TRUNKING IN THE TELEPHONE INDUSTRY
The telephone industry discovered years ago that the growing demand for its service rapidly was outgrowing the available wire lines. It became apparent that some method was needed to use the lines between geographic areas more efficiently. (The major lines between geographic areas, or central stations, also are called trunks.)
In a telephone system, the users are called subscribers. The goal of the simple system is to provide a communications link between every subscriber whenever the subscriber desires it. The system can provide communications for any number of subscribers. In this example, we ll assume there are 12 subscribers. Six are assigned to each of two telephone centraI switching stations. It is a technical given that communications between any two subscribers connected to a common telephone central station have a high communications probability
Six hard-wire telephone lines are between Station 1 and Station 2. Assuming no mechanical failures, the probability is 100 percent that subscriber 4 will be able to talk to subscriber 11. Why? Because six wire lines are standing by ready to carry the conversation.
Now% consider another scenario.In this area of growth, the telephone company decides to use more modern technology to control cost and to conserve a natural resource, copper wire. Also note the introduction of what is called the trunk manager. The communications environment now has changed in that the probability of communications no longer stands at 100 percent. Is this really unacceptable? The consideration in the case of telephone service is the need to balance cost with the need to communicate. History and the marketplace show the average subscriber is not willing to pay extraordinary costs simply to ensure the absolute certainty of routine communications. The corollary of this statement is that the average subscriber will accept up to a certain level of calls that cannot be completed the first time they’re placed, provided that the failures do not exceed a group need established over time. From a practical point of view, the majority7 of subscribers are willing to concede that a 911 call has absolute priority over a personal communication, provided it does not interrupt a personal communication need too often.
What happens when a subscriber attempts to complete a call but can’t? A busy signal is something every subscriber experiences from time to time. Subscribers most often interpret the busy signal to mean that the subscriber being called is using the telephone. However, this is not the only reason a busy signal is received. Trunks between telephone-switching stations could be 100-percent utilized at the time the subscriber places a call, requiring that the call be routed through another switching station. Typically, the “trunk busy” signal is given at a faster rate than the “subscriber busy” signal. Nevertheless, the result is the same: The connection cannot be completed.
How does the telephone company identify the number of trunk lines needed between telephone switching stations? It monitors the duration and frequency of telephone calls placed between the stations. The standard operating procedures of telephone companies define a level of “probability of trunk availability” required for acceptable service. Trunks normally are switchable and assigned between certain stations during peak usage times and reassigned between other stations when peak usage times fall between the same stations. It is a very complicated system of communications path management. Again, the goal is to balance some acceptable level of probability that a trunk line is available against the costs of facilities and natural resources.
The trunk manager typically is a computer with a sophisticated program that watches the condition of trunk availability and call loading. It operates on predetermined probabilities.
The telephone system trunk management task becomes very obvious when the topic of long-distance telephone service is considered. Longdistance trunk management in the telephone industry not only includes management of wire-lines but also extends into radio communications spectrum management (resource management) when the contribution of microwave system channels to long-distance telephone communications is considered.*
