Last month in this space, we examined the overarching issues associated with using IP networks to provide the backbone for voice traffic in a public-safety radio system. This month, we explore the specifics, including priority/emergency procedures, control, reliability, the radio environment, and cost — perhaps the most important element.

Priority/Emergency: Public-safety organizations are top-down hierarchical paramilitary organizations. Chiefs must be able to override captains, and captains must be able to override police officers or firefighters. Public-safety radio systems always have preserved this command hierarchy by giving console dispatchers priority over field units and by providing emergency signaling and alert tones in both directions.

Radio-over-IP (RoIP) technology must perform in a similar fashion. In some RoIP systems, a packet-switching computer is used to provide the intelligence necessary to arbitrate priorities among packets and provide for emergency transmissions. Other systems use IP multicast techniques to distribute the voice packets, but this technology provides a “party line,” usually without priorities.

Control: IP systems in wired environments always have afforded system administrators the ability to control and configure individual devices — each having a unique IP address — on the area network.

Such control is even more important in a public-safety system. However, some RoIP systems are based upon non-native IP platforms and implement the IP protocol via middleware that uses IP. This means that there are “translators” at various places in the system to convert the protocols used in the system to IP and back again.

In such cases, end-user devices such as officer radios don't have an IP address, making system control and administration more complex.

Reliability: Public-safety systems must operate during storms, disasters, blackouts and peak periods caused by multiple incidents. Traditional land mobile radio systems have been designed to ensure this. They have overcapacity. They don't have a single point of failure, and the critical backbone links connecting base stations have utilized ring, mesh or star (with dual links) configurations for maximum reliability. RoIP systems must be able to provide these same reliability levels.

For example, if a packet switch is used in the system, it should be able to be backed up, either on site or at a geographically redundant site. Ideally, such packet switches also should be capable of being distributed throughout the network at key locations to improve traffic flow and to provide increased reliability.

Radio Environment: IP originally was envisioned for wired systems, so some adaptations are necessary for it to work in the radio environment. For example, the vocoders used in VoIP or computer telephony often utilize high bandwidths, and they usually do not provide sufficient error correction for a fading mobile radio link.

However, IMBE/AMBE vocoders have been used in public safety, and they work very well over radio, providing sufficient fidelity to recognize both speaker and emotion. Ideally, the voice will undergo only one vocoding process during transport from one part of the system to the other. Should the voice be re-coded several times during transport, considerable degradation can occur because of differences in vocoder characteristics.

Cost: When looking at RoIP systems, agencies should look at total lifecycle costs. For example, links to distant base station sites can be a large recurring cost.

To reduce costs, some systems use compression on these links to allow the use of lower-speed lines. The RoIP system also should have a future — by using open standards and interfaces — and provide for migration paths to future radio air interfaces or other features, without costly forklift upgrades.

John Facella is the director for public-safety markets at M/A-COM. Prior to joining M/A-COM, he spent 20 years at Motorola in public-safety wireless communications.