Update on Various P25 Interfaces

September 17, 2007

Editor’s Note: The following information is an excerpt of information supplied by P25 Interface Committee Chairman Harrison Reves.

Common Air Interface (CAI). The CAI represents the most important collection of standards and the subsequent interface that flows from those standards. The conventional side of the standards was completed in 1995, only two years after the P25 steering committee entered into a memorandum of understanding (MoU) with TIA. The completion of the two standards accelerated the development of the new P25-based radios. As planned, those radios included both analog and digital capabilities to satisfy graceful migration requirements and full backward compatibility. Those standards were enhanced through the approval of FDMA trunking standards in 1996, one of the state and local government users’ highest priorities. The GAO report also fails to recognize that from 1993 – 2007 these and other standards were constantly updated, revised, and renewed as needed or required.

Subscriber A Interface. The original circuit- and packet-data standards were also completed in 1995 and were supported at that time by the CAI. The subscriber interface or “A” interface was then defined as an electrical RS-232 interface (another pre-existing TIA standard). Given the diversity of protocols in use at the time, the steering committee didn’t believe working on a standardized protocol was a priority, when the few interested users were buying an array of protocol-specific data equipment. Since then, the interface has been revised to also include a standard USB interface. As the use of IP technology expands, we will reconsider the need for a standardized IP protocol for this “A” interface. Fixed Station Interface (FSI). The FSI message and procedures document was completed in June 2006, and the conformance document was published in April 2007. The documents will provide industry the basic standardized information they need to produce products with this interface. Console Sub-System Interface (CSSI). The CSSI overview is scheduled for publication in 2007 with continuing work being done on the conventional messages and procedures documents, which we expect to complete in late 2007. While we have focused on conventional services to meet the needs of large federal agencies, we haven’t forgotten the need for a trunked CSSI, which will be highly dependent on the next round (Scope 2) of ISSI documents scheduled for completion in late 2007 and early 2008.

Network Management Interface. In 1994, the P25 steering committee accepted the recommendations of TIA to adopt signaling network management project (SNMP) as the primary network management interface. This ensures each system manufacturer can create its own proprietary network management systems that include the ability to deal with proprietary technology they use in their radio sub-system interface. To do otherwise would have created a massive effort to find ways of protecting each company’s proprietary operational and management software, with little benefit to either the process or users. However, with the completion of the ISSI and other critical standards, we will revisit this issue in early 2008. Data Interface. The issues associated with the Data Interface mirror those associated with the CSSI and are also supported by the CAI. Significant progress on this interface will not take place until Scope 2 of the ISSI is completed.

Telephone Interfaces. Currently, the Telephone Interface is in an integral part of each product provider’s proprietary system components. Those systems are supported by the CAI and allow the transparent distribution of dual-tone multifrequency (DTMF) signaling and associated tones. As might be expected, the systems also use all appropriate Bell standard electrical interfaces. GAO doesn’t recognize what an extremely low user priority the telephone interconnect is. GAO also didn’t give credence to the level of cellular coverage existing in North America or the limited spectrum public safety has available to complete its daily mission-critical functions, let alone worry about providing quasi-telephone service.

Inter Sub-System Interface (ISSI). The message and procedures document was published in 1996 and updated in 2006. The following chart shows an update on ISSI documents as of September 2007.