Telephone exchange how does it work

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In the modern age, switchboards exist and operate in generally the same way but with modern equipment and electronics, but the big rooms and hundreds of operators can still be seen. International Wired Call: The switchboard will likely have access to a physical cable that is connected to France via the north sea, so these types of calls are trickier but still quite easy to access.

Your call will be sent into space, then be reflected on a satellite and sent back down to the person who you are calling. The conflicts between electricity wires, phone wires, cable TV cables and trees is still largely unresolved.

After WW2 some countries made efforts to put the phone lines underground but this decision was often made on other grounds than aesthetics. In Australia, for instance, the decision was strongly influenced by the cost of repairs to overhead lines following storms and bushfires.

The development of Internal Bearer Cable a bundle of wires wrapped around a steel wire core to take the weight extended the life of pole construction and it is still common today. The photo at left, courtesy of Marvin Hodges, is a typical U.

Back at the exchange, calls were connected by an operator using a cord with a plug at each end. As a caller rang in, a small metal plate "shutter" would drop, revealing the caller's number. The operator would take one end of a cord and plug it into a socket jack corresponding to their number. The operator would ask for the number they wanted "number, please" by pressing a switch that cut her headset into the circuit, and partly plug the other end of the cord into the wanted party's socket.

She could hear if there was a call in progress "busy" or "engaged". Once the call was established she would restore the drop shutter to its position, then handle another call. A firm called American Electric soon invented a system that rang the called number as soon as the plug was fully inserted. At the end of the call, callers were expected to "ring off" - ring their generators to signal the operator they had finished.

She would then remove the plugs. Although male telephonists boys were used in the early days, young ladies were soon found preferable as they were quieter, more stable, not cheeky to the customers, and could be paid less. The telephone exchange was one of the few forms of employment for a young lady for many decades, and the girls were ruthlessly supervised. They were expected, of course, to resign on marrying. The unit hanging from the top of the board is a transmitter for the operator to talk into.

It was soon replaced with the operator's "head and breast set", an early earphone and microphone set that could be plugged into a special socket on the switchboard. At the top of the board are the drop shutters to indicate an incoming call. Under these are the panel of sockets for each subscriber's line, into which the cords were plugged. The cords hanging from the bottom of the board are the connecting cords, weighted at the bottom so they will automatically retract back to the flat shelf when disconnected.

One improvement to the system was introduced steadily from the late s, and was almost universal for larger exchanges by the s. It allowed a bank of batteries at the exchange to power the entire system, doing away with the batteries in the customer's phone. This reduced maintenance and allowed the telephones to be made smaller. Signalling the exchange was done by simply picking up the handset. As current flowed through the phone,it turned on a small lamp on the exchange switchboard.

There were different versions of the system introduced by many manufacturers, each having its advantages and technical improvements. It still required a room full of operators to switch the calls, though.

The growth in the number of customers meant that the switchboards were getting bigger. So big, in fact, that the Multiple system had to be invented so an operator could handle a few hundred customers and pass calls to another switchboard that handled the other party's number. The exchange was now taking on the look that it would keep for the best part of the twentieth century.

Left: Manual exchange, Photo courtesy Marvin Hodges. The photo gives an idea just how large and crowded a manual exchange could be in terms of numbers of staff.

Remember, this is only one shift. As well as the telephonists working other shifts, there were technicians, linesmen and administrative staff.

The telephone company was a major employer in most communities. The exchange was now a major building in its own right, not the small "office" of the earlier days. Typically the exchange would have a huge room full of switchboards and operators, a separate room for the power supply, generators and batteries, offices for the administrative staff, and a room for the technical and maintenance staff.

The incoming cables, which could now be many inches thick, were split up and wired to a Main Distribution Frame. Technicians could test lines or connect and disconnect them at this point. A pair of wires for each phone line led from the MDF to the appropriate switchboard. Left: Incoming cables from the cable tunnels were brought up from underground into the exchange. These cables were usually lead-sheathed and often pressurised with dry air to keep moisture out.

From Poole "Practical Telephony Handbook" Left: The cables were broken out of their sheath and each pair of wires was connected to a set of terminals on a Main Distribution Frame. The MDF also carried fuses. It allowed lines to be connected or disconnected , and tested for faults. In bigger exchanges the MDF often occupied a room of its own. Pairs of wires from the MDF were led usually overhead to the switchboards.

Photo courtesy Marvin Hodges, ca Hundreds of operators worked on each switchboard in towns and cities, and the service was efficient and largely confidential. In smaller neighbourhoods, the switchboard might be operated by a single individual. The rapid increase in demand for telephones and reliable connections posed a problem for the telephone companies.

More lines meant more switchboard operators—an expense they were keen to control. And fast as they worked, operators could be overwhelmed by the volume of calls at busy times, leaving callers queuing for a line.

Many inventors began to put their minds to automatic methods of handling switching, so that customers could dial each other directly. Strowger apparently suspected that a telephone operator was diverting requests for his services to a rival firm of undertakers. The key component of the system was a telephone with a rotary dial. Just like a modern keypad, the dial had 10 positions numbered from 1 to 0. The caller would place a finger in the hole over the number required, and rotate the dial to its stopping point.

As the dial rotated back to its starting position, it sent electrical signals down the line. This controlled the stepping of the selectors. The Post Office opened the first automatic exchange on the public network at Epsom in Surrey in May It took decades for automatic dialling to roll out across the country. Installing an automatic exchange was expensive, and while savings were made by making operators redundant, it took time for this to generate cash.

The two world wars also had a depressing effect on investment. One of the last manual switchboards in the UK—and the last to run in London—was at Enfield, north London. It switched to automatic connection at In the early days of automation, there were no area codes. In order to automate large cities such as London, the GPO introduced three-letter exchange codes such as WIM for Wimbledon at the beginning of each seven-digit number.