For an example of asymmetric communications, see Asymmetric Digital Subscriber Line (ADSL).
In telecommunications, the term asymmetric (also asymmetrical or non-symmetrical) refers to any system in which the data speed or quantity differs in one direction as compared with the other direction, averaged over time. Asymmetrical data flow can, in some instances, make more efficient use of the available infrastructure than symmetrical data flow, in which the speed or quantity of data is the same in both directions, averaged over time.
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Consider an Internet connection in which downstream data (from an Internet server to the subscriber) flows over a broadband satellite downlink, while upstream data (from the end user to the server) is sent over a twisted-pair telephone line. This is an example of asymmetric communications. The hardware for reception of satellite signals is simple, consisting of a small dish antenna, an amplifier/converter, and a modem. While the downstream data might flow at 1 MBps or more, upstream data is limited to 56 kpbs (often much less). In most Web browsing applications, this is a major improvement over a connection in which the upstream and downstream data both must flow through the twisted pair. This is because most of the bytes come downstream as relatively large graphics, sound, multimedia, and HTML files, while upstream data consists mainly of new content requests by the subscriber, which, in comparison, contain few bytes. In this environment, it would not make sense to supply the subscriber with the sophisticated hardware necessary for a broadband satellite uplink.
In some situations, asymmetric communications is not satisfactory. An example is two-way, full-motion videoconferencing, in which broadband data must flow in two directions between two end users. Another example is File Trasfer Protocol (FTP) applications in which the volume of upstream data is considerable. For communications of this type, broadband cable modem, optical fiber, or broadband wireless Internet access is available in some locations. At the time of this writing, symmetric broadband is not generally available outside of metropolitan areas.