Wireless MAN Technologies
Many of the wireless MAN installations utilize proprietary
technologies operating in licensed bands. The licensing avoids potential RF
interference by ensuring that nearby systems are using different frequencies.
Even though end users follow a process of obtaining a license, it isn't time
consuming because it is only done once. The problem, however, is that licensed
band components are expensive.
As a result, companies prefer to utilize equipment based on
standards, which generally results in fewer initial costs and lower support
costs. If a manufacturer no longer supports a particular product, the company
can go to a different manufacturer when modifying the network. Standards
certainly improve the longevity of the system.
802.11 and Wi-Fi
Many companies deploy wireless MANs using wireless LAN
standards, such as 802.11 and Wi-Fi. Chapter 5, "Wireless LANs: Networks for Buildings and
Campuses," gives details on these standards. The difference is that a wireless
MAN utilizes directional antennae to establish a point-to-point link between
fixed points in the system. The hardware includes a wireless bridge that
implements wireless LAN standards.
The use of wireless LAN hardware for metropolitan-sized
networks decreases costs, but 802.11 has performance limitations when supporting
larger numbers of users needing guaranteed bandwidth. In addition, RF
interference is often a significant problem with 802.11 when covering large
areas because of license-free operation. A competitor might install an 802.11
network that interferes with yours, and users will suffer due to sporadic
performance. There is no solution because there are no legal grounds to remedy
the situation.
802.16
The IEEE 802 group initiated the IEEE 802.16 working group to
create standards for broadband wireless access in order to offer a high-speed,
high-capacity, low-cost, scalable solution to extend fiber-optic backbones. The
first IEEE 802.16 standard, published in
April 2002, defines the Wireless MAN Air Interface for wireless MANs. These
systems are meant to provide network access to homes, small businesses, and
commercial buildings as an alternative to traditional wired connections.
With wireless base station equipment targeted at under $20,000,
802.16 can economically serve up to 60 customers with T-1 (1.5 Mbps) speed
connections. That's really attractive to the typical WISP that's short on cash.
In addition, 802.16 can provide a feasible backhaul for connecting wireless LAN
hotspots together.
802.16 supports point-to-multipoint architecture in the 10–66
GHz range, transmitting at data rates up to 120 Mbps. At those frequencies,
transmission requires line of site, and roofs provide the best mounting
locations for base and subscriber stations. The base station connects to a wired
backbone and can transmit wirelessly up to 30 miles to a large number of
stationary subscriber stations, possibly hundreds.
To accommodate non-line-of-site access over lower frequencies
for locations without line of site, IEEE published 802.16a in January 2003,
which includes support for mesh architecture. 802.16a operates in the licensed
and unlicensed frequencies between 2–11 GHz using Orthogonal Frequency Division
Multiplexing (OFDM).
The 802.16 MAC layer supports many different physical layer
specifications, both licensed and unlicensed. Through the 802.16 MAC, every base
station dynamically distributes uplink and downlink bandwidth to subscriber
stations using time-division multiple access (TDMA). This is a dramatic
difference from the 802.11 MAC, with current implementations operating through
the use of carrier sensing mechanisms that don't provide effective bandwidth
control over the radio link.
The next step for the IEEE 802.16 working group is to add
portability and mobility to the standard. In March 2002, the group began the
802.16e study group on Mobile Broadband Wireless Access. This group will address
many different mobility issues, including providing connectivity to moving
vehicles within a base station's sector.
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