[TriLUG] interoperability: 802.11g and n
Joseph Mack NA3T
jmack at wm7d.net
Sun Oct 4 14:20:04 EDT 2009
>> There's a decent article right on Wikipedia...
This, and a list of references to the mesh routing demons,
set me on the right track.
My original interest was if the new dd-wrt running on the
WRT54G-TM (with 32M memory) could do mesh routing. It looks
like it can.
In the meantime I got a whole lot of other stuff straight.
802.11n is not mesh routing (as I thought). It's 100Mbps
using MIMO (multiple in, multiple out) antennas, ie 2 or
more antennas at each end. The multiple antennas are used to
reduce interference, through among other things, on the fly
phasing for each packet. Unless a large number of elements
is in use, this seems to be a computationally intense
procedure for not much gain to my mind. I didn't find the
packet protocol underneath. I didn't find any examples of
devices using it. I don't know if it's designed for trunking
or for leaf nodes or if it's node agnostic. AFAIK it's not
here yet.
802.11s is mesh routing, like used in the OLPC. The routing
is described independantly of the packet protocol.
Presumably a node could use 802.11a,b,g for the packets, and
802.11s for the routing. I don't know if an OLPC can join an
802.11g network out of the box or not. It seems in principle
it could.
Most interest in mesh routing is for VOIP and in particular
the military is interested in it for battlefield operations.
People like you and me don't rate in the setting the pace
and direction of 802.11s.
>From what I can see, the routing used for current 802.11g
links is just the standard wired ethernet routing with a
default gw. In 802.11s, nodes (or leaf computers) have a
view of the network much like that of a wired router running
an internet routing protocol - no default gw and a large
routing table.
There's a couple of differences between wired internet
routing and 802.11s
o all hops aren't equal - there is reliability factor
(weighting) for each hop
o packets arrive and leave via the same interface.
o routing is proactive - the nodes are looking for and
updating routes continually even if there is no traffic
asking for those routes. This means each node has to hold a
table of all the routes and a large amount of traffic is
involved in updating the routes. A wired network is
reactive, nodes don't search for a route till traffic
arrives for that route. How discovery works and updates are
propagated in 802.11s I didn't discover - ie what is the
equivalent of icmp. At least one 802.11s protocol uses UDP
packets to a low port for some part of this. The large
amount of traffic for route updates means that nodes that
are asleep most of the time (eg sensors which wake up once
an hour to send a reading) can't be 802.11s. I'm not sure
how a non-s leaf node fits in (joins, leaves) to a 802.11s
network.
Joe
--
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!
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