Trunk seize for emergency calls?


#1

If someone places an emergency call, but all trunks are in use, is there some way to have seize/drop an existing call so that the emergency call can go out?

Perhaps something like Grandstream UCM’s Disable Line Hunt for Emergency Calls? “If this option is not enabled, when the lines of trunks which the coming emergency call routes by are completely occupied, the line-grabbing function will automatically cut off a line from all busy lines so that the coming emergency call can seize it for dialing out.”

Thanks!


#2

If you detect an emergency then from a shell

rasterisk -x 'channel request hangup all'

would achieve that , you could be more selective of course on the channel(s) to hangup


(David55) #3

I’m finding the terminology confusing here. A trunk means a collection of lines between the same two places. That means that a 100 member trunk is in use when only 1% of its capacity is being used.

Lines also implies that there is an identifiable resource for each call. That pretty much restricts it to analogue and primary rate connections, when most people are using VoIP, which has no hard limits. The technological limits depend on the level of contending traffic. Even if service providers impose commercial limits on simultaneous calls, they will only apply them after they have seen the destination number. If they prioritise emergency calls, they are in a better place to shed load to accommodate them, and if they don’t, you may throw away a call just to get congestion in the service provider, as another customer grabs your released capacity.

In the outside world, some emergencies can generate multiple calls, so one would need to avoid releasing other emergency calls.

It also takes time to release a call to the point where the physical resource is re-usable, or the call count at the provider end drops. Asterisk handles releases asynchronously, so issuing hangup doesn’t meant that the emergency call will succeed. I’d suggest it is better to keep one line free (set VoIP calls one less than the commercial limit) for the first emergency call.


(Dave Burgess) #4

I’m gonna guess (based on some of his other posts) that he’s dealing with an ATA, so his description might not be too far off.

@TechGuyMike, if I may, let me help a little:

BLUF: Don’t seize the trunk - add inexpensive capacity.

In analog telephony, a trunk is a collection of lines. A trunk is in use if 1 line is in use (as @david55 points out), but if the trunk is full (all lines are in use), you need to find a way to force capacity. Where I come from, we call this “Flash” and “Flash Override”, which releases assets to place your call. In fact, “Flash Override” is the analog equivalent of what @dicko suggested. It hangs up everyone’s calls and clears the lines for Emergency Action Messaging. It also prevents “non Flash” traffic from getting onto the switch.

To answer your original question requires us all to agree on the vocabulary we’re using. One way that can help is to differentiate a “SIP Trunk” and an “Analog Trunk” (or a T1, a Primary Rate Interface, or PRI trunk). The reason it’s important is because Analog trunks provide a limited number of lines in hardware, so there’s no flexibility for capacity. When you hit line 24 on a T1, you’re done. That’s there Flash comes in.

With most VOIP providers, the limit is contractual. You get a certain number of outbound simultaneous calls in your “package” and if you exceed that number, you pay extra. In this scenario, you don’t have to worry about the trunk being full, because you just get charged more for the extra lines.

Most VOIP providers also charge extra for 911 support for a trunk. For example, Voip Innovations (now a Sangoma company) charges a couple dollars a month to support 911, and $300 for a rogue 911 call on a non-911 trunk.

IIRC, @TechGuyMike, you’re using an ATA to support 4 analog lines from the phone company. If that’s the case, it’s up to you how you’re going to handle this. You have several choices:

  1. Dedicate one of your analog lines to Emergency Services and use the available commands in your ATA to seize that line for outbound emergency calls.
  2. Get an account on an ITSP that supports e911 services (Telnyx and VI are both reasonable choices in my experience) and only use that SIP connection for Emergency Services.
  3. Move away from the ATA altogether and move to a more flexible (and probably considerably cheaper) SIP primary solution. You can certainly leave one of your analog lines (since they have B+ support) as your emergency line.

So, in the olden days, your question would have been one that we would have had to debate and decide the least disruptive solution, but with modern telephony solutions, the choices are all flexible and inexpensive. Bottom line: add flexible capacity to support your anticipated e911 needs and mitigate the concern you have about your trunk config.


(system) closed #5

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