Specifications

Cisco 1751 Router Software Configuration Guide

OL-1070-01

Voice over IP Overview

Voice over IP (VoIP) enables a Cisco 1751 router (hereafter referred to as the router) to carry voice

traffic (for example, telephone calls and faxes) over an IP network. Cisco’s voice support is

implemented using voice packet technology. In VoIP, the digital signal processor (DSP) segments the

voice signal into frames and stores them in voice packets. These voice packets are transported using IP

in compliance with the International Telecommunications Union-Telecommunications (ITU-T)

specification H.323, the specification for transmitting multimedia (voice, video, and data) across a

network. Because it is a delay-sensitive application, you need to have a well-engineered, end-to-end

network to successfully use VoIP. Fine-tuning your network to adequately support VoIP involves a

series of protocols and features to improve quality of service (QoS). Traffic shaping considerations must

also be taken into account to ensure the reliability of the voice connection.

VoIP is primarily a software feature; however, you must install the voice interface cards (VICs) in the

router. For more information about installing a VIC in the router, refer to the Cisco WAN Interface

Cards Hardware Installation Guide.

Voice Primer

The Voice Primer section provides supplementary information for those users unfamiliar with voice

telephony. To understand Cisco’s voice implementations, it helps to have some understanding of the

analog and digital transmission and signaling. This section provides some very basic, abbreviated voice

telephony information as background to help you configure VoIP, Voice over Frame Relay, Voice over

ATM, and Voice over HDLC and contains the following topics:

• How VoIP Processes a Typical Telephone Call

• Numbering Scheme

• Analog Compared with Digital

• CODECs

• Delay

• Echo

• Signaling