Subject: RE: [manet] Interference in ad hoc network

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 8 Aug 2002 18:23:17 -0400

Martin wrote:

> You assume that there is a direct path, which may not be the case.

Agreed. If there isn't a direct path, and there are lots of loud echo paths (Rayleigh propagation), you're probably close to the transmitter(s).

> > At these longer ranges, C/(N+I) --> C/N rather than C/I.
> 
> I don't quite see why the interference should become smaller over
> longer distances. Interferers may be close to the intended receiver,
> irrespective of the transmission range.

Well, perhaps you should specify the distribution of MANET hosts, then. Let's assume for the sake of argument that the distribution is even. Approximately the same density throughout the MANET. Now make the area large enough that hosts toward opposite edges of the coverage area are just out of direct communication range, even with high power transmitters. In that sort of MANET, if you reduce power, you can cause more and more hosts to be out of *direct* range of each other. Which allows clusters of hosts within this MANET that are far apart to use the same spectrum simultaneously. This happens because for a good many of the MANET hosts, the C and the I in the C/(N+I) ratio are both low with respect to N, when hosts are far apart as described. They will drop off the horizon quickly, as you reduce power.

Remember that it's not just the inverse square law that works for you here, but also the fact that signals at these freqs will be propagating into space.

You say above, "Interferers may be close to the intended receiver, irrespective of the transmission range." I assume you mean that a host is attempting to receive a signal from far away directly, and that there are close-by hosts causing interference.

Now, what happens if you reduce everyone's power? The far-away host will not be capable of reaching you directly. Those packets will be relayed. Meanwhile, the hosts closer to you will be communicating on that same frequency. *On average*, more packets will be transferred at any given time, in the aggregate. 

> If you consider the total amount of packets transmitted over 
> the lifetime
> of the battery, I agree. If energy consumption (or lifetime) 
> is not an issue,
> I don't. Then boosting power will increase the throughput.
> To be more precise, maybe we should rather speak of transport capacity
> of the network (bit-meters per second). This quantity 
> certainly increases
> with increased transmit power.

I was assuming infinite battery life. If you add in battery life, the problem becomes difficult. Low power means more hosts have to be involved in relaying packets hop by hop, which means more batteries will die without their owners even being aware they are being used. High power means shorter battery life for those actually transmitting packets, but at least there's less relaying going on because fewer hops would be involved in data paths. The article I had mentioned went into this, using more and more elaborate constraint sets:

IEEE Communications Magazine, June 2001, "Maximum Battery Life Routing to Support Ubiquitous Mobile Computing in Wireless Ad Hoc Networks," C.K. Toh, Georgia Tech, pp. 138-147.

Bert

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

---

From: John Tapsell <[email-masked]>
Date: Fri, 9 Aug 2002 14:33:33 +0100

Hi,
  I'm trying to setup the rsvp on a small test network to have a play with
it.
  I grabbed the ISI's rsvpd, but when I run it I get "No multicast daemon
found".  I don't need/want multicast - can I start it without this?
If not, how do I runa multicast daemon, and what is it?

JohnFlux

--

---

From: John Tapsell <[email-masked]>
Date: Fri, 9 Aug 2002 14:33:33 +0100

Hi,

JohnFlux

--

---

From: Sudarshan N Raghavan <[email-masked]>
Date: Fri, 09 Aug 2002 13:44:08 -500

DSR is supposed to work over unidirectional links. But how does DSR   
discover unidirectional links ?? Do we need to explicitly configure   
DSR to work over uni/bi directional links ??                          
                                                                      
Assume DSR is configured for uni-directional links:                   
1. A single RREQ broadcast can trigger multiple RREQ broadcasts from  
intermediate nodes that have a route to the destination in their cache
(assuming that the nodes are not able to find out that the source has 
already chosen a RREP)                                                
                                                                      
2. DSR expects each node along the source route to get an ack from the
next downstream node. If the links are unidirectional and the nodes do
not have a route to their upstream neighbor along the source route, it
can trigger N RREQ broadcats for a source route of length N           
                                                                      
3. The same is the case with route maintainance if the node reporting 
a link failure does not have a route to the source                    
                                                                      
This seems like an enormous number of broadcasts which has the        
potential to bring down the network if there is lot of network        
activity.                                                             
                                                                      
Have there been any studies/simulations to analyze chain reactions and
the performance of DSR when configured for unidirectional links ?     
                                                                      
Regards,                                                              
Sudarshan                                                             
                                                                      
****                                                                  
Sudarshan N Raghavan                                                  
Graduate Student, Computer Sc.                                        
North Carolina State University

---

From: Sudarshan N Raghavan <[email-masked]>
Date: Fri, 09 Aug 2002 13:44:08 -500

Regards,                                                              
Sudarshan                                                             
                                                                      
****                                                                  
Sudarshan N Raghavan