WiMax Operator's Manual

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.