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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.

Addendum — 02/26/2009

This article has stirred spirited discussion in several forums. The following information has been culled from some of these discussions.

San Diego County operates a large regional trunked radio system that is, arguably, one of the best designed and managed systems of its kind. Many of the busies that occurred during the 2003 and 2007 wildland fires were caused by the problem I describe above. The system managers advise that the key to solving the problem is: 1) recognizing the potential for it to occur and geographically limiting user talk groups to the area they normally operate in; 2) limiting the number of “system wide” talk groups to the minimum number necessary, and 3) designing a system capacity (number of channels) in the smaller sites to allow for local operations plus a credible number of wide area conversations at the same time.

One of the foremost experts in the field reports another example occuring in Colorado during a State Chiefs Association meeting in Denver a few years ago. The chiefs busied out the local 5-channel CCNC site because many of them dragged their primary dispatch channel with them across the State. The same thing happened during a tornado at Windsor, Colorado in 2008 when many of the responders tried to maintain communication on their primary talkgroup and busied out the 5-channel CCNC site that covers Windsor.

The I-35 bridge collapse in Minnesota had a similar problem, but not at the scene of the incident where they had almost no busies. However, a few counties away sites were busied out by everyone trying to listen to the various I-35 response talkgroups… again, the “casual listener.” Proper system design, configuration and user taining can preclude this situation from happening.

Another reader reports this issue in Orange County and Orlando Florida during hurricanes and other major events. The system managers had to disable wide-area talkgroups that were not dedicated to interoperability. They had about 75 wide area talkgroups between the two systems, and during these high-volume events, went down to 10.

The facts are clear… It is critically important for radio system users, engineers, and system managers to understand this issue. San Diego RCS is a leader in the field of user education. Contact Scott Gillis <scott.gillis at> for information about his research in this area.