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First read FAQ HTS and FAQ CSMA

Networking on Purpose -- by Tadd, KA2DEW

By the late 80s it was possible to configurate a packet network router with two or more radios. It was now possible to have sites to site communications which would happen without that communications impacting or being heard by another site. Having multiple channels upped the cost of the network routers, made some stations not see each other since they'd be on different channels, and made network routing difficult. However, having the ability to select frequencies gave us the opportunity to pick and plan networking strategies and to ease congestion.

This web page talks about different options we have when building out a network of packet stations and compares each. If you can think of a configuration I missed or can critique this page, please email me at my QRZ-listed address.

The TARPN packet network is made up primarily of low technology digital modems and off-the-shelf radio hardware. Packet radio with this kind of equipment has a bad reputation because packet networks with no design intent, or with poor design, have performed badly. Typical solutions using packet radio try to legislate the problem away by specifying who, or what-for, and when, certain parts of the network may be used.

I think with good design we can get much better results. For the purposes of this discussion, 80 characters per second between any two stations would be good results.

Here is a fiction (based mostly in fact) about how the systems we have seen come and go were implemented.

Five hams make a packet network -- mesh-on-channel

If you have five people sharing a single 1200 baud channel, each using a radio and a TNC, it looks like the group would have a maximum of 1200 bits per second (max of about 90 characters per second in packets) to share between the five people.
csma_5_stations002_no_hill
But, because the stations would naturally transmit at the same time if you let them, each of the five stations is set up to implement CSMA using SlotTime and Ppersistence calculations. Even with no collisions, an 80 character packet message and acknowledgement takes 5 seconds to exchange on this shared channel. So the group of five is now sharing 16 characters per second. If there was a magical supervisor guiding the stations to transmit at the appropriate times, the channel could achieve 90 characters per second, but there is no magic here. Instead we have Ppersistance and SlotTime.

Practically speaking it is worse than that, because as the number of users grows, not everybody can hear everybody. Take this example:
csma_5_stations002_one_frequency
The grey thick lines are radio paths between the stations. Everybody is on the same frequency.
Sergey can hear Bob and Bimaldo, but not Sigmund or Anna-Mae. That means that Sigmund might start transmitting while Sergey is already on the air.
Before reading further, check out this article: FAQ HTS

If Sergey sends a packet to Bimaldo, and Sigmund goes on the air at the same time, Bimaldo would miss both packets. Sergey will not retry until his FRACK delay runs down. Using TNC default numbers, that's a delay of about 4 seconds. If Sigmund is sending to Bimaldo, his packet also would have to be resent. Sigmund would be able to talk to Anna-Mae, and get acknowledgements despite Sergey's retries. However, when Bimaldo finally does answer Sergey, that could interfere with Anna-Mae and Sigmund's communications. These packet collisions result in retries which result in more transmission time just to keep up with the packets ready to go out. This temporarily decreases the channel available capacity. Also, in order for Bob to talk to Anna-Mae, his packets have to survive the gauntlet of collisions, and the delays of Ppersistence 3 times out, and 3 times back. Even if his packet was the only message in flight, it would take 18 seconds for him to get a response, best case. If the channel was loaded with all five stations being active, it would take much longer than that. Collisions in an HTS environment are really a bad problem.

Advantages:

Disadvantages: One feature which is worth noting is that Bob and Sergey can converse without interfering with Anna-Mae and Sigmund.

Mountain top relay -- mesh-on-channel

Historically, the next advancement for this little network would be to activate a relay station on Hill Mountain, enabling Bob to talk to Anna-Mae by way of the hilltop.
csma_5_stations002_digipeater

The mountaintop situation makes things better, but only if nobody is actually sending packets. If 3 stations become active generating packets, the mountaintop makes things much worse for individuals. It has some social results as well.
Since the mountaintop relay exists, operators no longer need to maximize the antenna at their home station to encompass as much of the network as possible. Instead they can focus on getting a good signal into and out of the mountain. This results in the creation of even more hidden transmitters. Before, when you only had stations operating from their houses, a collision would occur when the several other stations within range of one station were all on the air. Now a collision can occur with any two stations trying to send to the mountain that are not within range of each other.

The mountain-top station may also be located at a site which cannot fall under the same service regimine as the home stations. This is especially true if the access to the mountain is conditional on 3rd parties. Hams tend to fix their own stuff, especially during an emergency and regardless of worsening conditions. Commercial vendors tend to fix the high priority stuff but sometimes not during emergencies. It is not unheard of that the mountaintop site, if commercial or government, would be unusable at the very moment when it is most needed.

Impact of inconsistant channel capacity
Key factors in keeping collisions under control are the Ppersistence and SlotTime figures. The point of these figures is to delay transmission, making it less likely that two stations transmit at the same time. In a local network where everybody can hear everybody else the SlotTime figure is set to the amount of time that a station takes to move from listening to the channel to transmitting on the channel. The Ppersistence covers the likelihood that some other station will go on the air. Ppersistence must be calculated from the number of other stations that can collide at the receiver of the message, not the transmitter. Thus if a mountain top location can hear 20 stations currently involved in packet operations, then every station coming into the mountain needs to set Ppersistence to work out to a chance of one transmission in 20. SlotTime is supposed to be the slowest time of any participating station going from receive to transmit, i.e. switching off its receiver during PTT until its transmitter actually starts putting out packet noise.

