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Incoming Calls |
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Outgoing Calls |
Lines/Trunks
In this section on PRI and T1, lines are the representations that appear on extension telephones or that are put into pools. They represent the type of service requested on a call. Trunks are the facilities that link switches. For all facilities except DS1, lines have a one-to-one correspondence to trunks because there are 24 transmission channels for each DS1 connection. With PRI, lines are further removed from trunks because the type of service is not linked to the B-channel (trunk). The system has an intermediary called a B-channel group (BCG). Lines are used to place and receive calls, and a BCG links B-channels to lines. B-channel groups may be either a single B-channel or multiple B-channels grouped together.
The figure "PRI Lines and B-Channel Groups" shows how lines, B-channels, and B-channel groups function together. For outgoing calls, a user selects a PRI line that routes the call to a B-channel group. The BCG selects an open B-channel and connects over the PRI connection. For incoming calls, the network selects an open B-channel and the BCG directs the call to the PRI line for which it has been intended: it matches the Called Party Number with the line's programmed telephone number. In addition, the Dial Plan Routing feature may be used to further direct the call to a specific extension (SA button) or Calling Group by matching some portion of the Called Party Number against the system dial plan. Dial plan routing is similar to Direct Inward Dial.
Each DS1 module is given 24 lines, whether or not it is used for emulation of lines/trunks or for PRI.