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James Hawkins

ISDN30 Channel Grouping Support


I have a customer in the UK that has an older ISDN30 circuit that uses the British Telecom DASS2 standard. This pre-dates the ISDN30e service that is now commonly provided across Europe.

DASS2 is not supported by the Cisco 3945 gateway that we are deploying so I have located a supplier of DASS2 to ISDN30e convertors.

So far so good but the customer has told me that they have two blocks of DDI (DID in US parlance).

The first block goes to channels 1-25. The second block goes to channels 26-30.

I have never encountered a configuration like this. If anyone has and can let me know whether there is any special configuration needed on the gateway I would be most grateful.

Note that this circuit is only used for inbound calls. Outbound calls are routed via a different gateway.    

Gordon Ross

CUCM doesn't handle this type of configuration well. (CUCM Can't handle fractional ISDN-30s)

If these channels are just for incoming calls, and you don't need to strip different ammounts of digits at the gateway for each group, then you should be able to lie to CUCM and tell it that it's a got a normal 30-channel ISDN-30.

Of course, the best solution, would be to get the DASS-2 converted to Q.931 and get the two DDI ranges sent over all 30 channels. DASS-2/DPNSS<>Q931/QSIG convertors are "OK" but can give headaches. Plan to get rid of them ASAP.


PS I didn't think DASS-2 was still a supported service from BT.

Please rate all helpful posts.

Hi Gordon,

Thanks for the response. Unfortunately due to project schedules and less than rigourous pre-sales work I am stuck with using convertors.

I do not need to any special digit manipulation so if I just do a standard gateway config do you think it should work ok - I cannot see why it should not but cannot afford to get it wrong.

Re. DASS2 I still come across quite a few circuits when deploying CUCM upgrades and there are documents dated 2009 on BT's web site that describe it as still being available.



A good converter will take care of all the protocol details transparently.

Of course, the best solution would be to have telco upgrade to PRI.

Also, CM can handle 'fractional PRIl' with either the preferred H.323 setup, or a channel busyout tecnique in MGCP.


Just an update to this thread. The DASS2 converters are installed and working well. The supplier was able to configure them so that calls to the channel groups on the DASS2 side are just presented on ascending channels on the Cisco gateway side.

All pretty painless! - link below to the converter webpage for anyone who comes across a similar requirement.

Very good. Thanks for the nice rating and good luck!

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