WAVES ON THE OCEAN

·         A complete wave is from one peak to the next, or one trough to the next one.

·         Usually, one wave every few seconds

·         The frequency of them can be expressed like this:     One wave per five seconds.

·         Radio transmissions often thought of as waves
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RADIO WAVES

·         These waves pass by much faster than ocean waves.   At the very least around 20,000 per second.    But at the most can be upwards of many millions per second.

·         1 wave per second can be expressed as 1 cycle/sec, but in the modern world there is a special measurement unit called the Hertz.     1 c/s = 1 Hz.         1000 Hz is expressed as 1 kHz.   One million Hz is expressed as 1 MHz.       1 billion Hz is a GHz.

 

THE SPECTRUM

·         The electromagnetic spectrum shown above includes radio waves at the left in the diagram, all the way up to gamma rays at the far right.  The frequency increases left to right. We are only concerned here with just the radio portion, and only a small part of it.

·         At the lower end of the radio spectrum the signals can travel around the curve of the Earth and/or can bounce off the ionosphere, so we can get long distance coverage... maybe even to the other side of the globe.  But this is generally unreliable as conditions change with the time of day and the weather.

·         In the middle and higher ends of the radio spectrum, the transmissions do not usually go around the curve of the earth and are essentially line-of-sight.    Most public safety radio is VHF (Very High Frequency) and UHF (Ultra High Frequency), and these fit into the line-of-sight characteristic.   “Line of sight” is not an absolute cut-off factor, as transmissions do regularly go somewhat beyond that point.  For example I can hear transmissions from my local airport even on the other side of a hill, but the quality is much reduced.   To some extent they can go through buildings and forest, but these can make a big difference.   In certain conditions and situations VHF and to some extent UHF signals can bounce off atmospheric layers or passing meteors and be received well beyond the horizon.  Hobbyists and others refer to this unusual distant reception as “skip” or “dx”.

While it may seem to be a negative that these signals do not usually carry much beyond the horizon, it is actually a positive because it does mean that frequencies can be re-used as long as there is sufficient distance between these uses.   For example, many different fire departments could use the same paging frequency without interfering with each other.  Typically in our region there would be 100 km or more between the same assignment, but could be less if there is a height of land in between.
 

RADIO TYPES  

·         Radios that transmit and receive are properly called transceivers, but for our purposes here, we will just call them radios. 

·         Portable radios, sometimes called handhelds, are self-contained with antenna directly attached.   Relatively low powered.    The term “walkie-talkie” is sometimes encountered but this is not usually used in professional circles.

·         Some TMR users only have portables and may be using them inside a vehicle, which is a very significant metal barrier.    If encountering difficulties in communicating such users should exit the vehicle.    In Nova Scotia, coverage with portables out in the open is good to excellent, but there are some areas that are problematic.

·         Portables are used commonly by officers out on foot and can be very much affected by wind noise.    Other issues include the limitations caused by the radio being up against the body, and simply turning around might make a difference.         

·         Many users have a mobile radio mounted in their vehicle.  This is higher power, and has the antenna located separately, outside the metal body of the vehicle.  Coverage in NS is generally excellent for mobile radios.    

·         There are also base radios that essentially are mobile radios mounted inside a building and powered from the regular household power.  The antenna might be on the roof and coverage will be good.     

SIMPLEX

·         When radios transmit back and forth to each other on one single frequency, this is called SIMPLEX.     One transmits, the other listens, and vice versa, taking turns.    This is different from cell phones and landlines, in which the transmissions are DUPLEX, i.e. going in both directions at the same time.  Ever notice how you can hear the other person on the phone even while you are talking, and you don’t push a transmit button to talk.

·         If both are portables or mobiles on the ground the range is limited…  maybe only a couple of km for portables.     Mobiles a bit more due to higher power and better antenna.     But can be a long way… e.g. mountain top to mountain top, airplane to controllers on ground, space station to ground.  

·         Simplex is the standard for ships and for aircraft communications.

·         In public safety not used very much.   Most commonly for traffic control, and other very localized operations.    

·         Since only local, cannot connect back to, or be heard by, dispatch. 

REPEATERS

·         Public safety organizations need to have both wider coverage between units than Simplex can provide, AND connectivity to dispatch, which might be quite far away.

