Overview of Multicast Streaming - Scalable Continuous Media Streaming Systems - page 287

16

Overview of Multicast Streaming

The parallel server architectures solved the scalability problemof themedia streaming server
by combining multiple low-cost media servers into one high-capacity streaming system. In
very-large-scale systems such as providing video-on-demand services to millions of users,
the resultant network traffic generated will be very substantial. For example, to stream a
media of 4Mbps (e.g., high-quality video encoded in MPEG2 or MPEG4) to just 10,000
users, we will need an aggregate bandwidth of 40Gbps which only optical fiber can carry
in today's networks.
Interestingly, as many studies have shown, most users will probably request a small
portion of popular media contents, such as popular songs, movies, or documentaries. Thus,
within the huge volume of network traffic there are in fact a great deal of duplicated data
being transferred from the servers to the clients. Now if the data are the same, can we send
a single copy instead of sending many duplicated copies to the clients?
This is the question being addressed in Part III of the topic. In particular, we investigate
the use of network multicast to deliver media data from the servers to the clients. Unlike
the unicast model used in today's Internet, network multicast allows a sender to multicast
a single packet to be received by multiple receivers. This has the potential to significantly
reduce the network traffic in sending duplicated media data to a large number of users.
In this chapter, we give an overview of network multicast in general, and IP multicast
in particular, and discuss how media streaming can be carried over network multicast. In
the subsequent chapters we will cover in more detail three classes of multicast streaming
algorithms, namely, closed-loop algorithms, open-loop algorithms, and hybrid algorithms.

16.1 Introduction
Today's Internet employs a form of data delivery known as unicast. In unicast a sender sends
a data packet through the network which is eventually received by one and only one receiver,
thus unicast is also known as point-to-point communications. This unicast data delivery model
works well for a wide variety of applications, including the World-Wide-Web, email, file
transfer, and even streaming continuous media such as audio and video.