Large, multi-zone digital trunked radio systems can be so fragile that too many users merely listening to the system can cause problems. Such was the case during the aftermath of a catastrophic collapse of the Interstate 35W bridge over the Mississippi River in Minneapolis on the evening of August 1, 2007.

The State of Minnesota Department of Public Safety commissioned GeoComm Consultants to evaluate the performance of the State’s Motorola SmartZone digital trunked radio system known as ARMER. The report is available for your review by clicking on the link below. The report identifies a dilemma that’s well-known by experienced digital trunked radio system managers, but rarely considered by police and fire incident commanders and executives.

Most digital trunked radio systems that serve large geographic areas are comprised of subsystems for specific geographic areas. These subsystems are linked together and controlled by a centralized computer so that the subsystems can act as one large system. Individual agency radios are “homed” on the subsystem that provides the best coverage for their primary geographic area. When an individual radio is operated outside of its home zone, the central computer will attempt to make the user’s talkgroup active in the zone where the user is currently affiliated. Merely turning a radio on will cause the radio to affiliate with the talkgroup. The act of merely listening can consume limited resources and negatively affect communication at the scene of the incident.

A trunked talkgroup will assume the characteristics of whatever sites get added to a call. If a user is on a site that has only a few channels available, the system may not be able to assign a channel for the talkgroup and the user who is attempting to transmit a message will get a busy signal, even if there are enough channel resources at every other site involved in the communication. To illustrate this point, consider a major incident such as the bridge collapse where hundreds of radio users throughout the large coverage area want to listen to the critical incident, even if they are not directly involved. This can cause the talkgroup that is being used for the incident to become active in many zones where it wouldn’t normally be used. If channels aren’t available in all of the zones where the non-participant listeners are affiliated, the trunked radio system will return a busy signal for the users who are directly involved in the incident even if channels are available in the area near the incident.

There are workarounds for this problem, such as using a system mode that says “go ahead and transmit the communication even if all sites do not have a channel available.” Unfortunately, this mode of operation also means that users will never be sure if messages are being broadcast in all of the zones where the talkgroup is active. Managing talkgroups in a large multi-zone system is extremely complex. Training non-technical first responders to understand the limitations of the digital trunked radio system is equally difficult. Failure to address both issues will result in ineffective communication for first responders and perceived system failures.

Generally speaking, communications systems should only be as complex as needed to meet the business requirements of the mission. Making a system unnecessarily complex in anticipation of the unlikely need for wide area communication may reduce the system’s effective reliability for routine activities.