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Chapter Chapter 11
Multiple Access ProtocolsMultiple Access Protocols
UponUpon completioncompletion ofof thisthis chapterchapterstudentsstudents shouldshould bebe ableable toto identifyidentifyMultipleMultiple AccessAccess ProtocolsProtocols::11-- RandomRandom AccessAccess ProtocolsProtocols
ALOHA, CSMA, CSMA/CD, ALOHA, CSMA, CSMA/CD, CSMA/CACSMA/CA
22-- ControlledControlled Access ProtocolsAccess ProtocolsReservation, Polling, Token Passing
33-- ChannellizationChannellization AccessAccess ::FDMA, TDMA, CDMA
CLO1:CLO1:•• Identify different LAN Identify different LAN components, media access control components, media access control protocols, internetworking devices protocols, internetworking devices and their functions.and their functions.
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Data link layer divided into two functionalityData link layer divided into two functionality--oriented oriented sublayerssublayers
Data Link layer divided into two sublayers.
The upper sublayer is responsible for datalink control,
The lower sublayer is responsible for resolving access to the shared media.
Data Link Data Link SublayersSublayers
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33/47/47Taxonomy of multipleTaxonomy of multiple--access protocols discussed in this chapteraccess protocols discussed in this chapter
Multiple-Access Protocol
When nodes or stations are connected and use a common link, called a multipoint or broadcast link,
We need a multiple-access protocol to coordinate access to the link.
Taxonomy of multipleTaxonomy of multiple--access protocolsaccess protocols
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2.1 2.1 RANDOM ACCESSRANDOM ACCESS
InIn random accessrandom access oror contentioncontention methods, no station is methods, no station is superior to another station and none is assigned the control superior to another station and none is assigned the control over another. No station permits, or does not permit, another over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a send uses a procedure defined by the protocol to make a decision on whether or not to send. decision on whether or not to send.
ALOHA (Pure & Slotted)Carrier Sense Multiple Access (CSMA) using Persisetant Methods Carrier Sense Multiple Access with Collision Detection (CSMA/CD)Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Topics discussed in this section:Topics discussed in this section:
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Random Access DescriptionRandom Access Description
Random Access
In a Random access method, each station has the right to the medium without being controlled by any other station.
If more than one station tries to send, there is an access conflict –COLLISION – and the frames will be either destroyed or modified.
To avoid access conflict, each station follows a procedure.
When can the station access the medium ?
What can the station do if the medium is busy ?
How can the station determine the success or failure of the transmission ?
What can the station do if there is an access conflict ?
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• The earliest random-access method, was developed at the Univ. of
Hawaii in the early 1970s.
• Base station is central controller
• Base station acts as a hop
• Potential collisions, all incoming data is @ 407 MHz
ALOHA network – Multiple Access
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Frames in a pure ALOHA networkFrames in a pure ALOHA network
Pure ALOHA Random Access
Each station sends a frame whenever it has a frames to send.
However, there is only one channel to share and there is the possibility of collision between frames from different stations.
Pure ALOHA (1/4)
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Procedure for pure ALOHA protocolProcedure for pure ALOHA protocol
Pure ALOHA (2/4)
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Pure ALOHA (3/4)Pure ALOHA (3/4)
Protocol Rule
After sending the frame, the station waits for an acknowledgment
If it does not receive an acknowledgement during the
2 times the maximum propagation delay between the two widely separated stations (2 x Tp)
It assumes that the frame is lost; it tries sending again after
a random amount of time (TB =R x TP , or R x Tfr ).
Tp (Maximum propagation time)= distance / propagation speed TB (Back off time) : common formula is binary exponential back-off
– R is random number chosen between 0 to 2k - 1 – K is the number of attempted unsuccessful transmissions
Tfr (the average time required to send out a frame)
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The throughput for pure ALOHA isS = G × e −2G .
The maximum throughputSmax = 0.184 when G= (1/2).
S is the average number of successful transmissions,
called throughput.
G is the average number of frames generated by the system
during one frame transmission time.
Pure ALOHA (4/4)
* If oneIf one--half a frame is generated during one frame transmission half a frame is generated during one frame transmission time, then 18.4 percent of these frames reach their destination time, then 18.4 percent of these frames reach their destination successfully.successfully.
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AA purepure ALOHAALOHA networknetwork transmitstransmits 200200--bitbit framesframes onon aasharedshared channelchannel ofof 200200 kbpskbps.. WhatWhat isis thethe throughputthroughput ifif thethesystemsystem (all(all stationsstations together)together) producesproducesaa.. 10001000 framesframes perper secondsecondbb.. 500500 framesframes perper secondsecondcc.. 250250 framesframes perper secondsecond..
