Token Ring06

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Computer Networks: Token Ring and FDDI 1

Token Ring

andFiber Distributed DataInterface (FDDI)




IEEE 802.5 Token RingProposed in 1969 and initially referred to as a

Newhall ring.Token ring :: a number of stations connected by

transmission links in a ring topology. Informationflows in one direction along the ring from sourceto destination and back to source.

Medium access control is provided by a small frame,

the token , that circulates around the ring when allstations are idle. O nly the station possessing thetoken is allowed to transmit at any given time.




Token Ring OperationWhen a station wishes to transmit, it must wait for the token to pass by and s eize the token . ± O ne approach: change one bit in token which transforms it

into a ³start-of-frame sequence´ and appends frame for transmission.

± S econd approach: station claims token by removing itfrom the ring.

The data frame circles the ring and is removed bythe transmitting station.

Each station interrogates passing frame. If destinedfor station, it copies the frame into local buffer.{Normally, there is a one bit delay as the framepasses through a station.}




Wiring center

A

B

CD

E

Figure 6.58Leon-Garcia & Widjaja: C ommunication Network sCopyright ©2000 The McGraw Hill Companies

Token Ring Networkwith star topology




Token Insertion Choices1. multi-token: insert token after station has

completed transmission of the last bit of theframe.

2. single-token: insert token after last bit of busytoken is received and the last bit of the frame istransmitted.

3. single-frame: insert token after the last bit of theframe has returned to the sending station.

P erformance is determined by whether more thanone frame is allowed on the ring at the sametime and the relative propagation time.




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 =4 90 , reinsert

token

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 6.59Leon-Garcia & Widjaja: Communication NetworksCopyright ©2 000 The McGraw Hill Companies

S ingle frame operation




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 entersring, 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 6.6 0Leon-Garcia & Widjaja: Communication NetworksCopyright ©2 000 The McGraw Hill Companies

S ingle token operation




IEEE 802.5 Token Ring4 and 16 Mbps using twisted-pair cabling with differentialManchester line encoding.Maximum number of stations is 25 0 .4Mbps 802.5 token ring uses s ingle frame operation .4 Mbps IBM token ring uses s ingle token operation .Both 80 2.5 and IBM 16Mbps token rings use multi-tokenoperation.80 2.5 has 8 priority levels provided via two 3-bit fields

(priority and reservation) in data and token frames.Permits 16-bit and 48 -bit addresses (same as 80 2.3).




Token RingUnder light load ± delay is added due to waiting for the token {on average the delay is one half ringpropagation time} .Under heavy load ± ring is ³round-robin´.² Performance is fairer and better than

Ethernet!! The ring must be long enough to hold the completetoken.A dvantages ± fair access, no collisions.Disadvantages ± ring is single point of failure, ringmaintenance is complex due to token malfunctions.




Token Maintenance Issues

W hat can go wrong?Loss of token (no token circulating)Duplication of token (forgeries or mistakes)The need to designate one station as theactive ring monitor .Persistently circulating frameDeal with active monitor going down.




S D DestinationA ddress

S ourceA ddress

Information FCS

1 4

EDFC2 or 6 2 or 61 1

A C1

FS1

S D A C EDToken Frame Format

P P P T M R R R A ccesscontrol

PPP Priority; T Token bitM Monitor bit; RRR Reservation

Framecontrol

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

Endingdelimiter

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

S tartingdelimiter

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


IEEE 80 2.5 Token and data frame structure




Fiber Distributed Data

Interface (FDDI)FDDI uses a ring topology of multimode or singlemode optical fiber transmission links operating at100 Mbps to span up to 2 00 kms and permits up to500 stations.E mploys d ual counter-rotating rings.16 and 48 -bit addresses are allowed.

In FDDI, token is absorbed by station and releasedas soon as it completes the frame transmission{ multi-token operation }.




A

E

DC

B


FDDI ² Dual Token Ring




FDDI Repair




FDDI

RingOperation




FDDITo accommodate a mixture of stream and burstytraffic, FDDI is designed to handle two types of traffic:

± S ynchronou s frames that typically have tighter delayrequirements (e.g., voice and video) ± As ynchronou s frames have greater delay tolerances

(e.g., data traffic)

FDDI uses TTRT (Target Token Rotation Time)to ensure that token rotation time is less than somevalue.




FDDI Data Encoding

Cannot use differential Manchester because100 Mbps FDDI would require 2 00 Mbaud!

Instead each ring interface has its own localclock. ± O utgoing data is transmitted using this clock. ± Incoming data is received using a clock that is

frequency and phase locked to the transitions inthe incoming bit stream.




FDDI Data EncodingData is encoded using a 4B/5B encoder. ± For each four bits of data transmitted, a corresponding

5-bit codeword is generated by the encoder. ± There is a maximum of two consecutive zero bits in each

symbol.The symbols are then shifted out through a NRZIencoder which produces a signal transition whenever a 1 bit is being transmitted and no transition when a0 bit is transmitted.Local clock is 125MHz. This yields 1 00 Mbps ( 80% due to 4 B/5B).




FDDI




S D DestinationA ddress

S ourceA ddress

Information FCS

8 4

EDFC2 or 6 2 or 61 11

FS1

PRE

Preamble

S D FC EDToken Frame Format PRE

FrameControl

Data Frame Format

CLFFZZZZ C = S ynch/ A synchL = A ddress length (16 or 48 bits)FF = LLC/M A C control/reserved frame type

Figure 6.63Leon-Garcia & Widjaja: C ommunication Network s

FDDI frame structure




More FDDI DetailsFDDI Transmission on optical fiber requires A S K.The simplest case: coding is done via the absence or

presence of a carrier signal { I nten s ity Mo d ulation }.S pecific 5-bit codeword patterns chosen to guarantee

no more than three zeroes in a row to provide for adequate synchronization.1300 nm wavelength specified.Dual rings (primary and secondary) ± transmit in

opposite directions.Normally, second ring is idle and used for redundancyfor automatic repair (self-healing).




Differences between 802.5 and

FDDI802.5 Token RingS hielded twisted pair 4 , 16 Mbps

No reliability specifiedDifferential Manchester Centralized clock Priority and Reservation bits

A ll three token operations possible

FDDIO ptical Fiber 100 Mbps

Reliability specified (dual ring)4 B/5B encodingDistributed clockingTimed Token Rotation Time

Multi-token operation

Token Ring06

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