PRI Line & advantages and limitations
of PRI circuits
This article explains what a PRI line is, what are the
benefits of having PRI lines for the telephony requirements of an organization,
and the dis-advantages of a PRI line.
What is a PRI line?
There is only one line physically terminating on the
customer PBX but still a PRI line can receive/send 30 calls simultaneously! A
PRI line is end to end digital circuit.
A PRI (Primary Rate Interface) line is a form of ISDN
(Integrated Services Digital Network) line which is a telecommunication
standard that enables traditional phone lines to carry voice, data and video
traffic, among others.
A PRI circuit consists of two pairs of copper lines
terminating on a modem from a service provider premises to the customer
premises. It uses multiplexing/de-multiplexing techniques to carry more than
one channel in a single circuit. There are two common forms of PRI lines – E1
(which carry 30 channels in the two pairs of copper lines, common in Europe,
India) and T1 (which carry 23/24 channels in the two pairs of copper lines,
common in United States).
Each channel in a PRI line provides 64 Kbps for data
transmission.
A PRI line can connect to both Analog/Mixed EPABX systems
and also the newer IP PBX systems. A PRI Card / Interface might be required to
terminate the PRI circuit on the PBX.
A PRI line can also be used to connect two PBX systems
thereby providing 30 channels between them for interoperability.
Advantages of PRI Lines:
If thirty separate analog trunks are taken instead of one
PRI line,
The cost of terminating all the thirty analog trunk lines
becomes higher than terminating one PRI line.
There would be thirty rentals to be paid instead of one
consolidated lower rental for a PRI line.
Some analog trunks might be used more (uneven distribution
of calls) and some lines may not have even crossed the free calls limit.
Terminating 30 analog trunks in a PBX also requires more
free slots/cards than the one slot usually occupied by one or even two PRI
trunk cards.
Direct Inward Dialing: For each PRI line, the service
provider would provide more around 100-500 numbers which can be used by
outsiders to call the extension directly, instead of having to go through the
PBX Auto-attendant.
Caller ID: Since all the extensions have their own number,
this unique number will be displayed in the phones that they are calling to.
Some call centre applications are based on the unique caller ID number for
differentiation of services.
It is possible to offer both voice and data in the PRI line.
Some service providers have dynamic offerings where data is transmitted in all
the channels that are free (not occupied by voice) at that given point of time.
Call hunting (Where the call lands in any channel that is
free, instead of the called number specifically – For example, if there is one
board number but a number of people are calling in at the same time and still a
channel is allocated to them .With analog lines, if one number is busy, they
need to call in another number manually) is possible by default with a PRI
connection, but for the analog trunks this facility needs to be extended by the
service provider and enabled on the PBX, involving additional cost at times.
PRI lines can be used for voice connectivity, data
connectivity, video conferencing, faxing, and all the above can be done
simultaneously too (on different channels).
PRI lines are end-to-end digital lines and hence the clarity
is much better than analog trunk lines.
Since they are digital lines, PRI lines are more reliable
and trouble shooting is also easier with them. They are mostly on a fiber core
ring and hence there is some redundancy.
It is harder to tap into digital lines and listen to the
conversations.
There are flexible billing options available with most of
the PRI service providers. The billing can be centralized or distributed
(department wise, etc).
PRI lines take lesser time to establish calls then analog
trunk lines.
Some service providers offer flexible plans where instead of
the full 30 channels, they provide and charge for only 20 channels etc. This
makes PRI lines more economical for smaller companies.
Dis-advantages of PRI lines:
A PRI line is economical only if the minimum rental charged
by the service provider for a PRI line is more than the average value of calls
with analog trunk lines every month in an organization. Otherwise, the usage
may not even cross the free call value provided by the service provider for a
PRI line.
A PRI line is not so economical for long distance/
international calling. An ITSP or SIP trunk service provider who takes the
calls over the internet might charge much lesser for international long
distance calls.
Inter branch communication between the branches is not free
of cost with PRI lines (Some PRI service providers provide this facility, but
all your branches may need to have PRI lines from the same service provider and
there also might be a minimum revenue commitment for the same). With VOIP
systems, inter-branch communication can be done over internet/ leased lines
hence reducing the cost drastically.
The cost of a single PRI card to connect to your EPABX/ IP
PBX is still very high. Most of these cards are proprietary, meaning you can
buy them only from your EPABX vendor.
