1. 13.1 Chapter 13 Wired LANs: Ethernet Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Overview A local area network (LAN) is a computer network that is designed for a limited geographic area such as a building or a campus. Although a LAN can be used as an isolated network to connect computers in an organization for the sole purpose of sharing resources, most LANs today are also linked to a wide area network (WAN) or the Internet. 13.2

3. Overview The LAN market has seen several technologies such as Ethernet, Token Ring, Token Bus, FDDI, and ATM LAN. Some of these technologies survived for a while, but Ethernet is by far the dominant technology. 13.3

4. 13.4 13-1 IEEE STANDARDS In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols. Data Link Layer Physical Layer Topics discussed in this section:

5. IEEE STANDARDS The relationship of the 802 Standard to the traditional OSI model is shown in Figure 13.1. The IEEE has subdivided the data link layer into two sub layers : logical link control (LLC) and media access control (MAC). IEEE has also created several physical layer standards for different LAN protocols. 13.5

6. 13.6 Figure 13.1 IEEE standard for LANs

7. Data Link Layer As we mentioned before, the data link layer in the IEEE standard is divided into two sublayers: LLC and MAC. Logical Link Control (LLC) In Chapter 11, we discussed that data link control handles framing, flow control, and error control. In IEEE Project 802, flow control, error control, and part of the framing duties are collected into one sublayer called the logical link control. Framing is handled in both the LLC sublayer and the MAC sublayer. 13.7

8. Data Link Layer The LLC provides one single data link control protocol for all IEEE LANs. In this way, the LLC is different from the media access control sublayer, which provides different protocols for different LANs. A single LLC protocol can provide interconnectivity between different LANs. Figure 13.1 shows one single LLC protocol serving several MAC protocols. 13.8

9. Data Link Layer Framing LLC defines a protocol data unit (PDU) that is somewhat similar to that of HDLC(High-level Data Link Control). The header contains a control field like the one in HDLC; this field is used for flow and error control. The two other header fields define the upper-layer protocol at the source and destination that uses LLC. These fields are called the destination service access point (DSAP) and the source service access point (SSAP). 13.9

10. Data Link Layer Framing The other fields defined in a typical data link control protocol such as HDLC are moved to the MAC sublayer. In other words, a frame defined in HDLC is divided into a PDU at the LLC sublayer and a frame at the MAC sublayer, as shown in Figure 13.2. 13.10

11. 13.11 Figure 13.2 HDLC frame compared with LLC and MAC frames

12. Data Link Layer Need for LLC The purpose of the LLC is to provide flow and error control for the upper-layer protocols that actually demand these services. 13.12

13. Data Link Layer Media Access Control (MAC) In Chapter 12, we discussed multiple access methods including random access, controlled access, and channelization. IEEE Project 802 has created a sublayer called media access control that defines the specific access method for each LAN. For example, it defines CSMA/CD as the media access method for Ethernet LANs and the token passing method for Token Ring and Token Bus LANs. 13.13

14. Data Link Layer As we discussed in the previous section, part of the framing function is also handled by the MAC layer. In contrast to the LLC sublayer, the MAC sublayer contains a number of distinct modules; each defines the access method and the framing format specific to the corresponding LAN protocol. 13.14

15. Physical Layer Dependent on the implementation and type of physical media used. IEEE defines detailed specifications for each LAN implementation. For example, although there is only one MAC sublayer for Standard Ethernet, there is a different physical layer specifications for each Ethernet implementations as we will see later. 13.15

16. 13.16 13-2 STANDARD ETHERNET The original Ethernet was created in 1976 at Xerox’s Palo Alto Research Center (PARC). Since then, it has gone through four generations. We briefly discuss the Standard (or traditional) Ethernet in this section. MAC Sublayer Physical Layer Topics discussed in this section:

17. 13.17 Figure 13.3 Ethernet evolution through four generations

18. MAC Sublayer In Standard Ethernet, the MAC sublayer governs the operation of the access method. It also frames data received from the upper layer and passes them to the physical layer. 13.18

19. 13.19 Figure 13.4 802.3 MAC frame

20. MAC Sublayer Preamble : The first field of the frame contains 7 bytes (56 bits) of alternating 0s and 1s that alerts the receiving system to the coming frame and enables it to synchronize its input timing. The pattern provides only an alert and a timing pulse. Start frame delimiter (SFD) : The second field (1 byte: 10101011) signals the beginning of the frame. The SFD warns the station or stations that this is the last chance for synchronization. The last 2 bits is 11 and alerts the receiver that the next field is the destination address. 13.20

