8.0 Chapter Introduction
8.0.1 Chapter IntroductionPage 1:
Upper OSI layer protocols prepare data from the human network for transmission to its destination. The Physical layer controls how data is transmitted on the communication media.
The role of the OSI Physical layer is to encode the binary digits that represent Data Link layer frames into signals and to transmit and receive these signals across the physical media - copper wires, optical fiber, and wireless - that connect network devices.
This chapter introduces the general functions of the Physical layer as well as the standards and protocols that manage the transmission of data across local media.
In this chapter, you will learn to:
- Explain the role of Physical layer protocols and services in supporting communication across data networks.
- Describe the purpose of Physical layer signaling and encoding as they are used in networks.
- Describe the role of signals used to represent bits as a frame is transported across the local media.
- Identify the basic characteristics of copper, fiber, and wireless network media.
- Describe common uses of copper, fiber, and wireless network media.
8.0.1 - Chapter Introduction
The diagram depicts the O S I model with the Physical Layer highlighted. A group of people are shown next to an IP phone, a desktop PC, and a laptop. An arrow points from these to the Physical Layer of the O S I model. Photographs are shown of coaxial cable attached to a Cable Modem Termination System (CMTS), Ethernet twisted pair cabling attached to a LAN patch panel, and a cellular/microwave wireless antenna tower. The Physical Layer interconnects our data networks.
The diagram depicts the O S I model with the Physical Layer highlighted. A group of people are shown next to an IP phone, a desktop PC, and a laptop. An arrow points from these to the Physical Layer of the O S I model. Photographs are shown of coaxial cable attached to a Cable Modem Termination System (CMTS), Ethernet twisted pair cabling attached to a LAN patch panel, and a cellular/microwave wireless antenna tower. The Physical Layer interconnects our data networks.
8.1 The Physical Layer - Communication Signals
8.1.1 Physical Layer - PurposePage 1:
The OSI Physical layer provides the means to transport across the network media the bits that make up a Data Link layer frame. This layer accepts a complete frame from the Data Link layer and encodes it as a series of signals that are transmitted onto the local media. The encoded bits that comprise a frame are received by either an end device or an intermediate device.
The delivery of frames across the local media requires the following Physical layer elements:
- The physical media and associated connectors
- A representation of bits on the media
- Encoding of data and control information
- Transmitter and receiver circuitry on the network devices
It is also the job of the Physical layer to retrieve these individual signals from the media, restore them to their bit representations, and pass the bits up to the Data Link layer as a complete frame.
8.1.1 - Physical Layer - Purpose
The diagram depicts a source node and a destination node, each with an O S I model next to it. The PDU at each layer is shown as data moves from the source node to the destination node.
O S I Layers 7, 6, and 5 - PDU is data.
O S I Layer 4 - PDU is a segment.
O S I Layer 3 - PDU is a packet.
O S I Layer 2 - PDU is a frame.
O S I Layer 1 - PDU is bits.
Signals on the physical media are depicted by a line that moves up and down, representing zeros and ones or bits of data.
The diagram depicts a source node and a destination node, each with an O S I model next to it. The PDU at each layer is shown as data moves from the source node to the destination node.
O S I Layers 7, 6, and 5 - PDU is data.
O S I Layer 4 - PDU is a segment.
O S I Layer 3 - PDU is a packet.
O S I Layer 2 - PDU is a frame.
O S I Layer 1 - PDU is bits.
Signals on the physical media are depicted by a line that moves up and down, representing zeros and ones or bits of data.
8.1.2 Physical Layer - Operation
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The media does not carry the frame as a single entity. The media carries signals, one at a time, to represent the bits that make up the frame.
There are three basic forms of network media on which data is represented:
- Copper cable
- Fiber
- Wireless
Identifying a Frame
When the Physical layer encodes the bits into the signals for a particular medium, it must also distinguish where one frame ends and the next frame begins. Otherwise, the devices on the media would not recognize when a frame has been fully received. In that case, the destination device would only receive a string of signals and would not be able to properly reconstruct the frame. As described in the previous chapter, indicating the beginning of frame is often a function of the Data Link layer. However, in many technologies, the Physical layer may add its own signals to indicate the beginning and end of the frame.
To enable a receiving device to clearly recognize a frame boundary, the transmitting device adds signals to designate the start and end of a frame. These signals represent particular bit patterns that are only used to denote the start or end of a frame.
The process of encoding a frame of data from the logical bits into the physical signals on the media, and the characteristics of particular physical media, are covered in detail in the following sections of this chapter.
8.1.2 - Physical Layer - Operation
The diagram depicts the digital signal on three basic forms of network media: copper cable, fiber, and wireless.
Copper cable signals: A line moves up and down indicating changes in voltage levels to represent digital zeros and ones.
Fiber-optic signals: Light wave pulses vary, representing digital zeros and ones.
Microwave and wireless signals: Analog electromagnetic waves vary in frequency (Frequency Modulation - FM), amplitude (Amplitude Modulation - AM) and phase (Phase Modulation - PM) to represent radio signals or digital zeros and ones.
The diagram depicts the digital signal on three basic forms of network media: copper cable, fiber, and wireless.
Copper cable signals: A line moves up and down indicating changes in voltage levels to represent digital zeros and ones.
Fiber-optic signals: Light wave pulses vary, representing digital zeros and ones.
Microwave and wireless signals: Analog electromagnetic waves vary in frequency (Frequency Modulation - FM), amplitude (Amplitude Modulation - AM) and phase (Phase Modulation - PM) to represent radio signals or digital zeros and ones.
8.1.3 Physical Layer - Standards
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The Physical layer consists of hardware, developed by engineers, in the form of electronic circuitry, media, and connectors. Therefore, it is appropriate that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.
By comparison, the protocols and operations of the upper OSI layers are performed by software and are designed by software engineers and computer scientists. As we saw in a previous chapter, the services and protocols in the TCP/IP suite are defined by the Internet Engineering Task Force (IETF) in RFCs.
Similar to technologies associated with the Data Link layer, the Physical layer technologies are defined by organizations such as:
- The International Organization for Standardization (ISO)
- The Institute of Electrical and Electronics Engineers (IEEE)
- The American National Standards Institute (ANSI)
- The International Telecommunication Union (ITU)
- The Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA)
- National telecommunications authorities such as the Federal Communication Commission (FCC) in the USA.
8.1.3 - Physical Layer - Standards
The diagram compares Physical Layer standards and upper layer standards. The top portion of the Data Link Layer and the layers above it are implemented in software. The bottom portion of the Data Link Layer and the Physical Layer are implemented in hardware.
TCP/IP standards are set by the IETF. Physical Layer standards are set by I S O, ANSI, IEEE, I T U, EI A/T IA, and the FCC.
The diagram compares Physical Layer standards and upper layer standards. The top portion of the Data Link Layer and the layers above it are implemented in software. The bottom portion of the Data Link Layer and the Physical Layer are implemented in hardware.
TCP/IP standards are set by the IETF. Physical Layer standards are set by I S O, ANSI, IEEE, I T U, EI A/T IA, and the FCC.
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Physical Layer Technologies and Hardware
The technologies defined by these organizations include four areas of the Physical layer standards:
- Physical and electrical properties of the media
- Mechanical properties (materials, dimensions, pinouts) of the connectors
- Bit representation by the signals (encoding)
- Definition of control information signals
Hardware components such as network adapters (NICs), interfaces and connectors, cable materials, and cable designs are all specified in standards associated with the Physical layer.
8.1.3 - Physical Layer - Standards
The diagram depicts standards for the Physical Layer that specify signals, connectors, and cables.
Signals: Various digital signal forms are shown. Signal standards allow a variety of devices to interoperate.
Connectors: Network adapter modules are shown with I S O 8877 RJ-45 jacks.
Cables: UTP cables are shown next to a NIC with an RJ-45 adapter. Standards allow different companies to make cables and NIC's, knowing that they will work together.
The diagram depicts standards for the Physical Layer that specify signals, connectors, and cables.
Signals: Various digital signal forms are shown. Signal standards allow a variety of devices to interoperate.
Connectors: Network adapter modules are shown with I S O 8877 RJ-45 jacks.
Cables: UTP cables are shown next to a NIC with an RJ-45 adapter. Standards allow different companies to make cables and NIC's, knowing that they will work together.
8.1.4 Physical layer Fundamental Principles
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The three fundamental functions of the Physical layer are:
- The physical components
- Data encoding
- Signaling
Encoding
Encoding is a method of converting a stream of data bits into a predefined code. Codes are groupings of bits used to provide a predictable pattern that can be recognized by both the sender and the received. Using predictable patterns helps to distinguish data bits from control bits and provide better media error detection.
In addition to creating codes for data, encoding methods at the Physical layer may also provide codes for control purposes such as identifying the beginning and end of a frame. The transmitting host will transmit the specific pattern of bits or a code to identify the beginning and end of the frame.
Signaling
The Physical layer must generate the electrical, optical, or wireless signals that represent the "1" and "0" on the media. The method of representing the bits is called the signaling method. The Physical layer standards must define what type of signal represents a "1" and a "0". This can be as simple as a change in the level of an electrical signal or optical pulse or a more complex signaling method.
