A Hybrid Architecture - Scalable Continuous Media Streaming Systems - page 349

The number of channels required for NVoD is a constant value and equal to 360. The channel
requirement of TVoD increases with the arrival rate and quickly exceeds that of NVoD. The
channel requirements of SS-VoD and UVoD are significantly lower than both TVoD and NVoD.
For higher arrival rates, SS-VoDoutperforms UVoDby awidemargin. For example, the channel
requirements at one request per second are 114 and 36 for UVoD and SS-VoD respectively;
and the channel requirements at 5 requests per second are 225 and 38 for UVoD and SS-VoD
respectively. This result demonstrates the performance gain achieved by replacing the dynamic
unicast channels in UVoD with dynamic multicast channels in SS-VoD.

19.4.5 Performance at Light Loads
The previous results are computed using relatively high arrival rates. Intuitively, the perfor-
mance gains will decrease at lower arrival rates as fewer requests will be served by a dynamic
multicast channel. To investigate this issue, we define a percentage of channel reduction over
TVoD, denoted by G ,as

min { n | W TVoD ( n ) ≤ 1 , ∀ n = 0 , 1 ,... } − min { n | W ( n ) ≤ 1 , ∀ n = 0 , 1 ,... }

100%
(19.19)
where W TVoD ( n ) and W ( n ) are the latency, given there are n channels, for TVoD and SS-VoD/
UVoD respectively.
Figure 19.14 plots the channel reduction for arrival rates from 1

G = ×

min { n | W TVoD ( n ) ≤ 1 , ∀ n = 0 , 1 ,... }

10 − 4 to 0.01 for SS-VoD
and UVoD. The results show that SS-VoD requires fewer channels than TVoD as long as arrival
rates are over 1

×

10 − 4 requests per second. Note that at this low arrival rate, both TVoD and

. 8 ×

100

50

0

1 . 10 − 4

1 . 10 − 3

0.01

Arrival Rate (request/second)

SS-VoD over TVoD
UVoD over TVoD

Figure 19.14 Channel reduction over TVoD at very low arrival rates