encapsulating a packet (data). Above the frame is Network Layer 3 and below is Physical Layer 1. The Data Link Layer provides an interface between the Network and Physical layers.
The diagram depicts a frame at Layer 2 with a header and trailer encapsulating a packet (data). Above the frame is Network Layer 3 and below is Physical Layer 1. The Data Link Layer provides an interface between the Network and Physical layers.
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Formatting Data for Transmission
When data travels on the media, it is converted into a stream of bits, or 1s and 0s. If a node is receiving long streams of bits, how does it determine where a frame starts and stops or which bits represent the address?
Framing breaks the stream into decipherable groupings, with control information inserted in the header and trailer as values in different fields. This format gives the physical signals a structure that can be received by nodes and decoded into packets at the destination.
Typical field types include:
- Start and stop indicator fields - The beginning and end limits of the frame
- Naming or addressing fields
- Type field - The type of PDU contained in the frame
- Control - Flow control services
- A data field -The frame payload (Network layer packet)
Not all protocols include all of these fields. The standards for a specific Data Link protocol define the actual frame format. Examples of frame formats will be discussed at the end of this chapter.
7.1.3 - Data Link Layer - Creating a Frame
The diagram depicts how data is formatted for transmission using a frame structure. The header contains frame start, addressing, type, and control fields. A specific bit pattern indicates the start of the frame. The trailer contains error detection and frame stop fields. Another specific bit pattern indicates the end of the frame.
The diagram depicts how data is formatted for transmission using a frame structure. The header contains frame start, addressing, type, and control fields. A specific bit pattern indicates the start of the frame. The trailer contains error detection and frame stop fields. Another specific bit pattern indicates the end of the frame.
7.1.4 Data Link Layer - Connecting Upper Layer Services to the Media
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The Data Link layer exists as a connecting layer between the software processes of the layers above it and the Physical layer below it. As such, it prepares the Network layer packets for transmission across some form of media, be it copper, fiber, or the atmosphere.
In many cases, the Data Link layer is embodied as a physical entity, such as an Ethernet network interface card (NIC), which inserts into the system bus of a computer and makes the connection between running software processes on the computer and physical media. The NIC is not solely a physical entity, however. Software associated with the NIC enables the NIC to perform its intermediary functions of preparing data for transmission and encoding the data as signals to be sent on the associated media.
7.1.4 - Data Link Layer - Connecting Upper Layer Services to the Media
The diagram depicts connecting upper layer services to the media using a PC NIC. The O S I model shows the Data Link Layer highlighted. The Data Link Layer links the software and hardware layers. Physical devices devoted to the Data Link Layer have both hardware and software components. The NIC operates at Layers 1 and 2. Layer 1 and a portion of Layer 2 are implemented in hardware. A portion of Layer 2 and the remaining O S I upper layers are implemented in software.
The diagram depicts connecting upper layer services to the media using a PC NIC. The O S I model shows the Data Link Layer highlighted. The Data Link Layer links the software and hardware layers. Physical devices devoted to the Data Link Layer have both hardware and software components. The NIC operates at Layers 1 and 2. Layer 1 and a portion of Layer 2 are implemented in hardware. A portion of Layer 2 and the remaining O S I upper layers are implemented in software.
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Data Link Sublayers
To support a wide variety of network functions, the Data Link layer is often divided into two sublayers: an upper sublayer and an lower sublayer.
- The upper sublayer defines the software processes that provide services to the Network layer protocols.
- The lower sublayer defines the media access processes performed by the hardware.
The two common LAN sublayers are:
Logical Link Control
Logical Link Control (LLC) places information in the frame that identifies which Network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IP and IPX, to utilize the same network interface and media.
Media Access Control
Media Access Control (MAC) provides Data Link layer addressing and delimiting of data according to the physical signaling requirements of the medium and the type of Data Link layer protocol in use.
7.1.4 - Data Link Layer - Connecting Upper Layer Services to the Media
The diagram depicts the Data Link Layer divided into two sublayers, called the Logical Link Control and Media Access Control. A Network Layer packet is encapsulated into a frame at the Data Link Layer. The frame is encoded into a signal (zeros and ones) at the Physical Layer.
Logical Link Control sublayer:
- Frames the Network Layer packet.
- Identifies the Network Layer protocol.
Media Access Control sublayer:
- Addresses the frame.
- Marks the beginning and end of the frame.
The diagram depicts the Data Link Layer divided into two sublayers, called the Logical Link Control and Media Access Control. A Network Layer packet is encapsulated into a frame at the Data Link Layer. The frame is encoded into a signal (zeros and ones) at the Physical Layer.
Logical Link Control sublayer:
- Frames the Network Layer packet.
- Identifies the Network Layer protocol.
Media Access Control sublayer:
- Addresses the frame.
- Marks the beginning and end of the frame.
7.1.5 Data Link Layer - Standards
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Unlike the protocols of the upper layers of the TCP/IP suite, Data Link layer protocols are generally not defined by Request for Comments (RFCs). Although the Internet Engineering Task Force (IETF) maintains the functional protocols and services for the TCP/IP protocol suite in the upper layers, the IETF does not define the functions and operation of that model's Network access layer. The TCP/IP Network Access layer is the equivalent of the OSI Data Link and Physical layers. These two layer will be discussed in separate chapters for closer examination..
The functional protocols and services at the Data Link layer are described by engineering organizations (such as IEEE, ANSI, and ITU) and communications companies. Engineering organizations set public and open standards and protocols. Communications companies may set and use proprietary protocols to take advantage of new advances in technology or market opportunities.
Data Link layer services and specifications are defined by multiple standards based on a variety of technologies and media to which the protocols are applied. Some of these standards integrate both Layer 2 and Layer 1 services.
Engineering organizations that define open standards and protocols that apply to the Data Link layer include:
- International Organization for Standardization (ISO)
- Institute of Electrical and Electronics Engineers (IEEE)
- American National Standards Institute (ANSI)
- International Telecommunication Union (ITU)
For example, a device implementing the Data Link layer on a computer would be the network interface card (NIC). For a laptop, a wireless PCMCIA adapter is commonly used. Each of these adapters is the hardware that complies with the Layer 2 standards and protocols.
http://www.iso.org
http://www.ieee.org
http://www.ansi.org
http://www.itu.int
7.1.5 - Data Link Layer - Standards
The diagram depicts standards for the Data Link Layer as developed by the standards organizations I S O, i e e e, I T U and ANSI.
I S O:
- HDLC (High Level Data Link Control)
i e e e:
- 8 0 2 dot 2 (LLC)
- 8 0 2 dot 3 (Ethernet)
- 8 0 2 dot 5 (Token Ring)
- 8 0 2 dot 11 (Wireless LAN)
I T U:
- Q dot 9 2 2 (Frame Relay Standard)
- Q dot 9 2 1 (ISDN Data Link Standard)
- HDLC (High Level Data Link Control)
ANSI: 3T9 dot 5
- ADCCP (Advanced Data Communications Control Protocol)
The diagram depicts standards for the Data Link Layer as developed by the standards organizations I S O, i e e e, I T U and ANSI.
I S O:
- HDLC (High Level Data Link Control)
i e e e:
- 8 0 2 dot 2 (LLC)
- 8 0 2 dot 3 (Ethernet)
- 8 0 2 dot 5 (Token Ring)
- 8 0 2 dot 11 (Wireless LAN)
I T U:
- Q dot 9 2 2 (Frame Relay Standard)
- Q dot 9 2 1 (ISDN Data Link Standard)
- HDLC (High Level Data Link Control)
ANSI: 3T9 dot 5
- ADCCP (Advanced Data Communications Control Protocol)
7.2 Media Access Control Techniques
7.2.1 Placing Data on the MediaPage 1:
Regulating the placement of data frames onto the media is known as media access control. Among the different implementations of the Data Link layer protocols, there are different methods of controlling access to the media. These media access control techniques define if and how the nodes share the media.
Media access control is the equivalent of traffic rules that regulate the entrance of motor vehicles onto a roadway. The absence of any media access control would be the equivalent of vehicles ignoring all other traffic and entering the road without regard to the other vehicles.
However, not all roads and entrances are the same. Traffic can enter the road by merging, by waiting for its turn at a stop sign, or by obeying signal lights. A driver follows a different set of rules for each type of entrance.
In the same way, there are different ways to regulate the placing of frames onto the media. The protocols at the Data Link layer define the rules for access to different media. Some media access control methods use highly-controlled processes to ensure that frames are safely placed on the media. These methods are defined by sophisticated protocols, which require mechanisms that introduce overhead onto the network.
The method of media access control used depends on:
- Media sharing - If and how the nodes share the media
- Topology - How the connection between the nodes appears to the Data Link layer
7.2.1 - Placing Data on the Media
The diagram depicts media access control methods. Three PC's are connected to a common shared media providing an example of no control. No control at all would result in many collisions. Collisions cause corrupted frames that must be resent.
Three PC's are connected to a common shared media providing an example of taking turns, a method that enforces a high degree of control. This prevents collisions, but the process has high overhead.
Methods that enforce a low degree of control have low overhead, but there are more frequent collisions.
The diagram depicts media access control methods. Three PC's are connected to a common shared media providing an example of no control. No control at all would result in many collisions. Collisions cause corrupted frames that must be resent.
Three PC's are connected to a common shared media providing an example of taking turns, a method that enforces a high degree of control. This prevents collisions, but the process has high overhead.
