Subject: RE: [manet] 802.11

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 9 May 2002 10:00:15 -0400

> -----Original Message-----
> From: Miguel Sanchez [mailto:[email-masked]]
> 
> Hi,
> 
> > 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?
> 
> 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).

This sounds right to me. In CDMA cellular telephones, multiple spreading codes are used, because each mobile sets up a different connection with the base station (and from there, the PSTN). You don't want two mobiles interfering with one another. You don't want two conversations to be superimposed. The connections of other phones sharing the same frequency band appear as background noise (the codes are not completely orthogonal).

But IEEE 802.11 is meant to be Ethernet-like, in the sense that every mobile host is _supposed_ to hear the others in that same wireless LAN. The LAN's bandwidth is shared by collision detection and backoff, rather than by assigning different codes to each mobile. Much as the medium is shared in a wired Ethernet.

Using different codes would create multiple wireless LANs.

> > 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?
> 
> 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.

Sounds right to me as well. Any kind of spread spectrum is a mechanism to allow sharing a given frequency band. The only question here is whether or not the users of _an_ IEEE 802.11 network want to hear on another or not, and I believe the answer is that they must hear one another.

Bert
[email-masked]

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