ATM Data Types and Quality of Service

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.