Methods that enforce a low degree of control have low overhead, but there are more frequent collisions.
7.2.2 Media Access Control for Shared Media
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Some network topologies share a common medium with multiple nodes. At any one time, there may be a number of devices attempting to send and receive data using the network media. There are rules that govern how these devices share the media.
There are two basic media access control methods for shared media:
- Controlled - Each node has its own time to use the medium
- Contention-based - All nodes compete for the use of the medium
Controlled Access for Shared Media
When using the controlled access method, network devices take turns, in sequence, to access the medium. This method is also known as scheduled access or deterministic. If a device does not need to access the medium, the opportunity to use the medium passes to the next device in line. When one device places a frame on the media, no other device can do so until the frame has arrived at the destination and has been processed by the destination.
Although controlled access is well-ordered and provides predictable throughput, deterministic methods can be inefficient because a device has to wait for its turn before it can use the medium.
Contention-based Access for Shared Media
Also referred to as non-deterministic, contention-based methods allow any device to try to access the medium whenever it has data to send. To prevent complete chaos on the media, these methods use a Carrier Sense Multiple Access (CSMA) process to first detect if the media is carrying a signal. If a carrier signal on the media from another node is detected, it means that another device is transmitting. When the device attempting to transmit sees that the media is busy, it will wait and try again after a short time period. If no carrier signal is detected, the device transmits its data. Ethernet and wireless networks use contention-based media access control.
It is possible that the CSMA process will fail and two devices will transmit at the same time. This is called a data collision. If this occurs, the data sent by both devices will be corrupted and will need to be resent.
Contention-based media access control methods do not have the overhead of controlled access methods. A mechanism for tracking whose turn it is to access the media is not required. However, the contention-based systems do not scale well under heavy media use. As use and the number of nodes increases, the probability of successful media access without a collision decreases. Additionally, The recovery mechanisms required to correct errors due to these collisions further diminishes the throughput.
CSMA is usually implemented in conjunction with a method for resolving the media contention. The two commonly used methods are:
CSMA/Collision Detection
In CSMA/Collision Detection (CSMA/CD), the device monitors the media for the presence of a data signal. If a data signal is absent, indicating that the media is free, the device transmits the data. If signals are then detected that show another device was transmitting at the same time, all devices stop sending and try again later. Traditional forms of Ethernet use this method.
CSMA/Collision Avoidance
In CSMA/Collision Avoidance (CSMA/CA), the device examines the media for the presence of a data signal. If the media is free, the device sends a notification across the media of its intent to use it. The device then sends the data. This method is used by 802.11 wireless networking technologies.
Note: CSMA/CD will be covered in more detail in Chapter 9.
7.2.2 - Media Access Control for Shared Media
The diagram depicts media access control for shared media and compares the controlled and contention-based methods. Three PC's are connected to a common shared media. All three PC's have text bubbles that state: We need rules for how to share the media.
Controlled Access Method:
PC text bubbles:
- PC1 - I have a packet to send, but it is not my turn. I'll wait.
- PC2 - I have nothing to send.
- PC3 - It is my turn to send. I will send now.
Characteristics:
- Only one station transmits at a time.
- Devices wanting to transmit must wait their turn.
- No collisions.
- Some deterministic networks use token passing.
Examples:
- Token Ring
- FDDI
Contention-based Access Method:
PC text bubbles:
- All PC's have text bubbles that state: I try to send when I am ready.
Characteristics:
- Stations can transmit at any time.
- Collisions exist.
- Mechanisms exist to resolve contention:
- CSMA/CD for Ethernet networks
- CSMA/CA for 8 0 2 dot 11 wireless networks
Examples:
- Ethernet
- Wireless
The diagram depicts media access control for shared media and compares the controlled and contention-based methods. Three PC's are connected to a common shared media. All three PC's have text bubbles that state: We need rules for how to share the media.
Controlled Access Method:
PC text bubbles:
- PC1 - I have a packet to send, but it is not my turn. I'll wait.
- PC2 - I have nothing to send.
- PC3 - It is my turn to send. I will send now.
Characteristics:
- Only one station transmits at a time.
- Devices wanting to transmit must wait their turn.
- No collisions.
- Some deterministic networks use token passing.
Examples:
- Token Ring
- FDDI
Contention-based Access Method:
PC text bubbles:
- All PC's have text bubbles that state: I try to send when I am ready.
Characteristics:
- Stations can transmit at any time.
- Collisions exist.
- Mechanisms exist to resolve contention:
- CSMA/CD for Ethernet networks
- CSMA/CA for 8 0 2 dot 11 wireless networks
Examples:
- Ethernet
- Wireless
7.2.3 Media Access Control for Non-Shared Media
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Media access control protocols for non-shared media require little or no control before placing frames onto the media. These protocols have simpler rules and procedures for media access control. Such is the case for point-to-point topologies.
In point-to-point topologies, the media interconnects just two nodes. In this arrangement, the nodes do not have to share the media with other hosts or determine if a frame is destined for that node. Therefore, Data Link layer protocols have little to do for controlling non-shared media access.
Full Duplex and Half Duplex
In point-to-point connections, the Data Link layer has to consider whether the communication is half-duplex or full-duplex.
Click the tabs in the figure to see the differences in the two methods.
Half-duplex communication means that the devices can both transmit and receive on the media but cannot do so simultaneously. Ethernet has established arbitration rules for resolving conflicts arising from instances when more than one station attempts to transmit at the same time.
In full-duplex communication, both devices can transmit and receive on the media at the same time. The Data Link layer assumes that the media is available for transmission for both nodes at any time. Therefore, there is no media arbitration necessary in the Data Link layer.
The details of a specific media access control technique can only be examined by studying a specific protocol. Within this course, we will study traditional Ethernet, which uses CSMA/CD. Other techniques will be covered in later courses.
7.2.3 - Media Access Control for Non-Shared Media
The diagram depicts full-duplex and half-duplex media access control for non-shared media. Two PC's are connected to a common media. The two PC's have text bubbles that state: Only you and I communicate on this line. We can talk any time.
Full Duplex:
The two PC's have text bubbles that state: We can send and receive at the same time.
Half Duplex:
The two PC's have text bubbles that state:
We can send and receive, but not at the same time.
A text box points to an incoming frame for one of the PC's and points to a frame that needs to be sent. It states: Wait until this frame is received before sending this one.
The diagram depicts full-duplex and half-duplex media access control for non-shared media. Two PC's are connected to a common media. The two PC's have text bubbles that state: Only you and I communicate on this line. We can talk any time.
Full Duplex:
The two PC's have text bubbles that state: We can send and receive at the same time.
Half Duplex:
The two PC's have text bubbles that state:
We can send and receive, but not at the same time.
A text box points to an incoming frame for one of the PC's and points to a frame that needs to be sent. It states: Wait until this frame is received before sending this one.
7.2.4 Logical Topology vs Physical Topology
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The topology of a network is the arrangement or relationship of the network devices and the interconnections between them. Network topologies can be viewed at the physical level and the logical level.
The physical topology is an arrangement of the nodes and the physical connections between them. The representation of how the media is used to interconnect the devices is the physical topology. These will be covered in later chapters of this course.
A logical topology is the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network independent of their physical layout. These logical signal paths are defined by Data Link layer protocols. The Data Link layer "sees" the logical topology of a network when controlling data access to the media. It is the logical topology that influences the type of network framing and media access control used.
The physical or cabled topology of a network will most likely not be the same as the logical topology.
Logical topology of a network is closely related to the mechanism used to manage network access. Access methods provide the procedures to manage network access so that all stations have access. When several entities share the same media, some mechanism must be in place to control access. Access methods are applied to networks to regulate this media access. Access methods will be discussed in more detail later.
Logical and physical topologies typically used in networks are:
- Point-to-Point
- Multi-Access
- Ring
7.2.4 - Logical Topology versus Physical Topology
The diagram depicts three logical topologies: point-to-point, multi-access, and ring.
Point-to-Point: Two routers are connected to a network cloud via serial WAN links.
Multi-access: Multiple PC's share a common bus-style media.
Ring: Multiple PC's are arranged in a ring.
The diagram depicts three logical topologies: point-to-point, multi-access, and ring.
Point-to-Point: Two routers are connected to a network cloud via serial WAN links.
Multi-access: Multiple PC's share a common bus-style media.
Ring: Multiple PC's are arranged in a ring.
7.2.5 Point-to-Point Topology
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A point-to-point topology connects two nodes directly together, as shown in the figure. In data networks with point-to-point topologies, the media access control protocol can be very simple. All frames on the media can only travel to or from the two nodes. The frames are placed on the media by the node at one end and taken off the media by the node at the other end of the point-to-point circuit.
In point-to-point networks, if data can only flow in one direction at a time, it is operating as a half-duplex link. If data can successfully flow across the link from each node simultaneously, it is a full-duplex link.
Data Link layer protocols could provide more sophisticated media access control processes for logical point-to-point topologies, but this would only add unnecessary protocol overhead.
7.2.5 - Point-to-Point Topology
The diagram depicts a point-to-point topology. Two routers, Node1 and Node2, are connected to a network cloud via serial WAN links. Point-to-point topologies are limited to two nodes.
The diagram depicts a point-to-point topology. Two routers, Node1 and Node2, are connected to a network cloud via serial WAN links. Point-to-point topologies are limited to two nodes.
