Call Types in Telephone Switches: Originating, Terminating, and More

This page defines call types in a telephone switch, focusing on terms used in teletraffic engineering. The call types we’ll cover are: Originating (O), Terminating (T), Outgoing, Incoming (I), Intra-office, Tandem, O+T, O+I, and others. These terms are particularly relevant when discussing telephone switches.

Let’s define each call type with respect to the figure above, keeping in mind that all relate to the MSC (Mobile Switching Center).

Originating Call (O)

• A call placed by a subscriber of the telephone office.
• Example: If caller A calls callee B, then for subscriber A, it’s considered an originating call.

Terminating Call (T)

• A call received by a subscriber of the office.
• Example: If caller A calls callee B, then for subscriber B, it’s considered a terminating call.

Outgoing Call

• A call going out of the telephone switch or office. This signifies a connection being established with a destination outside of the current switch’s network.

Incoming Call (I)

• A call coming into the telephone office from another MSC. This indicates a connection originating from a different switch.

Intra-office Call

• A call which originates from and terminates to the same MSC. Essentially, the call remains within the local exchange.

Tandem Call

• Calls which come in on a trunk from another MSC and go out over a trunk to a different MSC. This type of call is routed through an intermediate switch (the “tandem” switch) to reach its final destination.

O+T Call

• The measure of traffic load on the line side of the MSC, accounting for both calls originating within the switch and those terminating from other telephone switches. It represents the total traffic handled on the subscriber lines.

O+I Call

• The measure of incoming trunk circuit traffic load and overall traffic load on the MSC. This gauges the load imposed by calls originating from other switches and impacting the MSC.