ISSI Switch
We’ve determined that the open source Asterisk PBX appears to be a good framework on which to build a P25 ISSI switch. Asterisk has a mature SIP stack and already has the ability to transparently pass RTP frames between SIP channels. The National Institute of Standards and Technology (NIST) has made an open source program for ISSI testing freely available to the P25 community. A full-featured open source P25 ISSI switch is clearly achievable.
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P25 Trunking Controller
Developing an open source P25 trunking controller is achievable, but much more challenging than developing an ISSI switch. The P25 specifications have placed the trunking controller within the RFSS (RF Sub-System) and allowed for proprietary communications protocols between the trunking controller and the base station. The fixed station interface (FSI) isn’t robust enough to pass the necessary data structures from subscriber equipment to the trunking controller. We have identified ways to workaround this problem, however significant work remains to be done in this area. One base station manufacturer has indicated a willingness to consider making a low-level interface to its radios available.
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Funding Issues & Business Plan
Our goal is to award one or more contracts to professional software developers for this open source project. There is a possibility that this project would be eligible for grant funding and we are pursuing this avenue. Concurrently, we are exploring the possibility of a private funding consortium comprised of manufacturers and system integrators.
We are beginning work to identify intellectual property rights issues related to this project. Even though P25 is somewhat of an open standard, certain components are protected by patents and other legal devices. At this time, we believe that we can accomplish our objective without infringing on the intellectual property rights of others.