How a SmartZone system works

by Jim Walls

This is an exposť on the basics of how a Motorola SmartZone system works. The contents of this article originated from posts to the SoCalScan message group by Jim Walls who was kind enough to let us share this with you. Please forward any questions, comments or corrections to
OK, you asked for it. WARNING! This will get long, but I'll try to give a short answer first. Please understand that Motorola will happily teach you this for a large sum at a training facility in Schaumburg, Ill. in 2 weeks so I am condensing this A HUGE AMOUNT. It's another week for SmartNet Systems, another week for simulcast, etc.
A SmartZone system can be analog only (such as the Southern California Edison system I baby-sit), digital only (a lot of Navy systems, and I'm sure others), or mixed mode (such as the Orange County system). From what Steve Hronek of Glendale said yesterday ICIS will be mixed mode capable system. He did not indicate which (if any) operations will be digital, at least to start. Eventually I would expect that most (if not all) operation will be digital as narrow-banding continues.

As to monitoring a SmartZone system, if you can monitor a SmartNet system, you will be able to monitor a SmartZone system. That's the short answer. There is a longer answer later on.


How does a SmartZone system work?
I'm not covering basic trunking here. I'm assuming you already know that at least a bit. A SmartZone system is a group of SmartNet trunking systems tied together. That's the REALLY short (and not 100% correct answer). A single site trunking system (such as a SmartNet system) has a controller that controls the assignment of channels for each conversation. A SmartNet system may actually have multiple sites using voting receivers, simulcast transmitters, or both. In a SmartZone system, each SmartNet site (whether it's simulcast or not is irrelevant for this discussion) is tied to a Zone Controller that under normal circumstances takes over the channel assignment role. Each radio tells the Zone Controller what site and talkgroup it is on. This is called affiliating. The Zone Controller uses that affiliation information to keep track of each radio that is turned on so that it knows what talkgroup it is on, and also what site it's on. When a radio wants to transmit on a talkgroup, that radio sends a data burst (called an Inbound Signaling Word or ISW) on the control channel of the site on which it is operating requesting a channel assignment. The Site Controller forwards that request via a serial data circuit to the Zone Controller. The Zone Controller then determines that the radio ID making the request is valid, the talkgroup is valid, the talkgroup is valid on the site that the radio is on, and (and this is the important part) what other radios are on that talkgroup, and what sites those radios are on. The Zone Controller then makes a channel assignment for each of the sites that have radios affiliated on that talkgroup, and sends that assignment information to each of the Site Controllers that are involved in this particular call. The Site Controllers then transmit a data burst as part of the data on the control channel (called an Outbound Signaling Word or OSW) to every radio indicating that talkgroup nnn has been assigned to channel xxx. All radios that are monitoring talkgroup nnn then switch to the assigned channel.

That whole paragraph just gets data moved around which gets the radios on the right channels, but we still need to route audio to the various sites. In a SmartNet system, received audio is retransmitted locally in each repeater. That's simple enough (and not a complete story, but close enough for this discussion). In a SmartZone system, the received audio from each channel is sent to a fancy audio switch that you may hear described as an Ambassador or an Embassy. There is a much longer answer to what's really the Ambassador and what's the Embassy, but for this discussion they are interchangeable, and I will use the abbreviation AMB to refer to the audio switch. The AMB is controlled via a serial data circuit from the Zone Controller. The AMB takes the received audio from the radio site that is currently receiving for a talkgroup and routes that audio to outbound audio circuits to whichever site or sites each particular call has affiliated radios on.

This might be easier to visualize this if I give an example. For this particular example, talkgroup 800101 has radios affiliated on sites 1, 4, 11, and 17 at this instant. A radio on site 4 wants to transmit on talkgroup 800101. For this example the Zone Controller assigns the following channel assignments for this call: Site 1 channel 5, Site 4 channel 3, Site 11 channel 8, and Site 17 channel 4. The radio currently transmitting is on site 4 so the AMB routes the received audio from Site 4 channel 3 to Site 1 channel 5, Site 4 channel 3, Site 11 channel 8, and Site 17 channel 4. Note that the repeated audio from site 4 is sent back to site 4. In other words, if there are two people operating on a given site, when one transmits, his audio goes through the AMB before it gets transmitted back to the other person on the same site. It is not repeated locally. There are two exceptions to this which I'll get to a bit later.

A couple of general points. Each trunking system has a system ID number. In a SmartZone system, there is only one system ID number for the system. Each site in a SmartZone system has a site number. If subscriber radios are programmed with a button to display it (ours at Edison are), pushing the button will display the site number, and pushing the button for more than about a second will force the radio to attempt to find a different site on the system. Subscriber radios will search to find the best receive signal in the event it is in an area that more than one repeater site covers. When a subscriber radio moves out of coverage of the site it's currently affiliated to, it will find another site and automatically switch to that new site and affiliate with the system (so the Zone Controller will know where to find it). A SmartNet subscriber radio will work fine on a SmartZone system except that it wont change radio sites automatically.


