Ring LANs - cs.virginia.edujorg/teaching/cs457/slides/lanp2.pdf · Token Ring LANs • Note: – If...

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© Jörg Liebeherr, 1998,1999 CS457

Ring Local Area Network

Ring interface(1-bit buffer)

Ring interface

To station From station


Ring LANs

• The ring is a series of bit repeaters, each connected by aunidirectional transmission link

• All arriving data is copied into a 1-bit buffer and then copiedout again (1-bit delay)

• Data in the buffer can be modified before transmission• Ring interface can be in one of three states:

– Listen State– Transmit State– Bypass State

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• Listen State: Incoming bits are copied to output with 1-bit delay

• Transmit State: Write data to the ring



States of the Ring Interface


States of the Ring Interface

• Bypass State: Idle station does not incur bit-delay

3


Ring LANs

• If a frame has traveled once around the ring it is removed bythe sender

• Ring LANs have a simple acknowledgment scheme:– Each frame has one bit for acknowledgment.– If the destination receives the frame it sets the bit to 1.– Since the sender will see the returning frame, it can tell if

the frame was received correctly.


What is the ‘Length’ of a Ring?

• The length of a ring LAN, measured in bits, gives the totalnumber of bits which are can be in transmission on the ring ata time

• Note: Frame size is not limited to the “length” of the ring sinceentire frame may not appear on the ring at one time. Why?

• Bit length = propagation speed · length of ring · data rate ++ No. of stations · bit delay at repeater

4


Example|:

• Calculate the length of the following ring LAN:• 3 km ring• 1 Mbps data rate• 5 µs/km propagation speed• 20 stations @ 1 bit delay

• Bit length =


Ring LAN

• Advantages:• Can achieve 100 % utilization• No collisions• Can achieve deterministic delay bounds• Can be made efficient at high speeds

• Disadvantages:– Long delays due to bit-delays

• Solution: Bypass state eliminates bit-delay at idle station– Reliability Problems

• Solution 1: Use a wire center• Solution 2: Use a second ring (opposite flow)

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Ring LANS: Wire Center (802.5)

Station

Station

Station

Station

Connector

Cable

Bypass Relay

Wire Center


Ring LANS: Use a second ring

failure

This is known as a dual-redundant counter-rotating ring

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• Token is a small packetthat rotates around thering

• When all stations are idle,the token is “free” andcirculates around the ring

• Possible Problem: Allstations are idle and in theBypass state. What is theproblem?

Token

A

B

C

D

Token Ring LANS


802.5(Token Ring) MAC Protocol

• In order to transmit a station must catch a “free” token

• The station changes the token from “free” to “busy”

• The station transmits its frame immediately following the busytoken

• IF station has completed transmission of the frame AND thebusy token has returned to the station THEN station inserts anew free token into the ring

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Token Ring LANs

• Note:– If the bit length of the ring is less than the packet length,

then the completion of a packet transmission implies returnof busy token

– Only one station can transmit at a time. If a stationreleases a free token, the next station downstream cancapture the token


Transmission in a Token Ring

• Sender looks for freetoken

A

B

C

D

Free token

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• Sender changes freetoken to busy tokenand appends data tothe token

A

B

C

D

Busy token

Data



A

B

C

D

Busy token

• Receiver recognizesthat it is the destinationof the frame

• Receiver copies frameto station

• Note: Frame alsoreturns to sender

Data

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A

B

C

D


Data

• Receiver recognizesthat it is the destinationof the frame

• Receiver copies frameto station

• Note: Frame alsoreturns to sender



A

B

C

D

Data

Free token

• Sender generatesfree token when it isdone transmitting(Note: The busytoken has returned)

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Properties of the 802.5 Token Ring

• No collisions of frames• Full utilization of bandwidth is feasible• Transmission can be regulated by controlling access to token• Recovery protocols is needed if token is not handled properly,

e.g., token is corrupted, station does not change to “free”, etc


Token Format / Data Frame Format:

Destinationaddress

Sourceaddress Data Checksum

1 1 1 2 or 6 2 or 6 No Limit 4Bytes

PreambleStart delimiter

Frame control End delimiter

SD AC FC ED FS

Frame status

1 1 1

SD AC ED

Bytes

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IEEE 802.5 Frame Format

• One 3-byte token circulates if all stations are idle.AC = “PPPTMRRR” where

“PPP” Priority fields“RRR” Reservation fields“T” Indicates “Token” or “Data frame”

