Subject: [manet] 802.11

From: Yi Lv <[email-masked]>
Date: Wed, 8 May 2002 10:24:19 -0400 (EDT)

Hi, all:

First, i'm not sure if it's appropriate to put my questions here.:)

As you know, 802.11 defines not only the MAC layer, but also the physical
layer. I am not very familiar with the physical layer, such as DSSS. On my
understanding, DSSS should be fighting for some interference. Does that
mean from the MAC layer point of view, it is possible that physical layer
offers the unicast service? While the mac layer seems to assume only
broadcast can be done by physical layer. If unicast can be implemented by
physical layer, the probability of collisions should be less than the 
current assumption.

Can anyone tell me  what's the prototype for the model of physical layer
in ns-2 or some other simulators?

Thanks a lot!
Yi Lu

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From: "Miguel Sanchez" <[email-masked]>
Date: Wed, 8 May 2002 18:46:07 +0200

Hi,

> physical layer, the probability of collisions should be less than the

It depends on what your assumptions are about the available antenna(s). The
use of some kind of directional antennas can change the broadcast nature of
the physical medium. Some papers have proposed such a scheme, either tied to
a single radio or to multiple transceivers per node.
> in ns-2 or some other simulators?

ns-2 has two propagation models: free space and two-ray ground reflection
(equations shown below , copied from the source code comments).

        /*
         * Friis free space propagation equation:
         *
         *       Pt * Gt * Gr * (lambda^2)
         *   P = --------------------------
         *       (4 *pi * d)^2 * L
         */

        /*
         *  if d < crossover_dist, use Friis free space model
         *  if d >= crossover_dist, use two ray model
         *
         *  Two-ray ground reflection model.
         *
         *      Pt * Gt * Gr * (ht^2 * hr^2)
         *  Pr = ----------------------------
         *           d^4 * L
         *
         * The original equation in Rappaport's book assumes L = 1.
         * To be consistant with the free space equation, L is added here.
         */

I hope this helps,

Miguel Sanchez
--
Universidad Politecnica de Valencia
Camino de Vera, 14
46022 VALENCIA (Spain)

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From: "Miguel Sanchez" <[email-masked]>
Date: Wed, 8 May 2002 18:46:07 +0200

Hi,

        /*

        /*

I hope this helps,

Miguel Sanchez

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From: Mineo Takai <[email-masked]>
Date: Wed, 08 May 2002 14:41:38 -0700

Hello Yi,

The IEEE 802.11 DSSS PHY specifies a single 11-bit spreading code, which
means that its DSSS does not help reduce interference from other 802.11
radios. It should help if the interference source is something else,
like microwave ovens, hair dryers, or jammers in military scenarios.

The IEEE 802.11 DSSS PHY models in common manet simulators (ns-2,
GloMoSim, QualNet and OPNET) do not simulate spreading/despreading, and
that should be fine unless non-802.11 noise sources are involved in the
simulation scenario.

Mineo Takai

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From: Mineo Takai <[email-masked]>
Date: Wed, 08 May 2002 14:41:38 -0700

Hello Yi,

Mineo Takai

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From: [email-masked]
Date: Wed, 8 May 2002 21:16:16 -0400 (EDT)

Hi Mineo,

> like microwave ovens, hair dryers, or jammers in military scenarios.

I am not sure that this is correct. Even if there is a *single* spreading
sequence, but the sequence is with low autocorrelation function (for all
time shifts, except at the orgin), then the spreading *does* provide some
degree of interference reduction. The key here is that the transmissions
from different nodes will not be chip-synchronized. Thus, due to the low
autocorrelation, there will be some rejection of the other transmissions.

Best,

Zygmunt.
==--==-=-===
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Wireless Networks Laboratory		 http://wnl.ece.cornell.edu
School of Electrical Engineering         
Cornell University			 tel: +1-607-255-3454
323 Frank Rhodes Hall			 fax: +1-607-255-9072
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From: [email-masked]
Date: Wed, 8 May 2002 21:16:16 -0400 (EDT)

Hi Mineo,

Best,

Zygmunt.
==--==-=-===

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From: Yi Lv <[email-masked]>
Date: Wed, 8 May 2002 21:47:27 -0400 (EDT)

Hi, Mineo:

> like microwave ovens, hair dryers, or jammers in military scenarios.

What's the reason why only one spreading code is used? Is that because
it's hard to deploy different spreading codes to each node?

Since only one code is used, what's the benifit of DSSS? Only to avoid the
interference with other devices, which use the different codes?

Thanks a lot!
Yi Lu

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From: Yi Lv <[email-masked]>
Date: Wed, 8 May 2002 21:47:27 -0400 (EDT)

Hi, Mineo:

Thanks a lot!
Yi Lu

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From: "Miguel Sanchez" <[email-masked]>
Date: Thu, 9 May 2002 09:46:38 +0200

Hi,

> it's hard to deploy different spreading codes to each node?

A single shared channel is achieved when all the nodes use the same channel.
If you use different codes/channels you make MAC (and multihop routing) more
complex than if you just use a common channel. (Most receivers can only
correlate receiver signal with one predifined code).

> interference with other devices, which use the different codes?

It is my understanting that DSSS allows a more graceful degradation as noise
floor raises than narrow band transmission. Using 802.11 an unlicensed band,
where pontetially you can have multiple sources of interference, the goal
was to create a interference resilient system.

Regards,

Miguel Sanchez

---

From: "Miguel Sanchez" <[email-masked]>
Date: Thu, 9 May 2002 09:46:38 +0200

Hi,

Regards,

Miguel Sanchez

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From: Mineo Takai <[email-masked]>
Date: Thu, 09 May 2002 14:43:25 -0700

Hi Zygmunt,

I admit that DSSS may provide some processing gain even if interfering
signals use the same spreading code, but I do not know how much gain we
would get. I do not usually simulate systems in chip level, but will
talk to EE friends if it is worth analyzing.

Regards,

Mineo

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From: Mineo Takai <[email-masked]>
Date: Thu, 09 May 2002 14:43:25 -0700

Hi Zygmunt,

Regards,

Mineo

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From: Mineo Takai <[email-masked]>
Date: Thu, 09 May 2002 15:02:58 -0700

Hi Yi,

I think other folks have already answered your questions. FYI, IEEE
802.11b includes CCK (Complementary Code Keying), which changes
spreading codes dynamically. However, different codes are not used to
reduce the interference, but to realize higher data rates.

Regards,

Mineo

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From: Mineo Takai <[email-masked]>
Date: Thu, 09 May 2002 15:02:58 -0700

Hi Yi,

Regards,

Mineo

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From: "Philippe Jacquet" <[email-masked]>
Date: Mon, 13 May 2002 11:24:31 +0200

Hi, all,

I think the DSSS code is mainly used to fight the interference between the
multipath from the same source. Fighting interference from different sources
is more the job of the MAC CSMA.

Philippe

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From: "Philippe Jacquet" <[email-masked]>
Date: Mon, 13 May 2002 11:24:31 +0200

Hi, all,

Philippe

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From: Yi Lv <[email-masked]>
Date: Wed, 15 May 2002 11:55:44 -0400 (EDT)

Hi, folks:

Because i am not familiar with the OLSR configuration, does anyone send
me a sample tcl script to support the running of OLSR?

Thank you very much!

BR/yi lu

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From: Yi Lv <[email-masked]>
Date: Wed, 15 May 2002 11:55:44 -0400 (EDT)

Hi, folks:

Thank you very much!

BR/yi lu