On Distributed Communications Series
IV. Priority, Precedence, and Overload
Commercial Telephone Traffic Overload Protection Techniques
The following techniques are either in use by or are being considered by the Bell Telephone Company to minimize system degradation under overload.
1. Reduced Inter-sender Timing
The length of time spent by a sender waiting for dial pulses from the subscriber's telephone is reduced under heavy traffic conditions. For example, upon lifting the telephone from its cradle a dial tone is heard. If you do not start to dial immediately, you needlessly tie up shared common-control equipment. Under overload, the period that the "sender unit" waits before assuming that the subscriber is balking is reduced. This forces the subscriber to dial his number more quickly.
2. Recorded Message
When there is an overload in the Direct Distance Dialing system, calls can be passed to an automatic playback device. A canned recording announces an overload condition, and requests that the caller wait and place his call later. A variation on this recording is used in emergency. Such recorded messages have proved to be highly effective in the past. When people are specifically requested not to use the telephone unless it is an emergency, they will generally refrain from doing so.
3. Operator Spacing
During dial overload, a manual operator can cut in to ask for the number being called. The operator will tell the calling party that she will call back later when the line is free. This effects a delayed spreading of calls during peak periods.
4. Variable Divisions of Circuits
Whenever only two circuits are left in a group of trunks between two cities in the Direct Distance Dialing switching hierarchy, these two lines are reserved for calls approaching from the higher level of the switching hierarchy. This procedure provides precedence for calls that have proceeded a greater distance into the system than incoming calls.
5. Network Management
On certain peak days, such as Christmas, network management policies are enforced which prohibit alternate routing. A traffic supervisory console is used to prevent round-about alternate routes during periods of heavy network use, and to restrict traffic to short efficient routes. The decision to invoke such network management is limited to those in the Regional Offices of the DDD system. (All Regional Control Centers intercommunicate on a single "hot-wire" loop circuit, which permits each center to inform the others of its actions.)
6. Switching Operator Position
During conditions of high overload in which the pattern of demand changes in a predetermined manner (on Mothers' Day, for example), there is a shortage of operator positions in the suburbs and a surplus in the cities. It is possible to remotely switch operator positions and transfer trunks by key switches to process local suburban traffic. An operator position in downtown New York can be wired to handle calls originating in a particular suburb (alleviating overload peaks).
7. Line Load Control
Line load control, as presently practiced, divides all subscribers into three separate groups. First there are the subscribers who are deemed to merit communications in emergencies. Such users include police, fire, mayors, newspapers editors, pay telephone booths, etc. These users are in turn divided into two groups. Each group comprises about 40-45 per cent of all subscribers. During emergencies, a switch at the Central Office can be closed to deny access to new calls to one group of 45 per cent of the users. After ten minutes, service is then denied the second 45-per-cent group. No one's connection is interrupted during line load control; it is simply that no new calls will be accepted from the 45 per cent to whom access is denied.
It is said to be possible to call the operator in an emergency, to identify yourself, tell her it is an emergency, and expect that she will handle the call in the appropriate manner (this is sometimes called Category "O" Priority within the Bell System).