PBX Terminology | Cisco Voice Over IP

The following is a list of terms that you should be comfortable using when working with PBXs.

• T-1 24 voice channels (DS-0s), with total bandwidth of 1.544 Mbps.

• ISDN PRI – T-1 Uses T-1 framing, but uses one DS-0 for upper layer signaling (23 voice channels).

• E-1 European standard. 2.048 Mbps (32 voice channels).

• ISDN PRI – E-1 Uses two channels for signaling and framing, (30 voice channels).

• E&M Analog signaling method. Used for trunk or tie lines between switches (network-to-network), and for connections to voice mail or legacy PBX systems.

• Foreign Exchange Station Link between the switch and an extension. Sometimes used to describe a connection that services an analog device attached to the PBX.

• Foreign Exchange Office Link between the PBX and the central office. It is a analog DS-0 tariffed at a flat-rate.

• Loop start Removing the receiver from the hook closes a circuit and creates a loop, allowing connections.

• Ground start Earth ground is needed to complete the loop and allow connectivity.

• Central office Local telephone company termination point for all numbers in a given area, and commonly connects to PBXs via T-1s.

• Coordinated Dial Plans (CDP) Defines numbers on your network and how callers will reach numbers outside your dial plan (for example, a coordinated dial plan may require a nine to be dialed to reach an external number).

• Call routing Physical act of routing a call through the network, and processing the call. This is static in PBX systems.

• Tip-and-Ring In single pair copper connections, identifies which end supplies the voltage on the wire.

• Direct Inward Dial (DID) Establishes a relationship between the extension and a public number. Assigns a block of numbers to a trunk line from the telephone provider to the PBX, and the PBX administrator can route those numbers to related extensions. Figure 1 illustrates the logical mapping of number 415-555-1706 to extension 51706. Please note that it is quite common to create five-digit extensions in North America that relate to the assigned DID numbers.

Figure 1: Direct Inward Dialing Illustrated

Our discussion so far has focused on traditional means of providing voice telephone service. The information provided thus far will help you to transition to accomplishing the same tasks using IP networks and techniques.