Information Technology Reference
In-Depth Information
convenience, these eight packets altogether are called an
ensemble
. In this case,
the FEC scheme is capable of recovering at most three missing packets out of
an ensemble as shown in Figure 1.35. As shown in the example, even though
three out of eight parents have failed, the receiver can still recover all of the
original packets as well.
Figure 1.36 shows a possible topology where each peer has three different
parents. Each parent peer can then forward partial streaming content to a
child peer (i.e., one packet per ensemble).
Figure 1.37 shows the analytical packet loss probability of the multisource,
multicast structure with a packet drop rate of 0.1 under different FEC pro-
tection levels. The average recovery time is 30 seconds and the average ser-
vice time is 10 minutes. As one can see, the packet loss accumulates rapidly
with increasing depth when there is no FEC protection. With a low level FEC
protection, such as FEC(5,4), the packet loss performance improves, but still,
the packet loss accumulates as the depth increases. However, an interesting
result is that when a sufficiently strong FEC protection is used, the packet
loss accumulation stops. That means when an appropriate FEC protection is
incorporated into a multisource system, the impact of a packet loss due to the
dynamic behavior of peers can be overcome. Further detailed mathematical
analysis and discussion of multisource, multicast structures can be found in
Wu, Hwang, Lee, Gau, and Kao [67].
The benefits of using multiple sources and a packet level FEC is that once a
few parents fail or the connections to some parents are suffering from bad con-
ditions, the remaining parents can still provide most of packets that can be used
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Packet Level FEC
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Figure 1.35
An example of packets recovery with three sources failed in an FEC(8,5) system.
