Point-Multipoint communication under radio systems

  • Thread starter Carlos Araya Mackenney
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Thread Starter

Carlos Araya Mackenney

Hi everybody:
Somebody know a way to make a communication by exception under a radio system point-multipoint, using one frequency? This media allows polling protocols, but some people said there is a
software like TCP/IP to solve the collisions and allow a net system with radios.

Carlos Araya Mackenney [[email protected]]

Darold Woodward

I would suggest you contact a vendor of point-to-point radio systems directly to determine if their systems have carrier detection capabilities.

Protocols designed for SCADA operation like DNP 3.00 (see www.dnp.org for more info) have a collision avoidance mechanism that monitors a carrier detect line. There is also a form of collision resolution possible by using Application or Data link confirmations. (This is similar to Ethernet's CSMA/CD but does not
include Collision Detection)

You must pick a physical channel (multi-point radios, etc) that have carrier detection and feed that information back to the device doing the communicating.

So, you must answer a few important questions:
- What physical channel will you use?
- Do your RTUs, etc. connect to the radios in a way that allows carrier detection?
- Does your SCADA protocol have collision avoidance, carrier detection, or collision detection support?

Darold Woodward PE
SEL Inc.
[email protected]

Nicholas Lock


This reply is from one vendor's perspective, although it applies broadly in radio telemetry.

The stock-standard technique in radio telemetry is one variant or another of "CSMA/CD" - carrier-sense-multiple-access / collision-detect"

All RTUs "listen" on a common channel, and contend for that channel when they need to talk to another. (That's the MA - multiple access part.)

If another RTU is transmitting, my receiver will detect presence of its carrier, and inhibit my transmission until the channel becomes clear, plus a defined quiet-time period. (That's the CS - carrier sense) part.

The CD - collision detect - works in that if I didn't receive an ACK within a defined timeout, then I assume there was a message collision, and (after a defined, pseudo-randomised timeout), retransmit. (Number of re-tries also defined)

The other station will probably also have failed its message because of the collision, and will also retry. If we can both 'hear' each other, then the CS portion comes into effect, and the second RTU will not transmit until my carrier has cleared. If they can't hear each other, and there is again a collision, then both RTUs will again retry. The setting and optimising of the timeouts and pseudo-random delay times are part of the system design process.

It doesn't require specific protocols such as TCP/IP to these communications, as this traffic management is more to do with comms port-firmware functionality than protocol definitions. Apart from our own native KF2 protocol, we have implemented these concepts in radio networks
using Modbus, DNP-3, and probably others.

In two frequency systems, configured in a "star" network, the hub can hear all outstations, and all outstations can hear the hub. But, outstations cannot hear each other, because they are listening to the hub tx frequency. So, the Carrier-Sense (CS) will not work between outstations. Several techniques for improving on this include
- make the 'hub' a talk-thru repeater
- Have the hub transmit a carrier when it is receiving from any RTU, to inhibit others from transmitting
- Make it a single tx/rx frequency

The latter is common (at least here in Australia) in such applications as small town water & sewerage telemetry. It works especially well if all RTUs are sufficiently close together to actually hear each other.

Action Controls Pty Ltd

Nicholas Lock

Manager, Systems Engineering
[email protected]

Tel: (03) 9535 6200
Fax: (03) 9562 8470
Web: www.action-controls.com.au [The homepage for Kingfisher telemetry!]

Bill Gausman

There is a protocol called X.25 that is used in this way. http://www.protocols.com/pbook/x25.htm
Amateur radio operators have made good use of this in a mode called "packet radio", or AX.25. Packet modems using this protocol are available very inexpensively through the Amateur Radio
market. One manufacturer that makes commercial packet modems is Kantronics, www.kantronics.com.

Amateur radio operators have also implemented TCP/IP via radio connections, but I am not familiar with the requirements.

Bill Gausman
System One Control, St. Paul, MN
Another source of technical data about digital radio communications is:

Tucson Amateur Packet Radio
8987-309 E. Tanque Verde Rd., #337
Tucson, AZ 85749-9399

These are the hams (amateur radio operators) who developed many of the experimental setups to show how digital wireless communications could work for systems that are few feet apart to literally around the world. Most of the early work dates from the early 1980's. The packet radio system described does use collision detection to
retransmit data. They have CD-ROMs available and some downloadable files from their Web site with much more information.

One interesting newer application is the APRS, Automatic Packet Reporting System which can interface a GPS receiver with a packet radio system "...that allows you to automatically track moving objects (such as vehicles, people, and weather) in real time using maps displayed on
your computer monitor." Sure seems to be something that a number of commercial interests would want.

Russ Kinner
AVCA Corporation
Maumee, OH

Dave Gunderson

Hi Carlos,

Nick gave an excellent explanation of 'CSMA/CD' systems. If you search the archives - you should get a good idea of what it takes for a successful telemetered system. I suggest that you check out the postings on 'RF Radio Modems' or 'RF Data' . We've had several excellent postings within the last year that may help answer your question.

To augment Nick's good information, I can only add the following -

You HAVE to look closely at the Data Flow requirements of the fielded systems. Can they work as a simple 3 wire interface [Txd, Rxd & Gnd] that requires NO Flow Control Lines -or- an interface that requires Hardware Flow Control???

Some systems require some sort of Flow Control to 'wake up' a serial port. And sometimes [depends on the system] you cannot tie the Flow Control Signals 'Active' [as these may need to be
toggled - to satisfy some hardware or firmware requirements].

The Carrier Detect Signal from the receiving Radio is often used as the 'wake up' Signal to the Serial Port on some systems. On a simple 1200 baud telemetry system, Carrier Detect is developed by a Bell 202 RF MODEM when the Data is received from the Radio.

Your selection of telemetry radio is important. If you select one that passes Data only [the 3 wire interface] - passing control signals for
your system - will be part of the Data transmitted [as protocol overhead] - the programmer's responsibility.

By the way, Carlos, the 'net system' that you describe using TCP/IP will work on a 'CSMA/CD' system provided that the RF Radio/MODEMS have the ability to pass Flow Control Signals as part of the 'internal overhead'. I have fielded one such system using a spread spectrum radio net.

Currently on the road somewhere 'on holiday',
Dave Gunderson Hoover Dam Control Section
Web Page http://www.river.f2s.com

Adolfo Jimmy Saldivias Valarezo

There is a product manufactured by national Intruments called Lookout who can support multiple protocols through the same port.
You may want to check it at:
Hope it helps

Jimmy Saldivias
Phone: 591-4-523438
Fax: 591-4-523413
website: http://tecsim.trading.net