In a situation where the participants are transmitting to a mountain-top relay, the stations will be heard by the mountain-top the entire duration of the transmission but not by most of the stations transmitting to the mountain. SlotTime needs to be the length of the entire transmission. That's 4 seconds or so. That could mean that on average, a station will wait for 20 x 4 seconds before transmitting. That's slow, but also the participants will never put up with that. Even if agreements were reached to make things work that way, human nature would have some of the participants cheating. The default values for Ppersistence in a KPC-3plus is 64, accounting for 5 stations on the LAN. The default value for SlotTime is 100mS or 1/10th of a second. For a mountain top situation the user station delay numbers are completely ineffective. This means that as soon as there are more than a few stations on-the-air, retries will start occurring, resulting in an even higher loading of the channel. The extra loading will build once it starts, result in all of the packet stations getting disconnected -- 0 characters per second. This is what we call a catastrophic channel meltdown.

Practically speaking this means that the packet channel will only be useful so long as only 2 stations are on the air within the range of the mountain top relay station, or if the stations sending data are doing it at much less than the channel capacity, leaving the channel 80% empty, from the perspective of the mountaintop receiver, i.e. as low as 8 character per second. In calculating the channel bandwidth it is not necessary to count the transmissions from the mountaintop relay since everybody can hear it, but that is only true if there is one and only one mountaintop relay on that frequency and in range of the mountaintops!
Note! One way to dramatically increase channel capacity is to strike the requirement for acknowledgements and retries. This makes collisions much less relavent. This is the mode APRS runs in.

to restate: If the TNCs were set up for optimal operation via the mountaintop relay, the total channel capacity (all users combined) would be about 4 characters per second through the mountain top relay. If the TNCs were set up to default values, the channel capacity would be about 10 characters per second but it would only work if there were only 2 stations plus the mountain. However, the likely hood is that as soon as the hungry packeteers see that the channel is actually working again, they will join in and the capacity now goes back to 0.

Advantages of using a mountaintop relay:

Disadvantages:

Digital repeater as packet relay

One solution for the hidden transmitter problem is to convert the Hill Mtn site into a repeater. The repeater would use a TNC modem for reception and for transmission but no error checking would be done. Very low latency is required to permit CSMA to operate.
csma_5_stations002_repeater
Advantage to a repeater vs a single-frequency relay: The total capacity of a repeater channel is about 20 characters per second with five users if the stations are setting Ppersistence correctly.

Disadvantages:

With the repeater in place, Anna-Mae and Bob can see each other's stations on the channel. If they have their Ppersistence and SlotTime set up appropriately (the defaults actually) then the latency on the link will be about 4 seconds and they will be able to send an 80 character packet through in about 4 seconds assuming they are alone on the repeater. If there are five different transmissions with acknowledgements in flight, they will have considerably less throughput, between 20 seconds per packet (4 chars per second) and much slower. Latency would go up to a variable rate between 4 seconds and maybe 30 seconds.

TARPN dedicated point‑to‑point links

The TARPN solution is to use Dedicated point‑to‑point Links. Every channel has only two stations on it in any give area. The TNCs are set up so Ppersistence and SlotTime are removed from the equation. The rate of packet transfer is one packet every 2 seconds giving us a throughput of from 40 to 90 characters per second depending on radio tx/rx/tx switchover times and depending on size of packet.
csma_5_stations002_dedicated_links
Now, while Hill Mountain may still be applied as a hilltop relay, we're using it only as a link between Bob and Anna-Mae. This doubles the throughput around the mountain but is not unique in that if it were to go away, the stations still have the other route.
The latency across any link is depending on packet backlog but generally works out to about 2 seconds per hop for a large packet and 1 second per hop for short packets. There aren't any collisions or load-cause catastrophic channel meltdowns. The cost of the system is about twice what it would cost for single radios at each site but the throughput is from 5 to 500 times as good, depending on loading. Each of the network sites is notionally expandable by adding more radios, though doing so at a commercial tower site can be a bit hard. Adding links at the individual ham-shack level is pretty easy.

Anna-Mae and Bob can connect through the network to each other. If they go through the hill mountain site they will have a latency of as low as 4 seconds for an 80 character packet and a throughput of better than 40 characters per second. If they went around the mountain through Sergey and Bimaldo the latency goes up to at least 6 seconds but the throughput is still better than 40 characters per second, assuming they are the only channel users. If the channel usership goes up (due to stations elsewhere in the network passing through the same nodes), the latency will increase and the throughput will decrease, but at no point does everybody get dumped. The throughput is deterministic in that there are no multiplicative costs for any particular number of users. Collisions do not occur. Life is good.

There is a timeout on the channel. If it takes more than L4TIMEOUT to transact any single packet across the network, then that connection will get disconnected. L4TIMEOUT is usually quite a bit higher than the typical latency. The disadvantage of a high L4TIMEOUT is that if the link is broken for some reason, it takes you a long time to find out.

Advantages for dedicated links over any other system:

Disadvantages:

Pre-arranging connections, and tying up radios, are the big objections for most poll respondents.

In this day and age, the biggest real limits to implementing a dedicated link network are:

TARPN project -- FAQ: Dedicated Point‑to‑Point Links summary

The TARPN project aims to make setting up networks of dedicated links practical. We want friendly, easy, cheap, reliable, and expandable. We're prepared to try it without commercial tower locations though that may not be reasonable in mountainous environs.

Dedicated links give us excellent performance and easy upgrade paths.

© Tadd Torborg, 2014↝2025 -- all rights reserved