• SD, ED: Start/End delimiter of a frame• FC: Identifies type of a control frame• FS: Contains address recognized bit (A bit) and

frame copied bit (C bit)– Receiver sets A=1 when frame arrives– Receiver sets C=1 when frame has been copied


Priority of Transmission in 802.5

• Eight levels of priorities– Priorities handled by 3-bit priority field and 3-bit reservation

field

– Define:Pm = priority of the message to be transmittedPr = token priority of received tokenRr = reservation priority of received token

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IEEE 802.5 ( Token Ring)

1. A station wishing to transmit a frame with priority Pm must waitfor a free token with Pr ≤ Pm

2. The station can reserve a future priority-Pm token as follows:– If busy token comes by, then set Rr ← Pm (if Rr < Pm)

– If free token comes by, then set Rr ← Pm (if Rr < Pm and Pm < Pr)


IEEE 802.5 ( Token Ring)

3. If a station gets a free token, it sets the reservation field to“0”, and leaves the priority field unchanged and transmits

4. After transmission send a free token with• Priority = max (Pr , Rr , Pm)• Reservation = max (Rr , Pm)

5. Station which upgraded the priority level of a token must alsodowngrade the priority (if no one used the token)

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Ring Maintenance

• Token ring selects one station as the monitor station

• Duties of the monitor:– Check that there is a token– Recover ring if it is broken– Detect garbled frames– Make sure the token (24-bit) is shorter than the ring length


Terms in IEEE 802.5

• IEEE 802.5 requires to maintain a large number ofcounters

THT: Token Holding Timer (one per station)

Limits the time that a station can transmit (Default 10 ms)

TRR: Time-to-Repeat Timer (one per station)

Limits the time that a station waits for return of own message(Default 2.5 ms)

TVX: Valid Transmission Timer (in monitor station)

Verifies that station which accessed the token actually used it(Default: THT + TRR = 12.5 ms)

TNT: No-Token Timer (one per station)

If expire, a new token is generated(Default: N * (THT + TRR))

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Performance of Token Rings

• Parameters and Assumption:• End-to-end propagation delay a• Packet transmission time 1• Number of stations N

• Assume that each station always has a packet waiting fortransmission

• Note: The ring is used either for data transmission or forpassing the token



• Define:

• T1 = Average time to transmit a frame. Perassumption, T1=1

• T2 = Average time to pass the token

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1

TTT

OverheadTimeFrameTimeFrame

ThroughputMaximum

+=

+=

=

15


NaS

+=

11

NaaS

+= 1

Effect of propagation delay

Effect of propagation delay on throughput:• Case 1: a < 1 (Packet longer than ring)

• T2 = time to pass token to the next station

= a/N• Case 2: a > 1 (Packet shorter than ring)

• Note: Sender finishes transmissionafter T1 = 1, but cannot release thetoken until the token returns

• T1 + T2 = min (1, a) + a/N



• Illustration of Analysis ( a>1 )

Workstation

t0

Workstation

t0+a+1

Workstation

t0+a

Workstation

t0+1

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• Illustration of Analysis ( a<1 )

Workstation

t0

Workstation

t0+a+1

Workstation

t0+1

Workstation

t0+a


Ethernet vs. Token Ring

• Maximum throughput as a function of “a”

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Ethernet vs. Token Ring

• Maximum throughput as a function of “N”


FDDI

• Some Facts:• FDDI = Fiber Distributed Data Interface• FDDI is a high-speed token ring• Fiber-optic (dual redundant counter rotating) ring LAN• Multimode fiber• Standardized by ANSI and ISO X3T9.5 committee• 100 Mbps data rate• Maximum frame size is 4500 bytes• Allows up to 1000 connected stations• Maximum ring circumference 200 km

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FDDI

• FDDI distinguishes 4 Service Classes:

– Asynchronous

– Synchronous

– Immediate (for monitor and control)

– Isochronous (only in FDDI-II)


FDDI - Protocol Architecture

LLC (IEEE 802.2)Logical Link Control

SMT

initialization

performancemonitoring

maintenance

allocation ofbandwidth

configuration

MACring accessdata transfer

PHYcoding / decoding

transmit / receive datastream

PMDConversion to optical signal

SMT: Station Management

MAC: Medium Access Control

PHY: Physical Layer Protocol

PMD: Physical Layer Medium Dependent

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Dual Redundant Counter Rotating Ring

• Second ring adds a certain level of fault tolerance


SMT

LLC LLC

MAC MAC

PHY PHY

BYPASS

To Next StationFrom Last Station

Station Types - Class A Station

• Two PHY (and one or two MAC) entities• Connects to another Class A station or to a concentrator

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Station Types - Class B Station

• Class B station has one PHY (and one MAC) entity

• Connects to a concentrator

SMT

LLC

MAC

PHY

To Concentrator


Station Types - Concentrator

• Connects Class A and Class B stations into one of thecounter rotating rings.

