LIS 525 - Streaming Media
Files in streaming formats
can begin playing
as soon as the first few packets are received.
Even on a fast line (T1), however,
the results can be unsatisfactory,
with poor sound quality,
and low resolution and dropouts in video.
At the provider's end,
encoding into streaming formats,
though simple with the right tools,
can take a lot of computer time.
RealAudio is a format for supplying streaming audio;
RealVideo is a corresponding format for streaming video.
RealAudio and RealVideo files have the extensions
.ra, .ram, and .rm.
there is a RealText (.rt) format for streaming text,
and a RealPix (.rp) format for streaming GIF and
Microsoft has its own Advanced Streaming Format
.wma, .wmv, .wm);
links on Web pages may actually be to Windows Media Metafiles
(extension .asx, .wvx, .wax)
which contain playlists and other information
and point to the actual streaming format files.
RealNetworks produces Helix Server,
which supports only RealAudio and RealVideo,
and Helix Universal Server,
which supports over 55 media formats,
including Windows Media and QuickTime.
RealSystem Producer Basic,
available for free download (in exchange for a bit of information),
can be used for simple translation to RealAudio and RealVideo format.
Streaming is one of the features of Microsoft Windows Media Services,
which is built into Windows 2000 Server and Advanced Server
and Windows Server 2003
and is available for download for Windows NT 4.0.
Microsoft Windows Media Services does not stream files in Real format.
(RAW radio clips are delivered by a special server
media.fims.uwo.ca or 188.8.131.52,
not by the main FIMS Internet server.
This server responds to pinging,
but not to basic HTTP requests.)
QuickTime Streaming Server is part of the Mac OS X Server.
Web hosting services may offer streaming as an additional option
for an extra charge
(e.g., US$25 setup fee plus US$20 per month
for 5 RealAudio/RealVideo streams)
or included in higher-end plans.
Most Internet traffic is unicast:
a separate copy of data is sent from the source
to each client that requests it.
a single copy of data is sent to all clients,
even those that do not request it.
a single copy of data is sent to all clients
that request it,
and only to those clients.
To receive a multicast,
the client listens to a particular IP address;
the server does not need to know who has decided to receive the multicast.
Multicasts are announced in advance
so that clients know when one will be available.
A client may need to send
an initial message to inform a local router
that it wants to receive multicasts
directed at a particular group of clients.
The most widely used multicast network
is the Internet Multicast Backbone (the MBone),
which has been in place since 1992.
The MBone consists of those portions of the Internet
on which multicasting has been enabled,
connected by unicast tunnels.
Multicasts are typically announced
using the Session Description Protocol (SDP).
Why HTTP Is Not Suited to Streaming
HTTP (which is based on TCP) is suited to the transfer of Web
but performs less well for streaming media.
Some reasons follow.
- TCP enforces reliability without regard to timeliness.
- TCP changes the data transfer rate of the client-server
according to the availability of bandwidth,
not the needs of the media.
- HTTP has only rudimentary mechanisms for random access.
- TCP is not suited to multicast.
Many Web hosting services
deliver streaming content via HTTP,
but some, such as Hostway,
use streaming protocols.
RTP is the Internet-standard protocol for the transport of real-time data,
including audio and video.
The initial target networking environment for RTP
this is a protocol used instead of TCP/IP
to provide better speed,
which checksums its data to ensure integrity,
but provides few error recovery services.
Efforts have been made, however,
to make RTP independent
so that it can be used over other protocols.
RTP can run over IP version 4 or IP version
No end-to-end protocol can ensure in-time delivery,
but RTP provides functionality suited for carrying real-time
including mechanisms for synchronizing different streams.
RTP data is carried on an even UDP port number and
the corresponding RTCP (control) packets
are carried on the next higher (odd) port number.
Two proposed standards,
RTSP (Real Time Streaming Protocol) and
both use RTP to format packets of multimedia content.
RTSP is designed for videoconferencing,
while H.323 is designed for broadcast.
RTSP is supported by Helix Universal Server,
Quick Time Streaming Server,
and Windows Media Player 9,
and as a plugin in Windows Media Services.
(Microsoft Media Server), a proprietary protocol developed by Microsoft,
TCP is typically used for sending and receiving media control commands,
and UDP for the actual streaming.
(RAW Radio clips are accessed with MMS.)
For More Information
- CBC. 2007.
CBC.ca - Listen to CBC Radio.
(Links to streaming audio of local CBC broadcasts.)
- Dalrymple, J. 2004.
"Who's Winning the Streaming Media Wars?"
- Evers, J. 2002.
"RealNetworks Issues a Streaming Media Challenge".
- Microsoft. 2007.
Windows Media Player multimedia file formats.
- Microsoft. 2007.
Windows Media Services.
- Niccolai, J. 2005.
"RealNetworks Promotes Wireless Streaming".
("Updated Helix Server Unlimited
supports streaming multimedia to handhelds.")
- RealNetworks. 2007.
Media Creation > RealProducer.
(Tool to produce audio and video files for streaming
from files or directly from media input devices;
Plus version, US$199.95;
Basic version, free.)
- RealNetworks. 2007.
(Products support a number of protocols,
including RTP and RTSP;
free downloads for evaluation.)
- Schulzrinne, H. 2007.
RTP: About RTP and the Audio-Video Transport Working Group.
(Information about RTP.)
- Sinclair, J.T. 2003.
Getting started with streaming media.
(Introduction, focusing on RealMedia.)
- University of Western Ontario,
Faculty of Information and Media Studies. 2007.
raw radio index.
(Sample streaming audio clips.)
Last updated October 31, 2007.
This page maintained by
Prof. Tim Craven
E-mail (text/plain only): firstname.lastname@example.org
Faculty of Information and
University of Western
Canada, N6A 5B7