[Cu-wireless] an article on an 802.11 rooftop network

[David Young]

BTW, there are parts available from Maxim (maxim-ic.com) and others for
building 900MHz radios. I do not doubt that an ambitious EE undergrad
could put together a versatile 900MHz digital radio that would plug into
a PCMCIA card slot.

Dave

On Thu, Jul 25, 2002 at 10:11:06PM -0500, stephane_alnet at ureach.com wrote:
> 
> Dave,
> 
> > Sorry, I think I transposed the S and V: it is DSDV that they use.
> 
> Oh, I was just pointing out that from what I understood of the MIT paper,
> they did most if not all of the testing using static "routes". The paper
> effectively mentions DSDV in the intro; I admit I went a bit rapidly
> through the last part of the document and jumped to the part that said
> "conclusion", so I may have missed the section where they went into DSDV.
> :-\
> 
> > Direct Sequence Distance Vector, I think.  It is in the book Ad Hoc
> > Networking, isn't it?
> 
> It is, with Perkins noting it's mostly there for historical purposes and
> that it was designed "quite before [they] thought about "on-demand", so
> DSDV looks primitive by comparison with modern protocols." [p.23]
> (You can bet I skipped that chapter.)
> 
> > I forget what other reasons that guy gave that 802.11b is not appropriate
> > for ad hoc networks.
> 
> The two main reasons I've seen (and can remember) are (a) RTS/CTS is a
> pain and (b) 802.11 doesn't behave well with asymmetrical links (which is
> also a problem for AODV at L3).
> 
> I had read about RTS/CTS somewhere else, but the MIT paper is the first
> paper (I've seen) to point out the issue with asymmetry at L2 (and the
> fact that asymmetry is quite common). Actually this is the first paper
> I've seen that goes this far in analyzing the issues we are going to face
> with 802.11b -- reading just the conclusion of the paper can give one a
> good overall idea. (The DSR paper had some testing, with the three cars on
> a loop between the two buildings, but that was 900MHz technology; all the
> other papers/protocols seem to use simulations to "prove" they are
> better.)
> 
> > I think that there are some challenges, and 802.11b is certainly not the
> > best (I think 900MHz is a more suitable band, and there's gotta be a
> > better MAC protocol), but I think 802.11b is "good enough."
> 
> And it's cheap too. And it has very good cross-platform support. And it's
> available off-the-shelf. -- No point intended; I was just sharing my
> "reading comments". :-]
> 
> > There are different MAC protocols than 802.11 [...].
> 
> That's where the DoD went if I understand properly. Until such point we
> can trick the firmware (if ever), we'll have to live with some of 802.11
> shortcomings. (Knowing what they are may help us until such time.)
> 
> But again, I haven't been impressed with what I've seen so far as far as
> layer 3 routing is concerned; it sounds like "scalability" in the ad-hoc
> world means "50 nodes (on paper)". As far as I can tell from my limited
> reading on the subject, it doesn't seem anybody has actually tested any of
> this stuff in the real world with a proper "scale" that would match what
> we are shooting for. Which is something you said a couple weeks ago. ;)
> 
> S.

-- 
David Young             OJC Technologies
dyoung at onthejob.net     Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933