Subject: RE: [manet] Carrier sense threshold/range control

From: "#LI ZHIFEI#" <[email-masked]>
Date: Thu, 10 Jul 2003 22:06:21 +0800

>What you say is correct, but only to a point and with a reservation.
>First of all, although you can avoid collisions on the data channel, I
>don't think you can avoid them completely on the busy tone channels.

As for my understanding, in the busy channels, other potentailly interfering nodes need not interpret the content but just sensing the existence of busy carrier.
So what do you mean for the collisions on the busy channel? 

>The more significant problem is that you are reserving part of your
>available spectrum for signaling.  Whether this is done with a separate
>radio channel or with time division on a single channel, you are
>allowing only a fraction of the allotted band for data.  Thus, you need
>to determine how much of the channel(s) need to be reserved for
>signaling.  This would be a design matter and probably a tuning
>parameter, since the distribution of message lengths would determine the
>optimal signaling/data ratio.

>This is not to say that such a thing is a bad idea, just that if not
>done properly, it could be an inefficient use of the available spectrum.

I accept your viewpoint without any doubt. In addition to the issue about the spectrum efficience, complexity may be another issue by introducing multiple channels.  

Best regards

Li zhifei

-----Original Message-----
From: John Mullen [mailto:[email-masked]]
Sent: dijous, 10 / juliol / 2003 21:51
To: [email-masked]
Subject: RE: [manet] Carrier sense threshold/range control 

Hi Li,

What you say is correct, but only to a point and with a reservation.
First of all, although you can avoid collisions on the data channel, I
don't think you can avoid them completely on the busy tone channels.
The more significant problem is that you are reserving part of your
available spectrum for signaling.  Whether this is done with a separate
radio channel or with time division on a single channel, you are
allowing only a fraction of the allotted band for data.  Thus, you need
to determine how much of the channel(s) need to be reserved for
signaling.  This would be a design matter and probably a tuning
parameter, since the distribution of message lengths would determine the
optimal signaling/data ratio.

This is not to say that such a thing is a bad idea, just that if not
done properly, it could be an inefficient use of the available spectrum.

John Mullen

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
#LI ZHIFEI#
Sent: Wednesday, July 09, 2003 11:29 PM
To: [email-masked]
Subject: RE: [manet] Carrier sense threshold/range control 

It seems that the RTS/CTS handshake in single channel cannot elimate the
hidden-terminal and esposed-terminal problem. 
But the DBTMAC seems to solve these two problems completely with two
additional busy tone channel (rather than send the control frames in the
same data channel).

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

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

---

From: "John Mullen" <[email-masked]>
Date: Thu, 10 Jul 2003 12:05:00 -0600

Hi Li,

I'm not entirely sure how you use the busy tone channels, so my
statement may not be correct.  However, a fundamental problem is in how
a terminal gets the right to use a channel.  If terminals transmit RTSs
as the need arises, there is always a chance that those of different
terminals will collide with each other.  This contention will have to
occur somewhere, whether on the signaling channel or the data channel.
So, wherever this signaling occurs is where the problem will be.

Another problem is the short, but significant, transmission delay.  A
terminal may sense that the busy channel is idle and begin to transmit,
unaware that another terminal has made the same decision.  Because the
carriers takes a finite, though brief, time to reach the other terminal
and there is inherent delay in the sensing and control processes, by the
time the carrier could be sensed, the terminal will already be sending.

Later,
John Mullen 

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
#LI ZHIFEI#
Sent: Thursday, July 10, 2003 7:06 AM
To: John Mullen
Cc: [email-masked]
Subject: RE: [manet] Carrier sense threshold/range control 

>don't think you can avoid them completely on the busy tone channels.

As for my understanding, in the busy channels, other potentailly
interfering nodes need not interpret the content but just sensing the
existence of busy carrier. So what do you mean for the collisions on the
busy channel? 

>parameter, since the distribution of message lengths would determine 
>the optimal signaling/data ratio.

>done properly, it could be an inefficient use of the available 

I accept your viewpoint without any doubt. In addition to the issue
about the spectrum efficience, complexity may be another issue by
introducing multiple channels.  

Best regards

Li zhifei

---

From: "Chi-Hsiang Yeh" <[email-masked]>
Date: Thu, 10 Jul 2003 16:37:55 -0400

Hi, John,

> Hi Li,
> 
> I'm not entirely sure how you use the busy tone channels, so 
> my statement may not be correct.  However, a fundamental 
> problem is in how a terminal gets the right to use a channel. 
>  If terminals transmit RTSs as the need arises, there is 
> always a chance that those of different terminals will 
> collide with each other.  This contention will have to occur 
> somewhere, whether on the signaling channel or the data 
> channel. So, wherever this signaling occurs is where the 
> problem will be.

What you said is correct in that contention will occur somewhere. But
DBTMA uses busy tone and such busy tone does not have the collision
problem (as Zhifei pointed out). Using this and other properties, data
packet transmissions are collision-free in DBTMA even when some RTS/CTS
messages are collided, "under certain assumptions" (e.g., when the
following Problems (1-3) do not exist). We have another protocol (that
does not require busy tone) where data packets and RTS/CTS control
messages are both collision-free, under certain assumptions. See
 
You, T., C.-H. Yeh, and H. Hassanein, "A new class of
collision-prevention MAC protocols for ad hoc wireless networks," ICC,
May 2003.

