A Hybrid Architecture - Scalable Continuous Media Streaming Systems - page 333

. . .

. . .

N D Dynamic
Multicast
Channels

.

. . .

N S Static
Multicast
Channels

.

Figure 19.2 Transmission schedules for static and dynamic multicast channels

We note that while using more sophisticated open-loop multicast algorithms (cf. Chapter 18)
can achieve better resource savings, they often require more client-side bandwidth and client-
side buffer. More importantly, these multicast schedules require the client to switch between
multiple multicast channels during a video session to achieve the resource savings. For large-
scale systems comprising millions of users, the channel switching overhead can present a
significant burden to the network.
Let us use IP multicast as an example. A client wishing to switch from one multicast channel
to another will need to send an IGMP message to the edge router to stop it from forwarding
data in the current multicast group. Another IGMP message will then be sent to request the
edge router to start forwarding data from the new multicast group. Unlike processing data
packets, these control messages and group management processing are performed in software
running on the router CPU. Hence, the more channel switching it requires, the more chance
that a router could become overloaded. This could lead to missed schedule and/or data loss,
resulting in client playback hiccup and/or visual quality degradation.
Another advantage of using the simple staggered multicast schedule in SS-VoD is in the sup-
port of interactive playback control. In particular, interactive controls such as pause-resume,
slow motion, and seeking can be supported in SS-VoD without incurring any additional re-
sources or processing at the video server, nor any additional buffer at the client (cf. Section
19.2).
The next section presents the admission procedure for starting a new video session and we
explain in Section 19.1.3 how the client is merged back into one of the static multicast channels.

19.1.2 Admission Control
To reduce the response time while still leveraging the bandwidth efficiency of multicast, SS-
VoD allocates a portion of the multicast channels and schedules them dynamically according
to the request arrival pattern.
Specifically, a new request always goes to the admission controller. Knowing the complete
transmission schedule for the static multicast channels, the admission controller then deter-
mines if the new user should wait for the next upcoming multicast transmission from a static