Nonblocking Minimal Spanning Switch - Practical Implementations of Switches

Practical Implementations of Switches

As soon as the algorithm was discovered, Bell system engineers and managers began discussing it. After several years, Bell engineers began designing electromechanical switches that could be controlled by it. At the time, computers used tubes and were not reliable enough to control a phone system (phone system switches are safety-critical, and they are designed to have an unplanned failure about once per thirty years). Relay based computers were too slow to implement the algorithm. However, the entire system could be designed so that when computers were reliable enough, they could be retrofitted to existing switching systems.

Eventually, a lock-step dual computer was developed using transistors. In this system, two computers performed each step, checking each other. When they disagreed, they would diagnose themselves, and the correctly-running computer would take up switch operation while the other would disqualify itself and request repair.

It's not difficult to make composite switches fault-tolerant. When a subswitch fails, the callers simply redial. So, on each new connection, the software tries the next free connection in each subswitch rather than reusing the most recently released one. The new connection is more likely to work because it uses different circuitry. As fewer connections pass through a subswitch, the software routes more test signals through a subswitch to a measurement device, and then reads the measurement. This does not interrupt old calls that remain working. If a test fails, the software isolates the exact circuit board by reading the failure from several external switches. It then marks the free circuits in the failing circuitry as busy. As calls using the faulty circuitry are ended, those circuits are also marked busy. Some time later, when no calls pass through the faulty circuitry, the computer lights a light on the circuit board that needs replacement, and a technician can replace the circuit board. The next test succeeds, the connections to the repaired subswitch are marked "not busy", and the switch returns to full operation.

The diagnostics on Bell's early electronic switches would actually light a green light on each good printed circuit board, and light a red light on each failed printed circuit board. The printed circuits were designed so that they could be removed and replaced without turning off the whole switch.

The eventual result was the Bell 1ESS switch (electronic switching system 1). Initially it was installed on long distance trunks in major cities, the most heavily used parts of each telephone exchange. On the first Mother's Day that major cities operated with it, the Bell system set a record for total network capacity, both in calls completed, and total calls per second per switch; which resulted in a record for total revenue per trunk.