Subject: RE: [manet] doubt regarding 802.11 IFSs

From: "#LI ZHIFEI#" <[email-masked]>
Date: Mon, 8 Sep 2003 15:10:26 +0800

Hi,

My answer to your questions are as follows:

>aSlotTime = aCCATime +aRxTxTurnaroundTime + aAirPropagationTime +
>aMACProcessingDelay.

The SotTime is specified as above to ensure that the transmission by the winning node (say A) will be sensed within the same slot by the MAC layer of all the other nodes(e.g., node B) . 
Otherwise, even though nodes A and B choose different backoff values (say 6 and 7 slots, respectively), node B will transmit after node A has already begin to transmit, resulting in a collision. Note that even with the above rule for the SlotTime,  the collision between two nodes having different backoff values may not be avoided when the time is not synchronized.

>aSlotTime = aCCATime +aRxTxTurnaroundTime + aAirPropagationTime +
>aMACProcessingDelay.

To my knowledge, the SIFS should be 28 us for FHSS while it should be 10 us for DSSS in IEEE 802.11b rather than being 9 us. 

The SIFS (as well as the SlotTime) is determined by the physical layer and thus you cannot set as you want.

The SIFS is designed as above to ensure that it is equal to the minimum time required by a node to turn from the receiving state to transmit state (e.g., from receiving RTS to transmit CTS). That is also why it needs not include the aAirPropagationTime.

>>WHY aDIFS = aSIFS + 2 x aSlotTime ? why cannot it be someother value
>>say 30 microsec instead of 34 microsec in 802.11b protocol?

A->B 
  C

Consider the above scenario where a frame exchange sequence is in progress between nodes A and B while node C is deferring.
After A transmited a RTS to B, before node B sends back a CTS, it takes a SIFS time for B to turn from receiving node to transmitting node.
Then, it may take a SlotTime for node C senses the transmission of CTS. Therefore, to give priority for the ongoing transmission, the DIFS must be greater than (aSIFS + aSlotTime).
I think the reason that the standard chooses  2 x aSlotTime rather than  1 x aSlotTime is just to make the protocol more relialbe.

Moreover, the DIFS value should be 50 us in DSSS of IEEE 802.11b.

Best regards
Li zhifei

-----Original Message-----
From: T. Bheemarjuna Reddy [mailto:[email-masked]]
Sent: dimecres, 3 / setembre / 2003 15:47
To: [email-masked]
Subject: [manet] doubt regarding 802.11 IFSs

Hi all,

      In 802.11 MAC protocol, the "slot time" id defined as

  aSlotTime = aCCATime +aRxTxTurnaroundTime + aAirPropagationTime +
aMACProcessingDelay.

My doubt is why above all parameters be considered for determining "slot
time" value. In otherwords, in 802.11b protocol, SIFS is set to 9
microsec, why cannot it be someother value say 5 microsec?

Further, SIFS is defined as 

 aSIFS = aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaround
time.

My doubt is why "aAirPropagationTime" used in aSlotTime is not taken into
consideration during calculation of aSIFS value?

and 

WHY aDIFS = aSIFS + 2 x aSlotTime ? why cannot it be someother value
say 30 microsec instead of 34 microsec in 802.11b protocol?

If anybody knows, please clarify my doubts.
Thanx inadvannce.

              -----arjun

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From: "T. Bheemarjuna Reddy" <[email-masked]>
Date: Mon, 8 Sep 2003 16:14:43 +0530 (IST)

Dear Li,

          thanx for ur kind response. Based on ur reasoning i think
the following is the reason for setting DIFS as SIFS + 2*slottime.

A-->B AP(PC)
    C 

In the above scenario, nodes C and AP(PC) are in the Tx range of node B.
Since AP has higher priority (in dual mode of 802.11 protocol) it should
wait a minimum of PIFS (=SIFS + slottime) before transmitting Beacon to
initiate CFP (contention free period). If at all a transmission is
happening from B to A, AP detects it within PIFS period and defers. 

If node C wants to transmit, first it must ensure that transmission
between A and B has finished and AP is not transmitting its Beacon.

ASSUME THAT AP is NOT willing to TX a Beacon. 
Then, to know that node B has finished its transmission, node C takes
(PIFS=SIFS + slotime) period. Further, node C takes one more "slottime" to
confirm that AP is not willing to tx its Beacon (If willing, AP would have
started Tx after PIFS and node C would have detected it in a slottime that
follows PIFS). Hence DIFS = SIFS + 2*slottime.

If dual mode is not there, for pure DCF mode, i think, it is enough to
take DIFS = SIFS + slottime. Is it right?

regards
arjun 

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---

From: "Alaa Muqattash" <[email-masked]>
Date: Sun, 21 Sep 2003 19:04:50 -0700

Hi,

In [1], I believe that the RTS, CTS, Data and Ack packets are power
controlled, and not only the Data packets as many people assume.

This can be understood from the following authors statement in [1] "When the
MAC times out while waiting for a CTS or DATA or ACK message from a node, we
increment the transmit power level field by one and update the dr_pwr field"
Thus, since if the MAC times out for a  *CTS*, the node increments the
transmit power level, that means that the *RTS* is sent at a  controlled
transmit power level and not at the maximum power level.

This is quite different  from the approach of PARO in [2], where the RTS/CTS
packets are *always transmitted at maximum power*.

The reason why I bring this up is that many papers refer to the two
approaches as if they similar at the time that they are fundamentally
different. Please correct me if I'm wrong.

[1] Agarwal, S.; Katz, R.H.; Krishnamurthy, S.V.; Dao, S.K.; "Distributed
power control in ad-hoc wireless networks"   in PIMRC'01

[2] Javier Gomez, Andrew T. Campbell, "Conserving Transmission Power in
Wireless Ad Hoc Networks" ,  ICNP'01

Best regards,
Alaa Muqattash

=================================================
Alaa Muqattash
Ph.D. Student & Research Assistant
Broadband Networking Laboratory
Dept. of Electrical & Computer Engineering
The University of Arizona
Rm. 109, ECE Bldg.
Tucson AZ 85721
Tel  : (520) 626-9249
Fax  : (520) 621-3862
Email : [email-masked]
Web: http://www.ece.arizona.edu/~alaa/

"Imagination is more important than knowledge"
Albert Einstein
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