CSC 336 Data Communications and Networking

1. CSC 336Data CommunicationsandNetworking Lecture 7b: Local Area Networking (Token Ring – 802.5) Dr. Cheer-Sun Yang Spring 2001

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5. Topologies • Bus: A single communication line, typically a twisted pair, coaxial cable, or optical fiber, represents the primary medium. • Ring: packets can only be passed from one node to it’s neighbor. • Star: A hub or a computer is used to connect to all other computers. • Tree: no loop exists (logical connection).

6. Token Passing • Token Ring (802.5) : P. 183, Section 6.3 • Token Bus (802.4) : P. 186, Section 6.4

8. Token Passing • The difficulty with many networks is that no central control or authority makes decisions on who sends when. • Token passing is designed to deal with this issue and hopefully the link utilization can be increased.

9. Token Passing • In order to send, a station must obtain an admission pass, called a token. • In a token ring, the token is passed from one station to another. • When a station does not need it, it simply passes it on. • Token ring network must pass the token orderly to it’s neighbor. • Token bus network can pass a token to any other station directly.

10. Token Passing • However, a token bus network cannot be added as simply as with the CSMA/CD bus. • All stations must know who and where its neighbor is in a token bus.

11. 6.3 Token Ring: IEEE 802.5 • Each repeater connects to two others via unidirectional transmission links • Single closed path • Data transferred bit by bit from one repeater to the next • Repeater regenerates and retransmits each bit • Repeater performs data insertion, data reception, data removal • Repeater acts as attachment point • Packet removed by transmitter after one trip round ring

12. Token Ring (802.5) • MAC protocol • Small frame (token) circulates when idle • Station waits for token • Changes one bit in token to make it SOF for data frame • Append rest of data frame • Frame makes round trip and is absorbed by transmitting station • Station then inserts new token when transmission has finished and leading edge of returning frame arrives • Under light loads, some inefficiency • Under heavy loads, round robin

13. Dedicated Token Ring • 1997 update to IEEE 802.5 • Central hub • Acts as switch • Full duplex point to point link • Concentrator acts as frame level repeater with immediate access possible • No token passing if using a switch to connect all stations

14. 802.5 Physical Layer • Data Rate 4 16 100 • Medium UTP,STP,Fiber • Signaling Differential Manchester • Max Frame 4550 18200 18200 • Access Control TP or DTR TP or DTR DTR • Note: 1Gbit in development

15. Ring Repeater States

16. Listen State Functions • Scan passing bit stream for patterns • Address of attached station • Token permission to transmit • Copy incoming bit and send to attached station, while forwarding each bit • Modify bit as it passes • e.g. to indicate a packet has been copied (ACK)

17. Transmit State Functions • Station has data • Repeater has permission • May receive incoming bits • If ring bit length shorter than packet • Pass back to station for checking (ACK) • May be more than one packet on ring • Buffer for retransmission later

18. Bypass State • Signals propagate past repeater with no delay (other than propagation delay) • Partial solution to reliability problem (the failure of one station can cause the network failure). • Improved performance

19. Ring Media • Twisted pair • Baseband coaxial • Fiber optic • Not broadband coaxial • Would have to receive and transmit on multiple channels, asynchronously

20. Two observations • 1. Ring contention is more orderly than • with an Ethernet. No wasted bandwidth.

21. Two observations 2. The failure of one station can cause network failure. More discussion will be provided in next slide.

22. Advantage of Token Ring • The flexible control over access that it provides. • The access is fair. • It is easy to provide priority and guaranteed bandwidth services.

23. Disadvantage of Token Ring • Token maintenance requires extra work. • Loss of token prevents further utilization of the ring. • Duplication token can disrupt the operation. • A monitor station is required. It becomes a crucial point for a single point failure.

24. Potential Ring Problems • Break in any link disables network • Repeater failure disables network • Installation of new repeater to attach new station requires identification of two topologically adjacent repeaters • Method of removing circulating packets required • With backup in case of errors • Mostly solved with star-ring architecture (the wire center approach).

25. Network Failure Problem The failure of one station can cause network failure: This problem can be solved by using a wire center (Fig. 6.11). Instead of connecting neighboring stations directly, they all communicate through a wire center. The wire center contains a bypass relay. If a station fails, the bypass relay will allow a frame to bypass the station. This architecture is called a Star Ring Architecture.

