[MLB-WIRELESS] [TECH] Dipole antennas, and melbwireless structure

[Tony Langdon, VK3JED]

At 06:24 PM 19/03/2002 +1100, Ben Anderson wrote:

> > Some good ideas in here, but I've got a couple of comments.
>
>Comments, exactly what I wanted.  Goodie goodie :)

:)

>Even more difficult is a 'moving node' -- a node in a car shifting between
>cells at 100Km/h, could change routing cells every 10 seconds...  Being able
>to maintain a reliable bi-directional connection is going to be *tough*

Yes, mobile applications are really problematic.  If we can pull that one 
off, it'd be interesting. :)

> > This might be a necessary evil.  I'm one who might get a bit of a raw
>deal,
> > because I'm not in a particularly good spot RF wise, and probably 45-60
> > degrees of my field of view is useless, because of Essendon Airport (and
>the
> > lack of LOS ensures I won't be able to pass the airport either, not to
> > mention the effect of aircraft movements! :) ).  However, I may be able to
> > offer tunneled routes, as long as I can limit their bandwidth to 64 kbps.
> > Doesn't sound like much, but perhaps our protocols can mesh tunnels as
>well
> > one day. :)
>
>Shortcuts across the network I think will be all important to getting this
>working widespread (ie over bigger areas than mobile mesh already scales
>to).  64kbits might not sound much, but if it's a low-latency connection
>across a congested network, it could be an extremly valueable route (read
>lots of mojo).  If you're in a disadvantaged location, there's nothing
>stopping you running a node up a tree in a congested part of the network to
>accrue mojo for you to spend :)

We must support tunneling, as it is the most feasible way to rapidly link 
segments right now, until more wireless links take over.


> > Given the huge number of unknowns, I am somewhat skeptical of this
>approach,
> > and am not sure how well it will work in practice.  There are many factors
> > that affect propagation at 2.4 GHz which can't be expressed by a 2
> > dimensional lat/long coordinate set.  These include:
> >
> > Terrain
> > Antenna pointing
> > Stray reflections (I am going to have some interesting aircraft
>bounce! ) ).
> > noise and interference (OK, I'll keep my 2.4 GHz TV transmissions to 1W
>and
> > avoid transmitting during game tournaments! ;) )
>
>if a node can receive a packet, then that node can work out the best route
>to send the packet out of (if it has multiple interfaces) to get that packet
>physically closer to the destination...   One thing that would be cool is to
>be able to somehow 'route backwards' if there's a 'cheap' shortcut across
>the network -- ie say your neighbour has a fibre connection that goes _past_
>you  (but not connects with you) that means that to use that to route
>traffic out, the wireless node would actually have to transmit data
>backwards (well, this example would actually work correctly, because the
>shortcut is inside the broadcast zone of the original node...  if there's
>one layer of indirection, it breaks...  and i haven't thought of a simple
>way of dealing with this except perhaps maintaining an x hop accurate
>routing map (ie OSPF like)... that way the size of the 'hole' the shortcut
>can service doesn't simply extend behind it (which it naturally does with
>the system I've described), but in front of it as well by x hops.

Again, how are the nodes going to "know" this if the only information they 
have is lat/long of nodes?  This only reinforces my argument that routers 
still need to do some discovery broadcasting to work out the topology of 
the network near them.

> > I feel each node is going to have to discover its environment, but as a
>lot
> > of the variables are relatively static, or vary be relatively small
>amounts,
> > a lot of the node availability information could be cached, and "neighbour
> > discovery" may be able to be kept to a minimum.  For many sites, their RF
> > coverage will be anything _but_ a simple approcimation of a circle.
>
>I make no assumptions about the size or shape of the broadcast zone.. if a
>node hears the packet, and it's in the direction of the destination node, it
>will rebroadcast on the best outgoing route it has.  When the connection is
>'established' the best route will have been discovered, and known by the
>destination node, and that info can be sent back to the source in the ack to
>do source-routed packets.

Again, this suggests that a _LOT_ more than lat/long information is 
needed.  For example, here, the best route to a node north of me is likely 
to be _away_ from it, to the south! :)  So we're back to some sort of route 
discovery mechanism. :)

>No porting is really necessary for the encryption, because the whole network
>layer in the OSI model is being replaced, if the system compiles for a
>target system, the encryption will come along for the ride.

Well, the code will have to be ported (ever seen Windows source compile 
cleanly on a *NIX box and vice versa? - "Hello World" examples don't count 
;) ).

> > Well, for that sort of thing, we're now well outside the realm of ISM
>class
> > licence conditions.  Get a ham ticket and tinker to your heart's content,
> > that's _precisely_ what the ham bands are for - personal experimentation
>and
> > self training in radio techniques. :-)  And I, for one, would love a good
> > excuse to play with high speed data on microwaves...   Need a testing
> > partner? ;)
>
>The ISM bands can be still used, as long as the device complies to the
>restrictions set down in the class licence.  There are a lot of bands that

One of those restrictions is that the equipment needs to be type approved 
for the purpose by the ACA, AFAIK.

>Even if we have to get ham licences, for the benefit of this system, I'm
>sure we could get piles of people licenced... they're not that pricey...
>and there's some you don't even have to know morse for (I do already know
>morse though...)

I'd separate the highly experimental hardware from the "general" 
network.  This will avoid potential regulatory issues on the network.  If 
you're successful, it could mean there's two networks - a full featured ham 
network and a more encompassing "citizens network", based on commercially 
available gear (which is the original concept of Melb wireless :) ).

And you're right, one doesn't need Morse to get a ticket that allows 
microwave access. :)

73 de Tony, VK3JED
http://vk3jed.vk.irlp.net