Failure modes and redundancy
I'll start at the smallest level. If a single repeater fails, that channel is turned down and will no longer have traffic assigned to it until is it deemed (by the self diagnostics) to be working again. There are some problems that are hard failures and wont restore until someone fixes it (such as a power amp failure, blown fuse, etc.) and there are some types of failures that are transitory in nature. When these types of failures clear, the channel will automatically return to service and the Zone Controller will resume assigning traffic to that channel. The only redundancy at the repeater failure level is that there are multiple repeaters and they are all shared so failure of one only reduces available channels and may increase number or duration of busy signals.

The next level of failure is a Site Controller. In the event of a failure of a Site Controller, that particular site will go into failsoft. In failsoft, each repeater operates conventionally (non-trunked) and each subscriber radio will switch to one of those channels - which one depends on how the radio was programmed. Unlike a SmartNet system, in as SmartZone system, subscriber radios will try to find a radio site that is operating in wide area trunking and will switch to a fully working site even if a site in failsoft has a stronger signal. There are two types of site controllers in use in a SmartNet or SmartZone system. The original type is commonly referred to as a 6809 controller and is quite old. A 6809 Site Controller can either be non redundant, or have a backup controller that is switched into service if the primary controller fails. In newer systems that use Quantar or
Quantro repeaters, each repeater is capable of operating as the site controller. Under normal operation, one of the repeaters will operate as the site controller. The repeaters all communicate over a LAN with each other so if the repeater that is operating as the Site Controller fails, one of the other repeaters will take over that role. There is one other failure besides the site Controller that will put a site into failsoft. Each repeater site can have up to 4 channels available as control channels. In the event of failures of all available control channels, the site will go into failsoft mode. Failsoft is one of the two exceptions I mentioned earlier where received audio is repeated locally.

The highest level of failure is failure of the Zone Controller. If the Zone Controller fails, each site will revert to a trunking on a stand alone bases where each Site Controller takes over channel assignments for that one site's trunking. This is called Site Trunking. Depending on how the subscriber radios are programmed, the user may not even know that the system is not in wide area trunking Users on any site will be able to communicate with each other on their trunked talkgroups just as they did before the Zone Controller failure. The exception is that they can only talk to other radios on the same site that they are operating on. Site Trunking is the second of the two exceptions I mentioned earlier where received audio is repeated locally. Normally all important cards in the Zone Controller are redundant and hot-swappable. I personally have walked up to Edison's Zone Controller and pulled the active CPU card out of the shelf and operation continued on the standby CPU without any interruption. I will point out that this was done during a scheduled maintenance outage, but it really does work. Besides the redundancies in the Zone Controller, a system can have a backup Zone Controller (frequently located somewhere other than where the primary Zone Controller is). If the primary Zone Controller fails, the backup Zone Controller can be switched into service.

The other top level failure is that of the AMB audio switch. Because all repeated audio routs through the AMB, failure of the AMB will also put the individual radio sites into Site Trunking (where the audio is repeated locally). The AMB can have a backup switch.

Consoles in a SmartZone system are connected via the AMB audio switch (short description). In the event of failure of the SmartZone or AMB, the consoles will not work. Dispatchers can use control stations to communicate on the talkgroups on the radio sites they have an RF path to. This works pretty well on a small system or with distributed dispatching, but on a wide area system with centralized dispatching it does not work so well.


A few things about control channels
Just like a SmartNet system, each radio site can have up to 4 control channels. In most SmartNet systems, the control channel rotates (normally daily). Generally in a SmartZone system the control channel stays the same unless the primary control channel fails. Yes, this means that the primary control channel may operate continuously for years. Part of the OSW datastream is a listing of the frequency for the (up to 3) alternate control channels at that site. The subscriber radios store that information in memory. Also part of the OSW datastream is the current control channel frequency at up to 7 adjacent radio sites. The subscriber radios also store that in memory.

If the event that a subscriber radio stops receiving control channel data, it will first look at the 3 channels stored that are the alternate control channels at the current radio site (in case the site switched control channels). If the radio can't find control channel on one of those three frequencies, it will try the 7 adjacent site control channels (in case the radio drove out of range of the previous site, or the previous site failed).

If the subscriber radio still can't find a control channel, it will look at a list of up to 32 channels that are programmed into the radio to find a control channel. Normally every possible control channel at all sites in the system are programmed into that list.

The last option is that if this feature is enabled on the radio, it will search every channel in the band looking for a control channel. Earlier in this paragraph I referred to up to 7 adjacent sites. The system has no way of "automatically" knowing what are the adjacent site, but that information is entered by us humans into the SmartZone Manager system.


