A Staggered-Push Parallel Server Architecture - Scalable Continuous Media Streaming Systems - page 220

. . .

Transmission

Server 0

. . .

Retrieval

. . .

Transmission

Server 1

. . .

Retrieval

2 nd new video session
1 st new video session

Figure 12.4 Inconsistent schedule assignment arising from server clock jitter

. . .

Transmission

Server 0

. . .

Retrieval

Fully-occupied

. . .

Transmission

Server 1

. . .

Retrieval

new video session

Figure 12.5 Micro-round overflow due to inconsistent schedule assignment

(Figure 12.5). While one can delay the new video session at the overflowed server until the
next available micro-round, the transmission schedule will be delayed significantly and will
result in severe traffic overlapping with transmissions from another server (see Section 12.4).
To solve this inconsistent schedule assignment problem, we introduce an external admission
scheduler between the servers and the clients to centralize schedule assignment. To initiate a
new video session, a client will first send a request to the admission scheduler. Using the same
clock-synchronization protocol, the admission scheduler maintains the same clock jitter bound
with the servers. As new sessions are assigned solely according to the admission scheduler's
clock, the scenario depicted in Figures 12.4 and 12.5 will not occur. However, to ensure that
the assigned micro-round has not started in any of the servers due to clock jitter, the admission
scheduler must add an extra delay to the assignment:

Theorem 12.1. If the admission scheduler delays the start of a new video session by

T F

=

(12.2)

micro-rounds, then it guarantees that the assigned micro-round has not started in any of the
N S servers.