Chapter 6: MAC Protocols For Ad-Hoc Wireless Networks
Chapter 6: MAC Protocols For Ad-Hoc Wireless Networks
Chapter 6: MAC Protocols For Ad-Hoc Wireless Networks
Wireless Networks
Introduction Contention-based Protocols
Issues with reservation mechanisms
Design Goals Contention-based Protocols
Classifications without Scheduling
mechanisms
Contention-based Protocols
MAC Protocols that use
directional antennas
Other MAC Protocols
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Issues
The main issues need to be addressed while designing a MAC
protocol for ad hoc wireless networks:
• Bandwidth efficiency is defined at the ratio of the bandwidth used for
actual data transmission to the total available bandwidth. The MAC protocol
for ad-hoc networks should maximize it.
• Quality of service support is essential for time-critical applications. The
MAC protocol for ad-hoc networks should consider the constraint of ad-hoc
networks.
• Synchronization can be achieved by exchange of control packets.
• Hidden and exposed terminal problems
• Error-Prone Shared Broadcast Channel
• Distributed Nature/Lack of Central Coordination
• Mobility of Nodes: Nodes are mobile most of the time.
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Design goals of a MAC Protocol
Design goals of a MAC protocol for ad hoc wireless networks
• The operation of the protocol should be distributed.
• The protocol should provide QoS support for real-time traffic.
• The access delay, which refers to the average delay experienced by any
packet to get transmitted, must be kept low.
• The available bandwidth must be utilized efficiently.
• The protocol should ensure fair allocation of bandwidth to nodes.
• Control overhead must be kept as low as possible.
• The protocol should minimize the effects of hidden and exposed terminal
problems.
• The protocol must be scalable to large networks.
• It should have power control mechanisms.
• The protocol should have mechanisms for adaptive data rate control.
• It should try to use directional antennas.
• The protocol should provide synchronization among nodes. 3
Classifications of MAC protocols
Ad hoc network MAC protocols can be classified into three types:
• Contention-based protocols
• Contention-based protocols with reservation mechanisms
• Contention-based protocols with scheduling mechanisms
• Other MAC protocols
MAC Protocols for Ad Hoc
Wireless Networks
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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.
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Contention-based protocols
MACAW: A Media Access Protocol for Wireless LANs is based on
MACA (Multiple Access Collision Avoidance) Protocol
MACA
• 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. The problem is solved by MACAW.
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MACA examples
MACA avoids the problem of hidden terminals
• A and C want to
send to B
• A sends RTS first
RTS
• C waits after receiving
CTS from B CTS CTS
A B C
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Contention-based protocols
Busy Tone Multiple Access Protocols (BTMA)
• The transmission channel is split into two:
• a data channel for data packet transmissions
• a control channel used to transmit the busy tone signal
• When a node is ready for transmission, it senses the channel to check
whether the busy tone is active.
• If not, it turns on the busy tone signal and starts data transmissions
• Otherwise, it reschedules the packet for transmission after some random
rescheduling delay.
• Any other node which senses the carrier on the incoming data channel
also transmits the busy tone signal on the control channel, thus, prevent
two neighboring nodes from transmitting at the same time.
Dual Busy Tone Multiple Access Protocol (DBTMAP) is an
extension of the BTMA scheme.
• a data channel for data packet transmissions
• a control channel used for control packet transmissions (RTS and CTS
packets) and also for transmitting the busy tones. 11
Contention-based protocols
Receiver-Initiated Busy Tone Multiple Access Protocol (RI-
BTMA)
• The transmission channel is split into two:
• a data channel for data packet transmissions
• a control channel used for transmitting the busy tone signal
• A node can transmit on the data channel only if it finds the busy tone to be absent
on the control channel.
• The data packet is divided into two portions: a preamble and the actual data packet.
MACA-By Invitation (MACA-BI) is a receiver-initiated MAC
protocol.
• By eliminating the need for the RTS packet it reduces the number of
control packets used in the MACA protocol which uses the three-way
handshake mechanism.
Media Access with Reduced Handshake (MARCH) is a receiver-
initiated protocol. 12
Access method DAMA: Reservation-
TDMA
Reservation Time Division Multiple Access
• every frame consists of N mini-slots and x data-slots
• every station has its own mini-slot and can reserve up to k data-slots using
this mini-slot (i.e. x = N * k).
• other stations can send data in unused data-slots according to a round-robin
sending scheme (best-effort traffic)
reservation 1 2 3 4 5 6 7 8 time-slot
ACDABA-F frame1 A C D A B A F
ACDABA-F frame2 A C A B A
AC-ABAF- frame3 A B A F collision at
reservation
A---BAFD frame4 A B A F D attempts
t 14
ACEEBAFD frame5 A C E E B A F D
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