A little about scanning in a SmartZone system
I'm actually referring to the intended users of the system, but the information is relevant to people using TrunkTracker scanners. Remember earlier that I said that the Zone Controller will assign a channel for a conversation at the sites that have radios currently affiliated on that talkgroup.

In the example I used many paragraphs ago, talkgroup 800101 had radios affiliated on sites 1, 4, 11 & 17. These are the only sites that the Zone Controller will assign a channel for a conversation on talkgroup 800101. I'll get to the exception in a minute. If a subscriber radio is affiliated on one of these 4 sites, and "scanning" talkgroup 800101 but affiliated on another talkgroup, that radio will hear the conversation. If however that radio is on another site, they will not hear the conversation on "scan". As a TrunkTracker user, you will hear traffic on a given talkgroup only if there is a radio affiliated with that particular talkgroup at that time. The exception I mentioned earlier is that a system administrator (like me) can tell the Zone Manager that any particular site (or sites) are required for any talkgroup. In other works, I could tell the system that it always must assign a channel for talkgroup 800101 on site 2. There are several reasons why this might want to be done. In our system, it is not normally done.


A few words about Private Calls
First of all, they are private only to users of the system. Scanner users and even people monitoring conventionally can hear them just fine. This is something that we constantly tell our users. Just because it's a "Private Call" does not mean that no one can monitor it. The guy that does most of our training for users, often demonstrates that in training classes using a TrunkTracker scanner.

One thing to know about a Private Call however from a monitoring perspective. If the two radios in a private call are on different sites, you will only hear one side of the conversation. This is because the AMB only routes audio to the site of the person receiving. In other words, if one radio in the private call is on site 1 and the other is on site 2, when the radio on site 1 is talking, the audio is only routed to site 2. When the radio on site 2 is talking, the audio is only routed to site 1. Since you are monitoring only one site, you will only hear one side of the conversation if the two radios are on different sites. If the two radios in the Private Call are on the same radio site, you will hear both sides of the conversation normally.


A little bit about telephone interconnect
Subscriber radios that have interconnect enabled in the Zone Manager, can place phone calls from their radio. It is also possible for a phone caller to dial a certain number, enter a radio ID and the radio will be rang. Once a phone call is started, it stays on the same channel until it is terminated. This makes it very easy to monitor even on a conventional scanner. In theory, you will only hear the person on the phone. In reality however, it does not always work that way. The problem is with the phone on the far end, not the radio system. What actually happens is that audio transmitted by the radio, is routed via the telephone system to the phone on the far end, where it returns to the radio
system. Sometimes you can hear both sides of the conversation almost equally well and sometimes you can't hear the person on the radio at all.


For those of you that really deal with SmartZone systems, you likely found a few minor technical errors in this discussion. This was generally intentional for brevity or to keep it reasonably simple. The Motorola manuals fill a small bookshelf, and I'm trying to repeat them in that level of detail here for obvious reasons.

If anyone has specific questions they would like to ask, I will attempt to answer them, I spend a large portion of my work day dealing with a very large SmartZone system (and have for the last 6 years), so I can likely come up with answers to your questions. You can direct your questions either to the SoCalScan list or directly to me at

Jim Walls - K6CCC
AMSAT member 32537 / WSWSS member 395

Post Scripts
Fail-soft frequencies
Q: I've been monitoring the Orange County Public Safety TRS with the Pro96 in Control Channel Mode only and have noticed that the first frequency of sites 1 thru 5 never seem to have any traffic. I've heard CW IDs on the Countywide and North sites, but have yet to hear any voice or digital traffic on the following frequencies...
In a Motorola SmartZone system (what I babysit for a living, and what Orange County has), there are several levels of protection that will prevent the system from assigning traffic unless there are no other available channels. These are:
  • Base Station Identifier (BSI) channel
  • Alternate control channels
  • Protected channels

The least protected channel is the BSI. In a trunked system, the FCC requires that only the lowest RF frequency channel that is not a control channel transmit a station identifier. To prevent the IDer from transmitting on top of someone's conversation, the system restricts voice traffic on the BSI channel unless no other channels (except more protected channels) are available.

The next higher level of protection is the alternate control channels. A SmartZone system can have up to 4 control channels. The channels that are able to be used for control channels, but are not currently active as a control channel are protected from activity unless there are no other available voice channels (except more protected channels). The reason for this is that if the active control channel fails, the next control channel will IMMEDIATELY become the active control channel. If there is voice traffic on that channel, it will be ruthlessly pre-empted so that the channel will become the active control channel. This means that the the existing conversation will go away.

The most protected are protected channels. A protected channel is designated by the system operator (me in the case of the SCE system). A protected channel will never carry any traffic unless there are no other available channels. A system operator may set a channel as protected for whatever reason that he wants. The system neither knows nor cares what the reason is.

A channel that has a failsoft designation is NOT protected in any way at all. In fact the system has no way of knowing what channel is used for what traffic as that is programmed only in the subscriber radios.