[NLRS] Re: hsmm and msmm (long)

[Doug Reed]

Hi Clare.

What do you consider HSMM and MSMM?

Most of what I'd call HSMM has been based on using WiFi equipment for 
long range links at 1-10Mb rates. Usually either 802.11b/g at 2.4GHz, or 
802.11a at 5.6GHz. The usual sources quote 25-35 mile range with 2' 
dishes at each end. Some have used it as Part 97 equipment which allowed 
bigger dishes and more power. I believe a WiFi "shootout" in Colorado 
reached over 70 miles (100 miles?) using 8' dishes and higher power than 
allowed under Part 15.

I understand that several locals have reached 4-5 miles fairly easily 
without exceeding Part 15 rules. The main limiting factor tends to be 
height above ground and TREES. As you start looking around, most web 
sites will warn you that you need a clear optical path and the Fresnel 
Zone along the path must be clear as well. You could think of the 
Fresnel Zone as being the 6dB or 10dB points of the antenna pattern. If 
you get trees or buildings into that zone it can cause reflections and 
multipath that the signal can't recover from. So you really need a HIGH 
location to make those long paths work.

And of course you have interference problems from all the other Part 15 
users, no matter what the band. Yes, Part 97 might take precedence, but 
how long are you going to look for all the people causing interference 
to make them change channels? And what about next week?

One thing that was briefly mentioned in the QST article about using WiFi 
in the Shenandoah Valley was the link time-outs. Once you reach about 12 
miles, the path delay causes the WiFi card to time-out and repeat the 
packet because it didn't get an answer. This leads to packet collisions 
and lower data rates. If you have a path over 10 miles, you should buy 
equipment or software that allows you to change the default delay times 
to avoid this problem. I've got an email somewhere that explains this 
better....

The D_STAR 1.2GHz system runs 128Kb data rates. That seems to usually be 
considered the bottom of the HSMM rates or possibly the top of the MSMM 
rates. The single biggest advantage this has over competing technology 
is that it is not a Part 15 band. It is relatively quiet. I could wish 
for more power and better sensitivity, but 6-10 watts isn't bad. For 
whatever reason, ICOM usually specs about 25 miles as the maximum range 
between users. Maybe that is omni-to-omni rather than beams, I don't 
know....

Another existing technology is the old Ricochet modems. (If anyone has a 
bunch sitting in the closet, I'll take them! :-) The GS version of the 
modem was most common around here. It ran spread spectrum 128Kb data 
rates and 1 watt maximum in the 902-928MHz band and was about the size 
of a paperback book. It was said that two units with the built-in 
antennas could usually communicate over 1/2 to 1 mile. You can extend 
that to maybe 2 miles if there is a local pole-top radio between the two 
user radios. Since the entire metro network has been shut-off, the only 
other option is to replace the built-in antenna with an external gain 
antenna. Rumor has it you should then get 12-15 mile range between units.

One thing the Ricochet is very good at is point-to-point links. The 
modem uses the Hayes AT command set and any two units can call each 
other by simply using the ATDT dial command along with the serial number 
of the other unit. Once you have the connect, it sends and receives data 
as if it were a telephone dial-up modem. This could work very well in 
certain circumstances. The modem also has a "star-mode" capability for 
multi-point applications but it is not supported except under Linux, and 
then just barely.... The TwinsLAN club has been playing with these for 
some time now and Max has finally got them working pretty well. But we 
still haven't tried using them over long paths.

At the bottom end of the scale is regular packet radio, plus some of the 
newer sound card modes designed for HF use. None of them exceed 2400 or 
3600bps. These modes, especially 1200 bps packet are still popular 
because they don't require anything special in mods to the radio.

The faster 9600 bps or 19200 bps or 56kbps modes all require mods to the 
radios which make it much trickier to get them working and keep them 
working. (Wasn't there once a 9600 bps 2M data repeater in Winona?) 
TwinsLAN is still doing 9600 bps links and user ports, but we're 
spending more time looking at Ricochet and other newer technologies for 
future expansion.

As for the emergency services aspects of the system, there are a number 
of proposals floating around the metro that would use D-STAR radios to 
link various EOCs and served agencies via 1.2GHz data repeaters. For the 
most part this pertains to the served agency and on-scene command level, 
not the individual shadow level. But if you have a D-STAR voice 
repeater, you also have the option of sending data over that link to the 
users or automatically getting GPS position reports from the users. I 
expect to see people with a Palm Pilot and GPS linked to their IC-91AD 
HT radio when this happens. For on-scene connectivity, we've had pretty 
good luck with standard 802.11b/g WiFi at the Twin Cities Marathon 
finish line.

73, Doug Reed, N0NAS.