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Quality of Service
Quality of Service is a subject of crucial importance to your success with VoIP. Not
surprisingly, QoS technologies aren’t often well-understood by traditional telephony
people. But those with a data background have never had to use them, either. So this
chapter will introduce you to QoS concepts and protocols, the problems they solve,
and the complexities they introduce. Don’t let that scare you, though. There are QoS
approaches for networks that have a few dozen endpoints, and there are approaches
for giant, high-capcacity networks, too.
QoS Past and Present
In traditional telephony, quality of service for each and every phone call is guaran-
teed by the constant availability of dedicated bandwidth. Whenever a channel or
“loop” is established across the network, the bandwidth allocated to that channel is
steadfast and unchanging. Most digitally encoded call paths on the PSTN use the
same codec, G.711, so transcoding isn’t necessary. Almost no processing bottle-
necks will be found on the PSTN, and since the system isn’t generally packet-based,
there is almost never degradation in perceived call quality as a result of congestion.
As a circuit-switched network, the PSTN provides quality of service by
having almost no latency or congestion issues. The option of lowering
call quality in order to increase call capacity never existed on the
If the PSTN and SS7 can’t establish a full-bandwidth path through the network, the
call just doesn’t get connected—and the caller hears a busy tone. The designers of
the PSTN felt a breakdown of connectivity would be preferable to a breakdown of
Of course, packet networks work the other way. When bandwidth availability drops,
as more packets are sent on the network, throughput slows. Until a certain breaking
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