Subject: RE: [manet] MAC layer

From: "#LI ZHIFEI#" <[email-masked]>
Date: Mon, 24 Nov 2003 13:13:13 +0800

Hi,

I think your question is: why CDMA is not used in ad hoc networks? I also try to figure out the reasons.
I have the following reasons: 
(1) since ad hoc network is fully distributed, how to make the code orthogonal, and how to make the sender be aware of the code it should use before transmitting?
(2) again, due to the distributed nature, how to do the power control to cope with the inteference which is ensential for cdma system?

However, the above difficulties do not mean that we should not try to apply CDMA into ad hoc networks. In fact, there is a paper talking about CDMA in ad hoc networks 
in mobihoc 2003. The author is also on this list, and maybe he can give us his understanding.

Also, it seems that in the literature about packet radio networks ( ad hoc networks developed for military use during 1970s), they normally use CDMA for security reason (i.e., jamming as you have mentioned). However, now there are very few papers talking about CDMA in ad hoc networks. Reasearch is really a little bit strange :).

Best regards
Li zhifei

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Ioannis C. Avramopoulos
Sent: dissabte, 22 / novembre / 2003 12:33
To: [email-masked]
Subject: [manet] MAC layer

Dear fellows,

I have a question on the MAC layer used in ad hoc networks.

In 802.11 nodes share the medium, i.e. they use the same
frequency hopping pattern or chip sequence. Had nodes used
orthogonal hopping patterns, spectrum utilization would
significantly increase and, furthermore, the impact of
malicious nodes with respect to jamming would be severely 
mitigated.

I am interested in the reasons that schemes of the latter
type are not considered in the ad hoc community. I would
appreciate it if you could point me to any papers discussing
such issues.

Kind regards,
Yannis

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---

From: "Chi-Hsiang Yeh" <[email-masked]>
Date: Mon, 24 Nov 2003 01:57:02 -0500

Hi, Zhifei,

I have the following reasons: 
(1) since ad hoc network is fully distributed, how to make the code
orthogonal, and how to make the sender be aware of the code it should
use before transmitting?

Transmissions cannot be made orthogonal in MANET even if orthogonal
codes are known and used by nearby nodes and all nodes are perfectly
synchronized. The reason is that propagation delays will make the
beginning of codes arrive at a node at different times.

Some previous papers assumed that nearby clusters used orthogonal codes
and would not interfere with each other. Such assumptions are not valid
when the chip rate is moderate or high (e.g., >= 1 M chips per second).

But in my opinion, this will not necessarily prohibit CDMA from being
used in ad hoc networks. You can still use a set of codes that have
small cross-correlation.

Regards,

Chihsiang

---

From: Ali Nabi Zadeh <[email-masked]>
Date: Mon, 24 Nov 2003 10:14:35 -0500

Hi Li,

> (1) since ad hoc network is fully distributed, how to make 
> the code orthogonal, and how to make the sender be aware of 
> the code it should use before transmitting?

Transmissions from different users cannot be synchronous due to
propagation delay and multipath fading. Even in the downlink of the
current cellular networks, where a base station can use synchronous CDMA
for transmissions to different users, due to multipath fading, the
receiver observes several replicas of the whole set with different
delays. In effect the final reception is not synchronized. So, I don't
think this is a major problem in Ad hoc networks. Sousa and Silvester
have a paper on how to assign spreading codes in this environment.

E. S. Sousa and J. A. Silvester, "Spreading code protocols for
distributed spreadspectrum
packet radio networks," IEEE Transactions on Communications, vol. 36,
no. 3, pp. 272-281, Mar. 1988.

> (2) again, due to the distributed nature, how to do the power 
> control to cope with the inteference which is ensential for 
> cdma system?

Again there are some old papers on using this structure without power
control, however I've not seen the exact performance measurment of this
system. 
So what is the drawback of using a channel inversion power control in a
master-slave environment? For example in a bluetooth system, each master
node can act as a base station with power control capability. Actually,
we have used this method for performance modeling of this structure.

A. N. Zadeh, B. Jabbari, R. Pickholtz, and B. Vojcic, "Self-organizing
packet radio ad
hoc networks with overlay (SOPRANO)," IEEE Communications Magazine, vol.
40,
no. 6, pp. 149-157, June 2002.

Best regards
Ali Zadeh.

---

From: Rex Buddenberg <[email-masked]>
Date: 24 Nov 2003 13:47:01 -0800

You may find the question clearer if you look at it from a security
vulnerability point of view.

But first, we need to sort out layers properly.  CDMA in packet switched
radio networks is a physical layer function not a MAC layer one.  Per
802 committee usage:
	- LLC sublayer (802.1, 802.2)	\data link layer
	- MAC sublayer			/
	- PMI sublayer			\physical layer
	- Phy sublayer			/

CDMA (or FDMA or QAM) all fall in the Phy sublayer (or some purists
would argue below that).

MAC, by contrast, has to do with how stations take turns.

Now to the security questions.  What security attributes to we need at
each layer to provide a complete solution?
	- at the MAC sublayer, the vulnerability to be worried about is traffic
analysis and traffic flow analysis.  The keys to that vulnerability are
the MAC addresses (in ethernet terms, the factory-assigned hardware
addresses).  One proper approach to plugging this vulnerability is to
encrypt these addresses is link encryption.  
	- at the Phy sublare the vulnerabilities include Low Probability of
Intercept/Detection and jam resistance.  In commercial 802.11 jam
resistance was properly considered; LPI/LPD was not.  The single direct
sequence spread spectrum code addresses jamming, but any netstumbler can
find the correct PN code and intercept.  Addition of more spreading
codes would complicate the LPI/LPD problem (and probably complicate
system configuration as well).  

	(for completeness, layer 3 security measures provide for enclaving or
perimeter control.  Note that so far, all these measures protect the
_infrastructure_, not the data directly.
	Layer 7 protections (SSL, ssh, S/MIME, all with PKI) are protections of
the data and are consequently media-independent and can be arranged
end-to-end -- something you can't do with any of the lower layer
remedies.)

-- 
b