Trunks to PBX to supoort E911: PRI or DS0?

Unanswered Question
Nov 16th, 2007

I have looked through the Administration documentation for CER and I can only finds PRI references to the gateway trunk. I have the capability of doing both DS0 or PRI. Is there a requirement for the trunk to be a PRI?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
rob.huffman Fri, 11/16/2007 - 09:54

Hi Jeffrey,

Yes, you should go with a full PRI not a T1/DS1 due to the full Name and Number display capabilities. Here are some design notes;

Obtain CAMA or PRI Trunks to the PSTN

To handle emergency calls, you must obtain PRI or CAMA trunks to connect to your service provider. Your service provider might support only one type of trunk. If you have an option, work with your service provider to decide on the type of connection that works best for you.

Consider these issues:

PRI-If you use a PRI connection for emergency calls, you can share the connection with regular telephone traffic. If you use the trunk for regular traffic, monitor trunk usage to ensure there is sufficient available bandwidth to handle emergency calls. If your capacity is inadequate, an emergency caller might get a busy signal when trying to make the call. Ensure you do capacity planning based on emergency call requirements.

When you configure the PRI trunk, you must configure it so that it sends the actual calling party's number rather than a generic number (such as the main number of the site). Otherwise, the PSAP will not receive the expected ELIN, and the emergency call might not be routed to the right PSAP.

CAMA-CAMA trunks are dedicated to emergency calls, and are available in most areas. You do not need to do any capacity planning for CAMA trunks, because they are never used by regular voice traffic.

Hope this helps!


jeffrey.girard Fri, 11/16/2007 - 10:13

Rob -

Thanks for your post. However, virtually all of your text is word for word out of the CER Admin Guide page 1-18 and 19. The text that I am questioning is on page 1-18 "To handle emergency calls, you must obtain PRI or CAMA trunks to connect to your service provider". Do I take this as an absolute REQUUIREMENT that my trunk MUST be a PRI? Why would a DS0 not work? You mentioned the full name and number display capabilities in your post. Can you elaborate? Why is this capability possible in PRI but not T1?


rob.huffman Fri, 11/16/2007 - 10:34

Hi Jeff,

Yes, this text is from the link I included so you could see Cisco's requirements. Sorry if I did not make that clear :(

Generally, at least in my area a simple 24 B-channel T1 does not support Name and Number display (a must for the PSAP - E911 Dispatch) whereas a 23 B + 1 D-Channel full PRI does have the Name and Number support. The D-Channel or Signalling Channel carries this information. If in your area the Telco can provide this information via a T1 then you would be good to go :)

Hope that helps! Sorry for the confusion in the first post.


jeffrey.girard Fri, 11/16/2007 - 11:06

Rob -

No sweat on the confusion part, its just I try to do my homework before I raise my hand in class.... ;)

Name and number display being carried by the D channel.....that is what I was thinking when I read the must part. Do you have a link/pointer to anything that specifically lays this out? I have scanned through the entire Admin guide and - though the guide continually makes references to ONLY CAMA and PRI, there is no discussion (that I could find) of sending any information over the D channel and not B channels.


rob.huffman Fri, 11/16/2007 - 12:29

Hi Jeff,

Good stuff! I don't think that you will find this discussion anywhere in the CER docs. This is a common practice in Cisco documentation where certain ideas are "assumed" to be understood by us as readers (I know what happens when you assume :)

The truth of the matter is that many people interchange the terms PRI and T1 when in reality they are two similar but different services. With one (T1) offering 24 B-Channels (or DS0's) for Voice only and the other (PRI) offering 23 B-Channels (or DS0's) for Voice and 1 D-Channel for sending information like Name and Number.

Hope this helps!



This Discussion