Newsgroups: rec.radio.amateur.packet

In rec.radio.amateur.packet, brian@ucsd.edu (Brian Kantor) writes:

    Bdale and crew were working on doing something similar - the goal is to
    have a box that you can plug into a PC(or mac, or sun)'s Ethernet hole,
    and point its antenna at a similar box, yielding a highspeed link.

    I think they got up to about 3mbit a year or more ago.  Dunno where it's
    gone since then.


  I'm not sure what is going on in Colorado either. Perhaps new-parenthood
has stolen a few extra cycles (:>). (Bdale, you still there? )

  In any case, even the stuff I built with radar gun/motion detector
sources would receive full 10 Mbps and generally transmit something that
bore a resemblance to data up to about 8 Mbps or so.  I didn't work very
hard trying to improve the tx bandwidth of the modulator since we ended
up not using Ethernet cards and didn't necessarily want to spend the
system C/N (or DX) on it anyway.  As Zack pointed out, at the time we
were limited by digital h/w capability and not by the RF stuff.  Things
really haven't changed much in that respect.  While MaComm or similar
sources with wider bandwidth electronic tuning can be used to run with
Ethernet cards I haven't been particularly interested in pursuing that
since it doesn't seem to be a great help in the general problem of
building a better amateur network with amateur resources.  It attracts
the XWindows_at_home crowd and a few other CS types but doesn't do much
toward getting the amateur community as a whole involved in working
together in a coordinated way in order to create a moderate or high
speed wide area *network*.  It's chief value is in higher speed
backbones and it's all we can do to support the current ~250kbps
development going on at the moment (on hamradio budgets) without trying
to give *users* 10 Mbps or faster service.  Getting them to work
together to install well located local hubs and connect their clusters
together with backbones is presently challenge enough at these speeds.

  As I've said before, the microwave link article was intended as a
teaser as much as anything else.  I wanted to demonstrate that higher
rate information transfer was not expensive if you got into territory
where you could effectively communicate the associated energy. To answer
a question of an early poster in this string, parts cost was on the order
of $100-$150 per end for the 2 Mbps 10 GHz link.

    However, RCE and his crew have shown that this kind of performance is
    probably ONLY available on point-to-point links unless you use some
    sort of spread spectrum scheme that can deal with multipath, since in
    any non-PtP application with high-speed data, the multipath just chews
    the life out of your bitstream.  Even 56kb at 433 MHz dies from that.

    Note that a repeater system can be equivalent to a PtP link, since each
    participating station points his directional antenna at the repeater.

 I think this is what Brian meant but I'd modify by the above to use the
phrase Line-of-Sight (LOS) rather than PTP to avoid confusion and
emphasize that this is a physical layer issue.  Even Ethernet has this
problem.  At the physical layer it is *not* broadcast.  There is a
shield on the coax.  This is crucial to its success.  It's also
interesting to me that Ethernet's DX which is often thought of as being
limited by the CD requirement and the packet length/transmission delay
issues is on the verge of being limited by multipath and losses.  I
haven't done an exhaustive analysis but when the effects of cable losses
are combined with reasonable estimates of reflections due to mismatches
(at the taps and terminations) it looks to me like inter-symbol
interference would kill things even if the minimum packet length were
increased to keep the CD function alive.  As I remember from my
estimates, the present maximum Ethernet DX is about halfway to this
limit.

  For amateur radio, adding spreading is not a panacea either.  From an
economics and resources viewpoint, the additional loss incurred once one
deviates from LOS is so bad that even if multipath weren't an issue, I
don't believe that hams would be able to afford the systems that would
be necessary to get highspeed data through; even in the microwave bands
where we have the best (only) shot at it.  We don't mind an extra 30 dB
loss most of the time when we're jawing on our 10 watt 2M nbfm radio via
the local repeater and have 60 dB to spare but when that is applied to a
higher information rate system running with much smaller margins it's a
killer.

  We developed the Hubmaster protocol, which has a lot of similarity to
the user/repeater model Brian mentions, as *a* means to implement an
intermediate solution given ham resources and population densities.  It
doesn't solve the problem for the sparse user situation.  Even with the
best LOS/microwave solution it's going to co$t to pipe information over
DX.  Some of the really long haul situations (coast to coast, continent
to continent) may be served by satellites operating in a controlled
point-to-multipoint manner but many local backbones between clusters of
users (not to mention the cluster itself) are going to have to funded by
the local users.  This is a model similar to the local nbfm repeater
club with the extension that it is a linked repeater system.  The good
news here is that hams probably have the best line on access to the
primo local sites and know the local powers-that-be (if they aren't them
themselves).  For once we have an asset/resource toward building a
physically distributed wireless network that even the big guys have
trouble financing.

  The Hubmaster protocol is a compromise in the direction of full pt-pt
LOS paths which are probably not currently practical given the amateur
radio environment.  It allows a multiplexed server 
(a polling protocol and radio with an antenna which is designed
to serve multiple users in the azimuthal plane while having gain in the
elevation plane) while letting users pick up some of the advantages of
directional antennas in order to make system requirements and spectral
reuse reasonable at higher data rates.

  When the day comes that an amateur network has to have 100 Mbps (or 1
Gbps) to the user to be "interesting" and "competetive" then we'll all
have to have pt-pt protocols and narrow beamwidth (millimeter
wavelength?)  antennas or else *real* LOS/point-point hardware ...
optical fiber to our doors.
  In the meantime we're trying to find an affordable and interesting
intermediate speed solution using amateur radio.  I'm trying to get
things ready to put the first of the 256 kbps radios with an MIO
digital interface/control on top of a local hilltop this coming weekend.
Wish me luck.


Glenn Elmore n6gn

N6GN @ K3MC      
amateur IP:	glenn@SantaRosa.ampr.org
Internet:	glenne@sr.hp.com 









Newsgroups: rec.radio.amateur.packet