METROPOLITAN AREA NETWORKS
MAN serves an adddrop function placing traffic into the MAN and taking traffic from
the node in its "drop" function. For added protection against vandalism, the fiber-optic
cable in question is buried. Several dark fibers are placed along with the active fibers to
serve an expansion capability for future growth.
The ring formation of the fiber-optic MAN is based on one of several possible
ANSI/Telcordia standards under the umbrella term "automatic protection switching"
(Refs. 3, 4). These techniques can improve availability by a factor of 100 or better. Fiber
offers two approaches for future growth: (1) Add spare dark fibers to handle growth
and/or (2) use WDM techniques particularly in the 1550-nm band when the user needs
additional capacity. A fiber-optic MAN is much less versatile if assets have to be moved
or topology changed when compared to its IEEE 802.11 radio-wireless counterpart. Once
the maximum bit rate of a single thread is reached for an IEEE 802.11 system, we can
expand no further unless a new access point is added.
IEEE 802.11 SYSTEM
The IEEE 802.11a system operates in the 5-GHz band employing orthogonal frequency
division multiplex modulation (OFDM) offering data rates from 6 to 54 Mbps. IEEE
802.11b specifies operation in the 2.4-GHz band providing data rates of 1 or 2 Mbps of
the initial 802.11b specification and an added capability for 6.6 Mbps and 11 Mbps with
CCK (complementary code keying) modulation which makes more efficient use of the
The 802.11 series are commonly referred to as wireless LANs (WLANs). These
WLANs can be used either to replace wired LANs or as an extension of the wired
LAN infrastructure. In the WLAN arena we can have a basic service set (BSS) consisting
of two or more wireless nodes or stations (STAs) which have recognized each other and
have established communications. In the most basic form, stations communicate directly
with each other on a peer-to-peer level sharing a given call coverage area. This type of
network is often formed on a temporary basis and is commonly referred to as an ad hoc
network, or independent basic service set (IBSS).
In most instances, the BSS contains an access point (AP). The main function of an
AP is to form a bridge between wireless and wired LANs. The AP is analogous to a
base station used in cellular phone networks. When an AP is present, stations do not
communicate on a peer-to-peer basis. All communication between stations or between a
station and the wired network client go through the AP. APs are not mobile, and they
form part of the wired network infrastructure. A BSS in this configuration is said to be
operating in the infrastructure mode.
The extended service set (ESS) illustrated in Figure 13.1 consists of a series of over-
lapping BSSs, each containing an AP and are connected together by means of a DS
(distributed system). Although the DS could be any kind of network, it is almost invari-
ably an Ethernet LAN. Mobile nodes can roam between APs, and seamless campus-wide
coverage is possible.
The MAC layer in the IEEE 802.11 standard is a set of protocols that is responsible
for maintaining order in the use of a shared medium. The 802.11 standard specifies
a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this
protocol, when a node receives a packet to be transmitted, it first listens to ensure no
other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise,
it chooses a random "backoff factor" that determines the amount of time the node must
wait until it is allowed to transmit its packet. During periods in which the channel is