Replacing PBX Trunks: Out with the Old, In with the New


PBX systems often span a company's geographic locations, as shown in Figure 3-3. The connections used to tie the PBXs together are trunks, and a company pays monthly recurring charges to its telephone carrier for these trunk connections. However, more than calls are carried over these trunks. PBXs have signaling protocols used to communicate call setup information between them. For example, dialed digits and on-hook/off-hook conditions are sent across these trunk connections, too.

Figure 3-3. PBXs Before Convergence


These trunks are likely candidates for replacement as a company begins its migration toward a VoIP network. This initial migration step doesn't necessitate throwing out the PBXs. The PBXs can continue to service the telephony needs of the company. However, the difference, as shown in Figure 3-4, is that the PBXs are interconnected by the IP WAN instead of the trunk lines. By eliminating these trunk lines, the company can eliminate the recurring cost of the dedicated trunk lines.

Figure 3-4. PBXs After Convergence


The connection from a PBX to a local router can take one of several approaches. In Figure 3-4, notice that the PBX connects to the router using an Ear and Mouth (E&M) connection. An E&M interface is an analog interface present in many of today's PBX systems. Many say that the E stands for Ear, and the M stands for Mouth. Other authorities say that the E stands for Earth, and the M stands for Magneto. You might also read that the E is the E in rEceive, and the M is the M in transMit. Personally, I use the Ear and Mouth definition because it gives me a great visual image of the E lead being used for the receive function, and the M lead being used for the transmit function.

Note that the actual voice path doesn't use the E or M leads (that is, wires). The E&M leads are used for call signaling (that is, setting up and tearing down a call), but an E&M connection still uses tip and ring wires to transmit the actual voice. Instead of always having a single tip wire and a single ring wire, in some instances, two wires are used for "tip," and two wires are used for "ring," as shown in Figure 3-5.

Figure 3-5. E&M Connections


If a company already has an E&M interface in its PBX being used to form a trunk with a remote PBX, we can help preserve the company's original investment in its PBX E&M interface by connecting the PBX's E&M interface to an E&M interface in a router. The router then connects into the IP WAN using a traditional WAN interface, such as a serial interface. We don't have to use an E&M interface, however, to interconnect a PBX and a router. Other analog and digital interfaces are also available, as we'll discuss in the next couple of sections.




Voice over IP First-Step
Voice over IP First-Step
ISBN: 1587201569
EAN: 2147483647
Year: 2005
Pages: 138
Authors: Kevin Wallace

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