Understanding Radio Scanning

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Conventional Radio Scanning

Conventional radio scanning is a relatively simple concept. Each group of users in a conventional system is assigned a single frequency (for simplex systems) or two frequencies (for repeater systems) to use. Any time one of them transmits, their transmission always goes out on the same frequency. Up until the late 1980’s this was the primary way that radio systems operated. Even today, there are a lot of 2-way radio users who operate using a conventional system:

  • Aircraft
  • Amateur Radio
  • FRS/GMRS Users
  • Broadcast AM/FM/TV Stations
  • Many Business Radio users

When you are scanning a conventional system, the scanner stops very briefly on each channel to see if there is activity. If there isn’t, the scanner quickly moves to the next channel. If there is, then the scanner pauses on the transmission until it is over.

Simplex Operation

Simplex systems use a single frequency for both transmit and receive. Most radios using this type of operation are limited to line-of-sight operation. This type of radio is frequently used at construction job sites, and with inexpensive consumer radios such as GMRS/FRS radios. The range is typically 1-5 miles, depending upon the terrain and many other factors.

Repeater Operation

Repeater systems use two frequencies: one transmits from the radio to a central repeater; the other transmits from the repeater to other radios in the system. With a repeater based system, the repeater is located on top of a tall building or on a radio tower that provides great visibility to the area of operation. When a user transmits (on an input frequency), the signal is picked up by the repeater and retransmitted (on an output frequency). The user’s radios always listen for activity on the output frequency and transmit on the input frequency. Since the repeater is located very high, there is a very large line of sight. Typical repeater systems provide coverage out to about a 25 mile radius from the repeater location.

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 systems can be implemented under APCO 25 Phase 1 or Phase 2 standards. The BCD396T and BCD996T are able to decode all unencrypted digitized voice traffic on either mixed mode or digital-only APCO 25 Phase 1 systems.

One big difference you will notice with digital versus analog transmissions, is that with analog systems, you might be able to hear weak signals interspersed with hissing. As you move further away from the system, the interference gradually increases until you are unable to make out the transmission. With digital systems, the cutoff point is much more abrupt. You might have a small area where partial decoding which case you will hear partial and garbled audio. However, once the scanner is unable to receive the data well enough to decode it, the audio stops entirely. For the best range, antenna selection and placement is critical. See “Attaching an Antenna” for more information.

EDACS Trunking

EDACS trunking works in much the same way as Motorola trunking with a couple of major differences. In an EDACS system, each frequency used by the system is assigned a Logical Channel Number (LCN) so that less data needs to be transmitted by the control channel. Also, talkgroups are assigned in an Agency- Fleet-Subfleet (AFS) hierarchy. Also, there is one variation of EDACS called SCAT that the BCD396T and BCD996T can monitor.

  • Logical Channel Numbers – each frequency used by the system is assigned an LCN. This information is programmed into each user radio. When a user presses PTT, their radio sends their AFS information to the control channel. The computer then assigns that talkgroup to a channel and sends the LCN so that all other radios in that talkgroup will switch to the correct channel. To program a EDACS system in BCD396T and BCD996T scanners, you will need to know both the frequencies used by the system and the LCN for each frequency so that you can program the frequencies in LCN order.
  • Agency-Fleet-Subfleet – talkgroup ID’s for EDACS systems are assigned in a way that makes it easy to see at a glance the affiliation of the user. Each radio is assigned a 2-digit agency identifier from 00 – 15. For example, 01 might be used by the police, 02 by ambulance service, 03 by the fire department, and so on. Each agency is then subdivided up to 16 times to provide fleet identification, and then 8 more times to identify subfleets. For example, the complete AFS for the Police Department West District’s dispatch channel might be 01-062. 01 identifies the agency as the police department, 06 identifies the fleet as the West district, and 02 identifies the subfleet as the dispatch channel. While these assignments are somewhat arbitrary and vary from system to system, there are many resources on the web for finding the assignments for most systems. Because of the logical hierarchy of the AFS system, the BCD396T and BCD996T let you assign wildcard ID’s that let you, for example, use only one ID memory to identify all units in either an agency or a fleet.
  • EDACS SCAT – EDACS SCAT (Single Channel
    Autonomous Trunking) systems operate on a single channel and alternate control data with analog voice traffic. While BCD396T and BCD996T cannot track ID’s in this system, it can eliminate the control data so that all you hear is the voice transmissions when you monitor this type of system.

LTR Trunking

LTR® (Logic Trunked Radio) systems are trunking systems used primarily by business or private communications service providers, such as taxicabs, delivery trucks, and repair services. These systems encode all control information as digital subaudible data that accompanies each transmission, so there is no separate control channel. Users on an LTR system are assigned to specific talkgroups, which are identified by the radio as six digit numbers. These numbers are in the form AHHUUU, where:

A= Area code (0 or 1)
H= Home repeater (01 through 20)
U= User ID (000 through 254)

When the scanner receives a transmission on a channel set to the LTR mode, it first decodes the LTR data included with the transmission. In the ID Search mode, the scanner stops on the transmission and displays the talkgroup ID on the display. In the ID Scan mode, the scanner only stops on the transmission if the LTR data matches a talkgroup ID that you have stored in the bank’s talkgroup ID list and have not locked out.

LTR systems are frequently programmed so that each radio has a unique User ID.

LTR systems also need to be programmed into BCD396T and BCD996T scanners in channel-order.

Since many LTR systems use only odd-numbered channel slots, you would program these systems using only the corresponding odd-numbered channels in a bank (for example, you would program a system with channels at 1, 3, 5, and 9 into Trunk 2 channels 101, 103, 105, and 109).

Understanding Banks and Channels

The memory in BCD396T and BCD996T scanners is organized into 10 banks of 100 channels each. Each bank can contain conventional channels as well as 1 trunking system. For each trunking system, each bank can also store 10 groups of 10 talkgroup ID’s (100 per bank).

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