Open-Loop Algorithms - Scalable Continuous Media Streaming Systems - page 311

Table 18.1 Summary of notations

Symbol Definition

L The length of the video (sec)

b The playback rate of the video (Mbps)

K The total number of logical channels

L i The size of the i th video segment (sec)

N The total number of video segments

B The total network bandwidth (Mbps)

B i The network bandwidth for the video segment L i (Mbps)

C The client access bandwidth constraint (Mbps)

T The maximum start-up latency (sec)

H The maximum client buffer requirement (Mb)

The client receives from the beginning of a media segment as soon as it encounters it in any
of the broadcasting channels. In the worst case, the client has to receive data from all channels
simultaneously. The maximum start-up latency T is equal to the broadcast duration of the first
media segment L 0 .

18.2.2 Variable-Segment Fixed-Bandwidth Schemes
Variable-segment fixed-bandwidth schemes (e.g., [1-3]) divide a media stream into variable-
size media segments for broadcast over fixed-bandwidth network channels (e.g., b Mbit/sec).
A notable example is the Skyscraper Broadcasting scheme [2] proposed by Hua et al. in
1997 as an improvement on the Pyramid Broadcasting scheme proposed by Viswanathan
and Imielinski [1]. Unlike the Pyramid Broadcasting scheme, where the media segment sizes
increase according to a geometric series, the Skyscraper Broadcasting scheme divides a media
stream into N video segments according to a predefined data partition function. They also
limited the maximum media segment size to a given length W to reduce the client buffer
requirement. The network bandwidth B is then divided equally into N channels (i.e., same as
the number of media segments), each with a bandwidth equal to the media playback bit-rate
b . Video segment L i ( i 1) is then repeatedly broadcast over channel i .

The client always caches media data from the beginning of a media segment (instead
of from anywhere in between). The client begins by caching data from the next broadcast
of the media segment L 0 . Then it caches the subsequent media segments L i in the order of
i

= 0 , 1 ,..., N −

1 at the earliest time after it started playing back the media segment L i .
The client receives data from up to two channels simultaneously and the maximum start-up
latency T is equal to the broadcast duration of the first media segment L 0 . The client buffer
requirement is equal to L 0 b ( W

= 1 , 2 ,..., N −

1) [2].
Another notable example is the Greedy Disk-Conserving Broadcasting scheme [3] proposed
by Gao et al. in 1998. It is a greedy algorithm that minimizes the number of server channels
needed to guarantee a givenmaximum start-up latency T and client I/Obandwidth requirement.
Unlike the Skyscraper Broadcasting scheme, GDB allows the client to receive media segments

−