Voice T1, ISDN PRI

I commonly run into the question what is the difference between a voice T1 and a PRI? I know the config is completely different from the router prespective, but what's the reasoning for choosing one over the other?


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You can run ISDN over a PRI. You get all advanced features on a PRI by default like DNIS/DIDs without having to negotiate back and forth with the telco to see if they've got wink, double wink, etc. all setup correctly on both sides, or paying extra for them. Most telco's still only allow DID as seperate inbound only trunks on T1s. PRIs can use any channel either way with full DID inbound.

Some telco's are actually cheaper on PRI than DSS T1s. Downsides of PRIs are that they are usually a tad more expensive, and you lose a whole channel for signalling (if you have a whole stack of them, you can regain some of the signalling channels back with NFAS).

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Doug McIntyre

Voice T1 uses "robbed bit" signaling, in which the signaling is sent in-band along with the data, thereby reducing the capacity of each channel. Voice T1 is also known as CAS, or Channel Associated Signaling. PRI uses a separate channel, the "D" channel for out-of-band signaling, allowing each of the remaining 23 channels to send a full 64K.

The physical layer for both are the same

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