mobile ad hoc networks - lokeshchouhan.comlokeshchouhan.com/gallery/mac_manet.pdf · macaw •macaw...
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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: [email protected]
Dr. Lokesh ChouhanAssistant Professor
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Mobile Ad Hoc Networks
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
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