The IEEE 1 defines traffic as messages that are transmitted and received over a communications channel. It quantifies...
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usage. There are three types of traffic: voice, data and image. Each type has its own set of requirements for transmission.
Voice traffic is measured by the Erlang, which is the unit of (telephone) traffic intensity defined as the number of (telephone) call arrivals per mean service time. One Erlang is equal to the number of call-seconds divided by 3600, which is equal to a fully loaded (voice) circuit over a one-hour period. (1) With data transmission the Erlang may be used in message loading of telephone facilities.
In most cases a circuit carrying voice telephony the traffic has a "spurty" nature reflecting spaces between spoken words and the "talk-listen" effect. It is assumed that on a two-way circuit, one party is talking and the other is listening. In the world of "broadcast" voice circuits may be one-way with a coordinating order wire in the other direction.
On data circuits, one-way transmission is not uncommon. Transmission may also be one-way with a low-rate return channel for error correction. It also may be one-way, a frame, at a time. After each frame is transmitted, there is a wait period to receive an acknowledgment. Still other data circuits may operate in a full-duplex mode, with full data rate in each direction, frame by frame. Here positive acknowledgment is assumed. If a downstream frame is received in error, an upstream service frame is transmitted to service the error.
With video (image) communication, traffic is more difficult to define. It may consist of a still picture(s) or image(s), slow scan motion pictures, or a normal scan motion picture (32 frames per second). Conference television also has a number of variants whether still picture, slow scan or normal motion and may be in just one direction or in both directions. Facsimile is generally considered to be the transmission of one still picture in one direction.
1 IEEE 100 The Authoritative Dictionary of IEEE Standard Terms, 7th edition, IEEE NY 1999.
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