Specifications

160 IBM Voice Over Frame Relay Perform Guide
The considerations for tuning the 2216 (in the diagram shown earlier) are the
same as for the 2212 since it is performing voice forwarding between the voice
adapters in the 2212s and the 9783.
It has been found in testing, that a 60-ms delay in the voice at the FR interface
was tolerable. This means that the PVC’s burst interval, Tc, should be set to 60
ms. Tc is not directly configurable. It is indirectly configurable via the burst
interval (Bc) since Bc = Tc x CIR. If we have a CIR of 64 kbps, the Bc should be
set to 0.06 x640000 = 3840 bits. This means that the PVC can send 3840 bits
(480 bytes) per 60 ms.
The four voice calls will each generate one 25-byte frame every 15 ms. This
means that in a 60 ms interval, the voice ports will be sending 400 bytes (25
bytes x 4 frames x 4 calls) per interval. This leaves 80 bytes per Tc to send data.
Therefore, the fragment size should be set to 74 (80 - 6 bytes of overhead). For
Tc to be strictly honored for a PVC, you must enable CIR monitoring for this
interface.
As another example, assume you have only two voice calls over the same PVC
above. Tc will still be 60 ms; thus Bc will be configured as 3840. However, the
fragment size will change since a larger fragment may now be sent in the same
Tc interval with the voice packets. In this case the fragment size should be set to
274 bytes (480-(25 x 4 x 2)-6).
FR supports a minimum Tc of 30 ms. The voice packets are by rule small, so
lowering the Tc will not affect the performance of the voice traffic. However, using
a small Tc forces the fragment size to be smaller and small fragment sizes are
inefficient in terms of bandwidth, processor utilization, and global buffer use. It is
best to find the largest Tc and therefore the largest fragment size that can be
used to maintain good voice quality.
5.1.6 Tuning PVCs with No Voice Traffic
The 2210 is not sending any voice traffic. However, it is communicating with the
2216 interface that is sending voice traffic. In this case, the 2210 does not need
any special tuning other than enabling the PVC for fragmentation. The 2210 PVC
does not really need to fragment its outgoing packets, but fragmentation must be
enabled to allow it to receive fragmented packets. Therefore, the fragmentation
size for this PVC should be set to the MTU for the interface or 8190, which is the
maximum MTU for an FR interface. In either case, the 2210 will not be
fragmenting frames it sends but will be reassembling those sent to it by the 2216.
5.1.7 Buffers
Depending on the number of voice calls supported and the access rates, the
number of input buffers per interface may also need to be increased. This is
because of the queuing delays caused by FR running burst timers. What
generally happens on a T1 line is that the PVC will fill its burst size immediately
and then pause for Tc (60 ms in the above example) before sending again. This
means that the circuit will queue 60 ms worth of voice frames before sending
again. Flow control mechanisms in the router can cause voice packets to be
discarded before voice quality would be affected. This causes voice calls to be
hung up by the voice adapter or not able to initiate a voice call even when the
bandwidth is available. Therefore, it may be necessary to increase the number of
receive buffers on both the FR and voice card interfaces. The best way to monitor