CS3502:CS3502:

Data and Computer NetworksData and Computer Networks

Local Area Networks - 1Local Area Networks - 1

introduction and early broadcast introduction and early broadcast protocols protocols

CS3502 , LANs. Objectives CS3502 , LANs. Objectives

1. describe LAN topologies/transmission media2. describe MAC protocols -> in detail3. compare/contrast different LANs4. verify basic LAN protocols5. describe and compare LAN throughputs 6. describe and analyze bridges/LAN switches7. describe basic router function, differentiate from bridge.

local area networks : general local area networks : general infoinfo

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

LANs : classes, topologiesLANs : 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

local area networks : broadcastlocal 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

LANs : ALOHA (pure) 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.

LANs : ALOHA (pure)LANs : ALOHA (pure) works for when transmissions are rare; but

quickly degenerates as load increases performance analysis, based on assumed

Poisson distribution, shows max utilization of 18%. (following slides) .

User Load- the amount of traffic attempting to get through the channel

Throughput - amount of traffic getting through the channel

Utilization - is the fraction of time that the channel is transmitting data (throughput/max throughput)

LANs : Aloha performance LANs : Aloha performance analysis analysis

based on several assumptions: 1. Transmission attempts are generated by an infinite number of users.2. 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 LANs : Aloha performance analysis 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)

LANs : Aloha performance LANs : Aloha performance analysisanalysis

Let S = number of successful packet transmissions per packet time (equals channel utilization)

G = average number of attempted transmissions per packet time(user load+retransmissions).Then,S = P[successful transmission]= P[1 attempt] P[no other attempts]= P[X=1] P[X=0]= G x e -2G, pure Aloha

Q : what is the maximum utilization? (take the derivative, set to 0, plug back in)

LANs : performance analysis 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., utilization 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) 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 utilization improve?

LANs: ALOHALANs: 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?

LANs : ALOHA, 2 power levels 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.

LANs : ALOHA summaryLANs : 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

LANs: CSMALANs: 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).

LANs: CSMALANs: 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

LANs: CSMALANs: CSMA what are 2 ways that collisions can occur in

CSMA? What is their likelihood?

Will CSMA improve throughput over Aloha?

nonpersistent CSMA: when carrier is busy, wait a random time.

Will this improve utilization?

LANs : CSMA, p-persistent LANs : CSMA, p-persistent variation of CSMA; generalizationfor 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 : CSMALANs : CSMA will this improve utilization?

for low values of p, maximum utilization is highest; what about user friendliness?

Since even if the channel is free one defers transmission

with p=> 0 the average wait time =>

LANs : Aloha/CSMA , summaryLANs : Aloha/CSMA , summary

Compare approximate utilization of the protocols pure Aloha slotted Aloha 1-persistent CSMA p-persistent CSMA nonpersistent CSMA

Compare the throughput as a function of user Load