Download CS3502-LANs

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

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, topologies
 broadcast
(contention); bus or wireless
 Aloha,
CSMA, CSMA/CD (802.3) *,
 wireless LANs (802.11)
 broadcast
 bit

(controlled)
map protocol, token bus
ring
 802.5
token ring *, FDDI (token), slotted rings
 star
 ATM
LAN
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
LANs : ALOHA (pure)
 radio
frequencies OR any broadcast medium
U
of Hawaii, early 1970s. Prof.. N. Abramson,
funded by ARPA.
 simplest
a
possible protocol
station with a message simply transmits it to
completion. If no collision, message gets through,
otherwise wait random time and retransmit.
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
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!
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)
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)
 how
can ALOHA be improved?
 need
to reduce collisions
 slotted
slots
ALOHA : restrict transmissions to time
 divide
time into “slots”
 station waits until next time slot to transmit
 slots must be synchronized, somehow
 how
much will throughput improve?
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?
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 summary

simple communications (simple is good)
 relatively
 good
 not
cheap, simple to implement
for sparse, intermittent communication.
a good LAN protocol because of
 poor
utilization
 potentially
 stations
utilize it
infinite delay
have listening capability, but don’t fully
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).
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
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.
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).
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.
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?