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Logical Point-to-Point Networks
The end nodes communicating in a point-to-point network can be physically connected via a number of intermediate devices. However the use of physical devices in the network does not affect the logical topology. As shown in the figure, the source and destination node may be indirectly connected to each other over some geographical distance. In some cases, the logical connection between nodes forms what is called a virtual circuit. A virtual circuit is a logical connection created within a network between two network devices. The two nodes on either end of the virtual circuit exchange the frames with each other. This occurs even if the frames are directed through intermediary devices. Virtual circuits are important logical communication constructs used by some Layer 2 technologies.
The media access method used by the Data Link protocol is determined by the logical point-to-point topology, not the physical topology. This means that the logical point-to-point connection between two nodes may not necessarily be between two physical nodes at each end of a single physical link.
7.2.5 - Point-to-Point Topology
The diagram depicts a logical point-to-point topology. Two routers, Source Node and Destination Node, are connected to a network cloud via Frame Relay links. A rollover displays physical connections within the cloud, along with the following text: Adding intermediate physical connections may not change the logical topology. The logical point-to-point connection is the same.
The diagram depicts a logical point-to-point topology. Two routers, Source Node and Destination Node, are connected to a network cloud via Frame Relay links. A rollover displays physical connections within the cloud, along with the following text: Adding intermediate physical connections may not change the logical topology. The logical point-to-point connection is the same.
7.2.6 Multi-Access Topology
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A logical multi-access topology enables a number of nodes to communicate by using the same shared media. Data from only one node can be placed on the medium at any one time. Every node sees all the frames that are on the medium, but only the node to which the frame is addressed processes the contents of the frame.
Having many nodes share access to the medium requires a Data Link media access control method to regulate the transmission of data and thereby reduce collisions between different signals.
The media access control methods used by logical multi-access topologies are typically CSMA/CD or CSMA/CA. However, token passing methods can also be used.
A number of media access control techniques are available for this type of logical topology. The Data Link layer protocol specifies the media access control method that will provide the appropriate balance between frame control, frame protection, and network overhead.
Play the animation to see how nodes access the media in a multi-access topology.
7.2.6 - Multi-Access Topology
The animation depicts how nodes access the media in a multi-access topology. Multiple PC's, A, B, C, D, and E, share a common bus-style media. As the animation progresses, the nodes access the media as follows.
PC A text bubbles:
- I need to transmit to E.
- I check for other transmissions.
- No other transmissions are detected.
- Transmitting.
The animation shows the frame successfully arriving at PC E.
PC B text bubbles:
- I need to transmit to D.
- I check for other transmissions.
- Transmission detected. I'll wait.
- No other transmissions are detected.
- Transmitting.
The animation shows the frame successfully arriving at PC D.
The animation depicts how nodes access the media in a multi-access topology. Multiple PC's, A, B, C, D, and E, share a common bus-style media. As the animation progresses, the nodes access the media as follows.
PC A text bubbles:
- I need to transmit to E.
- I check for other transmissions.
- No other transmissions are detected.
- Transmitting.
The animation shows the frame successfully arriving at PC E.
PC B text bubbles:
- I need to transmit to D.
- I check for other transmissions.
- Transmission detected. I'll wait.
- No other transmissions are detected.
- Transmitting.
The animation shows the frame successfully arriving at PC D.
7.2.7 Ring Topology
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In a logical ring topology, each node in turn receives a frame. If the frame is not addressed to the node, the node passes the frame to the next node. This allows a ring to use a controlled media access control technique called token passing.
Nodes in a logical ring topology remove the frame from the ring, examine the address, and send it on if it is not addressed for that node. In a ring, all nodes around the ring- between the source and destination node examine the frame.
There are multiple media access control techniques that could be used with a logical ring, depending on the level of control required. For example, only one frame at a time is usually carried by the media. If there is no data being transmitted, a signal (known as a token) may be placed on the media and a node can only place a data frame on the media when it has the token.
Remember that the Data Link layer "sees" a logical ring topology. The actual physical cabling topology could be another topology.
Play the animation to see how nodes access the media in a logical ring topology.
7.2.7 - Ring Topology
The animation depicts how nodes access the media in a logical ring topology. Multiple PC's, A, B, C, and D, are arranged with the media in a ring formation. PC A transmits a frame that travels around the ring as it is passed from PC A to PC B to PC C and then to PC D. As the animation progresses, the nodes access the media as follows.
PC A text bubble:
- I need to transmit to D.
PC B text bubbles:
- Is this frame for me?
- No.
PC C text bubbles:
- Is this frame for me?
- No.
PC D text bubbles:
- Is this frame for me?
- Yes.
The animation depicts how nodes access the media in a logical ring topology. Multiple PC's, A, B, C, and D, are arranged with the media in a ring formation. PC A transmits a frame that travels around the ring as it is passed from PC A to PC B to PC C and then to PC D. As the animation progresses, the nodes access the media as follows.
PC A text bubble:
- I need to transmit to D.
PC B text bubbles:
- Is this frame for me?
- No.
PC C text bubbles:
- Is this frame for me?
- No.
PC D text bubbles:
- Is this frame for me?
- Yes.
7.3 Media Access Control Addressing and Framing Data
7.3.1 Data Link Layer Protocols - The FramePage 1:
Remember that although there are many different Data Link layer protocols that describe Data Link layer frames, each frame type has three basic parts:
- Header
- Data
- Trailer
The Data Link layer protocol describes the features required for the transport of packets across different media. These features of the protocol are integrated into the encapsulation of the frame. When the frame arrives at its destination and the Data Link protocol takes the frame off the media, the framing information is read and discarded.
There is no one frame structure that meets the needs of all data transportation across all types of media. As shown in the figure, depending on the environment, the amount of control information needed in the frame varies to match the media access control requirements of the media and logical topology.
7.3.1 - Data Link Layer Protocols - The Frame
The diagram depicts Data Link Layer protocol characteristics in a fragile environment and in a protected environment.
Fragile environment: The diagram show two routers, each one connected to a satellite dish that is communicating with a satellite. A rain cloud above the satellite indicates bad weather to illustrate a fragile environment where a higher potential exists for data transmission errors. In a fragile environment, more controls are needed to ensure delivery. The header and trailer fields are larger because more control information is needed. Greater effort is needed to ensure delivery, resulting in higher overhead and slower transmission rates.
Protected environment: The diagram shows three PC's communicating on a wired media illustrating a more stable environment. In a protected environment, the frame arrives at its destination. Fewer controls are needed, resulting in smaller fields and smaller frames. Less effort is needed to ensure delivery, resulting in lower overhead and faster transmission rates.
The diagram depicts Data Link Layer protocol characteristics in a fragile environment and in a protected environment.
Fragile environment: The diagram show two routers, each one connected to a satellite dish that is communicating with a satellite. A rain cloud above the satellite indicates bad weather to illustrate a fragile environment where a higher potential exists for data transmission errors. In a fragile environment, more controls are needed to ensure delivery. The header and trailer fields are larger because more control information is needed. Greater effort is needed to ensure delivery, resulting in higher overhead and slower transmission rates.
Protected environment: The diagram shows three PC's communicating on a wired media illustrating a more stable environment. In a protected environment, the frame arrives at its destination. Fewer controls are needed, resulting in smaller fields and smaller frames. Less effort is needed to ensure delivery, resulting in lower overhead and faster transmission rates.
7.3.2 Framing - Role of the Header
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As shown in the figure, the frame header contains the control information specified by the Data Link layer protocol for the specific logical topology and media used.
Frame control information is unique to each type of protocol. It is used by the Layer 2 protocol to provide features demanded by the communication environment.
Typical frame header fields include:
- Start Frame field - Indicates the beginning of the frame
- Source and Destination address fields - Indicates the source and destination nodes on the media
- Priority/Quality of Service field - Indicates a particular type of communication service for processing
- Type field - Indicates the upper layer service contained in the frame
- Logical connection control field - Used to establish a logical connection between nodes
- Physical link control field - Used to establish the media link
- Flow control field - Used to start and stop traffic over the media
- Congestion control field - Indicates congestion in the media
7.3.2 - Framing - Role of the Header
The diagram depicts a simple frame structure with a focus on the frame header role. The following components are shown from left to right.
Header:
- Start frame field - Tells other devices on the network that a frame is coming along the medium.
- Address field - Stores the source and destination data-link addresses.
- Type/Length field - Optional field used by some protocols to state either what type of data is coming or possible the length of the frame.
Data
FCS
Stop Frame
The diagram depicts a simple frame structure with a focus on the frame header role. The following components are shown from left to right.
Header:
- Start frame field - Tells other devices on the network that a frame is coming along the medium.
- Address field - Stores the source and destination data-link addresses.
- Type/Length field - Optional field used by some protocols to state either what type of data is coming or possible the length of the frame.
Data
FCS
Stop Frame
7.3.3 Addressing - Where the Frame Goes
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The data Link layer provides addressing that is used in transporting the frame across the shared local media. Device addresses at this layer are referred to as physical addresses. Data Link layer addressing is contained within the frame header and specifies the frame destination node on the local network. The frame header may also contain the source address of the frame.
Unlike Layer 3 logical addresses that are hierarchical, physical addresses do not indicate on what network the device is located. If the device is moved to another network or subnet, it will still function with the same Layer 2 physical address.
Because the frame is only used to transport data between nodes across the local media, the Data Link layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from source host to destination host regardless of the number of network hops along the route.
