efore we present some of the important MAC protocols for wireless ad hoc networks,
we will give a brief overview of some among the possible criteria for classifying those
protocols; the reader will thus be able to grasp main features of different MAC protocols
and identify the important similarities as well as differences among them.
Mechanismforaccessingthemedium. Probablythemostintuitiveamongtheclassification
criteria is the manner of accessing the medium, which comes in three main flavours:
• Contention-based protocols are those in which a potential sender node must compete
with all others in order to gain access to the medium and transmit its data.
• Bandwidth reservation-based protocols have provisions for requesting and obtaining
bandwidth (or time) allocations by individual senders.
• Finally, scheduling-based protocols, in which the transmissions of individual senders
arescheduledaccordingtosomepredefinedpolicywhichaimstoachieveoneormore
of the objectives outlined above, such as the maximization of throughput, fairness,
flow priority, or QoS support.
Note that the third option requires the presence of an entity which is responsible for
implementing the aforementioned policy. In most cases, this requirement translates into
the requirement for a permanent or temporary central controller. Note also that the policy
to be pursued should be adaptive, depending on the traffic and/or other conditions in the
network. The presence of a central controller is sometimes needed in protocols that use the
second option as well.
Quite a few among the existing MAC protocols offer more than one of those mecha-
nisms. This may be accomplished by slicing the available time into intervals of fixed or
variable size, referred to as cycles or superframes (IEEE 2003a, 2006; O’Hara and Petrick
1999), and assigning certain portions of those intervals to different categories of access
from the list above. For example, the IEEE 802.11 Point Coordinator Function (PCF) uses
superframes in which the first part is reserved for (optional) contention-free access, while
the second part is used for contention-based access (ANSI/IEEE 1999; O’Hara and Petrick
1999). A similar approach is adopted in the IEEE 802.15.4 protocol in its beacon enabled,
slottedCSMA-CAmode(IEEE 2006), exceptthat thecontentionaccessperiod precedesthe
contention-free period in the superframe. More details on the structure of the superframe
are presented in the next chapter.
On the other hand, some MAC protocols offer optional features which modify the
manner in which the protocol operates, and effectively introduce a different mechanism for
medium access control. For example, the IEEE 802.11 Distributed Coordinator Function
(DCF) utilizes pure contention-based access in its default form, but allows bandwidth
reservation on a per-packet basis through the optional RTS/CTS handshake (ANSI/IEEE
1999).
Alternativeclassificationsonthebasisofmediumaccessmechanism. Analternativeclassi-
fication criterion could be devised by assuming that contention-based access will always be
present, and then using the presence or absence of the latter two access mechanisms as the
basisforclassification.Thisapproachresultsinthecommon(andmarginallymorepractical)
classification into pure contention-based MACs, contention-based MACs with reservation
mechanisms, and contention-based MACs with scheduling mechanisms (Ram Murthy and
Manoj 2004). A variant of this approach distinguishes between contention- or random
access-based protocols, scheduling or partitioning ones, and polling-based ones. Yet even
these classifications are neither unambiguous, as the presence of optional features out-
lined above leads to the same protocol being attached to more than one category, nor
comprehensive, as some of the existing protocols cannot be attached to any single cate-
gory (Ram Murthy and Manoj 2004); on account of these shortcomings, it is listed as an
alternative only.
Mechanism used for bandwidth reservation and its scope. These two criteria applies only
to MAC protocols that employ some form of bandwidth reservation, and thus actually rep-
resent sub-classifications within the previous one based on the mechanism used to access
the medium. With respect to the mechanism used for bandwidth reservation, we can dis-
tinguish between the protocols that use some kind of handshake, e.g., RTS/CTS, and those
that use out-of-band signalling, most notably the Busy Tone approach which is an extension
of the familiar concept from the traditional telephony systems.
With respect to the scope of bandwidth reservation, we can distinguish between the
protocols which request bandwidth for a specified time (i.e., for a single packet or for
a group of consecutive packets, commonly referred to as a burst) and those that request
bandwidth allocation for an unspecified time. In both cases, time can be measured in
absolute units or in data packets. In the former case, bandwidth allocation is valid for the
transmission of a specified number of packets only, while in the latter, it has to be explicitly
revoked by some central authority, or perhaps waived by the requester itself.
Another scheme based on the concept related to bandwidth reservation is the family of
the so-called multi-channel MAC protocols. Namely,most communication technologiesuse
only onechannelout of severalavailablein the given frequencyband.Multi-channel MACs
exploit this feature to employ channel hopping in order to improve bandwidth utilization
and/or reduce congestion.
Presence and scope of synchronization. The presence or absence of time synchronization
among the nodes in the network is another criterion that can be used to classify MAC
protocols for wireless ad hoc networks. Synchronization, if present, may be required to
extend to all the nodes in the network (global synchronization); alternatively, it may apply
to just a handful of nodes which are physically close to one another (local synchronization).
Intheformercase,acentralcontrollermaybeneededtoinitiateandbroadcastthenecessary
synchronization information.
Synchronization is most often required in protocols that use scheduling or bandwidth
reservation, as basic synchronization intervals serve to apportion the available bandwidth
to appropriate sender nodes. However, bandwidth reservation and allocation can be ac-
complished in an asynchronous manner, in particular when reservation is requested on a
per-packet basis, while synchronous protocols can be used even with pure contention-based
access. For example, the IEEE 802.15.4 protocol in its beacon enabled, slotted CSMA-CA
mode without guaranteed time slots uses pure contention-based access,yet all transmissions
must be synchronized to the beacon frames periodically sent by the network coordinator
(IEEE 2006).
Synchronization is one of the most important factors that may affect scalability of
the network. As the size of the network grows, synchronization becomes more difficult
and more costly to establish and maintain. In particular, protocols which rely on global
synchronization will suffer the most degradation; for example, it has been shown that the
construction and maintenance of a globally optimal schedule in a multi-level Bluetooth
network (a scatternet) is an NP-complete problem (Johansson et al. 2001).
Presence of a controller and its permanence. Another possible classification criterion is
the presence and permanence of a central network controller or coordinator. While wireless
ad hoc networks, by default, should be able to function without a permanent or dedicated
central controller, quite a few protocols rely on certain monitoring and control functions
that can only be provided by a local or global controller. This is the case with several
of the MAC protocols that use bandwidth reservation, as well as with all of the MAC
protocols which use scheduling. In fact, even some pure contention-based protocols rely
on the presence of a controller for administrative tasks such as time synchronization and
sometimes even node admission.
Again,thepresenceofacontrolleraffectsthescalabilityofthenetwork,astheamountof
work the controller has to do – most of which is administrative and control overhead – must
grow with the number of nodes. Hierarchical decomposition or layering is often used to
reduce this overhead, but it leads to additional problems regarding synchronization and
delays.
Interdependence of the classification criteria. As can be seen, not all of the classification
criteria outlined above are entirely independent of each other; rather, they exhibit a certain
overlap or redundancy. Still, they are useful in the study of MAC protocols, as they tend
to highlight different aspects of their design and operation.
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