Posted by: Mudassir Ali | June 18, 2010

Using Analogue E&M Ports to Interface to Overhead Paging Systems

Using Analogue E&M Ports to Interface to Overhead Paging Systems

Introduction

This document details the background theory and configurations that allow a router Ear and Mouth (E&M) voice port to interface to an overhead paging system.

Prerequisites

Requirements

Ensure that you meet these requirements before you attempt this configuration:

· Analog E&M signaling theory and router voice port operation

· Cisco IOS® configuration and Cisco CallManager configuration

Components Used

The information in this document is based on these software and hardware versions:

· Cisco 2610 Router

· Cisco IOS version 12.2.7a with an IP Plus feature set

· NM-2V voice carrier card and a VIC-2E/M (E&M Voice Interface Card (VIC))

· External paging amplifier

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Background Information

Many sites with an existing PBX also have a paging system which allows users to call an extension on the PBX that forwards the audio broadcast to overhead loudspeakers. This concept is useful in workshops, parking lots, and open plan areas where a called party is not near a telephone handset. PBX manufacturers can provide dedicated line cards that interface with external paging amplifiers. These PBX paging cards have an isolated audio output, which prevents faults on the paging amplifier that results in damage to the PBX, and a control or relay output that is used to activate the paging amplifier.

As IP-based PBXs and Voice over IP (VoIP) networks become more common, the need to integrate voice-equipped routers into legacy installations is apparent. New paging systems are available. These systems use loop start trunks that interface directly to PBX extension ports and have Voice Operated Relays (VOX) that control the paging amplifiers. Many customers do not wish to purchase new interfaces or replace existing hardware as they transition to IP-based systems. Fortunately, Cisco voice products are flexible enough to cover many of these cases. This document details the method of how to use analog E&M voice interfaces to provide an audio and control output to an interface with an external paging amplifier. Many dedicated PBX paging cards are based on normal PBX E&M line cards.

The difference between a conventional two wire telephone interface, such as Foreign Exchange Station or Office (FXS or FXO), and an E&M interface is that the E&M interface has wires that pass the audio signals plus additional wires to act as an input (to sense an incoming call) or an output (to indicate an outgoing call). These control leads are normally called the E lead (input) and the M lead (output). The signaling leads can be controlled if you connect them to the ground, switch a negative 48Volt DC source, or complete a current loop between the two devices. This depends on the type of E&M interface.

E&M interfaces normally have the option of two or four wire operation. Rather than referring to the total number of physical connections on the port, two or four wire operation relates to the how audio is passed between the devices. Two wire operation means the audio signals that transmit and receive are passed through a single pair of wires (one pair equals two wires). Four wire operation separates the direction of the signal and uses one pair to transmit and another pair to receive audio.

By default, Cisco E&M ports use wink start signaling. Wink start operation dictates that when the voice port goes off hook (E leads the state changes from on hook to off hook), it expects to receive a 200 millisecond wink (on hook/off hook/on hook) transition on the M lead as the acknowledgement allows digits to be sent. The E lead stays in the off hook condition for the duration of the call.

A simpler form of E&M signaling is called immediate start. In this mode, when the voice port goes off hook (E lead changes from on hook to off hook), there is a brief pause. The router then sends the digits without any acknowledgment from the external device. As with wink start, the E lead stays in the off hook condition for the duration of the call.

When the voice port is configured as two wire with immediate start, an outgoing call (from the IP side toward the external device) makes the E lead change from an open circuit, to a short circuit, to the ground. Control leads can be used to switch a relay or push-to-talk control and the audio path opened on the transmit/receive (T/R) leads.

Configure

In this example, the customer has a requirement to interface an older paging system into a new Cisco CallManager installation. A Cisco 2610 Router with an E&M VIC is used. The paging amplifier has an audio input and an external push-to-talk control input. These interface pinouts are used between the router E&M voice port and the amplifier:

T1 (Pin 4)

Using Analogue E&M Ports to Interface to Overhead Paging Systems.pdf

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