The medium access control (MAC) protocol is that part of the overall network functionality
that deals with problems of achieving efficient, fair, and dependable access to the medium
shared by a number of different devices (Stallings 2002). The role of the MAC protocol is
particularly important in wireless networks which differ from their wired counterparts in
many aspects. The most important among those differences stem from the very nature of
the wireless communication medium, where two devices need not be explicitly connected
in order to be able to communicate–instead, it merely suffices that they are within the radio
transmission range of each other.
For example, when two or more packets are simultaneously received, the receiver may
encounter problems. At best, the unwanted packets are treated as noise which impairs
the reception of the packet intended to be received but can be filtered out. At worst, the
correct packet may be damaged beyond repair and the receiver may be unable to make any
sense out of it; this condition is referred to as a collision. Collisions waste both network
bandwidth and power resources of individual devices, transmitters and receivers alike, and
active measures should be taken to reduce the likelihood of their occurrence.
Common approaches for collision minimization in wired networks include detection
and avoidance. Collision detection is widely used in wired networks, where it involves
the simple act of listening while transmitting. However, this is not feasible in wireless
communication, where few devices are equipped with the required capability (Stallings
2002). Furthermore, packet collisions in wireless networks may occur in scenarios that
cannot occur in wired ones, such as the so-called hidden and exposed terminal problems
(Ram Murthy and Manoj 2004).
Since collision detection is not available, MAC protocols for wireless networks must
rely on collision avoidance techniques, which include explicit scheduling, bandwidth reser-
vation, and listening to the medium before attempting to transmit a packet. This last
procedure is commonly known as clear channel assessment (IEEE 2003a, 2006; O’Hara
and Petrick 1999), although other terms may be occasionally encountered as well.
Obviously,MACprotocolsinwirelessnetworksfacebothtraditionalchallengesencoun-
tered in wired networks andnew onesthat stem from the useof the wireless communication
medium. According to Ram Murthy and Manoj (2004), the most important features of
MACs in ad hoc wireless networks can be summarized as follows:
• The operation of the protocol should be distributed, preferably without a dedicated
central controller. If the use of such a controller cannot be avoided, the role should
be only temporary, and devices with appropriate capabilities must be allowed to
undertake it for a certain period of time.
• The protocol should be scalable to large networks.
• The available bandwidth must be utilized efficiently, including the minimization of
packet collisions and minimization of the overhead needed to monitor and control
network operation. In particular, the protocol should minimize the effects of hidden
and exposed node problems.
• The protocol should ensure fair bandwidth allocation to all the nodes. Preferably, the
fairness mechanism should take into account the current level of congestion in the
network.
• The MAC protocol should incorporate power management policy, or policies, so as
to minimize the power consumption of the node and of the entire network.
• The protocol should provide quality of service (QoS) support for real-time traffic
wherever possible. Real-time, in this context, implies data traffic with prescribed
performance bounds; these may include throughput, delay, delay jitter, and/or other
performance indicators.
Two additional issues deserve to be mentioned. First issue is time synchronization among
the nodes, which is required for the purpose of bandwidth reservation and allocation. Time
synchronization is usually achieved by having one of the nodes periodically broadcast some
sortof synchronization signal(the beacon)which isthen usedby other nodes.While theuse
of periodic beacon transmissions facilitates the process of placing the reservation requests
andsubsequentbroadcastingof reservationallocations,it requiresthatsomenodeis capable
of, and willing to, act as the central controller – somewhat contrary to the distributed, self-
organizingcharacterofanadhocnetwork.Inparticular,additionalprovisionsmustbemade
to replacethe controller node whenit departsfrom the network or experiencesa failure; this
is part of the self-healing property of ad hoc networks described above. Furthermore, the
use of beacons consumes the bandwidth and affects the scalability of the MAC algorithm.
The second issue is related to the interference from neighbouring nodes. As this in-
terference is harmful, steps have to be taken to reduce it, most often through appropriate
multiplexing techniques. According to Stallings (2002), multiplexing techniques are avail-
able in the following domains:
• in the frequency domain (FDMA), wherein different frequency bands are allocated
to different devices or subnetworks;
• in the code domain (CDMA), wherein different devices use different code sequences;
• in the time domain (TDMA), wherein different devices transmit at different times;
and/or
• in the space domain, where the range and scope of transmissions are controlled
through the use of transmitter power control and directional antennas, respectively.
Strictly speaking, all these techniques belong to the PHY layer; while the MAC layer is
completely oblivious to the first two techniques, it can utilize the latter two (multiplexing
in time and space domain), or even integrate them to a certain extent. (For example, time
multiplexing is a close relative of scheduling.) This cross-layer integration and optimization
allow the MAC protocol to better address the requirements outlined above. We note that
such integration is not too common in ad hoc networks, where the MAC layer is more
likely to cooperate with the network and, possibly, transport layers above it, than with the
PHY layer below; however, MAC protocols exist that make use of it (Ram Murthy and
Manoj 2004).
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