That RS232/serial run is in my view way too long if you are running at
115K2, even when shielded. I would suspect that noise is being
introduced into the circuit. RS232 isnt a balanced system like RS485 or
XBaseT ethernet so you dont gain the benefits of common mode rejection.
You may even have earth loops occurring in your computer/mains/ground
setup that puts main noise onto the cable shield. The simple test is to
reduce the DTE rate from the computer to the radio to the next step down
to see if that helps. If the modems offer hardware compression you might
lose your peak speed but gain a higher reliability. Generally speaking
the slower the bit rate the longer the cable can be.

The data rate problem is the same thing that gfilion was fighting on. I
understand the Aerocomm fixes the TX/RX ratio at 50% to make PPP work
and thus you get the low data rate. You may of course also be suffering
from radio interference. I remember the Cisco Aironet boxes use to keep
stats as to their performance. If the Aerocomm boxes do that may be a
place to check. One simple thing you can do if you think it is another
transmitter is to change to horizontal polarisation in the hope that
most other users will be on vertical.

I have had a look around for Ethernet radios on 900MHz and they arent
real cheap (like USD700/pair etc). The serial offerrings all seem to
have the same rate limitations you have encountered and as yet I havent
seen a USB connected one (to gain a higher rate than 115K2). They seem
to be more for telemetry/SCADA use rather than a FDX internet link.

My flippant suggestion to get another Aerocomm pair and connected them
for full duplex operation might be cost viable for you. You'll need to
play with the hardware a bit but I dont see any problems if you can tell
the modems to stay in the right mode. I get the impression that this
will then get you 57K6.

I am seriously thinking of taking some of my works 2.4GHz RF modules for
TX and RX and supplying my own QAM modulation scheme to them. The idea
being to run maybe 1MB/sec via the USB port. I can do that as a radio
amateur but will need to check whether I can use the ISM band in that
manner. I can certainly tune it to the FCC spec but dont know if I have
to go through a type approval process. A very long term project I am
afraid. In that vein I am also looking for AX25/packet QAM I/Q modems
that would do that part for me.

Good luck!

Cheers Bob

Braath Waate wrote:

Thanks Bob for all of your good suggestions.

Yes, good suggestion. I am running the serial lines at 115K, I will try
to run at 57 or 38 or even 19 and see if that improves the link.

Connexlink does have a signal strength register (report last RSSI) and
has some DOS software to manage it, but I have been reluctant to spend
the time on the radio section because the 19K limit is disappointing
enough for web access, even if it were reliable.

I'm sorry I don't understand the radio pair suggestion; wouldn't the
pair of radios just interfere with each other? Is the idea to hook the
tx lead of the serial port to one radio and the rx lead to the other?
How would they hook to the antenna?

-- Braath

Hi Braath

The kind of stats I am talking about is more the bit error rate one. ie
what percentage of the radio packects are mangled or not getting
through. It is common in a radio system to have some percentage of
failure. I use to administer two non licensed links, one on 2.4Ghz and
the other on 5.7. Towards the end of its useful life the 2.4GHz link had
about 90% retries mainly from interferring sources. In that case we got
much better reliability limiting it to the 1MB speed (as against 11).
The newer less populated 5.7GHz link ran about 20% retries.

The fullduplex radio pair configuration I suggested would use separate
frequencies and maybe even antenna polarisation/space separation to
reduce interference. ie two antennas at each end. Using a diplexor to
allow two boxes on the one antenna is probably a bit cost wasteful. The
idea is that at one end you force one box into transmit mode and the
other to receive. You also have to specify a fixed channel instead of
allowing automatic operation. The node names would also have to be
paired. Connection to the computer is as you stated but some
configuration of the handshaking lines (RTS, CTS, DTR etc) would also be
needed. I expect that Connexlink would have already done this with their
equipment so there may even be an application note on it available. It
would of course be smart to run all lines back to the PC and do the
patching there, in case you want to send commands to the unit itself and
need both TX/RX lines available.

Cheers Bob


Braath Waate wrote:
--

I'm sorry I don't understand the radio pair suggestion; wouldn't the
pair of radios just interfere with each other? Is the idea to hook the
tx lead of the serial port to one radio and the rx lead to the other?
How would they hook to the antenna?