 - 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.
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.
| LAST UPDATED: |
14 Aug 2006
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