Subject: [manet] turning off RTS/CTS

From: "D Kothris, Electrical & Electronic Engineering" <[email-masked]>
Date: Fri, 10 Oct 2003 10:36:09 +0100

Hi all,

There is a point for which i would like help to clearify:

Is it common practice to turn off the RTS/CTS handshake part of the 802.11b 
DCF MAC protocol, due to high collisions?

If this is the case then why use the protocol at all-since it is also 
complicated to implement?

Cheers

Dimos

_______________________________________________________________________
***********************************************************************
Dimosthenis Kothris	      Centre for Communications Research,
			      University of Bristol,
			      Network & Protocols group

Address:		      Contact numbers: 	
_______________________________________________________________________
Network & Protocols
Center for Communcations research
Merchant Venturers Building         Tel   :            +44 117 903 1104
Woodland Road                       Fax   :            +44 117 954 5206
Bristol BS8 1UB                     Mobile:           +44 7779 40 48 42
United Kingdom                      Email :     [email-masked]
***********************************************************************

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

---

From: [email-masked]
Date: Fri, 10 Oct 2003 04:57:11 -0500 (CDT)

hi;
RTS/ CTS are two parts of 4-way handshake mechanism that is used to sense
and dected the channel to send  packets. i do not think you should turn them
off.. because they are essential part of DCF

-salah

---

From: Justin Lipman <[email-masked]>
Date: Fri, 10 Oct 2003 20:05:25 +1000

Hi,

> and dected the channel to send  packets. i do not think you should turn
> them off.. because they are essential part of DCF

Im not sure if its part of the spec, but I think RTS/CTS/NAV are not used for 
small data frames. I would be interested to know what the threshold value for 
this is. It makes sense for smaller data frames, but for large frames, it 
will obviously increase collisions. 

Cheerio,
Justin

-- 
PhD Candidate
Telecommunications & Information Technology Research Institute
School of Computer, Electrical & Telecommunications Engineering
University of Wollongong, Australia 
http://www.titr.uow.edu.au/~justin

---

From: Justin Lipman <[email-masked]>
Date: Fri, 10 Oct 2003 20:13:14 +1000

Hello,

On Fri, 10 Oct 2003 07:36 pm, D Kothris, Electrical & Electronic Engineering 
> complicated to implement?

I think the whole point of the RTS/CTS/NAV is to alleviate problems during 
situations with high contention and to reduce problems associated with hidden 
nodes etc... Therefore, I doubt its common practice. As in my previous email. 
RTS/CTS is not used for smaller data frames as far as I understand.  But what 
the byte threshold is, I do not know. 

Perhaps someone on the list can provide more insight...

regards,
Justin

---

From: "D Kaleshi, Electrical & Electronic Engineering" <[email-masked]>
Date: Fri, 10 Oct 2003 14:47:47 +0100

Hi Dimos,

> Is it common practice to turn off the RTS/CTS handshake part of the
> 802.11b DCF MAC protocol, due to high collisions?

I understand that this is the normal practice, especially in non-enterprise 
networks - makes life easier in terms of deployment - but I would like very 
muhc to hear if it is otherwise.

One argument is that activating RTS/CTS at an access point does not make 
much sense as every node around it is covered (which is not always true - 
tertiary collisions are still possible). Practical advice is to play around 
with the values of RTS/CTS threshold (value range in 0-2347 bytes, with 
default being 2347) ONLY if it is an issue (after traffic measurements 
indicate a high level of collisions due to packet sizes); the 
recommendation is to set it to circa 1500bytes and see if the throughput in 
the network throughput improves.

According to the 802.11 spec the value (in the 802.11 MAC MIB) is 
controlled by the dot11RTSThreshold attribute, and it can be set on a 
per-node basis. But even if a node has RTS disabled, it still has to react 
to received RTS/CTS sequences by updating its NAV, and it must always 
respond to an RTS packet addressed to it even if RTS is disabled. I believe 
the default value for dot11RTSThreshold in the IEEE 802.11 spec is 2347 
bytes (i.e. by default the RTS/CTS is disabled, as per spec).

> complicated to implement?

To solve the hidden terminal problem (tertiary collisions). And possible 
better performance for TCP traffic over RTS-enabled networks (there has 
been research work reported on this). This is probably the main reason why 
you would find it more likely to be enabled in enterprise networks.

cheers,

	Dritan
> Dimos

--------------------------------------------------------
Dritan Kaleshi
Department of Electrical and Electronic Engineering
University of Bristol, UK
--------------------------------------------------------

---

From: Dave Maltz <[email-masked]>
Date: Fri, 10 Oct 2003 09:51:42 -0400

>will obviously increase collisions. 

I was amused to learn that RTS/CTS is completely disabled by default in
many popular 802.11b drivers under windows.

For example, if you dig down through the configuration layers of the
Orinoco driver, you will find the following radio check box:

RTS/CTS Medium Reservation

[X] Off (default)
[ ] On  (use when troubleshooting poor performance)

According to the help pages, "off" means completely off: never send RTS pkts.

Considering the prevalence of 802.11b machines running windows, I've often
wondered whether we should all just turn off RTS/CTS exchange in our
simulations in order to make their results more applicable to the real world.

Cheers,
 -dam

---

From: Justin Lipman <[email-masked]>
Date: Sun, 12 Oct 2003 13:32:08 +1000

> many popular 802.11b drivers under windows.
....
> simulations in order to make their results more applicable to the real

Curiously... against better judgement (allthough I know RTS/CTS is not 
perfect), is there some sane obscure reason for this? :-)

I wonder if this is done to improve the performance (throughput) ratings 
provided by many computing mags when evaluating 802.11 cards. They seem to 
only perform simple two node performance tests as not everyone has a base 
station.

cheers,
Justin