Science Fair Project Encyclopedia
Primary rate interface
All data and voice channels are (ISDN) and operate at 64 kbit/s.
Fewer active B channels (also called user channels) can be used for a fractional T1. More channels can be used with more T1's, or with a fractional or full T3 or E3.
See T-carrier for DS0, T1, E1, T3, E3 and other carrier communications terms.
Sources: From Federal Standard 1037C
* Asterisk Telephony wiki PRI 
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