Subject: [manet] Re: MANET definition of a neighbor

From: <[email-masked]>
Date: Thu, 4 May 2006 12:54:59 -0500

The definition of a neighbor is why wireless MANETs are so much more 
difficult than wireline networks.  In fact, one could say that the problem 
of determining who is a MANET neighbor and controlling the link to MANET 
neighbors sums up the problems and complexities associated with MANETs - 
at least to a first order approximation!

The old DARPA Packet Radio and SURAN work (from 1970s through 1980s) used 
the term neighbor to indicate nodes that had been heard over the air 
within a given time period.  [for example, see Jubin, J., and Tornow, J. 
D. (January 1987). "The DARPA Packet Radio Network Protocols". Proceedings 
of the IEEE]

If it was judged that the probability of transmitting and receiving 
packets to/from the neighboring node was estimated to be above some 
threshold, say 80%, then the neighbor was considered to be a GOOD NEIGHBOR 
and it was considered to be a neighbor for routing.

If it was judged that the probability of transmitting or receiving packets 
to/from the neighboring node was estimated to be below that same 
threshold, then the node was considered to be a BAD NEIGHBOR and it was 
not considered to be a neighbor for routing.

Information was maintained about the ability of the link between good 
neighbors to carry traffic at different data rates.  Stateful control 
loops were used to decide when to switch a node between {"not-a-neighbor", 
"bad-neighbor", and "good-neighbor"} states.  Nodes could get out of synch 
with one another, i.e. node A thinks it is a good neighbor to node B while 
node B thinks they are bad neighbors.  And the neighbor states can change 
dynamically, i.e. as nodes move about, two nodes will dynamically move 
from bad-neighbor to good-neighbor and back to bad-neighbor.

Variants of Least Interference Routing (LIR) in the DARPA SURAN work even 
went further to consider an additional state for potential interfering 
nodes & added an additional state called INTERFERING NEIGHBOR - which 
might be nodes 2 hops away from a routing sense.  [Stevens, J, 1988, 
"Spatial Reuse Through Dynamic Power and Routing Control in Common-Channel 
Random-Access Packet Radio Networks", UTD PhD thesis] This would be 
neighbors that are so far away that they cannot be heard well enough to be 
considered even a bad-neighbor, but are close enough that their 
transmissions can interfere at the channel access layer. 

These cross-layer interactions in MANET networks are what make MANETs so 
difficult to optimize.  This is why DARPA has started their new CBMANET 
work.

As Dr. Mullen noted, many simulations use simplistic assumptions, such as 
all nodes within a radius X are neighbors (i.e. can transmit and receive 
packets) while all nodes outside that radius are not neighbor (i.e. cannot 
transmit and receive - not even to interfere with one another).  These 
simplifying assumptions speed up the simulation development and execution, 
but can lead to significantly inaccurate results.

So why am I making this soliloquy?  I guess it is to encourage all of us 
working in MANETs to think about what it means to be a neighbor and the 
fact that being a neighbor is not just hearing a protocol beacon.  And we 
need to apply control theory to our algorithms that determine neighbor 
state (as well as the link state to our neighbors) to keep the algorithm 
output stable.

Jim Stevens
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
James A. Stevens, PhD
Chief Network Architect
Rockwell Collins Government Systems
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From: Kenneth Brayer <[email-masked]>
Date: Thu, 04 May 2006 14:16:13 -0400

It strikes me that a neighbor of Node A is any node that A can setup a 
link to, whether it be next door or 300 miles away. A node that the 
second node can setup a link to but that A cannot setup a link to is a 
neighbor once removed, etc. Once you know who the neighbors are you can 
build a network routing table.

See
Packet switching for mobile earth stations via low-orbit satellite network
Brayer, K.;
Proceedings of the IEEE 
<http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=5>
Volume 72,  Issue 11,  Nov. 1984 Page(s):1627 - 1636

Available online in IEEE XPLORE

Ken

-- 
Kenneth Brayer
Fellow of the IEEE
M/S M370
The MITRE Corporation
202 Burlington Road
Bedford, MA 01730

e-mail: [email-masked]
tele: 781-271-5254
fax: 781-271-2841

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From: Chip Elliott <[email-masked]>
Date: Thu, 04 May 2006 17:47:36 -0400

I think Jim has stated the fundamental challenge with MANETs
very succinctly, and his comment is worth some reflection:

>at least to a first order approximation!

 From my own perspective, even the term "link" can lead to
great mental confusion. And once one relies on simulations where
everyone within range has 100% reception, and everyone outside
a range has 0%, one has missed most of what makes a
real MANET so challenging.

Mobility adds more complexity, but the probabilistic,
self-noisy, shared channel is the core problem, I think. And as
Jim points out, layering doesn't help and can even hurt.

Nice statement of the problem, Jim.

Chip Elliott
Principal Engineer, BBN