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Bit Rate (Kbps) Call Duration (secs) Burstiness (%)
Min Max Min Max Min Max
telemetry 0.01 10 1 10 1 10
videotex 1 64 100 1000 10 10
video surveillance 10 30,100 1,000 10,000 10 100
teletex 10 10 100 100 100 100
electronic mail 10 10 100 100 50 50
voice mail 16 64 10 100 30 50
video telephony 64 70,100 100 100 50 100
telephony (POTS) 64 64 100 100 30 100
facsimile 64 64 10 100 100 100
hifi audio 768 768 1,000 10,000 100 100
video conference 1,000 70100 1,000 10,000 50 100
broadband videotex 1,000 70,100 100 1,000 10 100
video mail 1,000 70,100 10 1,000 10 50
TV 30,100 70,100 1,000 10,000 100 100
HDTV 140,100 565,100 1,000 10,000 100 100
North American CCITT (International)
Digital Signal No. of Voice Data Rate Level No. of Voice Data Rate
Number Channels (Mbps) No. Channels (Mbps)
DS-1 24 1.544 1 30
2.048
DS-1C 48 3.152 2 120
8.448
DS-2 96 6.312 3 480
34.368

Table 2-3. Characteristics of PSTN users

Inherently, the PSTN is a constant bit rate medium. The PSTN data streams, such as DS-1, are fixed bit rate streams. In the case of DS-1, is fixed at 1.544 Mbps by regulatory bodies. Each of the 24 voice channels comprising the DS-1 pipe have a fixed bit rate of 64 Kbps, regardless of the particular service that might be using the individual channels moment to moment. However, a glance at Table 2-3 reveals that most user services operate at bit rates other than 64 Kbps. Additionally, each user does not transmit continuously, even when switched to the destination. The "burstiness" is a measure of the amount of time the service actually transmits while connected to the PSTN. For example, some telemetry source may be switched (connected) to a destination for 1 to 10 seconds, yet during the time period the source is connected, it is actually transmitting data 1 to 10 percent of the time. For the remaining 90 to 99 percent of the time, the switched circuit is actually idle, waiting for the telemetry source to do something.

Channel A in Figure 2-9 shows the channel bandwidth usage of a data service that operates with a burstiness of 10 percent, such as videotex. Channel B shows the usage by a service that utilizes the available channel bandwidth with a burstiness of 30 percent, such as POTS. Channel C shows a service that utilizes the bandwidth with a burstiness of 50 percent. Obviously, regardless of the call duration, much bandwidth is wasted if a single channel is reserved for the exclusive use of most data services.


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