If the packet in the frame must pass onto another network segment, the intermediate device - a router - will decapsulate the original frame, create a new frame for the packet, and send it onto the new segment. The new frame will use source and destination addressing as necessary to transport the packet across the new media.
Addressing Requirements
The need for Data Link layer addressing at this layer depends on the logical topology.
Point-to-point topologies, with just two interconnected nodes, do not require addressing. Once on the medium, the frame has only one place it can go.
Because ring and multi-access topologies can connect many nodes on a common medium, addressing is required for these typologies. When a frame reaches each node in the topology, the node examines the destination address in the header to determine if it is the destination of the frame.
7.3.3 - Addressing - Where the Frame Goes
The diagram depicts addressing factors for a logical point-to-point topology and a logical multi-access topology.
Logical Multi-Access Topology:
The diagram shows multiple PC's sharing a common bus-style media. A multi-access frame has many possible destinations. Data Link Layer addresses are required.
Logical Point-to-Point Topology:
The diagram shows two routers connected to a network cloud via serial WAN links. A point-to-point frame has only one possible destination. Data Link Layer addresses are not required.
The diagram depicts addressing factors for a logical point-to-point topology and a logical multi-access topology.
Logical Multi-Access Topology:
The diagram shows multiple PC's sharing a common bus-style media. A multi-access frame has many possible destinations. Data Link Layer addresses are required.
Logical Point-to-Point Topology:
The diagram shows two routers connected to a network cloud via serial WAN links. A point-to-point frame has only one possible destination. Data Link Layer addresses are not required.
7.3.4 Framing - Role of the Trailer
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Data Link layer protocols add a trailer to the end of each frame. The trailer is used to determine if the frame arrived without error. This process is called error detection. Note that this is different from error correction. Error detection is accomplished by placing a logical or mathematical summary of the bits that comprise the frame in the trailer.
Frame Check Sequence
The Frame Check Sequence (FCS) field is used to determine if errors occurred in the transmission and reception of the frame. Error detection is added at the Data Link layer because this is where data is transferred across the media. The media is a potentially unsafe environment for data. The signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent. The error detection mechanism provided by the use of the FCS field discovers most errors caused on the media.
To ensure that the content of the received frame at the destination matches that of the frame that left the source node, a transmitting node creates a logical summary of the contents of the frame. This is known as the cyclic redundancy check (CRC) value. This value is placed in the Frame Check Sequence (FCS) field of the frame to represent the contents of the frame.
When the frame arrives at the destination node, the receiving node calculates its own logical summary, or CRC, of the frame. The receiving node compares the two CRC values. If the two values are the same, the frame is considered to have arrived as transmitted. If the CRC value in the FCS differs from the CRC calculated at the receiving node, the frame is discarded.
There is always the small possibility that a frame with a good CRC result is actually corrupt. Errors in bits may cancel each other out when the CRC is calculated. Upper layer protocols would then be required to detect and correct this data loss.
The protocol used in the Data Link layer, will determine if error correction will take place. The FCS is used to detect the error, but not every protocol will support correcting the error.
7.3.4 - Framing - Role of the Trailer
The diagram depicts a simple frame structure focusing on the frame trailer. The following components are shown from left to right.
Start Frame
Address
Type/Length
Data
Trailer
- FCS field - The Frame Check Sequence is used for error checking. The source calculates a number based on the frame's data and places that number in the FCS field. The destination then recalculates the data to check whether the FCS matches. If they do not match, the destination deletes the frame.
- Stop Frame field - Also called the Frame Trailer. An optional field that is used when the length of the frame is not specified in the Type/Length field. It indicates the end of the frame when transmitted.
The diagram depicts a simple frame structure focusing on the frame trailer. The following components are shown from left to right.
Start Frame
Address
Type/Length
Data
Trailer
- FCS field - The Frame Check Sequence is used for error checking. The source calculates a number based on the frame's data and places that number in the FCS field. The destination then recalculates the data to check whether the FCS matches. If they do not match, the destination deletes the frame.
- Stop Frame field - Also called the Frame Trailer. An optional field that is used when the length of the frame is not specified in the Type/Length field. It indicates the end of the frame when transmitted.
7.3.5 Data Link Layer Protocols - The Frame
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In a TCP/IP network, all OSI Layer 2 protocols work with the Internet Protocol at OSI Layer 3. However, the actual Layer 2 protocol used depends on the logical topology of the network and the implementation of the Physical layer. Given the wide range of physical media used across the range of topologies in networking, there are a correspondingly high number of Layer 2 protocols in use.
Protocols that will be covered in CCNA courses include:
- Ethernet
- Point-to-Point Protocol (PPP)
- High-Level Data Link Control (HDLC)
- Frame Relay
- Asynchronous Transfer Mode (ATM)
The Layer 2 protocol used for a particular network topology is determined by the technology used to implement that topology. The technology is, in turn, determined by the size of the network - in terms of the number of hosts and the geographic scope - and the services to be provided over the network.
LAN Technology
A Local Area Network typically uses a high bandwidth technology that is capable of supporting large numbers of hosts. A LAN's relatively small geographic area (a single building or a multi-building campus) and its high density of users make this technology cost effective.
WAN Technology
However, using a high bandwidth technology is usually not cost-effective for Wide Area Networks that cover large geographic areas (cities or multiple cities, for example). The cost of the long distance physical links and the technology used to carry the signals over those distances typically results in lower bandwidth capacity.
Difference in bandwidth normally results in the use of different protocols for LANs and WANs.
7.3.5 - Data Link Layer Protocols - The Frame
The animation depicts examples of changing Data Link Layer protocols as a packet traverses various links in a network.
As the animation progresses, the following occurs.
Step 1. Wireless PC1 sends an 8 0 2 dot 11 wireless frame to wireless router R1.
Step 2. The wireless router sends a PPP frame to router R2 over a point-to-point WAN link.
Step 3. Router R2 sends an HDLC frame to router R3 over a point-to-point WAN link.
Step 4. Router R3 sends a Frame Relay frame to router R4 over a WAN link through a Frame Relay cloud.
Step 5. Router R4 sends an Ethernet frame to switch S1 on a LAN, which then sends the Ethernet frame to wired PC2.
The animation depicts examples of changing Data Link Layer protocols as a packet traverses various links in a network.
As the animation progresses, the following occurs.
Step 1. Wireless PC1 sends an 8 0 2 dot 11 wireless frame to wireless router R1.
Step 2. The wireless router sends a PPP frame to router R2 over a point-to-point WAN link.
Step 3. Router R2 sends an HDLC frame to router R3 over a point-to-point WAN link.
Step 4. Router R3 sends a Frame Relay frame to router R4 over a WAN link through a Frame Relay cloud.
Step 5. Router R4 sends an Ethernet frame to switch S1 on a LAN, which then sends the Ethernet frame to wired PC2.
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Ethernet Protocol for LANs
Ethernet is a family of networking technologies that are defined in the IEEE 802.2 and 802.3 standards. Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies. Ethernet is the most widely used LAN technology and supports data bandwidths of 10, 100, 1000, or 10,000 Mbps.
The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet. However, the methods for detecting and placing data on the media vary with different implementations.
Ethernet provides unacknowledged connectionless service over a shared media using CSMA/CD as the media access methods. Shared media requires that the Ethernet frame header use a Data Link layer address to identify the source and destination nodes. As with most LAN protocols, this address is referred to as the MAC address of the node. An Ethernet MAC address is 48 bits and is generally represented in hexadecimal format.
The Ethernet frame has many fields, as shown in the figure. At the Data Link layer, the frame structure is nearly identical for all speeds of Ethernet. However, at the Physical layer, different versions of Ethernet place the bits onto the media differently.
Ethernet II is the Ethernet frame format used in TCP/IP networks.
Ethernet is such an important part of data networking, we have devoted a chapter to it. We also use it in examples throughout this series of courses.
7.3.5 - Data Link Layer Protocols - The Frame
The diagram depicts an Ethernet frame, the most common Data Link Layer protocol for LAN's. An Ethernet frame's field names and sizes are shown in the following sequence, with information describing each.
Preamble: 8 bytes. Used for synchronization. Also contains a delimiter to mark the end of the timing information.
Destination Address: 6 bytes. 48-bit MAC address for the destination node.
Source Address: 6 bytes. 48-bit MAC address for the source node.
Type: 2 bytes. Indicates which upper layer protocol receives the data after the Ethernet process is complete.
Data (or payload): 46 to 1500 bytes. This is the PDU, typically an IPv4 packet, that is to be transported over the media.
Frame Check Sequence (FCS): 4 bytes. Used to check for damaged frames.
The diagram depicts an Ethernet frame, the most common Data Link Layer protocol for LAN's. An Ethernet frame's field names and sizes are shown in the following sequence, with information describing each.
Preamble: 8 bytes. Used for synchronization. Also contains a delimiter to mark the end of the timing information.
Destination Address: 6 bytes. 48-bit MAC address for the destination node.
Source Address: 6 bytes. 48-bit MAC address for the source node.
Type: 2 bytes. Indicates which upper layer protocol receives the data after the Ethernet process is complete.
Data (or payload): 46 to 1500 bytes. This is the PDU, typically an IPv4 packet, that is to be transported over the media.
Frame Check Sequence (FCS): 4 bytes. Used to check for damaged frames.
