[EMAIL PROTECTED] wrote: > In addition to Neil's comment - you would screw up the APRS operation > if you do not listen and prevent your own APRS broadcast from going out > during another APRS transmission. You could key up on top of the > ongoing transmission and both your own and the other packets would be > QRMed. > > 73 - Jim W5ZIT
Not to continue to belabor how packet works on RB here too much, but packet being a network and not a point-to-point system has always suffered from the "hidden RF node" problem where : Station A & B can hear each other. Station B & C can hear each other. Station A & C transmit at the same time and collide at the receiver at station B. Multiply this "effect" by the number of nodes in a particular geographic area, and things get mighty interesting (and conjested) pretty quick. Someone did the math once to show the maximum density available in an area before contention completely clogged the network, but I have no idea where that whitepaper is today. No amount of "listening" will ever fix that on Packet. Add in that some packet stations use a standard COR type squelch circuit and others use a soft-squelch where the radio's squelch circuit is always open and the packet engine or TNC simply listens for tones, and the problem gets a little more complex. A system using soft-squelch won't even "hear" a voice transmission and will just key up over the top of one. Our club did a VHF packet digital-regenerative repeater many years ago with about 150 miles coverage that worked well to completely eradicate this problem -- any node that could access the repeater would be guaranteed to be heard by the others. It removed all the variables of antenna, power, etc. You either got your bits to the repeater or you didn't. We shut it down a few years ago due to lack of use and site costs. So while I wholeheartedly understand all the sentiments expressed by those who said that keying up for a quick voice call on a simplex packet frequency could be a) problematic due to unintentional interference or b) against the FCC rules for monitoring before transmitting... In practice it really just doesn't matter. Collisions happen. Fact of life on Packet. The nodes that need to get information to another location will continue retrying in a connected state, and in non-guaranteed delivery systems like APRS where the method is "fire and forget", you'll just miss a couple of packets of information -- a known operational lost packet issue of the network protocol itself, inherent in the system. Since the practice of monitoring on 144.39 with 100 Hz CTCSS tone was first recommended (as best as I can tell) by Bob Bruninga who created the APRS software as a specifically good configuration for a Kenwood D-700 (which he helped Kenwood develop and licensed code to), I don't think anyone's all that concerned about a brief voice QSO to QSY somewhere else on 144.39. In fact, there's been some discussion that 144.39 is the most widely monitored VHF frequency in the U.S. due to APRS stations all being on it, moreso than 146.52. How many of us leave a radio on 146.52 for travellers or other assistance 24/7? How many of us have APRS stations on 144.39 24 hours a day? (Actually I have neither right now, but that's how the argument goes...) By the way, to keep this on-topic, I tossed a note to Bob about that mystery packet. He said it really didn't look like it was in any APRS format he recognized, and wasn't sure what it actually was. I got sucked into this whole thing just from remembering my former experiences with TCP/IP over Packet radio in the early 1990's. Back then there was no APRS, and Packet BBS's were on every channel from 145.01 through 145.09 and people even had to go down into the 144's, annoying the satellite crowd. Some of us fired up TCP/IP over Packet using the venerable old 44.x.x.x network around here on 145.07 and the BBS owner on .07 and the local "packet association" were always up in arms because our systems used much bigger packets than the standards used for BBS access, and "hogged" the channel. Many meetings, and fire and brimstone conversations both on the air and off, and a few years later -- packet's popularity had dropped off so dramatically no one cared anymore. APRS then hit the scene and worked so well, after everyone decided on 144.39, that everyone but the die-hards were doing APRS, and the conversations and complaints and everything else (and most of the utility of the large network that had been grown in our area) started to dwindle. Today around here, there isn't much other than some dedicated folks running VHF to HF gateways and an FBB BBS or two... it's fairly dead. Nothing like it was in 1991-1993. The Emergency Services crowd always wanted us to keep our high-level packet repeater on-air, stating that they would use it in emergencies, but in practice, they might use it for 5 minutes to establish comm between two packet stations, then QSY to simplex, find out that they had a path, and they were gone... they didn't want the repeater pushing their "sensitive" emergency services comms out over 150 mile radius, and there were "privacy" issues with doing that. So basically the repeater was un-used, even by those asking to keep it. We gave a brief thought to replacing the repeater with a standard digipeater, but the site costs were the same either way, based on number of antennas on the tower and square footage in the building used, along with power (which was so negligible it didn't matter). Paying to keep it up there for no one to use, when there was already a working network of 50-100 APRS nodes that most of which would happily act as digipeaters for passing message traffic already scattered everywhere nearby on 144.39, made keeping it a lost cause. The repeater (like everything else the club used to do on packet) went the way of the dodo bird. Nate WY0X
