WiMax Operator's Manual

164 CHAPTER 7 ■ SERVICE DEPLOYMENTS OVER PUBLIC WIRELESS MANS
Subjective assessments of quality, or additional dimension of QoS, generally apply to
audio or video presentations. Here the user will be alert to artifacts, which are departures
from fidelity because of data loss or the imperfect operation of compression algorithms.
Such departures do not lend themselves to easy quantification or the assignment of onedimensional
metrics, and the sensitivity of individual users to such aberrations may vary
considerably. To one individual, MP3 audio compression at an 11-to-1 ratio may be perfectly
transparent while to another may be unacceptable in terms of sound quality, and that is why
such assessments are deemed subjective. Normally, service-level agreements, considered at
length in the following section, avoid subjective considerations and focus on the readily
quantifiable.
Marketing QoS Through Service-Level Agreements
QoS, regardless of how it is achieved, is inherently more expensive for the service provider than
is best-effort data delivery, given an equal allocation of bandwidth. This being the case, service
providers will have the expectation of commanding a premium for QoS in their networks.
In a packet network, QoS will vary from application to application and from customer to
customer. Normally, part of the available bandwidth will be given over to best-effort services
and part to services demanding QoS. Incidentally, such apportionment is possible only within
a packet network. A pure circuit network such as a SONET ring can offer bandwidth to subscribers
only in fixed increments, each with the inherent QoS that comes with wholly
predictable bandwidth assignments.
Some types of services such as voice telephony and regular video programming provoke in
the subscriber an expectation of high presentation quality simply because such services were
delivered exclusively over circuit networks in the past and typically exhibited excellent fidelity.
For this reason no one is likely to opt for grainy, pixilated video presentation even at a reduced
fee or for robotic voice quality and intermittent phone service. In such cases the network operator
simply cannot sell on the basis of a guaranteed minimum level of service quality; rather,
the service has to be perceived as good to excellent.
Nevertheless, for many business services, including conferencing and LAN connectivity,
the operator can offer stratified services with differential pricing. For example, videoconferencing
may be offered at a medium throughput of 400Kbps or at a high throughput exceeding
700Kbps, with the latter commanding premium pricing. The subscriber can expect near broadcast
quality at the higher figure, and some video artifacts at the lower figure, but because
700Kbps pipes are quite expensive over long distances, the subscriber may opt for some degradation
in picture quality to stay within a budget. Because the previously prevalent Integrated
Services Digital Network (ISDN) videoconferencing was so compromised in fidelity, it set
expectations for the service at a fairly low level—unlike the case with voice telephony where
the original analog transmissions established a standard of excellence that has still not entirely
been met by packet services—and thus many users are perfectly satisfied with compromised
quality if the price is right.
Where tiered services are offered, a service-level agreement will establish the performance
that the subscriber may expect. Normally, performance will be established in terms of minimums
that must be met by the service provider rather than the best performance possible over
a lightly loaded network. In most cases, the subscriber will be entitled to a discount on the
monthly fees when service levels are not maintained according to the agreement.