PRI stands for Primary Rate Interface. It is a telecommunications technology that is used in an Integrated Services Digital Network (ISDN). It allows for up to 30 “channels” of voice calls over a single copper wire to the PSTN.

Channels are the number of possible voice calls that can be placed over the ISDN service. There are two different types of PRI service. In the US, Canada, and Japan, these channels are limited to 23 PRI + 1 data (D) channels. Europe and Australia can obtain 30 PRI channels + 1 data (D) channel. The PRI D-channel uses timeslot 16 on an E1, while 24 is used on a T1.

T-carrier (T1) transmission is used for delivering PRI services in the US, Canada, and Japan. The total bandwidth available on a T1 is 1.533 Mbit/s (24×64-kbit/s).

E-carrier (E1) is used in Europe and Australia. The total bandwidth available on an E1 is 1.048 Mbit/s (30×64-kbit/s).

PRI is a method of using time division multiplexing (TDM) to give a time slot to each channel meaning that a single copper wire can carry more than 1 call. The voice traffic is broken up into lots of little bits, split up, and then sent over the network. The telecom carrier then “repackages” the PRI traffic back into its original order so that a steady stream of voice can be heard. There are other features of PRI connections but these are all becoming legacy and no longer in use.

PRI ISDN services are normally utilized by medium to large enterprises however in many countries, PRI connections are being retired in favor of an IP-based digital network. There are many advantages to an IP network including much more resilient toll fraud capabilities. In some examples, businesses need to retain a Primary Rate Interface connection for added layers of resiliency should the IP network fail.

Traditional and hybrid IP PBX systems can connect a PRI service by means of a piece of equipment called a gateway. The PRI service plugs into the physical gateway device which then converts the traffic and connects to the PBX for the users to make and receive calls over the lines. PRI/ISDN services can be added to a phone system using a supported VoIP gateway.