Different types of data require a different type of network implementation. The discussion in this article is based upon GPS data, but there are many other types of data that are sent over radio and cellular systems. Each type of data is handled best by one particular type of network architecture which may or may not handle a different type of data efficiently. This discussion about data centers around GPS data which is the most popular type of data being sent over radio networks, but not the only type of data being sent
In the beginning, all radio transmissions were either voice communications or Morse code which was used in lieu of voice due to the simplicity of transmitting the code. As time went on and technology improved, there became a need and desire to transmit data. Many data transmission schemes have been developed for two-way radio over the years. Some are still in use and many have fallen by the wayside.
When transmitting data over two-way radio, there are a few methods that are still in common use. These are CTCSS, DCSS, Fleetsync and MDC-1200. Although CTCSS and DCSS are almost always used by almost every fleet of radios to allow a fleet of radios to identify all radios from that fleet and prevent the user from being forced to listen to radio transmission from another fleet, there are a few other signaling applications that are occasionally deployed with these signal schemes. Fleetsync and MCD-1200 are signaling schemes that are used for caller ID, selective call and sending “canned” messages from one radio to another. LTR signaling is still commonly used for LTR Trunking and DTMF is commonly used in analog two way radio for telephone dialing and some signaling. Tone remote signaling is still common, but slowly being replaced by IP consoles. POCSAG signaling is still common with pagers as well as Golay signaling, but Golay is winding down and being replaced with POCSAG. Older signaling systems such as burst tone, 2 tone sequential, Reach, Quick Call I, Quick Call II, 1500/500Hz signaling, 2805Hz signaling, IMTS, AMTS, 5/6 Tone and Motorola MODAT are mostly history. See our article on “Signaling Methods for Radio Systems” for a full explanation of these signaling schemes which can be used for data transmission.
The explosive need for GPS data requires some method of data transport. These days, there are a few methods of transporting the data, each with their own advantages and disadvantages. The first method is for the vehicle operator to tell someone verbally which is a very unreliable, slow and cumbersome method of data transport which is not recommended. The second method is to take the data and send it over a cellular system to a central location which is displayed on a website on the internet. Another method is to use a conventional radio system to transmit the data. A fourth method is to send the data over an analog trunked radio system. The last method is to send the data over a digital trunked radio network with multiple sites, similar to using a cellular radio system. Each method of sending the data has its advantages and disadvantages.
DATA NETWORK DESIGN
Data networks have several designs which optimize the efficiency of sending data. Networks that send both voice and data over the same channel have inefficiencies that need to be resolved. Networks that send voice over one channel and data over another channel can optimize the voice channels for voice operation while the data channels are optimized for data transmission. These types of networks operate more efficiently because only one type of transmission is present and the throughput can be optimized for the one type of transmission that occurs through the network.
The first type of network uses random transmissions. This is a common method of sending data, but it causes data collisions whenever the network starts to have a lot of transmissions. If the units are programmed to wait for an acknowledgement (ACK) response, typically the unit will resend the data if the ACK is not received. This causes additional traffic on the channel, thus making it busy. Unfortunately, at some point, there is a cascade action where the data collisions cause even more data collisions at which point you get runaway data collisions, kind of like the process of runaway meltdown of a nuclear reactor except there is no radiation or physical damage to anything. The data collisions keep the data from reaching destination, so you do not get the data throughput. Another type of network uses timed transmissions in which each GPS transmitter is programmed to send its data in a specific time slot. This type of network requires a lot of record keeping and manual programming to keep everything operating in a “timely” fashion. Upgraded networks have the ability to remotely program the GPS modem to transmit at a specific time. The last type of network uses polling to tell the GPS modem to report its position which can be controlled automatically by the network if the software is up properly.
Cellular data is a good method of transporting the data as the cost is reasonable and the reliability is good. When using cellular data, the GPS uses a cellular data modem which transmits the data over the cellular network. Some (but not all) cellular carriers route the data through a separate network just for data that is optimized for data transmission, while other carriers share the voice network for data, but send the data on the channel in-between voice calls. Cellular carriers route the data through their network backbone and collect it at a central site. The data is then made available through a website which is set up to display the GPS data on a map. There are often several map options which allow you to zoom in and out, display each vehicle with an icon that can be chosen to display the type of vehicle (such as a trash truck, taxi, tow truck, police car or other type of vehicle) , set up geo-fences (which alert you when a vehicle enters or leaves a particular area), get statistics about the timing, route taken, vehicle speed and direction.
The disadvantage of using cellular data is that there is a separate piece of equipment in the vehicle and a separate fee each month for operating the GPS tracking of the vehicle. It is often more cost effective to have your GPS and voice communications using the same network. Another disadvantage of cellular data is that you have not control over the coverage offered by the cellular carrier. If there are dead zones, there is little or nothing that can be done to fill in those areas. Data that needs to work in underground areas usually does not work as well as many rural areas have no cellular service. If the carrier has voice service in an area, there is no guarantee that there will be data service.
ANALOG CONVENTIONAL RADIO
Whenever a single radio site can provide all the radio coverage required, things are much simpler and inexpensive because all that is needed is the one tower site which keeps the tower site rent under control and eliminates the need to switch tower sites which can be a problem for the network. Unfortunately, most data networks need coverage from more than one tower site. Our discussion will start with a single site network.
The GPS data can operate with any of the methods described above under the section Data Network Design. Systems that operate with the timed transmissions or polling have greater throughput and reliability. Data controllers can be designed to operate remotely assign time slots to the GPS modems so that they report at the correct time. If everything is operating smoothly, approximately 3 GPS positions per second can be reported or 180 GPS locations per minute. The network can send an ACK to acknowledge the receipt of the GPS message if necessary to let the GPS unit know that the data message has been received.
When coverage is insufficient from one tower site, a method of switching sites is needed. Some networks rely on the user to switch tower sites manually to keep the GPS signal operating properly, however, some networks use the lack of an ACK to trigger the GPS unit to switch to another tower site. This method does not know which site will work best and relies on multiple failures to cause the unit to switch to the next site. The selected site may not be optimal, so it may take several attempts to find a good site for the location of the unit. Another method of selecting the site is through geo-fencing in the GPS modem so it automatically switches sites in different areas. This method requires separate data links to the main office (unless utilizing a IP site connect system) which require multiple inputs for data collection so that the GPS can be displayed on the associated map program. (The IP site connect typically has a common data system which can be connected through a gateway to the input of the data system so that you only need one input to the data system.) Typically, this type of setup is purchased by the GPS user.
Conventional radio systems work more reliably when they are on exclusive channels. Exclusive channels can be legally obtained in most rural areas due to lack of congestion, but in the urban areas, they can be difficult to impossible to obtain. When operating on shared channels, the data system has to be set up to prevent or minimize interference to the co-channel user which can significantly compromise the performance of the data system.
Conventional radio networks have the advantage of cost when you have a large number of units that need data and they have the advantage of being able to control the coverage by adding tower sites to the network. However, depending upon the cost of site rent, maintenance, subscriber units, coverage needs, spectrum costs, radio interference (on shared channels), installation and equipment costs, it may be more expensive or unreliable to operate a conventional data network.
For more information about conventional radio systems, please see our article entitled “Conventional Radio”.
DIGITAL CONVENTIONAL RADIOS
Digital conventional radios offer all the same challenges of analog conventional radios, plus the challenge of being a digital radio. Digital radios do not transmit data well through the microphone input that is used in many analog data systems. All voice is digitized in a digital radio using the AMBE+2 vocoder which is tailored to the human voice and does not transmit other types of sounds well. Therefore, the data system that has a built in modem is typically not going to work with a digital radio.
Digital radios work by shifting frequency according to the data stream of 1s and 0s that comes from the GPS unit or other data encoder. The digital radio normally has a data input that will transmit the 1s and 0s properly, but it must store the data stream to be sent upon certain conditions that allow for the transmission of the data. This requires a tighter integration between the GPS source and the radio that transmits the data.
For more information on Kenwood NEXEDGE digital radios, please see our article entitled “The NEXEDGE Digital Advantage”.
The use of an analog trunking system has similar issues to a conventional system, but with an additional problem associated with trunking. The problem with trunking is that when the transmit signal is sent to the radio to send the GPS signal, it takes potentially random amount of time for the transmitter to send the data. This requires the GPS unit to be able to hold the information until the GPS is sent which may happen within 0.3 seconds and may take up to several seconds. If the GPS data is sent prior to the transmitter accessing the LTR system, then the transmitted data goes into the bit bucket and is lost. A handshake between the GPS modem and the LTR radio needs to be instituted to prevent this problem from happening. Most other issues associated with conventional radios apply to the LTR trunked radios.
LTR trunked radio systems, like any other radio system work more reliably when they are on exclusive channels. Exclusive channels can be legally obtained in most rural areas due to lack of congestion, but in the urban areas, they can be difficult to impossible to obtain. Some LTR systems operate exclusively on clear channels, but some trunking systems operate hybrid systems where certain channels in the LTR system are exclusive and other channels in the LTR system are shared. This can cause some messages not to be received which necessitate the message to be skipped or necessitating it to be resent.
For more information about LTR Trunking, see our article on “LTR Trunking”.
DIGITAL RADIO NETWORKS
Digital networks have significant advantages over trunking or conventional radio. These networks have the ability to control the GPS data modem so that it operates efficiently, especially when used solely for GPS data. When the radio is shared between voice and GPS, sometimes the radio may miss a voice call depending upon the setup and the network design. If the network is designed properly, the voice and data will operate independently with very little interaction between the two functions which allows it to operate efficiently as both a voice and a data radio at the same time.
With digital radios, there is a choice between two methods of getting your GPS units displayed on the map. The first method is having your own mapping software and base stations to collect the mapping data. A base station is required for each site on the network where a GPS unit will be operating, so several base stations are necessary. The alternative method is similar to cellular where the network completely controls the GPS radio and the GPS data is sent via the IP infrastructure through a gateway to a website where your GPS data is displayed. This is the more popular method of obtaining GPS through the network as you get the benefit of all of the sites in the network being connected to the IP infrastructure of the network so all the data collection base stations are eliminated.
For more information about our Diga-Talk Network, please see our Diga-Talk Network page on the website
There are many ways to accomplish the task. Each method has its advantages and disadvantages so the method of accomplishing the task needs to be selected after the task is properly identified. The MRA Diga-Talk Network is optimized to send GPS data and display the data on our MRA Webmail website so that you can get efficient display of your GPS information.