Survivable Remote Site Telephony

Gateway operation

In this blog, we will discuss SRST in more detail. Remote site MGCP gateways must be provisioned with MGCP fallback to route calls to and from the PSTN when the MGCP call agent (CUCM) is unreachable. This blog will cover the options to configure TDM call routing over a remote site gateway in SRST mode.
MGCP fallback will allow inbound call routing over any MGCP controlled endpoint (TDM voice port).

MGCP fallback does not implicitly know the internal dial plan and makes no default attempt at performing digit translation. If the PSTN provider was routing inbound calls with a 10 digit DNIS and the internal dial plan was 5 digits, all inbound calls would fail. The following command syntax will map 10 digit dialing in the 212 area code and 551 exchange to the internal five digit extensions beginning with a 1:

Router(config)#call-manager-fallback
Router(config-cm-fallback)#dialplan-pattern 1 212551.... extension-length 5

Multiple dialplan-pattern commands can be used for sites that have different DID ranges. The dialplan-pattern command will translate incoming calls for Cisco IP phones, but not for analog devices connected to foreign exchange station (FXS) ports. Translation profiles can be used to match on inbound calls from an ISDN interface destined to an FXS port. Since FXO ports do not support DID, FXO ports will need the command “connection plar opx [extension]” to redirect inbound calls to analog FXO ports to devices. Translation profiles have been covered in previous blogs.

The gateway will also need a configuration to route outbound calls to the PSTN during an SRST event. There are two ways to achieve outbound call routing:

1) An entire H.323 dial plan could be provisioned for SRST events. H.323 dial plans allow class of restrictions lists which will be covered in a future blog. H.323 gateways have been covered in previous blogs.

2) An access-code can be configured in SRST configuration mode. When the access code is dialed, the SRST gateway would redirect the call to an outbound TDM interface and the calling party would then hear a stutter (secondary) dial tone. At this time, the calling party would be able to dial any pattern the PSTN provider is capable of routing. The configuration appears below:

Router(config-cm-fallback)#access-code pri 9 direct-inward-dial

There is plenty more SRST configuration to talk about. We’ll continue the SRST conversation in the next blog.

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• SRST H.323 MGCP mgpc-fallback