Trunked Scanning, Motorola Trunking | Motorola BC796D, APCO25

Trunked Scanning

While conventional scanning worked great while there were only a few groups wanting to use the frequencies, with the advent of smaller, lower-cost radios more and more agencies and businesses wanted to take advantage of the utility of 2-way radio. As a result, the bands that were used most became full, so new users were not able to take advantage of the technology as quickly as they wanted.

Trunking solved this frequency shortage by allowing multiple groups to use the same set of frequencies in a very efficient way. While each type of trunking system operates a little differently (see the next few sections), they all work on the same basic premise: even in a system with a lot of users, only a few users are ever transmitting at any one time.

Instead of being assigned a frequency, as with conventional systems, each group is assigned a Talkgroup ID. A central computer controls the frequency each group operates on...and this frequency selection is made each time a user transmits. So, while on a conventional system queries, replies, and follow-ups are all on a single frequency, they could each be on completely different frequencies on a trunked system. This semi-random frequency assignment made monitoring such a system impossible prior to Uniden’s invention of the Trunktracking scanner.

Motorola Trunking

While there are 4 different types of Motorola trunking systems, they all use the same basic trunking method. The system consists of one control channel plus one or more voice channels (typically 10, 20, or 30 total channels). When a user presses Push To Talk (PTT) to transmit, their radio first sends their talkgroup information to the control channel. The computer then assigns that talkgroup to a specific voice channel and transmits that data over the control channel. All radios in that talkgroup switch over to the assigned voice channel and the user can begin speaking. This all typically takes place in about a second...the person transmitting hears a beep from their radio when the channel is assigned and it is OK to start talking.

The four systems in use are:

•Motorola Type I – the radios send the radio ID, the fleet and subfleet talkgroup ID to the control channel each time they transmit. To program a Type I system, you need to know the system’s fleet map. The most common fleet maps are included at the back of this manual. You can also find fleet map resources on the web.

•Motorola Type II – the radios only send the radio ID and radio channel code to the control channel. The central computer keeps a database of radio ID’s and which talkgroup is assigned to which channel code for each radio, so with this system the user’s radio sends only about 1/3 the data as a Type I system with each transmission. Type II systems do not use Fleet-subfleet talkgroups; instead they use a 5-digit ID for each talkgroup.

•Type IIi Hybrid — these systems support a mix of both Type I and Type II users. Like Type I systems, you must know the system’s fleetmap to ensure proper tracking.

•Motorola Astro Digital — for channel control purposes, this type of system operates just like a Type II system — although the control channel can be a 3600 bps data rate (for mixed analog/digital systems) or a 9600 bps (for digital-only systems). Pure digital sys- tems can be implemented under APCO 25 Phase 1 or Phase 2 standards. Your BC796D is able to decode all unencrypted digitized voice traffic on either mixed mode or digital-only APCO 25 Phase 1 systems.