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If I understand correctly, ACK frames on either 802.11b or 802.11g will be transmitted at the speed satisfied both conditions :
- highest speed that belongs to the BasicRateSet, *and*
- smaller or equal to the speed of the frame that this ACK frame acks.

I make some tests with three Cisco APs ( that support 802.11b/g ) which have basic rates to set to 1 and 2, and all others ( from 5.5 to 54 ) are optional. For some reason, the ACK frames ( which acknowleges the data frames transmitted at 54Mbps) from these AP are transmitted at 24. I thought these ACKS should be transmitted at only 2mbps because it is the highest speed of Basic Rates.

Do I misunderstand the standards somewhere ?


Thanks,

Vu

Vu Pham,

I haven't reviewed this in detail and compared it to what you are reporting but it seems like it may be relevant. You may be seeing the highest mandatory rate that uses the same modulation. This is from the 802.11g-2003 amendment.

Clause 9.6 Multirate support:

To allow the transmitting STA to calculate the contents of the Duration/ID field, a STA responding to a received frame shall transmit its Control Response (either CTS or ACK) frames at the highest rate in the BSSBasicRateSet that is less than or equal to the rate of the immediately previous frame in the frame exchange sequence (as defined in 9.7) and that is of the same modulation type as the received frame. If no rate in the basic rate set meets these conditions, then the control frame sent in response to a received frame shall be transmitted at the highest mandatory rate of the PHY that is less than or equal to the rate of the received frame, and that is of the same modulation type as the received frame. In addition, the Control Response frame shall be sent using the same PHY options as the received frame, unless they conflict with the requirement to use the BSSBasicRateSet.

An alternative rate for the control response frame may be used, provided that the duration of the control response frame at the alternative rate is the same as the duration of the control response frame at the originally chosen rate and the alternative rate is in either the BSSBasicRateSet or the mandatory rate set of the PHY and the modulation of the control response frame at the alternative rate is the same type as that of the
received frame.

Clause 19.1.2 Extended Rate Phy Specification-Introduction:

The ERP builds on the payload data rates of 1 and 2 Mbit/s, as described in Clause 15, that use DSSS
modulation and builds on the payload data rates of 1, 2, 5.5, and 11 Mbit/s, as described in Clause 18, that use DSSS, CCK, and optional PBCC modulations. The ERP draws from Clause 17 to provide additional
payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. Of these rates, transmission and reception
capability for 1, 2, 5.5, 11, 6, 12, and 24 Mbit/s data rates is mandatory.

Regards,

moe

moe Escribi?3:

[...]
An alternative rate for the control response frame may be used, provided that the duration of the control response frame at the alternative rate is the same as the duration of the control response frame at the originally chosen rate and the alternative rate is in either the BSSBasicRateSet or the mandatory rate set of the PHY and the modulation of the control response frame at the alternative rate is the same type as that of the
received frame.
[...]
Of these rates, transmission and reception
capability for 1, 2, 5.5, 11, 6, 12, and 24 Mbit/s data rates is mandatory.

moe

Thanks, Moe. So I think in the case with these APs, they use the alternative set.

One more question : does the above statement of alternative rate apply to 802.11b ?

The reason I ask is when I use the 802.11b cards, the APs reply the ACK at the speed of 11Mbps when the beacons show that only 1 and 2 are in the basic rate set.

( I have nothing to do with Ciso, the reason I often ask about Cisco APs is that my company uses Cisco devices for the company network and for customers therefore most of my testing equipments are from Cisco )

Thanks,

Vu

Vu Pham,

If you are using b/g APs with b only cards then I suspect that the reason the AP replies with an ACK at 11 Mbps is because that is the rate at which the STA sent the data frame. 11 Mbps is a mandatory rate.

1 and 2 Mbps are DSSS-DPSK modulated.
11 Mbps is HR/DSSS-CCK modulated.

To allow the transmitting STA to calculate the contents of the Duration/ID field, a STA responding to a received frame shall transmit its Control Response (either CTS or ACK) frames at the highest rate in the BSSBasicRateSet that is less than or equal to the rate of the immediately previous frame in the frame exchange sequence (as defined in 9.7) and that is of the same modulation type as the received frame.

The basic rates of 1 and 2 Mbps are not the same modulation as the 11 Mbps rate. So you are following the next part:

If no rate in the basic rate set meets these conditions, then the control frame sent in response to a received frame shall be transmitted at the highest mandatory rate of the PHY that is less than or equal to the rate of the received frame, and that is of the same modulation type as the received frame.

Data Frame sent at 11Mbps HR/DSSS; ACK sent at 11Mbps HR/DSSS. Mandatory rate requirement met; Modulation requirement met.

Regards,

moe

Moe, thanks a lot for your explanation. I didn't pay attention to the modulation.

Vu