WiMax Operator's Manual

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CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE 71 central office and should not contemplate doing so in spite of the excellent facilities maintained by the incumbents. Additional Base Stations Once network operators have secured central office facilities, they must then turn their attention to the matter of access points. Initially, the network may have but a single access point that will be situated within or adjacent to the central office, but, as the network expands, additional access points will be required. Determining precise requirements in this regard is no simple matter, but it is essential to the success of the network. Several factors should play a role in the optimal siting of access points in the network, including the nature of the equipment (line-of-sight or NLOS and beam-forming antenna, or lack thereof), the effective range of the access point transmitter, the number of potential customers within range of the transmitter, the extent to which frequency reuse can be achieved within a given area, and the availability of suitable locations at an affordable price. None of these factors should be considered in isolation. A network based on a large number of NLOS subscribers is going to have to put the subscribers closer to the base stations—in other words, more base stations will be needed. Density of the subscriber population and the reach of the transceivers will also have a bearing on where base stations are situated. The first Multichannel Multipoint Distribution Service (MMDS) networks that transmitted over licensed spectrum at relatively high power and could maintain links over distances exceeding ten miles could serve thinly scattered customers over a large area with a single base station. Network operators using unlicensed spectrum and transmitting at 1 watt maximum power are unlikely to be able to duplicate that and instead will require a number of base stations for the same area and the same customer density. I want to generalize in this regard and say that at such-and-such customer density over so many square miles at 5.8 gigahertz (GHz) this many base stations will be required, but it is never that simple, because the same equipment may perform differently in different radio frequency (RF) environments. Wireless network operators add base stations reluctantly—doing so only when the reach and/or capacity of the existing base stations is insufficient to serve what is seen as a sizable number of potential customers. One knows when one is approaching that point because of increasing network congestion, and one then sets out to find a location of the new base station. Insofar as possible, the network operator should attempt to plot traffic patterns at least two years into the future, however, and should plan the location of future base stations long before they are actually required. This brings you to the final topic regarding base stations: identifying suitable locations for base stations. Particularly in the case of line-of-sight equipment, one wants a base station with a minimum of obstructed pathways to potential customers. In most cases, the base station antenna should occupy an elevated position where sight lines extend well over the tops of the tallest intervening structures. In recent years the tendency in American cities has been to place antennas for wireless networks on towers specially constructed for the purpose and owned by companies such as American Tower and SBA, which have developed businesses based on the lease of space to network operators. The rise of such companies has been the direct result of the proliferation of mobile telephone cell sites and the determination on the part of municipal governments to
72 CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE limit their number. By obliging commercial operators to collocate on few approved sites, that proliferation has been checked, and, coincidentally, a new industry has emerged. Broadband wireless operators can certainly elect to occupy such a site, but they may not necessarily be welcome or able to afford the lease if they are. Tower owners do not charge uniform rates, but a rough average is $1,000 a month. This is nothing to a cellular operator with tens of thousands of customers, but for a struggling wireless broadband operator it is a sig- nificant expenditure especially when that operator is expected to sign a 30-year lease in the bargain. Another issue with towers is that they are primarily intended for hosting lowerfrequency mobile services rather than networks operating above 2GHz, and they may not provide optimal elevation since the mobile services lack line-of-sight requirements. If the broadband wireless network operators are disinclined or unable to gain access to a tower, they can always attempt to negotiate roof rights on a multistory building. Here it is difficult to generalize, because rates can vary tremendously. In some instances, if the network operator is willing to provide data services to the tenants of the structure in question, recurrent fees may be reduced or even waived. Backhaul Backhaul refers to the connection from an access point or base station back to a central office facility. In a broadband wireless network, backhaul, ironically enough, generally occurs over wireline connections, although wireless links are perfectly possible as well. Obtaining backhaul connections at reasonable rates is essential if the network is to operate profitably, particularly as the network expands and more and more backhaul is required. For this reason the operator must determine the means of obtaining backhaul for given locations before a single network element is put in place. One simply cannot build first and then start casting about for backhaul solutions. Backhaul itself is a rather complex subject. Several distinct physical media have been used for providing it, and a number of different arrangements are possible with companies offering backhaul capacity. One needs to explore all options during the planning stage in order to make certain that the essential backhaul component is being obtained in the most costeffective manner. Backhaul itself represents the aggregation of network traffic, the sum of each transmission to or from a base station and an individual subscriber node. For this reason the capacity of the backhaul must be significantly greater than the capacities of the individual access airlinks. Nevertheless, the capacity need not and should not equal that which would be required if all subscribers in a cell were transmitting simultaneously, because such an eventuality is highly unlikely. The capacity of the backhaul can be as little as a tenth of the aggregate capacity of all individual airlinks, though a four-to-one or six-to-one ratio is more prudent. If, for instance, 100 customers are each provided with a 10 megabits per second (Mbps) connection within a single cell, then the combined capacity of all of the individual airlinks is 1 gigabits per second (Gbps). A 100Mbps backhaul could suffice in that situation, though 250Mbps would be ideal. Quasi-Backhaul It should be noted here that there are a couple of ways to set up wireless broadband networks requiring little or no backhaul. These involve two variant network architectures, the mesh network and the ring or point-to-consecutive-point network, both of which will be discussed at further length later in this chapter.
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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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Page 55 and 56: 34 CHAPTER 3 ■ STRATEGIC PLANNING
Page 85 and 86: 64 CHAPTER 4 ■ SETTING UP PHYSICA
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Page 123 and 124: 102 CHAPTER 5 ■ STRATEGIES FOR SU
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122 CHAPTER 5 ■ STRATEGIES FOR SU
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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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