Voice Loss Considerations

Routers discard packets for a variety of reasons. The two biggest reasons for packet loss in routers are

QoS cannot help much with bit errors. However, QoS can help a great deal with loss due to queue space. Figure 1-27 contrasts FIFO queuing (one queue) with a simple queuing method with one queue for voice payload, and another for everything else.

Figure 1-27 FIFO Queuing Versus Imaginary Two-Queue System (One Queuefor Voice, One for Everything Else)

Figure 1-27 FIFO Queuing Versus Imaginary Two-Queue System (One Queuefor Voice, One for Everything Else)

tail Drop tail Drop

Queue for non-Voice

Queue for non-Voice

Suppose, for instance, that four packets arrive almost instantaneously, numbered 1 through 4, with packet 1 being the first to arrive. In the FIFO scheme, the router places the packets into the FIFO output queue, in the same order as arrival. What happens, however, if the output queue only has space for three packets, as shown in the figure? Well, the fourth packet is tail dropped.

Now suppose that the fourth packet is a voice packet, and the two-queue system is used. Each queue has space for three packets. In this case, the router does not drop the voice packet (packet 4). In fact, assuming the router serves the voice queue so that any packets always get to be sent next, this router reduces the delay for the voice packet.

NOTE A queue of size 3 is too small; however, showing a queue of size 40 just seemed to be a little clumsy in the figure!

With the simple example in Figure 1-27, the router does not drop the voice packet. However, the real power of this two-queue system for avoiding lost voice packet shines through with a little closer examination. Suppose that CAC allows only two concurrent G.729a calls to run through this router, and suppose the router does not use cRTP. The bandwidth required would be 26.4 kbps for each call, or a total of 52.8 kbps. Now imagine that the queuing method always sends the voice packets at next opportunity when a voice packet arrives, only waiting for the "currently being sent" packet to finish. Also imagine that the queuing method guarantees at least 60 kbps of this 128-kbps link for the voice queue. With all these features, the voice queue should never get very long (assuming the following parameters):

• The correct choices for maximum queue length.

• Queuing that always takes voice packets at first opportunity.

• Call admission control that prevents too many voice calls.

• LFI that shrinks the time a voice packet must wait for the packet ahead of it to be serialized.

• The voice queue would never fill and voice packets would not be tail dropped on this interface.

Another type of QoS tool, call admission control (CAC), provides a very important component of a strategy to avoid packet loss, and thereby improve voice quality. The best router-based queuing tools for voice include a setting for the maximum amount of bandwidth used by the voice queue. If exceeded, when the interface has other packets waiting, the excess voice packets are discarded. Literally, adding one call too many can make what was a large number of quality voice calls all degrade to poor quality. Some form of CAC must be considered to avoid loss for all calls.

Finally, one additional feature helps when a single voice payload packet is lost. G.729 codecs compress the voice payload in part by predicting what the next few milliseconds of voice will look like. G.729 uses this same logic to perform a function called autofill when converting from digital to analog at the receiving side of the call. G.729 autofill makes an educated guess at what the next few milliseconds of sound would have been if the next packet in sequence has been lost. Autofill makes this educated guess, filling in up to 30 ms of "lost" voice. Because IP Phone and IOS gateways default to sending 20 ms of voice per packet, with G.729, a single packet can be lost, and the G.729 autofill algorithm will play out a best guess as to what was in the missing voice packet.

Loss considerations for voice can be summarized as follows:

• Routers drop packets because of many reasons; the most controllable reason is tail drop due to full queues.

• Queuing methods that place (isochronous) voice into a different queue than bursty data reduce the chance that voice packets will be tail dropped.

• The QoS tools that help voice already, particularly queuing and LFI, reduce the chance of the voice queue being full, thereby avoiding tail drop.

• Whereas other QoS tools protect voice from other types of packets, CAC protects voice from voice.

• Single voice packet loss, when using G.729, can be compensated for by the autofill algorithm.

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