From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: rec.radio.amateur.policy
Subject: Re: HF Packet and a question...
Date: 30 Jul 92 21:00:16 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <1992Jul29.181844.19026@news.mentorg.com> rstory@simhp720.mentorg.com (Ron Story) writes:
>What is the advantage to using HF packet as opposed to VHF packet?
>
>	I realize that HF packet will cover more ground per hop, but
>the VHF stuff is persistant and a whopping 1200 Baud. ;^)  The extent
>of VHF packet as far as coverage seems pretty good and the response 
>time seems "reasonable" for what it doing.  Is HF packet this used 
>outside North America frequently?  Or have I missed the boat....

You've missed the boat. Depending on terrain and individual station
antenna height, the range of VHF packet is around 35 miles (+/- 35
miles). Under ideal non-contending conditions, the effective baud-
rate is halved each time the signal is relayed. So two relay hops
reduce a 1200 baud signal to an effective 300 baud signal. That
gives a range of 0 to 140 miles on relayed VHF with the same
effective baudrate as direct HF. Now direct HF has regional, national,
or global coverage depending on band and time of day. To get a NY to
CA link on VHF would require about 100 relays. With a halving of
effective baudrate at each juncture, that gives an effective baudrate
of 1200/2^10, or 1.17 baud. That's about 300 times worse than 300
baud HF.

Now real world channels are not non-contending, not noise free, and
require positive acknowledgements, header overhead, and turnaround
delays. That makes the situation even more grim for effective 
*communications* data rate. Single hop 1200 baud under contention
conditions can have an effective communications data rate as low
as .0005 bps under typical urban conditions if the station is at a
high site, the so called exposed terminal problem. One high site
here in Atanta routinely experiences hold off delays of 2 *minutes*
or more between packets. It can hear several hundred active stations
at once. Divide that by 2^10 and a transcontinental circuit passing
through urban areas would have an effective data rate of 4.9E-7 bps. 
Now that's grim. HF starts to look mighty fast at 300 baud.

*If* we had dedicated high speed non-contending full duplex trunks 
forming a national grid, then VHF/UHF/Microwave transcontinental 
relaying would be practical. But we don't, and getting such a system
built requires the cooperation and resources of thousands of people.
Worse than that, it requires the *continuing* dedication of thousands
of people to *keep* the system operational. One critical node failure
can close down the entire transcontinental circuit. Transoceanic
circuits are basically impossible to setup at all except by satellite
relay.

Amateur packet relaying's most scarce resource is not money or technology,
it's manpower to keep the relay nodes going. HF can get by with a few
hundred operators. A VHF/UHF/Microwave relay system would require tens
to hundreds of times more people depending on whether it was regional
or national in scope. US amateur radio doesn't have that many dedicated
node and system operators. Nearly every VHF packet circuit now in use
has a critical node. If that station went down, due to equipment failure,
changing interest of the owner, or the owner's moving or dying, the
total system becomes disconnected. A national system would have hundreds
of such critical nodes. On HF, *every* system can access *every* other
system on some band at some time of day. This incredible flexibility
means that the loss of one system or group of systems doesn't totally
crash the network. Dynamic routing is still a thorny technical problem,
but it pales compared to the operator problem.

Gary KE4ZV