The document discusses several MAC protocols for ad hoc networks including MACA, MACAW, and PAMAS. MACA uses RTS and CTS packets to avoid collisions but does not provide ACK. MACAW is a revision of MACA that includes ACK. It significantly increases throughput but does not fully solve hidden and exposed terminal problems. PAMAS uses a separate signaling channel for RTS-CTS and a data channel. It allows nodes to power down transceivers when not transmitting to save energy.

1. MAC Protocols of ADHOC
network
By Shashi Gurung
Assistant Professor
CTIEMT

2. MAC protocols
• The 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 problem
– Exposed terminal problem

3. Hidden and Exposed Terminals
• Hidden terminals
– A 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 terminals
– B sends to A, C wants to send to another terminal (not A or B)
– C senses carrier, finds medium in use and has to wait
– A is outside the radio range of C, therefore waiting is not necessary
– C is “exposed” to B
A B C

7. 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.
7

8. 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.

9. 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.

10. – 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.

11. 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
11
RTS
CTS CTS
A B C
RTS
CTS
RTS
A B C

12. • 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

13. 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. 13

14. 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

27. Other Mac Protocols

29. 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