Store and Forward with Radios

[Scada Tom 0]

How does the AB Store and Forward work when used with a radio network? Where is the recived data held? In the PLC or radio?

[Ken Roach 194]

Store-and-forward is done by the controller. Say that Station 5 gets an incoming DF1 frame and finds that it is addressed from Station 0 to Station 12. Instead of ignoring the packet like it would have in DF1 Half Duplex, it listens to the packet and stores it internally, making note of the target address, the Transaction Sequence Word value, and the CRC. If the CRC is correct, Station 5 will check its Store-and-Forward table to see if the frame's destination address (Station 12) is in that table. Station 5 then waits a few milliseconds (configurable) and re-transmits the DF1 frame to Station 12, preserving the original source Station # and TNSW and CRC value. Then Station 5 kicks back and listens to the network. If Station 5 hears a reply from Station 12 to Station 0, it will do the same function and re-transmit that frame. If Station 5 hears the same command that it just forwarded (remember, it stored the Transaction Sequence Number so it can tell if it's an old or new command) it will refrain from forwarding it again. That's how multiple repeaters can be used in series. In my experience the hardest thing about DF1 Radiomodem protocol is situations where a remote station can sometimes hear the base station transmitting but most of the time only hears the repeater transmitting. This leads to the remote station transmitting its reply to the base station right about the same time the repeater comes on the air sending the repeated request. Of course, these kind of propagation problems only happen on the coldest, clearest nights.... I've seen a lot of constellations from outside sewage pump stations in the middle of January. There's an elegance to it.

[Scada Tom 0]

I know what you mean about 2 am callouts. I have been on many traffic signal calls in the wee hours, where its was stuck took 5 minutes to cycle, and is really about 90 seconds. OK, your explaination is kind of what I thought. Controller 5 resends the information to the radio, but the radio won't send it unless there is no carrier detect. If its clear off goes the signal, then it comes back, into the radio and back to the controller 0. All of this adds perhaps 50% latency. I have a site that does one store and forward, in a simplex radio system, with 5 total sites. Because the master location is being moved, I have two sites that I need hit via store and forward. This simplex radio system is time based polling, each site gets polled every 30 seconds, about 5 seconds apart. My concern is with three being "stored and forwarded" from one site, is the radio traffic will get congested and slow due to all the information being sent and received, and may not get out due to the time based polling. Options: 1. Use another site for store and forwarding - spread it out over multiple sites 2. Go to sequential polling instead of time based.

[Scada Tom 0]

I know what you mean about 2 am callouts. I have been on many traffic signal calls in the wee hours, where its was stuck took 5 minutes to cycle, and is really about 90 seconds. OK, your explaination is kind of what I thought. Controller 5 resends the information to the radio, but the radio won't send it unless there is no carrier detect. If its clear off goes the signal, then it comes back, into the radio and back to the controller 0. All of this adds perhaps 50% latency. I have a site that does one store and forward, in a simplex radio system, with 5 total sites. Because the master location is being moved, I have two sites that I need hit via store and forward. This simplex radio system is time based polling, each site gets polled every 30 seconds, about 5 seconds apart. My concern is with three being "stored and forwarded" from one site, is the radio traffic will get congested and slow due to all the information being sent and received, and may not get out due to the time based polling. Options: 1. Use another site for store and forwarding - spread it out over multiple sites 2. Go to sequential polling instead of time based.

[Bill Linne 0]

And then there is the option of using radios that do the work for you. In the system we presently use, the radio knows the path to the master, and the master knows the path to each field site. So, if station five sends a message to the master, but station five needs to be "repeated" by station three to get to the master, all that configuration is in the radio at station five. Nice part of that is, if the PLC at station three heads south, the communication link continues to work. You can put a radio out in the field without a PLC attached, and repeat radio transmissions with it forever. I know this is slightly off the specific question you asked, but thought it worth mentioning. Bill