multiple access.ppt

8/9/2019 multiple access.ppt

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Media

AccessControl

(MAC)


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Outline

11 RANDOM ACCESS RANDOM ACCESS

2 2 CONTROLLED ACCESSCONTROLLED ACCESS

3 3 CHANNELIZATION CHANNELIZATION


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Figure : Taxonomy of multiple-access protocols


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11 RANDOM RANDOM ACCESS ACCESS

In random-access or contentionno station is superior to anotherstation and

none is assigned control overanother.

At each instance, a station thathas data to send uses a

procedure dened by the protocolto mae a decision on !hether or

not to send.


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ALOHA ALOHA

ALOHA, the earliest random access method, wasdeveloped at the University o Hawaii in early !"#$% &t was desi'ned or a radio (wireless) LAN, *+t it

can *e +sed on any shared medi+m%

&t is o*vio+s that there are potential collisions inthis arran'ement%he medi+m is shared *etween the stations% -hen

a station sends data, another station may attempt to

do so at the same time%he data rom the two stations collide and *ecome

'ar*led%


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Figure : Frames in a pure A$%&A net!or


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Figure : 'rocedure for pure A$%&A protocol


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The stations on a wireless ALOHA network are a maximum

of 600 km apart. If we assume that signals propagate at

3 × 10 m!s" we fin# T p $ %600 × 103& ! %3 × 10& $ ' ms. (or

) $ '" the range of * is +0" 1" '" 3,. This means that T- an

/e 0" '" " or 6 ms" /ase# on the outome of the ran#omaria/le *.

(xample )


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Figure : *ulnerable time for pure A$%&A protocol


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A pure ALOHA network transmits '002/it frames on a

share# hannel of '00 k/ps. hat is the re4uirement to

make this frame ollision2free5

Solution

Aerage frame transmission time Tfr is '00 /its!'00 k/ps or1 ms. The ulnera/le time is ' × 1 ms $ ' ms. This means no

station shoul# sen# later than 1 ms /efore this station starts

transmission an# no station shoul# start sen#ing #uring the

perio# %1 ms& that this station is sen#ing.

(xample +


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A pure ALOHA network transmits '002/it frames on a

share# hannel of '00 k/ps. hat is the throughput if the

sstem %all stations together& pro#ues

a. 1000 frames per seon#5

b. 700 frames per seon#5

c. '70 frames per seon#5

(xample

Solution

The frame transmission time is '00!'00 k/ps or 1 ms.

a.

If the sstem reates 1000 frames per seon#" or 1 frame

per milliseon#" then 8 $ 1'. In this ase 9 $ 8 × e:'8 $

0.137 %13.7 perent&. This means that the throughput is

1000 × 0.137 $ 137 frames. Onl 137 frames out of 1000


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b. If the sstem reates 700 frames per seon#" or 1!' frames

per milliseon#" then 8 $ 1!'. In this ase 9 $ 8 × e:'8

$ 0.1 %1. perent&. This means that the throughput is

700 × 0.1 $ ;' an# that onl ;' frames out of 700 will

pro/a/l surie.


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Figure : Frames in a slotted A$%&A net!or


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Figure : *ulnerable time for slotted A$%&A protocol


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A slotte# ALOHA network transmits '002/it frames using a

share# hannel with a '002k/ps /an#wi#th. (in# the

throughput if the sstem %all stations together& pro#ues

a. 1000 frames per seon#.

b.700 frames per seon#. c. '70 frames per seon#.

Solution

This situation is similar to the preious exerise exept thatthe network is using slotte# ALOHA instea# of pure

ALOHA. The frame transmission time is '00!'00 k/ps or 1

ms.

(xample


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a& In this ase 8 is 1'. 9o 9 $ 8 × e:8 $ 0.36 %36.

perent&. This means that the throughput is 1000 ×

0.036 $ 36 frames. Onl 36 out of 1000 frames will

pro/a/l surie.


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. CSMA. CSMA

o minimi/e the chance o collision and,thereore, increase the perormance, the CSMA

method was developed% he chance o collision can *e red+ced i a

station senses the medi+m *eore tryin' to +se it% Carrier sense m+ltiple access (CSMA) re0+ires

that each station irst listen to the medi+m (or chec1

the state o the medi+m) *eore sendin'%

&n other words, CSMA is *ased on the principle

2sense *eore transmit3 or 2listen *eore tal1%3


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Figure : pace/time model of a collision in 01A


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Figure : *ulnerable time in 01A


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Figure : 2ehavior of three persistence methods


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Figure : Flo! diagram for three persistence methods


