Inter-System Interface

The Inter-System Interface (ISI) is the standard way to connect TETRA networks together.

Some radio users regularly operate in an area that is at the border of two radio networks – either regional ones, or different country networks. It’s likely that they work together and communicate with people who use the other network.

Therefore, it would be best for them if they could easily use their own radio in both networks. To communicate with their own organizations, but also with people in the other network.

In other words, they need to roam to another TETRA network. They also need cooperation talk groups that have members from both networks.

ISI makes this possible.

4 questions and answers

Is ISI available?

Yes. TETRA System Releases 6 and above from Airbus DS include ISI functionality.

Is ISI interoperable?

Yes. Multivendor implementations are possible.

Does ISI support data?

Yes. In addition to group calls and one-to-one calls, the ISI standard includes covers status and text messages.

What is the physical connection between networks?

The physical connection is via fractional E1. A call occupies a 64 kbit/s E1 slot.

Since software Release 6, the TETRA systems from Airbus have included ISI functionality that meets the following standard specifications:

• radio can migrate to another network (visited network)
• radio authentication is ensured from the home network over ISI
• the radio user can communicate in cross-border group calls
• the radio user can communicate over one-to-one calls from one network to another
  status and SDS messages can be sent and received.

As long as the two networks support ISI functionality that adheres to the current ETSI standard, communication across network borders is possible even when the two networks have a different vendor.

Smooth communication across network borders is possible when the vendors providing TETRA interconnectivity have similar commitment to ISI.