Subject: RE: [manet] Routing protocols for Wireless Mesh Networks

From: "Tan, Chiang Kang " <[email-masked]>
Date: Mon, 21 May 2007 10:26:05 +0100

Hi Nagarajan,
    I don't suppose there's any reason why these protocols can be not be for
wireless
mesh networks, if you find them suitable for your use...
 
cheers
Chiang

-----Original Message-----
From: nagarajan [mailto:[email-masked]]
Sent: 21 May 2007 10:20
To: manet
Subject: [manet] Routing protocols for Wireless Mesh Networks

Hi all,

       Can the following protocols

*	Ad-hoc On Demand Distance Vector Routing protocol - AODV

*	Optimized Link State Routing protocol - OLSR 

       be used for Wireless Mesh Networks??

Thanks and Regards,
Nagarajan

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From: Rex Buddenberg <[email-masked]>
Date: Mon, 21 May 2007 11:36:28 -0700

Nagarajan,

'mesh' is a somewhat abused term in this business.  But in general, it
connotes a layer 2 solution to problems like need to handoff from one
base station to another and need to daisy-chain multiple nodes together
(usually thought of as a hermaphrodite rig where a node is BS in one
segment and SS in another).  

Most of the mesh solutions we've seen, being at layer 2, are
technology-dependent.

And the problem with layer 2 solutions appears to be in loop behavior.
Loops in LAN protocols result when bridges are connected in such a way
that bridge forwarding returns a frame back to it's origin.  At layer 2,
a loop can result in frames 'ringing' around the network forever.
Obviously not a Good Thing.  So within layer 2, IEEE 802.1 worked up a
spanning tree protocol.  It deals with loops by detecting them and
inserting logical breaks in the forwarding chain.  Turning a graph back
into a tree.

The problem is that in a lot of situations where we want 'mesh'
solutions, we have some very pressing high availability engineering
requirements.  We also want to introduce lots of alternate routes to
account for those that fail at any point in time.  In other words, for
Ao reasons we want altroutes ... which are loops!  

Indeed, show me situations where high Ao isn't a trumping requirement.
Single-threaded networks are fragile.  And single threaded, multiple hop
radio networks are reeeealy fragile.  
	
So I tend to get suspicious about vendors telling me about their mesh
solutions when they tell me they are using spanning tree.  ... rig a
test to show what happens with loops.

A routed solution, which is what you're implying in your question, is
working at layer 3.  Here we have a set of protocols that have long
tolerated and used alternate routes.  The obvious example here is that
we're not frame-forwarding, but rather datagram forwarding.  And each
hop results in a decremented TTL value in the datagram header ...
eventually a ringing datagram will timeout and self-extinguish.  

Help?

On Mon, 2007-05-21 at 10:26 +0100, Tan, Chiang Kang wrote:
> Hi Nagarajan,
>     I don't suppose there's any reason why these protocols can be not
> be for wireless

---

From: "Herbert Rubens" <[email-masked]>
Date: Mon, 21 May 2007 16:41:26 -0400

Rex,

	It sounds like what you are looking for is a layer 2 routing
protocol which is instantaneously loop free. Most link-state protocols do
not have this property and are only loop free in their converged steady
state (this is true at regardless of whether they operate at layer 2 or
layer 3). However, it is possible to design protocols which have this
property. The most obvious example of such a protocol would be DSR, since it
uses source routing. It wouldn't make sense in the context of DSR to source
route a packet into a loop. There are definitely other protocols which
provide instantaneous loop freedom; DSR was just the most obvious example.

	Like you mentioned, the Spanning Tree Protocol can turn a graph into
a tree to 'try' and prevent routing loops. The problem is that packets which
are routed to destinations which are not the root of the tree probably could
have found a better route in the graph as opposed to the tree. If you
consider 3 wired switches connected in a loop, the spanning tree protocol
will remove one of the links to form a tree. This means that two of the
switches can't communicate across their direct wired connection and are
forced to hop through the intermediate switch. On a wired network this may
or may not be the end of the world, but in a wireless network two hops
instead of one 'could' result in losing half (or more) of your bandwidth
(depending on the link speeds). In general, running the 'standard' spanning
tree protocol to create a mesh is a very bad idea.

-Herb

--------------------------
Herbert Rubens
Persistent Systems, LLC
www.PersistentSystems.com

-----Original Message-----
From: Rex Buddenberg [mailto:[email-masked]] 
Sent: Monday, May 21, 2007 2:36 PM
To: Tan, Chiang Kang
Cc: manet
Subject: RE: [manet] Routing protocols for Wireless Mesh Networks

Nagarajan,

	

Help?

>                be used for Wireless Mesh Networks??
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> sender specifically states them to be the views of Thales Research &
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---

From: "Kenneth Peirce" <[email-masked]>
Date: Tue, 22 May 2007 09:36:37 -0400

FWIW - I know of a large bubble-era metropolitan-scale wireless
mesh(fixed) network which attempted to use STP. They tried all sorts of
variations on it, and it's performance was never really very good. At
first glance STP looks to be a natural fit for a wireless mesh, but the
reality of wireless link behavior is "non-synergystic".:)

Ken Peirce

-----Original Message-----
From: Herbert Rubens [mailto:[email-masked]] 
Sent: Monday, May 21, 2007 4:41 PM
To: 'Rex Buddenberg'; 'Tan, Chiang Kang'
Cc: 'manet'
Subject: RE: [manet] Routing protocols for Wireless Mesh Networks

Rex,

	It sounds like what you are looking for is a layer 2 routing
protocol which is instantaneously loop free. Most link-state protocols
do not have this property and are only loop free in their converged
steady state (this is true at regardless of whether they operate at
layer 2 or layer 3). However, it is possible to design protocols which
have this property. The most obvious example of such a protocol would be
DSR, since it uses source routing. It wouldn't make sense in the context
of DSR to source route a packet into a loop. There are definitely other
protocols which provide instantaneous loop freedom; DSR was just the
most obvious example.

	Like you mentioned, the Spanning Tree Protocol can turn a graph
into a tree to 'try' and prevent routing loops. The problem is that
packets which are routed to destinations which are not the root of the
tree probably could have found a better route in the graph as opposed to
the tree. If you consider 3 wired switches connected in a loop, the
spanning tree protocol will remove one of the links to form a tree. This
means that two of the switches can't communicate across their direct
wired connection and are forced to hop through the intermediate switch.
On a wired network this may or may not be the end of the world, but in a
wireless network two hops instead of one 'could' result in losing half
(or more) of your bandwidth (depending on the link speeds). In general,
running the 'standard' spanning tree protocol to create a mesh is a very
bad idea.

-Herb

-----Original Message-----
From: Rex Buddenberg [mailto:[email-masked]]
Sent: Monday, May 21, 2007 2:36 PM
To: Tan, Chiang Kang
Cc: manet
Subject: RE: [manet] Routing protocols for Wireless Mesh Networks

Nagarajan,

	

Help?

On Mon, 2007-05-21 at 10:26 +0100, Tan, Chiang Kang wrote:
> be for wireless mesh networks, if you find them suitable for your 
> use...
>  
> cheers
> Chiang
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> sender specifically states them to be the views of Thales Research & 
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