Developments in technology have broadened the scope of traditional interoperability solutions. Land mobile radio users can increase the power of their existing radio infrastructure using the diversity and flexibility provided by Internet protocol and session initiation protocol. Interoperability has evolved — there are now some unique applications available to radio network administrators.
With the wide acceptance of SIP-based technologies, interoperability is again taking on a new meaning.
When the concept of interoperability first emerged within the industry we referred to the idea of cross banding different radio frequencies such as connecting UHF (ultra high frequency) to (very high frequency) VHF to HF (high frequency) and so on. Our understanding of interoperability then developed further when we entered the world of IP solutions. It became possible to connect radio to IP, PBX (private branch exchange), leased lines and satellite. Radio over IP (RoIP) connections enabled reliable permanent point-to-point communications. IP provided a valuable new medium for connectivity but was often cumbersome because of its fixed "static" connection. The infrastructure paths were "set and forget" at the point of commissioning.
As a result of the introduction of SIP protocol, LMR users can establish multiple connections with various communication mediums. This has added a new dimension to interoperability.
What does this mean? In a county police department SIP-based interoperability solutions enable the sheriff to pick up a PBX phone in the office or at home and speak with remote radio users. Providing an efficient use of the network, these users could be at several locations throughout the county on different repeater sites, and each call can be made to a specific repeater site. Calls can be set up to talk to several sites, allowing greater connectivity over a wide area. After the conversation has finished, the call can be broken down, leaving the network free for other users. This means there is no need for a centralized operator to monitor and control all calls, as demonstrated in Diagram 1 on Page 72.
RoIP manufacturers have developed products that can perform basic point-to-point SIP functionality and combine this with multicasting and conferencing.
SIP is generally used to create point-to-point connections for phones. With the introduction of VoIP/SIP gateways, such as the Omnitronics IPR110 Plus, radio users can now enjoy the convenience of dialling SIP phone users via pre-programmed SELCAL and DTMF numbers from their existing mobile handheld radios. Likewise, phone users can simply dial radio channels via PBX extension numbers. Interoperability has never been so easy to implement. If the phone user is not using an SIP phone, a Web-based server (a service provider) can be used to connect radio to standard phone users.
These devices allow remote radio users to contact phone users at any time of the day without having to rely on an operator patching the call via telephone.
Users have traditionally required different interfaces depending on their duties, i.e. dispatcher, maintenance technician or manager. IP-based solutions offer many interface options including IP dispatch software, dedicated consoles or SIP compatible PBX phones. Wherever the Internet can be accessed, a technician can monitor and talk over the radio network. Users select the operator interface device best suited to their needs as outlined in Diagram 2 below.
Matching user requirements
Translator functions built into VoIP/SIP gateways convert SIP point-to-point connectivity to point-to-multi-point connectivity. A phone user can now access multiple repeater sites. This radical development further expands the communication reach for wide area networks and adds a new level of interoperability. Diagram 3 (on Page 73) demonstrates the power SIP protocol can bring to interoperability.