21. MAC Sublayer Destination address (DA) : The DA field is 6 bytes and contains the physical address of the destination station or stations to receive the packet. Source address (SA) : The SA field is also 6 bytes and contains physical address of the sender of the packet. 13.21

22. MAC Sublayer Length or type : This field is defined as a type field or length field. The original Ethernet used this field as the type field to define the upper-layer protocol using the MAC frame. The IEEE standard used it as the length field to define the number of bytes in the data field. Data : This field carries data encapsulated from the upper-layer protocols. CRC : The last field contains error detection information 13.22

23. Frame Length Ethernet has imposed restrictions on both the minimum and maximum lengths of a frame. An Ethernet frame needs to have a minimum length of 512 bits or 64 bytes (without preamble and SFD field). Part of this length is the header and the trailer. The standard defines the maximum length of a frame (without preamble and SFD field) as 1518 bytes. 13.23

24. 13.24 Figure 13.5 Minimum and maximum lengths

25. Frame Length If we count 18 bytes of header and trailer (6 bytes of SA, 6 bytes of DA, 2 bytes of length or type, and 4 bytes of CRC), then minimum length of data from the upper layer is 64 - 18 = 46 bytes and maximum length of data from the upper layer is 518 – 18 = 500 bytes. If the upper-layer packet is less than 46 bytes, padding is added to make up the difference. 13.25

26. Frame Length The minimum length restriction is required for the correct operation of CSMAlCD. The maximum length restriction has two historical reasons. First, memory was very expensive when Ethernet was designed: a maximum length restriction helped to reduce the size of the buffer. Second, the maximum length restriction prevents one station from monopolizing the shared medium, blocking other stations that have data to send. 13.26

27. Addressing Each station on an Ethernet network (such as a PC, workstation, or printer) has its own network interface card (NIC). The NIC fits inside the station and provides the station with a 6-byte physical address. As shown in Figure 13.6, the Ethernet address is 6 bytes (48 bits), normally written in hexadecimal notation, with a colon between the bytes. 13.27

28. 13.28 Figure 13.6 Example of an Ethernet address in hexadecimal notation

29. Addressing Unicast, Multicast, and Broadcast Addresses : A source address is always a unicast address-the frame comes from only one station. The destination address, however, can be unicast, multicast, or broadcast. Figure 13.7 shows how to distinguish a unicast address from a multicast address. 13.29

30. 13.30 The least significant bit of the first byte defines the type of address. If the bit is 0, the address is unicast; otherwise, it is multicast. Note

31. 13.31 The broadcast destination address is a special case of the multicast address in which all bits are 1s. Note

32. 13.32 Figure 13.7 Unicast and multicast addresses

33. 13.33 Define the type of the following destination addresses: a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EE c. FF:FF:FF:FF:FF:FF Solution a. This is a unicast address because A in binary is 1010. b. This is a multicast address because 7 in binary is 0111. c. This is a broadcast address because all digits are F’s. Example 13.1

34. Addressing The way the addresses are sent out on line is different from the way they are written in hexadecimal notation. The transmission is left-to-right, byte by byte; however, for each byte, the least significant bit is sent first and the most significant bit is sent last. This means that the bit that defines an address as unicast or multicast arrives first at the receiver. 13.34

35. 13.35 Show how the address 47:20:1B:2E:08:EE is sent out on line. Solution The address is sent left-to-right, byte by byte; for each byte, it is sent right-to-left, bit by bit, as shown below: Example 13.2

36. Physical Layer The Standard Ethernet defines several physical layer implementations; four of the most common, are shown in Figure 13.8. 13.36

37. 13.37 Figure 13.8 Categories of Standard Ethernet

38. Physical Layer Encoding and Decoding All standard implementations use digital signalling (baseband) at 10 Mbps. At the sender, data are converted to a digital signal using the Manchester scheme; at the receiver, the received signal is interpreted as Manchester and decoded into data. 13.38

39. 13.39 Figure 13.9 Encoding in a Standard Ethernet implementation

40. 10Base5: Thick Ethernet The nickname derives from the size of the cable and too stiff to bend with your hands. 10Base5 was the first Ethernet specification to use a bus topology with external transceiver (transmitter/receiver) connected via a tap to a thick coaxial cable. The transceiver is responsible for transmitting, receiving, and detecting collisions. The transceiver is connected to the station via a transceiver cable that provides separate paths for sending and receiving. This means that collision can only happen in the coaxial cable. 13.40

