Streaming - A Practical Guide to Video and Audio Compression - page 346

transmitted. A streaming server might do that, provided the requesting client can indicate
a starting position or in-point for the browse session. The metafiles that encapsulate video
streams for linkage to Web pages have a very sophisticated syntax that allows play lists,
combinations of clips, and specific in-points to be defined. This level of complexity isn't
always necessary, but it is worth reading the documents provided by your streaming ven-
dor in order to better understand what capabilities you have at your disposal.

20.3 How Streaming Works

The core idea behind streaming is that a server on a network for which your playback
client machine has rights to connect a session will deliver a sequence of bits through that
connection that are decoded and reconstructed to form a moving image and soundtrack.
For this to be useful, this sequence must be accurately synchronized and delivered
continuously without any break in transmission, and the amount of data must be suffi-
cient to make the viewing and listening experience enjoyable.
Given the way that the Internet is designed, this is quite a challenge from a technical
standpoint.
The Internet was not designed to deliver content in real time, nor was it designed to
deliver thousands of TV channels to millions of users. Originally, the Internet (or
ARPANET, as it was known then) was intended to reliably deliver e-mail between users
on different networks. Networking became progressively more advanced with the addi-
tion of the file transfer protocol (FTP). Then web-based services were introduced using
hypertext transfer protocol (HTTP). More recently, streaming audio and video via the real-
time transport protocol (RTP) and real-time streaming protocol (RTSP) provide media
services on the web.
The greatest strength of the Internet is the resilient nature of the connections and the
way that alternative routes between two end points are discovered automatically. The
downside is that it is actually quite hard to get a guaranteed level of throughput between
two points because a sudden burst of traffic between two other end points might saturate
the common parts of the link and consume all the available bandwidth.
Other protocols exist to help solve this, and you can operate the connections in a
variety of ways. Recent innovations allow for a single streaming service to be shared by
many users so that the number of listeners does not affect the outgoing bandwidth from a
streaming center.

20.4 Streaming Protocols

Using the user datagram protocol (UDP) and transmission control protocol (TCP) as a
foundation, the HTTP or RTSP protocol is layered on top to deal with higher-order func-
tionality. These describe content to be delivered, and in the case of RTSP, you can choose
TCP or UDP connections to improve the viewing experience. UDP is feasible because the
RTSP protocol supports buffering and skipping. The stream encoding has sufficient error