The Large Network Solution

Point-to-multipoint links in excess of 1,500 feet with ordinary equipment at the client side are very possible. Using high-gain antennas, sensitive receivers, and amplifiers if necessary, it is possible to achieve Ethernet-like speeds over 20+-mile point-to-point links. An experiment proved that it is theoretically possible to drive 802.11b signals well over 20 miles, using stock equipment. In fact, a 72-mile link from San Diego to San Clemente Island has been established by 'someone on earth' with some specialized 802.11 equipment on the 2.4-GHz band. In summary, 802.11b, by itself, is not limited to a range of 100m. Its maximum range is in excess of 20 miles. A comparison is that of the telephone central office, where the maximum range of the signal over copper wire is 18,000 feet or 3 miles without a repeater. It could be argued that the maximum, unboosted range of 802.11b exceeds that of the PSTN.

While a point-to-point 802.11b connection may have a range of 20 miles, and a point-to-multipoint connection that is somewhat shorter, building a wireless network to compete with the PSTN is considerably more complicated. Issues revolving around bandwidth sharing and frequency contention require a multitiered strategy for building a wireless metropolitan-area network (WMAN) to replace the PSTN in a given municipality.

Overcoming limitations of range can be achieved through proper architectural planning of a wireless network. Four elements of network architecture can be employed to extend the maximum range of 802.11b and its associated wireless protocols to cover an entire metropolitan area. First, a WMAN is fed from an IP backbone at a high bandwidth, say, 100 Mbps. This WMAN would operate at a licensed frequency to ensure a high quality of transmission devoid of interference. The chief subscribers of the WMAN would be wireless Internet service providers (WISPs). The WMAN would then feed lesser networks, the wireless wide-area networks (WWANs). The WWANs could operate at the 802.11a bandwidth (54 Mbps) at a frequency in the 5.8-GHz range. Subscribers of the WWAN would include large enterprises and smaller WISPs. The WWAN would, in turn, feed WLANs. WLANs would feed residences and small businesses. Wireless personal area networks (WPANs) would feed off WLANs to serve components within a given residence. Finally, an ad hoc peer-to-peer network, consisting of subscriber devices, intelligent access points, and wireless routers can extend the network even further with little infrastructure cost. Some new market entrants offer mobile applications of this technology. Fixed wireless includes local multipoint distribution service (LMDS), multichannel multipoint distribution service (MMDS), U-NII systems, and similar networks. Wireless cable usually refers to MMDS systems used to deliver television signals such as the instructional television fixed service (ITFS).

Covering Metropolitan Area

Two basic network topologies are supported by these systems. The simplest is a point-to-point system providing a high-speed wireless connection between two fixed locations. Bandwidth is not shared, but links typically require line of sight between the two antennas. The second topology is a point-tomultipoint network in which a signal is broadcast over an area (called a cell ) and communicates with fixed subscriber antennas in the cell. Because bandwidth in the cell is finite and is shared among all users, performance may be a concern in high-density cells. Systems of different frequencies may be combined to cover an area where terrain or other obstructions prevent full coverage. Other than frequency, the main difference between fixed wireless systems and cellular, WLAN, and WPAN networks is the mobility subscriber equipment. There has been some discussion about adding support for mobile subscriber equipment to fixed wireless systems. The addition of mobility support would enable these BWA systems to potentially function as fourth generation (4G) cellular networks, delivering subscriber speeds of several megabits.