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Point-to-Point Protocol for WANs
Point-to-Point Protocol (PPP) is a protocol used to deliver frames between two nodes. Unlike many Data Link layer protocols that are defined by electrical engineering organizations, the PPP standard is defined by RFCs. PPP was developed as a WAN protocol and remains the protocol of choice to implement many serial WANs. PPP can be used on various physical media, including twisted pair, fiber optic lines, and satellite transmission, as well as for virtual connections.
PPP uses a layered architecture. To accommodate the different types of media, PPP establishes logical connections, called sessions, between two nodes. The PPP session hides the underlying physical media from the upper PPP protocol. These sessions also provide PPP with a method for encapsulating multiple protocols over a point-to-point link. Each protocol encapsulated over the link establishes its own PPP session.
PPP also allows the two nodes to negotiate options within the PPP session. This includes authentication, compression, and multilink (the use of multiple physical connections).
See the figure for the basic fields in a PPP frame.
PPP protocol: http://www.ietf.org/rfc/rfc1661.txt?number=1661
PPP Vendor Extensions: http://www.ietf.org/rfc/rfc2153.txt?number=2153
7.3.5 - Data Link Layer Protocols - The Frame
The diagram depicts a Point-to-Point Protocol (PPP) frame, a common Data Link Layer protocol for WAN's. Field names and sizes are shown in the following sequence, with information describing each.
Flag: 1 byte. Indicates the beginning or end of a frame. The flag consists of the binary sequence 01111110.
Address: 1 byte. Contains the standard PPP broadcast address. PPP does not assign individual station addresses.
Control: 1 byte. Contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame.
Protocol: 2 bytes. Identifies the protocol encapsulated in the data field of the frame. The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).
Data: Variable number of bytes. Zero or more bytes that contain the datagram for the protocol specified in the protocol field.
Frame Check Sequence (FCS): 2 or 4 bytes. Normally, 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection.
The diagram depicts a Point-to-Point Protocol (PPP) frame, a common Data Link Layer protocol for WAN's. Field names and sizes are shown in the following sequence, with information describing each.
Flag: 1 byte. Indicates the beginning or end of a frame. The flag consists of the binary sequence 01111110.
Address: 1 byte. Contains the standard PPP broadcast address. PPP does not assign individual station addresses.
Control: 1 byte. Contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame.
Protocol: 2 bytes. Identifies the protocol encapsulated in the data field of the frame. The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).
Data: Variable number of bytes. Zero or more bytes that contain the datagram for the protocol specified in the protocol field.
Frame Check Sequence (FCS): 2 or 4 bytes. Normally, 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection.
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Wireless Protocol for LANs
802.11 is an extension of the IEEE 802 standards. It uses the same 802.2 LLC and 48-bit addressing scheme as other 802 LANs, However there are many differences at the MAC sublayer and Physical layer. In a wireless environment, the environment requires special considerations. There is no definable physical connectivity; therefore, external factors may interfere with data transfer and it is difficult to control access. To meet these challenges, wireless standards have additional controls.
The Standard IEEE 802.11, commonly referred to as Wi-Fi, is a contention-based system using a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) media access process. CSMA/CA specifies a random backoff procedure for all nodes that are waiting to transmit. The most likely opportunity for medium contention is just after the medium becomes available. Making the nodes back off for a random period greatly reduces the likelihood of a collision.
802.11 networks also use Data Link acknowledgements to confirm that a frame is received successfully. If the sending station does not detect the acknowledgement frame, either because the original data frame or the acknowledgment was not received intact, the frame is retransmitted. This explicit acknowledgement overcomes interference and other radio-related problems.
Other services supported by 802.11 are authentication, association (connectivity to a wireless device), and privacy (encryption).
An 802.11 frame is shown in the figure. It contains these fields:
Protocol Version field - Version of 802.11 frame in use
Type and Subtype fields - Identifies one of three functions and sub functions of the frame: control, data, and management
To DS field - Set to 1 in data frames destined for the distribution system (devices in the wireless structure)
From DS field - Set to 1 in data frames exiting the distribution system
More Fragments field - Set to 1 for frames that have another fragment
Retry field - Set to 1 if the frame is a retransmission of an earlier frame
Power Management field - Set to 1 to indicate that a node will be in power-save mode
More Data field - Set to 1 to indicate to a node in power-save mode that more frames are buffered for that node
Wired Equivalent Privacy (WEP) field - Set to 1 if the frame contains WEP encrypted information for security
Order field - Set to 1 in a data type frame that uses Strictly Ordered service class (does not need reordering)
Duration/ID field - Depending on the type of frame, represents either the time, in microseconds, required to transmit the frame or an association identity (AID) for the station that transmitted the frame
Destination Address (DA) field - MAC address of the final destination node in the network
Source Address (SA) field - MAC address of the node the initiated the frame
Receiver Address (RA) field - MAC address that identifies the wireless device that is the immediate recipient of the frame
Transmitter Address (TA) field - MAC address that identifies the wireless device that transmitted the frame
Sequence Number field - Indicates the sequence number assigned to the frame; retransmitted frames are identified by duplicate sequence numbers
Fragment Number field - Indicates the number for each fragment of a frame
Frame Body field - Contains the information being transported; for data frames, typically an IP packet
FCS field - Contains a 32-bit cyclic redundancy check (CRC) of the frame
7.3.5 - Data Link Layer Protocols - The Frame
The diagram depicts an 8 0 2 dot 11 Wireless Protocol frame, a commonly used Data Link Layer protocol for wireless LAN's. An 8 0 2 dot 11 frame contains these fields:
Frame Control Field: (2 bytes)
- Protocol Version
- Type
- Subtype
- To DS
- From DS
- More Fragments
- Retry
- Power Management
- More Data
- Wired Equivalent Privacy (WEP)
- Order
Duration/ID (2 bytes)
Destination Address (DA) (6 bytes)
Source Address (SA) (6 bytes)
Receiver Address (RA) (6 bytes)
Sequence Control (2 bytes)
- Fragment Number
- Sequence Number
Transmitter Address (TA) (6 bytes)
Frame Body (0-2312 bytes)
FCS field (4 bytes)
The diagram depicts an 8 0 2 dot 11 Wireless Protocol frame, a commonly used Data Link Layer protocol for wireless LAN's. An 8 0 2 dot 11 frame contains these fields:
Frame Control Field: (2 bytes)
- Protocol Version
- Type
- Subtype
- To DS
- From DS
- More Fragments
- Retry
- Power Management
- More Data
- Wired Equivalent Privacy (WEP)
- Order
Duration/ID (2 bytes)
Destination Address (DA) (6 bytes)
Source Address (SA) (6 bytes)
Receiver Address (RA) (6 bytes)
Sequence Control (2 bytes)
- Fragment Number
- Sequence Number
Transmitter Address (TA) (6 bytes)
Frame Body (0-2312 bytes)
FCS field (4 bytes)
7.4 Putting it All Together
7.4.1 Follow Data Through an InternetworkPage 1:
The figure on the next page presents a simple data transfer between two hosts across an internetwork. We highlight the function of each layer during the communication. For this example we will depict an HTTP request between a client and a server.
To focus on the data transfer process, we are omitting many elements that may occur in a real transaction. In each step we are only bringing attention to the major elements. Many parts of the headers are ignored, for example.
We are assuming that all routing tables are converged and ARP tables are complete. Additionally, we are assuming that a TCP session is already established between the client and server. We will also assume that the DNS lookup for the WWW server is already cached at the client.
In the WAN connection between the two routers, we are assuming that PPP has already established a physical circuit and has established a PPP session.
On the next page, you can step through this communication. We encourage you to read each explanation carefully and study the operation of the layers for each device.
7.4.1 - Follow Data Through an Internetwork
The diagram depicts a simple data transfer between two hosts across an internetwork.
Network Topology:
A user at a client PC, the requesting host, is sending a request to a server, the receiving host. The client PC is connected to router B. Router B is connected to router A via a WAN link. Router A is connected to the server.
The diagram depicts a simple data transfer between two hosts across an internetwork.
Network Topology:
A user at a client PC, the requesting host, is sending a request to a server, the receiving host. The client PC is connected to router B. Router B is connected to router A via a WAN link. Router A is connected to the server.
Page 2:
7.4.1 - Follow Data Through an Internetwork
The diagram depicts the steps involved in an HTTP request between a client and a server.
Network Topology: This example shows an HTTP request between a client and a server. The topology is the same as diagram 1 with the addition of the O S I model. The appropriate layer of the O S I model is highlighted, from top to bottom, as the request from the client to the server is processed. This process is explained further through a series of steps.
Step 1. A user on a LAN wants to access a web page stored on a server that is located on a remote network. The user starts by activating a link on a web page.
Step 2. The browser initiates an HTTP Get request. The Application Layer adds the Layer 7 header to identify the application and data type.
Step 3. The Transport Layer identifies the upper layer service as a World Wide Web (WWW) client. The Transport Layer then associates this service with TCP and assigns the port numbers. It uses a randomly selected source port that is associated with this established session (12345). The destination port (80) is associated with a WWW service.
Step 4. TCP also sends an acknowledgement number that tells the WWW server the sequence number of the next TCP segment that it expects to receive. The sequence number indicates where this segment is placed in the series of related segments. Flags are also set as appropriate to establish a session.
Step 5. At the Network Layer, an IP packet is constructed to identify the source and destination hosts. For the destination address, the client host uses the IP address associated with the WWW server host name that is cached in the host table. It uses its own IPv4 address as the source address. The Network Layer also identifies the upper layer protocol encapsulated in this packet as a TCP segment.
