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February 2003 1-5
1000-A2-GN22-00 1. Voice-over-Broadband Networking
Voice is more sensitive to network latency than data, so this
configuration ensures that voice traffic always has priority over
data. However, this configuration also enables voice traffic to
consume bandwidth when a call is in progress. Typically, few
subscriber's telephone lines are simultaneously in use, so the
majority of the bandwidth is usually available for data service.
For example, over a single 768 kbps symmetric DSL connection, an
ICP supports up to eight simultaneous telephone calls using 2:1
compression serving a KTS with 32 extensions at a P.01 grade of
service and still delivers data service at an average speed of
550 kbps (Figure 1–4).
Figure 1–4. Bandwidth Usage—768 kbps SDSL Circuit
However, many network operators have Frame Relay-based local
access networks interconnected with ATM networks. The CPX can
interface with the Frame Relay network through an Interworking
Function (IWF) between the Frame Relay and ATM (Figure 1–3).
The IWF is an industry-standard function that either the access
mux or a Frame Relay/ATM switch can execute.
In a Frame Relay network, each PVC uses an identifying Data Link
Connection Identifier (DLCI). ATM PVCs, however, use a Virtual
Path Identifier/Virtual Channel Identifier (VPI/VCI). The IWF
maps each Frame Relay DLCI to a corresponding ATM VPI/VCI.
This association between the Frame Relay DLCI and ATM VPI/
VCI is made when provisioning PVCs.
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Bandwidth (K)
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0202
Average bandwidth used by telephony
Average bandwidth available for data