Line Service
From TBwiki
Line service defines the payload type of a line interface or another line service. The line service chain stops when it reach the E1/J1/T1 line service.
The E1/J1/T1 line services are also known as trunks or spans.
Example
When configuring a DS3 line interface, it is required to add a DS3 line service then add E1/J1/T1 for that DS3 line service.
As illustrated in the picture below:
