TDMA, or Time-Division-Multiple-Access is a very effective way of allowing a lot of radios to share one radio channel.  Used extensively in GSM cellular and APCO public-safety systems, TDMA excels at allowing quick and reliable access to radio channels.  It allows 2-10 times more radios to share a radio channel than conventional carrier-sense methods.  This allows 2-10 times more tracking radios on one channel, as compared to radios that do not have TDMA capability.

The following diagram illustrates how it works.

When a RV-M7 GX wants to report its position and status, it waits until its assigned time-slot, and then transmits its data.  By default, TDMA time slot positions are assigned by unit-ID, so RV-M7 GX with ID 1 uses the first slot, and ID 2 uses the second slot, and so on.  This default slot assignment can be overriden by the SLOTNUM command or by using Raveon’s Radio Manager software, allowing each GX to have an ID that is different than the slot assignment.

A TDMA “Frame” time is the time it takes all units to transmit once.  This is configured with the TDMATIME xx command.  The factory default is 10 seconds, so every 10 seconds, each RV-M7 GX may transmit.  The TDMA frame must be set long enough for all units to transmit.  For example, if you have 50 RV-M7s, and use 200mS TDMA slots, then the TDMATIME should be set to 10 seconds.  The simplest way to set it the TDMATIME is to make it equal to the TXRATE, the rate you wish to report position

The duration of a TDMA time slot is programmed into the RV-M7 GX with the SLOTTIME command. If SLOTTIME is set to 200 milliseconds (factory default), then every 10 seconds, the RV-M7 will have a 200mS window to report its position in.

All TDMA frames are synchronized automatically in all RV-M7 GX Transponders to the top of the minute.  Slot 0, frame 0 is at the top of each minute. They use the internal GPS receiver to determine the current time, and calculate when their are supposed to transmit their position and status information.

A unit may be allocated additional time slots.  The SLOTQTY command sets the number of slots each unit receives.  It is normally set to 1.

After searching for and testing several UHF radios to upgrade a municipal water department’s wireless SCADA system, RAVEON’s Fireline modem was chosen as the best performance/value solution. The simple and versatile ability of the Fireline to be quickly configured easily beat out the other radios on the market. Thirty + radios have been installed and in service continuously operating with no problems for over two years.

Regards.

Chris Carda

President

Criterion Industrial

The standard M7-GX GPS tracking transponder has 2 separate reporting rates, the TXRATE and the IDLERATE.   In many deployments the TXRATE and IDLERATE are identical.  However, in some deployments the IDLERATE is set to transmit position less frequently than the TXRATE unless a specified event occurs.

Some of these events include:

Speed of travel greater than a user specified value (TRIGSPEED parameter)

Distance of travel greater than a user specified value (TRIGDX parameter)

Proximity to another GX transponder less than a user specified value (PROX parameter).

Here are three examples of using this flexibility in your system.

Example – Using TRIGSPEED

Picture a police squad car sitting idle or cruising slowly through a neighborhood on patrol.  In this instance it may not be important for the vehicle to transmit a position report more frequently than every 2 minutes.  If this is the case IDLERATE can be set for 120 (seconds).  However, the same squad car may later be move at higher speeds and reporting more frequently becomes important .  In our example the system administrator has set the fastest reporting frequency at 10 seconds (TXRATE 10).    When the car speed exceeds a certain threshold, let’s say 40kph (TRIGSPEED 40) the TXRATE is invoked and reports are now sent every 10 seconds.  When the car slows down, the IDLERATE again takes over, and transmissions are less frequent.

Example – Using TRIGDX

Your public works department has a number of vehicles out every day serving the community.  A vehicle and crew may stop for a while and work at a particular site.   While at the site they may even move the vehicle around a bit, to aid in the work.  While on the site a position report rate frequency of 5 minute intervals is perfectly adequate.  However, when they pick up and start moving to a new site, you’d like to be able to track their progress and know where they are more frequently, once they’ve moved 100 feet or so.  In this case your maximum system reporting frequency is twice a minute , or TXRATE 30 (seconds)   Set IDLERATE 300 (seconds), and TRIGDX 30 (meters).  30 meters is roughly 100 feet.

Example-Using PROX

You drive aheavy equipment in an open pit mine, or on a construction site.  The visibililty from your cab isn’t all that great, and you drive and operate your equipment as carefully as possible at all times.  In a move to further improve safety your company has placed a tracking display in your vehicle, allowing you to display the location of all of your other RavTrack transponders within radio range.  Your system administrator has set the following parameters in all of your transponders:

IDLRATE 10 (seconds)

TXRATE 2 (seconds)

PROX 15 (meters)

During your normal activities the display in your vehicle updates the location of all other transponders every 10 seconds.  Somewhere along the line, one of your coworkers, while on foot, comes close to your equipment.  Your co-worker  (Jim) is wearing on his belt the ATLAS PL personal locator.  When he gets within 50 feet of you (about 15 meters) your transponder, which was transmitting at 10 second intervals, starts  transmitting positions ever 2 seconds – and so does his ATLAS PL unit.  Now the blip on your display that represents your Jim starts pulsing every 2 seconds, perhaps a light has also lit on your dashboard, or a buzzer sounds in your cab, alerting you to Jim’s nearby presence.

