Jack,
Let me jump in with some more thoughts on wireless mesh:
I agree with you that RF engineering and RF limitations are not
being fully
considered in most mesh deployments. Most mesh designs I have seen are
theory based and assume the full use of the unlicensed spectrum at hand.
This will never be the case and therefore limits the overall
capacity. I saw
an RFP from the city of Miami Beach and they had done a pre-survey of
the
city and found the noise floor at 2.4 GHz at -70 db in most areas.
Now how
is one going to deploy a mesh network with the ability to overcome that?
Typical answer is build more nodes closer to each other so these PDAs
and
laptops get enough signal. This ignores the fact that all of these close
spaced nodes then create more noise for each other because they are
mounted
at a height where they hear each other. In high density nodes even
having 2
hops will bring these networks to their knees. There is not enough
spectrum
to make it work and be able to load the network up. An 802.11b based
system
can not deal with the hidden node problem effectively enough. Even if
you do
have all the internode traffic on other frequencies at the high density
placement required in most cities, the spectrum limits are still a
big issue
to have the channels to link all the nodes. I would still like to
hear of a
mesh network from any manufacturer that has been deployed and has a high
density of users that are the kids of today. I want to see what bit
torrent,
VOIP and audio streaming do to a mesh in multiple hops. While we can
make
the argument that those services can be limited, that is only a band-aid
approach as today's society is going to expect to be able to use these
services in one form or another, it may take a while but it will be
necessary. The cellular companies are already creating the
expectation for
this kids to be able to audio stream on demand. If someone has
knowledge of
a loaded mesh network please let me know. Don't get me wrong, I love the
idea of mesh and wish it could work and want to see it work. It's
just that
I've been in ham radio since 1989 and was in to the packet radio
technology,
we as hams built networks where we dealt with all of these issues (I
know it
was only 1200 baud but the problems remain). There are two major
problems
in mesh from my viewpoint. One, if you have a carrier sense based
collision
avoidance system, you always have limited capacity because only one
radio
can talk at a time (part of the HDX problem). Two, if you do not have a
carrier sense based system then you can overcome noise with a stronger
signal. This causes cell site shrinkage or breathing and changes the
coverage area. Most people deal with this by building transmitters
closer to
each other, problem is that there is limited unlicensed spectrum
which is
not enough room for most systems to deal with this.
I really would like to see mesh work and hope to be proven wrong.
There is
a lot of promise in mesh implementations out there but until I have seen
them under residential internet use loads I remain skeptical.
Thank You,
Brian Webster
www.wirelessmapping.com <http://www.wirelessmapping.com>
-----Original Message-----
From: Jack Unger [mailto:[EMAIL PROTECTED]
Sent: Monday, February 27, 2006 1:46 AM
To: WISPA General List
Subject: Re: [WISPA] Basic Mesh Theory
Jeromie,
You raise some good points... and here are some more differences between
Matt's fully-meshed WIRED network example and the real-world conditions
under which WIRELESS mesh networks are so often deployed today.
1) REROUTING - Only a node failure or a high peak traffic load would
normally force a routing path change on a fiber/copper network. On a
wireless mesh, routing path changes will also result from interference
caused by other same-network nodes, interference from other networks,
and interference from other wireless non-network sources. Routing path
changes will also be caused by the movement of obstructions and other
rf-reflective objects such as trees and vehicles.
2. CAPACITY - Fiber/copper networks typically start out with
high-capacity (compared to wireless) full-duplex links. Wireless mesh
networks start out with low-capacity half-duplex links.
3. CONNECTIVITY - Fiber/copper mesh network nodes have two or more paths
to other nodes. "Real-world" wireless mesh networks may contain nodes
that, in some cases (the traditional "mesh" definition not withstanding)
only have a path to one other node. For example, obstructions may block
paths to all but one (or even no) other nodes.
4. ENGINEERING - Fiber/copper mesh networks are typically properly
engineered for traffic-carrying capacity, QoS, latency, etc.
"Real-world" wireless mesh networks are typically deployed in near-total
ignorance of the Layer 1 (wireless layer) conditions. That's the great
attraction (IMHO) of muni-mesh networking today. These networks are
thrown up in the belief that they don't need any Layer 1 design or
engineering expertise and that this will allow for quick, widespread
deployment. Last time I looked however, there was still "no free lunch".
I predict that the muni
mesh networks that are "thrown up" today (Philadelphia will be a prime
example, unless it's re-engineered correctly) will fail and fail
miserably to meet the high expectations that have been raised like free
or low-cost broadband for all. In addition, muni mesh networks today
typically lack adequate traffic engineering and performance testing
under load.
The way that muni networks are being marketed today will likely lead to
a black eye for the entire license-free wireless broadband industry
within 18 to 24 months.
I'm not saying that wireless mesh networks should never be used. There
are certain (obstructed, short-link, low capacity) environments where
they will be the best, most economical solution. I'm just saying that
the false claims and marketing hype surrounding MOST (and let me repeat,
MOST) of today's mesh networking claims, particularly mesh network nodes
that contain just a single 2.4 GHz radio are going to come back to bite
both the vendors and the cities that deploy these networks without
sufficient wireless knowledge in the false belief that wireless mesh
networks are just "plug-and-play".
Sorry about my rant, but other than a few responsible
multiple-radio/multiple-band mesh equipment vendors, the current mesh
marketing/hype environment is in a word - disgraceful.
jack
