Subject: RE: [manet] Hiden- & exposed terminal problem in MANET

From: "Manfredi, Albert E" <[email-masked]>
Date: Mon, 23 Sep 2002 15:49:12 -0400

Sumeeth Nagaraj wrote, concerning

http://www2.elen.utah.edu/~schlegel/publications/KotSch01.pdf:

> I would think its considerably a different idea when compared to the
> cellular systems and applying the idea of multiple codes to 
> mobile ad-hoc
> networks should also be considered?(your inputs please).

Hi Sumeeth,

I have a knee jerk reaction, which is that spreading in IEEE 802.11b seems inadequate to support multiple users in a single 5 MHz frequency band. In fact, it seems that only 8 or 11 chips per symbol are used, respectively for the 5.5 and 11 Mb/s options and for the the 1 and 2 Mb/s options.

>  Every node, has a header  which is spread by a known 
> spreading code and the
> data part is spread by the random code sent to the destination by the
> source. This is done for each packet sent by the source and 
> the random code
> changes each time the source has some data packet to send 
> which is not the
> case in cellular systems.

Without digging through the details of cellular CDMA, however, the difference does not appear to be so great? In the cellular model, the base station negotiates the code for each channel with each mobile unit requesting service, on the synchronization channel(?), before the data stream begins. Seems to me that both systems have to start somewhere to establish the unique code for each data stream, and the only difference here is in the details of how the individual codes are assigned. Thereafter, you have non-shared channels on which data are sent, which seem dedicated to that user and the base station for the duration of the session? Unless I misunderstand the concept, seems to have aspects of circuit switching.

> The available bandwidth is pretty well utilized. The base 
> station still
> exists and they have not said anything to replace the base 
> station as noted
> by you.

Correct you are. But this makes it more similar, rather than less similar, to the telephony model. And less like a MANET.

> Although the success, is in proper detection of the 
> header, which
> is spread using a common spreading code, I would think this 
> should not be
> that problem, the same scenario exists in sending the RTS, 
> CTS , data packet
> and also the ACK in IEEE 802.11. The loss of any of the control/data
> packets, which is very likely to happen, for the Unicast 
> packets, involves
> starting the whole process of sending the RTS-CTS-Data and ACK again.
> Guessing the states of other nodes in the network(locally), 
> as noted by
> Cesar, is one tough job unless the node timeouts and starts 
> all over again.

Yes, I have no problem with the above. But depending on the base station removes this from MANET territiory. It would be interesting to see this channelized concept applied to a true MANET. That would require mobile hosts to simultaneously participate in the Aloha common channel, and then establish unique circuits to forward data between other hosts, using different quasi-orthogonal codes. Hmmm.

Bert
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From: Sumeeth Nagaraj <[email-masked]>
Date: Mon, 23 Sep 2002 15:09:06 -0600

Hello Bert,

My comments inline.

> I have a knee jerk reaction, which is that spreading in IEEE 802.11b seems
inadequate to support multiple users in a single 5 MHz frequency band. In
fact, it seems that only 8 or 11 chips per symbol are used, respectively for
the 5.5 and 11 Mb/s options and for the the 1 and 2 Mb/s options.

Well, I was thinking of using the spreading code in general for manet, and
not specifically to IEEE 802.11b.

> Without digging through the details of cellular CDMA, however, the
difference does not appear to be so great? In the cellular model, the base
station negotiates the >code for each channel with each mobile unit
requesting service, on the synchronization channel(?), before the data
stream begins. Seems to me that both systems >have to start somewhere to
establish the unique code for each data stream, and the only difference here
is in the details of how the individual codes are assigned. >Thereafter, you
have non-shared channels on which data are sent, which seem dedicated to
that user and the base station for the duration of the session? Unless I
>misunderstand the concept, seems to have aspects of circuit switching.

I would agree on the point that the above two scenarios are somewhat
related. Assignment of spreading codes can also be done in different way.
Pushing this idea of having multiple codes to MANET would surely be very
challenging. One advantage of this would be to avoid sending excess control
packets like the RTS and the CTS. The requirement of an ACK packet is
somewhat mandatory as this helps in finding the uni-directional links and
also makes sure that the destination receiver is listening to what is sent.

In scenario's where the density of nodes is very high, the usage of control
signals could cause congestion in the network. The channel access time
increases drastically and the routing protocols have to adapt to these
changes dynamically? With this in mind, In my opinion, the usage of multiple
codes could lessen the burden of using control packets and because of the
existence of multiple channels, each node can operate with relatively lesser
overhead?

Best Regards,
Sumeeth

"Anyone who has never made a mistake
      has never tried anything new" --Albert Einstein.

---

From: Sumeeth Nagaraj <[email-masked]>
Date: Mon, 23 Sep 2002 15:09:06 -0600

Hello Bert,

My comments inline.

Best Regards,
Sumeeth