• Concentrator can bypass failing stations.

LLC

MAC

PHY

PHY

BYPASS

To Next StationFrom Last Station

PHYPHY

SMTPHY

To Class B Stations

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Topology Example

P PP

P P

M

P

M

P

M

P

M

P

P P

P

P P

M M

Node 3

Node 4Node 5Node 6

P=PHYM=MAC


FDDI Media Access Control

• FDDI uses a Token Ring Protocol, similar to 802.5• Differences of FDDI and 802.5:

– To release a token, a station does not need to wait until the tokencomes back after a transmission. The token is released right after theend of transmission

– In FDDI, multiple frames can be attached to the token

– FDDI has a different priority scheme

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FDDI Token Ring Protocol

1. A awaits token

2. A seizes token, beginstransmitting frame F1addressed to C

A

B

A

B

T

A

F1

A

B

B



3. A appends token to end oftransmission

4. C copies frames F1 as itgoes by

A

B

A

B

C

D

F1C

D

F1

T

T

A

B

A

B

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5. C continues to copy F1; Bseizes token and transmitsframe F2 addressed to D

6. B emits token; D copies F2;A absorbs F1

A

B

A

B

C

D

F1

C

D

F1

T

A

B

A

B

F2

F2



7. A lets F2 and token pass; Babsorbs F2

8. B lets token pass

A

B

A

B

C

D

C

D

T

B

A

B

F2

T

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Preamble SD FC DA SA Info FCS ED FS

Bits 64 8 8 16 or 48 >=0 32 4 12

General Frame Format

Preamble SD FC

Bits 64 8 8

Token Frame Format

Frame and Token Format

• SD Starting Delimiter• FC Frame Control (type of frame)• DA Destination Address• SA Source Address• FCS Frame Check Sequence (CRC)• ED End Delimiter• FS Frame Status• Total Frame length ≤ 4500bytes


Timed Token Protocol

• FDDI has a timed token protocol which determines how longa station can transmit

• Each station has timers to measure the time elapsed since atoken was last received

• TTRT Target Token Rotation Time– Value of TTRT is negotiated during initialization (default is 8 ms)– Set to the maximum desired rotation time

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Parameters of Timed Token Protocol

• Station Parameters:• TRT Token Rotation Time

• Time of the last rotation of the token.• If TRT < TTRT, then token is “early”, asynchronous

traffic can be transmitted• If TRT > TTRT then token is “late”, asynchronous traffic

cannot be transmitted.• THT Token Holding Time

• Controls the time that a station may transmitasynchronous traffic.

• fi Percentage of the TTRT that is allocated forsynchronous traffic at station i.



• If a station receives the token it setsTHT:= TRTTRT:= TTRTEnable TRT (i.e., start the timer)

• If the station has synchronous frames are waiting the transmitsynchronous traffic for up to time TTRT • fi (with Σi <1)

• If the station has asynchronous trafficenable THTwhile THT > 0 transmit asynchronous traffic.

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• Note:

– Transmission is not interrupted if THT expiresduring a transmission.

– If a station does not utilize its maximumtransmission time (i.e., THT), the next station canuse it.


Analysis of FDDI

• Annalysis of– Synchronous traffic– Asynchronous trafic

• Synchronous Traffic:

– Recall that each station can transmit synchronous trafficfor up to time TTRT • fi (with Σi fi ≤ 1)

– If Σi fi = 1, the maximum throughput of synchronous traffic is100%.

