Subject: Asymmetric links

From: Jonas Nielsen <[email-masked]>
Date: 12 Feb 2002 15:23:35 +0100

In some ways DSR is said to be superior to AODV because it supports
asymmetric links. But in what scenarios are asymmetric links used. I
can only see that either a system is (almost) symmetric or highly
assymetric (e.g sattelite communications).

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From: Rajesh Bhairampally <[email-masked]>
Date: Tue, 12 Feb 2002 22:11:18 -0600 (CST)

I guess asymetry comes with the time; i mean, as the time passes, nodes
loose energy; different nodes may have different energy losses. so, there
can be a situation where a node can receive data  from another node, but
it doesn't has sufficient energy levels to send its data to the other
node. such situations create unidirectional links in the network which
originally started with bidirectional links.

rajesh

> assymetric (e.g sattelite communications).

Rajesh

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From: Kilian Weniger <[email-masked]>
Date: Wed, 13 Feb 2002 14:06:29 +0100 (CET)

You can have unidirectional links even if you have enough battery power:

Firstly, if you use power control alogrithms. This is obvious.
Secondly, interference leads to a decrease of the SNR at the receiver. If
the interference is much higher at one of both receivers, the link
may be unidirectional.

I guess there are even more examples of situation, which result in
unidirectional links.

Kilian

-- 
----------------------------------------------------------------------------------------
Dipl.-Ing. Kilian Weniger           phone: (+49)(0)721-608 6415
Institute of Telematics             fax:   (+49)(0)721-388097
Department of Computer Science      email: [email-masked]
University of Karlsruhe 	    homepage: http://www.tm.uka.de/~weniger/
D-76128 Karlsruhe

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From: Chip Elliott <[email-masked]>
Date: Wed, 13 Feb 2002 08:47:10 -0500

It's probably worth worrying about asymmetric links, especially
when using spread spectrum radios.

Many years ago, an interesting paper described simulations of
spread spectrum packet radio networks, in which a surprisingly
large number of simplex links were found. For example, using
assumptions that seemed plausible to me, the authors calculated
that 1/4 to 1/3 of all possible links were simplex, in a few
more or less random laydowns.

I'm not competent to judge their findings, but I'd be most
interested in hearing from anyone that is!

"Computer-Aided Modeling of Spread Spectrum Packet Radio
Networks," E. Sousa, J. Silvester, T. Papavassiliou, IEEE
JSAC, 9-1, January 1991.

-- Chip

Chip Elliott
Principal Engineer, BBN

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From: Jonas Nielsen <[email-masked]>
Date: 13 Feb 2002 15:13:04 +0100

Thanks for all the answers. What I really want to know is how often
simplex links is found in todays "personal" wireless network
standards. I'm particulary thinking of 802.11-based networks, but also
other technologies like Bluetooth. Any thougts ?

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From: Prithwish Basu <[email-masked]>
Date: Wed, 13 Feb 2002 11:13:47 -0500 (EST)

In our lab environment (and dept corridors) with 802.11 radios (cisco
aironet 350 series), we have observed occasional unidirectional links
between nodes which are on battery power (with APMD running) and those
which are on AC power. This happens at the lowest configurable power level
(1mW) usually.

This is not a systematic observation but just something which we have
observed sometimes.. I wonder if other people have experienced similar
behavior.

-Prithwish

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From: Mineo Takai <[email-masked]>
Date: Wed, 13 Feb 2002 09:00:15 -0800

Hello Jonas,

For unicast communication, there should not be a simplex link in 802.11-based
networks, because the MAC protocol requires the receiver to transmit ACK back to
the sender upon successful data reception. This is regardless of the RTS/CTS
option.

Mineo

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From: [email-masked] (Matt Read)
Date: Wed, 13 Feb 2002 10:15:32 -0800

Prithwish,

Your description is so familiar that I had to check to make sure you 
don't work here were I work. :-)

We run at 1mW or 5mW, Cisco 350's, "hallway" network, usually AC but 
sometimes battery, and we occasionally see unidirectional links.  It 
seems random, i.e. not always reproducible, but once it happens the 
condition usually persists until we move the node.  This confirms the
idea that it's due to asymmetrical radio propagation/interference.

Everyone,

AODV breaks when this happens.  As a relative newcomer to AODV, 
please pardon me if this has already been considered, but could the 
HELLO message include a list of the Neighbors from which the sender 
has recently heard HELLO's?  Then if a node hears a HELLO in which 
it does not find itself listed, it could assume there is no link and 
not try to send directly to that node (sending RREQ's instead).  
(There will be scalability and other issues, of course.)

- Matt

On Feb 13, 11:13, Prithwish Basu wrote:
> Subject: Re: Asymmetric links
> 

________________________________________________________________________
MattRead  [email-masked]  858/452-4608  HughesNetworkSystems  SanDiego,CA

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From: Prithwish Basu <[email-masked]>
Date: Wed, 13 Feb 2002 14:06:44 -0500 (EST)

Matt,

Since 802.11 needs an ACK at the MAC layer for a *unicast* pkt, a
unidirectional link won't show up at a higher layer. However, it is indeed
possible (as both of us have observed) for broadcast HELLO messages. (A
always hears B's broadcast but B never hears A's broadcast when A and B
are at certain fixed positions in space)

And yes, the condition does persist at times until one moves the node.

-Prithwish

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From: [email-masked]
Date: Wed, 13 Feb 2002 12:50:23 -0800

Matt,

> (There will be scalability and other issues, of course.)

Yes, by including a list of all neighbors in each HELLO, a node
can detect when a bidirectional link has become unidirectional.
This is what OSPF and OLSR do.  But a more efficient way is to
use differential HELLOs (as in TBRPF), so that each HELLO contains
only the neighbors whose states have recently changed.
Perhaps AODV should also use differential HELLOs.

Richard

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From: Erik Nordström <[email-masked]>
Date: 13 Feb 2002 23:23:03 +0100

Hi Matt + others.

Comments below.

> (There will be scalability and other issues, of course.)

I have also experienced unidirectional links during testing of my AODV
implementation. My implementation already include the feature/idea you
are requesting. The code, which just has been released in version 0.2,
can be downloaded from http://www.docs.uu.se/~henrikl/aodv/.

/Erik Nordström

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From: "Philippe Jacquet" <[email-masked]>
Date: Mon, 18 Feb 2002 20:00:43 +0100

Richard,

You must be careful with your definition of symmetric/asymmetric links. In
your draft you assume that a link is bidirectional even when it is made of
two unidirectional interfaces. This may fail with IEEE 802.11 ACK feedback
process. Please look at OLSR draft. Indeed you may meet weird situations
(the most uncomfortable one is when a node receives the same interface from
another node but on two different interfaces, among them one is asymmetric).

Philippe