In the next sections, you will examine different methods of signaling and encoding.
8.1.4 - Physical Layer Fundamental Principles
The diagram depicts Physical Layer fundamental principles. A Layer 2 frame arrives at the Physical Layer. The encoded digital zero and one signals are placed on the media.
The diagram depicts Physical Layer fundamental principles. A Layer 2 frame arrives at the Physical Layer. The encoded digital zero and one signals are placed on the media.
8.2 Physical Signaling and Encoding: Representing Bits
8.2.1 Signaling Bits for the MediaPage 1:
Eventually, all communication from the human network becomes binary digits, which are transported individually across the physical media.
Although all the bits that make up a frame are presented to the Physical layer as a unit, the transmission of the frame across the media occurs as a stream of bits sent one at a time. The Physical layer represents each of the bits in the frame as a signal. Each signal placed onto the media has a specific amount of time to occupy the media. This is referred to as its bit time. Signals are processed by the receiving device and returned to its representation as bits.
At the Physical layer of the receiving node, the signals are converted back into bits. The bits are then examined for the start of frame and end of frame bit patterns to determine that a complete frame has been received. The Physical layer then delivers all the bits of a frame to the Data Link layer.
Successful delivery of the bits requires some method of synchronization between transmitter and receiver. The signals representing the bits must be examined at specific times during the bit time to properly determine if the signal represents a "1" or a "0". The synchronization is accomplished by the use of a clock. In LANs, each end of the transmission maintains its own clock. Many signaling methods use predictable transitions in the signal to provide synchronization between the clocks of the transmitting and the receiving devices.
Signaling Methods
Bits are represented on the medium by changing one or more of the following characteristics of a signal:
- Amplitude
- Frequency
- Phase
As an example, with Non-Return to Zero (NRZ), a 0 may be represented by one voltage level on the media during the bit time and a 1 might be represented by a different voltage on the media during the bit time.
There are also methods of signaling that use transitions, or the absence of transitions, to indicate a logic level. For example, Manchester Encoding indicates a 0 by a high to low voltage transition in the middle of the bit time. For a 1 there is a low to high voltage transition in the middle of the bit time.
The signaling method used must be compatible with a standard so that the receiver can detect the signals and decode them. The standard contains an agreement between the transmitter and the receiver on how to represent 1s and 0s. If there is no signaling agreement - that is, if different standards are used at each end of the transmission - communication across the physical medium will fail.
Signaling methods to represent bits on the media can be complex. We will look at two of the simpler techniques to illustrate the concept.
8.2.1 - Signaling Bits for the Media
The diagram depicts ways to represent a signal on the medium. These include varying amplitude, frequency, and phase and clock bit times.
The diagram depicts ways to represent a signal on the medium. These include varying amplitude, frequency, and phase and clock bit times.
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NRZ Signaling
As a first example, we will examine a simple signaling method, Non Return to Zero (NRZ). In NRZ, the bit stream is transmitted as a series of voltage values, as shown in the figure.
A low voltage value represents a logical 0 and a high voltage value represents a logical 1. The voltage range depends on the particular Physical layer standard in use.
This simple method of signaling is only suited for slow speed data links. NRZ signaling uses bandwidth inefficiently and is susceptible to electromagnetic interference. Additionally, the boundaries between individual bits can be lost when long strings of 1s or 0s are transmitted consecutively. In that case, no voltage transitions are detectable on the media. Therefore, the receiving nodes do not have a transition to use in resynchronizing bit times with the transmitting node.
8.2.1 - Signaling Bits for the Media
The diagram depicts signaling bits for transmission using non-return to zero (NRZ). A one bit is represented by a positive voltage pulse, and a zero bit is represented by zero voltage. The height of the positive voltage pulse is the amplitude. The width of a pulse or absence of one is one bit-time.
Characteristics of NRZ include:
- Discrete pulses (not continuous)
- Can only have one of two states (1/0, on/off)
- Voltage jumps between levels
The diagram depicts signaling bits for transmission using non-return to zero (NRZ). A one bit is represented by a positive voltage pulse, and a zero bit is represented by zero voltage. The height of the positive voltage pulse is the amplitude. The width of a pulse or absence of one is one bit-time.
Characteristics of NRZ include:
- Discrete pulses (not continuous)
- Can only have one of two states (1/0, on/off)
- Voltage jumps between levels
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Manchester Encoding
Instead of representing bits as pulses of simple voltage values, in the Manchester Encoding scheme, bit values are represented as voltage transitions.
For example, a transition from a low voltage to a high voltage represents a bit value of 1. A transition from a high voltage to a low voltage represents a bit value of 0.
As shown in the figure, one voltage transition must occur in the middle of each bit time. This transition can be used to ensure that the bit times in the receiving nodes are synchronized with the transmitting node.
The transition in the middle of the bit time will be either the up or down direction for each unit of time in which a bit is transmitted. For consecutive bit values, a transition on the bit boundary "sets up" the appropriate mid-bit time transition that represents the bit value.
Although Manchester Encoding is not efficient enough to be used at higher signaling speeds, it is the signaling method employed by 10BaseT Ethernet (Ethernet running at 10 Megabits per second).
8.2.1 - Signaling Bits for the Media
The diagram depicts signaling bits for transmission using Manchester Encoding.
In the Manchester Encoding scheme, bit values are represented as voltage transitions. A transition from a low voltage to a high voltage represents a bit value of one. A transition from a high voltage to a low voltage represents a bit value of zero. One voltage transition must occur in the middle of each bit time.
The diagram depicts signaling bits for transmission using Manchester Encoding.
In the Manchester Encoding scheme, bit values are represented as voltage transitions. A transition from a low voltage to a high voltage represents a bit value of one. A transition from a high voltage to a low voltage represents a bit value of zero. One voltage transition must occur in the middle of each bit time.
8.2.2 Encoding - Grouping Bits
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In the prior section, we describe the signaling process as how bits are represented on physical media. In this section, we use of the word encoding to represent the symbolic grouping of bits prior to being presented to the media. By using an encoding step before the signals are placed on the media, we improve the efficiency at higher speed data transmission.
As we use higher speeds on the media, we have the possibility that data will be corrupted. By using the coding groups, we can detect errors more efficiently. Additionally, as the demand for data speeds increase, we seek ways to represent more data across the media, by transmitting fewer bits. Coding groups provide a method of making this data representation.
The Physical layer of a network device needs to be able to detect legitimate data signals and ignore random non-data signals that may also be on the physical medium. The stream of signals being transmitted needs to start in such a way that the receiver recognizes the beginning and end of the frame.
Signal Patterns
One way to provide frame detection is to begin each frame with a pattern of signals representing bits that the Physical layer recognizes as denoting the start of a frame. Another pattern of bits will signal the end of the frame. Signal bits not framed in this manner are ignored by the Physical layer standard being used.
Valid data bits need to be grouped into a frame; otherwise, data bits will be received without any context to give them meaning to the upper layers of the networking model. This framing method can be provided by the Data Link layer, the Physical layer, or by both.
The figure depicts some of the purposes of signaling patterns. Signal patterns can indicate: start of frame, end of frame, and frame contents. These signal patterns can be decoded into bits. The bits are interpreted as codes. The codes indicate where the frames start and stop.
8.2.2 - Encoding - Grouping Bits
The diagram depicts how the Physical Layer decodes framed media signals into binary code. Various frame signals correspond to various patterns of zeros and ones.
Physical Signal Patterns:
-A specific physical signal pattern indicates the start of the frame.
-Physical signals represent frame contents.
-A specific signal pattern indicates the end of the frame.
-Random unframed signals on physical media are from noise or interference.
Binary Bit Patterns:
-A specific bit pattern indicates the start of the frame.
-Bit pattern represents frame contents passed to the Data Link Layer.
-A specific bit pattern indicates the end of the frame.
-Unframed signals are not decoded and not passed to the Data Link Layer.
The diagram depicts how the Physical Layer decodes framed media signals into binary code. Various frame signals correspond to various patterns of zeros and ones.
Physical Signal Patterns:
-A specific physical signal pattern indicates the start of the frame.
-Physical signals represent frame contents.
-A specific signal pattern indicates the end of the frame.
-Random unframed signals on physical media are from noise or interference.
Binary Bit Patterns:
-A specific bit pattern indicates the start of the frame.
-Bit pattern represents frame contents passed to the Data Link Layer.
-A specific bit pattern indicates the end of the frame.
-Unframed signals are not decoded and not passed to the Data Link Layer.
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Code Groups
Encoding techniques use bit patterns called symbols. The Physical layer may use a set of encoded symbols - called code groups - to represent encoded data or control information. A code group is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns. For example, code bits 10101 could represent the data bits 0011.
As shown in the figure, code groups are often used as an intermediary encoding technique for higher speed LAN technologies. This step occurs at the Physical layer prior to the generation of signals of voltages, light pulses, or radio frequencies. By transmitting symbols, the error detection capabilities and timing synchronization between transmitting and receiving devices are enhanced. These are important considerations in supporting high speed transmission over the media.
Although using code groups introduces overhead in the form of extra bits to transmit, they improve the robustness of a communications link. This is particularly true for higher speed data transmission.
