Striving to meet the expectations
For the last few years, observers of the Project 25, or P25, standardization process must have noticed the increasing chorus of calls for finalization of wireline standards for P25 radio networks. Indeed, within the past two years, some in the federal government have expressed dissatisfaction with the pace of development of those standards and have considered developing their own to impose upon public-safety land mobile radio vendors.
The federal government has been the single largest purchaser of P25-compliant networks so far; in fact, the Department of Defense has bought P25-compliant systems from at least three different vendors. However, as the number of P25-compliant system suppliers has increased, the federal government has discovered that the ability of its personnel to communicate effectively across these networks is increasingly hamstrung by the inability to link disparate regional networks.
Furthermore, an effective intersystem interface — known in P25 standards forums as the inter-RF subsystem interface (ISSI) — is an important standard for the upcoming Integrated Wireless Network (IWN), which is being developed jointly by the Departments of Homeland Security, Justice and Treasury.
The lack of standards for the P25 wireline interfaces also has affected state and local first responders because the scope, complexity and capacity of first responder LMR networks have increased substantially over the last 15 years. Whereas it was common for organizations in the 1980s and 1990s to think of their radio systems as a necessity for communication among themselves, today agencies have a higher level of expectations regarding system interoperability. Organizations now know that they must not only communicate within their own teams, they also must plan for interoperable communications with others. Plus, they must have the capability to add agencies that they might not even be able to name today.
This is particularly true for public safety, which has learned from recent cataclysmic events such as Hurricane Katrina that it must have the ability to execute a coordinated, multi-agency response. The lessons learned from the communication problems that ensued in the Gulf Coast region have been well documented, and the designers of tomorrow’s critical communication networks have taken notice. As a result, radio systems providers now have to address interoperability issues related not only to radios, but also to infrastructure.
Examples of infrastructure-based interoperability requirements include the ability to join local LMR systems to create a single regional or statewide network that will enable on-demand roaming and mutual-aid connections between adjacent communities. Connecting these dissimilar networks requires a standardized ISSI to provide advanced digital features such as emergency declarations, talking unit IDs and encryption. But the ISSI is not the only wireline standard that interests P25 customers. Other standards, such as the P25 console interface (CI) and the P25 fixed station interface (FSI) also have significant value because they will enable an increased level of competition when LMR networks are procured.
There is a natural assumption that P25, as an LMR standard, already addresses wireline interfaces. Even agencies that are familiar with the nuances of P25 have mistakenly assumed that because they need wireline interfaces, P25 already must provide the required specifications, only to be frustrated to discover — sometimes after getting their new systems — the real status of the wireline interface standards.
The status of the wireline interfaces is in marked contrast to the status of the P25 air interfaces. A quick survey of the P25 equipment landscape reveals the stability of the Phase I air interfaces. Originally published in the mid-1990s, Phase I air interfaces allow a mix of multiple vendors’ equipment to operate in conventional or trunked mode, clear or encrypted mode, voice or data mode.
Clearly, when discussing standards for radio networks, the air interfaces are the prime differentiators for any particular network, and from a customer perspective, they define the applicability of any particular technology. So it is not surprising that initially the P25 technical committees chose to focus their energies on the development of the air interfaces and delay development of the associated wireline interfaces. The result is that for the most part, P25 has defined a level of air interface interoperability that allows any radio to function on multiple vendors’ systems.
The frustration of the federal government about the absence of wireline standards has certainly resonated with the P25 technical participants and has resulted in an increased level of attention and activity over the last 18 months. Key technical committees have moved from periodic bi-monthly meetings and limited e-mail correspondence to a disciplined weekly regimen of conference calls, interim working group meetings and extended face-to-face technical sessions to meet an aggressive development schedule. This higher level of commitment has overcome the many previous behaviors and obstacles that prevented the progress of the wireline standards for nearly eight years.
Based on input from user participants, the technical committees determined that an ISSI supporting digital encrypted voice between multiple vendors’ systems was of significant value. As a result, the initial release of the P25 ISSI was supported this service. Decisions were made that other services (for instance, numeric messaging and packet data) were to be addressed in a subsequent release. Driven by the acceptance of IP-based network infrastructure in the LMR market, the technical committee also embraced IP-based protocols for the P25 ISSI.
For the ISSI, call setup information is transmitted between ISSI-compliant systems using a subset of the session initiation protocol (SIP) that is customized for P25. Call traffic is transmitted using a variant of the real-time transfer protocol (RTP) with a specific well-defined profile for P25. Both SIP and RTP originally were designed for streaming multimedia Internet applications, which in practice are similar to LMR voice calls. However, significant changes were required to support both encryption and other key aspects of public-safety-grade communications.
The Telecommunication Industry Association, which handles the standards publication process for P25, has sent the ISSI standard to ballot. The expectation is that the ISSI will be approved and published within the next month. For vendors that have designed their networks around end-to-end IP, incorporating the ISSI should be relatively easy, and the expectation is that vendors will be able to provide compliant systems within 12 to 18 months after the publication of the standard.
The P25 FSI is finding similar success. Like the ISSI, the FSI initially was focused on providing support for conventional base stations, with the much more complicated support for P25 trunking to be done at a later time. This multiple step approach has greatly expedited development of the conventional P25 FSI, and it is likely that compliant equipment will be available this year. The progress in the P25 ISSI and FSI even has translated as de facto progress for the P25 CIs because the P25 trunked CI will be based on extensions of the P25 ISSI, while the simpler P25 conventional CI is anticipated to be based, in large part, on the P25 FSI.
There still remain significant issues for the P25 technical committees in the form of Phase II two-and four-slot TDMA standards, the completion of the wireline interfaces and even the resolution of outstanding interoperability issues with the Phase I conventional and trunked modes of operation. But all of the P25 participants should be gratified by the accomplishments of the last two years. For perhaps the first time, P25 standards have been developed that are truly the work of a consensus-driven process of commercial and technical equals. This level of support for the standards’ development is the direct result of a widening commercial participation in the P25 market, and ultimately, the related increase in P25 suppliers should make all public-safety users happy.
Paul May is business development manager for M/A-COM in its Lynchburg, Va., facility. He has worked in the LMR industry for the last 15 years.
