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.
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.
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.