802.5 and FDDI

TC625

802.5

• Token Ring

• Reference Stallings 6.3

• 4/16Mbps over STP

• 4Mbps over CAT3 UTP

History

• Developed by IBM

• Adopted by IEEE 802.5 virtually unchanged from IBM implementation

• Spec originally called for 4Mbps

• Upgraded to 16Mbps in 1988

• Going to 100Mbps in 1998??

Equipment

• STP cable was initial standard, UTP now supported too• MAU (Multistation Access Unit) in 802.5 is the hub that

implements the ring– It’s a passive physical device that just implements the PHY

• CAU (Controlled Access Unit) Intelligent MAU that interconnects LAMs

• LAM (Lobe Attachment Module) interconnects 20 lobes• Each connection to the ring is called a lobe• Topology is physically a star, but is logically a ring

The Rules

• You need to keep the ring withing certain physical limits– This is info for passive MAU, and type 1 cable– Equipment vendors provide platform specific info

• Adjusted Ring Lengh (ARL)– Sum of all trunk cables connecting MAUs PLUS– 5 Meters for each MAU PLUS– Length of the longest lobe cable

• Type 1 Cable has following limits:– 4Mbps 390m ARL– 16Mbps 175m ARL

MAC Frame

SD(1) AC(1) FC(1)

Destination Address (6)

Source Address (6)

Data (0 to 4K on 4Mbps or 18K on 16Mbps)

Frame Check Sequence (4)

ED(1) FS(1)

Frame Fields

• SD - Starting Delimiter

• AC - Access Control– Priority, Token, and Monitor– Discussed below

• FC - Frame Control– Data or management frame

Frame Fields Continued

• ED - End Delimiter

• FS - Frame Status– Contains A & C Bits– A - Address recognized– C - Frame copied

• AC = 00 Station Non-existant or inactive

• AC = 10 Station there, frame not copied

• AC = 11 Frame copied

Ring Operation

• MAC level ring management provides following services– Maintaining the ring– Locate and isolate ring faults– Identifying hard and soft errors– Providing node status

• We’ll develop each of these points in detail

Ring Maintenance

• Need a way to– Initialize the ring (start token going)– Handle lost, corrupt, or permanently circulating

frames and tokens

• Active Monitor provides these services

• One node elected Active Monitor– Monitor Contention process– Other nodes on ring act as standby monitors

Monitor Contention Process

• Process to elect new Active Monitor:• Each node starts transmitting Claim Token

frames– Transmit at fixed interval (10-20 milliseconds)– Keep transmitting these until:

• You receive a claim with a higher address (you lose) start passing on the higher claim token

• You receive your own claim token (you win)

• Strip lower address claims

Purge after the Election

• After Active Monitor is elected, AM purges ring• AM uses ring purge process to reset the ring in

general• AM sends Ring Purge frames every 10-20ms• AM stops sending the purge frames when it starts

to receive them• Ring starts running when AM creates a new token

and releases it

Corrupt Frames and Tokens

• Permanently circulating frames– AM sets Monitor bit to 1 on all frames that pass

by– If AM sees a frame where M bit is already 1,

AM resets ring

• Lost or corrupt frames– AM keeps a valid frame timer– If no valid frame shows up within the time

limit, AM resets ring

Active Monitor Present

• AM sends an Active Monitor Present frame every seven seconds

• Also called “Ring Polling”• Nodes use this process to keep tabs on AM

activity, and to get to know their upstream neighbor– AMP frame circulates around ring - nodes take note– Nodes then transmit Standby Monitor Present to

downstream neighbor– Stops when AM gets an SMP frame– NAUN - Nearest Active Upstream Neighbor

Joining the Ring

• Before joining stations do the following:– Interface self-test– Interface loopback test– Cable test - MAU loopback

• Duplicate Address Test

• Existing node detects new NAUN at next ring poll

Locating and Isolating Faults

• Bum electrical info coming from your NAUN– Could be cable break or fault

– Node transmits Beacon Frame reporting problem with NAUN

– Everyone drops what they’re doing and starts repeating the beacon frame

– When NAUN gets frame indicating that it is the source of problem, it removes itself from the ring… assuming it gets the beacon

Identifying Hard and Soft Errors

• REM (Ring Error Monitor) is a process that runs somewhere on the ring

• Detected errors are reported to the REM with a Report Soft Error Frame– Problems in information coming from NAUN

– Lost Frame Errors

– Congestion Errors (full buffers)

– Duplicate Address Errors

– Token Errors

Normal Ring Operation

• Got traffic?– Wait until you capture the token

• T=0 in AC field

– Send your data in a frame with T=1 • T=1 indicates frame is data frame not token

– When your data frame returns, strip frame release token

Early Token Release

• Introduced in 16Mbps rings

• Release token right after sending data

• Improves latency

• Backward compatible with non-ETR stations

FDDI

• Fiber Distributed Data Interface

• 100 Mbps token passing ring

• ANSI X3I9.5 not IEEE

• Reference Stallings Chapter 8

Differences from 802.5

• FDDI• Fiber, STP, UTP• 100 Mbps• MTU 4500 Bytes• Fault Tolerant Dual

Ring• Distributed Clock• Early Release

• 802.5• STP, UTP• 4/16 Mbps• MTU 4500-18K Bytes• No fault tolerance

built into spec• Centralized Clock• Release after receive

Clock Recovery

• FDDI uses distributed clock

• Each station recovers clock from data stream

• NRZI encoding– This fails with long strings of 0’s– Make the strings of 0’s go away– 4B/5B Encoding

4B/5B Encoding

• At the PHY level, FDDI encodes traffic as 5-bit symbols

• Symbols are divided into Data, Line State, and Control– Data: 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F– Line State: Q, I, H– Control: J, K, L, T, R, S

More Encoding

• Number of consecutive 0’s limited to three

• This guarantees that you always have enough transitions to make NRZI work– 4B/5B NRZI gives you 80% efficiency– Line runs at 125 Mbps to give you the effective

100Mbps– 802.5 uses Manchester which requires 2x base

rate or 50% efficiency

Fiber Distributed Data Interface• FDDI is a token ring network designed for reliable LAN or WAN use• 100 Mbps data transfer rate using fiber optic cable or UTP

Media type Segment length Signalingfiber optic 2 km 4B5B+NRZI UTP 100 m MLT-3

• Up to 500 stations; up to 100 km!

