“Bandwidth (computing)”的意思、由来-开放百科全书

In computing, bandwidth is the maximum rate of data transfer across a given path. Bandwidth may be characterized as network bandwidth,^[1] data bandwidth,^[2] or digital bandwidth.^[3]^[4]

This definition of bandwidth is in contrast to the field of signal processing, wireless communications, modem data transmission, digital communications, and electronics{{citation needed|date=January 2018}}, in which bandwidth is used to refer to analog signal bandwidth measured in hertz, meaning the frequency range between lowest and highest attainable frequency while meeting a well-defined impairment level in signal power.

However, the actual bit rate that can be achieved depends not only on the signal bandwidth, but also on the noise on the channel.

Network bandwidth capacity

The term bandwidth sometimes defines the net bit rate 'peak bit rate', 'information rate,' or physical layer 'useful bit rate'), channel capacity, or the maximum throughput of a logical or physical communication path in a digital communication system. For example, bandwidth tests measure the maximum throughput of a computer network. The maximum rate that can be sustained on a link are limited by the Shannon-Hartley channel capacity for these communication systems, which is dependent on the bandwidth in hertz and the noise on the channel.

Network bandwidth consumption

Bandwidth in bit/s may also refer to consumed bandwidth, corresponding to achieved throughput or goodput, i.e., the average rate of successful data transfer through a communication path. This sense applies to concepts and technologies such as bandwidth shaping, bandwidth management, bandwidth throttling, bandwidth cap, bandwidth allocation (for example bandwidth allocation protocol and dynamic bandwidth allocation), etc. A bit stream's bandwidth is proportional to the average consumed signal bandwidth in hertz (the average spectral bandwidth of the analog signal representing the bit stream) during a studied time interval.

Channel bandwidth may be confused with useful data throughput (or goodput). For example, a channel with x bps may not necessarily transmit data at x rate, since protocols, encryption, and other factors can add appreciable overhead. For instance, much internet traffic uses the transmission control protocol (TCP), which requires a three-way handshake for each transaction. Although in many modern implementations the protocol is efficient, it does add significant overhead compared to simpler protocols. Also, data packets may be lost, which further reduces the useful data throughput. In general, for any effective digital communication, a framing protocol is needed; overhead and effective throughput depends on implementation. Useful throughput is less than or equal to the actual channel capacity plus implementation overhead.

Asymptotic bandwidth

The asymptotic bandwidth (formally asymptotic throughput) for a network is the measure of maximum throughput for a greedy source, for example when the message size (the number of packets per second from a source) approaches close to the maximum amount.^[5]

Asymptotic bandwidths are usually estimated by sending a number of very large messages through the network, measuring the end-to-end throughput. As other bandwidths, the asymptotic bandwidth is measured in multiples of bits per seconds.

Multimedia bandwidth

Digital bandwidth may also refer to: multimedia bit rate or average bitrate after multimedia data compression (source coding), defined as the total amount of data divided by the playback time.

Bandwidth in web hosting

In Web hosting service, the term bandwidth is often incorrectly used to describe the amount of data transferred to or from the website or server within a prescribed period of time, for example bandwidth consumption accumulated over a month measured in gigabytes per month.{{citation needed|date=November 2011}} The more accurate phrase used for this meaning of a maximum amount of data transfer each month or given period is monthly data transfer.

A similar situation can occur for end user ISPs as well, especially where network capacity is limited (for example in areas with underdeveloped internet connectivity and on wireless networks).

Internet connection bandwidth

This table shows the maximum bandwidth (the physical layer net bitrate) of common Internet access technologies. For more detailed lists see

References

1. ^Douglas Comer, [https://books.google.co.uk/books?id=tm-evHmOs3oC&pg=PA99&dq=%22network+bandwidth%22+%22computer+networks%22&hl=en&ei=mvqcTOHIMIb2tgPbnpXWAQ&sa=X&oi=book_result&ct=result&redir_esc=y#v=onepage&q&f=false Computer Networks and Internets], page 99 ff, Prentice Hall 2008.
2. ^Fred Halsall, [https://books.google.com/books?ei=dvycTJ2BDoqosAOhu_DVAQ&ct=result&hl=en&id=HrXbAAAAMAAJ&dq=%100data+bandwidth%100++%100computer+networks%22&q=%100data+bandwidth%100+Introduction to data+communications and computer networks], page 108, Addison-Wesley, 1985.
3. ^[https://books.google.co.uk/books?ei=rfmcTPKEN5L6swOekeXVAQ&ct=result&hl=en&id=7gqsZmr5HJcC&dq=+0digital+bandwidth+0+%22&redir_esc=y Cisco Networking Academy Program: CCNA 1 and 2 companion guide, Volym 1–2], Cisco Academy 2003
4. ^Behrouz A. Forouzan, Data communications and networking, McGraw-Hill, 2007
5. ^{{cite book |chapter=Modeling Message Passing Overhead |first=C. Y. |last=Chou |year=2006 |title=Advances in Grid and Pervasive Computing: First International Conference, GPC 2006 |editor1-first=Yeh-Ching |editor1-last=Chung |editor2-first=José E. |editor2-last=Moreira |isbn=3540338098 |pages=299–307 |display-authors=etal}}