Subject: Re: Does 802.11b do multi-hop well?

From: "Dr. Chane L. Fullmer" <[email-masked]>
Date: Thu, 08 Nov 2001 15:35:42 -0800

At 09:25 AM 11/8/2001 -0500, Phillip Neumiller wrote:
>Hi,
>
>In the June 2001 IEEE Communications Magazine, 802.11 is berated for
>extremely poor performance as a multi-hop MAC.  Has anybody got any
>further evidence to either corroborate or show counter examples to
>the claims that are made.  These claims are:
>
>1).  It is pointed out that 802.11 still has the hidden node problem
>      when configured as a multi-hop network.

The work JJ pointed out provides details, but basically 802.11 fails in 
multi-hop configurations because the CTS from the receiver (which provides 
the notice to potential interferers not to transmit) may not be heard by 
all receiver's neighbors, and as such, allows them to transmit and clobber 
the packet destined to the receiver.

>2).  The exposed node problem is not handled at all by the 802.11 MAC.

Here, the RTS from the sender blocks potential transmitters around it, even 
if they could transmit and not interfere at the intended receiver.  There 
can be no more than one sender in any given 1-hop neighbor hood, even if 
their receivers are isolated from the others transmissions.   Additionally, 
there can only be one receiver in a 1-hop neighbor hood, as the first CTS 
would block any additional receivers from sending their own CTS to 
acknowledge an RTS for itself.

>3).  Carrier Sensing as used in 802.11 is bad since the sensing and
>      interference range is greater than the effective communication
>      range.

It is true that the carrier detect time is much greater than the one-way 
propagation delay, which in turn increases the vulnerability period and 
eats away at performance.  But I wouldn't say Carrier Sense is a bad thing 
-- certainly better than no carrier sense ala ALOHA/MACA/MACAW..

I think scheduling is one way to avoid using carrier sense, but this 
requires synchronized radios, and that has it's own problems..

>4).  The binary exponential back off always favors the latest successful
>      node leading to severe unfairness.

Well, in 802.11 I believe that as a node is more successful, it in turn 
lowers it's backoff value.. making it easier to get back in the next time, 
while its neighbors that failed will increase their time and have less of a 
chance to get in.. leading to an unfair situation..

>Any anecdotal information is welcome.  I feel this discussion is important
>to the MANET list since many researchers use various forms of 802.11 in
>their tests beds.

Hope this is helpful..
--chane

---

From: "Samir R. Das" <[email-masked]>
Date: Thu, 8 Nov 2001 20:41:47 -0500

I believe what Phil wants to point to in this item is a little
different. He probably means that carrier sense range is higher
than packet reception range. This means that a node can hear carrier from a
far away transmitter, but cannot receive a packet successfully
from this transmitter. This is because carrer sense threshold
and packet reception threshold are different at the receiver.

I don't know whether this is really a property of 802.11 or
property of certain radios like wavelan. Having a higher
carrier sense range gives better immunity from hidden terminals,
but obviously reduces spatial reuse.

Make that range twice that of the packet reception range
and you don't even need RTS/CTS for avoiding hidden terminals!

Samir

---

From: Yang Xiao <[email-masked]>
Date: Fri, 9 Nov 2001 08:35:48 -0800 (PST)

I would like to know whether there is a pratical
protocal that can avoid exposed node problem by using
the fact that carrier sense range is higher than
packet reception range without too much unconvenience.

Moreover, for a multi-hop ad hoc network with dynamic
topology change, I am wondering whether it is
worthwhile doing such an effort since it will decrease
immunity for hidden terminals like you said,it will
suffer the tradeoff of the comlexity and benefits, and
it might be difficult to make precise and pratical
judgement in a dynamic envirement. A wrong judgement
will cause collisions that will will waste more
resource.

I think that the paper's authors unfairly treat IEEE
802.11 MAC,and they blame 802.11 MAC more than 6 times
throughout their paper.This might offend some readers.

I would like to raise an question: when and where does
the TCP works very well in a wireless envirement in
pratice? Please let know.

Yang

--------------------
Yang Xiao, Ph.D.
Micro Linear Co.

--- "Samir R. Das" <[email-masked]> wrote:
> > eats away at performance.  But I wouldn't say
> Carrier Sense is a
> > bad thing

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---

From: Kui Wu <[email-masked]>
Date: Fri, 9 Nov 2001 10:54:53 -0700 (MST)

> packet reception range without too much unconvenience.

I think carrier sense range is larger than reception range is a natural
thing due to the fact that the power requirements are different. It might
be possible to alleviate the  hidden & exposed terminal problems
if a node has larger sense range. (Intuitively, that node could have more
knowledge about its environment)

But I do not know which protocol utilizes that property to avoid
hidden & exposed terminal problems (Please tell me if there are). You are
right. Before any effective MAC protocol appears, we should not be too
picky about IEEE 802.11.

Kui

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