27. Star Ring Architecture • Feed all inter-repeater links to single site • Concentrator • Provides central access to signal on every link • Easier to find faults • Can launch message into ring and see how far it gets • Faulty segment can be disconnected and repaired later • New repeater can be added easily • Bypass relay can be moved to concentrator • Can lead to long cable runs • Can connect multiple rings using bridges

28. Timing Jitter • Clocking included with signal • e.g. differential Manchester encoding • Clock recovered by repeaters • To know when to sample signal and recover bits • Use clocking for retransmission • Clock recovery deviates from midbit transmission randomly • Noise • Imperfections in circuitry • Retransmission without distortion but with timing error • Cumulative effect is that bit length varies • Limits number of repeaters on ring

29. Solving Timing Jitter Limitations • Repeater uses phase locked loop • Minimize deviation from one bit to the next • Use buffer at one or more repeaters • Hold a certain number of bits • Expand and contract to keep bit length of ring constant • Significant increase in maximum ring size

30. A Closer Look on Token Passing • 20 stations, each separated by 10 meters, for a total ring length of 200 meters • Transmission at 4Mbps, or one bit every 0.25 sec • Propagation speed of 2*108 m/s • How long does it take to travel around the ring once? - 1 sec • How many bits can a station send in 1 sec?

31. A Closer Look on Token Passing • A: 4 bits. • To get more bits on the ring, each station delays one bit-time allowing it to examine each bit before deciding whether to copy it or repeat it. (one bit-time = 0.25 sec) • How many bits can a station send then?

33. Token and Frame Formats • Start Delimiter (SD), End Delimiter (ED): 1 octet • Access Control (AC) : 1 octet, 3 prioritybits, 1 tokenbit, 1 monitor bit, 3 reserved bits. • Token bit determines the frame type, i.e. token frame or data frame. • Priority bit can be used to set the token’s priority. • Monitor bit and reserved bits are used for ring maintenance.

34. Token and Frame Formats • Frame Control (FC): used to distinguish control frame from data frame. • Frame Status(FS): 1 octet (acxxacxx) a: address recognized bit, c: frame copied bit, x: undefined bit. • a = 0, c = 0: dest not present or not power up • a = 1, c = 0: dest present but frame is not accepted • a = 1, c = 1: dest present and frame copied.

35. Token RingOperation

36. Priority Scheme • A station having a higher priority frame to transmit than the current frame can reserve the next token for its priority level as the frame passes by. • When the next token is issued at a station A, it will be at the reserved priority level. The station reserving the token can use this token to transmit data frame. • The station A is responsible to down-grade the priority of the token later.

37. Priority Scheme • A sends a frame to B at priority 0. • When the frame passes by D, D makes a reservation at priority 3. • When the token is sent back to A, A changes the priority to 3 and issues a new token. • D can use this token to send a frame to any station. • After the data is seized by the destination and the token is passed back to A, A is responsible for changing the priority back to 0. (Why A?)

38. Priority Scheme • In this case, station A is called a stacking station. • When it generates a new token with a higher priority, it also keeps the old priority in a stack locally. • Thus, a stacking station is the only station in which the old priority is kept.

39. Reserving and Claiming Tokens A B token C D

40. Reserving and Claiming Tokens A B Station A requests the token and sends its data to D C D

41. Reserving and Claiming Tokens A B C D Station C can reserve the next open token By entering its priority code in the AC field.

42. Reserving and Claiming Tokens A B Station D copies the frame and sends the data back to the ring. C D

43. Reserving and Claiming Tokens A B Station A receives the frame and releases the token C D

44. Reserving and Claiming Tokens A B C D Station C can send its data now.

46. Priority Scheme

47. Time Limits • Token holding time: the time duration a station is allowed to hold the token • Token rotation time: the total time a token is allowed to rotate around the ring. • TRT >= N * THT

48. Ring Maintenance Things can go wrong. For example: • A station sends a short frame over a long ring and subsequently crashes. It is not able to drain the token. This frame is called an orphan frame. • A station receives a frame or token crashes before it can send it. Now there is no token circulating. • Line noise damages a frame.

49. Ring Maintenance Some problems can be handled by giving one of the stations a few different responsibilities and designating it a monitor station. • When a monitor station receives a frame, it sets the monitor bit to 1. If the frame is received the second time and the monitor bit is still set to 1, the monitor station deletes the frame.

50. Ring Maintenance 2. The monitor station also detect a lost token using a built-in timer which is determined based on the length of the ring, number of stations, and maximum frame size. Whenever the monitor sends a frame or token, it starts the timer. If the monitor does not receive another frame or token before the timer expires, it assumes that the token is lost. It then creates another one.