Essential Guide to VoIP PBX
This article provides comprehensive information about voIP PBX.
What is a PBX?
A PBX (Private Branch Exchange) consists of equipment and software that follow the PSTN (Public Switched Telephone Network) within an organization, and provides paths into the PSTN. It is important to remember that voice services are based on TDM (time division multiplexed) streams of data, the basis for most connections. These systems can be categorized into four primary areas, with each area containing one or more functions:
Extension termination
Resources on the private side of the PBX are called extensions, and have a direct one-for-one connection to a port on the PBX. While typically digital, there are analog extensions. The PBX must also provide these extensions with dial tone generation, just as the PSTN switch provides for non-PBX attached phones. These interfaces also pass the DTMF tones to the call-processing engine.
Trunk Termination
Most PBX systems are connected by at least one T-1 circuit to either the PSTN or another PBX. A trunk is a T-1 or other circuit that can carry multiple channels, or TDM data stream. A T-1 can support up to 24 voice connections, while E-1 allows 30 user channels. Tie lines are trunks that connect PBXs to each other.
Call Processing and System Logic
Dial plans compare DTMF tones to route calls along paths configured on the PBX. These tones represent the numeric values of the buttons, in addition to the asterisk (*) and pound (#) keys. This addressing is used to route calls.
Switching
Switching maps a channel on one interface to a channel on another interface. This may involve linking a DS-0 to a DS-1 (T-1), or an FXS port to a T-1 trunk on another PBX. The logic that decides which path to be taken is part of the call processing function.
PBX Terminology
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.
The discussion so far has focused on traditional means of providing voice telephone service. However, such information provided thus far is also applicable when using IP networks and techniques.
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