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

2. LAN LAN is a computer network that is designed for a limited geographic area such as a building or a campus. It can be used as an isolated network to connect computers in an organization for the sole purpose of sharing resources. LANs are also linked to a WAN or the Internet. 13.2

3. LAN Technology 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. Relation of 802 Standard and Traditional OSI model 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. Logical Link Layer In IEEE Project 802, flow control, error control, and part of the framing duties are collected into one sub-layer called the logical link control. Framing is handled in both the LLC sub-layer and the MAC sub-layer. The LLC provides one single data link control protocol for all IEEE LANs. MAC sub-layer provides different protocols for different LANs. A single LLC protocol can provide interconnectivity between different LANs because it makes the MAC sub- layer transparent. 13.7

8. Framing LLC defines a protocol data unit (PDU) that is somewhat similar to that of HDLC. 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). 13.8

9. 13.9 Figure 13.2 HDLC frame compared with LLC and MAC frames

10. Continued…. The other fields defined in a typical data link control protocol such as HDLC are moved to the MAC sub-layer. In other words, a frame defined in HDLC is divided into a PDU at the LLC sub-layer and a frame at the MAC sub-layer. 13.10

11. 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. However, most upper-layer protocols such as IP, do not use the services of LLC. 13.11

12. Media Access Control (MAC) IEEE project 802 has created a sub-layer called media access control that defines access method (random access, controlled access and channelization) for each LAN. Example: CSMA/CD for the Ethernet LANs Token Passing for Token Ring and Token Bus LANs. 13.12

13. Continued….. A part of framing function is also handled by the MAC layer. In contrast to the LLC sub-layer, the MAC sub-layer contains a number of distinct modules; each defines the access method and the framing format specific to the corresponding LAN protocol. 13.13

14. Physical Layer The physical layer is 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 sub-layer for standard Ethernet, there is different physical layer specifications for each Ethernet implementations (as we will see later). 13.14

15. 13.15 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:

16. 13.16 Figure 13.3 Ethernet evolution through four generations

17. Standard Ethernet (MAC Sub- layer) In Standard Ethernet, the MAC sub-layer governs the operation of the access method. It also frames data received from the upper layer and passes them to the Physical layer. 13.17

18. Frame Format The Ethernet frame contains seven fields: Preamble, SFD, DA, SA, length or type of protocol data unit (PDU), upper-layer data, and the CRC. Ethernet does not provide any mechanism for acknowledging received frames, making it what is known as an unreliable medium. Acknowledgments must be implemented at the higher layers. 13.18

19. 13.19 Figure 13.4 802.3 MAC frame

20. Frame Length Ethernet has imposed restrictions on both the minimum and maximum lengths of a frame. The minimum length restriction is required for the correct operation of CSMA/CD. 13.20

21. 13.21 Figure 13.5 Minimum and maximum lengths

22. 13.22 Frame length: Minimum: 64 bytes (512 bits) Maximum: 1518 bytes (12,144 bits) Note

23. 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. Ethernet address is 6 bytes (48 bits), normally written in hexadecimal notation, with a colon between the bytes. 13.23

24. 13.24 Figure 13.6 Example of an Ethernet address in hexadecimal notation

25. Unicast, Multicast, and Broadcast Addresses A source address is always a unicast address, the frame comes from only one station. The destination can be unicast, multicast or broadcast. 13.25

26. 13.26 Figure 13.7 Unicast and multicast addresses

27. 13.27 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

28. 13.28 The broadcast destination address is a special case of the multicast address in which all bits are 1s. Note

29. 13.29 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 To find the type of the address, we need to look at the second hexadecimal digit from the left. If it is even, the address is unicast. If it is odd, the address is multicast. If all digits are F’s, the address is broadcast. Therefore, we have the following: 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

30. 13.30 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

31. 13.31 Figure 13.8 Categories of Standard Ethernet

32. 10Base 5 : Thick Ethernet First implementation. Nick name derives from the size of the cable, which is roughly the size of a garden hose and too stiff to bend with your hands. 10 base 5 was the first Ethernet specification to use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick coaxial cable. 13.32

33. 13.33 Figure 13.10 10Base5 implementation

34. 10 base 2: Thin Ethernet Second Implementation Cheaper-net Uses bus topology Cable is much thinner and more flexible. The cable can be bent to pass very close to the stations. In this case, transceiver is normally part of the network interface card (NIC), installed inside the station. 13.34

