A Staggered-Push Parallel Server Architecture - Scalable Continuous Media Streaming Systems - page 231

4

3

Upper limit for which
ORT can reduce
network congestion.

2

1

0

0 0.2 0.4 0.6 0.8 1

Max. Server Clock Jitter (seconds)

Over-Rate Tx (Q = 64KB)
Over-Rate Tx (Q = 128KB)
Over-Rate Tx (Q = 256KB)
2 x Video Bit Rate
1 x Video Bit Rate

Figure 12.14 Transmission rate versus server clock jitter

particular, if we consider the network traffic between a server and a client, it is easy to see
that the traffic will be in the form of bursts with an average inter-burst interval of ( N S − 1) T F

seconds. By contrast, servers in concurrent push transmit to a client continuously at a constant
rate, allowing easy integration with QoS offered by existing networks. Staggered push will not
be able to make use of QoS available in today's QoS-enabled networks.
In practice, if the VoD system is deployed in dedicated networks with a priori bandwidth
planning, then staggered push can still be used effectively. This is because the over-rate trans-
mission scheme already guarantees that network congestion due to traffic overlapping will not
occur, and the aggregate traffic going from the servers to a client will be close to constant bit-
rate, with small gaps in between (due to over-rate transmission).

12.8 Summary
In this chapter, we have presented and analyzed a staggered-push parallel server architecture for
implementing linearly scalable media streaming systems. The architecture employs fixed-size
block striping for data storage, and a staggered-push scheduling algorithm for co-ordinating
transmissions among multiple autonomous servers. We incorporated the effect of server clock
jitter to address the inconsistent schedule assignment problem and the traffic overlapping
problem. We tackled the former problem by an external admission scheduler and the latter