41. 13.41 Figure 13.10 10Base5 implementation

42. 10Base5: Thick Ethernet The maximum length of the coaxial cable must not exceed 500 m, otherwise, there is excessive degradation of the signal. If a length of more than 500 m is needed, upto five segments, each a maximum of 500-meter,can be connected using repeaters. 13.42

43. 10Base2: Thin Ethernet Also uses a bus topology. Cable is much thinner and more flexible. The cable can be bent to pass very close to the stations. This implementation is more cost effective than 10Base5 because thin coaxial cable is less expensive than thick coaxial. The length of each segment cannot exceed 185 m (close to 200 m). 13.43

44. 13.44 Figure 13.11 10Base2 implementation

45. 10BaseT: Twisted Pair Ethernet Uses a physical star topology. The stations are connected to a hub via two pairs of twisted cable, as shown in Figure. Note that two pairs of twisted cable create two paths (one for sending and one for receiving) between the station and the hub. Any collision here happens in the hub. The maximum length of the twisted cable here is defined as 100 m. 13.45

46. 13.46 Figure 13.12 10Base-T implementation

47. 10BaseF: Fiber Ethernet Although there are several types of optical fiber 10-Mbps Ethernet, the most common is called 10Base-F. 10Base-F uses a star topology to connect stations to a hub. The stations are connected to the hub using two fiber- optic cables. 13.47

48. 13.48 Figure 13.13 10Base-F implementation

49. 13.49 Table 13.1 Summary of Standard Ethernet implementations

50. 13.50 13-3 CHANGES IN THE STANDARD The 10-Mbps Standard Ethernet has gone through several changes before moving to the higher data rates. These changes actually opened the road to the evolution of the Ethernet to become compatible with other high-data-rate LANs. Bridged Ethernet Switched Ethernet Full-Duplex Ethernet Topics discussed in this section:

51. Bridged Ethernet The first step in the Ethernet evolution was the division of a LAN by bridges. Bridges have two effects on an Ethernet LAN: - They raise the bandwidth and - They separate collision domains. 13.51

52. Bridged Ethernet Raising The Bandwidth In an unbridged Ethernet network, the total capacity (10 Mbps) is shared among all stations with a frame to send; the stations share the bandwidth of the network. If only one station has frames to send, it benefits from the total capacity (10 Mbps). But if more than one station needs to use the network, the capacity is shared. 13.52

53. 13.53 Figure 13.14 Sharing bandwidth

54. Bridged Ethernet Raising The Bandwidth A bridge divides the network into two or more networks. Bandwidth-wise, each network is independent. For example, in Figure 13.15, a network with 12 stations is divided into two networks, each with 6 stations. Now each network has a capacity of 10 Mbps. 13.54

55. 13.55 Figure 13.15 A network with and without a bridge

56. Bridged Ethernet Separating Collision Domains Figure 13.16 shows the collision domains for an unbridged and a bridged network. You can see that the collision domain becomes much smaller and the probability of collision is reduced tremendously. Without bridging, 12 stations contend for access to the medium; with bridging only 3 stations contend for access to the medium. 13.56

57. 13.57 Figure 13.16 Collision domains in an unbridged network and a bridged network

58. Switched Ethernet The idea of a bridged LAN can be extended to a switched LAN. Instead of having two to four networks, why not have N networks, where N is number of stations on the LAN? In this way, the bandwidth is shared only between the station and the switch (5 Mbps each). In addition, the collision domain is divided into N domains. 13.58

59. 13.59 Figure 13.17 Switched Ethernet

60. Full Duplex Ethernet One of the limitations of 10Base5 and lOBase2 is that communication is half-duplex (10Base-T is always full- duplex); a station can either send or receive, but may not do both at the same time. The next step in the evolution was to move from switched Ethernet to full-duplex switched Ethernet. The full-duplex mode increases the capacity of each domain from 10 to 20 Mbps. 13.60

61. 13.61 Figure 13.18 Full-duplex switched Ethernet

62. 13.62 13-4 FAST ETHERNET Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. MAC Sublayer Physical Layer Topics discussed in this section:

63. Fast Ethernet The goals of Fast Ethernet can be summarized as follows: 1. Upgrade the data rate to 100 Mbps. 2. Make it compatible with Standard Ethernet. 3. Keep the same 48-bit address. 4. Keep the same frame format. 5. Keep the same minimum and maximum frame lengths. 13.63

