CS4550:CS4550:

Computer Networks IIComputer Networks II

high speed networks, high speed networks, part 1 : FDDI & 100baseTX part 1 : FDDI & 100baseTX

high speed networkshigh speed networks FDDI : fiber distributed data interface (1)

100 Mbps, fiber, dual-ring

Fast Ethernet

100 Mbps, twisted pair

Frame Relay (2)

ATM : asynchronous transfer mode (2)

fast packet switching; fiber; high speeds

FDDI : fiber dist. data interface FDDI : fiber dist. data interface

ANSI standard ASC X3T9.5; for MAC, physical layer and station mgt.

dual rings, data flows in opposite directions; 2nd ring provides redundancy

each ring has max diameter of 100 Km; so can be used as either MAN or a super-LAN

data rates : 100 Mbps max frame size 4.5 K media : fiber; t.p. possible for short links

FDDI FDDI

SASDAS

CON

LAN

FDDI... station typesFDDI... station types

DAS - dual attached station attached to both rings

SAS - single attached station attached only to main ring in case of failure, will be taken out

CON - concentrator connects multiple slower machines to

the ring

FDDI...FDDI...

2 major traffic typessynchronous : for real-time, time critical traffic

asynchronous : non time critical traffic timers

TRT - token rotation timer THT - token holding timer

key variable : Late_Ct keeps track of token, “early” or “late”

FDDI...FDDI...

TTRT: target token rotation time; upper bound on average token rotation time

2 main timing specifications

1. max. token rotation time: < 2 * TTRT

(max time for any single lap)

2. average token rotation: < TTRT (average time per lap)

FDDI.. token rotation time FDDI.. token rotation time

examplesuppose TTRT= 10 ms; suppose that in the

1st 10 rounds since startup, 60 ms has passed. (average of 6 ms each)

then the next rotation could take 50 ms and keep the average (2); but because of (1) can take no more than 20 ms.

FDDI ... synchronous allotmentsFDDI ... synchronous allotments

each station allowed a synchronous allotment, SA - a minimum time it is allowed to transmit synch. traffic

together with the timing specification, guarantees a minimum bandwidth

sum of SAs for all stations must be less than the TTRT

stations may only transmit asynchronous data if the token is “early”

FDDI FDDI

token rotation time negotiated at initialization; set according to strictest station

sum of SAs must be slightly less than the TTRT, to allow for overhead (small but measurable), and completion of last frame transmission

(when TRT expires during a frame)

FDDI protocolFDDI protocol

TRT set when ring starts up; always running; reset when token arrives (early) or when it expires (TRT <-- TTRT)

Late_Ct initialized to 0; incremented each time the TRT expires; reset when token arrives.

Thus : when token arrives, if Late_Ct =0, token is early; otherwise token is late.

FDDI protocolFDDI protocol

TkTRT=2Lt_Ct =1

here token is late, so only synch. data can be transmitted; no asynchronous

TTRT=10ms

FDDI protocol FDDI protocol

TkTRT=4Lt_Ct =0

TTRT=10ms

here token is early, so both types of data may be Xmitted (how much asynch may be xmitted?)

FDDI protocol FDDI protocol

upon arrival of the token, if token is early, then 1. THT <-- TRT;

2. TRT <-- TTRT & keeps running; 3. Xmit synch data, for time SA (or until done); 4. start THT, and Xmit asynch data until done or THT expires5. Xmit token to next station

(continued next slide)

FDDI protocol FDDI protocol

else (the token is late), 1. Late_Ct <-- 0; {TRT not reset; keeps running}2. Xmit synch traffic for SA time (or until done);3. Xmit token to next station

Round 1: ring operates with backlog on all stations. Station 1 gets 7 msec of asych dataStation 1 (t=0) Station 2 (t=38) Station 3(t=69)

A R SY AS L A R SY AS L A R SY AS LTRT 07 100 70 63 62 100 100 70 - 69 100 100 70 - 69LC 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0THT na 7 7 00 na na 0 0 0 na na 0 0 0 na

Round 2: ring operates with backlog on all stations. Station 2 gets 7 msec of asych dataStation 1 (t=100) Station 2 (t=131) Station 3(t=169)A R SY AS L A R SY AS L A R SY AS L

TRT 100 100 70 0 69 07 100 70 63 62 100 100 70 - 69LC 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0THT na 0 0 0 na na 7 7 00 na na 0 0 0 na

Round 3: ring operates with backlog on all stations. Station 3 gets 7 msec of asych dataStation 1 (t=200) Station 2 (t=231) Station 3(t=262)A R SY AS L A R SY AS L A R SY AS L

TRT 100 100 70 70 69 100 100 70 70 69 07 100 70 63 62LC 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0THT na 0 0 0 na na 0 0 00 na na 7 7 0 na

Round 4: ring operates with backlog on all stations. Station 3 gets 7 msec of asych dataStation 1 (t=300) Station 2 (t=331) Station 3(t=362)A R SY AS L A R SY AS L A R SY AS L

TRT 100 100 70 70 69 100 100 70 70 69 100 100 70 70 69LC 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0THT na 0 0 0 na na 0 0 00 na na 0 0 0 na

Repeat round 1 again now (t=393) with 7Msec to spare. Note the 7 msec of asynch allocation is round robin distributed around the ring.

Notes on FDDI Notes on FDDI

if Late-Ct exceeds 1 in any station ring is crashed

data Xmitted in 5-bit units - “symbol” 4B/5B/NRZI

symbols passed between MAC and PHY for transmission

symbol : 16 data values, special values, (frame delimiters, etc.), some unused.

delay: delay of 60 bits per station

FDDI - to think aboutFDDI - to think about

explain why token orbit can never exceed 2 TTRTs

explain why average must be less than the TTRT

what kind of throughput should FDDI get? can you think of a way to increase

throughput? can FDDI be used as a voice network?

explain how or why not.

FDDI - EFFICENCYFDDI - EFFICENCY Efficiency in general =

useful activity time/total time Network Efficiency =

Utilization = Throughput/data rate Example in 100 station 20km FDDI ring?

1 station wants to sends continuously?SA = 2 msSend 2 ms * 100Mbps = 200kbWait for token to rotate T =100stations*60bits per station/100Mbs +

20000/2x10^^8.Efficiency = 2ms / 2.16 = 92%

* What if all stations wants to send? What if some stations send?

Fast Ethernet - generalFast Ethernet - general

Speed 100 Mbps

Topology Star

Media Twisted pair Cat5

Access CSMA/CD

Collision domain Hub connected

Compatibility 10Mbps Ethernet

Spec Designation 100Base-Tx

Fast Ethernet - collision domainFast Ethernet - collision domain

hub hub

64 byte minimum message

2* d/c = 2*td < (64 bytes*8b/byte)/100Mbps

d < 512m

100 m max

GBit Ethernet - generalGBit Ethernet - general

Speed 1000 Mbps

Topology Star

Media fiber, four Cat 5

Access CSMA/CD

Collision domain Hub connected

Compatibility 10,100Mbps Ethernet

Spec Designation 1000Base-T, 1000Base-TX

GBit Ethernet - EnhancementsGBit Ethernet - Enhancements

Carrier Extension – minimum frame 4096 bit times up from 512 bit times for 10 and 100 Mbs systems.

Frame Bursting – Multiple short frames with a single CSMA/CD access.

Use of switching hubs becoming common.