When there are two separate channels for data packets and control
messages, the hidden terminal problem and the exposed terminal problem
can both be solved easily and simultaneously, but only theoretically. In
practice, due to the following problems DBTMA and our protocol are not
really collision-free, and most previous MAC protocols (including DBTMA
and our ICC'03 protocol) still suffer from some forms of  hidden/exposed
terminal problems:

(1) The propagation properties of the control channel and data channel
may be different. As a result, a node that receives a CTS message
actually may not collide the associated reception (and thus a form of
the exposed terminal problem exists). Moreover, a node that did not
receive a CTS message may actually collide the associated reception (and
thus a form of the hidden terminal problem exists). No matter how you
adjust the power levels of the control channel and busy tone, one or
both of the problems exist.

(2) In most papers it is assumed that transmission radius (TR) =
interference radius (IR). When IR > TR, then a node with distance
between TR and IR will collide a reception. This problem might be fixed
when you have separate channels -- by sending RTS/CTS messages with a
radius at least IR or at an appropriate power level.

(3) Assume that (1) and (2) are not problems. Then two or more nodes
that did not receive a CTS message may still collide the associated
reception due to their "additive interference" (and thus a form of the
hidden terminal problem exists). This problem is more difficult to solve
than Problem (2). PCMA might "solve" this problem, but it suffers from
Problem (1) and overhead/complexity/hardware cost.

(4) When two or more nearby nodes transmit busy tone simultaneously, the
strength received at a node is increased. So a node that should not have
received any busy tone may actually detect such "additive busy tone" and
defer its transmissions (and thus a form of the exposed terminal problem
exists).

Note that the above problems are not the hidden terminal problem and the
exposed terminal problem identified in the Tobagi and Kleinrock paper
(in IEEE Trans. Communications, 1975). They have some similarity in
principle, but are not exactly the same problems.

It's not impossible to solve all these problems, but the resultant
protocol will not necessarily outperform existing protocols due to its
overhead, and the resultant hardware may be more expensive.

Chihsiang

___________________________________________________________

Chi-Hsiang Yeh (Mr./Dr./Prof.)
Dept. of Electrical & Computer Eng.
Queen's University
Kingston, Ontario, K7L 3N6
Canada

phone:  +1 613-533-6368
fax:    +1 613-533-6615
e-mail: [email-masked] , [email-masked]
URL:    http://www.ece.queensu.ca/hpages/faculty/yeh/
___________________________________________________________

> Another problem is the short, but significant, transmission 
> delay.  A terminal may sense that the busy channel is idle 
> and begin to transmit, unaware that another terminal has made 
> the same decision.  Because the carriers takes a finite, 
> though brief, time to reach the other terminal and there is 
> inherent delay in the sensing and control processes, by the 
> From: [email-masked] [mailto:[email-masked]] On 
> Behalf Of #LI ZHIFEI#
> As for my understanding, in the busy channels, other 
> potentailly interfering nodes need not interpret the content 
> but just sensing the existence of busy carrier. So what do 
> you mean for the collisions on the busy channel? 
> 
> 
> I accept your viewpoint without any doubt. In addition to the 
> issue about the spectrum efficience, complexity may be 
> another issue by introducing multiple channels.  
> 
> 
> Best regards
> 
> Li zhifei
> 
> 
>

---

From: Jing Deng <[email-masked]>
Date: Thu, 10 Jul 2003 16:58:02 -0400 (EDT)

Dr. Mullen,

I agree with you. It will be very difficult to eliminate collisions when
distributed nodes (such as those in MANETs) are competing for a single
shared channel. Due to signal propagation delays, carrier sensing delays,
and processing delays, collisions will still exist even if carrier sensing
technique is used (as you have assumed).

However, if we use the busy tone or signaling channels properly, data
packet collisions can be avoided. Only channel requests (RTS packets) are
subject to possible collisions. Since the RTS packets are of much shorter
length, performance gain of throughput can still be achieved.

In what ratio the single channel is split affects the performance of the
MAC scheme significantly (as you pointed out in your earlier post),
especially when the RTS/CTS dialogues are to be transmitted on the control
channel. We have found some interesting results and shown them in the
following paper:

J. Deng, Y. S. Han, and Z. J. Haas, "Analyzing Split Channel Medium Access
Control Schemes with ALOHA Reservation," ADHOC-NOW '03, Montreal, Canada,
October 8-10, 2003. (to appear)

I will be happy to share it with anyone who is interested.

Cheers,

Jing

-----------------------------------------------------------
Dr. Jing Deng               http://www.cis.syr.edu/~jdeng01
2-191 CST, CASE Center      Email: [email-masked]
Dept. of EECS               Phone:  (315) 443-3029
Syracuse University         Fax:    (315) 443-4745
Syracuse, NY 13244
--------------------------- @_@ ---------------------------