Streaming media

From Wikipedia, the free encyclopedia

Streaming media is media that is continuously received by, and normally displayed to, the end-user whilst it is being delivered by the provider. The name refers to the delivery method of the medium rather than to the medium itself. The distinction is usually applied to media that are distributed over telecommunications networks, as most other delivery systems are either inherently streaming (e.g. radio, television) or inherently non-streaming (e.g. books, video cassettes, audio CDs). The verb 'to stream' is also derived from this term, meaning to deliver media in this manner.

History

Attempts to display media on computers date back to the earliest days of computing, in the mid-20th century. However, little progress was made for several decades, due primarily to the high cost and limited capabilities of computer hardware.

Academic experiments in the 1970s proved out the basic concepts and feasibility of streaming media on computers.

During the late 1980s, consumer-grade computers became powerful enough to display various media. The primary technical issues with streaming were:

having enough CPU power and bus bandwidth to support the required data rates
creating low-latency interrupt paths in the OS to prevent buffer underrun

However, computer networks were still limited, and media was usually delivered over non-streaming channels, such as CD-ROMs.

The late 1990s saw:

greater network bandwidth, especially in the last mile
increased access to networks, especially the Internet
use of standard protocols and formats, such as TCP/IP, HTTP, and HTML
commercialization of the Internet

These advances in computer networking combined with powerful home computers and modern operating systems to make streaming media practical and affordable for ordinary consumers. Stand-alone Internet radio devices are offering listeners a "no-computer" option for listening to audio streams.

In general, multimedia content is large, so media storage and transmission costs are still significant; to offset this somewhat, media are generally compressed for both storage and streaming.

A media stream can be on demand or live. On demand streams are stored on a server for a long period of time, and are available to be transmitted at a user's request. Live streams are only available at one particular time, as in a video stream of a live sporting event.

Streaming bandwidth and storage

Streaming media storage size (in the common file system measurements megabytes, gigabytes, terabytes, and so on) is calculated from streaming bandwidth and length of the media with the following formula (for a single user and file):

storage size (in mebibytes) = length (in seconds) · bit rate (in kbit/s) / 8,388.608

$\mathrm{size (MiB) = \left (length(s) \cdot bit\ rate\left ({kbit \over s} \right ) \right ) \cdot {1000\ bit \over 1\ kbit} \cdot {1\ byte \over 8\ bits} \cdot {1\ MiB \over 1,048,576\ bytes}}$

(since 1 mebibyte = 8 * 1,048,576 bits = 8,388.608 kilobits)

Real world example:

One hour of video encoded at 300 kbit/s (this is a typical broadband video for 2005 and it's usually encoded in a 320×240 pixels window size) will be:

(3,600 s · 300 kbit/s) / 8,388.608 = 128.7 MB of storage

if the file is stored on a server for on-demand streaming. If this stream is viewed by 1,000 people, you would need

300 kbit/s · 1,000 = 300,000 kbit/s = 300 Mbit/s of bandwidth

This is equivalent to 125.68 GiB per hour.

Protocol issues

Designing a network protocol to support streaming media raises many issues.

Datagram protocols, such as the User Datagram Protocol (UDP), send the media stream as a series of small packets. This is simple and efficient; however, packets are liable to be lost or corrupted in transit. Depending on the protocol and the extent of the loss, the client may be able to recover the data with error correction techniques, may interpolate over the missing data, or may suffer a dropout.

The Real-time Streaming Protocol (RTSP), Real-time Transport Protocol (RTP) and the Real-time Transport Control Protocol (RTCP) were specifically designed to stream media over networks. The latter two are built on top of UDP.

Reliable protocols, such as the Transmission Control Protocol (TCP), guarantee correct delivery of each bit in the media stream. However, they accomplish this with a system of timeouts and retries, which makes them more complex to implement. It also means that when there is data loss on the network, the media stream stalls while the protocol handlers detect the loss and retransmit the missing data. Clients can minimize the effect of this by buffering data for display.

Another issue is that firewalls are more likely to block UDP-based protocols than TCP-based protocols.

Unicast protocols send a separate copy of the media stream from the server to each client. This is simple, but can lead to massive duplication of data on the network. Multicast protocols undertake to send only one copy of the media stream over any given network connection, i.e. along the path between any two network routers. This is a more efficient use of network capacity, but it is much more complex to implement.

Furthermore, the most prominent of multicast protocols, IP Multicast, must be implemented in the network routers, as well as the servers. As of 2005, most routers on the Internet however do not support IP Multicast, and many firewalls block it. IP Multicast is most practical for organizations that run their own networks, such as universities and corporations. Since they buy their own routers and run their own network links, they can decide if the cost and effort of supporting IP Multicast is justified by the resulting bandwidth savings.

Peer-to-peer (P2P) protocols arrange for media to be sent from clients that already have them to clients that do not. This prevents the server and its network connections from becoming a bottleneck. However, it raises technical, performance, quality, business, and legal issues.

Newer camcorders stream video to a computer over a FireWire connection. This uses a system of time-based reservations to ensure throughput, and can be received by multiple clients at once.

Testing issues

Widespread deployment of streaming media raises scaling and Quality of Service issues. Testing service deployments is a significant problem. Vendors offer equipment to test streaming services across a number of test domains including Scalability, Quality of Service, Quality of experience, and protocol conformance.

Social and legal issues

Some streaming broadcasters use streaming systems that interfere with the ability to record streams for later playback, either inadvertently, through poor choice of streaming protocol, or deliberately, because they believe it is to their advantage to do so. Broadcasters may be concerned that copies will result in lost sales or that consumers may skip commercials. Whether users have the ability and the right to record streams has become a significant issue in the application of law to cyberspace.

In principle, there is no way to prevent a user from recording a media stream that has been delivered to their computer. Thus, the efforts of broadcasters to prevent this consist of making it inconvenient, or illegal, or both.

Broadcasters can make it inconvenient to record a stream, for example, by using unpublished data formats or by encrypting the stream. Of course, data formats can be reverse engineered, and encrypted streams must be decrypted with a key that resides—somewhere—on the consumer's computer, so these measures are security through obscurity, at best.

Efforts to make it illegal to record a stream may rely on copyrights, patents, license agreements, or—in the United States—the DMCA.

References

Schneier, Bruce (May 15, 2001). The Futility of Digital Copy Prevention Crypto-Gram Newsletter.
Schneier, Bruce (August 2000). The Fallacy of Trusted Client Software Information Security Magazine . also at The Fallacy of Trusted Client Software.
Schneier, Bruce (October 15, 2001). SSSCA Crypto-Gram Newsletter.

Streaming media

From Wikipedia, the free encyclopedia

History

Streaming bandwidth and storage

Protocol issues

Testing issues

Social and legal issues

References

See also

Streaming media technologies

Stream and transport protocols

Test equipment vendors

Providers

Media container formats