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Configuring Voice Ports

Configuring Digital Voice Ports

VC-54

Cisco IOS Voice, Video, and Fax Configuration Guide

Troubleshooting Tip for Cisco 803 and 804 Routers

Check to ensure that all cables are securely connected.

Configuring Digital Voice Ports

The digital voice port commands discussed in this section configure channelized T1 or E1 connections;

for information on ISDN connections, see “Configuring ISDN Interfaces for Voice” in this configuration

guide.

The T1 or E1 lines that connect a telephony network to the digital voice ports on a router or access server

contain channels for voice calls; a T1 line contains 24 full-duplex channels or timeslots, and an E1 line

contains 30. The signal on each channel is transmitted at 64 kbps, a standard known as digital signal 0

(DS0); the channels are known as DS0 channels. The ds0-group command creates a logical voice port

(a DS0 group) from some or all of the DS0 channels, which allows you to address those channels easily,

as a group, in voice-port configuration commands.

Digital voice ports are found at the intersection of a packet voice network and a digital, circuit-switched

telephone network. The digital voice port interfaces that connect the router or access server to T1 or E1 lines

pass voice data and signaling between the packet network and the circuit-switched network.

Signaling is the exchange of information about calls and connections between two ends of a

communication path. For instance, signaling communicates to the call’s end points whether a line is idle

or busy, whether a device is on-hook or off-hook, and whether a connection is being attempted. An end

point can be a CO switch, a PBX, a telephony device such as a telephone or fax machine, or a

voice-equipped router acting as a gateway. There are two aspects to consider about signaling on digital

lines: one aspect is the actual information about line and device states that is transmitted, and the second

aspect is the method used to transmit the information on the digital lines.

The actual information about line and device states is communicated over digital lines using signaling

methods that emulate the methods used in analog circuit-switched networks: FXS, FXO, and E&M.

The method used to transmit the information describes the way that the emulated analog signaling is

transmitted over digital lines, which may be common-channel signaling (CCS) or channel-associated

signaling (CAS). CCS sends signaling information down a dedicated channel and CAS takes place

within the voice channel itself. This chapter describes CAS signaling, which is sometimes called

robbed-bit signaling because user bandwidth is robbed by the network for signaling. A bit is taken from

every sixth frame of voice data to communicate on- or off-hook status, wink, ground start, dialed digits,

and other information about the call.

In addition to setting up and tearing down calls, CAS provides the receipt and capture of dialed number

identification (DNIS) and automatic number identification (ANI) information, which are used to support

authentication and other functions. The main disadvantage of CAS signaling is its use of user bandwidth

to perform these signaling functions.

For signaling to pass between the packet network and the circuit-switched network, both networks must

use the same type of signaling. The voice ports on Cisco routers and access servers can be configured to

match the signaling of most COs and PBXs, as explained in this chapter.

This section discusses the following topics:

• Prerequisites for Configuring Digital Voice Ports, page 55

• Preparing Information to Configure Digital Voice Ports, page 56

• Platform-Specific Digital Voice Hardware, page 58

• Configuring Basic Parameters on Digital T1/E1 Voice Ports, page 61

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