Subject: RE: [manet] 802.11: infrastructure mode and adhoc mode

From: "Pei, Ke (Peak)" <[email-masked]>
Date: Thu, 6 Jun 2002 14:11:36 +0800

I think that is depend on standard.

Pei, Ke (Peak)
Bell Labs China
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*Email:  [email-masked]

-----Original Message-----
From: Yu-Ching Hsu [mailto:[email-masked]]
Sent: Thursday, June 06, 2002 1:19 PM
To: [email-masked]
Subject: [manet] 802.11: infrastructure mode and adhoc mode

Dear all,
  I have an interesting question regarding the IEEE 802.11b products.
Does anyone know why stations cannot opperate in the hybrid mode, i.e.
infrastructure + ad-hoc. For example, in the following scenario, why
sta1 cannot send to sta2 directly?

                             AP1
                            /   \
                           /     \
                          /       \
                       sta1 ---- sta2

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---

From: Miguel Sanchez <[email-masked]>
Date: Thu, 06 Jun 2002 10:06:08 +0200

Hi,

And what the standard tells you is that MAC behavior is not the same 
when a station is in ad-hoc mode (DCF) than when it is in infrastructure 
mode (PCF).

Nodes can get random access to the media in a distributed way or a 
coordinated access by means of the access point (polling).

However, in the scenario you present it is still possible for sta2 and 
sta1 to switch (on the fly) to ad-hoc mode to enable direct 
communication. The problem is that they will have to share the medium 
with ap1 and with any other nearby node -if any-  (and PCF has a higher 
access priority) so, unless they also change to an unused channel, it is 
possible to get a longer delay and lower performance when sta1 talks 
direct to sta2 than when they use the infrastructure mode.

Regards,

Miguel Sanchez
Universidad Politecnica de Valencia, Spain

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From: Miguel Sanchez <[email-masked]>
Date: Thu, 06 Jun 2002 10:06:08 +0200

Hi,

Regards,

Miguel Sanchez

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From: Sumeeth Nagaraj <[email-masked]>
Date: Thu, 06 Jun 2002 03:38:24 -0600

Hi all,

In IEEE 802.11 DCF DSSS mode,  are all the nodes chip synchronized? if not
to some extent it will hear the other node as noise(interference). If there
is no chip synchronization among all the nodes( neighbors) how does
broadcast work,  Broadcast packets are meant to be sent to all the
neighbors, if there is no chip synchronization with all the neighbors some
of the broadcast packets could be dropped. Isn't this a problem? or am I
missing some thing here?

-Sumeeth

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From: Sumeeth Nagaraj <[email-masked]>
Date: Thu, 06 Jun 2002 03:38:24 -0600

Hi all,

-Sumeeth

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From: Yu-Ching Hsu <[email-masked]>
Date: Thu, 6 Jun 2002 19:54:13 +0800 (CST)

Hi,
  I think there is a mistake here. Infrastructure mode does not imply PCF.
Stations in infrastructure mode can also opperate in DCF mode.

---

From: Yu-Ching Hsu <[email-masked]>
Date: Thu, 6 Jun 2002 19:54:13 +0800 (CST)

Hi,

---

From: John Stine <[email-masked]>
Date: Thu, 06 Jun 2002 09:30:21 -0400

Sumeeth,

	Potential destinations synchronize to the transmission during the
training sequence at the beginning.  Once the signal is captured there
is a potential advantage over other interfering signals.  The DSSS
spreading code is a 11 bit Barker sequence (when using 1 Mbs and 2 Mbs
rates) which means the code is orthogonal to itself in all displaced
positions.  So assuming the time of arrival of an interfering signal is
random, a good assumption since a chip with the 1 Mbs data rate
corresponds to a difference of ~28 meters propagation distance, there is
about a 10/11 chance that the codes will not align and the captured
signal will be received with a 10+ dB processing gain.  When
broadcasting in 802.11, the RTS-CTS exchange is disabled so you have a
pure CSMA-CA protocol that does not prevent hidden nodes.  Thus, this
processing gain is a nice feature.

	Note that the 4 data rates used in 802.11b are not equivalent.  The
bandwidth for a channel is constrained to 22MHz.  Increasing the data
rate to 11Mbs removes the processing gain that comes from spreading. 
The higher data rates also use a different modulation.  The propagation
and interference characteristics of each data rate and the interference
characteristics between data rates differ.  Exploring and defining how
to model these differences would be a nice contribution if someone were
interested.  I have never seen it defined anywhere and don't believe it
is adequately modeled in any of the simulation environments that I am
familiar with that are used to study ad hoc networks.

John

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From: John Stine <[email-masked]>
Date: Thu, 06 Jun 2002 09:30:21 -0400

Sumeeth,

John

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From: John Stine <[email-masked]>
Date: Thu, 06 Jun 2002 14:53:32 -0400

Sumeeth,

Sumeeth Nagaraj wrote:
> Hello John,
> Potential does mean all neighbors should synchronise with the node that is
> transmitting right?(The chances being low) . Suppose a node synchronises
> itself to one of its neighbors and hears its other neighbor transmitting at
> the same time(Since no RTS-CTS-ACK exchange takes place in broadcast), as
> noise, packets are lost  from one of its neighbor because of no
> synchronisation with it.

Since access attempts are temporally random it is unlikely two nodes
will start to send packets at the same time.  Say a node starts to
broadcast a packet, all nodes that are within range will then try to
receive it.  This involves synchronizing to the chip sequence.  Say all
nodes within range have now synchronized and are receiving a packet and
then a second node starts to transmit that was not within range of the
first source.  THe probability any one of the original destinations
receives the first packet will depend on its local SIR characteristics. 
If the interfering nodes chip sequence is out of sync with the first
packet transmission then the first packet will have a 10 dB SIR
advantage.  THe final SIR will depend on a number of factors, e.g. path
loss, antenna gains, etc, of each of the transmissions.
> 
> On the other hand, if the receiver node is synchronised with the both
> transmitter's  then there is collision at the receiver because of two
> transmissions. Data packets are lost in both the cases either
> synchronisation exists are not.

The fact that a node is within range of two transmissions does not mean
that it is bound not to receive either.  It is likely to be synchronized
to the first it receives and the probability of reception of this first
packet depends on the noise offered by the second transmission.  A way
of looking at the processing gain is that it reduces the interference
noise that the second transmission causes and thus improves the odds
that the first packet will be received.  

> 
> Could this be a potential problem for network disconnectivity. If the
> broadcast fails to that node for more than the some threshold(RREQ_RETRIES)
> in finding routes to destination?

Possibly.  It really depends on many factors not just the chance that
two broadcast transmissions interfere at a destination.  However, the
performance of access protocols and in turn routing protocols will be
very sensitive to congestion (more nodes within range of each other) as
these hidden node transmissions will occur more frequently.

John

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From: John Stine <[email-masked]>
Date: Thu, 06 Jun 2002 14:53:32 -0400

Sumeeth,

John

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From: Sumeeth Nagaraj <[email-masked]>
Date: Thu, 06 Jun 2002 11:55:10 -0600

Hello John,

-Sumeeth

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From: Sumeeth Nagaraj <[email-masked]>
Date: Thu, 06 Jun 2002 11:55:10 -0600

Hello John,

-Sumeeth