PRI stands for Primary Rate Interface. PRI is an industry standard based for transporting multiple voice and data streams between a user and network. PRI is part of ISDN technology (Integrated Services Digital Network) and implemented on E1 & T1 lines. PRI is the standard for delivering telecom services to business locations.
On E1 lines operating at 2048 Kbps, 30 channels of E1 are used as B-channels (Bearer channels) and 2 channels are used as D-channels (for control and signaling). Hence, the frame structure of an E1 PRI connection is said to be 30B+2D where the 2 D-channels are on timeslots 0 and 16 and the remaining channels are used as B-channels (B channels may be used for Voice or Data traffic). The timeslot assignment of channels is –
Channel 0 – D-channel (Used mainly for clocking and synchronisation)
Channel 1-15 – B-channel (Voice)
Channel 16 – D-channel (Signalling)
Channel 17-31 – B-channel (Voice)
On T1 lines operating at 1.544 Mbps, PRI provides 23 B channels (for user data and voice) and 1 D channel (for control and signalling). Hence, the frame structure of a T1 PRI connection is said to be 23B+D where the D-channels is on timeslot 23 and the remaining channels are used as B-channels (B channels may be used for Voice or Data traffic).
To summarize –
- PRI system is an end-to-end digital circuit which enables the delivery of 30 or 23 calls simultaneously.
- There are number of D-channels and B-channels which are responsible for data transfer, video conferencing, voice calling along with other controlling and signaling information.
- PRI line consists of 23/30 B-channels and D-channels over T1/E1 lines.
- T1 PRI standard is popular in America.
- E1 PRI standard is popular in India and European countries.
- PRI uses Q.931 protocol over D-channel.
Benefits of PRI Line –
- No need to have dedicated auto-attendant to address every call. As the service provider of the PRI system provides around nearly 500 numbers for each extension, any outsider can call the extension directly.
- Each extension can have own number which helps caller ID is another vital benefit. Every extension has its own number and that is shown while they are calling to.
- Call hunting is simpler. This may incur additional cost for that.
- Voice and data access is possible with PRI line.
- PRI line is always digital as compared to analog lines which is better technology due to high reliability and ease of maintenance.
- PRI being a digital line, it’s not possible to tap a digital line which gives it more security.
- PRI provides data, fax, video conferencing, etc. facilities which can be simultaneously performed.
- CAPEX and Opex for terminating a PRI line is much lower compared to Analog Lines (considering 30/23 channels of PRI vs 30/23 analog lines)
- PRI management and termination is simpler than analog lines. For eg – 1 slot to terminate PRI line as compared to 30 slots/ports to terminate analog lines.
- PRI lines take lesser time to establish calls then analog trunk lines