ExampleExample
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TheThe frameframe transmissiontransmission timetime isis 200200//200200 kbpskbps oror 11 msms..aa.. IfIf thethe systemsystem createscreates 10001000 framesframes perper second,second, thisthis isis 11 frameframe perper
millisecondmillisecond.. TheThe loadload isis 11.. InIn thisthis casecase SS == GG×× ee−−22 GG oror SS == 00..135135 ((1313..55percent)percent).. ThisThis meansmeans thatthat thethe throughputthroughput isis 10001000 ×× 00..135135 == 135135 framesframesOnlyOnly 135135 framesframes outout ofof 10001000 willwill probablyprobably survivesurvive..
bb.. IfIf thethe systemsystem createscreates 500500 framesframes perper second,second, thisthis isis ((11//22)) frameframe perpermillisecondmillisecond.. TheThe loadload isis ((11//22)).. InIn thisthis casecase SS == GG ×× ee −−22GG oror SS == 00..184184 ((1818..44percent)percent).. ThisThis meansmeans thatthat thethe throughputthroughput isis 500500 ×× 00..184184 == 9292 andand thatthatonlyonly 9292 framesframes outout ofof 500500 willwill probablyprobably survivesurvive.. NoteNote thatthat thisthis isis thethemaximummaximum throughputthroughput case,case, percentagewisepercentagewise..
cc.. IfIf thethe systemsystem createscreates 250250 framesframes perper second,second, thisthis isis ((11//44)) frameframe perpermillisecondmillisecond.. TheThe loadload isis ((11//44)).. InIn thisthis casecase SS == GG×× ee −−22GG oror SS == 00..152152 ((1515..22percent)percent).. ThisThis meansmeans thatthat thethe throughputthroughput isis 250250 ×× 00..152152 == 3838.. OnlyOnly 3838framesframes outout ofof 250250 willwill probablyprobably survivesurvive..
ExampleExample--SolutionSolution
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The throughput for slotted ALOHA isS = G × e−G .
The maximum throughputSmax = 0.368 when G = 1.
Slotted ALOHA
• If a frame is generated during one frame transmission time, If a frame is generated during one frame transmission time, then 36.8 percent of these frames reach their destination successfully.then 36.8 percent of these frames reach their destination successfully.
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AA slottedslotted ALOHAALOHA networknetwork transmitstransmits 200200--bitbit framesframes onon aasharedshared channelchannel ofof 200200 kbpskbps.. WhatWhat isis thethe throughputthroughput ifif thethesystemsystem (all(all stationsstations together)together) producesproducesaa.. 10001000 framesframes perper secondsecondbb.. 500500 framesframes perper secondsecondcc.. 250250 framesframes perper secondsecond..
ExampleExample
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TheThe frameframe transmissiontransmission timetime isis 200200//200200 kbpskbps oror 11 msms..aa.. IfIf thethe systemsystem createscreates 10001000 framesframes perper second,second, thisthis isis 11 frameframe perper
millisecondmillisecond.. TheThe loadload isis 11.. InIn thisthis casecase SS == GG×× ee−G−G oror SS == 00..368368 ((3636..88percent)percent).. ThisThis meansmeans thatthat thethe throughputthroughput isis 10001000 ×× 00..03680368 == 368368 framesframesOnlyOnly 368368 framesframes outout ofof 10001000 willwill probablyprobably survivesurvive..
bb.. IfIf thethe systemsystem createscreates 500500 framesframes perper second,second, thisthis isis ((11//22)) frameframe perpermillisecondmillisecond.. TheThe loadload isis ((11//22)).. InIn thisthis casecase SS == GG ×× ee−G−G oror SS == 00..303303 ((3030..33percent)percent).. ThisThis meansmeans thatthat thethe throughputthroughput isis 500500××00..03030303 == 151151..
OnlyOnly 151151 framesframes outout ofof 500500 willwill probablyprobably survivesurvive..
cc.. IfIf thethe systemsystem createscreates 250250 framesframes perper second,second, thisthis isis ((11//44)) frameframe perpermillisecondmillisecond.. TheThe loadload isis ((11//44)).. InIn thisthis casecase SS == GG ×× ee −G−G oror SS == 00..195195 ((1919..55percent)percent).. ThisThis meansmeans thatthat thethe throughputthroughput isis 250250 ×× 00..195195 == 4949.. OnlyOnly 4949framesframes outout ofof 250250 willwill probablyprobably survivesurvive..
ExampleExample--SolutionSolution
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Carrier Sense Multiple Access (CSMA)Carrier Sense Multiple Access (CSMA)
To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed.
CSMA is based on the principle “sense before transmit” or “listen before talk.”
CSMA can reduce the possibility of collision, but it cannot eliminate it.
The possibility of collision still exists because of propagation delay; a station may sense the medium and find it idle, only because the first bit sent by another station has not yet been received.
