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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