Streaming media
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Streaming multimedia is multimedia that is constantly received by, and normally displayed to, the end-user while it is being delivered by the provider (the term "to display" is used in this article in a general sense that includes audio playback.) 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.
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[edit] 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, primarily due to the high cost and limited capabilities of computer hardware.
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[citation needed]
However, computer networks were still limited, and media was usually delivered over non-streaming channels, such as CD-ROMs.
The late 1990's 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 made 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 is 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.
Research in streaming media is ongoing and representative research can be found at the Journal of Multimedia.
[edit] 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):
(since 1 megabyte = 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*1024) give around 130 MB of storage
If the file is stored on a server for on-demand streaming and this stream is viewed by 1,000 people at the same time using a Unicast protocol, you would need:
- 300 kbit/s · 1,000 = 300,000 kbit/s = 300 Mbit/s of bandwidth
This is equivalent to around 125 GiB per hour. Of course, using a Multicast protocol the server sends out only a single stream that is common to all users. Hence, such a stream would only use 300 kbit/s of serving bandwidth. See below for more information on these protocols.
[edit] Protocol issues
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Designing a network protocol to support streaming media raises many issues, such as:
- 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, there is no mechanism within the protocol to guarantee delivery. It is up to the receiving application to detect loss or corruption and recover data using error correction techniques. If data is lost, the stream 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.
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Unicast protocols send a separate copy of the media stream from the server to each client. In terms of difficulty of implementing technically, these protocols are the most simplistic. At the cost of this simplicity, there can be massive duplication of the data being sent on the network.
Multicasting broadcasts the same copy of the multimedia over the entire network to all clients - Multicast protocols were developed to try to cut down on the duplication that Unicast protocols cause. These protocols send only one copy of the media stream over any given network connection, i.e. along the path between any two network routers. Many of these protocols require special routing hardware capable of broadcasting the stream. These multicasts are one-way connections which very closely mirror the functionality of over the air television in that viewers lose their on-demand viewing abilities. Some of these lost viewing abilities include rewinding and fastforwarding a media file. There exist streaming media servers which combine Unicast and Multicast solutions to both cut down on the bandwidth requirements and provide users most of the on-demand functionality of a pure unicast.[1]
- IP Multicast, the most prominent of multicast protocols, must be implemented in all nodes between server and client including network routers. As of 2005, most routers on the Internet however do not support IP Multicast, and many firewalls block it.[citation needed] 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.
