Specifications

158 IBM Voice Over Frame Relay Perform Guide
means that any given component in the network must minimize its portion of the
delay and yet maximize its efficiency.
When multiplexing voice and data over the same PVC, the FR burst size and
burst interval are important in reducing the amount of delay incurred by voice
packets. The burst interval, or Tc, is calculated by Bc/CIR (committed burst size
divided by CIR). This specifies the duration of the burst. The burst size is the
number of bits the router is configured to send during Tc. It is normally Bc+Be but
can be more or less depending on whether CIR or congestion monitoring is
enabled and whether any congestion indications have been received.
Assume for example, you have a CIR of 64 kbps, a Bc of 64 kbps and a Be of 0.
Tc is therefore equal to 1 second (Tc = CIR/Bc). The router will burst up to 64
kbps anytime during that 1 second period. If there is data queued for the circuit,
then the 64 kilobits will be sent at the beginning of the interval. The router must
now wait until the next Tc, the next second, before it can send any more data.
This works well for file transfers and also works well for voice alone since the
voice card sends data to the FR interface at a steady, predictable rate thereby
eliminating the burst. But if the PVC is being used to transmit both voice and data
traffic, then the voice could be queued for up to 1 second waiting for the next Tc
interval, which is unacceptable.
Assume in the configuration above that the 2212s, 2210, and 2216 each have T1
a access rate link to the FR network. The 2212s and 2210 each have a single
PVC leading to the 2216. The 2216 therefore has a single PVC to each of the
routers. PVCs to other routers are assumed to be on the same link. The 2216
also has a back-to-back FR link to the 9783 at T1 speed.
The following sections describe the configuration considerations that must be
made when configuring the 2212s.
5.1.2 Consider Voice Bandwidth Utilization
Irrespective of whether a PVC is carrying voice and data or just voice, you need
to be aware of the amount of data each voice call generates to ensure you have
the bandwidth available to carry the traffic. The amount of data a voice port
generates depends on the vocoder and rate configured. The following table
details the bandwidth used for specific rates: