ISSI and CSSI functionality

For mobile or portable radios roaming across or between networks, the ISSI supports:

• P25 addressing
• Secure and clear voice calls (both individual and group)
• Authentication of roaming radios
• Mobility management
• Call control
• Push-to-talk management
• P25 supplementary data service (e.g., call alert)
• P25 packet data on the Data Network Interface (including OTAR)
• ISSI support of the P25 CSSI
• P25 conventional voice service (including mixed trunked/conventional voice service)

The CSSI supports exactly the same services because (and this may come as a surprise) it is essentially the same as the ISSI. The difference lies in the use of each interface, rather than its composition.

The ISSI is used for P25 internetworking, whereas the CSSI is used for P25 trunked console connection. Both rely on standard IP protocols such as the Real-time Transport Protocol (RTP) for voice transport and the adaptable Session Initiation Protocol (SIP)–which originally was developed for Internet telephony—for signalling. The ISSI was designed deliberately to expedite CSSI operations across the ISSI, so that consoles in different P25 systems could communicate with and support each other. Essentially, the CSSI was conceived as an ISSI connection in which one radio subsystem, the console, has no RF.

Management of P25 addressing lies at the heart of the ISSI. Radios have identities that can be tracked and controlled when they roam from one network to another. They have “home” networks that they normally belong to and “visited” networks to which they roam. Similarly, they have home consoles (also with IDs) that operate in their home network and which must communicate with consoles in visited networks, and they have a home talk group to which they belong, and talk groups they wish to connect to when they visit a different network.

As a result, the trunking concepts of registration (for mobility management) and authentication (for security), as well as call privileges permitted to the visitor, must be carefully negotiated between the home and visited networks.

In conclusion, the ISSI and CSSI components of the P25 standards suite represent a vital technical and business enhancement to digital radio technology, as they remove the last major proprietary links between components of such networks. They create new, more flexible and cost-effective options for network design. And, perhaps more important, they encourage interoperation and vendor competition.

Dr. Jan Noordhof is principal consultant for Tait Communications.