Implementing TEHO with PSTN Backup in Cisco Unified Communications Manager v8.x Covers basics for TEHO

Posted: April 3, 2012 in CUCM

To reduce PSTN connectivity charges, system administrators might want to route calls to off-net destinations by using the IP network to bring the egress point to the PSTN as close as possible to the called number. At the same time, if the call’s preferred TEHO route is not available, it might be necessary to use the calling phone’s local gateway to send the call to the PSTN. This can be achieved by allowing all phones partaking in TEHO routing for a given type of number to match the same route pattern that matches the specific destination number and that points to a route list containing the TEHO egress gateway-of-choice as the first entry and the Standard Local Route Group as the second entry.

Within a centralized call processing cluster with N sites, you can implement Tail-End Hop-Off (TEHO) using one of the following methods:
-TEHO with centralized failover
This method involves configuring a set of N route patterns in a global partition, with each pattern pointing to a route list that has the appropriate remote site route group as the first choice and the central site route group as the second choice.
–TEHO with local failover
This method involves configuring N sets of N route patterns in site-specific partitions, with each pattern pointing to a route list that has the appropriate remote site route group as the first choice and the local site route group as the second choice. In order to implement local failover TEHO routes to Brazil, a site in Paris, France would require a dedicated route pattern and route list to route the calls to the TEHO gateways in Brazil as a first choice or to the Paris gateways as a second choice. Because the pattern is linked to a site-specific route list, it cannot be reused at any other site. Likewise, the site in Ottawa, Canada requires its own dedicated route pattern pointing to an Ottawa-specific route list to allow local failover to a gateway in Ottawa.

– TEHO Without Local Route Group

While this second approach allows for an optimal failover scenario when the remote gateway or the IP WAN is unavailable, it also introduces a high level of complexity into the dial plan because it requires a minimum of N2 route patterns and N2 route lists, as opposed to the N route patterns and N route lists needed with the first approach.

TEHO with local failover with Local Route Group
The Local Route Group allows for the local failover of TEHO routes to be implemented without having to create route patterns for each site. A single TEHO pattern and route list is used by both the Paris and Ottawa sites. Because the user input for these two sites is not the same (French users dial Brazilian destinations differently that Canadian users do), the configuration relies on translation patterns to globalize the user input. The global form is then used to match a single, cluster-wide route pattern pointing to a route list whose first entry is the Brazil route group and whose second entry is the Standard Local Route Group. The local route group is resolved to the Paris route group when the calling device is in a Paris device pool, and to an Ottawa route group when the calling device is in an Ottawa device pool.

– TEHO With Local Route Group

When appropriate for your national numbering plan, you may configure an additional translation pattern at each site to catch local PSTN calls dialed as long-distance calls and to translate them into the proper abbreviated form. This translation pattern should be accessible only from phones located within the site. Such a configuration also helps simplify the AAR configuration

Do not use the multilevel precedence and preemption (MLPP) feature to assign higher priority to emergency calls. An emergency-related call might not appear as such to the IP Telephony system, and you would risk terminating an existing emergency call to place another call to the main emergency service routing number. For example, an emergency situation might prompt someone to place a call to a regular ten-digit number to reach a medical professional. Preemption of this call would abort the ongoing emergency communication and could delay handling of the emergency. Also, incoming calls from emergency service personnel would be at risk of preemption by MLPP.

You can implement Tail-End Hop-Off (TEHO) across multiple Unified CM clusters by following these guidelines:
Add specific route patterns for the relevant E.164 ranges to the source (originating) Unified CM cluster, and point them to a route list that has the IP WAN route group as the first choice and the Standard Local Route Group as the second choice
Within the Cisco IOS gatekeeper configuration, add zone prefix statements for all the relevant E.164 ranges and point them to the appropriate Unified CM cluster
Ensure that the intercluster trunk calling search space in the destination Unified CM cluster includes partitions featuring route patterns that match the local PSTN numbers, and that digit manipulation is applied as needed (for example, stripping the area code before sending the call to the PSTN) by using appropriate Called Party Number Transformation Patterns.

To help you decide which approach is best suited for your needs, consider the following high-level design questions:
How many sites will eventually be served by the IP Telephony system?
What are the calling patterns between sites or branches?
What do users dial within a site and to reach another site?
Are there any calling restrictions applied to on-net inter-site calls?
What transport network (PSTN or IP WAN) will be used for most inter-site calls?
What (if any) CTI applications are being used?
Is there a desire for a standardized on-net dialing structure using site codes?

Reference from Cisco SRND 8.x


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