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Cisco VISM Installation and Configuration Guide
Release 3.0, Part Number OL-2521-01 Rev. D0, June 2004
Chapter2 Telephony Appli cations Using VISM
Multiservice Access
Each of the two lower shelf cellbuses can sustain a bandwidth of one OC-3/STM-1 link. This bandwidt h
limits the number of E1 ports on the lower shelf, when using t he G.711 codec, to 78 (approximately 10
VISM cards).
Multiservice Access
A Cisco MGX 8000 series shelf, combined with one or more VISM cards, provides multiservice access
between a customer’s TDM network and a voice gateway over a packet network. The voice gateway
provides the interface to the telephone network.
When voice traffic is conveyed over a packet network using VISM and an MGX 8850—multiservice
access—the MGX 8850 is located either at the customer’s premises (prob ably the case for large cus tomer
installations) or at the central office. Use VISM in the V oIP switch ing or switched AAL2 PVC operating
mode to enable this application.
This application is very similar to the tandem switch offloading application, except that instead of
performing as the voice gateway, VISM provides access to the voice gateway.
VISM operates in conjunction with an MGCP or SGCP compatible call agent via an edge router/switch
on the packet network. Signaling is backhauled from VISM to the call agent through this connection.
The call agent connects to the SS7 network and handles call setup and teardown across the packet
network. The VISM connects to the ATM network and handles the voice payload between the TDM
voice/data network and a voice gateway. For transmitting the voice payload to the network, VISM uses
either VoIP transported in AAL5 ATM cells or VoAAL2.
Other data services (such as frame relay) can also be accommodated by configuring the MGX 8000
series shelf with the appropriate service modules (for exa mple, FRSM) and using s epara te PVCs in to the
packet network.
AAL2 Trunking
A Cisco MGX 8000 series shelf, in combination with VISM cards, provides A AL 2 tru nking be twee n a
voice TDM network and voice gateways over a packet network. Use VISM in the AAL2 trunking
operating mode to enable this application.
Figure 2-3 shows a trunk with a VISM-equipped MGX 8850 shelf at one en d of a t run k (a t t he c en tra l
office) and a Cisco 3810 Multiservice Access Concentrator and a Cisco MGX 8220 edge concentrator at
the other end of the trunk (customer premises).
Figure2-3 AAL2 Trunking—One End
Cisco
3810
Cisco
3810
V
PBX
MGX 8850
with VISM
T1 lines
ATM Packet
Network
PST
N
CPE
PBX
Central Office
AAL2 (Bearer & CAS)
AAL5 (CCS)
31438
Cisco
MGX 8830