There are some situations where one needs to interconnect two (or more) EPABX/ IP PBX or IP Telephony Servers. This is mainly required because companies want to protect their existing investments in legacy PBX systems or even other IP PBX models that have limited expandability. In this article, let us explore some ways in which two or more PBX systems can be interconnected with each other.
E1/ T1/ PRI Trunks:
If you are not sure what these terms mean, you might want to have a look at the articles on PRI / E1 Lines & T1 Lines. Basically, E1/ PRI interfaces enable 30 channels to be carried on the same line (through multiplexing/ de-multiplexing techniques) simultaneously and T1 lines carry 23/24 channels on the same line.
Many PBX models come with built-in E1/T1/PRI ports that are meant to connect to appropriate digital trunk lines. But these ports in two different PBX models can also be used to interconnect the two PBX systems with 24/30 channels in-between them, for every interconnected port.
Its simple: You take a cable and interconnect the empty E1/T1/PRI ports on two PBX models, and they are interconnected. Of course, you need to enable PRI trunking and most of the PBX models can be programmed to enable it. If there are no spare ports, additional cards can be purchased to provide the required number of E1/T1/PRI ports. Even PBX models of different makes can be interconnected using E1/T1/PRI Trunking method, provided both the models support this functionality.
IP Trunks/ SIP Trunks:
This is the easiest way to interconnect two PBX models (especially IP PBX) over the Network (LAN) or even WAN (Internet/ MPLS, etc). Both the PBX models that need to be interconnected should be connected to, and accessible from the same network. Once this is taken care, its just a matter of enabling IP Trunking/ SIP Trunking between them using the PBX management module.
Certain PBX vendors might not have enabled SIP Trunking by default, and one needs to program the PBX to enable the same. Certain PBX models require a specialized IP Trunking Card/ SIP Trunking card to be purchased (at both ends) for this purpose.
The number of channels available between the PBX models so connected depends on the make/ model of the PBX & the capacity of IP Trunk/ SIP Trunk Cards. SIP is a standard and can be used to connect two or more IP PBX models manufactured by different vendors, as long as all of them support SIP Trunking. IP Trunking is a proprietary protocol that is used to connect two or more PBX models manufactured by the same vendor, over the LAN/ WAN Network.
Using FXS/ FXO Ports:
Certain IP PBX models can be interconnected to legacy PBX in a limited way using FXS and FXO ports/ FXS-FXO Cards & Modules. In this case, the number of channels between the two PBX models are pretty much limited and some PBX functionalities maybe lost.
Analog Tie-lining is another method used to interconnect two PBX models. In this method, the FXS port of one PBX is connected to the FXO port of the other PBX and vice versa. As you can see, this requires complex connectivity and a number of cables are required to enable few channels between the two PBX models. Thats one reason why the FXS/ FXO port method of interconnecting two PBX is not used frequently.
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