Signaling Methods for Radio Systems

There are many different signaling methods for different types of radio systems that have been developed over the years.  Each signaling system has its advantages which make the system work well for its designed purpose and has disadvantages which make the signaling system work poorly for other applications.  The key to any signaling system is to have the advantages outweigh the disadvantages.  Often, a signaling scheme is an old system that is inefficient and may have a better solution, but due to concerns of maintaining compatibility with legacy systems, it continues to be used today.  This has happened more times than most of us would imagine.

Why would a signaling scheme need to be implemented?  Radio is like the internet in that there is a single pipe (the radio channel for radio and the internet connection for the internet) that transmits the information and the pipe is shared by many users.  If you listen to everything in the pipe, you will listen to many things that are of no interest to your use of the radio or internet.  There needs to be some method of directing the radio or internet traffic that is intended for you to reach you and not to reach all the others who are listening to the pipe.  Unlike your home telephone which is connected to the Public Switch Telephone Network (PSTN), the network switches phone calls to your phone that are intended for only your phone.  The internet
and radio are not switched because your radio or computer receives all traffic on the frequency (or internet connection) and it is your responsibility to set up your equipment to ignore traffic that is not intended for you.

Radio signaling is broken down into two basic categories, one being audible and the other being subaudible.  The human ear can hear from 20Hz to 20,000Hz, but all that is needed for normal voice communications is 300Hz to 3000Hz.  A home phone only responds to 300Hz to 3000Hz, just the same as a cellular telephone.  The rest of the audio spectrum is needed for high fidelity radio when listening to music and for other similar purposes.   Low frequencies are associated with feeling the sound such as rumbling and explosions. Dogs can hear up to somewhere between 45,000Hz and 60,000Hz.  Most signaling is within the 300 to 3000Hz range which means that it is audible if you are listing to the radio transmission.  However, many services mask the signaling by muting the audio while the signaling is occurring so that you do not hear it out of the speaker or earpiece of the radio, so it gives the illusion of not being audible.  However, with two-way radio applications, the signaling is often heard out of the speaker and during the signaling period, the radio is not available for voice communications except for CTCSS, DCSS and LTR which are subaudible signaling and can occur simultaneously with voice. Since the signaling is at a low frequency that is below the normal audio spectrum for voice communications, it is generally not heard out of the speaker and can in most cases, run simultaneously with voice or other audible signaling

SINGLE TONE aka BURST TONE

Let’s start with the simplest and oldest signaling system known as burst tone or single tone.  This scheme was developed in the early
days of radio and was a very simple system that used a single frequency audible tone that would pass through the audio circuits of the radio that would last for about 1.0 second and cause the decoder to latch open the audio gate in the receiver to hear the radio.  (Some people would call this a whistle up system because you could open the audio decoder by whistling at the correct pitch.) The audio gate would remain open until you picked up the microphone to transmit a response, at which time the radio would no longer need the burst tone to hear anything on the channel.  The latching circuit would be reset by picking up the microphone or by the carrier squelch closing depending upon how the module was wired into the radio, so those who were listening to the conversation and not participating in the conversation had to pick up the microphone or depress the PTT button to cause the decoder to reset so that a new burst tone was needed to hear any transmissions again.

The following table represents the most common burst tone frequencies.  Other burst tone frequencies are used from a low of 600Hz to a high of 3150Hz with typically either 25Hz or 50Hz between burst tones.  When the frequencies get too close together, they tend to “false decode” or “false” for brevity.

The issue with burst tone is that it was a single burst at the beginning of the transmission.  If you missed the burst tone, you would not hear the entire transmission.  Once the first transmission was concluded and someone answered the call, another burst tone was sent which will trigger the decoder (if the burst tone was heard by the radio) to open the audio gate, so you will hear most, but not all of the conversation.  Another issue with the system is that it was easily “falsed” by women who typically have higher pitched voices.  The audio tones used for burst tone were typically between 1000Hz and
2500Hz frequency range.  Standard frequency values were established and each user on a radio channel would be assigned a different burst tone frequency. The last issue is that the users who are listening, but not participating in the conversation must perform some manual reset function to silence their radio so that they did not have to continue listening to the radio traffic on the channel.  Few if any of these systems continue to operate today because there has been enough time for newer technology to take over and replace this older
signaling system.