– One can show that the maximum delay until a frame iscompletely transmitted is:

Maximum Access Delay ≤ 2 • TTRT

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Analysis of FDDI

Station1 Station1 Station3

D

T-D

3T-D

• Asynchronous Traffic

• Parameters:

D Ring latency

n Number of activesessions (all

heavily loaded)T Value of TTRT

• Assumption:– No synchronous

traffic


• From the Example we see:• Cycle in a system has a length of: nT + D• Time in a cycle used for transmission: n(T - D)

• We obtain for the maximum throughput for asynchronoustraffic is:

• ... and the maximum access delay for asynchronous traffic:

DnTD)-n(T

Throughput Max.+

=

2D1)-T(n Delay Access .Max +=

Analysis of FDDI

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Analysis of FDDI

• Numerical Example– Number of stations: 16– Length of fiber: 200 km– Speed of Signal 5 ms/km– Delay per station: 1 ms/station– TTRT: 5 msec

• Ring Latency D = (20 km) × (5 msec/km) + + (16 stations) × 1 msec/station

= 0.12 ms.


75.24ms0.1221)-5(16

Delay Access .%5.97

12.05160.12)-16(5

Throughput Max.

=×+=

==

+×=

=

Max

Analysis of FDDI

Synchronous Traffic • Asynchronous Traffic

10ms5ms 2

Delay Access .%100

Throughput Max.

=⋅=

==

=

Max

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Numerical Results

• We show plots for 3 different FDDI networks.

– “Typical” FDDI:– 20 stations (single MAC)– 4 km ring

– “Big” FDDI:– 100 stations (single MAC)– 200 km ring

– “Largest” FDDI:– 500 stations (dual MAC)– 200 km ring


Throughput vs. TTRT

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Maximum Access Delay vs. TTRT


Supplemental Material (Token Bus )

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IEEE 802.4 (Token Bus)

• Problems with 802.3:• Collisions of frames can lead to unpredictable delays• In some real-time scenarios, collisions and

unpredictable delays can be catastrophic

• Solution via Token Bus:• A control packet (Token) regulates access to the bus• A station must have the token in order to transmit• A station can hold the token only for a limited time• The token is passed among the stations in a cyclic order

This structures the bus as a logical ring


Token Bus

A

B

C

D

Succ=DPred=C

Succ=APred=B

Succ=CPred=D

Succ=BPred=A


• Stations form a logical ring• Each station knows its successor and predecessor in the ring

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Feature of Token Bus

• Bandwidth is 1, 5, or 10 Mbps

• The token bus MAC protocol is very complex

• Typically, token bus is free of collisions

• Defines priority transmissions and can offer boundedtransmission delays



• 802.4 requires each station to implement the followingmanagement functions:

– Ring Initialization– Addition to ring– Deletion from ring– Fault management

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Adding a Station to the Token Bus

• Each node periodically sends a solicit successor packet whichinvites nodes with an address between itself and the nextnode to join the ring

• Sending node waits for response for one round trip• One of the following three cases apply

(1) No Response:– Pass token

(2) Response from one node:– Reset successor node– Pass token to new successor node

(3) Response from more than one node:– Collision has occurred– Node tries to resolve contention


Add a station to the Token Bus

• Assume: Response from more than one node has resulted ina collision.– Station sends a resolve contention packet and waits for four windows

(window = 1 round trip time) for a response:– In window 1, stations with address prefix “00” can reply– In window 2, stations with address prefix “01” can reply– In window 3, stations with address prefix “10” can reply– In window 4, stations with address prefix “11” can reply

– If there is a another collision, procedure is repeated for the second pairof bits. Only the nodes which replied earlier can join the next round

– First successful reply joins the ring

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• Four priority levels:• Levels 6, 4, 2, 0• Priority 6 is the highest level

• Token Holding Time (THT):• Maximum time a node can hold a token

• Token Rotation Time for class i (TRTi):• Maximum time of a full token circulation at which priority

i transmissions are still permitted


Token Bus Transmission Rules

• Each station can transmit class 6 data for a time THT• For i= 4, 2, 0:

– Transmit class i traffic if all traffic from class i+2 or higher istransmitted

– and the time of the last token circulation (including thetransmission time of higher priority packets during thecurrent holding of the token) is less than TRTi.

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Token Bus Priority Scheme

Class 6 ToSend ?

THT Timerexpired ? Send Frame

Y N

UseToken

Class 4 ToSend ?

TRT4 Timerexpired ? Send Frame

Y N

Class 2 ToSend ?


Y N

Class 0 ToSend ?


Y N

N

N

N

Y

Y

Y

More DataTo Send ?

Want ToStay inRing?

Send Set-SuccessorFrame to

PredecessorN N

NY

UseToken

Y Y

Ring LANs - cs.virginia.edujorg/teaching/cs457/slides/lanp2.pdf · Token Ring LANs • Note: – If...

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