DLL MAC Protocols

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Types of MAC

• Control: – Distributed. – Centralized.

• How they coordinate medium access: – Round-robin. – Scheduled-access.

– Contention-based.

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MAC Protocols• Contention-based

– ALOHA and Slotted ALOHA. – CSMA. –

CSMA/CD.• Round-robin : token-based protocols.

– Token bus. – Token ring.

• Scheduled Access Based: – Centralized or distributed scheduling based MAC – Ex. GSM, WIMAX, LTE, WiFi networks.

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Pure ALOHA•

In pure ALOHA, frames are transmitted at completelyarbitrary times.

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Collisions

• Invalid frames may be caused by channel noise or

• Because other station(s) transmitted at the same time:collision .

• Collision happens even when the last bit of a frameoverlaps with the first bit of the next frame.

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Pure ALOHA: Performance• Vulnerable period for the shaded frame .

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ALOHA’s Performance (Cont’d)

• S = G e -2G , where S is the throughput (rate of successfultransmissions) and G is the mean offered load per frameduration.

• S = S max = 1/2e = 0.184 for G=0.5.

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Slotted Aloha

• Time divided in uniform slots• Need central clock (or other sync mechanism)• Frames can only be transmitted at beginning of slot: “discrete”

ALOHA.

• Frames either miss or overlap totally• Vulnerable period is half of pure ALOHA.• S = G e -G.• S = S max = 1/e = 0.368 for G = 1.• Doubles performance of pure ALOHA.

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ALOHA Protocols: Performance

• Throughput versus offered traffic for ALOHAsystems.

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ALOHA Protocols: Summary

• Simple.• poor utilization (network efficiency)• Applicable only in the network having very low user

density

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CARRIER SENSE MULTIPLE ACCESS(CSMA)

•CSMA protocol was developed to overcome the problem found in ALOHAi.e. to minimize the chances of collision, so as to improve the performance.

•CSMA protocol is based on the principle of ‘carrier sense’.

•The chances of collision can be reduce to great extent if a station senses thechannel before trying to use it.

• Although CSMA can reduce the possibility of collision, but it cannot eliminateit completely.

•The chances of collision still exist because of propagation delay.

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CSMA Protocol• 1-Persistent CSMA

– Continuous sensing – Transmits packet with probability 1 in immediate idle time slot – Collision rate is high

• Non-Persistent CSMA – Senses carrier at random time – Network efficiency is comparatively high – Collision rate is also comparatively low

• P-Persistent CSMA – Time divided into slots – if idle time sensed Transmits packet with probability p – Network efficiency is comparatively high – Collision rate is low

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CSMA /CD protocol

• Carrier Sense Multiple Access/ Collision Detection• Used in Ethernet LAN• Transmitting device transmits packets and continuously detects

collision• If collision detected, differs transmission for random time

• For Random time delay Binary exponential back off algorithm

is used.

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Collision Free Protocols

• Reservation protocol – Like token base access

• Bit Mapped Protocol using contention period – transmit time is divided in to contention and Frame time – In contention period one bit is reserved (0 nothing to transmit, 1 to

transmit) – Contention period followed by number of frames = No. of “1” in

contention period•

Binary count down Protocol using contention period – Instead reserving one bit for all users in contention period user devices

transmits their binary addresses

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Binary Count down contention protocol

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In contention period user nodes transmits their binary address (only those who wantsto transmit)

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Limited-Contention Protocol

• Contention Protocol are more suitable for low load – As load increases contention delay also increases – As load increases packet collision probability also increases

• Collision free protocols are more suitable for heavy load – As load increases channel efficiency also improves at the cost of

additional overhead

• Limited-Contention Protocol combines the best properties ofcontention and collision free protocols

– At low load uses contention Protocol due to less delay – At high load uses collision free protocol due to good channel efficiency

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Limited-Contention Protocol Cont.

• Adaptive Tree walk Protocol – Low load

• Every body contends – Collision

• Reduces number of stations

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Adaptive Tree walk algorithm

First contention all stations are permitted to contendIf collision then in next slot only stations under node 2 contendsIf success then in next slot stations under node 3 contendsIf collision then in next slot stations under node 4 contendsIf success then in next slot stations under node 5 contends

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Binary exponential back-off algorithm

• Used for randomization• The contention slot is decided as 2 power n• Where n= 0,1,2,….N •

For first time collision n=1, station transmits in slot 0 or 1 .• For second time collision n=2, station transmits in any one slot

from 0 to 3 .• For third time collision n=3, station transmits in any one slot from

0 to 7.• And so on.

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DLL MAC Protocols

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