Step 6. The Data Link Layer refers to the Address Resolution Protocol (ARP) cache to determine the MAC address that is associated with the interface of router B, which is specified as the default gateway. It then uses this address to build an Ethernet frame to transport the IPv4 packet across the local media. The MAC address of the laptop is used as the source MAC address, and the MAC address of the FA0/0 interface of router B is used as the destination MAC address in the frame.
Step 7. The frame also indicates the upper layer protocol of IPv4 with a value of 0800 (hex) in the Type field. The frame begins with the Preamble and ends with a cyclic redundancy check (CRC) in the Frame Check Sequence at the end of the frame for the error detection. It then uses CSMA/CD to control the placing of the frame onto the media.
Step 8. The Physical Layer begins encoding the frame onto the media, bit by bit. The segment between router B and the source host is a 10 Base-T segment; therefore, the bits are encoded using the Manchester Differential encoding. Router B buffers the bits as they are received.
Step 9. Router B examines the bits in the preamble looking for the two consecutive 1 bits that indicate that the synching process is completed and the beginning of the frame. Router B then begins buffering the bits as part of the reconstructed frame. When the entire frame is received, Router B generates a CRC of the frame. It then compares this to the FCS at the end of the frame to determine that the frame was received intact. When the frame is confirmed as a good frame, the destination MAC address in the frame is compared to the MAC address of the interface (FA0/0). Because it matches, the headers are removed, and the packet is pushed up to the Network Layer.
Step 10. At the Network Layer, the destination IPv4 address of the packet is compared against the routes in the routing table. A match is found that is associated with the next-hop out interface S0/0/0. The packet inside router B is then passed to the circuitry for the S0/0/0 interface.
Step 11. Router B creates a PPP frame to transport the packet across the WAN. In the PPP header, a flag of 01111110 binary is added to indicate the start of the frame. Following that, an address field of 11111111 is added, which is equivalent to a broadcast (it means "send to all stations"). Because PPP is point-to-point and is used as a direct link between two nodes, this field has no real meaning.
Step 12. Also included is a Protocol field with a value of 0021 (hex) to indicate that an IPv4 packet is encapsulated. The frame trailer ends with a CRC in the FCS for error detection. A Flag value of 01111110 binary indicates the end of a PPP frame.
Step 13. With the circuit and PPP session already established between the routers, the Physical Layer begins encoding the frame onto the WAN media, bit by bit. The receiving router (router A) buffers the bits as they are received. This type of bit representation and encoding is dependent on the type of WAN technology being used.
Step 14. Router A examines the bits in the flag to identify the beginning of the frame. Router A then begins buffering the bits as part of the reconstructed frame. When the entire frame is received, as indicated by the flag in the trailer, router A generates a CRC of the frame. It then compares this to the FCS at the end of the frame to determine that the frame was received intact. When the frame is confirmed as a good frame, the headers are removed, and the packet is pushed up to the Network Layer of router A.
Step 15. At the Network Layer, the destination IPv4 address of the packet is compared against the routes in the routing table. A match is found that is directly connected to interface FA0/0. The packet inside router A is then passed to the circuitry for the FA0/0 interface.
Step 16. The Data Link Layer refers to the ARP cache of router A to determine the MAC address that is associated with the interface of the Web Server. It then uses this MAC address to build an Ethernet frame to transport the IPv4 packet across the local media to the server. The MAC address of the FA0/0 interface of router A is used as the source MAC address, and the MAC address of the server is used as the destination MAC address in the frame. The frame also indicates the upper layer protocol of IPv4 with a value of 0800 (hex) in the Type field. The frame begins with the Preamble and ends with a CRC in the FCS at the end of the frame for the error detection. It then uses CSMA/CD to control the placing of the frame onto the media.
Step 17. The Physical Layer begins encoding the frame onto the media, bit by bit. The segment between router A and the server is a 100 Base-T segment; therefore, the bits are encoded using 4B/5B encoding. The server buffers the bits as they are received.
Step 18. Router B examines the bits in the preamble, looking for the two consecutive 1 bits that indicate that the synching process is completed and the beginning of the frame. The server then begins buffering the bits as part of the reconstructed frame. When it has received the entire frame, the server generates a CRC of the frame. It then compares this to the FCS at the end of the frame to determine that the frame was received intact.
Step 19. When the frame is confirmed as a good frame, the destination MAC address in the frame is compared to the MAC address of the NIC in the server. Because it matches, the headers are removed and the packet is pushed up to the Network Layer.
Step 20. At the Network Layer, the destination IPv4 address of the packet is examined to identify the destination host. Because this address matches its own IPv4 address, the packet is processed by the server. The Network Layer identifies the upper layer protocol as TCP and directs the contained segment to the TCP service at the Transport Layer.
Step 21. At the Transport Layer of the server, the TCP segment is examined to determine the session to which the data contained in the segment belongs. This is done by examining the source and destination ports. The unique source and destination port identifies an existing session to the web server service. The sequence number is used to place this segment in the proper order to be sent upward to the Application Layer.
Step 22. At the Application Layer, the HTTP Get request is delivered to the Web Server service (httpd). The service can then formulate a response to the request.
The diagram depicts the steps involved in an HTTP request between a client and a server.
Network Topology: This example shows an HTTP request between a client and a server. The topology is the same as diagram 1 with the addition of the O S I model. The appropriate layer of the O S I model is highlighted, from top to bottom, as the request from the client to the server is processed. This process is explained further through a series of steps.
Step 1. A user on a LAN wants to access a web page stored on a server that is located on a remote network. The user starts by activating a link on a web page.
Step 2. The browser initiates an HTTP Get request. The Application Layer adds the Layer 7 header to identify the application and data type.
Step 3. The Transport Layer identifies the upper layer service as a World Wide Web (WWW) client. The Transport Layer then associates this service with TCP and assigns the port numbers. It uses a randomly selected source port that is associated with this established session (12345). The destination port (80) is associated with a WWW service.
Step 4. TCP also sends an acknowledgement number that tells the WWW server the sequence number of the next TCP segment that it expects to receive. The sequence number indicates where this segment is placed in the series of related segments. Flags are also set as appropriate to establish a session.
Step 5. At the Network Layer, an IP packet is constructed to identify the source and destination hosts. For the destination address, the client host uses the IP address associated with the WWW server host name that is cached in the host table. It uses its own IPv4 address as the source address. The Network Layer also identifies the upper layer protocol encapsulated in this packet as a TCP segment.
Step 6. The Data Link Layer refers to the Address Resolution Protocol (ARP) cache to determine the MAC address that is associated with the interface of router B, which is specified as the default gateway. It then uses this address to build an Ethernet frame to transport the IPv4 packet across the local media. The MAC address of the laptop is used as the source MAC address, and the MAC address of the FA0/0 interface of router B is used as the destination MAC address in the frame.
Step 7. The frame also indicates the upper layer protocol of IPv4 with a value of 0800 (hex) in the Type field. The frame begins with the Preamble and ends with a cyclic redundancy check (CRC) in the Frame Check Sequence at the end of the frame for the error detection. It then uses CSMA/CD to control the placing of the frame onto the media.
Step 8. The Physical Layer begins encoding the frame onto the media, bit by bit. The segment between router B and the source host is a 10 Base-T segment; therefore, the bits are encoded using the Manchester Differential encoding. Router B buffers the bits as they are received.
Step 9. Router B examines the bits in the preamble looking for the two consecutive 1 bits that indicate that the synching process is completed and the beginning of the frame. Router B then begins buffering the bits as part of the reconstructed frame. When the entire frame is received, Router B generates a CRC of the frame. It then compares this to the FCS at the end of the frame to determine that the frame was received intact. When the frame is confirmed as a good frame, the destination MAC address in the frame is compared to the MAC address of the interface (FA0/0). Because it matches, the headers are removed, and the packet is pushed up to the Network Layer.
Step 10. At the Network Layer, the destination IPv4 address of the packet is compared against the routes in the routing table. A match is found that is associated with the next-hop out interface S0/0/0. The packet inside router B is then passed to the circuitry for the S0/0/0 interface.
Step 11. Router B creates a PPP frame to transport the packet across the WAN. In the PPP header, a flag of 01111110 binary is added to indicate the start of the frame. Following that, an address field of 11111111 is added, which is equivalent to a broadcast (it means "send to all stations"). Because PPP is point-to-point and is used as a direct link between two nodes, this field has no real meaning.
Step 12. Also included is a Protocol field with a value of 0021 (hex) to indicate that an IPv4 packet is encapsulated. The frame trailer ends with a CRC in the FCS for error detection. A Flag value of 01111110 binary indicates the end of a PPP frame.
Step 13. With the circuit and PPP session already established between the routers, the Physical Layer begins encoding the frame onto the WAN media, bit by bit. The receiving router (router A) buffers the bits as they are received. This type of bit representation and encoding is dependent on the type of WAN technology being used.
Step 14. Router A examines the bits in the flag to identify the beginning of the frame. Router A then begins buffering the bits as part of the reconstructed frame. When the entire frame is received, as indicated by the flag in the trailer, router A generates a CRC of the frame. It then compares this to the FCS at the end of the frame to determine that the frame was received intact. When the frame is confirmed as a good frame, the headers are removed, and the packet is pushed up to the Network Layer of router A.
Step 15. At the Network Layer, the destination IPv4 address of the packet is compared against the routes in the routing table. A match is found that is directly connected to interface FA0/0. The packet inside router A is then passed to the circuitry for the FA0/0 interface.