Advantages using code groups include:
- Reducing bit level error
- Limiting the effective energy transmitted into the media
- Helping to distinguish data bits from control bits
- Better media error detection
To properly detect an individual bit as a 0 or as a 1, the receiver must know how and when to sample the signal on the media. This requires that the timing between the receiver and transmitter be synchronized. In many Physical layer technologies, transitions on the media are used for this synchronization. If the bit patterns being transmitted on the media do not create frequent transitions, this synchronization may be lost and individual bit error can occur. Code groups are designed so that the symbols force an ample number of bit transitions to occur on the media to synchronize this timing. They do this by using symbols to ensure that not too many 1s or 0s are used in a row.
Limiting Energy Transmitted
In many code groups, the symbols ensure that the number of 1s and 0s in a string of symbols are evenly balanced. The process of balancing the number of 1s and 0s transmitted is called DC balancing. This prevents excessive amounts of energy from being injected into the media during transmission, thereby reducing the interference radiated from the media. In many media signaling methods, a logic level, for example a 1, is represented by the presence of energy being sent into the media while the opposite logic level, a 0, is represented as the absence of this energy. Transmitting a long series of 1s could overheat the transmitting laser and the photo diodes in the receiver, potentially causing higher error rates.
Distinguish Data from Control
The code groups have three types of symbols:
- Data symbols - Symbols that represent the data of the frame as it is passed down to the Physical layer.
- Control symbols - Special codes injected by the Physical layer used to control transmission. These include end-of-frame and idle media symbols.
- Invalid symbols - Symbols that have patterns not allowed on the media. The receipt of an invalid symbol indicates a frame error.
Better Media Error Detection
In addition to the data symbols and control symbols, code groups contain invalid symbols. These are the symbols that could create long series of 1s or 0s on the media; therefore, they are not used by the transmitting node. If a receiving node receives one of these patterns, the Physical layer can determine that there has been an error in data reception.
8.2.2 - Encoding - Grouping Bits
The diagram depicts using a set of encoded symbols called code groups to represent encoded data or control information. A code group is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns. A series of data bits generates a code symbol to indicate that they are data bits, and the code symbol is signaled onto the media.
The diagram depicts using a set of encoded symbols called code groups to represent encoded data or control information. A code group is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns. A series of data bits generates a code symbol to indicate that they are data bits, and the code symbol is signaled onto the media.
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4B/5B
An example, we will examine a simple code group called 4B/5B. Code groups that are currently used in modern networks are generally more complex.
In this technique, 4 bits of data are turned into 5-bit code symbols for transmission over the media system. In 4B/5B, each byte to be transmitted is broken into four-bit pieces or nibbles and encoded as five-bit values known as symbols. These symbols represent the data to be transmitted as well as a set of codes that help control transmission on the media. Among the codes are symbols that indicate the beginning and end of the frame transmission. Although this process adds overhead to the bit transmissions, it also adds features that aid in the transmission of data at higher speeds.
4B/5B ensures that there is at least one level change per code to provide synchronization. Most of the codes used in 4B/5B balance the number of 1s and 0s used in each symbol.
As shown in the figure, 16 of the possible 32 combinations of code groups are allocated for data bits, and the remaining code groups are used for control symbols and invalid symbols. Six of the symbols are used for special functions identifying the transition from idle to frame data and end of stream delimiter. The remaining 10 symbols indicate invalid codes.
8.2.2 - Encoding - Grouping Bits
The diagram depicts 4B/5B code symbols using a table that converts 4-bit data patterns to 5-bit symbols. Data codes and control and invalid codes are shown.
Data Codes Table:
There are 16 4B values and corresponding 5B symbols in this table. Examples include:
4B Code: 0000
5B Symbol: 11110
4B Code:0001
5B Symbol:01001
4B Code: 1111
5B Symbol: 11101
Control and Invalid Codes Table:
There are 16 4B values and corresponding 5B symbols in this table. Examples include:
4B Code: idle
5B Symbol: 11111
4B Code: start of stream
5B Symbol: 11000
4B Code: invalid
5B Symbol: 1100
The diagram depicts 4B/5B code symbols using a table that converts 4-bit data patterns to 5-bit symbols. Data codes and control and invalid codes are shown.
Data Codes Table:
There are 16 4B values and corresponding 5B symbols in this table. Examples include:
4B Code: 0000
5B Symbol: 11110
4B Code:0001
5B Symbol:01001
4B Code: 1111
5B Symbol: 11101
Control and Invalid Codes Table:
There are 16 4B values and corresponding 5B symbols in this table. Examples include:
4B Code: idle
5B Symbol: 11111
4B Code: start of stream
5B Symbol: 11000
4B Code: invalid
5B Symbol: 1100
8.2.3 Data Carrying Capacity
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Different physical media support the transfer of bits at different speeds. Data transfer can be measured in three ways:
- Bandwidth
- Throughput
- Goodput
The capacity of a medium to carry data is described as the raw data bandwidth of the media. Digital bandwidth measures the amount of information that can flow from one place to another in a given amount of time. Bandwidth is typically measured in kilobits per second (kbps) or megabits per second (Mbps).
The practical bandwidth of a network is determined by a combination of factors: the properties of the physical media and the technologies chosen for signaling and detecting network signals.
Physical media properties, current technologies, and the laws of physics all play a role in determining available bandwidth.
The figure shows the commonly used units of bandwidth.
8.2.3 - Data Carrying Capacity
The diagram depicts a tabular listing of units of bandwidth.
Unit of Bandwidth: bits per second
Abbreviation: bps
Equivalence: 1 bps = fundamental unit of bandwidth
Unit of Bandwidth: Kilobits per second
Abbreviation: kbps
Equivalence: 1 kbps = 1,000 bps = 10 to the third power bps.
Unit of Bandwidth: Megabits per second
Abbreviation: Mbps
Equivalence: 1 Mbps = 1,000,000 bps = 10 to the sixth power bps.
Unit of Bandwidth: Gigabits per second
Abbreviation: Gbps
Equivalence: 1 Gbps = 1,000,000,000 bps = 10 to the ninth power bps.
Unit of Bandwidth: Terabits per second
Abbreviation: Tbps
Equivalence: 1 Tbps = 1,000,000,000,000 bps = 10 to the twelfth power bps.
The diagram depicts a tabular listing of units of bandwidth.
Unit of Bandwidth: bits per second
Abbreviation: bps
Equivalence: 1 bps = fundamental unit of bandwidth
Unit of Bandwidth: Kilobits per second
Abbreviation: kbps
Equivalence: 1 kbps = 1,000 bps = 10 to the third power bps.
Unit of Bandwidth: Megabits per second
Abbreviation: Mbps
Equivalence: 1 Mbps = 1,000,000 bps = 10 to the sixth power bps.
Unit of Bandwidth: Gigabits per second
Abbreviation: Gbps
Equivalence: 1 Gbps = 1,000,000,000 bps = 10 to the ninth power bps.
Unit of Bandwidth: Terabits per second
Abbreviation: Tbps
Equivalence: 1 Tbps = 1,000,000,000,000 bps = 10 to the twelfth power bps.
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Throughput
Throughput is the measure of the transfer of bits across the media over a given period of time. Due to a number of factors, throughput usually does not match the specified bandwidth in Physical layer implementations such as Ethernet.
Many factors influence throughput. Among these factors are the amount of traffic, the type of traffic, and the number of network devices encountered on the network being measured. In a multi-access topology such as Ethernet, nodes are competing for media access and its use. Therefore, the throughput of each node is degraded as usage of the media increases.
In an internetwork or network with multiple segments, throughput cannot be faster than the slowest link of the path from source to destination. Even if all or most of the segments have high bandwidth, it will only take one segment in the path with low throughput to create a bottleneck to the throughput of the entire network.
Goodput
A third measurement has been created to measure the transfer of usable data. That measure is known as goodput. Goodput is the measure of usable data transferred over a given period of time, and is therefore the measure that is of most interest to network users.
As shown in the figure, goodput measures the effective transfer of user data between Application layer entities, such as between a source web server process and a destination web browser device.
Unlike throughput, which measures the transfer of bits and not the transfer of usable data, goodput accounts for bits devoted to protocol overhead. Goodput is throughput minus traffic overhead for establishing sessions, acknowledgements, and encapsulation.
As an example, consider two hosts on a LAN transferring a file. The bandwidth of the LAN is 100 Mbps. Due to the sharing and media overhead the throughput between the computers is only 60 Mbps. With the overhead of the encapsulation process of the TCP/IP stack, the actual rate of the data received by the destination computer, goodput, is only 40Mbps.
8.2.3 - Data Carrying Capacity
The diagram depicts the concepts of throughput and goodput using a source node and a destination node, each with an O S I model next to it. As data moves from the source node to the destination node, throughput is represented as bits along the Physical Layer. Goodput is data that arrives at the Application Layer.
Data throughput is actual network performance. Goodput is a measure of the transfer of usable data after protocol overhead traffic has been removed.
The diagram depicts the concepts of throughput and goodput using a source node and a destination node, each with an O S I model next to it. As data moves from the source node to the destination node, throughput is represented as bits along the Physical Layer. Goodput is data that arrives at the Application Layer.