·         Repeaters are remote radios consisting of the radio itself and an antenna.   The antenna will ideally be up on a hilltop or on as high as tower as is practical. 

·         Because the repeater is up high, it is in line of sight of a large area, and the users on the ground can access the repeater and hear back from it.

·         This can overcome many of the limitations of the curve of the Earth, and of terrain such as intervening hills.  

·         The area covered by a repeater is often called its “footprint”.    This can extend out to 50 km or more depending on terrain and antenna height.

·         The users transmit on a frequency that is received by the repeater (the input frequency), and simultaneously retransmitted from the repeater on a second frequency (the output frequency).    All the users transmit on the input and receive on the output.   Because there are two frequencies involved, but you still have to take turns speaking, some people refer to this as SEMI-DUPLEX, but I myself do not use this term.

·         Repeaters are usually located on a plot of land with a shed and a tower of some kind.  The actual radio is in the shed, and the antenna is up higher on the tower.   While some are solar powered, most are powered by the regular electric system, but there is likely a power back-up system such as a battery pack or generator.   There will usually be a fence around the installation.   The whole thing is often referred to simply as a tower or a tower site.     In many sites there is an alarm system for technical faults or intrusions, that alerts a distant monitoring site.

·         Many users of repeaters have multiple sites, and these are linked together so that individual units can talk from within one repeater’s area to another, and back to dispatch.   The linking can be constant (“hard-linked”) or on command, either by the individual users or by dispatch.

WHAT IS A CHANNEL?

 

In conventional radio, a channel is a predetermined frequency or pair of frequencies that is normally is assigned for a particular use.    It avoids having to refer to the frequency or frequencies involved.   In some services such as television and marine radio and FRS/GMRS, there are numbered channel plans in which the users never refer to the frequencies.   In others, there are local names for the channels.  For example the fire department might have a channel for dispatch, another for fire ground, and another for the fire investigators.  They would each refer to a frequency or frequency pair, but the same frequencies would have different “names” in other communities outside of the first community’s radio range.

CONVENTIONAL RADIO

·         Users manually select and use channels that vary depending on location.

·         Different agencies or departments have different channels.   

·         A channel equates to a particular frequency (if simplex) or pair of frequencies (if a repeater is involved).      These are assigned by the federal government.  

·         In simple systems there would be just one channel, but in larger ones there can be different channels for different purposes or for different areas

·         While some channels might be very busy, there may also some with only a few minutes of use in a typical day.  This means very inefficient use of increasingly scarce frequencies.

·         Users of wide-area systems must manually select a different channel for the next repeater, when travelling into a different area.

·         Because each agency has its own repeater and channels, it is difficult for one agency to talk with another, unless all the radios have each others’ channels, or dispatch can patch channels together.    All of this can very cumbersome.

·         May be able to talk to distant areas but must either manually dial up the correct repeater with the correct code OR have dispatch set up a link.  

·         For dispatch to talk to units in a certain area, must select the correct repeater to talk through.

·         All of this is labour intensive, prone to confusion, as well as using up valuable radio frequencies.

·         The NS provincial communications system prior to the TMR being introduced in 2000 was a conventional system.   That system was called the Nova Scotia Integrated Mobile Radio System.  The basics of the system as it existed then and as well the modern remnants of NSIMRS are described in a separate article on this site.   The RCMP had a similar system before they also migrated to the TMR.

NSIMRS

·         This is an abbreviation or acronym that is commonly pronounced as a word:  “NIZMIRZ”.

·         Nova Scotia Integrated Mobile Radio System, but this full form name is now only rarely used.

·         In the 1980’s and 90’s this was the provincial government’s primary communications system.

·         A network of VHF repeater sites covered the province. 

·         Each site had a separate repeater for DOT, DNR, GSS, Ambulance, VFD. 

·         Users in each service had access to their own channels, and would select the correct one for the area they were in.  

·         Users had a guide booklet, or at least a map, if they If they traveled into different areas, and would consult this to determine which channels to use.

·         In normal operations when transmitting the user’s voice would be going out over the local area repeater and only be heard within that repeater’s footprint.  For many users, such as highways maintenance this was the only way they operated, as there was no need to talk further.

