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CS4550: Computer Networks II high speed networks, part 1 : FDDI & 100baseTX

CS4550: Computer Networks II high speed networks, part 1 : FDDI & 100baseTX . high 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)

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CS4550: Computer Networks II high speed networks, part 1 : FDDI & 100baseTX

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  1. CS4550:Computer Networks IIhigh speed networks, part 1 : FDDI & 100baseTX

  2. high 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

  3. 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

  4. FDDI DAS SAS CON LAN

  5. FDDI... 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

  6. FDDI... • 2 major traffic types synchronous : 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”

  7. 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)

  8. FDDI.. token rotation time • example suppose 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.

  9. FDDI ... 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”

  10. 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)

  11. FDDI 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.

  12. FDDI protocol TRT=2 Lt_Ct =1 Tk TTRT=10ms here token is late, so only synch. data can be transmitted; no asynchronous

  13. FDDI protocol TRT=4 Lt_Ct =0 Tk TTRT=10ms here token is early, so both types of data may be Xmitted (how much asynch may be xmitted?)

  14. 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 expires 5. Xmit token to next station (continued next slide)

  15. 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

  16. Round 1: ring operates with backlog on all stations. Station 1 gets 7 msec of asych data Station 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 L TRT 07 100 70 63 62 100 100 70 - 69 100 100 70 - 69 LC 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 THT 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 data Station 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 - 69 LC 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 THT 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 data Station 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 62 LC 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 THT 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 data Station 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 69 LC 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 THT 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.

  17. 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

  18. FDDI - 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.

  19. FDDI - 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 ms Send 2 ms * 100Mbps = 200kb Wait 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?

  20. Fast 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

  21. Fast Ethernet - collision domain hub hub 100 m max 64 byte minimum message 2* d/c = 2*td < (64 bytes*8b/byte)/100Mbps d < 512m

  22. GBit 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

  23. GBit 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.

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