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Channel access method or multiple access

method allows several terminals connected

to the same multi-point transmissionmedium to transmit over it and to share its

capacity


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ALOHA

ALOHA is a ground-based radio broadcastingsystem

It is first multiple-access protocol for sharinga transmission channel by enabling the

transmitter to access the channel at randomtimes

The decisions to transmit are made withoutpaying attention to what other nodes might be

doing ALOHA is of two types-

1.pure and2.slotted


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Pure ALOHA

Frames are transmitted at completely arbitrarytimes

Whenever two frames try to occupy the

channel at the same time, there will be acollision and both will be garbled If the first bit of a new frame overlaps with just

the last bit of a frame almost finished, both

frames will be totally destroyed and both willhave to be retransmitted later If there is no collision, the node has successfully

transmitted its frame.


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The central node acknowledges the correctframes it receives.

when a node does not get an acknowledgmentwithin a specific timeout, it assumes that itsframe collided.

when a frame collides, the transmitting nodeschedules a retransmission after a random delay.


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For any frame-time, the probability of therebeing k transmission-attempts during that

frame-time is:

The average amount of transmission-attemptsfor 2 consecutive frame-times is 2G. Hence, forany pair of consecutive frame-times, theprobability of there being k transmission-

attempts during those two frame-times is:


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Therefore, the probability (Probpure) of therebeing zero transmission-attempts between t-Tand t+T (and thus of a successful transmissionfor us) is:

Probpure = e 2G

The throughput can be calculated as the rate of

transmission-attempts multiplied by theprobability of success, and so we can conclude

that the throughput (Spure) is: Spure = Ge

2G


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Slotted ALOHA

Uses discrete timeslots and increased themaximum throughput

A station can send only at the beginning of atimeslot, and thus collisions are reduced

When a node has a fresh frame to send, it waitsuntil the beginning of the next slot and transmitthe entire frame.

In slotted ALOHA, the best we can hope for is37% success, 37% slots empty, and 26%collisions


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The probability of there being zero transmission-attempts in a single timeslot is:

Probslotted = e G

The probability of k packets is:

Probslottedk = e G(1 e G)k 1

The throughput is:Sslotted = Ge

G


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Carrier Sense Multiple AccessProtocols(CSMA)

Probabilistic Media Access Control (MAC)protocol in which a node verifies the absence ofother traffic before transmitting on a sharedtransmission medium

"Carrier Sense" describes the fact that atransmitter uses feedback from a receiver that

detects a carrier wave before trying to send


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Types of CSMA Persistent CSMA:

The station transmits with a probability of 1whenever it finds the channel idle, if the channel isbusy, it waits until it becomes idle. If a collisionoccurs, the station waits a random amount of timeand starts all over again.

The station transmits with a probability of 1 when itfinds the channel idle Non-persistent CSMAThe station transmits if the channel is idle, if the

channel is busy, it waits a random time and tries

againBetter Channel utilization but longer delays


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P-persistent CSMA (slotted):

Transmits with a probability of p whenever itfinds the channel idle, with a probability of 1-p,itwaits until the next slot

If another station has begun transmitting, itacts as if there had been a collision. It waits arandom time and starts again


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CSMA with collision detection (CSMA/CD:

Abort a transmission as soon as they detect acollision. Quickly terminating damaged framessaves time and bandwidth.

Collisions can be detected by looking at thepower or pulse width of the received signal andcomparing it with the transmitted signal.

After a station detects a collision, it aborts its

transmission, waits a random period of time,and then tries again, assuming that no otherstation has started transmitting in themeantime.


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Collision-Free protocol

Two types of Collision-Free ProtocolA Bit-Map Protocol:

each contention period consists of exactly Nslots

If station 0 has a frame to send, it transmits a 1bit during the zeroth slot

Stations agrees on who goes next, there will

never be any collisions After the last ready station has transmitted its

frame, an event all stations can easily monitor,another N bit contention period is begun


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If a station becomes ready just after its bit slothas passed by, it is out of luck and must remain

silent until every station has had a chance andthe bit map has come around again


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Binary Countdown Broadcasts channel address as a binary bit string

Bits in each address position from differentstations are BOOLEAN ORed together

Implicitly assumes that the transmission delays

are negligible so that all stations see assertedbits essentially instantaneously.

The channel efficiency is d/(d + log2 N). If,however, the frame format has been cleverly

chosen so that the sender's address is the firstfield in the frame, even these log2 N bits are notwasted, and the efficiency is 100 percent


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Limited Contention Protocol

Combines the best properties of the contentionand collision-free protocols

used contention at low load to provide low delay Suppose that k stations are contending forchannel access. Each has a probability p oftransmitting during each slot. The probability

that some station successfully acquires thechannel during a given slot is then kp(1 - p)k- 1.


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To find the optimal value of p, we differentiatewith respect to p, set the result to zero, and solvefor p. Doing so, we find that the best value of p is1/k. Substituting p = 1/k, we get

The Adaptive Tree Walk Protocol

The stations are the leaves of a binary tree

In the first contention slot following a successfulframe transmission, slot 0, all stations arepermitted to try to acquire the channel


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If one of them does so, fine. If there is a collision,then during slot 1 only those stations falling

under node 2 in the tree may compete. If one ofthem acquires the channel, the slot following theframe is reserved for those stations under node 3


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Wavelength Division Multiple Access

Protocols Spectrum is divided into channels (wavelength

bands)

Each station is assigned two channels Each channel is divided into groups of time slots

The protocol supports three traffic classes : (1)constant data rate connection-oriented traffic,

such as uncompressed video, (2) variable datarate connection-oriented traffic, such as filetransfer, and (3) datagram traffic, such as UDPpackets


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Wireless LAN Protocols

MACA (Multiple Access with CollisionAvoidance) (Karn, 1990)

Sender to stimulate the receiver into outputtinga short frame

Stations nearby can detect this transmission andavoid transmitting for the duration of theupcoming (large) data frame

It uses RTS (Request To Send) frame and CTS(Clear to Send) frame to communicate betweentwo stations


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MACAW

Bharghavan et al. (1994) developed the MACAW

protocol from MACA

Use an ACK frame after each successful dataframe

Keep a station from transmitting an RTS at thesame time another nearby station is also doingso to the same destination, so carrier sensingwas added.

Mechanism for stations to exchange informationabout congestion and a way to make the backoffalgorithm react less violently to temporaryproblems, to improve system performance

Documents

Dynamic Channel Access Protocol(Barga Deori)