WiMax Operator's Manual

168 CHAPTER 7 ■ SERVICE DEPLOYMENTS OVER PUBLIC WIRELESS MANS
of the broadband wireless access provider but of the owner of backbone fiber and/or Internet
access points.
Furthermore, several protocols exist for managing connections end to end across the
backbone, including Resource Allocation Protocol (RAP) and the aforementioned RSVP.
Although the signals may originate in the metro, the actual resource allocation will take place
on the backbone.
Making Sense of QoS Protocols
QoS protocols, as previously mentioned, are often used in multiples. RSVP and DiffServ, for
instance, are often used in conjunction with MPLS to support a wide variety of applications.
Other protocols may be used more selectively; for example, RSTP would be used for streaming
video but not for voice telephony. Given the proliferation of protocols and the complexity of
their interactions, engineering an end-to-end IP network for QoS is not an easy task, and it is in
fact nearly impossible where protocols are not carried consistently across the network. Unfortunately,
configuring intervening switches and routers is generally out of the hands of the local
access provider.
With content demanding very stringent QoS such as high-definition video or Super Audio
Compact Disc (SACD)–quality audio, delivery over a multimode network may simply be infeasible,
and if the source is geographically remote, the local access provider may choose to utilize
a satellite service to transmit the content in one hop to the local network where it can be
cached on a server until needed. It can then be transmitted over a short distance where QoS is
entirely under the access provider’s control.
Other Methods for Supporting QoS
I should not leave the subject of QoS without mentioning a long-standing argument as to the
need of special protocols for supporting it over packet networks. Some in fact would contend
that QoS can be achieved through other means and that special reservation and prioritization
protocols are not the answer because of the difficulty of implementing them across heterogeneous
networks.
The simplest technique for getting around reservation requirements is to undersubscribe
available bandwidth. If everyone on a network enjoys, say, a median throughput rate of
50Mbps, QoS is likely to be satisfactory for almost any application absent any additional
provisions for ensuring QoS. And indeed some in the industry have suggested that as network
throughputs inch ever upward, the need for special QoS protocols will disappear over time.
Given enough bandwidth, surely such representations are correct, but for now bandwidth
is still expensive, particularly in metropolitan networks, and with some services such as lifeline
telephony or backup storage, the user cannot tolerate momentary suspensions of service,
however infrequent. For such services a reservation of bandwidth simply has to be in place
against every contingency.
Another way to attempt to get around the need for reservation is to create a sort of arti-
ficial bandwidth by means of data compression technologies. Compression algorithms reduce
the bandwidth required for any given application, often dramatically, and 50-to-1 reductions
in the amounts of data required to transmit a signal are commonplace today. In practical terms
this may approximate a 50-fold increase in bandwidth, though in reality matters are not so
simple. Compression almost always introduces appreciable latency in a transmission, which