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There isn't really a direct "conversion" of PRI to BRI. The ATLAS can route calls from a BRI to a PRI, and vice versa. A PRI has 23 B-channels and 1 D-channel, whereas a BRI has 2 B-channels and 1 D-channel per BRI interface. So, as far as routing goes, you can have 11 BRI's "active" or placing calls out the PRI (for a total of 22 B-Channels) plus one additional B-channel from the 12th BRI - for a total of 23 B-Channels active at the same time.
So as far as bandwidth goes, you can have a total of 23 B-Channels active on the PRI at one time, which is equal to 11 BRI's but an additional channel from the 12th BRI. You can have more than 12 BRI ports CONFIGURED in the ATLAS (with 4 Quad BRI modules you can have 16 BRI ports total), as long as you understand there is a maximum of 23 B-Channels that can be active on the PRI at one time.
Hope this makes sense,
@lance001 - I had to set up a radio station that had a point to point T1 between sites for data, a PRI for incoming/outgoing calls on one side, and 2 BRI modems at each site. This ended up being one of the most interesting jobs I ever have done. Adtran TS helped me all the way!
Before the Atlas 550 the 2 sites had 2 ISDN BRI lines for each site (they had 2 radio stations so 1 BRI per station). They needed to get a stereo signal to site B so they had a BRI modem for each station at each site. Site A would establish a 2 B-Channel call to site B and broadcast a stereo signal.
We were able to use the PTP T1 and ISDN BRI-U interfaces on the Adtran to accomplish the above. It ended up far less complicated and easier to troubleshoot since the station was no longer dependent on the LEC (except for the PTP T1). It has been in place for over 10 years and it works phenomenally.
They had a 3rd BRI they could use for remote but now they have smart phone apps that do the job more easily.
We have also used the 550 to connect Video Conferencing equipment and have it use a PRI.
I just wanted to post to demonstrate it is possible and people have been there, done that, and got the mug...
Like Patrick explained, you can have as many BRI as you need and have active calls up to the limit of your PRI.
Think of the Atlas as a mini Central Office in your hands. One of the greatest things ever made!