Point-to-Multipoint

For networks utilizing low microwave frequencies, a point-to-multipoint architecture will represent
the norm. Point-to-multipoint will enable the network operator to reach the greatest
number of subscribers at the lowest cost and will sharply limit the number of routers and
switches required for the network. Figure 4-3 shows a point-to-multipoint network.
Point-to-multipoint deployments have frequently been advocated for millimeter wave
frequencies as well, but few have actually been built. The problem lies in the topography of
most large cities, which would be the prime markets for such services. Given the high cost of
base stations, minimally $100,000 for equipment alone, operators will probably be unable to
populate the market with more than two or three such facilities. But at the same time it will be
hard to reach all potential customers within the radius of a given base station because of blockage.
Because of the unhappy experiences of first-generation LMDS network operators in
attempting to sign a sufficient number of customers to achieve profitability, many authorities
today have concluded that point-to-multipoint architectures are seldom if ever advisable for
millimeter wave networks. Point-to-multipoint deployments generally involve what are known as sectoral antennas
consisting of an arrangement of highly directional parabolic dishes distributed around a central
pole. Each dish defines a sector, an area where frequencies can be reused. Generally the
higher the frequency, the more sectors can be created and the higher the degree of frequency
reuse. Sectors can also be formed by using what are known as phased array antennas where a
number of omnidirectional pole antennas are grouped together and beams are formed by
varying the phase relationships of their outputs. The phase relationships are electronically
manipulated, and, in the case of adaptive beam forming or “smart antennas,” the system will
adjust beam width and direction on the fly to provide the best signal to a subscriber. Adaptive
beam forming antennas, discussed at great length in Chapter 4, are likely to become the predominant
technology at some point but are still quite expensive today.