Data throughput is actual network performance. Goodput is a measure of the transfer of usable data after protocol overhead traffic has been removed.
8.3 Physical Media - Connecting Communication
8.3.1 Types of Physical MediaPage 1:
The Physical layer is concerned with network media and signaling. This layer produces the representation and groupings of bits as voltages, radio frequencies, or light pulses. Various standards organizations have contributed to the definition of the physical, electrical, and mechanical properties of the media available for different data communications. These specifications guarantee that cables and connectors will function as anticipated with different Data Link layer implementations.
As an example, standards for copper media are defined for the:
- Type of copper cabling used
- Bandwidth of the communication
- Type of connectors used
- Pinout and color codes of connections to the media
- Maximum distance of the media
This section will also describe some of the important characteristics of commonly used copper, optical, and wireless media.
8.3.1 - Types of Physical Media
The diagram depicts physical characteristics of various forms of Ethernet and wireless media.
Ethernet Technologies and Media Used:
Ethernet Standard: 10 Base-T
Media: EIA/TIA Category 3, 4, 5 UTP - four pair
Max segment length: 100 meters or 328 feet
Topology: Star
Connector: I S O 8877 (RJ-45)
Ethernet Standard: 100 Base-TX
Media: EIA/TIA Category 5 UTP - two pair
Max segment length: 100 meters or 328 feet
Topology: Star
Connector: I S O 8877 (RJ-45)
Ethernet Standard: 100 Base-FX
Media: 50/62.5 multi-mode fiber
Max segment length: 2 kilometers or 6,562 feet
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-CX
Media: STP
Max segment length: 25 meters or 82 feet
Topology: Star
Connector: I S O 8877 (RJ-45)
Ethernet Standard: 1000 Base-T
Media: EIA/TIA Category 5 (or greater) UTP - four pair
Max segment length: 100 meters or 328 feet
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-SX
Media: 50/62.5 multi-mode fiber
Max segment length: Up to 550 meters (1,804 feet) depending on fiber used
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-LX
Media: 50/62.5 multi-mode fiber (MMF) or 9 micron single mode fiber (SMF)
Max segment length: 550 meters (MMF) or 10 kilometers (SMF)
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-ZX
Media: 9 micron single mode fiber
Max segment length: Approximately 70 kilometers
Topology: Star
Connector: Various
Ethernet Standard: 10G Base-ZR
Media: 9 micron single mode fiber
Max segment length: Up to 80 kilometers
Topology: Star
Connector: Various
Wireless media:
Standard: Bluetooth 8 0 2 dot 15
Speed: <1 Mbps
Range: Short
Applications: Peer-to-peer device-to-device
Standard: 8 0 2 dot 11(a, b, g, n), HiperLAN 2
Speed: 1 to 54+ Mbps
Range: Medium
Applications: Enterprise networks
Standard: MMDS, LMDS
Speed: 22 Mbps+
Range: Medium-long
Applications: Fixed, last mile access
Standard: GSM, GPRS, CDMA, 2.5 to 3G
Speed: 10 to 384 Kbps
Range: Long
Applications: PDA's, mobile phones, cellular access
The diagram depicts physical characteristics of various forms of Ethernet and wireless media.
Ethernet Technologies and Media Used:
Ethernet Standard: 10 Base-T
Media: EIA/TIA Category 3, 4, 5 UTP - four pair
Max segment length: 100 meters or 328 feet
Topology: Star
Connector: I S O 8877 (RJ-45)
Ethernet Standard: 100 Base-TX
Media: EIA/TIA Category 5 UTP - two pair
Max segment length: 100 meters or 328 feet
Topology: Star
Connector: I S O 8877 (RJ-45)
Ethernet Standard: 100 Base-FX
Media: 50/62.5 multi-mode fiber
Max segment length: 2 kilometers or 6,562 feet
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-CX
Media: STP
Max segment length: 25 meters or 82 feet
Topology: Star
Connector: I S O 8877 (RJ-45)
Ethernet Standard: 1000 Base-T
Media: EIA/TIA Category 5 (or greater) UTP - four pair
Max segment length: 100 meters or 328 feet
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-SX
Media: 50/62.5 multi-mode fiber
Max segment length: Up to 550 meters (1,804 feet) depending on fiber used
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-LX
Media: 50/62.5 multi-mode fiber (MMF) or 9 micron single mode fiber (SMF)
Max segment length: 550 meters (MMF) or 10 kilometers (SMF)
Topology: Star
Connector: Various
Ethernet Standard: 1000 Base-ZX
Media: 9 micron single mode fiber
Max segment length: Approximately 70 kilometers
Topology: Star
Connector: Various
Ethernet Standard: 10G Base-ZR
Media: 9 micron single mode fiber
Max segment length: Up to 80 kilometers
Topology: Star
Connector: Various
Wireless media:
Standard: Bluetooth 8 0 2 dot 15
Speed: <1 Mbps
Range: Short
Applications: Peer-to-peer device-to-device
Standard: 8 0 2 dot 11(a, b, g, n), HiperLAN 2
Speed: 1 to 54+ Mbps
Range: Medium
Applications: Enterprise networks
Standard: MMDS, LMDS
Speed: 22 Mbps+
Range: Medium-long
Applications: Fixed, last mile access
Standard: GSM, GPRS, CDMA, 2.5 to 3G
Speed: 10 to 384 Kbps
Range: Long
Applications: PDA's, mobile phones, cellular access
8.3.2 Copper Media
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The most commonly used media for data communications is cabling that uses copper wires to signal data and control bits between network devices. Cabling used for data communications usually consists of a series of individual copper wires that form circuits dedicated to specific signaling purposes.
Other types of copper cabling, known as coaxial cable, have a single conductor that runs through the center of the cable that is encased by, but insulated from, the other shield. The copper media type chosen is specified by the Physical layer standard required to link the Data Link layers of two or more network devices.
These cables can be used to connect nodes on a LAN to intermediate devices, such as routers and switches. Cables are also used to connect WAN devices to a data services provider such as a telephone company. Each type of connection and the accompanying devices have cabling requirements stipulated by Physical layer standards.
Networking media generally make use of modular jacks and plugs, which provide easy connection and disconnection. Also, a single type of physical connector may be used for multiple types of connections. For example, the RJ-45 connector is used widely in LANs with one type of media and in some WANs with another media type.
The figure shows some commonly used copper media and connectors.
8.3.2 - Copper Media
The diagram depicts copper media. A coaxial cable and unshielded twisted pair (UTP) cables are shown. UTP cables with RJ-45 connectors are shown plugging into switch ports.
The diagram depicts copper media. A coaxial cable and unshielded twisted pair (UTP) cables are shown. UTP cables with RJ-45 connectors are shown plugging into switch ports.
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External Signal Interference
Data is transmitted on copper cables as electrical pulses. A detector in the network interface of a destination device must receive a signal that can be successfully decoded to match the signal sent.
The timing and voltage values of these signals are susceptible to interference or "noise" from outside the communications system. These unwanted signals can distort and corrupt the data signals being carried by copper media. Radio waves and electromagnetic devices such as fluorescent lights, electric motors, and other devices are potential sources of noise.
Cable types with shielding or twisting of the pairs of wires are designed to minimize signal degradation due to electronic noise.
The susceptibility of copper cables to electronic noise can also be limited by:
- Selecting the cable type or category most suited to protect the data signals in a given networking environment
- Designing a cable infrastructure to avoid known and potential sources of interference in the building structure
- Using cabling techniques that include the proper handling and termination of the cables
8.3.2 - Copper Media
The diagram depicts external signal interference sources that can affect copper media. Large bundles of under-floor UTP copper cables are shown along with sources of interference, including fluorescent lighting (office environment), electric motors (drill), and radio waves (radio tower).
The diagram depicts external signal interference sources that can affect copper media. Large bundles of under-floor UTP copper cables are shown along with sources of interference, including fluorescent lighting (office environment), electric motors (drill), and radio waves (radio tower).
8.3.3 Unshielded Twisted Pair (UTP) Cable
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Unshielded twisted-pair (UTP) cabling, as it is used in Ethernet LANs, consists of four pairs of color-coded wires that have been twisted together and then encased in a flexible plastic sheath. As seen in the figure, the color codes identify the individual pairs and wires in the pairs and aid in cable termination.
The twisting has the effect of canceling unwanted signals. When two wires in an electrical circuit are placed close together, external electromagnetic fields create the same interference in each wire. The pairs are twisted to keep the wires in as close proximity as is physically possible. When this common interference is present on the wires in a twisted pair, the receiver processes it in equal yet opposite ways. As a result, the signals caused by electromagnetic interference from external sources are effectively cancelled.
This cancellation effect also helps avoid interference from internal sources called crosstalk. Crosstalk is the interference caused by the magnetic field around the adjacent pairs of wires in the cable. When electrical current flows through a wire, it creates a circular magnetic field around the wire. With the current flowing in opposite directions in the two wires in a pair, the magnetic fields - as equal but opposite forces - have a cancellation effect on each other. Additionally, the different pairs of wires that are twisted in the cable use a different number of twists per meter to help protect the cable from crosstalk between pairs.