CONTINUOUS
TONE CODED SUBAUDIBLE SQUELCH (CTCSS)

There are many different systems that were developed over the years to accomplish the need to squelch the radio so that you did not have
to listen to conversations that have nothing to do with your job at hand. (These different systems will be discussed in depth in another document that will be on the website entitled “Conventional Signaling Methodology”.) Motorola, who is the largest manufacturer of LMR two-way radios developed a system that they called Private Line (PL).  General Electric developed the same system which they called Channel Guard (CG). Other manufacturers came out with the same thing which they called Quiet Channel (QC), Private Call (PC), Call Guard (CG) which are all trade names for the same thing with is generically known as CTCSS.  All these systems are compatible with each other, so one manufacturer’s radio will be compatible with every other manufacturer’s radios.

CTCSS is a signaling system that keeps the radio speaker quiet until a transmission is heard that you are supposed to hear.  Each radio from a given fleet of radios is set to send an analog coded signal that is unique to that fleet of radios.  The coded signal is a very low frequency signal that is below the normal audio spectrum that is used for voice communications.  This allows a fleet of radios to identify radios from that same fleet.  The radios with the correct coded signal will be sent to the speaker for the radio user to hear and the radios sending the incorrect coded signal will not enable the speaker to play the audio for the radio user to hear.  This was a significant  advancement in conventional radio operation because it freed the user from having to listen to the annoyance of radio transmissions
from other entities which tended to disturb their work.

The following table represents the audio frequencies used in CTCSS systems:

The above chart of tone frequencies for CTCSS includes all the standard tones.  There are some non-standard tones that are used on rare occasions but can cause problems with systems operating on adjacent tone frequencies which can either be “false decoded” by the non-standard tone or be “falsed” by the adjacent standard tone because the frequencies are very close together. Group A & B are the most common tones as Group C is used less often.  Also, the higher tones that are above 200Hz tend to be used less frequently because
the audio filters to not take out all the tone signal, so it is heard out of
the speaker at a low level which will annoy some people.

DIGITAL CODED SUBAUDIBLE SQUELCH (DCSS)

In the 1970s, Motorola developed an enhancement to their Private Line (PL) technology that they originally invented.  The original PL system used audio tones that are low in frequency so that they are not heard out of the speaker of the radio.  However, the original system of PL worked well, but was not perfect.  It would “false” from time to time which would irritate radio users who did not want to listen to the co-channel traffic.  Also, Motorola wanted to create a marketing  advantage that would keep the competition away from Motorola customers who had purchased Motorola radio systems since the
competition did not have radios that used DPL signaling.  Therefore, once Motorola sold a radio system that used DPL, no other competitor could sell the customer another radio.  Hence, the birth of the digital DCSS system that Motorola called Digital Private Line (DPL).

DPL works on a similar method as CTCSS except that it used
data bits instead of audio tones.  The system uses 134.4bps to send the DPL code which consists of a 3-digit octal number (9 data bits) plus 11 check bits plus another 3 bits that are fixed at 001.  This system is based upon a system of forward error correction developed by the mathematician Marcel Golay in 1949 so that the system is very resistant to being able to receive a false decode and it is capable of decoding correctly with up to 3 different data bits being obscured by interference, noise, weak signal or any other cause.

DCSS theoretically has 512 codes, but only uses 83 of them  to eliminate problems that can be caused by the decoder starting to decode the data bits at the wrong part of the data message which is being sent continuously and repeatedly.  Essentially, there are no sync bits to tell the decoder where to find the start of the data word, so this required elimination of most of the codes due to the possibility of a false decode.  The other issue is that some receivers can change from high side injection to the receiver mixer circuit to low side injection to eliminate beat notes, which causes the data bits to invert so the zeros turn into ones and the ones turn into zeros. Therefore, some codes were eliminated to prevent inverted codes from causing a
conflict.  The following table represents the standard 83 codes used in DCSS:

At the termination of a transmission, the radio will send a 134.4Hz sine wave for 200 milliseconds to force the muting of the audio.  This  eliminates the “squelch tail” that would occur if the radio relied on the carrier squelch to mute the audio.  Also, since the data word consists of 23 bits that are 7.5 milliseconds long, the data word takes 172.5 milliseconds to be sent.  Assuming the data word is decoded on the first try, it takes approximately 180 milliseconds to decode.

2 TONE SEQUENTIAL

Two tone sequential signaling is a method of sending one audible tone, then another audible tone.  Motorola has a trade name for the
signaling which they call Quick Call II, while General Electric called it Type 99 signaling.  Generically, it is called 1 + 1 signaling.  The first tone would typically be present for 1 second, then have a gap between tones for 0.05 – 1.0 seconds, then the 2^nd tone which would be present for approximately 3 seconds.  This signaling method was most common in paging, but it was used in two-way radio operations for selective calling, group calling, all calling, etc.

