MAC Protocols of ADHOC network

MAC Protocols of ADHOC networkBy Shashi GurungAssistant Professor CTIEMT

MAC protocolsThe topology is highly dynamic and frequent changes in the topology may be hard to predict.MAC is responsible for resolving the conflicts among different nodes for channel access.There are two problems Hidden terminal problemExposed terminal problem

Hidden and Exposed TerminalsHidden terminalsA sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails)collision at B, A cannot receive the collision (CD fails)A is hidden for C

Exposed terminalsB sends to A, C wants to send to another terminal (not A or B)C senses carrier, finds medium in use and has to waitA is outside the radio range of C, therefore waiting is not necessaryC is exposed to B

BA

C

Classifications of MAC ProtocolsContention-based protocolsSender-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 mechanismsSynchronous 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.7

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Classifications of MAC ProtocolsContention-based protocols with scheduling mechanismsNode 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.

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Contention-based protocols without reservation

Multiple Access with Collision Avoidance(MACA) MACA uses signaling packets for collision avoidanceRTS (request to send) :sender request the right to send from a receiver with a short RTS packet before it sends a data packetCTS (clear to send) :receiver grants the right to send as soon as it is ready to receiveSignaling packets containsender addressreceiver addresspacket sizeThe 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 examplesMACA avoids the problem of hidden terminalsA and C want to send to BA sends RTS firstC waits after receiving CTS from B

MACA avoids the problem of exposed terminalsB wants to send to A, C to another terminalnow C does not have to wait for it cannot receive CTS from A11

AB

C

RTS

CTS

CTS

AB

C

RTS

CTSRTS

LimitationsMACA does not provide ACKRTS-CTS approach does not always solve the hidden node problemExampleA sends RTS to BB sends CTS to A; At the same time, D sends RTS to CThe CTS & RTS packets collide at CA transmits data to B; D resends RTS to C; C sends CTS to DThe data & CTS packets collide at B

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MACAWMACAW (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.Neighborssee 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.CollisionsThere is no collision detection.The senders know collision when they dont receive CTS.They each wait for the exponential backoff time.13

MACAW (MACA for Wireless)RTS-CTS-DS-DATA-ACKRTS from A to BCTS from B to AData Sending (DS) from A to BData from A to BACK from B to ARandom wait after any successful/unsuccessful transmissionSignificantly higher throughput than MACADoes not completely solve hidden & exposed node problems

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Other Mac Protocols

PAMAS (Power aware medium access control with signaling)RTS-CTS exchanges over a signaling channelingData transmission over a separate data channelReceiver sends out a busy tone, while receiving a data packet over the signaling channelNodes listen to the signaling channel to determine when it is optimal to power down transceiversA node powers itself off if it has nothing to transmit and its neighbor is transmittingA node powers off if at least one neighbor is transmitting and another is receivingUse of ACK and transmission of multiple packets can enhance performance

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