64. Fast Ethernet MAC Sublayer Replaces bus topology with star topology For the star topology, there are two choices : half duplex and full duplex. In the half-duplex approach, the stations are connected via a hub; in the full-duplex approach, the connection is made via a switch. 13.64

65. Fast Ethernet MAC Sublayer A new feature added to Fast Ethernet called Auto negotiation. Auto negotiation allows two devices to negotiate the mode or data rate of operation. For example, a device with a maximum capacity of 10 Mbps can communicate with a device with a 100 Mbps capacity. 13.65

66. Fast Ethernet Physical Layer Fast Ethernet is designed to connect two or more stations together. If there are only two stations, they can be connected point-to-point. Three or more stations need to be connected in a star topology with a hub or a switch at the centre, as shown in Figure 13.19. 13.66

67. 13.67 Figure 13.19 Fast Ethernet topology

68. 13.68 Figure 13.20 Fast Ethernet implementations

69. 13.69 Figure 13.21 Encoding for Fast Ethernet implementation

70. 13.70 Table 13.2 Summary of Fast Ethernet implementations

71. 13.71 13-5 GIGABIT ETHERNET The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the standard 802.3z. MAC Sublayer Physical Layer Ten-Gigabit Ethernet Topics discussed in this section:

72. Gigabit Ethernet The goals of the Gigabit Ethernet design can be summarized as follows: 1. Upgrade the data rate to 1 Gbps. 2. Make it compatible with Standard or Fast Ethernet. 3. Use the same 48-bit address. 4. Use the same frame format. 5. Keep the same minimum and maximum frame lengths. 6. To support autonegotiation as defined in Fast Ethernet. 13.72

73. MAC Sublayer Gigabit Ethernet has two distinctive approaches for medium access: half-duplex and full-duplex. Almost all implementations of Gigabit Ethernet follow the full-duplex approach. However, we briefly discuss the half-duplex approach to show that Gigabit Ethernet can be compatible with the previous generations. 13.73

74. MAC Sublayer Full Duplex Mode There is a central switch connected to all computers or other switches. In this mode, each switch has buffers for each input port in which data are stored until they are transmitted. There is no collision in this mode. This means that CSMAlCD is not used. Lack of collision implies that the maximum length of the cable is determined by the signal attenuation in the cable, not by the collision detection process. 13.74

75. MAC Sublayer Half Duplex Mode Gigabit Ethernet can also be used in half-duplex mode, although it is rare. In this case, a switch can be replaced by a hub, which acts as the common cable in which a collision might occur. The half-duplex approach uses CSMAlCD. 13.75

76. Physical Layer Topology Gigabit Ethernet is designed to connect two or more stations. If there are only two stations, they can be connected point-to-point. Three or more stations need to be connected in a star topology with a hub or a switch at the centre. 13.76

77. 13.77 Figure 13.22 Topologies of Gigabit Ethernet

78. Physical Layer Implementation Gigabit Ethernet can be categorized as either a two-wire or a four-wire implementation. The two-wire implementations use fiber-optic cable (1000Base-SX, short-wave, or 1000Base-LX, long-wave), or STP (1000Base-CX). The four-wire version 1000Base-T was designed in response to those users who had already installed this wiring for other purposes such as Fast Ethernet or telephone services. 13.78

79. 13.79 Figure 13.23 Gigabit Ethernet implementations

80. 13.80 Figure 13.24 Encoding in Gigabit Ethernet implementations

81. 13.81 Table 13.3 Summary of Gigabit Ethernet implementations

82. Ten-Gigabit Ethernet The goals of the Ten-Gigabit Ethernet design can be summarized as follows: 1. Upgrade the data rate to 10 Gbps. 2. Make it compatible with Standard, Fast, and Gigabit Ethernet. 3. Use the same 48-bit address. 4. Use the same frame format. 5. Keep same minimum and maximum frame lengths. 6. Allow the interconnection of existing LANs into a MAN or a WAN. 7. Make Ethernet compatible with technologies such as Frame Relay and ATM (Chapter 18). 13.82

83. Ten-Gigabit Ethernet MAC Sublayer Ten-Gigabit Ethernet operates only in full duplex mode which means there is no need for contention. CSMA/CD is not used in Ten-Gigabit Ethernet. 13.83

84. Ten-Gigabit Ethernet Physical Layer The physical layer in Ten-Gigabit Ethernet is designed for using fiber-optic cable over long distances. Three implementations are the most common: 10GBase- S, 10GBase-L, and 10GBase-E. Table 13.4 shows a summary of the Ten-Gigabit Ethernet implementations. 13.84

