Trunked Radio System Types
Trunked public radio systems are increasingly the norm in the United States. Many smaller municipalities continue to use traditional "one frequency/one channel" systems. And some larger cities still use them for various operations. But trunked systems are here to stay.
There are many types of trunked radio systems, including analog and digital types. The vast majority are analog.
The most common types of trunked systems are Motorola Types I and II and Ericsson EDACS. These systems will be examined individually below.
Motorola Type I
The Motorola Type I system operates much as the typical trunking system described elsewhere on this site with a couple of distinctions. One is that when a radio operator keys the mic, the radio transmits its own identifying information, including ID information for the system it interfaces with. The radio also transmits fleet and applicable sub-fleet information.
A fleet-sub-fleet hierarchy allows the option to break fleets into sub-fleets. An example is a police department fleet with sub-fleets for crime investigation, traffic control, SWAT, and so forth. Programming a scanner for such operation involves mapping the fleets and sub-fleets.
Motorola Type II
This type is similar to the Motorola Type I, except that upon transmit, only the radio's ID is sent. It does not send information identifying the system it interfaces with, nor does it send fleet/sub-fleet information. Rather, a Type II system stores this information. This means a Type II system has leftover bandwidth that can be used to accommodate more users.
Ericsson EDACS (Enhanced Digital Access Communications System)
EDACS has many similarities to the Motorola Type II system, but includes enhanced functionality that includes Transmission Trunking. Unlike other trunked systems, this type of trunking always uses a new channel for each press of the radio mic. EDACS operates quickly and efficiently and can therefore support this type of operation. Transmission Trunking allows enhanced channel availability for a given number of users.
EDACS also features Message Trunking, which uses the same channel for an entire conversation. This type of channel selection works well for emergency situations in which channel availability must be assured, but it can tie up channels unnecessarily when critical communications are not required. An ideal scenario is one in which Message and Transmission Trunking are used as conditions warrant.
EDACS uses Decimal (DEC) and Agency-Fleet-Sub-fleet (AFS). Scanners capable of receiving EDACS can typically be switched between DEC and AFS.
Digital Trunked Systems
Digital trunked systems cannot be monitored by scanners which are capable of receiving analog-only systems. However, scanners capable of receiving digital systems are becoming more common.
The driving force behind the development and standards in digital trunked systems is APCO 25 (Association of Public Safety Communications Officials Project 25). Motorola's ASTRO digital trunking technology is an outgrowth of APCO 25, featuring advanced functionality such as direct interoperability with other groups in a radio system and more efficient (12.5 kHz) frequency spacing. ASTRO also allows digital communications directly between units (without repeaters) and facilitates a smooth upgrade path from analog to digital.
Digital trunking systems are legal for scanning enthusiasts to monitor as long as the transmissions are not scrambled or encoded for security purposes.