Computer Science and Engineering (CSE) Department

National Institute of Technology (NIT)Hamirpur (H.P.) INDIA

Website: http://nith.ac.in/newweb/computer-science-engineering/

E-mail: lokesh@nith.ac.in

Dr. Lokesh ChouhanAssistant Professor

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Mobile Ad Hoc Networks

MAC Protocols for Mobile Ad Hoc Networks

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Classifications of MAC Protocols• Contention-based protocols

– Sender-initiated protocols: Packet transmissions are initiated by the sender node.

• Single-channel sender-initiated protocols: A node that wins the contention to the

channel can make use of the entire bandwidth.

• Multichannel sender-initiated protocols: The available bandwidth is divided into

multiple channels.

– Receiver-initiated protocols: The receiver node initiates the contention resolution

protocol.

• Contention-based protocols with reservation mechanisms

– Synchronous protocols: All nodes need to be synchronized. Global time

synchronization is difficult to achieve.

– Asynchronous protocols: These protocols use relative time information for effecting

reservations.

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Classifications of MAC Protocols• Contention-based protocols with scheduling mechanisms

– Node scheduling is done in a manner so that all nodes are treated fairly and

no node is starved of bandwidth.

– Scheduling-based schemes are also used for enforcing priorities among flows

whose packets are queued at nodes.

– Some scheduling schemes also consider battery characteristics.

• Other protocols are those MAC protocols that do not strictly fall under the above

categories.

Contention-based protocols without reservation

Multiple Access with Collision Avoidance(MACA)

• MACA uses signaling packets for collision avoidance

– RTS (request to send) :sender request the right to send from a receiver

with a short RTS packet before it sends a data packet

– CTS (clear to send) :receiver grants the right to send as soon as it is

ready to receive

• Signaling packets contain

– sender address

– receiver address

– packet size

• The neighbor node that overhears an RTS packet has to defer its own

transmission until the associated CTS packet is transmitted.

– Then any node overhearing a CTS packet would defer for the length of

expected data transmission When a node wants to transmit a data packet,

it first transmit a RTS (Request To Send) frame.

• The receiver node, on receiving the RTS packet, if it is ready to receive

the data packet, transmits a CTS (Clear to Send) packet.

• Once the sender receives the CTS packet without any error, it starts

transmitting the data packet.

• If a packet transmitted by a node is lost, the node uses the binary

exponential back-off (BEB) algorithm to back off a random interval of

time before retrying.

• The binary exponential back-off mechanism used in MACA might

starves flows sometimes.

MACA examples• MACA avoids the problem of hidden terminals

– A and C want to

send to B

– A sends RTS first

– C waits after receiving

CTS from B

• MACA avoids the problem of exposed terminals

– B wants to send to A, C

to another terminal

– now C does not have

to wait for it cannot

receive CTS from A

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A B C

RTS

CTSCTS

A B C

RTS

CTS

RTS

• Limitations

– MACA does not provide ACK

– RTS-CTS approach does not always solve the hidden node problem

– Example

• A sends RTS to B

• B sends CTS to A; At the same time, D sends RTS to C

• The CTS & RTS packets collide at C

• A transmits data to B; D resends RTS to C; C sends CTS to D

• The data & CTS packets collide at B

MACAW• MACAW (MACA for Wireless) is a revision of MACA(without ACK).

– The sender senses the carrier to see and transmits a RTS (Request To Send)

frame if no nearby station transmits a RTS.

– The receiver replies with a CTS (Clear To Send) frame.

– Neighbors

• see CTS, then keep quiet.

• see RTS but not CTS, then keep quiet until the CTS is back to the sender.

– The receiver sends an ACK when receiving an frame.

• Neighbors keep silent until see ACK.

– Collisions

• There is no collision detection.

• The senders know collision when they don’t receive CTS.

• They each wait for the exponential backoff time. 11

MACAW (MACA for Wireless)

• RTS-CTS-DS-DATA-ACK

– RTS from A to B

– CTS from B to A

– Data Sending (DS) from A to B

– Data from A to B

– ACK from B to A

– Random wait after any successful/unsuccessful transmission

• Significantly higher throughput than MACA

• Does not completely solve hidden & exposed node problems

Other Mac Protocols

PAMAS (Power aware medium access control with

signaling)• RTS-CTS exchanges over a signaling channeling

• Data transmission over a separate data channel

• Receiver sends out a busy tone, while receiving a data packet over the signaling

channel

• Nodes listen to the signaling channel to determine when it is optimal to power

down transceivers

• A node powers itself off if it has nothing to transmit and its neighbor is

transmitting

• A node powers off if at least one neighbor is transmitting and another is

receiving

• Use of ACK and transmission of multiple packets can enhance performance