Cisco Systems (VISM)

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Cisco VISM Installation and Configuration Guide

Release 3.0, Part Number OL-2521-01 Rev. D0, June 2004

Chapter3 VISM Functional Description

ATM Voice Data Processing Function

3. Encapsulated in an IP packet.

4. Converted to AAL5 ATM cells for transmission to an edge router on the network.

Figure 3-7 shows the protocol stack for VoIP.

Figure3-7 VoIP Protocol Stack

Figure 3-8 shows how a voice sample is packetized and transmitted as cells.

Figure3-8 VoIP Cell Packetization and Transmission

At the layer containing RTP, a 12-byte header is added. At the layer containing UDP, an 8-byte header

is added and at the layer containing IP, a 20-byte header is added for a total of 120.

At the layer containing AAL5, the 8-byte AAL5 trailer is added and the data is padded with 16 bytes to

make an integral number of ATM cell payload bytes. The resulting protocol data unit (PDU) is 144 bytes

in length and is transported in three ATM cells.

A single PVC is set up between the Cisco MGX 8000 series platform and the router. All packets are sent

across the PVC regardless of their destination. The router extracts the IP addresses and routes the cells

accordingly.

RTP RTPxGCP xGCP

UDP UDP UDP

IP IPIP IP

ATM ATM AT M

AAL5 AAL5

VISM PXM Router Call Agent Remote Media

Gateway

Voice Payload (RFC 1889)

Signaling and Control (xGCP)

(RFC 1483)

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80 bytes of packetized PCM

(10 ms sample)

80-byte packet + 12-byte RTP header

= 92 bytes

IP (20 bytes) + UDP (8 bytes) + 92 pack

= 120

16 bytes padding

8 bytes AAL5 trailer

= 144 bytes of AAL5 PDU

3 ATM cells per 10 ms

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