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

high speed networks 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

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 SAS DAS CON LAN

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

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”

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  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.

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”

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

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

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)

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 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 LC THTna nana na na 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 LC THTna nana na na 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 LC THTna nana na na 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 LC THTna nana na na 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  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 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 - 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 /2x10^^8. Efficiency = 2ms / 2.16 = 92% * What if all stations wants to send? What if some stations send?

Fast Ethernet - general Speed100 Mbps TopologyStar MediaTwisted pair Cat5 AccessCSMA/CD Collision domain Hub connected Compatibility10Mbps Ethernet Spec Designation 100Base-Tx

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

GBit Ethernet - general Speed1000 Mbps TopologyStar Mediafiber, four Cat 5 AccessCSMA/CD Collision domain Hub connected Compatibility10,100Mbps Ethernet Spec Designation 1000Base-T, 1000Base-TX

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.