Backup Failed or Busy ISDN PRI / T1 DID Service with ISDN BRI & Redirected Number Delivery
(How to install a PRI and a half when you can't afford two PRI's)
Many businesses today use competitive local exchange carrier (CLEC) T1 or ISDN PRI (T1/PRI) facilities to deliver direct inward dial calls (DID) calls to their PBX. Some customers may be interested in a feature that would automatically redirect incoming calls to low cost backup/overflow trunks should their CLEC T1 or PRI fail, or if all of the DID trunks on the T1/PRI were busy.
When an attempt is made to route a call over a busy trunk group (say, between two Central Offices), your carrier's switched voice network can direct the call over another trunk group to the same destination, provided one is available. This capability is called 'alternate routing', and it can be applied to customer DID trunks, as well.
The ability to provide alternate DID routing to another DID trunk group when facilities fail is only possible if the DID trunks themselves are either 'riding' an ISDN Primary Rate or T1 facility. It won't work on individual analog trunks. However, it is always possible, to alternate route when all analog or T1/ PRI served DID trunks are service busy.
Alternate routing on digital trunk failures uses the Central Office Carrier Group Alarm (CGA) feature which is customarily assigned to trunks transported over a T1 facility. With CGA, CO can "sense" when the facility (and consequently the trunks) are out of service. This monitoring prevents attempting to erroneously route traffic over them.
As shown above, incoming calls would ordinarily be directed over route index A. CO switch digit Interpretation programs would translate the 7 or 10 digits dialed as belonging to a DID group on the CO switch. Normally, the route index would delete the NPA - NXX digits, and "point' to the appropriate trunk group, selecting an idle member, and then out pulsing the industry standard 4 digits. In our scenario, to maintain compatibility with the backup 10 digit redirected number delivery (RND) route, we provision 10 digit DID out pulsing from the carrier.
If all the trunks on route A are service busy, or if a T1/PRI facility failure has previously made the trunks maintenance busy, then the calls can be directed over the alternate trunk group B to the same PBX.
If desired, more backup routes can be assigned. Route index C provides another route using the incumbent carrier's ISDN BRI service. Thus, the customer can purchase low cost competitive carrier DID service and have a safety net of a few DID trunks from the incumbent carrier.
Since the CO can also detect a failed PBX trunk card, this is a viable, cost effective and "bullet proof" way to provide automatic redirection capability to DID served customers. CLEC's are generally willing to program this extra route for free. If your primary route is the incumbent carrier, they will charge you for the "next route" programming.
Note: Incumbent local exchange carrier's use other expensive and clumsy methods to provide alternate routes using service from the Advanced Intelligent Networking" AIN platform or manual switched redirect. Switch Redirect Service" can be used to manually redirect DID calls from one PBX to another, even changing the digits out pulsed if the receiving PBX needs this. When the customer wanted to activate redirection, a call to an automated voice response system or to a human is required to change the routing of calls to route B or C. Another call would return the route back to route A. The possible problem here is that detecting the failure, calling the phone company and making the actual change may take a long time, and valuable traffic could be lost in the process.