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4 CSMA5CD4 CSMA5CD

he CSMA method does not speciy the proced+re

ollowin' a collision%

Carrier sense m+ltiple access with collision detection

(CSMA5CD) a+'ments the al'orithm to handle the

collision%

&n this method, a station monitors the medi+m ater

it sends a rame to see i the transmission wass+ccess+l%

& so, the station is inished% &, however, there is a

collision, the rame is sent a'ain%


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Figure : 0ollision of the rst bits in 01A/03


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Figure : 0ollision and abortion in 01A/03

( l 4


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A network using =9>A!=? has a /an#wi#th of 10 >/ps. If

the maximum propagation time %inlu#ing the #elas in the

#eies an# ignoring the time nee#e# to sen# a @amming

signal" as we see later& is '7.6 s" what is the minimum siBe

of the frame5

Solution

The minimum frame transmission time is Tfr $ ' × T p $

71'.' s. This means" in the worst ase" a station nee#s to

transmit for a perio# of 71'.' s to #etet the ollision. Theminimum siBe of the frame is 10 >/ps × 71'.' s $ 71' /its

or 6 /tes. This is atuall the minimum siBe of the frame

for 9tan#ar# Cthernet" as we will see later in the hapter.

(xample 4


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Figure : Flo! diagram for the 01A/03


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Figure : (nergy level during transmission, idleness, orcollision


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Figure : 0ontention !indo!


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Figure : 01A0A and 5A*


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.%CONROLLED.%CONROLLED

ACCESS ACCESS

&n controlled access, the stations cons+lt

one another to ind which station has the ri'ht

to send%

A station cannot send +nless it has *een

a+thori/ed *y other stations%

-e disc+ss three controlled:access methods%

R iR ti


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Reservation Reservation

&n the reservation method, a station needs to

ma1e a reservation *eore sendin' data%

ime is divided into intervals%

&n each interval, a reservation rame precedes the

data rames sent in that interval%


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Figure : 6eservation access method

. ; lli. ; lli


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. ;ollin' . ;ollin'

;ollin' wor1s with topolo'ies in which one device is

desi'nated as a primary station and the other devicesare secondary stations%

All data e


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Figure : elect and poll functions in polling-accessmethod

4 1 ; i4 1 ; i


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4 o1en ;assin' 4 o1en ;assin'

&n the to1en:passin' method, the stations in a

networ1 are or'ani/ed in a lo'ical rin'%

&n other words, or each station, there is a

predecessor and a s+ccessor%

he predecessor is the station which is lo'ically

*eore the station in the rin'= the s+ccessor is the

station which is ater the station in the rin'%


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Figure : $ogical ring and physical topology in toen- passing

access method


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4 CHANNEL&>A&ON 4 CHANNEL&>A&ON

Channeli/ation (or channel partition, as it is

sometimes called) is a m+ltiple:access method

in which the availa*le *andwidth o a lin1 is

shared in time, re0+ency, or thro+'h code,

amon' dierent stations%

&n this section, we disc+ss three protocols8

9DMA, DMA, and CDMA%

! 9DMA! 9DMA


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! 9DMA! 9DMA

&n re0+ency:division m+ltiple access (9DMA),

the availa*le *andwidth is divided into re0+ency*ands%

Each station is allocated a *and to send its data%

&n other words, each *and is reserved or a

speciic station, and it *elon's to the station all the

time%

Each station also +ses a *andpass ilter to conine

the transmitter re0+encies% o prevent


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Figure : Fre7uency-division multiple access "F31A#

. DMA. DMA


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. DMA. DMA

&n time:division m+ltiple access (DMA), the stations

share the *andwidth o the channel in time%

Each station is allocated a time slot d+rin' which it

can send data%

Each station transmits its data in its assi'ned time

slot% 9i'+re shows the idea *ehind DMA%


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Figure : Time-division multiple access "T31A#

4 CDMA4 CDMA


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4 CDMA4 CDMA

Code:division m+ltiple access (CDMA) was

conceived several decades a'o%

Recent advances in electronic technolo'y have

inally made its implementation possi*le%

CDMA diers rom 9DMA in that only one

channel occ+pies the entire *andwidth o the lin1%

&t diers rom DMA in that all stations can send

data sim+ltaneo+sly= there is no timesharin'%


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Figure : imple idea of communication !ith code


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Figure : 0hip se7uences


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Figure : haring channel in 031A


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Figure : 3igital signal created by four stations in031A


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Figure : 3ecoding of the composite signal for one in031A


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Figure : 8eneral rules and examples of creating 9alshtables

(xample


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(in# the hips for a network with

a. Two stations

/. (our stations

(xample

(xample ;


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hat is the num/er of se4uenes if we hae ;0 stations in

our network5

(xample ;

(xample


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Droe that a reeiing station an get the #ata sent / a

speifi sen#er if it multiplies the entire #ata on the hannel

/ the sen#erEs hip o#e an# then #ii#es it / the num/er

of stations.

(xample

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