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 5. describe and compare LAN throughputs 6. describe and analyze bridges/LAN switches 7. describe basic router function, differentiate from bridge.

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

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

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

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

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

LANs : Aloha performance analysis u 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!

LANs : Aloha performance analysis u 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)

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)

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.

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

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

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

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

LANs: CSMA u corrects the obvious flaw in Aloha (blindly transmitting without first checking the medium) u 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).

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

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

LANs : CSMA, p-persistent u 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).

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

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