Cisco 3600 Series Configuration Manual - Page 8

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Configure IP Networks for Real-Time Voice Traffic

For more information about Multilink PPP, refer to the "Configuring Media-Independent PPP and

Multilink PPP" chapter in the Dial Solutions Configuration Guide.

Multilink PPP Configuration Example

The following example defines a virtual interface template that enables Multilink PPP with

interleaving and a maximum real-time traffic delay of 20 milliseconds, and then applies that virtual

template to the Multilink PPP bundle:

interface virtual-template 1

multilink virtual-template 1

Configure RTP Header Compression

Real-Time Transport Protocol (RTP) is used for carrying packetized audio traffic over an IP network.

RTP header compression compresses the IP/UDP/RTP header in an RTP data packet from 40 bytes

to approximately 2 to 4 bytes (most of the time), as shown in Figure 4.

This compression feature is beneficial if you are running Voice over IP over slow links. Enabling

compression on both ends of a low-bandwidth serial link can greatly reduce the network overhead if

there is a lot of RTP traffic on that slow link.

Typically, an RTP packet has a payload of approximately 20 to 160 bytes for audio applications that

use compressed payloads. RTP header compression is especially beneficial when the RTP payload

size is small (for example, compressed audio payloads between 20 and 50 bytes).

Figure 4

Before RTP header compression:

After RTP header compression:

VC-20

Voice, Video, and Home Applications Configuration Guide

ppp multilink

encapsulated ppp

ppp multilink interleave

ppp multilink fragment-delay 20

ip rtp reserve 16384 100 64

RTP Header Compression

20 bytes

8 bytes

12 bytes

UDP

RTP

Header

2 to 4 bytes

IP/UDP/RTP header

Payload

20 to 160 bytes

Payload

20 to 160 bytes