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Persistence methods (1/2)Persistence methods (1/2) Persistence strategy defines the procedures for a station that senses a busy
medium.
Three strategies have been developed;
1-persistent method : after the station finds the line idle, it sends its frame immediately (with probability 1)
non-persistent method : a station that has a frame to send senses the line. If the line is idle, it sends immediately. If the line is not idle, it waits a random amount of time and then senses the line
again.p-persistent method : After the station finds the line idle,
With probability p, the station sends its frames With probability q=1-p, the station waits for the beginning of the next time slot
and checks the line again.
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Flow diagram for three persistence methodsFlow diagram for three persistence methods
Persistence methods Persistence methods (2/2)(2/2)
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CSMA with Collision Detection (CSMA/CD) (1/3)CSMA with Collision Detection (CSMA/CD) (1/3)CSMA/CD adds a procedure to handle the collision.
A station monitors the medium after it sends a frame to see if the transmission was successful.
If so, the station is finished. If there is a collision, the frame is sent again.
Collision of the first bit in CSMA/CDCollision of the first bit in CSMA/CD
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Collision and abortion in CSMA/CDCollision and abortion in CSMA/CD
Minimum Frame Size
For CSMA/CD to work, we need a restriction on the frame size.
The frame transmission time Tfr must be at least two times the maximum propagation time Tp .
CSMA with Collision Detection (CSMA/CD) (2/3)CSMA with Collision Detection (CSMA/CD) (2/3)
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2121/47/47Flow diagram for the CSMA/CDFlow diagram for the CSMA/CD
CSMA with Collision Detection (CSMA/CD) (3/3)CSMA with Collision Detection (CSMA/CD) (3/3)
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ExampleExample
AA networknetwork usingusing CSMA/CDCSMA/CD hashas aa bandwidthbandwidth ofof 1010 MbpsMbps.. IfIf thethe maximummaximumpropagationpropagation timetime (including(including thethe delaysdelays inin thethe devicesdevices andand ignoringignoring thethe timetimeneededneeded toto sendsend aa jammingjamming signal,signal, asas wewe seesee later)later) isis 2525..66 μsμs,, whatwhat isis thetheminimumminimum sizesize ofof thethe frame?frame?
SolutionSolutionTheThe frameframe transmissiontransmission timetime isis TTfrfr == 22 ×× TTpp == 5151..22 μsμs.. ThisThis means,means, inin thetheworstworst case,case, aa stationstation needsneeds toto transmittransmit forfor aa periodperiod ofof 5151..22 μsμs toto detectdetect thethecollisioncollision.. TheThe minimumminimum sizesize ofof thethe frameframe isis 1010 MbpsMbps ×× 5151..22 μsμs == 512512 bitsbits oror6464 bytesbytes.. ThisThis isis actuallyactually thethe minimumminimum sizesize ofof thethe frameframe forfor StandardStandardEthernetEthernet..
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In CSMA/CA, if the station finds the channel busy, it does not restart the timer of the
contention window;it stops the timer and restarts it when the
channel becomes idle.
CSMA with Collision Avoidance (CSMA/CA) (1/2)CSMA with Collision Avoidance (CSMA/CA) (1/2)
In CSMA/CA, the IFS can also be used to define the priority of a station or a frame.
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CSMA with Collision Avoidance (CSMA/CA) (2/2)CSMA with Collision Avoidance (CSMA/CA) (2/2)
Flow diagram for CSMA/CAFlow diagram for CSMA/CA
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2.2 2.2 CONTROLLED ACCESSCONTROLLED ACCESS
In controlled access, the stations consult one another to findwhich station has the right to send. A station cannot sendunless it has been authorized by other stations. We discussthree popular controlled-access methods.
ReservationPollingToken Passing
Topics discussed in this section:Topics discussed in this section:
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Reservation
A station need to make a reservation before sending data
In each interval, a reservation frame precedes the data frames sent in that interval.
If there are N stations in the system, there are exactly N reservation minislots in the reservation frame. Each minislot belongs to a station.
When a station needs to send a data frame, it makes a reservation in its own minislot.
The stations that have made reservations can send their data frames after the reservation frame.
Reservation access methodReservation access method
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Polling (1/2)Polling (1/2)
Polling works with topologies in which one device is designed as a Primary Station and the other devices are Secondary Station.
The primary device controls the link;
The secondary devices follow its instruction.
Poll function : If the primary wants to receive data, it asks the secondaries if they have anything to send.
Select function : If the primary wants to send data, it tells the secondary to get ready to receive.
