wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Ad Hoc Networks 7-7
it will sense the medium as free and sends data to node-2 which will result in a collision. This is known as
hidden terminal problem; node-1 and node-3 are hidden from each other. This issue is solved by requestto-send/clear-to-send
(RTS/CTS) mechanism. Now, assume that node-3 knows by RTS/CTS mechanism
that node-1 and node-2 are communicating; it will refrain from communicating with node-4 as well,
though node-4 is beyond the range of node-2 and thus channel capacity will be unnecessarily wasted.
This is known as exposed terminal problem.
7.2.3.1 random Access Schemes
In random access schemes (also know as contention-based scheme), nodes contend for the shared
channel with the neighboring nodes (nodes within interference range) and the winning nodes get
hold of the medium. Such schemes cannot guarantee QoS because access to the channel is not guaranteed
and is random. In ALOHA [N70], a node simply sends data as soon as it is available. Slotted
ALOHA introduces time division multiple access (TDMA)–like time-slots to reduce the number of
collisions. Carrier sense multiple access (CSMA)–based schemes further enhance the performance by
carrier sense mechanism. The performance of all such schemes is adversely affected by hidden and
exposed terminal problem.
7.2.3.2 reservation-Driven Contention-Based Access
In order to overcome the problems of hidden and exposed terminal problems, dynamic reservation-based
protocols are designed, which operates on the basis of RTS/CTS mechanism. In such protocols, a node
initiates the reservation process if it has some data to send. The sending node sends an RTS packet to the
next hop. The next hop in turn replies by a CTS message. Such a CTS is heard by other nodes in the vicinity,
and they keep quiet unless the data are actually sent by the initiating node. MAC layer defined by IEEE
802.11 standard includes distributed coordination function (DCF) and point coordination function (PCF).
DCF is based on CSMA/CA and uses RTS-CTS-DATA-ACK mechanism for data transmission, while PCF
is designed for infrastructure-based networks. Other important protocols in this category includes multiple
access collision avoidance (MACA) [K90], MACA for wireless LANs (MACAW) [BDSZ94], and floor
acquisition multiple access (FAMA) [FG95], which are all based on CSMA/CA.
7.2.3.3 Scheduling-Driven Contention-Based Access
Such protocols are designed to provide a certain level of QoS. Scheduling can be a function of different
parameters, for example, remaining node energy, traffic loads, and/or bound on packet delays. Some
of the scheduling-driven MAC schemes include distributed priority scheduling and medium access
[KLSSK02] and distributed laxity-based priority scheduling scheme [KMS05].
7.2.4 Physical Layer
Physical layer translates communication requests from the upper layers into hardware-specific operations
to enable transmission or reception of signals. These operations normally involve time and frequency
synchronization while also dealing with harsh time variant and frequency-selective wireless
channels. Besides, the performances can be severely compromised by interference originating from
other terminals operating on nonorthogonal resources, thereby requiring efficient receive strategies
which give good complexity performance trade-off. Here, we give an overview of physical layer specification
for only two main standards supporting ad hoc wireless networks, i.e., the IEEE 802.11 standard
for WLANs [802.11] and the Bluetooth specifications for short-range wireless communications
[B01,FBP98]. The first one not only allows single-hop WLAN ad hoc network but can also be extended to
multihop networks covering areas of several square kilometers, while the second one can only be used
to build smaller scale ad hoc wireless body and PANs.
IEEE 802.11 is the first wireless local area network standard with data rates of up to 2.Mbps [802.11].
The original standard has then been extended to 802.11b, 802.11a, and 802.11g which operate in the
© 2011 by Taylor and Francis Group, LLC