[CQ-Contest] NIL's in FT8, and the Two Generals Problem

Tim Shoppa tshoppa at gmail.com
Sun May 17 09:12:50 EDT 2020

In his CTU talk, Ed W0YK introduced me to the "Two Generals Problem" - a
concept that was not discussed formally in communications theory until the
1970's - and its relation to contesting NIL's.

Of course contesters have been trying to reduce their NIL rate since before
the 1970's. And in CW, Phone, and RTTY I think our context in our
contesting exchanges was (from the school of hard knocks) designed to
reduce NIL's from the get-go. We can use our brains and ears, as well as a
lot of inferences about the clarity of a communication channel, to get low
NIL rates in those modes.

But the advent of FT8/FT4 - and exchanges not designed to minimize NIL's in
QRM and fading - has resulted in very high NIL rate both in FT4/FT8 DX'ing
and FT4/FT8 contesting. I think contesters could help the WSJT guys a lot
in coming up with better exchanges.

You can read in great detail about the "Two Generals Problem" in
communications theory here here:
https://en.wikipedia.org/wiki/Two_Generals%27_Problem . But broadly it's
all about the impossibility of proving that a communication is complete no
matter how many acknowledgements have gone back and forth.

Quoting Wikipedia:

"The first general may start by sending a message "Attack at 0900 on August
4." However, once dispatched, the first general has no idea whether or not
the messenger got through. This uncertainty may lead the first general to
hesitate to attack due to the risk of being the sole attacker.

To be sure, the second general may send a confirmation back to the first:
"I received your message and will attack at 0900 on August 4." However, the
messenger carrying the confirmation could face capture and the second
general may hesitate, knowing that the first might hold back without the

Further confirmations may seem like a solution—let the first general send a
second confirmation: "I received your confirmation of the planned attack at
0900 on August 4." However, this new messenger from the first general is
liable to be captured, too. Thus it quickly becomes evident that no matter
how many rounds of confirmation are made, there is no way to guarantee the
second requirement that each general be sure the other has agreed to the
attack plan. Both generals will always be left wondering whether their last
messenger got through."

Tim N3QE

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