·         For other users there was a need to talk to colleagues in distant areas or central offices, and they would have to consult the map and dial up the appropriate repeater by using a keypad.      Similarly, to talk to Dispatch (Shubie Radio) by dialing Zero.   

·         Shubie Radio was the central point, in Shubenacadie, in which users could be patched to the telephone system or connected with other agencies or simply just have information obtained.

·         To talk to other agencies, would have to ask Shubie to patch them together.

·         Shubie would have to keep track of locations of officers to be able to manually select which repeater to use when wanting to talk to them.

·          Became increasingly inadequate and was replaced by the TMR in 2000, but still exists to some extent today as a backup system.   There may be another page on this site describing NSIMRS in those days gone by, and in the present.

TRUNKED RADIO

·         Relatively modern development in radio communications.

·         Shares repeaters and their frequencies amongst agencies for much more efficient use.

·         Allows automatic roaming when travelling from one area to another, just like cell phones.

·         Allows officers to be on one channel even though very far apart.

·         Allows officers to talk to dispatch without dialing.

·         Allows dispatch to talk to officers without being concerned what area they are in, or what repeater to use.

·         Allows different agencies to easily talk together on mutual aid channels

HOW DO THEY DO IT?

·         This outline will use TMR2 as the example.

·         TMR2 is the present radiocommunications network for almost all public safety communications in Nova Scotia.     Main exception is some VFD communications, but all FD have at least some usage of TMR2.      

·         TMR2 consists of 100 tower sites in NS.   It is part of a larger system that covers all the Maritimes and is being established as well in Newfoundland and Labrador.   This system is described in detail in a separate article on this website.

·         Because in a conventional system each channel or repeater is generally not in use much of the time, many individual repeaters can be combined into just a few.   Instead of 6 or 10 or 20 agencies each having their own repeater and frequencies, they might be combined into sharing just a couple of repeaters.  How many repeaters are needed is based on the number or users in the area as well as the length and nature of their transmissions.       

·         Trunked systems are computer controlled.  User radios, when turned on, are automatically affiliated or connected to a site, usually the closest one. 

·         At each site there is a dedicated control channel that continuously sends and receives data to and from the affiliated radios in its footprint.   The control channel transmitter is never idle, unless it malfunctions, in which case every TMR site has a backup control channel and transmitter to take over instantly.  

·         The fact that the radio is affiliated to that site is sent by fibre cable to the central control processor.  That central computer is sometimes called the central switch.  For Nova Scotia and PEI the central switch is in Halifax.   The central processor or switch “knows” where all the active radios in the system are located, at least which site they are affiliated with.

·         The control channel at a site is separate from the voice channels over which the actual communication takes place.   The control channel operates “in the background” and users do not listen to it or otherwise have anything do with it. 

·         Each agency has at least one channel, but it does not have a specific frequency or pair of frequencies attached to it.   Because of this each of these channels is a virtual channel, more commonly called a “talkgroup”.     Regardless of location, anyone authorized to use it, can be on a particular talkgroup.

·         When a user in the field presses the PTT button, a data burst goes out via the data channel, and the local computer (processor) at the site allocates a repeater at the site to receive and transmit the conversation.    Let’s say there are three repeaters at the site (other than the ones for data)…   the processor might assign Rptr #2, and a data stream back to the radio tells the radio to go the frequency for Repeater #2,  By the time the person begins speaking, it is all set up.    

·         Also via the data burst, a route is formulated via the central Halifax processor to all the other sites that have someone in that talkgroup affiliated.   An instant later when he or she begins talking the route has already been established and the communication goes out to everyone on that talkgroup but not to any site that does not have anyone in the group affiliated.     This latter aspect stops repeaters from being used unnecessarily.     

·         Similarly, when dispatch wants to talk to one or more officers on a particular talkgroup, when the dispatcher presses the transmit button the route is set up out to all towers that have that group of officers affiliated to them.  There is no manual selection of sites.  

·         In the TMR, all users have talkgroups available other than their main one.  These can include regional and/or secondary ones for their own agency.    There are also interoperability talkgroups available for talking between different agencies.      

