|
|
| 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.
|