CSC581Communication Networks II

Chapter 6b: Local Area Network

(Token Ring - 802.5)

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, 6.6.2

• FDDI: 6.6.3

4

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 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

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

11

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

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

Wiring center

A

B

CD

E

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 tokenBy 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, returnof first bit

t=400, transmitlast bit

A

t=490, reinserttoken

t=0, A begins frame t=400, last bit of frame enters ring

t=840, return of first bit

t=1240, reinserttoken

(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, returnof first bit

t=210, return of header

A

t=400, last bit enters ring, reinsert token

t=0, A begins frame t=400, transmitlast bit

t=840, arrivalfirst frame bit

t=960, reinserttoken

(b) High Latency Ring

(a) Low Latency Ring

Figure 6.60

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

31

SDDestination

AddressSource Address

Information FCS

1 4

EDFC

2 or 6 2 or 61 1

AC

1

FS

1

SD AC EDToken Frame Format

P P P T M R R RAccess control

PPP Priority; T Token bitM Monitor bit; RRR Reservation

Frame control

FF frame typeZZZZZZ control bitF F Z Z Z Z Z Z

Ending delimiter

I intermediate-frame bitE error-detection bit

Framestatus

A address-recognized bitxx undefinedC frame-copied bit

I EJ K 1 J K 1

A C x x A C x x

Data Frame Format

Starting delimiter

J, K non-data symbols (line code)0 0J K 0 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

T1 = time to send a frame

T2 = time to send a token

%100TT

TU

21

1

44

Other Ring Networks: FDDI

• 100Mbps

• LAN and MAN applications

• Token Ring

45

FDDI Physical Layer

• Medium Optical Fiber Twisted Pair

• Data rate 100 100

• Signaling 4B/5B/NRZI MLT-3

• Max repeaters 100 100

• Between repeaters 2km 100m

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

46

A

E

DC

B

Figure 6.62

47

Encoding Schemes

• 4B/5B-NRZI

• MLT-3

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

48

SDDestination

AddressSource Address

Information FCS

8 4

EDFC

2 or 6 2 or 61 11

FS

1

PRE

Preamble

SD FC EDToken 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

50

51

FDDI MAC Protocol

• As for 802.5 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