THE TELEPHONE SYSTEM

Historical review

1876 ​​Alexander Graham Bell patents the telephone. These phones came in pairs and you had to run a wire between them.

1878: Bell forms the Bell Telephone Company which opens its first switching office. The company ran a cable to each customer's home or office. To make a call, the user first communicated with the switching office, and from this office connected manually with the receiver through a jumper cable.

When the need to make long-distance calls begins, Bell System began to connect the switching offices, however, the wiring problem reappears, generating second-level switching offices. Figure 2-14 c. Ultimately, the hierarchy grew to five levels.

1890: The three main parts of the telephone system are defined: the switching offices, the cables (twisted pairs) between customers and offices (local loops), and the long-distance connections between the switching offices (trunks).

System Evolution

Today, there is a hierarchy of offices. The trend is towards digital signage. The following figure shows a connection in case the called telephone is connected to the same end office. Called direct connection between two local loops. This connection remains intact for the duration of the call.

If the phone called is connected to another end office. Each end office has several outgoing lines to one or more nearby switching centers, called charge offices (if within the same local area, tandem offices). These lines are called charge connection trunks. When the caller and the called party have a connection trunk to the same charge office, the connection can be established within the charge office.

If the caller and the called party do not have a common charge office, it is divided into primary, sectional and regional offices that form a network that connects the charge offices through high-bandwidth intercharge trunks (or inter-office trunks). ).