Multiservice Networking, IPX Networking, and Security - CCIE Routing and Switching Exam Certification Guide - page 548

Example 12-1 VoIP Configuration Example (Continued)

!
dial-peer voice 8000 voip
destination-pattern 8....
session target ipv4:10.10.10.1
ip precedence 5
!
dial-peer voice 8001 voip
destination-pattern 7....
session target ipv4:10.20.20.1
ip precedence 5
!
interface serial 0
ip address 10.1.1.1 255.255.255.0
ip rtp header-compression
ip rtp compression-connections 25
!
voice-port 1/1
!
voice-port 1/2

Ports

Several ports can connect to voice end stations (phones) and voice switches:
•

Foreign Exchange Station (FXS) —Connects to an analog phone and fax machines;
provides dial tone and ring voltage.

•

Foreign Exchange Office (FXO) —An RJ-11 connector that allows an analog connection
to be directed at the PSTN's central office (CO) or to a station interface on a private branch
exchange (PBX). The FXO sits on the switch end of the connection and plugs directly into
the line side of the switch, so the switch thinks that the FXO interface is a telephone.

•

Ear and mouth (E&M) —An analog trunk that connects to a voice switch; supports tie-
line facilities or signaling between phone switches.

•

Channelized T1 (or E1) —A digital trunk line that connects to a phone switch where each
digital service zero (DS0) supports an active phone call connection.

Erlangs

An Erlang is a unit of telecommunications traffic measurement that represents the continuous
use of one voice path. In practice, it describes the total traffic volume of one hour. Erlangs deter-
mine voice call usage for bandwidth requirements for voice network designs, including VoIP.