WiMax Operator's Manual

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CHAPTER 7 ■ SERVICE DEPLOYMENTS OVER PUBLIC WIRELESS MANS 165 Achieving QoS in Broadband Wireless Packet Networks As indicated many times previously in this text, the core packet standards in common use today (namely, TCP/IP and Ethernet) were both originally intended for transmitting best-effort traffic only, not traffic requiring stringent QoS. As a consequence, equipment manufacturers and standards bodies have made a great deal of effort in recent years to enable packet networks to handle QoS through both ancillary protocols and revisions of the core protocols. The 802.16 standard has a number of provisions for ensuring QoS, and it permits the use of a wide range of additional protocols for further ensuring that quality levels will be met. In addition to the industry standards supporting QoS, there are a number of other strategies for achieving it, including overprovisioning of bandwidth, data compression, and special routing protocols combined, often as not, with Internet diagnostics and route analysis software. I will cover each of these later in this chapter. It should be clearly understood that establishing QoS is much easier within the confines of a metropolitan network than it is across great distances where the transmission must traverse several backbone networks on the way to its final destination. If, for instance, network operators want to send video material cached at the central office to a local subscriber, they can simply provision enough bandwidth to support the transmission and can rest assured that the quality will be acceptable, barring some catastrophic interruption of the link. If, on the other hand, operators are hosting a videoconference that spans the continent, they are at the mercy of those parties controlling intervening network segments. They can contract with a longdistance service provider that will agree to support a service-level agreement, but they cannot personally allocate bandwidth across the network to ensure that all relevant performance parameters will be met. The QoS provisions within the 802.16 standard are generally sufficient to provide for good QoS on applications running entirely within the network, including voice- and broadcastquality video. They are nearly useless, however, in the long haul, and the network operator must utilize additional protocols and, in most cases, forge agreements with other service providers controlling intervening data paths and network nodes. 802.16 Provisions for QoS At the physical layer, 802.16 is actually a circuit-based protocol. It allows for channel divisions based upon frequency divisions or time slots, with time slots expected to predominate in the radios conforming to the standard. Recurrent time slots separated by intervals of other time slots can be reserved entirely for certain subscribers or shared among numbers of subscribers. When sequences of slots are reserved, they function analogously to the time slots or temporal channels in a SONET network. Such strict reservation of the bandwidth in one set of slots provides inherent QoS for a subscriber, albeit at the cost of efficiency, as is the case with all session- or circuit-based approaches. Depending on the types of services the network operator has settled on, some sequences of slots may be used to provide circuit service while others will be allocated to best effort. However, it should be understood that circuit-like determinacy can be delivered by means other than reserving time slots for the exclusive use of a single party. The 802.16 standard also provides inherent support for ATM, covered next, and IP 6, which provides for flow labeling somewhat in the manner of MPLS, also covered next. In and of itself, IP 6 does not provide extensive support for QoS, however, and cannot be considered adequate absent the use of additional protocols along with it.
166 CHAPTER 7 ■ SERVICE DEPLOYMENTS OVER PUBLIC WIRELESS MANS Major Networking Standards for Supporting IP QoS To a considerable extent, the standards for achieving QoS with IP transmissions are complementary in the sense that none represents a complete solution. Frequently two or more protocols are used together. IP over ATM The 802.16 standard fully supports ATM, which itself offers excellent support for QoS on quite a number of applications. ATM is also supported almost universally by long-distance service providers, and unlike the case with MPLS, QoS functionality will be fully enabled end to end across the network when transmissions are directed through ATM core switches. ATM is, however, less efficient and less flexible than MPLS, and ATM services have not been competitively priced in the past. I consider ATM to be a legacy technology that is not ideally suited to the new service provider striving to compete on the basis of value with entrenched incumbents. Commonly Employed Ancillary IP Protocols for QoS From the middle 1990s onward, when the ascendancy of IP was becoming obvious, certain IP advocates sought to redress its equally obvious shortcomings in respect to determinacy by creating supplementary protocols. The fact that so many of these have been created would seem to argue the individual inadequacy of any one of them, and in fact they are frequently used in multiples. The protocols I will consider include RSVP, IntServ, DiffServ, IP Precedence, RTP, RTSP, and MPLS, among others. Most such protocols perform one of three functions. First, they request a certain allocation of bandwidth from the network deemed necessary for the execution of the application they are intended to support, and then they attempt to establish separate streams for individual applications where certain minimum network performance requirements will at all times be upheld. Thus, real-time video may go into one stream, toll-quality voice into another, and so on. Second, they append extra information to the packet in the form of an additional header or label. This assigns the packet to the correct stream and also requests special treatment by any network element the packet subsequently encounters; in effect it is a service classification label. Finally, they invoke specific mechanisms at the router for dealing with congestion and delay where different instructions for emptying queues, dropping packets, and retaining packets within queues over time are applied to different streams. It should be understood that the protocols themselves form only one aspect of a unified approach to QoS and that the use of the protocols as Band-Aids on an underengineered network is not going to produce satisfactory results. The protocols enable bandwidth to be used efficiently in supporting various services but they do not create bandwidth where it does not exist. In a sense compression can do that, but that is another issue for another section. RSVP Resource Reservation Protocol (RSVP) is a protocol for requesting bandwidth from routers along the path that a transmission will take. It does not guarantee that adequate resources will be provided, however, because one or more of the routers may themselves either lack adequate resources or may not have the protocol enabled. RSVP itself is not a routing protocol, and it makes no determination regarding the path that packet will take. Instead, it is a flow-based
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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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100 CHAPTER 4 ■ SETTING UP PHYSIC
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102 CHAPTER 5 ■ STRATEGIES FOR SU
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Page 152 and 153: CHAPTER 6 ■ ■ ■ Beyond Access
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Page 160 and 161: CHAPTER 6 ■ BEYOND ACCESS 139 ins
Page 162 and 163: Figure 6-4. Wireless public network
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Page 168 and 169: Beyond the Central Office CHAPTER 6
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Page 210 and 211: CHAPTER 9 ■ NETWORK SECURITY 189
Page 216 and 217: Index ■Numbers 2.4GHz to 928MHz b
Page 218 and 219: asic access and best-effort deliver
Page 220 and 221: ■D dark fiber, features of, 75-76
Page 222 and 223: harmonics, impact on AC power, 188
Page 224 and 225: challenges to, 85-86 and NLOS, 121
Page 226 and 227: PONs (passive optical networks), fe
Page 228 and 229: signal diversity, providing with ad
Page 230 and 231: ■V VDSL and entertainment service
Page 233: forums.apress.com FOR PROFESSIONALS
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