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Technical information

In the VoIP network of NIIF, the traditional private branch exchanges (PBX) are always connected through a VoIP gateway. For quality and maintenance reasons, we use ISDN interfaces exclusively.

In case of connecting software-based private branch exchanges (IP PBX) installing of ISDN interfaces are needless, the IP PBX can communicate with the central call director directly.

In this article, we describe the technical details of the each methods of connecting and suggested modes of configuration.

Traditional private branch exchange setup

Physical connection

The connection between the institution’s private branch exchanges (PBX) and the HBONE’s routers are made using ISDN PRI or BRI lines.

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Figure 1: Private branch connection

The connection router-side interface is a standard symmetrical G.703 (electrical specification for E1 ISDN line) standard with  RJ45-type clip. Some PBX’es can differ from that, so conversion may be needed. The conversion can be either a simple straight  cable (for example DB-15 – RJ45) or a balun. Balun is needed in case of the physical connection type of the PBX is asymmetric (75 ohm coaxial). Signal conversion may also be needed if the distance between the PBX and the router requires it. In case of a symmetric connection, the length of the cables for G.703 type interfaces can be up to 10 meters. In case of an asymmetric connection, this length can be up to 100 meters. In a simple cases, a symmetric connection could be enough. But if 10 meters isn’t enough, media converter or transport system should be used to connect the PBX and the router. Optical E1 transfer systems is such a system, it converts the data stream to optical signals and transfers them via a pair of optical lines. In case of multimode optical line, the maximum length can be 2000 meters, in case of single mode optical line, it can be more than 10 kilometers. Further solution could be  to use different E1 transports systems, multiplexed into a synchronous stream. The transport system can be either SDH, ATM based system, or a leased line service provided by the service provider.

Configuration settings

The configuration at Cisco routers can be done setting the parameters of the “controller”. The only parameter that has to be set is  the usage of CRC4 in the E1 frames.

Router side ISDN cabling

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Figure 2: Router side RJ-45 pin-out in case of using an ISDN PRI interface


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Figure 3: Router side RJ-45 pin-out in case of using an ISDN BRI interface

ISDN PRI connection

If the E1 connection set up and ready between the PBX and the router, the next step is possible to configure the ISDN connection. 

The ISDN connection is working  in master-slave manner: user side and network side (Cisco routers support both connection modes).

User side - the equipment acts as a user device. Interfaces towards public phone networks of PBX connections are such interfaces (main line interface).

Network side - the equipment acts as a service provider interface. This interface can be found in systems of service providers, it maintains connection towards the user (this type of interfaces can rarely be found in PBX’es, but it might be configurable).

Recommended connection mode:
Router – network side
PBX – user side

According to this setup , the suggested interface is the same as the one which connects the PBX to the service provider. The used ISDN protocol on this interface is a standardized ETSI Euro-ISDN/E-DSS1.

Software-based private branch exchange (IP PBX) connection

IP PBX can join to the VoIP network  using SIP or H.323 signaling protocol. In our network the typical IP PBX implementations are Asterisk and Cisco CallManager.