Wireless MANs offer connections between buildings and users within a city or campus area through several system configurations. In most cases, the wireless MAN beams RF or infrared light from one point to another using directive antennae.
A point-to-point solution uses RF or infrared signals that utilize either semidirectional or highly directional antennae to extend range across metropolitan areas, such as college campuses and cities. Range can be as high as 30 miles for RF systems using highly directional antennae. Figure 6-5 illustrates a point-to-point wireless MAN system.
A medical center, for example, can use a point-to-point wireless MAN to provide a communications link between the main hospital and a remote clinic within the same city. This resulting system, however, does not provide as much flexibility as point-to-multipoint solutions. However, if there is a need to connect only a couple sites, the cost of implementing a point-to-point system is less compared to a point-to-multipoint system.
A typical point-to-multipoint link (see Figure 6-6) utilizes a centralized omnidirectional antenna that provides a single transceiver point for tying together multiple remote stations. For example, a building within the center of a city can host the omnidirectional antenna, and other nearby metropolitan-area buildings can point directional antennae at the centralized location. The central transceiver receives and retransmits the signals.
A strong advantage of the point-to-multipoint wireless MAN is that it makes the addition of new connections easy. In fact, this approach can be less expensive compared to point-to-point systems when there are multiple sites to interconnect or connect to a central location. For example, a company headquarters having many remote warehouses and manufacturing plants within the same city or rural area would benefit from a point-to-multipoint system.
A packet radio system (see Figure 6-7) utilizes special wireless routers that forward data contained within packets to the destination. Each user has a packet radio NIC that transmits data to the nearest wireless router. This router then retransmits the data to the next router. This hopping from router to router occurs until the packet reaches the destination. This mesh type networking is not new. Amateur Ham radio operators have used it for decades, and companies such as Metricom have been deploying these types of systems in cities for nearly 10 years.
A city government might want to deploy a packet radio system to offer wireless connectivity for supporting applications through the entire city area. The installation of routers in strategic places through the city provides the necessary infrastructure. There's no need for wires for interconnecting the routers. Each router is capable of receiving and retransmitting?hopping? the packets to their destination.
This form of networking is also survivable. If one router becomes inoperative, perhaps because of a lightning strike or sabotage, adaptive routing protocols automatically update routing tables in each router so that data packets will avoid traversing the inoperative router.