Step 16. The Data Link Layer refers to the ARP cache of router A to determine the MAC address that is associated with the interface of the Web Server. It then uses this MAC address to build an Ethernet frame to transport the IPv4 packet across the local media to the server. The MAC address of the FA0/0 interface of router A is used as the source MAC address, and the MAC address of the server is used as the destination MAC address in the frame. The frame also indicates the upper layer protocol of IPv4 with a value of 0800 (hex) in the Type field. The frame begins with the Preamble and ends with a CRC in the FCS at the end of the frame for the error detection. It then uses CSMA/CD to control the placing of the frame onto the media.
Step 17. The Physical Layer begins encoding the frame onto the media, bit by bit. The segment between router A and the server is a 100 Base-T segment; therefore, the bits are encoded using 4B/5B encoding. The server buffers the bits as they are received.
Step 18. Router B examines the bits in the preamble, looking for the two consecutive 1 bits that indicate that the synching process is completed and the beginning of the frame. The server then begins buffering the bits as part of the reconstructed frame. When it has received the entire frame, the server generates a CRC of the frame. It then compares this to the FCS at the end of the frame to determine that the frame was received intact.
Step 19. When the frame is confirmed as a good frame, the destination MAC address in the frame is compared to the MAC address of the NIC in the server. Because it matches, the headers are removed and the packet is pushed up to the Network Layer.
Step 20. At the Network Layer, the destination IPv4 address of the packet is examined to identify the destination host. Because this address matches its own IPv4 address, the packet is processed by the server. The Network Layer identifies the upper layer protocol as TCP and directs the contained segment to the TCP service at the Transport Layer.
Step 21. At the Transport Layer of the server, the TCP segment is examined to determine the session to which the data contained in the segment belongs. This is done by examining the source and destination ports. The unique source and destination port identifies an existing session to the web server service. The sequence number is used to place this segment in the proper order to be sent upward to the Application Layer.
Step 22. At the Application Layer, the HTTP Get request is delivered to the Web Server service (httpd). The service can then formulate a response to the request.
Page 3:
In this activity, you can examine in further detail the step-by-step animation on the previous page.
7.4.1 - Follow Data Through an Internetwork
Link to Packet Tracer Exploration: Packet Tracing Across an Internetwork
In this activity, you can examine in further detail the step-by-step animation on the previous page.
Link to Packet Tracer Exploration: Packet Tracing Across an Internetwork
In this activity, you can examine in further detail the step-by-step animation on the previous page.
7.5 Labs and Activities
7.5.1 Investigating Layer 2 Frame HeadersPage 1:
In this activity, you can explore some of the most common Layer 2 encapsulations.
Click the Packet Tracer icon to launch the Packet Tracer activity.
7.5.1 Investigating Layer 2 Frame Headers
Link to Packet Tracer Exploration: Investigate the Layer 2 Frame Headers
In this activity, you can explore some of the most common Layer 2 encapsulations.
Link to Packet Tracer Exploration: Investigate the Layer 2 Frame Headers
In this activity, you can explore some of the most common Layer 2 encapsulations.
7.5.2 Lab - Frame Examination
Page 1:
In this lab, you will use Wireshark to capture and analyze Ethernet II frame header fields.
Click the Lab icon to for more information.
7.5.2 Lab - Frame Examination
Link to Hands-on Lab: Frame Examination
In this lab, you use Wireshark to capture and analyze Ethernet II frame header fields.
Link to Hands-on Lab: Frame Examination
In this lab, you use Wireshark to capture and analyze Ethernet II frame header fields.
7.6 Chapter Summary
7.6.1 Summary and ReviewPage 1:
The OSI Data Link layer prepares Network layer packets for placement onto the physical media that transports data.
The wide range of data communications media requires a correspondingly wide range of Data Link protocols to control data access to these media.
Media access can be orderly and controlled or it can be contention-based. The logical topology and physical medium help determine the media access method.
The Data Link layer prepares the data for placement on the media by encapsulating the Layer 3 packet into a frame.
A frame has header and trailer fields that include Data Link source and destination addresses, QoS, type of protocol, and Frame Check Sequence values.
7.6.1 - Summary and Review
In this chapter, you learned to:
- Explain the role of Data Link Layer protocols in data transmission.
- Describe how the Data Link Layer prepares data for transmission on network media.
- Describe the different types of media access control methods.
- Identify several common logical network topologies, and describe how the logical topology determines the media access control method for that network.
- Explain the purpose of encapsulating packets into frames to facilitate media access.
- Describe the Layer 2 frame structure and identify generic fields.
- Explain the role of key frame header and trailer fields, including addressing, Q o S, type of protocol, and Frame Check Sequence.
In this chapter, you learned to:
- Explain the role of Data Link Layer protocols in data transmission.
- Describe how the Data Link Layer prepares data for transmission on network media.
- Describe the different types of media access control methods.
- Identify several common logical network topologies, and describe how the logical topology determines the media access control method for that network.
- Explain the purpose of encapsulating packets into frames to facilitate media access.
- Describe the Layer 2 frame structure and identify generic fields.
- Explain the role of key frame header and trailer fields, including addressing, Q o S, type of protocol, and Frame Check Sequence.
Page 2:
7.6.1 - Summary and Review
This is a review and is not a quiz. Questions and answers are provided.
Question 1: How does the Data Link Layer prepare packets for transmission?
Answer: The Data Link Layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.
Question 2: Describe four general Data Link Layer media access methods. Suggest data communications environments in which these access methods may be appropriately implemented.
Answer:
Media access control methods for shared media:
Controlled - Each node has its own time to use the medium; a ring topology.
-Contention-based - All nodes compete for the use of the medium; a bus topology.
Media access control in point-to-point connections:
Half duplex - A node can only transmit or receive at one time; a long-distance, low-bandwidth link.
Full duplex - A node can both transmit and receive at the same time; a long-distance, high-bandwidth link.
Question 3: Compare and contrast the logical point-to-point and logical multi-access topologies.
Answer: A logical point-to-point topology connects two nodes directly together. In data networks with point-to-point topologies, the media access control protocol can be very simple. All frames on the media can only travel to or from the two nodes. The frames are placed on the media by the node at one end and taken off the media by the node at the other end. In point-to-point networks, if data can only flow in one direction at a time, it is operating as a half-duplex link. If data can successfully flow across the link from each node simultaneously, it is a full-duplex service.
A logical multi-access topology enables a number of nodes to communicate by using the same shared media. Data from only one node can be placed on the medium at any one time. Every node sees all the frames that are on the medium, but only the node to which the frame is addressed processes the contents of the frame. Having many nodes share access to the medium requires a Data Link media access control method to regulate the transmission of data and thereby reduce collisions between different signals.
Question 4: Describe the features of a logical ring topology.
Answer: In a logical ring topology, each node in turn receives a frame. If the frame is not addressed to a node, the node passes the frame to the next node. This allows a ring to use a controlled media access control technique called token passing.
The media usually carries only one frame at a time. If there is no data being transmitted, a signal (known as a token) can be placed on the media. A node can place a data frame on the media only when it has the token.
Question 5: Name five Layer 2 protocols.
Answer:
- Point-to-Point Protocol (PPP)
- Ethernet
- High-Level Data Link Control (HDLC)
- Frame Relay
- Asynchronous Transfer Mode (ATM)
Question 6: How do Data Link Layer addresses differ from Network Layer addresses?
Answer: Unlike Layer 3 logical addresses that are hierarchical, physical addresses do not indicate on which network the device is located. If the device is moved to another network or subnet, it still functions with the same Layer 2 physical address.
Because the frame is only used to transport data between nodes across the local media, the Data Link Layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from source host to destination host, regardless of the number of network hops along the route.
Question 7: What are the possible header field types in Data Link frames?
Answer: Typical frame header fields include:
- Start Frame field - Indicates the beginning of the frame.
- Source and Destination address fields - Indicates the source and destination nodes on the media.
- Priority/Quality of Service field - Indicates a particular type of communication service for processing.
- Type field - Indicates the upper layer service contained in the frame.
- Logical connection control field - Establishes a logical connection between nodes.
- Physical link control field - Establishes the media link.
- Flow control field - Starts and stops traffic over the media.
- Congestion control field - Indicates congestion in the media.
Question 8: Give the purpose of the Frame Check Sequence field in a Data Link frame trailer.
Answer: The media is a potentially unsafe environment for data. The signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent. To ensure that the content of the received frame at the destination matches that of the frame that left the source node, a transmitting node creates a logical summary of the contents of the frame. This is known as the Frame Check Sequence (FCS) and is placed in the trailer to represent the contents of the frame. When the frame arrives at the destination node, the receiving node calculates its own logical summary, or FCS, of the frame. The receiving node compares the two FCS values. If the two values are the same, the frame is considered to have arrived as transmitted. If the FCS values differ, the frame is discarded. There is always the small possibility that a frame with a good FCS result is actually corrupt. Errors in bits can cancel each other out when the FCS is calculated. Upper layer protocols would then be required to detect and correct this data loss.
This is a review and is not a quiz. Questions and answers are provided.
Question 1: How does the Data Link Layer prepare packets for transmission?
Answer: The Data Link Layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.
Question 2: Describe four general Data Link Layer media access methods. Suggest data communications environments in which these access methods may be appropriately implemented.
