The IEEE 802.11 protocol (O’Hara and Petrick 1999) is, strictly speaking, intended for
wireless local area networks (LANs), rather than wireless ad hoc networks. However, it
is interesting to examine it in some detail, mainly on account of its ubiquity, and because
it uses most of the main concepts which are reused in many MAC protocols for ad hoc
networks. The protocol covers the functional areas of access control, reliable data delivery,
and security; in the following we will focus on the first two areas, as the last one (security)
is beyond the scope of this chapter.
Reliable transfer is achieved through the use of special acknowledgment (ACK) pack-
ets or frames, sent by the destination node upon successfully receiving a data packet.
Medium access is regulated in two ways, the first of which is a distributed contention-
based mechanism known as Distributed Coordination Function (DCF), which does not
require a centralized controller. The DCF, based on the CSMA protocol described above,
operates as follows. The node that wants to transmit a packet first performs the clear chan-
nel assessment procedure, i.e., it listens to the medium, for a time equal to Interframe
Space (IFS). If the medium is found to be clear (or idle) during that time, the node can
transmit its packet immediately; otherwise, i.e., if another transmission is in progress, the
node waits for another IFS period. If the medium remain idle during that period, the node
backs off for a random interval and again senses the medium. During that time (referred
to as the backoff window or contention window), if the medium becomes busy, the back-
off counter is halted; it resumes when the medium becomes idle again. When the backoff
counter expires and the medium is found to be idle, the node can transmit the packet.
A possible scenario in which this procedure is applied is shown in Figure 1.1. There are
several points worth mentioning. First, the backoff interval is chosen as a random number
from a predefined range. After each collision, the range is doubled in order to reduce the
likelihood of a repeated collision. After each successful transmission, the range is reset to
its initial value, which is typically small. This approach is known as binary exponential
backoff, or BEB (Stallings 2002). In this manner, the protocol ensures a certain level of
load smoothing in case of frequent collisions caused by heavy traffic.
Second, in order to enhance reliability and avoid the hidden/exposed terminal prob-
lems to a certain extent, the RTS/CTS handshake – well known from wired communica-
tions – may optionally be used. In this case, the node that wants to send a data packet first
sends a Request To Send (RTS) packet to the designated receiver which, if ready, responds
with a Clear To Send (CTS) packet. Both RTS and CTS packets contain information about IFS
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