Another useful application of a Cisco voice gateway is to connect to a company PBX. One reason to do this is to eliminate dedicated tie trunks that connect PBX systems in different locations, thus reducing costs by eliminating the monthly charge for those circuits. You can connect a PBX to a voice gateway, and you can route calls across a WAN network to another voice gateway that is connected to the remote PBX. This extends the usefulness of the data bandwidth between sites, and it allows companies to bypass any long-distance tolls that might be charged.
Most companies are either actively investigating or currently involved in replacing their PBX systems with an IP telephony system, such as the Cisco CallManager or Cisco CallManager Express (CME). This rapid change is being driven by the many benefits that IP telephony provides, including applications and XML services which enhance business productivity, simplified upgrades to add or change services to quickly adapt as business needs change, and so on.
You can connect a voice gateway to a PBX to help facilitate this migration. The PBX connection allows you to route calls between the two networks during the migration period. In this case, it is not unusual to have both public switched telephone network (PSTN) connections and PBX connections on the voice gateway.
This chapter will help you to do the following:
Part I: Voice Gateways and Gatekeepers
Gateways and Gatekeepers
Part II: Gateways
Media Gateway Control Protocol
Session Initiation Protocol
Connecting to the PSTN
Connecting to PBXs
Connecting to an IP WAN
Influencing Path Selection
Configuring Class of Restrictions
SRST and MGCP Gateway Fallback
Using Tcl Scripts and VoiceXML
Part III: Gatekeepers
Part IV: IP-to-IP Gateways
Cisco Multiservice IP-to-IP Gateway
Appendix A. Answers to Chapter-Ending Review Questions