PSTN Circuit Selection Overview

Connecting to PBXs

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:

• Understand, properly configure, and troubleshoot the various analog connections available, including Foreign Exchange Office (FXO)/Foreign Exchange Station (FXS) and ear and mouth (E&M) connection types
• Understand how clocking works on digital circuits and be able to properly configure it for various circuit options
• Understand how ISDN works as a PBX tie trunk and be able to successfully configure and troubleshoot an ISDN PRI trunk
• Recognize the interoperability benefits of Q Signaling (QSIG) and understand how to configure a Cisco CallManager-controlled QSIG trunk
• Understand the application of Transparent Common Channel Signaling (T-CCS) and be able to configure and troubleshoot a T-CCS trunk between locations