The following is the scheme used by GE in their Type 99 signaling:

The diagonal tone is used whenever the first and 2^nd tone are the same as in the example of pager 255.  The first digit 2 indicates that the first tone comes from group B and the 2^nd tone comes from group B.  Therefore, the 2^nd digit of the pager would be the same tone as the 3^rd digit of the pager, thus making both the 1^st and 2^nd tone 922.5Hz.  In such a case, the first tone is substituted with the diagonal tone of 742.5Hz so that there are two distinct tones sent instead of one long tone consisting of 922.5Hz, then 922.5Hz again.

The Motorola plan is a bit more complicated as it has capacity for more pagers.  Below is the Motorola Quick Call II (1 + 1 signaling) tone chart:

Depending upon the first digit of the pager code, the first tone will be from Group X and the second tone will be from Group Y.

REACH

Reach was a manufacturer of paging and signaling equipment.
They manufactured the encoders and pagers.  They also made decoders that could be added to radios manufactured by others.  The Reach system was similar to the 1 + 1 signaling, except that it was faster.  Both tones were present for a shorter period of time without a gap between the two tones, which allowed for more traffic throughput in the system.  Like the two tone sequential, it could be used for selective calling using the decoders that could be installed in manufacturers of other radios.

The chart above indicates which tone group from the chart below is used to select the tone frequency based upon the TONE # in the left-hand column.

Therefore, 1000 pagers can be accommodated with this system on a radio channel.

5/6/7 TONE SEQUENTIAL

This system was developed primarily for the paging industry during the 1960s before digital pagers became common in the 1980s.  Paging transmitters operate on typically a single channel, yet it can page hundreds of thousands of pagers.  To alert a paging receiver, a series of 5, 6 or 7 tones would be sent with different audible tones to represent digits 0-9.  Each tone was sent in sequence to send the code for a specific pager and alert the paging receiver by dialing a phone number, waiting for the automated answer, input an “overdial” number to call the specific pager. After receiving the alert signal, the pager would beep to alert the user.  The unit would continue to beep for approximately 10 seconds.  The problem with the beep was that only one message was possible to assign to the beep which typically indicated that someone wanted you to call the office.  Since this was before cellular (BC) phones, one would have to find a pay phone and call the office to find out what was the message.  A few paging carriers came up with the idea of allowing multiple pages from a single phone call.  The multiple messages allowed the sender to send 1-5 pages in a sequence of every 15 seconds.  Now a different meaning could be assigned to the different number of pages such as one beep to call the office, 2 beeps to call home, 3 beeps is an emergency, 4 beeps means call your brother, 5 beeps is call the boss, or any other meaning that you wish to assign to the different number of beeps.  The following table lists the tones that are established by the Electronic Industries Association (EIA) for use by pagers and an older mobile data terminal system built by Motorola that is called Modat.

The above chart gives you the tone frequencies for 5, 6 or 7 tone

MOTOROLA QUICK-CALL I

Quick-Call was a system developed by Motorola and is commonly referred to as 2 + 2 signaling.  Two tones were sent simultaneously,
similar to the phone company “Touch Tone” push button signaling.  After the first two tones, there was a short gap and then another 2 tones sent simultaneously.  This scheme was used for selective calling and was typically used by government for the fire department alerting fire department personnel that a emergency response was required.  It was featured in an old TV show called Emergency! Which played in the 1970s and which centered around the LA County Fire  epartment.  Other governmental agencies used the system such as the Air Pollution Control District (APCD and now known as the AQMD) who would send out an alert to businesses during elevated smog days that they had to carpool, cut back on manufacturing or take other actions necessary to mitigate the excessive smog conditions.

The following is the Quick Call tone chart:

The following chart indicates which tones are selected for a 3-digit CAP code:

Therefore, a Quick Call page of a 500 series CAP code will have one tone from Series A tones and one tone from Series Z tones.

DTMF

DTMF stands for “dual tone multi frequency” which is essentially known by the phone company as “Touch Tone” which they used for push button dialing over the public switched telephone network  (PSTN) instead of the rotary dial.  The wired telephone network originally used all rotary dial phone which sent a series of pulses to effect dialing.  The DTMF signaling was simply audio tones that could also be sent over the radio because it sent signaling through the microphone circuit of the radio.