Answer:
Media access control methods for shared media:
Controlled - Each node has its own time to use the medium; a ring topology.
-Contention-based - All nodes compete for the use of the medium; a bus topology.
Media access control in point-to-point connections:
Half duplex - A node can only transmit or receive at one time; a long-distance, low-bandwidth link.
Full duplex - A node can both transmit and receive at the same time; a long-distance, high-bandwidth link.
Question 3: Compare and contrast the logical point-to-point and logical multi-access topologies.
Answer: A logical point-to-point topology connects two nodes directly together. In data networks with point-to-point topologies, the media access control protocol can be very simple. All frames on the media can only travel to or from the two nodes. The frames are placed on the media by the node at one end and taken off the media by the node at the other end. In point-to-point networks, if data can only flow in one direction at a time, it is operating as a half-duplex link. If data can successfully flow across the link from each node simultaneously, it is a full-duplex service.
A logical multi-access topology enables a number of nodes to communicate by using the same shared media. Data from only one node can be placed on the medium at any one time. Every node sees all the frames that are on the medium, but only the node to which the frame is addressed processes the contents of the frame. Having many nodes share access to the medium requires a Data Link media access control method to regulate the transmission of data and thereby reduce collisions between different signals.
Question 4: Describe the features of a logical ring topology.
Answer: In a logical ring topology, each node in turn receives a frame. If the frame is not addressed to a node, the node passes the frame to the next node. This allows a ring to use a controlled media access control technique called token passing.
The media usually carries only one frame at a time. If there is no data being transmitted, a signal (known as a token) can be placed on the media. A node can place a data frame on the media only when it has the token.
Question 5: Name five Layer 2 protocols.
Answer:
- Point-to-Point Protocol (PPP)
- Ethernet
- High-Level Data Link Control (HDLC)
- Frame Relay
- Asynchronous Transfer Mode (ATM)
Question 6: How do Data Link Layer addresses differ from Network Layer addresses?
Answer: Unlike Layer 3 logical addresses that are hierarchical, physical addresses do not indicate on which network the device is located. If the device is moved to another network or subnet, it still functions with the same Layer 2 physical address.
Because the frame is only used to transport data between nodes across the local media, the Data Link Layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from source host to destination host, regardless of the number of network hops along the route.
Question 7: What are the possible header field types in Data Link frames?
Answer: Typical frame header fields include:
- Start Frame field - Indicates the beginning of the frame.
- Source and Destination address fields - Indicates the source and destination nodes on the media.
- Priority/Quality of Service field - Indicates a particular type of communication service for processing.
- Type field - Indicates the upper layer service contained in the frame.
- Logical connection control field - Establishes a logical connection between nodes.
- Physical link control field - Establishes the media link.
- Flow control field - Starts and stops traffic over the media.
- Congestion control field - Indicates congestion in the media.
Question 8: Give the purpose of the Frame Check Sequence field in a Data Link frame trailer.
Answer: The media is a potentially unsafe environment for data. The signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent. To ensure that the content of the received frame at the destination matches that of the frame that left the source node, a transmitting node creates a logical summary of the contents of the frame. This is known as the Frame Check Sequence (FCS) and is placed in the trailer to represent the contents of the frame. When the frame arrives at the destination node, the receiving node calculates its own logical summary, or FCS, of the frame. The receiving node compares the two FCS values. If the two values are the same, the frame is considered to have arrived as transmitted. If the FCS values differ, the frame is discarded. There is always the small possibility that a frame with a good FCS result is actually corrupt. Errors in bits can cancel each other out when the FCS is calculated. Upper layer protocols would then be required to detect and correct this data loss.
Page 3:
In this activity, you will continue to build a more complex model of the Exploration lab network.
Click the Packet Tracer icon to launch the Packet Tracer activity.
7.6.1 - Summary and Review
Link to Packet Tracer Exploration: Skills Integration Challenge: Data Link Layer Issues
In this activity, you continue to build a more complex model of the Exploration lab network.
Link to Packet Tracer Exploration: Skills Integration Challenge: Data Link Layer Issues
In this activity, you continue to build a more complex model of the Exploration lab network.
Page 4:
To Learn More
Reflection Questions
How did the widespread adoption of the OSI model change the development of network technologies? How does today's data communications environment differ from that of twenty years ago because of the adoption of the model?
Discuss and compare Carrier Sense Multi-Access Data Link media access protocol features and operation with those of deterministic media access protocols.
Discuss and consider the issues that the developers of a new physical data communications medium have to resolve to ensure interoperability with the existing upper layer TCP/IP protocols.
7.6.1 - Summary and Review
The diagram depicts a collage of people using computers and networks.
The diagram depicts a collage of people using computers and networks.
7.7 Chapter Quiz
7.7.1 Chapter QuizPage 1:
7.7.1 - Chapter Quiz
1. Which frame field is created by a source node and used by a destination node to ensure that a transmitted data signal has not been altered by interference, distortion, or signal loss?
A. Transport Layer error check field
B. Frame Check Sequence field
C. User Datagram Protocol field
D. Error correction
E. Flow control field
2. Which Data Link Layer addressing scheme is used in a point-to-point logical topology?
A. IPv4 addressing
B. IPv6 addressing
C. Ring addressing
D. Multilayer addressing
E. Layer 2 addressing not required for this topology
3. What do network hosts use Data Link Layer addresses for?
A. Remote delivery
B. Local and remote delivery
C. Local delivery
D. Remote delivery using routers
4. Which three basic parts are common to all frame types supported by the Data Link Layer? (Choose three.)
A. header
B. type field
C. MTU size
D. data
E. trailer
F. CRC value
5. What are two characteristics of the controlled media access method? (Choose two.)
A. It is known as a deterministic access method.
B. There are no collisions when this type of method is in use.
C. Any station can transmit at any time.
D. Bandwidth is more efficiently utilized than in a contention-based access method.
E. Stations must determine if the media is carrying a signal before they can transmit.
6. Which of the following are sublayers of the Data Link Layer?
A. ACL, LMC
B. MAC, LAC
C. MAC, LLC
D. O S I, LLC
7. Which two of the following are Data Link Layer encapsulation details? (Choose two.)
A. A header and trailer are added.
B. Data is converted into packets.
C. Packets are packaged into frames.
D. Frames are divided into segments.
E. Packets are changed into bits for Internet travel.
8. What is achieved by the encapsulation process at the Data Link Layer?
A. Packets are put into frames.
B. Data is packaged into a packet.
C. Packets are divided into segments.
D. Data is converted for Internet transmission.
9. Match the characteristic to the associated media access control method. (Not all characteristics are used.)
Characteristics:
Deterministic
Ethernet
Physical ring topology
No collisions
Non-deterministic
Only one station can transmit at a time
Stations can transmit at any time
Token passing
More efficient use of bandwidth
Media access control methods:
Controlled access
Contention-based access
10. Match the characteristic to the topology type. (Not all characteristics are used.)
Characteristics:
Connects two nodes directly
CSMA/CD
Deterministic
Logical virtual circuit
Frame header not required
Token passing
Shared media
Topology types:
Point-to-Point
Multi-access
Ring
1. Which frame field is created by a source node and used by a destination node to ensure that a transmitted data signal has not been altered by interference, distortion, or signal loss?
A. Transport Layer error check field
B. Frame Check Sequence field
C. User Datagram Protocol field
D. Error correction
E. Flow control field
2. Which Data Link Layer addressing scheme is used in a point-to-point logical topology?
A. IPv4 addressing
B. IPv6 addressing
C. Ring addressing
D. Multilayer addressing
E. Layer 2 addressing not required for this topology
3. What do network hosts use Data Link Layer addresses for?
A. Remote delivery
B. Local and remote delivery
C. Local delivery
D. Remote delivery using routers
4. Which three basic parts are common to all frame types supported by the Data Link Layer? (Choose three.)
A. header
B. type field
C. MTU size
D. data
E. trailer
F. CRC value
5. What are two characteristics of the controlled media access method? (Choose two.)
A. It is known as a deterministic access method.
B. There are no collisions when this type of method is in use.
C. Any station can transmit at any time.
D. Bandwidth is more efficiently utilized than in a contention-based access method.
E. Stations must determine if the media is carrying a signal before they can transmit.
6. Which of the following are sublayers of the Data Link Layer?
A. ACL, LMC
B. MAC, LAC
C. MAC, LLC
D. O S I, LLC
7. Which two of the following are Data Link Layer encapsulation details? (Choose two.)
A. A header and trailer are added.
B. Data is converted into packets.
C. Packets are packaged into frames.
D. Frames are divided into segments.
E. Packets are changed into bits for Internet travel.
8. What is achieved by the encapsulation process at the Data Link Layer?
A. Packets are put into frames.
B. Data is packaged into a packet.
C. Packets are divided into segments.
D. Data is converted for Internet transmission.
9. Match the characteristic to the associated media access control method. (Not all characteristics are used.)
Characteristics:
Deterministic
Ethernet
Physical ring topology
No collisions
Non-deterministic
Only one station can transmit at a time
Stations can transmit at any time
Token passing
More efficient use of bandwidth
Media access control methods:
Controlled access
Contention-based access
10. Match the characteristic to the topology type. (Not all characteristics are used.)
Characteristics:
Connects two nodes directly
CSMA/CD
Deterministic
Logical virtual circuit
Frame header not required
Token passing
Shared media
Topology types:
Point-to-Point
Multi-access
Ring
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