Introduction
We have discussed in some detail how a VoIP application can deal with packet jitter. We now
brie
y describe several schemes that attempt to preserve acceptable audio quality in the presence
of packet loss. Such schemes are called loss recovery schemes. Here we de ne packet loss in a broad
sense: A packet is lost either if it never arrives at the receiver or if it arrives after its scheduled
playout time. Our VoIP example will again serve as a context for describing loss recovery schemes.
As mentioned earlier, retransmitting lost packets may not be feasible in a real-time conversational
application such as VoIP. Indeed, retransmitting a packet that has missed its playout deadline
serves absolutely no purpose. And retransmitting a packet that over
owed a router queue cannot
normally be accomplished quickly enough. Because of these considerations, VoIP applications often
use some type of loss anticipation scheme. Two types of loss anticipation schemes are forward error
correction (FEC) and interleaving.
Forward Error Correction (FEC)
The basic idea of FEC is to add redundant information to the original packet stream. For the cost of
marginally increasing the transmission rate, the redundant information can be used to reconstruct
approximations or exact versions of some of the lost packets. We now outline two simple FEC
mechanisms. The rst mechanism sends a redundant encoded chunk after every n chunks. The
redundant chunk is obtained by exclusive OR-ing the n original chunks. In this manner if any one
packet of the group of n + 1 packets is lost, the receiver can fully reconstruct the lost packet. But
if two or more packets in a group are lost, the receiver cannot reconstruct the lost packets. By
keeping n + 1, the group size, small, a large fraction of the lost packets can be recovered when
loss is not excessive. However, the smaller the group size, the greater the relative increase of the
transmission rate. In particular, the transmission rate increases by a factor of 1/n, so that, if n = 3,
then the transmission rate increases by 33 percent. Furthermore, this simple scheme increases the
playout delay, as the receiver must wait to receive the entire group of packets before it can begin
playout. The second FEC mechanism is to send a lower-resolution audio stream as the redundant
information. For example, the sender might create a nominal audio stream and a corresponding
low-resolution, low-bit rate audio stream. (The nominal stream could be a PCM encoding at 64
kbps, and the lower-quality stream could be a GSM encoding at 13 kbps.)
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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