DTMF signaling is based upon sending two simultaneous tones, one representing the row if the number and one representing the column of the number.  As an example, when you press the number 4 button which is located in the 2^nd row and the 1^st column, you will get 770Hz from the 2^nd row and you will get 1209Hz from the first column at the
same time.  Another example is the number 9 button which will  produce the 852Hz tone and the 1477Hz tone simultaneously.  Most phone dials only have columns 1-3.  The 4^th column is used for special applications in signaling.

Radios adopted the DTMF signaling for radio systems that were connected to the phone network to place and receive phone calls because it eliminated the need to convert from the phone company signaling to a different method over the radio, and eventually was used for other types of signaling over the radio which includes selective call, operating external switches at base station locations such as opening gates operating sprinkler systems, etc.

1500 / 600Hz MOBILE TELEPHONE

FOR BELL SYSTEM

This signaling scheme was used by the old manual mobile telephone systems in the 1950s, 1960s, 1970s and 1980s operated by the Bell Operating Companies where you spoke to the mobile telephone  operator and told her the phone number that you wished to have dialed, which were otherwise known as the Baby Bells.  Some of the mobile phones were duplex, so they could receive and transmit at the same time.  However, many of the mobile phones were not duplex, so the conversation with the person on the phone was half-duplex. The signaling was from the phone company to the mobile to alert the mobile user that someone wanted to speak to the mobile user.  The encoder would send audible tones of 1500Hz and switch to 600Hz based upon the pulses from a rotary phone dial that would send digits 0-9.  The decoder in the radio being used as a mobile phone would send out a multi-digit number, typically 7 digits like a land line phone number, but it could be any number of digits set by the phone company.

2805Hz MOBILE TELEPHONE

FOR RCC

Radio Common Carriers aka RCCs offered mobile telephone service during the same time period as the Bell Operating Companies, but they were on different radio channels and used a different signaling scheme.  Like the Bell System phones, some were duplex and some were half-duplex.  To place a call, you would have to call the mobile operator and tell the operator the phone number that you wish to call.  To receive a phone call, the RCC would send a signal over the air using an encoder that sent a 2805Hz signal that was interrupted by a rotary phone dial pulses to represent digits 0-9 like the Bell System encoders and would send typically 4-5 digits to activate the decoder in the mobile phone and alert the mobile subscriber.  Also, the RCC operator would take messages for subscribers who were away from the mobile telephone which the Bell System would not do for their customers.

IMTS / AMTS

Improved Mobile Telephone Service or Advanced Mobile Telephone Service was developed in the late 1060s which automated the process of placing a telephone call.  The radios were full duplex and used audible signaling to automate the process of establishing a phone call.  The following chart depicts the tones that were used in this method of signaling:

The IMTS and AMTS services were a considerable advance from the older manual service because a phone call was completely automated so that there was no need for a mobile telephone operator.

TAIT RADIO 5/6 TONE

Tait radio used a 5/6 tone sequential tone signaling scheme with their radios that was built into the software of the radio.  The timing of the tones was programmable by the radio technician for duration of the tones and gaps between the individual digits.  The sequence of tones was used to set off an alert tone, open the audio gate, stun the radio, cause the radio to transpond with a status, send a status message or many other functions.  The tones could be sent at the beginning of the transmission or the end of transmission depending what is being done with the signaling.

The frequencies, tone length, tone gap duration were all programmable, although there was a default setting for all values.  Therefore, no two Tait signaling systems are necessarily compatible.

TONE REMOTE SIGNALING

Tone remote signaling is used for controlling radio transmitters at remote locations.  Different systems have different requirements as to what functions need to be controlled remotely.  Remotes are normally connected to radio transmitters via a leased phone line which is a 600-ohm circuit that connects the two locations together.  In the old days, DC currents were used to control the transmitters, but the phone company has made it difficult to impossible to obtain DC circuits in most situations. Therefore, a system was developed to control radios over circuits by sending audible tones over the phone line to cause the transmitter to transmit, change frequency, monitor or perform any other function required.  A standard was developed that uses 3 tones in sequence, 1^st the burst tone is sent for 120ms at a level of +10dbm, then the function tone (if needed) to perform the remote control function for 40ms at a level of 0dbm, then a guard tone is sent continuously during transmit to hold the transmitter on at a level of -20dbm.  The guard tone is 2175Hz and is notched out of the transmit audio with a notch filter.

The chart above depicts the various frequencies and their functions based upon the standards set up by Motorola and agreed to by others.

