Defining traffic telecommunications style

The IEEE 1 defines traffic as messages that are transmitted and received over a communications channel. It quantifies 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

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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.

About the author:
Roger Freeman is a widely recognized expert in telecommunications system engineering. Roger has worked in telecommunications since 1946 when he joined the Navy and became an aviation radioman. Later, Roger served as a radio officer in the merchant marine for nearly 10 years. He then held several positions with ITT assigned to their Spanish Standard Electrica subsidiary. He also served the International Telecommunication Union as Regional Planning Expert for Northern Latin America based in Quito, Ecuador. Roger was principal engineer with the Raytheon Company, Marlboro, MA where he took early retirement in 1991 to establish Roger Freeman Associates, Independent Consultants in Telecommunications. He has been giving seminars in telecommunication disciplines at the University of Wisconsin, Madison for nearly 20 years. Roger has been writing books on various telecommunication subjects for John Wiley & Sons since 1973. There are seven titles, which he keeps current including the two-volume work, Reference Manual for Telecommunication Engineers, now in 3rd edition. He holds two degrees from NYU. His Web site is www.rogerfreeman.com and his e-mail address is rogerf67@cox.net.

This was first published in August 2005

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