Details on CWmin and CWmax (Contention Window Minimum and Maximum)

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CW = Contention Window. CWmin and CWmax mean Minimum Contention Window and Maximum Contention Windows. Let’s understand where we need these two parameters.

If you have already gone through the CSMA/CA post then it will be easy to understand where these are used. In CSMA/CA flow chart there is one step

=> “Select a random backoff value if one is not chosen already”.

This random backoff value is chosen from the defined CWmina and CWmax value for device.

♣ CWmin and CWmax table:

PHY	CWmin	CWmax
802.11b	31	1023
802.11g	15 or 31	1023
802.11a	15	1023
802.11n	15	1023
802.11ac	15	1023

♣ Example to understand easily:

Let’s say one device (802.11a supported) wants to send a frame.

According to the table,

CWmin=15 and Cwmax is 1023.

♦ 1^st attempt: This device has option to select a random value form

0 – 15 and following CSMA/CA rule the selected random value will be decremented for each slot time [As long as Wireless Medium is free].

♦ 2^nd attempt: If the 1^st attempt is failed then this device has below CW range.

0 – 31.   Why ☻?

Each time a device is failed to transmit any frame the CW window will be increased exponentially keeping the CWmax constant. As the device failed on 1^st attempt so the new CW range for choosing random backoff number is 0-31.

For any further failure the CW range will be

0-63

0-127

0-255

0-511

0-1023(CWmax)

♠ Note: Minimal value of CW is 0 but Maximum value of CW at any particular time always varies.

♣ When device stops transmitting a frame if there is contentious (nth attempt) failure?

In simple there below conditions:

♦ CWmax is reached as CW range. This means when device gets the CW range as 0-CWmax.

♦ Maximum retry count is reached for a particular frame.

♣ Understand exponential behavior:

We generally use ECWmin and ECWmax for representing “exponential CWmin” and “exponential CWmax”.

PHY	CWmin	CWmax	ECW Min	ECW Max
802.11b	31	1023	5	10
802.11g	15(bg) or 31 (g)	1023	4 or 5	10
802.11a	15	1023	4	10
802.11n	15	1023	4	10
802.11ac	15	1023	4	10

Let’s understand 802.11b row: 2⁵-1=31, 2¹⁰-1=1023

The CW range for 1^st attempt for 802.11b is: 0-31

The CW range for 2^nd attempt for 802.11b is: 0-(2⁶-1) = 0-63

The CW range for 3^rd attempt for 802.11b is: 0-(2⁷-1) = 0-127

Following this we can do the calculation for others easily.

♣ Where do we see CWmin, CWmax values in sniffer capture?

In Beacon or Probe Response we can see this value under WMM field.

♣ CWmin and CWmax values for WMM ACs:

Let’s assume CWmin = 15 and CWmax = 1023 for below table.

AC	AIFSN	CWmin	CWmax
BK	7	CWmin=15	CWmax=1023
BE	3	CWmin=15	CWmax=1023
VI	2	(CWmin+1)/2 – 1 =7	CWmin=15
VO	2	(CWmin+1)/4 – 1 =3	(CWmin+1)/2 – 1 =7

Here is screenshot for above values advertised in Beacon.

In another way:

Initial CW range for

BK: 0-15

BE: 0-15

VI: 0-7

VO: 0-3

With above configurations we can easily understand how VO,VI are given highest priority for transmitting frames.

♣ Conclusion:

From above article we understand where CWmin and CWmax are used and what the importance of these values are.

♥♥ If you have any doubts or query please let me know in comment section or send mail at feedback@wifisharks.com. ♥♥