Subject: Re: [manet] RE: NHDP - Link Quality measurement

From: "Omprakash Gnawali" <[email-masked]>
Date: Sun, 6 Jan 2008 04:52:04 -0800

>Other
>available L2 information: statistics on number of retransmits and
>transmit
>failed indication for unicast traffic (sender) and the 802.11 MAC Header
>Retry field also for unicast traffic (receiver).
>This information is also used by data rate selection algorithm (if
>used),
>data rates higher than the minimum is a good indication that the link
>quality is OK.

>>> NHDP isn't designed to run only over 802.11. And even when running
>>> over 802.11, different drivers and OSes make different amounts of
>>> information available.
...
> >>> To use bidirectional information you would need to transfer it. And
> >>> that is not what this is about. Receiver information is what NHDP
> >>> works on in general (do you receive HELLO messages?).
> Bidirectionality
> >>> is established by the (implicit) HELLO handshaking both agreeing
> that
> >>> the link is usable in their direction.
> [Teco:]
> Chris, I agree on your comments. But what I meant is that a node may use L2
> feedback on transmissions also, when this is available. This could speed up
> detecting a broken link. When problems with unicast packet transfer occur,
> this information can be used to qualify link quality. Usage with care, of
> course. And there is dependency on unicast traffic.

NHDP aims to run on non 802.11 platforms as well so I wanted to share
some experiences from designing and using such protocols on 802.15.4
platforms. On at least one of the platforms built on 802.15.4, packet
ack information is available which allows us to estimate link
qualities. It turns out ack provides enough information to know about
bi-directionality of a link. We go into more details of our design and
experience of our link quality estimation on this paper:
http://enl.usc.edu/papers/cache/Fonseca07.pdf

Our conclusion is link layer information is crucial to having
up-to-date information about the links. This information would be
required by protocols that rely on hop-by-hop retransmissions so we
believe it will be widely available on wireless platforms.
Universality, however, does not imply standard interface or API and in
our paper above, we are arguing for some standard in how this
information is presented to the protocols. Messages such as HELLO are
useful for bootstrapping the link quality information but often are
unable to provide fine-grain and up-to-date link quality information.

In the past bi-directional link quality estimation on some low-power
wireless platforms was done with protocols such as:
LEEP: http://tinyos.cvs.sourceforge.net/tinyos/tinyos-2.x/doc/txt/tep124.txt?view=markup

- om_p

_______________________________________________
manet mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/manet

---

From: Philip Levis <[email-masked]>
Date: Sun, 6 Jan 2008 10:51:15 -0800

> LEEP: http://tinyos.cvs.sourceforge.net/tinyos/tinyos-2.x/doc/txt/ 

Another interesting point is that in 15.4, a link's ARR  
(acknowledgement reception ratios) are often quite different than  
it's PRR (packet reception ratio): it is almost always higher.  
Therefore, computing link quality using data packets in both  
directions can be very pessimistic. This makes a bit of sense: an  
acknowledgement is a conditional transmission after a successful data  
packet (raising the chances the link is good), and the fact that  
there was just a data packet reduces the probability of a collision  
(CSMA)[1].

We haven't done similar experiments on 802.11 yet.

Phil

[1] Figure 6 in Kannan Srinivasan, Prabal Dutta, Arsalan Tavakoli,  
and Philip Levis.
"Some Implications of Low-Power Wireless to IP Routing." In  
Proceedings of the Fifth Workshop on Hot Topics in Networks (HotNets  
V), 2006.

http://sing.stanford.edu/pubs/ip-hotnetsV.pdf