UTP Cabling Standards
The UTP cabling commonly found in workplaces, schools, and homes conforms to the standards established jointly by the Telecommunications Industry Association (TIA) and the Electronics Industries Alliance (EIA). TIA/EIA-568A stipulates the commercial cabling standards for LAN installations and is the standard most commonly used in LAN cabling environments. Some of the elements defined are:
- Cable types
- Cable lengths
- Connectors
- Cable termination
- Methods of testing cable
Cables in higher categories are designed and constructed to support higher data rates. As new gigabit speed Ethernet technologies are being developed and adopted, Cat5e is now the minimally acceptable cable type, with Cat6 being the recommended type for new building installations.
Some people connect to data network using existing telephone systems. Often the cabling in these systems are some form of UTP that are lower grade than the current Cat5+ standards.
Installing less expensive but lower rated cabling is potentially wasteful and shortsighted. If the decision is later made to adopt a faster LAN technology, total replacement of the installed cable infrastructure may be required.
8.3.3 - Unshielded Twisted Pair (UTP) Cable
The diagram depicts the components of an unshielded twisted pair (UTP) cable.
Outer jacket: Protects the copper wire from physical damage.
Twisted pairs: Protects the signal from interference.
Color-coded plastic insulation: Electrically isolates wire from each other and identifies each pair.
The diagram depicts the components of an unshielded twisted pair (UTP) cable.
Outer jacket: Protects the copper wire from physical damage.
Twisted pairs: Protects the signal from interference.
Color-coded plastic insulation: Electrically isolates wire from each other and identifies each pair.
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UTP Cable Types
UTP cabling, terminated with RJ-45 connectors, is a common copper-based medium for interconnecting network devices, such as computers, with intermediate devices, such as routers and network switches.
Different situations may require UTP cables to be wired according to different wiring conventions. This means that the individual wires in the cable have to be connected in different orders to different sets of pins in the RJ-45 connectors. The following are main cable types that are obtained by using specific wiring conventions:
- Ethernet Straight-through
- Ethernet Crossover
- Rollover
Using a crossover or straight-through cable incorrectly between devices may not damage the devices, but connectivity and communication between the devices will not take place. This is a common error in the lab and checking that the device connections are correct should be the first troubleshooting action if connectivity is not achieved.
8.3.3 - Unshielded Twisted Pair (UTP) Cable
The diagram depicts three types of UTP cables used in networking: Ethernet straight-through, Ethernet crossover, and rollover.
Cable Type 1: Ethernet straight-through
Standard: Both ends T568A or both ends T568B
Application: Connecting a network host to a network device such as a switch or hub.
Cable Type 2: Ethernet crossover
Standard: One end T568A, other end T568B
Application: Connecting two network hosts.
Connecting two network intermediary devices (switch to switch, or router to router).
Cable Type 3: Rollover
Standard: Cisco proprietary
Application: Connecting a workstation serial port to a router console port, using an adapter.
The diagram shows two standards for wiring an Ethernet straight-through cable, T568A and T568B.
T568A:
Pair 1 is made up of pins 4 and 5, wire colors blue and blue/white.
Pair 2 is made up of pins 3 and 6, wire colors orange/white and orange.
Pair 3 is made up of pins 1 and 2, wire colors green/white and green.
Pair 4 is made up of pins 7 and 8, wire colors brown/white and brown.
T568B:
Pair 1 is made up of pins 4 and 5, wire colors blue and blue/white.
Pair 2 is made up of pins 1 and 2, wire colors orange/white and orange.
Pair 3 is made up of pins 3 and 6, wire colors green/white and green.
Pair 4 is made up of pins 7 and 8, wire colors brown/white and brown.
The diagram depicts three types of UTP cables used in networking: Ethernet straight-through, Ethernet crossover, and rollover.
Cable Type 1: Ethernet straight-through
Standard: Both ends T568A or both ends T568B
Application: Connecting a network host to a network device such as a switch or hub.
Cable Type 2: Ethernet crossover
Standard: One end T568A, other end T568B
Application: Connecting two network hosts.
Connecting two network intermediary devices (switch to switch, or router to router).
Cable Type 3: Rollover
Standard: Cisco proprietary
Application: Connecting a workstation serial port to a router console port, using an adapter.
The diagram shows two standards for wiring an Ethernet straight-through cable, T568A and T568B.
T568A:
Pair 1 is made up of pins 4 and 5, wire colors blue and blue/white.
Pair 2 is made up of pins 3 and 6, wire colors orange/white and orange.
Pair 3 is made up of pins 1 and 2, wire colors green/white and green.
Pair 4 is made up of pins 7 and 8, wire colors brown/white and brown.
T568B:
Pair 1 is made up of pins 4 and 5, wire colors blue and blue/white.
Pair 2 is made up of pins 1 and 2, wire colors orange/white and orange.
Pair 3 is made up of pins 3 and 6, wire colors green/white and green.
Pair 4 is made up of pins 7 and 8, wire colors brown/white and brown.
8.3.4 Other Copper Cable
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Two other types of copper cable are used:
1. Coaxial
2. Shielded Twisted-Pair (STP)
Coaxial Cable
Coaxial cable consists of a copper conductor surrounded by a layer of flexible insulation, as shown in the figure.
Over this insulating material is a woven copper braid, or metallic foil, that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield, also reduces the amount of outside electromagnetic interference. Covering the shield is the cable jacket.
All the elements of the coaxial cable encircle the center conductor. Because they all share the same axis, this construction is called coaxial, or coax for short.
Uses of Coaxial Cable
The coaxial cable design has been adapted for different purposes. Coax is an important type of cable that is used in wireless and cable access technologies. Coax cables are used to attach antennas to wireless devices. The coaxial cable carries radio frequency (RF) energy between the antennas and the radio equipment.
Coax is also the most widely used media for transporting high radio frequency signals over wire, especially cable television signals. Traditional cable television, exclusively transmitting in one direction, was composed completely of coax cable.
Cable service providers are currently converting their one-way systems to two-way systems to provide Internet connectivity to their customers. To provide these services, portions of the coaxial cable and supporting amplification elements are replaced with multi-fiber-optic cable. However, the final connection to the customer's location and the wiring inside the customer's premises is still coax cable. This combined use of fiber and coax is referred to as hybrid fiber coax (HFC).
In the past, coaxial cable was used in Ethernet installations. Today UTP offers lower costs and higher bandwidth than coaxial and has replaced it as the standard for all Ethernet installations.
There are different types of connectors used with coax cable. The figure shows some of these connector types.
8.3.4 - Other Copper Cable
The diagram depicts the components of a coaxial cable as well as various types of coaxial connectors. The outer jacket, braided copper shielding, copper conductor, and plastic insulation are identified. Connector types include BNC, N type and F type.
The diagram depicts the components of a coaxial cable as well as various types of coaxial connectors. The outer jacket, braided copper shielding, copper conductor, and plastic insulation are identified. Connector types include BNC, N type and F type.
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Shielded Twisted-Pair (STP) Cable
Another type of cabling used in networking is shielded twisted-pair (STP). As shown in the figure, STP uses four pairs of wires that are wrapped in an overall metallic braid or foil.
STP cable shields the entire bundle of wires within the cable as well as the individual wire pairs. STP provides better noise protection than UTP cabling, however at a significantly higher price.
For many years, STP was the cabling structure specified for use in Token Ring network installations. With the use of Token Ring declining, the demand for shielded twisted-pair cabling has also waned. The new 10 GB standard for Ethernet has a provision for the use of STP cabling. This may provide a renewed interest in shielded twisted-pair cabling.
8.3.4 - Other Copper Cable
The diagram depicts the components of a shielded twisted pair (STP) cable. The outer jacket, braided shield, and twisted pairs are identified.
The diagram depicts the components of a shielded twisted pair (STP) cable. The outer jacket, braided shield, and twisted pairs are identified.
8.3.5 Copper Media Safety
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Electrical Hazards
A potential problem with copper media is that the copper wires could conduct electricity in undesirable ways. This could subject personnel and equipment to a range of electrical hazards.
A defective network device could conduct currents to the chassis of other network devices. Additionally, network cabling could present undesirable voltage levels when used to connect devices that have power sources with different ground potentials. Such situations are possible when copper cabling is used to connect networks in different buildings or on different floors of buildings that use different power facilities. Finally, copper cabling may conduct voltages caused by lightning strikes to network devices.
The result of undesirable voltages and currents can include damage to network devices and connected computers, or injury to personnel. It is important that copper cabling be installed appropriately, and according to the relevant specifications and building codes, in order to avoid potentially dangerous and damaging situations.
Fire Hazards
Cable insulation and sheaths may be flammable or produce toxic fumes when heated or burned. Building authorities or organizations may stipulate related safety standards for cabling and hardware installations.
8.3.5 - Copper Media Safety
The diagram depicts copper media safety factors. These include:
- The separation of data and electrical power cabling must comply with safety codes.
- Cables must be connected correctly.
- Installations must be inspected for damage.
- Equipment must be grounded correctly.
The diagram depicts copper media safety factors. These include:
- The separation of data and electrical power cabling must comply with safety codes.
- Cables must be connected correctly.
- Installations must be inspected for damage.
- Equipment must be grounded correctly.
