Cisco Systems (VISM)

2-2

Cisco VISM Installation and Configuration Guide

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

Chapter2 Telephony Applications Using VISM

Tandem Switch Offloading

When a call is initiated, the central office can use either the Class 4 switch or VISM to handle the call.

When calls are passed to VISM, VISM backhauls the signaling to an SGCP or MGCP compliant call

agent (such as the Cisco VSC 3000). The call agent at the near end manages the call setup in conjunction

with the call agent at the far end and the calling and called VISM cards.

Note Although not shown in the diagram, an alternative method for handling CCS signaling is to transport

it directly between the central office and the call agent with no VISM involvement. With this

arrangement, there is no backhauling function performed in the VISM.

Each VISM card supports up to eight T1 or E1 lines for voice traf fic. You can use an alternati ve method

to connect the voice lines to the VISM cards—the TDM lines can be carried over a T3 line to an SRM

card in the Cisco MGX shelf where the individual T1 lines are b roken o ut a nd dist ributed to the VI SM

card internally. Refer to the Cisco MGX 8250, Release 1 and the Cisco MGX 8 230 inst alla tion an d

configuration documents for details of the SRM card.

The VISM connects to the ATM network using either VoAAL2 or VoIP (UDP/IP packets encapsulated

in AAL5 PVCs). VISM and the call agent communicate with each other and their activities are

coordinated through either SGCP or MGCP.

For VoIP, when the call setup procedure is complete, each VISM has the IP address of the other VISM

associated with the call. An end-to-end IP bearer circuit is established between the calling and called

parties. At this point, the voice conversation can proceed.

By way of example, Figure 2-1 shows only one location for the VISM and call agent; in reality there is

a similar arrangement for each tandem switch.

Figure 2-2 shows the connection from VISM to the call agent in greater detail.

Figure2-2 VISM Used as a Voice Gateway Application

The VISM/MGX 8000 shelf is connected to the network by an OC-3 line which is used for both the voice

payload and the communication with the call agent. A network edge router moves the voice traffic across

the network to the called party’s VISM and routes call control information between the VISM and the

call agent.

For reliability, two PVCs using separate physical links to two separate edge routers to the packet network

can be established. If the primary circuit fails, transmission automatically switches to the secondary

circuit. For enhanced reliability, the physical OC-3 links to the network can be protected by the SONET

APS feature.

Using all the available slots, the Cisco MGX 8850 and the Cisco MGX 8250 can be configured w ith up

to 24 VISM cards and the Cisco MGX 8230 can be configured with up to 8 VISM card s.

SS7 Network

MGX 8850

with VISM

Call Agent

Packet

Network

PBX

Voice

Gateway

Data

Services

PSTN

Voice

Customer's Premises

53234