35. 13.35 Figure 13.11 10Base2 implementation

36. Continued….. There is collision in thin coaxial. Implementation is inexpensive, thin coax is inexpensive than thick. Tee connections are much cheaper than taps. Length of each segment must do not exceed 185m due to high level of attenuation in thin coax cable. 13.36

37. 10 Base T: (Twisted Pair Ethernet) 3 rd implementation Uses a physical star topology. The stations are connected to a hub via two pairs of twisted cable. (one for sending and one for receiving) Any collision here happens in the hub. Compared to 10 base 5 or 10 base 2, the hub actually replaces the coaxial cable. The maximum length of the twisted cable here is 100m. 13.37

38. 13.38 Figure 13.12 10Base-T implementation

39. 10 Base F: Fiber Ethernet Uses a star topology to connect stations to a hub. The stations are connected to the hub using two fiber optic cables. 13.39

40. 13.40 Figure 13.13 10Base-F implementation

41. 13.41 Table 13.1 Summary of Standard Ethernet implementations

42. 13.42 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:

43. 13.43 Figure 13.14 Sharing bandwidth

44. 13.44 Figure 13.15 A network with and without a bridge

45. 13.45 Figure 13.16 Collision domains in an unbridged network and a bridged network

46. 13.46 Figure 13.17 Switched Ethernet

47. 13.47 Figure 13.18 Full-duplex switched Ethernet

48. 13.48 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:

49. Continued….. The goal of Fast Ethernet can be summarized as follows: Upgrade the data rate to 100 Mbps Make it compatible with standard Ethernet Keep the same 48-bit address Keep the same frame format Keep the same minimum and maximum frame lengths 13.49

50. MAC Sublayer A main consideration in the evolution of Ethernet from 10 to 100 Mbps was to keep the MAC sublayer untouched. The bus topology was replaced with star topology. For the star topology there are two topologies: half duplex and full duplex In half duplex the stations are connected via a HUB. In full duplex the connection is made via a switch with buffers at each port. 13.50

51. Continued….. Access method for CSMA/CD is half duplex For full duplex there is no need of CSMA/CD. However, the implementation keep CSMA/CD for backward compatibility with Standard Ethernet. 13.51

52. New Feature (autonegotiation) Allows a station or a hub a range of capabilities. (devices can negotiate with one another)Negotiation of mode and data rate of operation. To allow incompatible devices to connect one another. e.g, a device with a maximum capacity of 10 Mbps can communicate with a device of 100Mbps capacity. To allow a station to check a hub’s capabilities. 13.52

53. Physical Layer In Fast Ethernet the physical layer is more complicated than the one in standard Ethernet. Topology: If there are 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. 13.53

54. 13.54 Figure 13.19 Fast Ethernet topology

55. 13.55 Figure 13.20 Fast Ethernet implementations

56. 13.56 Table 13.2 Summary of Fast Ethernet implementations

57. 13.57 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:

58. Continued…. The goal of the gigabit ethernet design is: Upgrade the data rate to 1 Gbps Make it compatible with Standard or Fast Ethernet. Use the same 48-bit address. Use the same frame format Keep the same minimum and maximum frame lengths To support autonegotiation as defined in Fast Ethernet. 13.58

59. MAC Sublayer Gigabit Ethernet has two kinds of implementations: Half duplex Full duplex But now almost all implementations of gigabit ethernet follow the full duplex approach. The half duplex is there to be compatible with the previous generations. 13.59

60. 13.60 Figure 13.22 Topologies of Gigabit Ethernet

61. 13.61 Figure 13.23 Gigabit Ethernet implementations

62. 13.62 Table 13.3 Summary of Gigabit Ethernet implementations

63. TEN-Gigabit Ethernet The IEEE committee created ten-gigabit ethernet and called it standard 802.3ae. The goals of the ten-gigabit ethernet design are: Upgrade the data rate to 10 Gbps Make it compatible with standard, fast, and gigabit ethernet. Use the same 48-bit address. Use the same frame format. Keep the same minimum and maximum frame lengths. Works with only fiber optics. 13.63

64. MAC Sublayer Allow the interconnection of existing LANs into MAN or WAN. Make ethernet compatible with technologies such as frame relay and ATM. 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. 13.64

65. 13.65 Table 13.4 Summary of Ten-Gigabit Ethernet implementations