Token or Data frame circulates ring. Each station buffers 9-80 bits.

FDDI has 2 rings oriented in opposite directions. In normal operation only one ring is used.

Dual Ring OperationUnder normal conditions only one ring is used.

If cable or a node is damaged, the second cable is used to re-establish the loop.

Link down

Station Attachment

Dual Attachment Stations (DAS): nodes attached to both rings, as in the previous slide. A DAS interface includes circuitry to use either ring for input and output.

Single Attachment Stations (SAS): a node may be attached to only one ring, via a concentrator, to reduce interface cost. The concentrator contains an optical bypass to remove SAS node if it fails.

Concentrator (DAS)

SAS SAS SAS SAS

Upstreamneighbor

Upstreamneighbor

OperationEarly Token Release: sender generates a new token as soon as he finishes sending his frame. Since FDDI rings can be up to 100 km, this is needed to achieve good utilization.

Destination host copies the frame and sets 2 bits in the trailer, but doesn’t remove frame from the network.

destinationcopies frame

Token

Early Release Improves ThroughputAnother station can seize the toke

n and insert his own frame after the first sender’s frame.

B

A A

Second station also releases token early, so other stations can transmit (tailgate), too.

FDDI Ring Size and LatencyAn FDDI ring can be up to 100 km long. The ring latency is a significant factor on large rings. Each station buffers 9 - 80 bits of a frame at it passes by, but may retransmit bits before its buffer is full.

Suppose each node buffers 5 bits on average.

Delay in node = 5/100Mbps = 50 ns

Propagation delay in 1 km cable is:

1km/0.67c = 5 microseconds/km

For a network with 40 nodes and 10 km of cable the ring latency is:

40 x 50 ns + 10 x 5 microsec = 52 microseconds

FDDI Ring ProblemsWhat happens if a sender crashes while his own frame is still traversing the network?

Token

What happens is the token is lost or damaged?

FDDI Frame Formats

Data

0 - 4480

11Start

delimiter

1 byteToken Frame

Data Frame

Framecontrol

Enddelimiter

Dest.address

1Start

delimiter

1 byteFramecontrol

Sourceaddress

Enddelimiter

Framestatus

FrameCRC

6 6 4 11

Same address format as ethernet

No min. length;no bit stuffing

“error detected” bit

TYPEC L FT

bits: 1 1 2 4

C = class: synchronous or asynchronousL = address length (16 or 48 bit)FT = frame typeTYPE = for a control frame, the control type

Two bits for address recognized and frame copied (receiver sets)

Sync

8 byte

Sync

“Synchronous” Frames• Synchronous frames are designed to s

upport delay sensitive traffic, particularly voice

• Synchronous frames are small:up to 96 bytes of circuit switched data,16 bytes of non-circuit switched data

• Multiple stations may acquire a time slot (or slots) in the same synchronous frame. Once acquired, a station receives the same time slot in every synchronous frame of this series until it explicitly releases it -- guaranteed bandwidth

• Synchronous frame generation doesn’t wait for token

A designated station generates synchronous frames @ 8000/sec (125 sec), as needed for PCM transmission of voice.

Asynchronous Frames

• Asynchronous frames are normal data frames, which can contain up to 4,500 bytes of data.

• A station must wait for token before sending an asynchronous frame.

• The amount of data a station can send it limited by a maximum Token Holding Time (THT), which is determined by how “late” or “early” the token arrives.

• When token arrives, the measured Token Rotation Time (TRT) is compared with an agreed upon Target Token Rotation Time (TTRT) to determine if station can send any asynchronous frames.

Token Rotation TimeAn FDDI ring may contain 500 stations and 100km of fiber optic cable. Suppose that each station grabs the token and transmits a 1000 byte frame.The time for the token to come back to node #1 is:

TokenRotationTime =

500 x TransmissionTime + RingLatency

= 500 x (1000x8 / 100 Mb/s) + (100,000m / 0.65c)

= 40 msec

Too slow!!

#1

1,000 bytes

#2

#500

Target Token Rotation Time

To limit the Token Rotation Time (TRT), the stations agree on a Target Token Rotation Time (TTRT).The TTRT and observed TRT determine how long a station can transmit, called the Token Holding Time (THT).A station also keeps track of the time it uses sending synchronous data, called its synchronous allocation (SA).When a token arrives, the station checks:

TRT + SA > TTRT ?If TRT+SA > TTRT then the station doesn’t send any asynchronous frames. If TRT+SA < TTRT, then the station may transmit data for a time up to

THT = TTRT - TRT - SA

SMT

• Station management• Provides for:

– Connection Initialization– Find neighbors, test link

– Topology Control– Enforce FDDI topology rules

– Ring Initialization– Create token and get it spinning

– Parameter Setting– Timer