CSC581 Communication Networks II Chapter 6b: Local Area Network (Token Ring ) Dr. Cheer-Sun Yang

2 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).

3 Token Passing Token Ring (802.5) : 6.4.3, 6.4.4, 6.4.5, FDDI: 6.6.3

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5 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.

6 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.

7 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.

8 Token Ring: IEEE 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

9 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

10 Dedicated Token Ring Central hub Acts as switch Full duplex point to point link Concentrator acts as frame level repeater No token passing

Physical Layer Data Rate Medium UTP,STP,Fiber Signaling Differential Manchester Max Frame Access ControlTP or DTRTP or DTRDTR Note: 1Gbit in development

12 Ring Repeater States

13 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 –Whilst forwarding each bit Modify bit as it passes –e.g. to indicate a packet has been copied (ACK)

14 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

15 Bypass State Signals propagate past repeater with no delay (other than propagation delay) Partial solution to reliability problem (see later) Improved performance

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

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

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

19 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 Timing jitter Method of removing circulating packets required –With backup in case of errors Mostly solved with star-ring architecture (the wire center approach).

20 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.

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 21 Figure 6.58

22 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

23 Reserving and Claiming Tokens C B D A token

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

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

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

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

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

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 29 A A A A A A A t=0, A begins frame t=90, return of first bit t=400, transmit last bit A t=490, reinsert token t=0, A begins frame t=400, last bit of frame enters ring t=840, return of first bit t=1240, reinsert token (a) Low Latency Ring (b) High Latency Ring Figure 6.59

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 30 A A A A A A A t=0, A begins frame t=90, return of first bit t=210, return of header A t=400, last bit enters ring, reinsert token t=0, A begins frame t=400, transmit last bit t=840, arrival first frame bit t=960, reinsert token (b) High Latency Ring (a) Low Latency Ring Figure 6.60

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 31 SD Destination Address Source Address Information FCS 14 ED FC 2 or AC 1 FS 1 SDAC ED Token Frame Format P P PT M R R R Access control PPP Priority; T Token bit M Monitor bit; RRR Reservation Frame control FF frame type ZZZZZZ control bit F Z Z Z Ending delimiter I intermediate-frame bit E error-detection bit Frame status A address-recognized bit xx undefined C frame-copied bit IE J K 1 ACx AC Data Frame Format Starting delimiter J, K non-data symbols (line code) 00 J K 0 Figure 6.61

32 Token and Frame Formats Start Delimiter (SD), End Delimiter (ED): 1 octet Access Control (AC) : 1 octet, 3 priority bits, 1 token bit, 1 monitor bit, 3 reserved bits. 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.

33 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.

34 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.

35 Priority Scheme 1.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. 2.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. 3.The station A is responsible to down-grade the priority of the token later.

36 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?)

37 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

38 Ring Maintenance Things can go wrong. For example: 1.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. 2.A station receives a frame or token crashes before it can send it. Now there is no token circulating. 3.Line noise damages a frame.

39 Ring Maintenance Some problems can be handled by giving one of the stations a few different responsibilities and designating it a monitor station. 1.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.

40 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.

41 Ring Maintenance Some problems cannot be solved even with a monitor station. For example, what if the malfunction station is the monitor station? What if a break in the ring causes a lack of tokens? Sending new ones does nothing to correct the problem. These problems are handled using control frames.

42 Ring Maintenance Some example control frames: Claim token frame – for submitting bids to elect a monitor station. Active monitor present (AMP) frame – to notify others that a monitor station has been produced. Standby monitor present (SMP) – frame. Beacon frame – to inform stations that a problem has occurred and the token-passing protocol has stopped.

43 Ring Efficiency T 1 = time to send a frame T 2 = time to send a token

44 Other Ring Networks: FDDI 100Mbps LAN and MAN applications Token Ring

45 FDDI Physical Layer MediumOptical FiberTwisted Pair Data rate Signaling4B/5B/NRZIMLT-3 Max repeaters Between repeaters2km100m

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 46 A E D C B Figure 6.62

47 Encoding Schemes 4B/5B-NRZI MLT-3

Copyright 2000 McGraw-Hill Leon- Garcia and Widjaja Communication Networks 48 SD Destination Address Source Address Information FCS 84 ED FC 2 or FS 1 PRE Preamble SDFC ED Token Frame Format PRE Frame Control Data Frame Format CLFFZZZZ C = Synch/Asynch L = Address length (16 or 48 bits) FF = LLC/MAC control/reserved frame type Figure 6.63

49 FDDI MAC Frame Format

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51 FDDI MAC Protocol As for except: Station seizes token by aborting token transmission Once token captured, one or more data frames transmitted New token released as soon as transmission finished (early token release in 802.5)

52 FDDI Operation

53 Example Taken from William Stallings’s book.

54 Reading Chapter 6: 6.4.3, 6.4.4, 6.4.5, 6.6.2, 6.6.3