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Select and poll functions in polling access methodSelect and poll functions in polling access method
Polling (2/2)Polling (2/2)
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Token-passing network (1/2) A station is authorized to send data when it receives a special
frame called a token
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Logical ring and physical topology in tokenLogical ring and physical topology in token--passing access methodpassing access method
Token-passing network (2/2)
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2.3 2.3 CHANNELIZATIONCHANNELIZATION
ChannelizationChannelization is a multipleis a multiple--access method in which the access method in which the available bandwidth of a link is shared in time, frequency, or available bandwidth of a link is shared in time, frequency, or through code, between different stations. In this section, we through code, between different stations. In this section, we discuss three channelization protocols.discuss three channelization protocols.
Frequency-Division Multiple Access (FDMA)Time-Division Multiple Access (TDMA)Code-Division Multiple Access (CDMA)
Topics discussed in this section:Topics discussed in this section:
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The available bandwidth is shared by all stations.
The FDMA is a data link layer protocol that uses FDM at the physical layer
FDMA (1/2)
In FDMA, the available bandwidth of the common channel is divided into
bands that are separated by guard bands.
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FrequencyFrequency--division multiple access (FDMA)division multiple access (FDMA)
FDMA (2/2)
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TDMA
The entire bandwidth is just one channel.
Stations share the capacity of the channel in time
In TDMA, the bandwidth is just one channel that is timeshared between different
stations.
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TimeTime--division multiple access (TDMA)division multiple access (TDMA)
FDMA
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CDMA
CDMA differs from FDMA because only one channel occupies the entire bandwidth of the link.
It differs from TDMA because all stations can send data simultaneously; there is no time sharing.
In CDMA, one channel carries all transmissions simultaneously.
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Simple idea of communication with code
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Chip sequences
Data representation in CDMA
Chip sequences and dataChip sequences and data representationrepresentation
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Sharing Sharing channel in CDMAchannel in CDMA
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Digital Digital signal created by four stations in CDMAsignal created by four stations in CDMA
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Decoding Decoding of the composite signal for one in CDMAof the composite signal for one in CDMA
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General rule and examples of creating Walsh tablesGeneral rule and examples of creating Walsh tables
The number of sequences in a Walsh table needs to be N = 2The number of sequences in a Walsh table needs to be N = 2mm..
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ExampleExample
FindFind thethe chipschips forfor aa networknetwork withwithaa.. TwoTwo stationsstations bb.. FourFour stationsstations
SolutionSolution
We We can use the rows of Wcan use the rows of W22 and Wand W44 in previous slidein previous slidea. a. For a twoFor a two--station network, we have station network, we have
[+1 +1] and [+1 −1].[+1 +1] and [+1 −1].
b.b. For a fourFor a four--station network we have station network we have [+1 +1 +1 +1], [+1 −1 +1 −1], [+1 +1 +1 +1], [+1 −1 +1 −1],
[+1 +1 −1 −1],[+1 +1 −1 −1], and and [+1 −1 −1 +1].[+1 −1 −1 +1].
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Summary (1/4)Data link layer as two sublayers. The upper sublayer is responsible for data link control, and the lower sublayer is responsible for resolving access to the shared media.
Three groups: random access protocols, controlled access protocols, and channelization protocols.
In random access or contention methods, no station is superior to another station and none is assigned the control over another.
ALOHA allows multiple access (MA) to the shared medium. There are potential collisions in this arrangement. When a station sends data, another station may attempt to do so at the same time.
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Summary (2/4)
Carrier sense multiple access (CSMA) requires that each station first listen to the medium before sending. Three methods have been devised for carrier sensing: 1-persistent, non-persistent, and p-persistent.
In Carrier sense multiple access with collision detection (CSMA/CD), a station monitors the medium after it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is a collision, the frame is sent again
To avoid collision on wireless networks, carrier send multiple access with collision avoidance (CSMA/CA) was invented. Collisions are avoided through the use three strategies: the Interframe space, the contention window, and acknowledgments.
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Summary (3/4) In controlled access, a station cannot send unless it has been authorized by other station. Three popular controlled-access methods: reservation, polling, and token passing.
In the reservation access method, a station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames send in that interval.
In the polling method, all data exchanges must be made through the primary device even when the ultimate destination is a secondary device. The primary device controls the link; the secondary devices follow its instructions
In the token-passing method, the stations in a network are organized in a logical ring. Each station has a predecessor and a successor. A special packet called a token circulates through the ring.
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Summary (4/4) In Channelization the available bandwidth of a link is shared in time, frequency, or through code, between different stations. Three channelization protocols: FDMA, TDMA, and CDMA.
In FDMA, the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data. In other words, each band is reserved for a specific station, and it belongs to the station all the time
In TDMA, the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data. Each station transmits its data in its data in its assigned time slot.
In CDMA, the stations use different codes to achieve multiple access. One channel carries all transmissions simultaneously.
