Using G8BPQ software, not uncommon, at 1200 baud AX.25 with NETROM over top of it, the typical 1200 baud throughput is 15 to 25 bytes of payload per second, plus callsigns and flags and whatnot, and that is assuming that PPERSIST is set to some high number, like 200.?

The interesting thing, to me, is what happens when four stations are on the same frequency, attempting two different 1 on 1 conversations, each pair thinking they are alone on the channel. ?

KR3RAL <—> KW1BRB <—> K1DT <—> KE1AST?

KW1BRB in the west suburbs is talking to?KR3RAL, a few miles further out. ? ?

Then later K1DT, in downtown, is talking to KE1AST in the East suburbs.

And then KW1BRB and?KR3RAL?resume. ?But where?K1DT?can hear KW1BRB and KE1AST, the only station KE1AST can hear is?K1DT. ? So?KE1AST?is pinging away trying to get?K1DT?to answer, and?K1DT?is waiting for?KW1BRB to become silent before?K1DT?can transmit. ?Once in a while K1DT sees the channel quiet enough and goes and sends a frame to KE1AST, trashing the frame?KR3RAL?was sending. ? That’s what happens using AX.25. ? What does VARA FM do in this scenario?

The 1200 baud packet exercise, with this scenario, would be massacred pretty badly by bit-errors, plus when a station goes to transmit, it would usually end up waiting until the other stations it can hear are all un-keyed. ?It’s very dynamic. ?If 1200 baud AX.25 is set up with PPERSIST set to a low value on all stations, they’d be really slow in the first place, but the collisions may not be as horrific. ? It’s fun to watch, if one doesn’t take it too personally. ??

The much worse case is when there are a couple of conversations and the stations are using on-channel relays, perhaps more than one relay. ?That’s very common in AX.25 1200 packet. ? The throughput goes from intolerably slow to dropped call.?

I’m a big fan of self-coordinated dedicated point to point link frequencies for VHF or UHF FM data links, where the frequency WIKI has enough channels for people to not be on the same channel as another link within range. ? But this is only useful (in an automated real-time network) if there are more than a couple of radios+modems at each site to cross link the channels. ?See the maps on the ncpacket.net webpage. ?

VARA FM sounds great and I’d love to have something like it that we can build a real-time network out of. ?I wonder how long it will be before one could get a licensed copy in a device that costs under $200 or so, in order that one could build a bunch of multi-port network stations. ?This would put AX.25 to bed finally, maybe? ? ?The $40 NinoTNC on a FM voice radio is good for 2400bits per second, around 40 bytes per second on G8BPQ with PPERSIST=200 and with NETROM taking up time. ?That’s pretty slow compared to what Mark was talking about. ?But a half dozen NinoTNCs can be attached to a G8BPQ network node with half a dozen radios, if one was so motivated. ?In our local network, ncpacket, we have a large percentage of 4800 and 9600 baud capable radios, when matched with a $40 NinoTNC. ?So we’re getting 60 bytes per second to 110 bytes per second on many links. ?But the network stack does not make bit-rate or processing adjustments to compensate for fading links. ?So it gets pretty bad when we have multi-path phasing. ?

We need a network protocol that is more efficient than NETROM, more forgiving of fading links than NETROM, and compatible with VARA FM. ?

I’m partial to the Raspberry PI as a computing platform, but we’ll see how the market for VARA FM compatible embedded solutions develops. ?Using a set of Intel NUCs doesn’t seem completely ridiculous, and getting less and less ridiculous as time goes on? Maybe there is already a better deal than Intel NUC for a single port VARA FM instance of a multi-port node. ??

Tadd KA2DEW in NC. ? ? ?

?

On Jul 27, 2023, at 4:18 PM, Mark Davis <markad7ef@...> wrote:

Orb;

The simple answer is that VARA FM doesn’t do any collision avoidance.

However, when there is an active VARA FM session between two station, any additional stations attempting to connect on the same channel will not see a response from either of the connected stations. Data communications between the connected stations will probably be impaired until the interfering station exhausts its connection retries, but the session normally resumes at that time.

Also, VARA FM supports the busy-channel detection that is implemented in the Winlink Express VARA FM session… if there’s traffic on the channel, the operator trying to connect must click the “Connect Anyway” button.

