It is possible to have point-to-multipoint links in excess of over 1,500 feet with ordinary equipment at the client side. Using high-gain antennas, sensitive receivers, and
Figure 3-2: The range of 802.11b exceeds 20 miles.
In summary, 802.11b, by itself, is
limited to a range of 100
 Rob Flickinger, "A Wireless Long Shot," O'Reilly Network, www.oreillynet.com/pub/a/wireless/2001/05/03/longshot.html, May 3, 2001.
 Bob Brewin, "San Diego Wireless Net Installs 72-Mile, 2.4-GHz Link," Computer World, www.computerworld.com/mobiletopics/mobile/story/0,10801,75830,00.html, November 12, 2002.
Although a point-to-point 802.11b connection may have a range of 20 miles and a point-to-multipoint connection may have somewhat less range, building a wireless network to
Overcoming limitations of range can be achieved through properly planning the architecture 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
Figure 3-3: Covering a metro area with WMANs, WWANs, WLANs, and WPANs
The WMAN encompasses a range of radio- and laser-based technologies
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-to-multipoint network where a signal is broadcast over an area (called a
Other than frequency, the main difference between fixed wireless systems, and cellular, WLAN, and WPAN networks is the mobility of 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
LMDS is a fixed wireless, radio-based technology. In North America, LMDS operates in the 28 to 31 GHz frequency range, but it may
The network topology of LMDS uses a central transmitter that sends its signal over a cell with a radius of 5 km or less. Antennas are usually placed on rooftops for line of sight to the central transmitter. This is because first-generation (1G) LMDS equipment uses radio technology that is affected by hills, walls, trees, and other physical barriers. This limitation may be lessened as equipment starts to adopt more advanced spectrum utilization techniques such as orthogonal frequency division multiplexing (OFDM).
As a high-frequency outdoor radio technology, LMDS performance and range vary depending on weather conditions. LMDS has a range of less than 5 km and supports gigabit speeds, although services are usually
An 802.16 wireless service provides a communications
IEEE 802.16 addresses first-mile applications of wireless technology to link commercial and residential
Working Group 802.16 is now completing a draft of the IEEE 802.16 Standard Air Interface for Fixed Broadband Wireless Access Systems. The document includes a flexible
Media Access Control
(MAC) layer. The
For transmission from subscribers to a base station, the standard uses the
Demand Assignment Multiple Access-Time Division Multiple Access
(DAMA-TDMA) technique. DAMA is a capacity assignment technique that adapts as needed to respond to demand changes among multiple stations. TDMA is the technique of dividing time on a channel into a sequence of
With DAMA-TDMA, the assignment of slots to channels varies dynamically. For transmission from a base station to subscribers, the standard specifies two modes of operation: one targeted to support a continuous transmission stream (mode A), such as audio or video, and one targeted to support a burst transmission stream (mode B), such as IP-based traffic. Both are TDM schemes.
Above the physical layer are the functions associated with providing service to subscribers. These functions include transmitting data in frames and controlling access to the shared wireless medium, and are grouped into the MAC layer. The MAC protocol defines how and when a base station or subscriber station may initiate transmission on the channel. Because some of the
In the downstream direction (base station to subscriber stations), only one transmitter is available and the MAC protocol is relatively simple. In the upstream direction, multiple subscriber stations
The sequence of time slots across multiple TDMA frames that are dedicated to one subscriber forms a logical channel, and MAC frames are transmitted over that logical channel. IEEE 801.16.1 is intended to support individual channel data rates from 2 to 155 Mbps.
Above the MAC layer is a convergence layer that provides functions specific to the service being provided. For IEEE 802.16.1, bearer services include digital audio/video multicast, digital telephony, ATM, Internet access, wireless trunks in telephone networks, and frame relay. Figure 3-4 depicts how the 802.16 protocol works for WMANs. 
Figure 3-4: A WMAN
In a WMAN, the reliability of the network can be ensured by implementing consecutive point network (CPN) technology. Like a Synchronous Optical Network (SONET) fiber ring, the data flow of the network around the wireless ring would reverse flow in the event of a disruption in the network (see Figure 3-5). This ensures that only a limited part of the network is down due to a disruption.
Figure 3-5: CPNs-note that like a SONET ring, data flow reverses itself in case of disruption in the network.
 James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 55-56.
 James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 58-59.
 William Stallings, "IEEE 802.16 for Broadband Wireless," Network World, www.nwfusion.com/news/tech/2001/0903tech.html, September 3, 2001.
 Roger Marks, "Broadband Access: IEEE Takes on Broadband Wireless," EE Times, www.eetimes.com/story/OEG20010606S0008, January 4, 2002.
 William Stallings, IEEE 802.16 for Broadband Wireless, Network World , www.nwfiision.com/news/tech/2001/0903tech.html, September 3, 2001.