Cisco MGX 8850 VISM, Release 1.5 Installation and Configuration
Introducing the Cisco Voice Interworking Service Module (VISM)
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Introducing the Cisco Voice Interworking Service Module (VISM)

Table Of Contents

Introducing the Cisco Voice Interworking Service Module (VISM)

General Description


Physical Characteristics


Introducing the Cisco Voice Interworking Service Module (VISM)

This Chapter provides a brief description of the main features and functions of VISM.

General Description

The Cisco Voice Interworking Service Module (VISM) Release 1.5 is a front and back card set designed to operate on the Cisco MGX 8850 Wide Area Edge Switch platform.

A Cisco MGX 8850 Wide Area Edge Switch, when equipped with one or more VISM card sets, has the capability of transporting digitized voice signals across a packet network. Thus the VISM/MGX 8850 combination provides an interface or gateway between conventional voice TDM networks and networks based upon packet switching technology (see Figure 1-1).

Figure 1-1 MGX 8850 and VISM as a Network Gateway

VISM employs the concept of operating modes; a feature that permits it to be used in a variety of applications. In Release 1.5 there are two operating modes, namely, Voice over IP (VoIP) mode and AAL2 Trunking mode. These modes support three major applications as follows:

Using VoIP mode, this application provides the emulation of many of the functions of a Tandem (Class 4) switch. In this application VISM functions as voice gateway (also referred to as a Media Gateway) to an IP network and performs call control in conjunction with a Call Agent. The Call Agent (also referred to a Media Gateway Controller) initiates and controls the call control functions and VISM performs the transmission of the voice payload through a bearer circuit using VoIP and ATM AAL5 techniques.

Multiservice Access with Call Control
This application also uses VoIP mode. In this application the VISM/MGX 8850 combination is used to concentrate voice and data user services onto a single broadband circuit for transmission over the packet network again using VoIP and ATM AAL5 techniques. As with the Switching application, VISM can function with a Call Agent to perform Call Control.

AAL2 Trunking
This application uses AAL2 Trunking mode. In this application VISM/MGX 8850 combination is used to concentrate voice (and Fax/Modem) user services but with no call control. VISM merely passes (tunnels) bearer and signaling data across a packet network as a pre-povisioned AAL2 trunk.

Equipped with eight T1 or E1 ports, an array of Digital Signal Processors (DSPs), an HDLC framer, and a broadband interface to the packet network, VISM is ideally suited to processing high density digital voice circuits providing compression, echo cancellation, dejittering, and packetization on the fly.

VISM architecture provides:

Technology flexibility that allows the incorporation of new or improved technology as it becomes available

Application flexibility such that it can be used in a range of situations providing interoperability with a wide variety of equipment types

Modularity that allows equipment to be purchased and installed as it is needed in a step-by-step manner.


Key features of VISM card set are:

Provides 8 standard T1 or E1 interfaces with B8ZS, AMI & HDB3 line coding.

Support for voice over ATM using AAL 2 cells (multiplexing only, no LLC/SNAP encapsulation).

Voice over IP (VoIP) using AAL5 cells to RFC 1889.

Support for both PCM a-law and u-law.

Programmable 24, 32, 48, 64, 80, 96, 112, 128 ms taildelay near end echo cancellation.

Voice compression to G.711, G.726-32K, G.729a, G.729ab standards.

Nx64 clear channel (N = 1 only) support.

Voice activity detection (VAD) and comfort noise generation (CNG) using variable threshold energy (Cisco proprietary).

Configurable idle code suppression on AAL2 CIDs

Support for call agent Simple Gateway Control Protocol (SGCP) version 1.0, SGCP 1.1+, and Media Gateway Control Protocol (MGCP) 0.1.

Backhauling CAS signaling to Call Agent.

Support for CCS signaling transport across an AAL5 trunk.

Support for Fax and modem VoIP bearer transmissions.

Support for dual (redundant) virtual circuits across the packet network

Support for full continuity testing (COT). Supports origination and terminating loopback and transponder COT towards the packet bearer and the TDM sides.

Support for looptiming, payload and line loopbacks.

1:N cold redundancy using SRM-3T3 capabilities (bulk mode support for T1 lines only). Call do not persist during switchover.

Courtesy downing of ongoing voice calls when the VISM is taken out of service for maintenance or other reasons.

Physical Characteristics

Physically a VISM card set is installed in a MGX 8850 Wide Area Edge Switch as a front card and a back card.

There are two types of front card: a VISM-8T1 card that supports up to 8 T1 lines carrying digitized voice and a VISM-8E1 card that supports up to 8 E1 lines carrying digitized voice.

Available T1 back cards are:

RJ48-8T1-LM. This card supports 8 T1 lines using RJ48 connectors.

R-RJ48-8T1-LM. This card is used with a T1 front card in redundant configurations.

Available E1 back cards are:

RJ48-8E1-LM. This card supports 8 E1 lines using RJ48 connectors.

SMB-8E1-LM. This card supports 8 E1 lines using SMB connectors.

R-RJ48-8E1-LM. This card is used with a E1 front card in redundant configurations.

R-SMB-8E1-LM. This card is used with a E1 front card in redundant configurations.


This section defines words, acronyms, and actions that are used in this manual and which may not be readily understood.

AAL ATM Adaptation Layer

AIS Alarm Indication Signal

BERT Bit Error Rate Test

CA Calling Agent

CAS Channel Associated Signaling

CCS Common Channel Signaling

CID Channel Identifier

CLI Command Line Interface

CU Coding Unit

DSP Digital Signal Processor

DSPM Digital Signal Processing Module

DTMF Dual Tone Multi Frequency

ECAN Echo Cancellation

FDL Facility Data Link

HDLC High Level Data Link Control

ICS Idle Code Suppression

IP Internet Protocol

LM Line Module

LOS Loss Of Signal

MIPS Million Instructions Per Second

MG Media Gateway

MGC Media Gateway Controller

MGCP Medial Gateway Control Protocol

OAM Operation And Maintenance

PXM Processor Switch Module

PCM Pulse Code Modulation

PNNI Private Network-to-Network Interface

PVC Permanent Virtual Circuit

RAI Remote Alarm Indication

RTP Real-time Transport Protocol

SAR Segmentation and Reassembly

SGCP Simple Gateway Control Protocol

SRM Service Redundancy Module

SS-7 Signaling System 7

SU Signalling Unit

TSI Time Slot Interchange

UDP User Datagram Protocol

UFM Universal Frame-Relay Module

UVM Universal Voice Module

UXM Universal Switch Module

VAD Voice Activity Detection

VISM VoIce Interworking Service Module

VoIP Voice over IP

VSC Virtual Switch Controller

xGCP Generic acronym for a Gateway Control Protocol