Voip Protocol Stack; Figure; Voip Cell Packetization And Transmission - Cisco Voice Interworking Services (VISM) Installation And Configuration Manual

ATM Voice Data Processing Function
Encapsulated in an IP packet.
3.
Converted to AAL5 ATM cells for transmission to an edge router on the network.
4.

Figure 3-7

Figure 3-7
RTP
Figure 3-8
Figure 3-8
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.
Cisco VISM Installation and Configuration Guide
3-12
shows the protocol stack for VoIP.

VoIP Protocol Stack

Signaling and Control (xGCP)
xGCP
UDP
(RFC 1483)
IP
AAL5
ATM
ATM
VISM PXM
shows how a voice sample is packetized and transmitted as cells.

VoIP Cell Packetization and Transmission

80 bytes of packetized PCM
(10 ms sample)
80-byte packet + 12-byte RTP header
= 92 bytes
IP (20 bytes) + UDP (8 bytes) + 92 packet
= 120
3 ATM cells per 10 ms
Voice Payload (RFC 1889)
IP
AAL5
ATM
Router
16 bytes padding
8 bytes AAL5 trailer
= 144 bytes of AAL5 PDU
Release 3.0, Part Number OL-2521-01 Rev. D0, June 2004
Chapter 3 VISM Functional Description
xGCP RTP
UDP UDP
IP IP
Call Agent Remote Media
Gateway