A Hybrid Architecture - Scalable Continuous Media Streaming Systems - page 336

Legend

IDLE

State

Decision

Action

Event

have free
channel?

Yes

received EXTEND request,
update A L

No

WAITING TRANSMITTING

channel available

send START reply
transmit for A L seconds

Figure 19.4 State-transition diagram for a service node

It may appear that the previous admission procedure is unnecessarily complex and the
clients would be better off sending requests directly to the service nodes. However, this direct
approach suffers from poor scalability. In particular, recall that each service node serves a few
video titles to the entire user population. Therefore, as the user population grows, the volume
of requests directed at a service node will increase linearly and eventually exceed the service
node's processing capability.
By contrast, an admission controller generates at most two requests, one START request
and one EXTEND request, for each dynamically-started multicast transmission, irrespective of
the actual number of client requests arriving in an admission cycle (i.e., from receiving the
first request in a batch to sending the EXTEND request). Given that the number of admission
controllers is orders of magnitude smaller than the user population, the processing requirement
at the service nodes is substantially reduced. For extremely large user populations where even
requests from admission controllers can become overwhelming, one can extend this request-
consolidation strategy into a hierarchical structure by introducing additional layers of admission
controllers to further consolidate requests until the volume becomes manageable by the service
nodes.

19.1.3 Channel Merging
According to the previous admission control policy, a statically-admitted client starts receiv-
ing streaming video data from a static multicast channel for playback as depicted in Fig-
ure 19.5. For dynamically-admitted clients, video playback starts with video data received