Forum

Hi,
11b and 11g introduces some capability bits. Standard does not clearly define the validation criteria for these bits.

For example,

(DSS-OFDM is a optional modulation)
We have a station which implements DSS-OFDM and hence it sets the corresponding capability bit in the association request.Now whether the AP should reject the association request or accept it and send association response with DSS-OFDM bit unset.

Suppose if we reject the association, will it violate the standard?

Status code:
I am not able to understand the Status code (What it means and where it can be used?)

Status code 25, 26:

25 - Association denied due to requesting station not supporting Short Slot time.
26 - Association denied due to requesting station not supporting DSS-OFDM option.

Both are optional fields, so how can we deny a association based on these
options.I could not imagine a scenario where they can be used.

Please help me to understand the standard better......
(Currently iam working on 11g (testing))

With regards,
Murali

Murali,

I am watching this post to see what the experts think. I read the standard and it appears to me too that the DSSS-OFDM and short slot times are optional and that a BSS should adjust if a STA trying to associate does not support these capabilities. Like you I do not understand the reason for the status codes.

Can someone out there help clarify this?

Thanks,

moe

Hi Murali and Moe:

IEEE 802.11m section 7.3.1.4 Capability Information field
"The Capability Information field contains a number of subfields that are used to indicate requested or advertised optional capabilities."

Depending on the capability and the source a "1" will mean either "is allowed," "is supported," or "is required"; while a "0" will mean either "is disallowed," "is not supported," or "is not required".

A client offering to associate that says "short slot time capability=1" is saying "I support both short and long slot times," while 0 means "I can't operate with short slot times."

A beaconing AP that says "short slot time capability=1" is saying "I require all members of this BSS to use short slot time until further notice," while 0 means "I require all members of this BSS to use long slot time until further notice."

A client offering to associate that says "DSSS-OFDM capability=1" is saying "I support but do not require DSSS-OFDM," while 0 means "I can't operate with DSSS-OFDM."

A beaconing AP that says "DSSS-OFDM capability=1" is saying "I support but do not require DSSS-OFDM," while 0 means "DSSS-OFDM is not allowed in this BSS."

Some policies don't map neatly to capability information field bits. For instance an AP could be configured to disallow associations from clients that require long slot times. The client can't know this until it attempts association, fails, and reads the status code in the association response frame.

"Optional" means that vendors are not required to implement the capability to be compliant with the standard. Some options have never been implemented. DSSS-OFDM might be one of them.

I hope this helps. Thanks. /criss

Hi Criss and Moe,

Thanks for your support.

For past few days iam struggling to understand the preamble concept. Since it is closely related to physical layer, iam not able to understand it through protocol analyser or through standard.

Let me put my questions:

1) What does preamble capability bit in Association request means?
and What it means in association reponse?
(I think it means it supports)

(Aloynk card being 802.11g says it does not support short preamble)
[Anyhow i do not believe this card!]

(Even then association request is accepted by D-Link AP and in response it sets short preamble capability, so what does this mean?)
[Did D-Link assume that being ERP STA it can support it? ]

2) Then what does preamble capability bit in Beacon means?

(Does it means that BSS is using short preamble currently or the AP supports short preamble ?)

After a long station associates, then BSS should use long preamble but Capability bit for short preamble is still set in Cisco Fat AP.

3) What is the difference between barker preamble bit and short preamble
bit in capability? Or why is that barker bit required?

[It is not set in Cisco device when it is configured to support only long preamble and it is set only when a station associates?]

4)In an 11b/g AP when Beacons are sent out in 1 Mbps does it use OFDM preamble or short preamble or long preamble? (When there are no stations)?

My Present Understanding :

Preamble bit in Association request means that station can support that short preamble and in reponse AP tells what preamble the station/BSS should use.
Preamble bit in beacon says what AP supports.Barker preamble bit says what Network/BSS is using currently.

(one more question : 11b stations cannot decode ERP element, how do they know what network is using? (Physical may undertsand preamble, then what is the use of barker bit ?) )


From the above post , i think you are quite clear, how much iam confused?
Please help me to understand the standard better!


With regards,
Murali

Hi Murali:

A client offering to associate that says "Short Preamble capability=1" is saying "I support but do not require short preambles," while 0 means "I can't operate with short preambles."

A beaconing AP that says "Short Preamble capability=1" is saying "short preamble is allowed in this BSS," while 0 means "short preamble is not allowed in this BSS."

Again, if the BSS policy is to only allow short preamble, this cannot be revealed in capability information field bits. A client that does not support short preamble can't know that the BSS requires short preamble until the client attempts association, fails, and reads the status code in the association response frame.

I hope this helps. Thanks. /criss

Criss,

Does this mean the AP can communicate with some STAs using a short preamble and others using a long preamble- in other words when the AP sets the short preamble bit he is not requiring that STAs use it?

Thanks,

moe

Hi Moe:

That is correct. All PHYs that can recognize short preambles can also recognize long preambles.

"APs (as well as STAs in IBSSs) shall set the Short Preamble subfield to 1 in transmitted Beacon, Probe Response, Association Response, and Reassociation Response MMPDUs to indicate that the use of the short preamble, as described in 18.2.2.2, is allowed within this BSS. To indicate that the use of the short preamble is not allowed, the Short Preamble subfield shall be set to 0 in Beacon, Probe Response, Association Response, and Reassociation Response MMPDUs transmitted within the BSS."

Fortunately 2.4 GHz STAs that do not understand short preamble are rare these days. To them short preamble frames are nothing but RF interference. An AP that allows one to associate has to decide whether to ramp the entire BSS down to long preambles.

I hope this helps. Thanks. /criss

Thanks Criss - it does help and I sincerely appreciate that you take the time to monitor the forums and respond.

Yes Criss, whenever you publish a non-official CWNE guide, I will come down to Leesburg and personally get it signed. Thanks for breaking this down. The standards can be a bit confusing , your "gift" is respected by all here!

I humbly second that opinion.