P25 Inter-Networking Using ISSI

On occasion, P25 network operators need to connect their radio systems to the P25 systems operated by other agencies and organizations. System designers have addressed this challenge with a range of different methods to construct a large trunked “network-of-networks” out of smaller P25 systems. Until now, this interconnection could only be delivered by proprietary technologies, such as Motorola’s SmartZone or Omnilink. Now internetworking is delivered by the P25 standard itself.

The new P25 ISSI standard provides a standardized, non-proprietary interface to allow two or more P25-compliant trunked systems to be connected – even if they are from different vendors or operating in different frequency bands (e.g. UHF vs 700MHz) or using different versions of P25 (Phase 1 or Phase 2) or all of the above. The basic requirement is that each network must install an ISSI interface.

Given the capabilities of the ISSI, it is easy to imagine some scenarios for its deployment:

  • A network designer, faced with the task of designing a large trunked network while preserving performance, can break up the design into smaller sub-networks that inter-communicate using the ISSI.
  • Agencies that wish to set up mutual aid communications can interconnect their independent P25 systems (and consoles) through the ISSI and CSSI.
  • A Public Safety agency can deploy a radio system from its preferred vendor rather than joining a state-wide system. It nevertheless can honor its interoperability obligations by connecting to the state-wide system via the ISSI. The organizational advantages of the ISSI are also quite obvious:
  • Because the ISSI is part of the P25 standard, it qualifies for grant support.
  • It vastly extends the interoperability of radio systems without the need to fund super-large networks or install large, complex gateways.
  • It offers flexibility for the future, enabling agencies to think beyond a single frequency band, a monolithic system, or a single vendor. Expansion can be contemplated in terms of adding whole networks – not just sites.
  • It allows for competitive tendering. Agencies can choose to change vendors or add vendors to their list of preferred suppliers. This encourages true interoperability, ensuring that agencies are not locked into contracts built around proprietary functionalities

Historically dispatch centers have also had a connection issue, one that is apparently unrelated to inter-networking. Each radio vendor had its own proprietary solution for connecting console equipment to a radio network. Moreover, while P25 was moving towards digital IP connectivity, consoles typically still linked to radio systems using analog signalling schemes. This was alleviated for P25 conventional systems when the Digital Fixed Station Interface (DFSI) standard was issued, for consoles to use IP to connect in a standardized fashion. But trunked P25 systems needed to wait for the CSSI before they could do the same.

While the ISSI and CSSI core functionality has been fully defined, there are still standards yet to be issued. The outstanding documents concern testing (in particular, interoperability testing) and measurements. The ISSI functionality has been demonstrated by a variety of P25 equipment vendors (Tait, Cassidian, Motorola, Harris, EF Johnson) at trade shows since 2009. The CSSI has been implemented by several major console vendors (e.g. Zetron, Avtec), and radio network vendors (Tait, Cassidian, Raytheon).

In short, the ISSI and the CSSI are now ready for sale.

ISSI and CSSI functionality – what you get

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
  • PTT 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 inter-networking 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 – originally developed for Internet telephony) for signalling. The ISSI was deliberately designed to expedite CSSI operations across the ISSI, so that consoles in different P25 systems can communicate with and support each other. Essentially, the CSSI was conceived as an ISSI connection in which one radio sub-system, 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 that they roam to. They have ‘home’ consoles, also with IDs, that operate in their home network, which must communicate with consoles in visited networks. They have a ‘home’ talk group they belong to and groups they wish to connect to when they visit a different network.

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 additions to the P25 suite of standards represent a vital technical and business enhancement to P25 technology. They remove the last major proprietary links between components of P25 networks. They create new, more flexible and cost-effective options for network design. They encourage inter-operation and vendor competition.

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