8.3.6 Fiber Media
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Fiber-optic cabling uses either glass or plastic fibers to guide light impulses from source to destination. The bits are encoded on the fiber as light impulses. Optical fiber cabling is capable of very large raw data bandwidth rates. Most current transmission standards have yet to approach the potential bandwidth of this media.
Fiber Compared to Copper Cabling
Given that the fibers used in fiber-optic media are not electrical conductors, the media is immune to electromagnetic interference and will not conduct unwanted electrical currents due to grounding issues. Because optical fibers are thin and have relatively low signal loss, they can be operated at much greater lengths than copper media, without the need for signal regeneration. Some optical fiber Physical layer specifications allow lengths that can reach multiple kilometers.
Optical fiber media implementation issues include:
- More expensive (usually) than copper media over the same distance (but for a higher capacity)
- Different skills and equipment required to terminate and splice the cable infrastructure
- More careful handling than copper media
8.3.6 - Fiber Media
The diagram depicts a cross section of a fiber media cable from the outermost component to the innermost component. The outermost component consists of a jacket that is typically PVC. The next component consists of the strengthening material, which is typically Aramid yarn. The next component is the buffer, then the cladding, and lastly the core component. Photographs of various fiber cables and connectors are shown.
The diagram depicts a cross section of a fiber media cable from the outermost component to the innermost component. The outermost component consists of a jacket that is typically PVC. The next component consists of the strengthening material, which is typically Aramid yarn. The next component is the buffer, then the cladding, and lastly the core component. Photographs of various fiber cables and connectors are shown.
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Cable Construction
Optical fiber cables consist of a PVC jacket and a series of strengthening materials that surround the optical fiber and its cladding. The cladding surrounds the actual glass or plastic fiber and is designed to prevent light loss from the fiber. Because light can only travel in one direction over optical fiber, two fibers are required to support full duplex operation. Fiber-optic patch cables bundle together two optical fiber cables and terminate them with a pair of standard single fiber connectors. Some fiber connectors accept both the transmitting and receiving fibers in a single connector.
8.3.6 - Fiber Media
The diagram depicts fiber media cable design. Each fiber in a fiber-optic cable has a transmit (Tx) end and a receive (Rx) end. Fiber provides full duplex communications with a cable dedicated to each direction.
The diagram depicts fiber media cable design. Each fiber in a fiber-optic cable has a transmit (Tx) end and a receive (Rx) end. Fiber provides full duplex communications with a cable dedicated to each direction.
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Generating and Detecting the Optical Signal
Either lasers or light emitting diodes (LEDs) generate the light pulses that are used to represent the transmitted data as bits on the media. Electronic semi-conductor devices called photodiodes detect the light pulses and convert them to voltages that can then be reconstructed into data frames.
Note: The laser light transmitted over fiber-optic cabling can damage the human eye. Care must be taken to avoid looking into the end of an active optical fiber.
Single-mode and Multimode Fiber
Fiber optic cables can be broadly classified into two types: single-mode and multimode.
Single-mode optical fiber carries a single ray of light, usually emitted from a laser. Because the laser light is uni-directional and travels down the center of the fiber, this type of fiber can transmit optical pulses for very long distances.
Multimode fiber typically uses LED emitters that do not create a single coherent light wave. Instead, light from an LED enters the multimode fiber at different angles. Because light entering the fiber at different angles takes different amounts of time to travel down the fiber, long fiber runs may result in the pulses becoming blurred on reception at the receiving end. This effect, known as modal dispersion, limits the length of multimode fiber segments.
Multimode fiber, and the LED light source used with it, are cheaper than single-mode fiber and its laser-based emitter technology.
8.3.6 - Fiber Media
The diagram depicts single-mode and multimode fiber cable design.
Single-Mode:
Components from the outside in are:
- Polymeric Coating
- Glass Cladding 125 microns diameter
- Glass Core=8 to 10 microns
Characteristics include:
- Produces a single, straight path for light.
- Small core.
- Less dispersion.
- Suited for long distance applications (up to 100 Kilometers or 62.14 miles).
- Uses lasers as the light source, often within campus backbones for a distance of several thousand meters.
Multimode:
Components from the outside in are:
- Polymeric Coating
- Glass Cladding 125 microns diameter
- Glass Core=50/62.5 microns
Characteristics include:
- Allows multiple paths for light.
- Larger core than single-mode cable (50 microns or greater).
- Allows greater dispersion and, therefore, loss of signal.
- Used for long-distance application, but shorter than single-mode (up to approximately 2 kilometers or 6,560 feet).
- Uses L E D's as the light source, often within LAN's or distances of a couple hundred meters within a campus network.
The diagram depicts single-mode and multimode fiber cable design.
Single-Mode:
Components from the outside in are:
- Polymeric Coating
- Glass Cladding 125 microns diameter
- Glass Core=8 to 10 microns
Characteristics include:
- Produces a single, straight path for light.
- Small core.
- Less dispersion.
- Suited for long distance applications (up to 100 Kilometers or 62.14 miles).
- Uses lasers as the light source, often within campus backbones for a distance of several thousand meters.
Multimode:
Components from the outside in are:
- Polymeric Coating
- Glass Cladding 125 microns diameter
- Glass Core=50/62.5 microns
Characteristics include:
- Allows multiple paths for light.
- Larger core than single-mode cable (50 microns or greater).
- Allows greater dispersion and, therefore, loss of signal.
- Used for long-distance application, but shorter than single-mode (up to approximately 2 kilometers or 6,560 feet).
- Uses L E D's as the light source, often within LAN's or distances of a couple hundred meters within a campus network.
8.3.7 Wireless Media
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Wireless media carry electromagnetic signals at radio and microwave frequencies that represent the binary digits of data communications. As a networking medium, wireless is not restricted to conductors or pathways, as are copper and fiber media.
Wireless data communication technologies work well in open environments. However, certain construction materials used in buildings and structures, and the local terrain, will limit the effective coverage. In addition, wireless is susceptible to interference and can be disrupted by such common devices as household cordless phones, some types of fluorescent lights, microwave ovens, and other wireless communications.
Further, because wireless communication coverage requires no access to a physical strand of media, devices and users who are not authorized for access to the network can gain access to the transmission. Therefore, network security is a major component of wireless network administration.
8.3.7 - Wireless Media
The diagram depicts wireless media signals being affected by interference and the fact that wireless signals can be susceptible to a security risk. A wireless signal is shown between an access point and a laptop computer. A radio tower creates interference, distorting the signal. A wireless PDA creates a security risk as it is shown intercepting the wireless signal.
The diagram depicts wireless media signals being affected by interference and the fact that wireless signals can be susceptible to a security risk. A wireless signal is shown between an access point and a laptop computer. A radio tower creates interference, distorting the signal. A wireless PDA creates a security risk as it is shown intercepting the wireless signal.
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Types of Wireless Networks
The IEEE and telecommunications industry standards for wireless data communications cover both the Data Link and Physical layers. Four common data communications standards that apply to wireless media are:
- Standard IEEE 802.11 - Commonly referred to as Wi-Fi, is a Wireless LAN (WLAN) technology that uses a contention or non-deterministic system with a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) media access process.
- Standard IEEE 802.15 - Wireless Personal Area Network (WPAN) standard, commonly known as "Bluetooth", uses a device pairing process to communicate over distances from 1 to 100 meters.
- Standard IEEE 802.16 - Commonly known as Worldwide Interoperability for Microwave Access (WiMAX), uses a point-to-multipoint topology to provide wireless broadband access.
- Global System for Mobile Communications (GSM) - Includes Physical layer specifications that enable the implementation of the Layer 2 General Packet Radio Service (GPRS) protocol to provide data transfer over mobile cellular telephony networks.
In each of the above examples, Physical layer specifications are applied to areas that include: data to radio signal encoding, frequency and power of transmission, signal reception and decoding requirements, and antenna design and construction.
8.3.7 - Wireless Media
The diagram depicts wireless media standards and types.
WiFi: An access point and a laptop computer communicating represent WiFi standard IEEE 8 0 2 dot 11.
Bluetooth: Two PDA's communicating represent Wireless Personal Area Network (WPAN) Bluetooth standard IEEE 8 0 2 dot 15.
WiMAX: A PDA, a microwave tower, a satellite and dish, and a web cam communicating represent WiMAX Standard IEEE 8 0 2 dot 16.
The diagram depicts wireless media standards and types.
WiFi: An access point and a laptop computer communicating represent WiFi standard IEEE 8 0 2 dot 11.
Bluetooth: Two PDA's communicating represent Wireless Personal Area Network (WPAN) Bluetooth standard IEEE 8 0 2 dot 15.
WiMAX: A PDA, a microwave tower, a satellite and dish, and a web cam communicating represent WiMAX Standard IEEE 8 0 2 dot 16.
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The Wireless LAN
A common wireless data implementation is enabling devices to wirelessly connect via a LAN. In general, a wireless LAN requires the following network devices:
- Wireless Access Point (AP) - Concentrates the wireless signals from users and connects, usually through a copper cable, to the existing copper-based network infrastructure such as Ethernet.
- Wireless NIC adapters - Provides wireless communication capability to each network host.
