A Hybrid Architecture - Scalable Continuous Media Streaming Systems - page 332

Client

Client

.
.
.

Regional
Network

Service Node 1

Client

Service Node 2

Backbone
Network

Admission
Controller

.
.
.

Client

Client

Service Node N

.
.
.

Regional
Network

Client

Admission
Controller

Figure 19.1 The super-scalar video-on-demand architecture

where the same video is repeatedly multicast over several channels in a time-staggered man-
ner, limits the time during which a client may start a new video session. Depending on the
number of multicast channels allocated to a video title, this start-up delay can range from
a few minutes to tens of minutes. To tackle this initial delay problem, we employ batching
and patching to enable a client to start video playback at any time using a dynamic multicast
channel until it can be merged back into an existing multicast channel. The following sections
present these techniques in detail.

19.1.1 Transmission Scheduling
Each service node in the system streams video data into multiple multicast channels. Let M
be the number of video titles served by each service node and let N be the total number of
multicast channels available to a service node. For simplicity, we assume N is divisible by M
and hence each video title is served by the same number of multicast channels, denoted by
N M =

M . These multicast channels are then divided into two groups of N S static multicast
channels and N D =

N /

N S dynamic multicast channels.
The video title ismulticast repeatedly over all N S staticmulticast channels in a time-staggered
manner as shown in Figure 19.2. Specifically, adjacent channels are offset by

N M −

L
N S

T R = (19.1)

seconds, where L is the length of the video title in seconds. Transmissions are continuously
repeated, i.e., restart from the beginning of a video title after transmission completes, regardless
of the load of the server or how many users are active. These static multicast channels are used
as the main channel for delivering video data to the clients. A client may start out with a
dynamic multicast channel but it will shortly be merged back into one of these static multicast
channels.