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Voice: The voice data generated by telephone conversations has to
be transmitted as it is produced, in real time. Sending voice data is an ongoing
process; relatively small amounts of data move back and forth at regular intervals.
The data must be kept flowing in both directions without noticeable distortion
or delay. Losing a chunk now or then isn't a problem, but delays or receiving
them out of order is. Thus voice is "loss insensitive, delay sensitive."
Video: Video data is very much like voice data, real-time, ongoing,
and not too fussy about lost data. But it differs in that it is by no means
small: the average is high (from about 0.5 Mbps for low resolution, up to 6
Mbps for TV quality), with very high peaks. So video is "loss insensitive, delay
sensitive," and it also has a very high "peak cell rate."
Computer: Every single byte that's sent must reach its destination;
if anything is lost, it must be resent. Because the individual chunks aren't
much use until they're all received and reassembled, delays in transmission,
even ones that result in data being received out-of-order, can often be
tolerated. In many cases, the data transfer doesn't have to occur in real
time. Thus computer data is "loss sensitive, delay insensitive." (This
is, of course, overly simplified, but it is true for email and some
types of file transfer.)
Service Categories: "Cell loss" and "cell delay" are ATM Quality
of Service parameters, "peak cell rate" is one of its traffic parameters.
QoS and traffic parameters together determine the ATM service category.
In the long run, a connection's service category will be negotiated during setup,
but for now, all the ATM virtual circuits (see PVC
and SVC in the ATM and
Network Glossary) we have at UIC have predefined service categories.
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