Cisco Systems (VISM) Voice TDM Network Interface, ATM Network Interface

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Cisco VISM Installation and Configuration Guide

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

Chapter1 Overvi ew of the VISM and VISM-PR Cards

VISM and VISM-PR Card Features

Figure 1-6 shows the major functional blocks and interfaces for the VoIP switching and switched AAL2

PVC operating modes.

Figure1-6 VISM Block Diagram for VoIP Switching and Switched AAL2 PVC Operating Modes

The CAS signaling path on the TDM side is embedded in the voice stream but is separated at the bearer

processing function. The CAS signaling then joins the CCS signaling path for CAS/ CCS processing and

is backhauled to the call agent. The path between the call agent and bearer processing, via a connection

handling function, is for call setup and teardown.

Voice TDM Network Interface

The voice payload path is shown as a solid line along the bottom of Figure1-6. All external TDM

streams arrive and depart on the T1/E1 lines. Depending on the ap plica tio n, t hes e st rea ms co nsi st of

voice bearer channels (with or without CAS signaling) and separate CCS channels (if CCS signaling is

used). The TDM line handling function provides the physical layer interface, which includes framing ,

line codes, clocking, loopbacks, physical alarms, etc. Bearer channels, includi ng CAS, are sent to the

bearer processing function. CCS channels are sent to the CAS/CCS processing function.

Further processing of the bearer channels is performed by the DSPs. This processing provides ECAN,

compression, A/Mu law conversion, silence suppression, and fax/modem handling. If CAS signaling is

present, signaling bits are extracted at the DSP stage and sent to the CAS/CCS processing function.

ATM Network Interface

The ATM processing function receives the processed DS0 voice streams and prepares them for transport

over a packet network. The voice strea ms are divided into specific sample periods (f or e xam ple , 5 ms or

10 ms) and formatted into service specific convergence sublayer (SSCS) packets appropriate for the

method of transport over the ATM network. The available transport methods are VoIP (using AAL5) and

voice over AAL2. Processing of the ATM packets further segments the voice payload into ATM cells for

transport over the network using a SONET port on the PXM card.

TDM Line

Handling

Bearer

Processing ATM Processing

CAS/CCS

Processing

Connection

Handling

To ATM

Netwo

3231

CCS

CCS (Q.931)

CCS

CAS

Voice

CAS (xGCP)

Voice

Connection Setup

Teardown

xGCP

Call

Agent

VISM

Voice payload

1/E1 Lines