We will cover the integration of warning lights and buzzers in another tech blog.

For the more technically minded, here is the logic flow chart the GPS transponder tests internally every TXRATE.

If you are running your GPS tracking system using RavTrack PC on a Windows XP computer, you may want remote users to be able to access the tracked objects database from a remote computer that is not on your local netwrok.  You may be able to do this using a Virtual Private Network (VPN) connection.

This article describes how to setup VPN access to a Windows XP computer.   VPN access will allows you to network two computers together, even if they are not on the same local network.  It will work across the Internet, while still maintaining security and privacy.

You will need Windows XP Professional with SP2 or SP3 installed.

How to Configure VPN host on a Windows XP computer

1. From the Start menu, open the Windows Control Panel.
2. Double-click the Network Connections item in Control Panel. A list of existing network connections will appear.
3. Choose the ‘Create a new connection‘ item from the left-hand side of the window. The Windows XP New Connection Wizard will appear on the screen.
4. First click Next to begin the new connection wizard.
5. Select Setup Advanced Connection‘ item from the list and click Next.
6. Choose the ‘Accept Incoming Connections‘ option and click Next.
7. On the ‘Devices for Incoming Connections’, click Next.
8. Choose an option on the “Allow Incoming Connections“and click Next.
9. On the ‘Users Permissions’page, select all of the users you would like to allow VPN access to this computer.  Important: Every user that you wish to give VPN access, must have a Windows Login on both computers with the SAME login name and password.
10. Click on the Next button.
11. On the Network Software page, select TCP/IP and click on Properties.
12. On the Incoming TCP/IP Properties page, select Allow Callers to Access my Local Area if you want remote users to also access the LAN.
13. Click on the option that determines how the remote computer will get its IP address for this VPN connection.   Make sure the IP scheme is the same as on your server.
14. Click OK.
15. If the VPN server is behind a router, Port Mapping will need to be done on the router. Standard port usage is port number 1723 for PPTP.  You might also have to configure your router for ‘PPTP Passthrough’.  Port usage for IPSec is 500, 50-51.  These ports will have to be forwarded to the VPN server’s IP.

Microsoft has excellent technical articles about setting up VPN networks <here> and <here>.

Once you have VPN access configured to your RavTrack PC, you will need to connect to it from the remote client computer.  The remote client computer will need to have a VPN connection created on it.  Follow the steps below to create an VPN connection on the client computer.

Configure a VPN connection from a client computer

To set up a VPN connection on the client computer, follow these steps:

1. On the computer that is running Windows XP, confirm that the connection to the Internet is correctly configured.
2. Click Start, and then click Control Panel.
3. In Control Panel, double-click on the Network Connections.
4. Click Create a new connection.
5. On the Network Connection Wizard, click Next.
6. Click Connect to the network at my workplace, and then click Next.
7. Click Virtual Private Network connection, and then click Next.
8. If you are prompted to, do one of the following:
   • If you use a dial-up connection to connect to the Internet, click Automatically dial this initial connection, and then click your dial-up Internet connection from the list.
   • If you use a full-time connection such as a cable modem, click Do not dial the initial connection.
9. Click Next.
10. Give the connection a descriptive name, and then click Next.
11. Enter the host name or the Internet Protocol (IP) address of the computer that you wish to connect to, then click Next.
12. Click Anyone’s use if you want the connection to be available to anyone who logs on to the computer, or click My use only to make it available only when you log on to the computer, and then click Next.
13. Click Finish.
14. If you are prompted to ‘connect’, click No.
15. In the Network Connections window, right-click the new connection you created.
16. Click Properties, and then configure more options for the connection:
    • If you are connecting to a Windows domain, click the Options tab, and click to select the Include Windows logon domain check box to specify whether to request Windows logon domain information before trying to connect.

To use the new connection, follow these steps:

1. Use one of the following two methods:
   • Click Start, point to Connect To, and then click the new connection.
   • If you added a network connection shortcut to the desktop, double-click the shortcut on the desktop.
2. If you are not currently connected to the Internet, Windows offers to connect to the Internet.
3. After your computer connects to the Internet, the VPN server prompts you for your user name and password. Type your user name and password, and then click Connect.
4. Your network resources should be available to you in just like they are when you connect directly to the network.
5. To disconnect from the VPN, right-click on the icon for the connection, and click Disconnect.