The other side of the coin, however, is that the media is always streamed at the constant data transmis
sion rate needed to match the media's data rate i.e., in real time. So, with a faster connection, there
may be bandwidth left over. One of the advantages of streaming media is the predictable load per
stream, on both the server and on the network.
One of the disadvantages of RTP enabled streaming is that many network fi rewalls block the transmis
sion of UDP information, although this is changing as the corporate world embraces streaming.
RTSP controls streaming media
While RTP enables a one way stream, transmitting media from the server to the client, RTSP is a two
way protocol (similar to HTTP) which uses TCP to communicate, and is usually layered on top of RTP.
According to its developers, RTSP acts as a `network remote control' for multimedia servers.
20
Like
a VCR remote control, RTSP provides mechanisms that allow individual end users or designated Web
conference participants to specifi cally request streams from one or more servers, as well as a specifi c
transport type and destination(s) for delivery of the data; request information about the data in a
format specifi c fashion; start, stop, and pause the delivery of the data; and gain random access to
various portions of the data (where applicable not, for example, in the case of a real time live feed).
This very simplifi ed chart shows how
Viewing a Web page
Experiencing Streaming Media
protocols are layered one on top of
another and, on the protocol level,
Browser
Viewing/Playback
Media Player
how the system for viewing a Web page
relates to the system for experiencing
streaming media.
HTTP
Interactivity
RTSP
TCP
Transport
RTP/UDP
IP
Routing
IP
Physical Connection
Network
Physical Connection
Unicast and Multicast
When media is broadcast, a single stream is transmitted to all clients on the network. To understand
what broadcast means, one need only think about our traditional use of the term broadcast media,
where the network is a cable or satellite system, or the airwaves, and the clients are television or radio
receivers. Because only a single stream is transmitted, all the clients that are tuned in experience the
same portion of the media program, at the same time, whether the program is live or pre recorded.
When media is streamed over the Internet, it may be either unicast or multicast.
Unicast: A unicast is a one to one narrowcast, in which each end user gets a separate stream even if
they are experiencing the same media simultaneously. Because they each get their own stream, end users
can be given options for controlling the media, such as the ability to pause the stream, replay portions,
namic Medianamic Media
or to jump to a different part of the program. This type of control is, however, only possible with
yy
pre recorded content made available on demand, and this fl exibility comes at the expense of both server
capacity and bandwidth every end user must be served and sent a discrete stream.
Multicast: Multicasting, or IP multicasting, is also considered a narrowcast strategy, and it is
obe D
designed to conserve both server processing capacity and bandwidth. The server transmits only one
obe D
d
stream, which is replicated by special routers throughout the network, to be distributed to groups
d
AA
of multiple end users. Multicasting does not allow the same fl exibility for the end user as does
unicasting every end user must experience the same content concurrently.
17
17
20 Real Time Streaming Protocol (RTSP) , an Internet Draft, by H. Schulzrinne, A. Rao, R. Lanphier, Columbia U./Netscape/RealNetworks, Internet Engineering Task Force, 1998,
PDF available at http://www.cs.columbia.edu/~hgs/rtsp/
