FEC versus PRT - Scalable Continuous Media Streaming Systems - page 246

FEC if K min < K max . To compensate, we can increase K max from 7 to 8 and the systemMTTF

will become 10 17 hours with K min = 3, exceeding the system MTTF of FEC with K = 7.

Therefore, in this example we can reduce the bandwidth overhead from K = 7 redundancies

in the case of FEC to K min = 3 redundancies in the case of PRT.

We further illustrate the bandwidth overhead savings in Figure 13.6 by plotting the bandwidth
ratio, defined as the ratio K min /

K , versus the average failure-detection time. In all cases PRT
achieves the same or a better systemMTTF compare to FEC. We can observe that the reduction
in bandwidth overhead is very significant, at least half of the bandwidth overhead is saved
and the savings increase with shorter failure-detection time. Considering failure detection in
practice is likely to be in the order of seconds, this result clearly shows the feasibility and
superiority of the PRT protocol.

13.5.3 Storage Overhead
Compared to FEC, the PRT protocol has one trade-off. Reconsidering the previous example
with K

8 to achieve
the same or a better system MTTF than FEC. While the bandwidth overhead is reduced by
(7

= 7 in FEC, we need to configure PRT with K min = 3 and K max =

57%, the fact that K max is larger than K means more redundancies will need to
be stored, though not all are actively transmitted. In this case the additional storage overhead
in PRT is equal to ( K max −

− 3) / 7 =

14%.
Figure 13.7 plots the percentage increase in storage overhead due to PRT versus the target
systemMTTF requirements. From the results we can see that the additional storage requirement
decreases rapidly with higher target system MTTF. With the rapid decrease in storage cost,
most hard disks are I/O bound rather than storage bound. Thus, the added storage requirement
can be easily absorbed with little to no impact on the total cost of the system.

K ) / K =

100%

80%

60%

40%

20%

0%

1 10000000 1E+14 1E+21 1E+28 1E+35 1E+49 1E+56 1E+63 1E+70 1E+77 1E+84

1E+42

System MTTF (in hours)

Figure 13.7 Percentage increase in storage due to PRT for different system MTTF requirement