??? One item was still outstanding in terms of voice transmission
and the effect of the data network. This issue was related to the fact that nothing was stopping
Figure 1.6 Packet Switching
10 Chapter 1 ??? Unifi ed Communications
someone from transmitting a large fi le, streaming audio, and??”oh yes??”trying to make a phone
call across the same 100 MB port. To address this, Quality of Service was born.
Whatis.com defi nes QoS as follows:
???the idea that transmission rates, error rates, and other characteristics
can be measured, improved, and, to some extent, guaranteed in advance.
QoS is of particular concern for the continuous transmission of high-bandwidth
video and multimedia information. Transmitting this kind of content
dependably is diffi cult in public networks using ordinary ???best effort??™
protocols.???
Basically, it comes down to the fundamental design of IP transmission??”the fact that
messages are sent using a best effort, and retries are often necessary to complete a transmission.
As multiple data streams are passed over the same 100 MB pipe, each packet is, by default,
given the same priority in terms of delivery. The obvious problem here is that in a voice
conversation, a listener isn??™t going to understand the conversation if a packet is repeated at the
end of the message. QoS was particularly important for voice so that voice packets could be
???tagged??? and given priority in the data stream. Traditionally, QoS has been managed almost
exclusively on hardware devices such as routers and switches.
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