85. 13.85 Table 13.4 Summary of Ten-Gigabit Ethernet implementations

86. Token Passing Method In the token-passing method, the stations in a network are organized in a logical ring. In other words, for each station, there is a predecessor and a successor. The predecessor is the station which is logically before the station in the ring; the successor is the station which is after the station in the ring. 13.86

87. Token Passing Method The current station is the one that is accessing the channel now. The right to this access has been passed from the predecessor to the current station. The right will be passed to the successor when the current station has no more data to send. 13.87

88. Token Passing Method In this method, a special packet called a token circulates through the ring. The possession of the token gives the station the right to access the channel and send its data. When a station has some data to send, it waits until it receives the token from its predecessor. It then holds the token and sends its data. 13.88

89. Token Passing Method When the station has no more data to send, it releases the token, passing it to the next station in the ring. The station cannot send data until it receives the token again in the next round. 13.89

90. Token Management Stations must be limited in the time they can have possession of the token. The token must be monitored to ensure it has not been lost or destroyed.  For example, if a station that is holding the token fails, the token will disappear from the network. Another function is to assign priorities to the stations and to the types of data being transmitted. And finally, token management is needed to make low- priority stations release the token to high priority stations. 13.90

91. Token Bus 4-Mbps Local Area Network. A type of local-area network (LAN) that has a bus topology and uses a token-passing mechanism to regulate traffic on the bus. A token bus network is very similar to a token ring network, the main difference being that the endpoints of the bus do not meet to form a physical ring. Creates a “virtual ring” in the network. 13.91

92. Token Bus 13.92

93. Token Bus Token bus utilized a copper coaxial cable to connect multiple end stations (terminals, workstations, shared printers etc.) to the mainframe. The coaxial cable served as a common communication bus and a token was created by the Token Bus protocol to manage or 'arbitrate' access to the bus. 13.93

94. Token Bus Any station that holds the token packet has permission to transmit data. The station releases the token when it is done communicating or when a higher priority device needs to transmit (such as the mainframe). This keeps two or more devices from transmitting information on the bus at the same time and accidentally destroying the transmitted data. 13.94

95. Token Bus Token Bus suffered from two limitations. Any failure in the bus caused all the devices beyond the failure to be unable to communicate with the rest of the network. Second, adding more stations to the bus was somewhat difficult. Thus, token bus networks were seen as somewhat unreliable and difficult to expand and upgrade.

96. Token Ring It forms a physical ring in the network. Means end devices are connected with each other. 16-Mbps is the standard speed for token-ring.

97. Token Ring When a station sends the token to its successor, the token cannot be seen by other stations; the successor is the next one in line. This means that the token does not have to have the address of the next successor. The problem with this topology is that if one of the links- the medium between two adjacent stations fails, the whole system fails. 13.97

98. Token Ring 13.98

99. FDDI Fiber Distributed Data Interface An FDDI network supports data transfer speeds of 100 Mbps over a fiber-optic cable and uses a rotating token to define which system can send data at any given time. FDDI specifies a 100-Mbps token-passing, dual-ring LAN. Can be extended in range up to 200 kms. 13.99

100. FDDI FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications, each with a specific function. FDDI's four specifications are the Media Access Control (MAC), Physical Layer Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT) specifications. 13.100

101. FDDI MAC specification defines how the medium is accessed, including frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error-recovery mechanisms. The PHY specification defines data encoding/decoding procedures, clocking requirements, and framing, among other functions. 13.101

102. FDDI The PMD specification defines the characteristics of the transmission medium, including fiber-optic links, power levels, bit-error rates, optical components, and connectors. The SMT specification defines FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection. 13.102

103. FDDI FDDI uses dual ring topology which makes it fault- tolerant; a second (auxiliary) ring which operates in the reverse direction compared with the main ring. The second ring is for emergencies only (such as a spare tire for a car). If one of the links in the main ring fails, the system automatically combines the two rings to form a temporary ring. After the failed link is restored, the auxiliary ring becomes idle again. Note that for this topology to work, each station needs to have two transmitter ports and two receiver ports. 13.103

104. FDDI 13.104

105. FDDI End delimiter - Contains unique symbols; cannot be data symbols that indicate the end of the frame. Frame status - Allows the source station to determine whether an error occurred; identifies whether the frame was recognized and copied by a receiving station. 13.105