Standards include:
IEEE 802.11a - Operates in the 5 GHz frequency band and offers speeds of up to 54 Mbps. Because this standard operates at higher frequencies, it has a smaller coverage area and is less effective at penetrating building structures. Devices operating under this standard are not interoperable with the 802.11b and 802.11g standards described below.
IEEE 802.11b - Operates in the 2.4 GHz frequency band and offers speeds of up to 11 Mbps. Devices implementing this standard have a longer range and are better able to penetrate building structures than devices based on 802.11a.
IEEE 802.11g - Operates in the 2.4 GHz frequency band and offers speeds of up to 54 Mbps. Devices implementing this standard therefore operate at the same radio frequency and range as 802.11b but with the bandwidth of 802.11a.
IEEE 802.11n - The IEEE 802.11n standard is currently in draft form. The proposed standard defines frequency of 2.4 Ghz or 5 GHz. The typical expected data rates are 100 Mbps to 210 Mbps with a distance range of up to 70 meters.
The benefits of wireless data communications technologies are evident, especially the savings on costly premises wiring and the convenience of host mobility. However, network administrators need to develop and apply stringent security policies and processes to protect wireless LANs from unauthorized access and damage.
These wireless standards and Wireless LAN implementations will be covered in more detail in the LAN Switching and Wireless course.
8.3.7 - Wireless Media
The diagram depicts wireless LAN network devices. These include wireless access points and wireless NIC adapters.
The diagram depicts wireless LAN network devices. These include wireless access points and wireless NIC adapters.
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In this activity, you can explore a wireless router connected to an ISP in a setup typical of a home or small business. You are encouraged to build your own models as well, possibly incorporating such wireless devices.
Click the Packet Tracer icon for more details.
8.3.7 - Wireless Media
Link to Packet Tracer Exploration: Simple Wireless LAN Model
In this activity, you can explore a wireless router connected to an ISP in a setup typical of a home or small business. You are encouraged to build your own models as well, possibly incorporating such wireless devices.
Link to Packet Tracer Exploration: Simple Wireless LAN Model
In this activity, you can explore a wireless router connected to an ISP in a setup typical of a home or small business. You are encouraged to build your own models as well, possibly incorporating such wireless devices.
8.3.8 Media Connectors
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Common Copper Media Connectors
Different Physical layer standards specify the use of different connectors. These standards specify the mechanical dimensions of the connectors and the acceptable electrical properties of each type for the different implementations in which they are employed.
Although some connectors may look the same, they may be wired differently according to the Physical layer specification for which they were designed. The ISO 8877 specified RJ-45 connector is used for a range of Physical layer specifications, one of which is Ethernet. Another specification, EIA-TIA 568, describes the wire color codes to pin assignments (pinouts) for Ethernet straight-through and crossover cables.
Although many types of copper cables can be purchased pre-made, in some situations, especially in LAN installations, the termination of copper media may be performed onsite. These terminations include crimped connections to terminate Cat5 media with RJ-45 plugs to make patch cables, and the use of punched down connections on 110 patch panels and RJ-45 jacks. The figure shows some of the Ethernet wiring components.
8.3.8 - Media Connectors
The diagram depicts common copper media connectors. These include a 110 punch-down block, RJ-45 UTP plugs, and RJ-45 UTP sockets.
The diagram depicts common copper media connectors. These include a 110 punch-down block, RJ-45 UTP plugs, and RJ-45 UTP sockets.
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Correct Connector Termination
Each time copper cabling is terminated, there is the possibility of signal loss and the introduction of noise to the communication circuit. Ethernet workplace cabling specifications stipulate the cabling necessary to connect a computer to an active network intermediary device. When terminated improperly, each cable is a potential source of Physical layer performance degradation. It is essential that all copper media terminations be of high quality to ensure optimum performance with current and future network technologies.
In some cases, for example in some WAN technologies, if an improperly wired RJ-45-terminated cable is used, damaging voltage levels may be applied between interconnected devices. This type of damage will generally occur when a cable is wired for one Physical layer technology and is used with a different technology.
8.3.8 - Media Connectors
The diagram depicts copper media connectors and UTP cable termination using RJ-45 connectors. An example of a bad termination is when the wires are untwisted for too great a length. An example of a good termination is when the wires are untwisted to the extent necessary to attach the connector. Improper cable termination can impact transmission performance.
The diagram depicts copper media connectors and UTP cable termination using RJ-45 connectors. An example of a bad termination is when the wires are untwisted for too great a length. An example of a good termination is when the wires are untwisted to the extent necessary to attach the connector. Improper cable termination can impact transmission performance.
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Common Optical Fiber Connectors
Fiber-optic connectors come in a variety of types. The figure shows some of the most common:
Straight-Tip (ST) (trademarked by AT&T) - a very common bayonet style connector widely used with multimode fiber.
Subscriber Connector (SC) - a connector that uses a push-pull mechanism to ensure positive insertion. This connector type is widely used with single-mode fiber.
Lucent Connector (LC) - A small connector becoming popular for use with single-mode fiber and also supports multi-mode fiber.
Terminating and splicing fiber-optic cabling requires special training and equipment. Incorrect termination of fiber optic media will result in diminished signaling distances or complete transmission failure.
Three common types of fiber-optic termination and splicing errors are:
- Misalignment - the fiber-optic media are not precisely aligned to one another when joined.
- End gap - the media do not completely touch at the splice or connection.
- End finish - the media ends are not well polished or dirt is present at the termination.
A field test can be performed by shining a bright flashlight into one end of the fiber while observing the other end of the fiber. If light is visible, then the fiber is capable of passing light. Although this does not ensure the performance of the fiber, it is a quick and inexpensive way to find a broken fiber.
8.3.8 - Media Connectors
The diagram depicts photographs of various fiber media connectors.
- ST Connector - Straight Tip (ST) connector is widely used with multimode fiber.
- SC Connector - Subscriber Connector (SC) is widely used with single-mode fiber.
- Single-Mode Lucent Connector (LC)
- Multimode LC Connector
- Duplex Multimode LC Connector
The diagram depicts photographs of various fiber media connectors.
- ST Connector - Straight Tip (ST) connector is widely used with multimode fiber.
- SC Connector - Subscriber Connector (SC) is widely used with single-mode fiber.
- Single-Mode Lucent Connector (LC)
- Multimode LC Connector
- Duplex Multimode LC Connector
8.4 Lab - Media Connectors
8.4.1 Media Connectors Lab ActivityPage 1:
Effective network troubleshooting requires the ability to both visually distinguish between straight-through and crossover UTP cables and to test for correct and faulty cable terminations.
This lab provides the opportunity to practice physically examining and testing UTP cables.
Click the Lab icon for more details.
8.4.1 - Media Connectors Lab Activity
Link to Hands-on Lab: Media Connectors Lab Activity
Effective network troubleshooting requires the ability to both visually distinguish between straight-through and crossover UTP cables and to test for correct and faulty cable terminations.
This lab provides the opportunity to practice physically examining and testing UTP cables.
Link to Hands-on Lab: Media Connectors Lab Activity
Effective network troubleshooting requires the ability to both visually distinguish between straight-through and crossover UTP cables and to test for correct and faulty cable terminations.
This lab provides the opportunity to practice physically examining and testing UTP cables.
8.5 Chapter Summaries
8.5.1 Summary and ReviewPage 1:
Layer 1 of the OSI model is responsible for the physical interconnection of devices. Standards at this layer define the characteristics of the electrical, optical, and radio frequency representation of the bits that comprise Data Link layer frames to be transmitted. Bit values can be represented as electronic pulses, pulses of light, or changes in radio waves. Physical layer protocols encode the bits for transmission and decode them at the destination.
Standards at this layer are also responsible for describing the physical, electrical, and mechanical characteristics of the physical media and connectors that interconnect network devices.
Various media and Physical layer protocols have different data-carrying capacities. Raw data bandwidth is the theoretical upper limit of a bit transmission. Throughput and goodput are different measures of observed data transfer over a specific period of time.
8.5.1 - Summary and Review
In this chapter, you learned to:
- Explain the role of Physical Layer protocols and services in supporting communication across data networks.
- Describe the purpose of Physical Layer signaling and encoding as they are used in networks.
- Describe the role of signals used to represent bits while a frame is transported across the local media.
- Identify the basic characteristics of copper, fiber, and wireless network media.
- Describe common uses of copper, fiber, and wireless network media.
In this chapter, you learned to:
- Explain the role of Physical Layer protocols and services in supporting communication across data networks.
- Describe the purpose of Physical Layer signaling and encoding as they are used in networks.
- Describe the role of signals used to represent bits while a frame is transported across the local media.
- Identify the basic characteristics of copper, fiber, and wireless network media.
- Describe common uses of copper, fiber, and wireless network media.
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8.5.1 - Summary and Review
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Name two ways in which bits are encoded as voltages. How do they differ?
Answer: In NRZ (non-return to zero) encoding, a zero might be represented by one voltage level on the media during the bit time, and a one might be represented by a different voltage on the media during the bit time.
Manchester Encoding uses transitions, or the absence of transitions, to indicate a logic level. For example, a zero is indicated by a high-to-low transition in the middle of the bit time. For a one, there is a low-to-high transition in the middle of the bit time.
Question 2. Why might bits be encoded as symbols before transmission?
