Wireless Network Design: Optimization Models and Solution ...

128 Jeff Kennington, Jason Kratz, and Gheorghe Spiride
6.1 Introduction
Wireless local area networks (WLANs) connect two or more computers or devices
using spread- spectrum or OFDM modulation technologies that allow communication
over a limited area. Communication between devices on a WLAN typically
requires low or limited mobility and/or roaming, and the use of unlicensed, free
industrial, scientific, and medical (ISM) spectrum. This is in contrast with cellular
networks, which supports high mobility subscribers and operates in licensed
spectrum bands at much higher transmission power levels. Taking advantage of
ISM spectrum resources at low transmission power means that WLAN technologies
must exhibit robust error tolerance and tolerate interference with other devices
in the same spectrum bands, e.g. cordless phones, microwave ovens, and
garage door openers. WLANs are based on the IEEE 802.11 family of protocols
(http://standards.ieee.org/getieee802/) that specify the set of frequencies that can be
used to construct these wireless networks. The most widespread protocol is IEEE
802.11 b/g, which defines 11 channels in the 2.4 GHz portion of the spectrum as
listed in Table 1. Since a signal occupies approximately 22 MHz of the spectrum,
this results in 3 non-overlapping channels (1, 6, and 11).
Table 6.1 IEEE 802.11 b/g Channels
Frequency Spectrum (GHz) Channel Frequency Spectrum (GHz) Channel
2.412 1 2.442 7
2.417 2 2.447 8
2.422 3 2.452 9
2.427 4 2.457 10
2.432 5 2.462 11
2/437 6 - -
The structure of a WLAN comprises mobile devices (MDs) that communicate
with access points (APs). A wireless access point bridges the communication between
devices connected to the wireless network and a wired network, typically
Ethernet based. The main driving factors for the growing WLAN popularity are the
availability of low cost equipment, ease and speed of installation in a variety of settings,
and the effective and efficient use of ISM spectrum resources. The ease and
speed of installation of wireless access points is due, among other things, to the fact
that most deployments use Power Over Ethernet (PoE) systems to safely transfer
electrical power along with data over standard category 5 cable in an Ethernet network.
Current PoE standards (IEEE 802.3af and 802.3at) allow up to 25W of power
to be distributed to each access point device. This greatly simplifies installation and
reduces costs since deploying a new access point requires a single type of cabling
to be made available (Ethernet CAT 5) without separate electrical work and permitting.
While setting up a WLAN in a residential environment requires installing
one or two APs, wireless LANs that target the enterprise/university campus envi-