Traffic Types

Both lines and trunks are DS0's ! ! !  The definition of a line is that it goes from the customer location to the End Office, whereas a trunk goes from the end office to the Serving Wire Center.

In Switched Access terminology, inter-LATA calls are referred to as voice traffic, or Switched Access Traffic. Calls going out of
the LATA are considered originating traffic, while calls coming into the LATA are considered terminating traffic.  

Originating traffic uses a line-to-trunk connection. 
Terminating traffic uses a trunk-to-line connection.

Keep in mind that each line or trunk can carry only a single call at any one time. While there may be thousands of lines coming into an End Office, not all the end users will be making long distance calls at any one time. Interexchange Carriers only need the right number and mix of trunks to handle their share of the traffic.

Access customers can order trunks to carry originating traffic only, terminating traffic only, or 2-way Traffic.

Overflow - a key to designing trunks that will handle all calls is the concept of "overflow", where if one trunk fills up, another trunk will accept the traffic.  The IXC can order various types of voice-traffic trunks from the LEC, including Trunks that overflow and trunks that do not overflow  -  as well as trunks that accept overflow, and trunks that will not accept overflow.