PRI and bandwitdth

Hi -
I’m new to the PRI lines. I know PRIs have 24 channels - each channel is 64k = 1.5Mb.

My goal is to have 22 concurrent outside calls. will this work with 1 PRI line. I was thinking about using Ulaw


If you’re talking T1 PRI, then you can have up to 23 concurrent phone calls going–so you’re good to go (provided you have the correct interface card/other hardware running with Asterisk). The 24th channel is used for call control (setup/teardown).

European PRIs (E1) have 30 available speech channels. Channel 0 is for synchronisation and channel 16 for signalling, giving 32 in total. The preferred codec in Europe is A-Law.

Thanks for the quick reply!!

I have a quote from a phone - I told the company I wanted 22 concurrent outside phone calls. They said i would need a 3.0M bonded PRI handoff. (2 pri(s)) Why would they say that?


because they are a phone company and the more they sell the more money they make

Just throwing stuff out here…

I have two PRI’s … 23 B and 1 D. So on my system I have 46 available channels. 300 DIDs point to those PRI’s.

I had the option of having one PRI with 23 B and 1 D and the other with 24 B. They would have set it up so that the 1 D channel handled signaling for both. Downside to that is, if the 1st PRI went down…then all my lines would be down since I would not have the signaling channel.

I also had a T1 PRI, that had 24 channels. They did whats called Bit Robbing and so the signaling info was sent down some of the bits on each channel. The channels were reduced from 64K to 56K but for voice its not noticeable.

Well - I asked the the Telephone company - this is what they said

For IP Traffic there is also overhead to consider. A typical IP Call takes up about 85k. 85X23=1.9mb

From what i read that sounds right.

Any arguments?

That’s rubbish. If you have PRI telephony, you don’t have VoIP over the T1. If you have VoIP over it, is a data connection, not a PRI telephony one.

Either you have mis-described your connection, or the supplier doesn’t know what they are talking about. Please describe the complete path from the computer backplane to the supplier’s cable.

My bad - it’s a IP Trunk with a PRI Handoff. 85k (that’s with overhead) My Next question - Just say I was getting 30 channels -over that Ip trunk (3.0mb bonded t1) and it’s a pri handoff. A digium PRI card only know 23 channels? right? so I would need to 2 PRI cards.

Thanks again!

If they’re sending you 30 channels. they can’t hand off to one PRI. A PRI delivered over T1 only has 23 bearer channels. So, you’d never get more than 23 concurrent calls. A PRI delivered over E1 has 30 bearer channels, so you’ll get up to 30 concurrent calls.

Most providers who hand you a “PRI” are actually putting fiber to your area,building,floor,etc. and they’re converting it to PRI. In that scenario, if you want 30 channels, you’ll need two T1’s handed off from them.