WiMax Operator's Manual

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CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE 79 T1 Backhaul over Copper I mention this unsatisfactory expedient, T1 backhaul over copper, only because it has been used in the past. A T1 connection affords the user a throughput rate of only 1.5Mbps per second, a tiny fraction of the speeds supported by 802.16. Aggregations of T1s, which are offered by many telcos, are of course better, but they are still woefully inadequate. I simply do not see how a network operator can possibly succeed with such a legacy solution and strongly advise operators to exclude T1 categorically as a method of backhaul. Determining Basic Network Architecture I have already mentioned the basic network architectures: point-to-point, point-to-consecutivepoint, point-to-multipoint, and mesh. While hybrids consisting of one or more topologies are certainly possible, generally a single architecture is chosen for the entire locale to be served, and the choice reflects not just technical considerations but the nature and composition of the local market. The band or bands in which the network operator is transmitting limits the choice of network architecture. For example, transmissions in the highest band now commonly in use, 59GHz to 64GHz, can never be point-to-multipoint because spreading the beam to accommodate such an architecture would result in unacceptably short range and also a failure to reach many potential customers because of strict line-of-sight considerations. Similarly, a mesh architecture, while technically feasible in the millimeter wave frequencies, is apt to be quite expensive because each node essentially becomes a base station (one U.K. manufacturer, Radiant Networks, now out of business, did make a 28GHz mesh product, however). Finally, point-to-point connections are rarely encountered below 3GHz though they certainly are possible, because the bandwidth lends itself to point-to-multipoint deployments and the profit potential of the latter is generally much greater than that associated with the sale of the total bandwidth to a single customer via a narrow-beam bridge link. 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.
80 CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE Figure 4-3. A point-to-multipoint network 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. Point-to-Point Simple point-to-point links are the legacy architecture and arguably are not networks at all since they do not permit any-to-any connectivity. This is the architecture that has been most used in the upper microwave regions within the last few years, though it has a long prior history in telecommunications. For years and even decades, microwave operators have set up isolated links for LAN extension, for connecting remote sites with the PSTN, and for cellular backhaul. Indeed it is often forgotten today that microwave point-to-point links carried a large fraction of long-distance telephone and data traffic prior to the introduction of fiber-optic networks in the 1980s. Today point-to-point backhaul and point-to-point access for business high-rises comprise a solid but slowly declining business, one that represents a poor utilization of network resources. Simply put, a base station should serve a multitude of customers if it is to pay for itself. Where point-to-point connections are indicated by the nature of the equipment, as in the 60GHz band, then the best choice is to construct a sort of quasi-point-to-multipoint network topology by placing multiple radios at the same site and sharing one router or switch among all of them.
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WiMax Operator’s Manual Building
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This book is dedicated to my wife.
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Contents About the Author . . . . .
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■CONTENTS ix Performing Site Surv
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■CONTENTS xi Cyberwarfare . . . .
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About the Technical Reviewer ■ROB
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Introduction Broadband wireless has
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CHAPTER 1 ■ ■ ■ Wireless Broa
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CHAPTER 1 ■ WIRELESS BROADBAND AN
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14 CHAPTER 2 ■ ARCHITECTING THE N
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CHAPTER 6 ■ ■ ■ Beyond Access
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CHAPTER 6 ■ BEYOND ACCESS 133 Rou
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CHAPTER 6 ■ BEYOND ACCESS 135 (Qo
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CHAPTER 6 ■ BEYOND ACCESS 137 The
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CHAPTER 6 ■ BEYOND ACCESS 139 ins
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Figure 6-4. Wireless public network
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CHAPTER 6 ■ BEYOND ACCESS 143 sto
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Specialized Content Distribution Pl
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Beyond the Central Office CHAPTER 6
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CHAPTER 6 ■ BEYOND ACCESS 149 out
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CHAPTER 6 ■ BEYOND ACCESS 151 som
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CHAPTER 7 ■ ■ ■ Service Deplo
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CHAPTER 7 ■ SERVICE DEPLOYMENTS O
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178 CHAPTER 8 ■ NETWORK MANAGEMEN
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CHAPTER 9 ■ ■ ■ Network Secur
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CHAPTER 9 ■ NETWORK SECURITY 189
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Index ■Numbers 2.4GHz to 928MHz b
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asic access and best-effort deliver
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■D dark fiber, features of, 75-76
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harmonics, impact on AC power, 188
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challenges to, 85-86 and NLOS, 121
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PONs (passive optical networks), fe
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signal diversity, providing with ad
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■V VDSL and entertainment service
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