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CS3502 , LANs. Objectives

CS3502: Data and Computer Networks Local Area Networks - 1 introduction and early broadcast protocols. CS3502 , LANs. Objectives. 1. describe LAN topologies/transmission media 2. describe MAC protocols -> in detail 3. compare/contrast different LANs 4. verify basic LAN protocols

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CS3502 , LANs. Objectives

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  1. CS3502:Data and Computer NetworksLocal Area Networks - 1introduction and early broadcast protocols

  2. CS3502 , LANs. Objectives 1. describe LAN topologies/transmission media 2. describe MAC protocols -> in detail 3. compare/contrast different LANs 4. verify basic LAN protocols 5. describe and compare LAN throughputs 6. describe and analyze bridges/LAN switches 7. describe basic router function, differentiate from bridge.

  3. local area networks : general info • limited geographical area • relatively high transmission rates • simple topologies and routing • mostly baseband -- single channel • usually owned by 1 organization • characterized by topology, medium, and MAC protocol

  4. LANs : classes, topologies • broadcast (contention); bus or wireless • Aloha, CSMA, CSMA/CD (802.3) *, • wireless LANs (802.11) • broadcast (controlled) • bit map protocol, token bus • ring • 802.5 token ring *, FDDI (token), slotted rings • star • ATM LAN

  5. local area networks : broadcast • all nodes connected by ONE channel • if more than 1 node transmits simultaneously, signals interfere (collision): the message is lost • thus, the transmission medium is always in 1 out of 3 possible states: (1) (2) (3) • example: classroom? .... 2 channels

  6. LANs : ALOHA (pure) • radio frequencies OR any broadcast medium • U of Hawaii, early 1970s. Prof.. N. Abramson, funded by ARPA. • simplest possible protocol • a station with a message simply transmits it to completion. If no collision, message gets through, otherwise wait random time and retransmit.

  7. LANs : ALOHA (pure) • works for when transmissions are rare; but quickly degenerates as load increases • performance analysis, based on assumed Poisson distribution, shows max throughput of 18%. (following slides) • throughput (aka utilization) is the fraction of time that the network is transmitting data. • load, or offered load, is the amount of traffic attempting to get through the network

  8. LANs : Aloha performance analysis • based on several assumptions: 1. offered load is infinite number of users. 2. total offered load (transmission attempts) follows a Poisson distribution. 3. fixed packet size Def: Let X be a random variable, representing a nonnegative integer. X is a poisson random variable if p(i) = P[X=i] = (e - i )/i!

  9. LANs : Aloha performance analysis • note: Poisson distribution (discrete RVs) and exponential distribution (continuous RVs) are closely related. • the mean, or “average” of the poisson dist. is  E [X] lso note -- P[X=0] = p(0) = e - , and P[X=1] = p(1) = e - ( come from plugging 0, 1 into the formula)

  10. LANs : Aloha performance analysis Let S represent the throughput (utilization), and G the offered load. Then, S = G x P[successful transmission] = G x P[no other transmissions] = G x P[X=0] = G x e -2G, pure Aloha Q : what is the maximum throughput? (take the derivative, set to 0, plug back in)

  11. LANs : performance analysis derivative : ( G x e -2G)’ = (1)(e -2G ) + G(e -2G )(-2) setting to 0, e -2G - 2G e -2G = 0 => 1 - 2G = 0 => G = 0.5 I.e., throughput is max at G = 0.5. Plugging this into the original formula, S = G x e -2G yields a max value of 0.18.

  12. LANs : ALOHA (slotted) • how can ALOHA be improved? • need to reduce collisions • slotted ALOHA : restrict transmissions to time slots • divide time into “slots” • station waits until next time slot to transmit • slots must be synchronized, somehow • how much will throughput improve?

  13. LANs: ALOHA • when should station retransmit after a collision? • show why throughput should double with slotted Aloha over pure Aloha • what is the worst-case time a station will have to wait until getting a successful transmission? • how can Aloha be improved? • hint: what if we could use 2 power levels?

  14. LANs : ALOHA, 2 power levels • idea: when station transmits, flip a coin. Heads, use low power level. Tails, used high power level. • high power clobbers lower power; if same power, collision as before. • can be added to either pure or slotted. Improves max throughput to 26% (pure) or 52% (slotted) under same Poisson assumptions.

  15. LANs : ALOHA summary • simple communications (simple is good) • relatively cheap, simple to implement • good for sparse, intermittent communication. • not a good LAN protocol because of • poor utilization • potentially infinite delay • stations have listening capability, but don’t fully utilize it

  16. LANs: CSMA • corrects the obvious flaw in Aloha (blindly transmitting without first checking the medium) • CSMA(carrier sense multiple access) protocol: (1)sense the carrier; {LISTEN} if no signal detected then transmit message to end; {TALK} if collision occurred, then wait random time, go to (1) else END. else {carrier is busy} go to (1).

  17. LANs: CSMA • basic CSMA is “persistent,” or “1-persistent” -- it transmits as soon as it detects the open carrier. • suppose another station is transmitting; when will the station start to transmit? • what effect does propagation delay have on this protocol? • note that whenever transmission occurs, the whole message is sent: no way to abort

  18. LANs: CSMA • what are 2 ways that collisions can occur in CSMA? What is their likelihood? • Will CSMA improve throughput over Aloha? • specify CSMA formally as a Comm.Finite State Machine, then analyze it.

  19. LANs : CSMA, p-persistent • variation of CSMA; generalization for parameter p : real, in (0,1], --- (1)sense the carrier; if no signal detected then transmit message to end with probability p ; else {probability 1- p} wait random time, goto (1); if collision occurred, then wait random time, go to (1) else END; else {carrier busy} go to (1).

  20. LANs : CSMA • exercise: alter the CSMA specification (CFSM) to handle p-persistence • throughput: will this improve throughput? • for low values of p, maximum throughput is highest; what about user friendliness? • nonpersistent CSMA: when carrier is busy, wait a random time. Rewrite the specification for this change.

  21. LANs : CSMA • when collisions occur, how much time is wasted? • what is approximate likelihood of repeating the collision, with • CSMA, 1-persistent • CSMA, 0.1 persistent • CSMA, nonpersistent • How can time wasted be reduced?

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