POCSAG

The Post Office Code Standardization Advisory Group (POCSAG) code was developed by the British Post Office who used to operate most telecommunications in Great Britain before the time that they decided to privatize the telecommunications industry.  This is an open standard by which any manufacturer can build products.

Before cellular took over the world (BC which means Before Cellular, just ask your kids) in the late 1990s and early 2000s, paging was a huge business.  There were over 60 million pagers in the US alone,  which has been reduced to about 10% of that figure.  During the height of the paging rage, there was intense competition amongst the paging companies to provide reliable paging almost anywhere and the volume of paging traffic taxed the infrastructure.  The only way to get more traffic over the system was to make it more efficient so that one could send a page in less time while providing more information.  The original pagers only beeped to let you know that someone at the office wanted you to call back.  In the middle of the 1980s, digital pagers became popular because you could input a callback number; which allowed you to know who was the likely caller and have their phone number so that you did not have to call the office to find out who needed you if the call was from outside the company.

The POCSAG code was adopted by many paging carriers and pagers
that would work on POCSAG was plentiful and reasonably priced.  There were 3 data rates for POCSAG, however, in the big cities, only the fastest format was used which operated at 2400 baud.  (POCSAG was also available in 1200 and 512 baud.)  The radio transmitters used Frequency Shift Keying (FSK) to transmit the signal with a 4.5KHz frequency shift.  In order to extend battery life, pagers would cycle on and off, spending most of their time off to conserve the
battery.  The POCSAG signal would transmit a 576-bit preamble to wake up a group of receivers, give them time to synchronize the decoder to the preamble and then send the messages to large groups of individual pagers.

GOLAY

Golay is another paging format that was proprietary to Motorola.  It was based upon the work of the mathematician Marcel Golay who
developed the math to show the world that it was possible to send extra data bits that would allow a digital signal to miss some of the data bits and still be able to properly decode the signal.  The Golay paging format included the forward data correction and proved the viability of forward error correction scheme.

MOTOROLA MDC-1200

MDC-1200 is a data over two-way radio signaling system that uses AFSK tones of 1200Hz and 1800Hz to send the mark and space tones.  The data is sent over the audible voice channel of the radio, thus making the data somewhat annoying to some users.  By sending the data in a quick burst at the beginning of transmission or end of transmission, the voice channel is available for use by the radio user for most of the time.  MDC-600 is the same thing, but the data rate is cut in half, thus making it a slower format.

The MDC data can be used for multiple purposes such as for Caller ID, Status, Emergency Alert, Selective Calling, Caned Messages or in
various combinations of the above.  The MDC is built into the software of most Motorola analog radios and can be programmed with the Radio Service Software to accomplish the various tasks for which it can be used.  At least one other radio manufacturer has made radios with MDC signaling to be compatible with Motorola.

KENWOOD FLEETSYNC

Fleetsync is a signaling system built into Kenwood radios that operates in a similar fashion to MDC in Motorola radios.  The data system can be used for Caller ID, Status, Emergency Alert, Selective Calling and Caned Messages, just like the MDC system.  Kenwood Fleetsync is more versatile than MDC and has more ways it can be used along with higher capacity to allow for more units or not  duplicating codes.  Fleetsync is based upon AFSK and has the
following capabilities:

1. A 3-digit fleet code with a range of 100-349 giving 250 possibilities.
2. A 4-digit individual code with a range of 1000-4999 giving
   4000 possibilities.
3. A baud rate of either 1200 or 2400.  The higher speed will
   make the system operate faster, but it will not tolerate noisy signals as well because of the higher baud rate.
4. An emergency status can be programmed to sound an alert at
   the receiving point.
5. The Fleetsync message can be displayed on the radio or
   used to activate some function within the radio, or both.
6. Fleetsync II includes forward error correction, more features
   and significantly larger data words, thus making it take longer in
   transmission.
7. Status messages can be sent by the radio.
8. Special statuses can be sent or restricted by the sending
   radio.
9. Canned messages can be sent.  Free form messages can be
   sent when connected to a computer.
10. Fleetsync messages can be restricted so that it can only
    be sent within the fleet or they can be configured to be sent between fleets of radios.
11. Radio can be configured to send a caller ID at the
    beginning of transmission (BOT) or end of transmission (EOT).
12. Messages can be stored in a memory stack and retrieved.
13. Radio allows an alias list to be programmed so that one
    does not have to remember Fleetsync numbers.
14. Individual, group and all calls can be sent.

Fleetsync is a very versatile status / Messaging / ID system and is built into most current production Kenwood radios.