This article first appeared in the April 2000 issue of Monitoring Times.
Motorola has several competitors in the trunked public safety radio system marketplace. One popular alternative is the Ericsson/General Electric Enhanced Digital Access Communications System, or EDACS for short.
EDACS operates in VHF, UHF, 800, and 900 MHz bands, and is used by private businesses as well as public safety organizations. Second generation trunk-tracking scanners such as the Bearcat 245XLT and PRO-92 as well as publicly available computer software are capable of scanning these systems effectively.
Each repeater site in an EDACS system has a dedicated Control Channel that continuously transmits signaling and command information out to the mobile radios. Channel requests and other mobile messages are transmitted to the repeater on this channel as well. Listening to this channel on a normal scanner will result in just a constant buzz of digital information.
Each EDACS site, in addition to the Control Channel, may have as many as 23 Working Channels. These channels carry voice and data between mobile radios and dispatch centers.
From an operational perspective EDACS systems have better performance should equipment fail or interconnections be lost. If the central controller in a Motorola system fails or cannot communicate with a repeater, the repeater will revert to conventional mode, losing all ability to trunk and forcing users to share frequencies manually. EDACS, on the other hand, goes into a "failsoft" mode where trunking cards at each repeater site continue to provide basic trunking features.
Another difference between Motorola and EDACS is how channels are assigned. Control messages in a Motorola system use a FCC channel number to indicate the specific radio frequency to use, so listeners can enter those frequencies into trunk-tracking scanners in any order.
EDACS, however, assigns each radio frequency a Logical Channel Number (LCN). These LCNs are programmed into each radio in the system, and the control channel uses the LCN to instruct a radio to tune to the corresponding frequency. What this means is that a listener must enter EDACS frequencies in LCN order in order to track the system properly.
EDACS talkgroups are divided into agencies. Each agency has a number of fleets, and each of these fleets has a number of subfleets. This hierarchy is similar to a Motorola Type I system, although there are no limitations on the number of individual radios in each subfleet. This Agency-Fleet-Subfleet scheme is abbreviated AFS.
EDACS uses 11 binary digits (bits) to identify a talkgroup. These 11 bits are divided into three pieces, one piece for the Agency, one for the Fleet, and one for the Subfleet. Each of these pieces uses a certain number of the 11 total bits to represent the identifying number. Each EDACS system may divide these bits up differently, but the most common arrangement for public safety agencies is four bits for the Agency, four bits for the Fleet, and the remaining three bits for Subfleet. This is represented by the last entry in the table, which shows a maximum of 16 Agencies, 16 Fleets per Agency, and 8 Subfleets per Fleet.
POSSIBLE EDACS AFS ASSIGNMENTS
The AFS is usually shown in the format AA-FFS where AA is the Agency, FF is the Fleet and S is the Subfleet. Newer trunk-tracking scanners that support EDACS default to displaying talkgroups in AFS format rather than a simple decimal number. The AFS format makes it easier to scan entire Agencies and/or Fleets without needing to enter each individual talkgroup. The Bearcat 245XLT in particular has a feature called XPAND which is designed to do just that.
Some talkgroups have a special function. The first talkgroup in the system, 00-000, is known as "System All-Call." Every radio in the system will hear a message sent to this talkgroup.
Similarly, an "Agent All-Call" is the first talkgroup in an Agency, where the Fleet and Subfleet are both zero. For instance, a transmission to talkgroup 03-000 would be heard by all radios assigned to that Agency.
There is also a "Fleet All-Call" which is the first talkgroup in each fleet. A transmission to talkgroup 04-080, for example, would be heard by every radio in Fleet 8 of Agency 4.
Ocean City, Maryland
This resort town on Maryland's Eastern Shore operates an EDACS for several city agencies through two 800 MHz towers. The primary site has eleven repeaters while a backup site a few miles away has three.
Brevard County, Florida
Florida's "Space Coast," home to Merritt Island, Cape Canaveral, and the historic Launch Complex 39, lies within Brevard County. The county operates an interconnected EDACS system through 400 foot towers in Titusville, Rockledge, and Palm Bay.
One channel at each site is assigned as a Control Channel. Because any channel in an EDACS system has the capability of operating as a control channel, the assignment may change. The southern site, which includes the city of Melbourne, has more frequencies due to a higher level of activity. Telephone interconnect activity appears to be limited to Channel 2 frequencies.
Illinois State Police
The Illinois State Police in the District Chicago area operate two interconnected EDACS systems for a number of local, state, and federal agencies. District Chicago was formed five years ago out of the old District 3 (Chicago) and District 4 (Crestwood). Several sites in Cook County and surrounding suburbs provide coverage throughout Chicagoland.
The two systems are divided into North and South, with the Eisenhower Expressway as the dividing line. Each has ten channels.
The Illinois State Police operates three patrols in the District, North ("Nora"), Middle ("Mary") and South ("Sam"). Argonne is a National Laboratory operated by the Department of Energy located about 25 miles southwest of Chicago.
Dallas/Fort Worth Airport, Texas
The Dallas/Fort Worth Airport, home to American Airlines, uses an EDACS system for a variety of ground operations including security, fire, emergency medical services (EMS) and transportation.
That's all for this month. I welcome comments, corrections, additional lisitings, and questions via electronic mail at email@example.com. There is also more radio-related material on my website at http://www.decodesystems.com. Until next month, happy monitoring!
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