Though PBX itself is a complete equipment and can perform standalone, but it can also be connected to many other equipments like other PBX, computer, PSTN etc. With connecting two or more PBX together, it is possible to build a large private voice and data network spread over large geographical area.
The interfaces of a PBX machine could be divided as follows :-
1. Interface with other PBX or PSTN
a. PSTN analog line
PSTN analog lines could be used to connect “trunk lines” in a PBX. PSTN lines in turn could be used to establish voice call with any telephone number in the world.
b. Trunk lines
Trunk lines are ports which are used for connecting any analog telephone extension. It can be a PSTN telephone line or extension of another PBX system.
ISDN ( Integrated Services Data Network) is used to connecting other PBX, PSTN with digital protocol. It uses either E1 ( 2.048MBPS) or T1 ( 1.54MBPS) data link for connecting a PBAX machine from either PSTN network or PBX. It provides 30 voice channels while using E1 and 24 voice channels while using T1.
Multiple Frequency R2 is another digital protocol which is used for connecting exchange to exchange. Now a days, it is not much used giving way for much simpler digital interface like ISDN PRI or SS7.
SS7 is the latest and most modern protocol for connecting two or more PBX. It has many features and flexibility to build large networks and connect variety of data and voice networks. It is widely used now a days and has many powerful and useful features for telephone network.
Many PBX manufacturers use their own proprietary interface protocol for connecting their own PBX systems. Now a days, usage of proprietary protocols is on the decline.
2. Interface with Computer
a. CTI Cards
To connect to a computer in order to use PBX voice and network resources, CTI boards are used. And CTI boards are connected with PBX using any of the above interfacing ( #1 above). Now a days, CTI applications widely used to provide voice applications like IVR ( Interactive voice response) systems, Voice mail, CRBT, Auto Dialer etc.
b. RS232 com port
RS232 or COM port interface is mainly provided to program PBX using a computer. It is also used to monitor calls online and download CDR ( call data record).
LAN or local area networking port is also provided in many PBX for voice recording as well as other online monitoring purpose.