Keep in mind that the actual best-case?throughput for a single 1200bps connection is on the order of 500bps… under optimal signal conditions. Typical throughput on mildly impaired radio circuits is on the order of 300bps. The per-user throughput is a fraction of that best-case value when multiple users share a channel. This difference between the 1200bps raw data rate and 300-500bps throughput is primarily due to the overhead imposed by the AX.25 protocol.

In contrast, unlicensed VARA FM NARROW throughput is 5Kbps over moderately impaired radio channels where 1200bps packet connections are not even practical. Licensed VARA FM NARROW throughput is at least twice the unlicensed value under similar conditions. Double those numbers again for licensed and unlicensed VARA FM WIDE operation.

Realistically, minimum VARA FM NARROW throughput, for unlicensed use, is 10-15 times higher than single connection 1200bps packet… and 40-60 times higher for licensed VARA FM WIDE operation.

I believe that, compared to 1200bps packet operation, the throughput advantage of VARA FM (WIDE or NARROW), even for unlicensed use, more than makes up the lack of a collision detection.

Mark - AD7EF

On Jul 27, 2023, at 4:18 PM, Mark Davis <markad7ef@...> wrote:

Orb;

The simple answer is that VARA FM doesn’t do any collision avoidance.

However, when there is an active VARA FM session between two station, any additional stations attempting to connect on the same channel will not see a response from either of the connected stations. Data communications between the connected stations will probably be impaired until the interfering station exhausts its connection retries, but the session normally resumes at that time.

Also, VARA FM supports the busy-channel detection that is implemented in the Winlink Express VARA FM session… if there’s traffic on the channel, the operator trying to connect must click the “Connect Anyway” button.

Keep in mind that the actual best-case?throughput for a single 1200bps connection is on the order of 500bps… under optimal signal conditions. Typical throughput on mildly impaired radio circuits is on the order of 300bps. The per-user throughput is a fraction of that best-case value when multiple users share a channel. This difference between the 1200bps raw data rate and 300-500bps throughput is primarily due to the overhead imposed by the AX.25 protocol.

In contrast, unlicensed VARA FM NARROW throughput is 5Kbps over moderately impaired radio channels where 1200bps packet connections are not even practical. Licensed VARA FM NARROW throughput is at least twice the unlicensed value under similar conditions. Double those numbers again for licensed and unlicensed VARA FM WIDE operation.

Realistically, minimum VARA FM NARROW throughput, for unlicensed use, is 10-15 times higher than single connection 1200bps packet… and 40-60 times higher for licensed VARA FM WIDE operation.

I believe that, compared to 1200bps packet operation, the throughput advantage of VARA FM (WIDE or NARROW), even for unlicensed use, more than makes up the lack of a collision detection.

Mark - AD7EF

Orb;

The simple answer is that VARA FM doesn’t do any collision avoidance.

However, when there is an active VARA FM session between two station, any additional stations attempting to connect on the same channel will not see a response from either of the connected stations. Data communications between the connected stations will probably be impaired until the interfering station exhausts its connection retries, but the session normally resumes at that time.

Also, VARA FM supports the busy-channel detection that is implemented in the Winlink Express VARA FM session… if there’s traffic on the channel, the operator trying to connect must click the “Connect Anyway” button.

Keep in mind that the actual best-case?throughput for a single 1200bps connection is on the order of 500bps… under optimal signal conditions. Typical throughput on mildly impaired radio circuits is on the order of 300bps. The per-user throughput is a fraction of that best-case value when multiple users share a channel. This difference between the 1200bps raw data rate and 300-500bps throughput is primarily due to the overhead imposed by the AX.25 protocol.

In contrast, unlicensed VARA FM NARROW throughput is 5Kbps over moderately impaired radio channels where 1200bps packet connections are not even practical. Licensed VARA FM NARROW throughput is at least twice the unlicensed value under similar conditions. Double those numbers again for licensed and unlicensed VARA FM WIDE operation.

Realistically, minimum VARA FM NARROW throughput, for unlicensed use, is 10-15 times higher than single connection 1200bps packet… and 40-60 times higher for licensed VARA FM WIDE operation.

I believe that, compared to 1200bps packet operation, the throughput advantage of VARA FM (WIDE or NARROW), even for unlicensed use, more than makes up the lack of a collision detection.

Mark - AD7EF