Answer: One encoding technique is the use of symbols. The Physical Layer might use a set of encoded symbols to represent encoded data or control information called code groups. A code group is a consecutive sequence of code bits interpreted and mapped as data-bit patterns. For example, code bits 10101 can represent the data bits 0011.
While code groups introduce overhead in the form of extra bits to transmit, they improve the robustness of a communications link. This is particularly true for higher speed data transmission.
By transmitting symbols, the error detection capabilities and timing synchronization between transmitting and receiving devices are enhanced. These are important considerations in supporting high-speed transmission over the media.
Some of these advantages are:
- Reduced bit-level error.
- Limiting the effective energy transmitted into the media.
- Distinguishing data bits from control bits.
- Better media error detection.
Question 3. What safety issues must be considered when using copper cabling?
Answer: Electrical hazards
Copper wires can conduct electricity in undesirable ways. This could subject personnel and equipment to a range of electrical hazards. A defective network device can conduct currents to the chassis of another network device. Additionally, network cabling could present undesirable voltage levels when used to connect devices that have power sources with different ground potentials. Also copper cabling can conduct voltages caused by lightning strikes to network devices. Undesirable voltages and currents can damage network devices and connected computers or injure personnel.
Fire hazards
Cable insulation and sheaths might be flammable or produce toxic fumes when heated or burned. Building authorities or organizations might also stipulate related safety standards for cabling and hardware installations.
Question 4. In what situations is fiber-optic cabling preferred over copper cabling?
Answer: Given that the glass fibers used in fiber-optic media are not electrical conductors, the media is immune to electromagnetic interference and does not conduct unwanted electrical currents due to grounding issues. In addition, because optical fibers have relatively low signal loss, they can be operated at much greater lengths than copper media, without the need for signal regeneration.
Question 5. Name several copper and fiber-optic connecter types.
Answer: Copper: RJ-45, RJ-11
Fiber: Straight-Tip (ST), Subscriber Connector (SC), Lucent Connector (LC)
This is a review and is not a quiz. Questions and answers are provided.
Question 1. Name two ways in which bits are encoded as voltages. How do they differ?
Answer: In NRZ (non-return to zero) encoding, a zero might be represented by one voltage level on the media during the bit time, and a one might be represented by a different voltage on the media during the bit time.
Manchester Encoding uses transitions, or the absence of transitions, to indicate a logic level. For example, a zero is indicated by a high-to-low transition in the middle of the bit time. For a one, there is a low-to-high transition in the middle of the bit time.
Question 2. Why might bits be encoded as symbols before transmission?
Answer: One encoding technique is the use of symbols. The Physical Layer might use a set of encoded symbols to represent encoded data or control information called code groups. A code group is a consecutive sequence of code bits interpreted and mapped as data-bit patterns. For example, code bits 10101 can represent the data bits 0011.
While code groups introduce overhead in the form of extra bits to transmit, they improve the robustness of a communications link. This is particularly true for higher speed data transmission.
By transmitting symbols, the error detection capabilities and timing synchronization between transmitting and receiving devices are enhanced. These are important considerations in supporting high-speed transmission over the media.
Some of these advantages are:
- Reduced bit-level error.
- Limiting the effective energy transmitted into the media.
- Distinguishing data bits from control bits.
- Better media error detection.
Question 3. What safety issues must be considered when using copper cabling?
Answer: Electrical hazards
Copper wires can conduct electricity in undesirable ways. This could subject personnel and equipment to a range of electrical hazards. A defective network device can conduct currents to the chassis of another network device. Additionally, network cabling could present undesirable voltage levels when used to connect devices that have power sources with different ground potentials. Also copper cabling can conduct voltages caused by lightning strikes to network devices. Undesirable voltages and currents can damage network devices and connected computers or injure personnel.
Fire hazards
Cable insulation and sheaths might be flammable or produce toxic fumes when heated or burned. Building authorities or organizations might also stipulate related safety standards for cabling and hardware installations.
Question 4. In what situations is fiber-optic cabling preferred over copper cabling?
Answer: Given that the glass fibers used in fiber-optic media are not electrical conductors, the media is immune to electromagnetic interference and does not conduct unwanted electrical currents due to grounding issues. In addition, because optical fibers have relatively low signal loss, they can be operated at much greater lengths than copper media, without the need for signal regeneration.
Question 5. Name several copper and fiber-optic connecter types.
Answer: Copper: RJ-45, RJ-11
Fiber: Straight-Tip (ST), Subscriber Connector (SC), Lucent Connector (LC)
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In this activity, you will examine how Packet Tracer provides a representation of the physical location and appearance of the virtual networking devices you have been creating in logical topology mode.
Packet Tracer Skills Integration Instructions (PDF)
Click the Packet Tracer icon to launch the Packet Tracer activity.
8.5.1 - Summary and Review
Link to Packet Tracer Exploration: Skills Integration Challenge: Connecting Devices and Exploring the Physical View
In this activity, you examine how Packet Tracer provides a representation of the physical location and appearance of the virtual networking devices that you have been creating in logical topology mode.
Link to Packet Tracer Exploration: Skills Integration Challenge: Connecting Devices and Exploring the Physical View
In this activity, you examine how Packet Tracer provides a representation of the physical location and appearance of the virtual networking devices that you have been creating in logical topology mode.
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To Learn More
Reflection Questions
Discuss how copper media, optical fiber, and wireless media could be used to provide network access at your academy. Review what networking media are used now and what could be used in the future.
Discuss the factors that might limit the widespread adoption of wireless networks despite the obvious benefits of this technology. How might these limitations be overcome?
8.5.1 - 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.
8.6 Chapter Quiz
8.6.1 Chapter QuizPage 1:
8.6.1 - Chapter Quiz
1.Match the wire colors listed to the numbers that represent the wiring pinouts for 568B patch cables.
Wire Colors:
Orange
Blue
Green
Brown
Wiring Pinouts:
7 and 8
1 and 2
3 and 6
4 and 5
2.Match each pin in group 1 to the correct pin in group 2 to properly create a RJ-45 connector for a router console cable.
Group 1 Pins:
Pin 1
Pin 2
Pin 3
Pin 4
Pin 5
Pin 6
Pin 7
Pin 8
Group 2 Pins:
Pin 1
Pin 2
Pin 3
Pin 4
Pin 5
Pin 6
Pin 7
Pin 8
3.Refer to the following diagram description to answer the question.
Diagram description:
A PC is connected to a switch. At the PC end of the cable is an RJ-45 to DB-9 adapter labeled Terminal. At the switch end of the cable is an RJ-45 to RJ-45 adapter. The cable is labeled Rollover Cable.
What kind of connection is represented in the diagram based on the description?
A.Console connection
B.Ethernet connection
C.ISDN connection
D.Leased line connection
4.What is the purpose of encoding?
A.Identification of start and stop bits in a frame.
B.Denotes the physical layer connectors of computers in relation to the way they connect to the network media.
C.Controls the way frames are placed on the media at the Data Link Layer.
D.Represents the data bits by using different voltages, light patterns, or electromagnetic waves as they are placed onto the physical media.
5.What best describes the purpose of the Physical Layer?
A.Ensures reliable transmission of data across a physical link.
B.Determines connectivity and path selection between two end systems.
C.Establishes physical addressing, network topology, and media access.
D.Defines the functional specifications for links between end systems and the electrical, optical, and radio signals.
6.Through what process does UTP cable help to avoid crosstalk?
A.Shielding of cable
B.Twisting of pairs
C.Grounding the endpoints
D.Cladding in cable
7.Refer to the following diagram description to answer the question.
Diagram description:
PC A, B, and C, a switch, and a router are shown in specific locations on a building floorplan. The switch is connected to a router. The vertical dimension of the building floorplan is 75 meters. The horizontal dimension of the building floorplan is 125 meters.
The 3 PC's are connected to the switch with cables of varying lengths. The switch and router are located in the northeast corner of the floorplan. PC A is located in the southeast corner of the floorplan. PC B is located in the middle north of the floorplan, and PC C is located in the southwest corner of the floorplan.
The PC's have the following IP addresses:
PC A IP address: 192.168.1.13/24
PC B IP address: 192.168.1.11/24
PC A IP address: 192.168.1.12/24
The network in the diagram is wired with Cat 5e cable, and PC C is unable to communicate with the network. What Physical Layer problem would cause PC C to not connect to the network?
A.Wrong operating system
B.Incorrect IP address
C.Cable length
D.Incorrect prefix
E.Switch type
8.What are the advantages of using fiber optic cable over copper cable? (Choose three.)
A.Copper is more expensive
B.Immunity to electromagnetic interference
C.Careful cable handling
D.Longer maximum cable length
E.Efficient electrical current transfer
F.Greater bandwidth potential
9.What cable type would be used to connect a computer directly to another computer?
A.Straight-through
B.Rollover
C.Crossover
D.Console
10.Refer to the following diagram description to answer the question.
Diagram description:
The diagram shows fiber media cable components including, from the outermost to the innermost layer, the jacket (typically PVC), strengthening material (Aramid yarn), buffer, cladding, and core.
What is the purpose of cladding in fiber optic cables?
A.Cable grounding
B.Noise cancellation
C.Prevents light loss
D.EMI protection
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