·         A secondary characteristic of trunked systems such as the TMR is that each radio has a unique radio identification number (RID) that accompanies all of its transmissions.  This means that other users, including dispatchers, will see that RID on the screen.   Furthermore it is possible to create an alpha tag to take the place of the RID on screen.  For example, and this is hypothetical, let’s say that John’s portable radio has RID 234976.   The system or parts of it can have an alias substituted for that RID, so that the screen reads “John” instead of 234976. These alphatags are generally referred to as aliases.

CONSOLES VS RADIOS

·         Major dispatch centres in Nova Scotia use consoles for everyday communications with their users.  There will likely also be backup radios in these centres but that is what they are, for backup or to provide channels not installed on the consoles.   Note that the commercially operated  fire dispatch centres do not currently use consoles, though they may use something called consolettes in order to ease the operation of many radio links.

·         When the PTT on a radio is pressed the signal goes out via the antenna into the air and to the TMR site to which the radio is affiliated, and vice versa.

·         A console is different, and although the console might be referred to sometimes as a radio, it really is not a radio.  The console is really a remote terminal for the central controller (computer) in Halifax. When you press the transmit button, the signal goes via cable to the central processor in Halifax and then back out to whatever towers it needs to, and vice versa when we are receiving.      Someone listening with a scanner near one of these dispatch centres  would not detect anything as there are no signals going directly into the air from that centre when the consoles are in use.

·         Consoles allow the dispatcher to have many talkgroups available via screen icons, and the ability to easily patch these talkgroups together.  

 

Trunking Situations

First, to review.   A trunk radio tower site in Nova Scotia has at least 4 repeaters.  Two of these will be the control channel repeater and the alternate (backup) control channel repeater.  That means that there are at least two voice channel repeaters or “voice paths” at each TMR site.    Some sites in busier areas have more voice paths.  There are several with three or four voice paths, and some have up to around a dozen, particularly around the urban Halifax area.

In normal circumstances for any particular site, the equipment is working well, and the users are not overwhelming the site.  Studies have been made to predict how many voice paths are needed at any particular site in normal circumstances.  Keep in mind that most transmissions from any particular user are rather short and rather intermittent.   So usually there is no problem for any user having immediate access to service.     

Occasionally there will come a time when a user presses the transmit button and the voice paths at the affiliated site are all in use.   In this case the radio will emit a tone called a busy signal or “bonk”, and the transmission cannot go ahead.   Generally this condition lasts only for a few seconds, and when a voice path clears, the additional user can proceed as normal.     It is possible for users to force their radio to affiliate to a different site if at least one other is in range, but this is not something that most users would bother with.  Note that in the TMR radios do NOT automatically switch to another site if the affiliated one is busy. 

Most of the time the system works very well in regard to being overwhelmed.    The difficulties arise when a major incident takes place in an area served by a rural TMR site with few voice paths.   I am sure that cost-benefit analyses have been conducted to balance the major expense of installing and maintaining voice path repeaters against the projected and possible demand levels. 

Site Trunking

Occasionally a site might lose its fibre connection to the central processor in Halifax.  This is most likely due to a break in an above-ground cable, but can also happen when an in-ground cable is damaged or severed by construction equipment digging in the wrong place.    In these cases the users of that site cannot be connected to the rest of the network.    The on-site processor continues to recognize the users and allocate the voice paths to them, so there is still sharing of these paths, and communications go out directly over the air to other users in the footprint of the site.       There will be no connection to dispatch unless they have backup radios and the centre is in the particular site’s footprint as well. 

 

In Nova Scotia there is a limited capability to link a TMR site to the rest of the system by establishing a microwave link carrying one mutual aid talkgroup to the provincial communications centre where it can be further linked into the wider TMR system.  This allows users in the field to have emergency communications to the “outside” on this one talkgroup.   This is not an automatic process and requires manual procedures by communications centre operators.

Failsoft

Very rarely the on-site processor fails and the trunking process can no longer take place at the site, even for local communications.   In effect this turns the TMR site into a conventional repeater site, and the various voice paths operate independently, or only one repeater operates.   How the field users access and use this is unclear to this writer at this time.  

Keep in mind that for a TMR site to go completely out of service its power supply must be disrupted.  It would take a very prolonged power grid failure for this to happen.