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I see in the forum that there are a few questions regarding an ESS/ESSID but I dont think this one has been covered.

In the official v3 study guide page 332 it says:
"An ESS consists of two or more BSSs that share the same SSID"

but in the v3 courseware, page 153, slide 17 it says:
"ESS USUALLY share the same SSID"

The bit that is confusing me is the USUALLY in the second quote. I asked my asked instructor about this and he told me that the SSID does not have to be the same (although it usually is). If it does not have to be, what is the true definition of an ESS?

3.25 extended service set (ESS):
A set of one or more interconnected basic service sets (BSSs) and integrated
local area networks (LANs) that appears as a single BSS to the logical link control layer at any station associated with one of those BSSs.

802.11-1999 (R2003) also gives input in sections 5.2.2.1 and 5.6.

The standards are a free download from:
http://standards.ieee.org/getieee802/802.11.html

Devinator

Hi S:

CWNAv3 study guide, page 332, second paragraph, third sentence is incorrect.

The IEEE 802.11-1999 definition 3.25 is itself confused and IEEE 802.11n draft 5.2 proposes to improve the definition as follows:

"3.57 extended service set (ESS): A set of one or more interconnected basic service sets (BSSs) that appears as a single BSS to the logical link control (LLC) layer at any station associated with one of those BSSs."

1. An ESS is one or more (not two or more) interconnected infrastructure BSSs (not IBSSs).

2. The interconnection is the functional equivalent of a bridged catenet. This can be achieved in several ways. This is essential.

3. It is popular for all BSSs of an ESS to use a common SSID for the sake of roaming. This is not a requirement.

4. An ESS includes zero or more portals to integrated LANs but does not include the integrated LANs or their stations.

I hope this helps. Can you add your location to your forum profile? Thanks. /criss

CWNAv3 study guide, page 332, second paragraph, third sentence is incorrect.

I think the study guide is correct. "An ESS consists of two or more BSSs..."

because the standard says ""3.57 extended service set (ESS): A set of one or more interconnected basic service sets (BSSs)..."

An interconnection requires at least two BSS, right?

CMIIW

regards,
AH

Hi AH:

One isolated infrastructure BSS is, by IEEE 802.11 definition, also an ESS.

The essence of an ESS is the ability of LLC layers in any associated stations to reach one another across the bridged catenet of infrastructure BSSs. An ESS is a single data-link broadcast domain. This communications service does not disappear when the number of BSSs reduces to one BSS, nor even when the number of associated stations reduces to zero!

A STA starting a new BSS has to choose between independent and infrastructure -- IBSS or ESS.

I grant that the standards language is tortured, but it is also compact and precise.

I hope this helps. Thanks. /criss


P.S. An IBSS is also a single data-link broadcast domain but why several of them cannot be bridged together is a story for another thread.

Hi Criss, thanks for the explanation

it's very confusing and i come with one conclusion: BSS = ESS

is there any different between BSS and ESS?

tia
AH

Well seems like the ESS is more like a virtual group within which a station can roam. So it would be BSSs within the proximity to which the station can authenticate and hopefully associate.
Thus if you have on BSS the station could authenticate to it.
If you had 2 in range it could authenticate to 2 of them potentially.

anyway just my interpretation of what written above

Chris,
Was the following a typo?

A STA starting a new BSS has to choose between independent and infrastructure -- IBSS or ESS.

isnt a independent one a IBSS and an Infrastructure a BSS?

Thanks in advance for all insight,
rico

Hi AH:

A BSS is a set of stations that share a timing function.

A BSS may be independent (include zero AP) or infrastructure (includes one AP).

A set of one or more infrastructure BSSs that appears as a single BSS ... is an ESS. (See thread above for LLC details.)

One Independent BSS = IBSS
One Infrastructure BSS = ESS
Several combined Infrastructure BSSs = ESS

When the standard uses the term ESS it means literally "one or more". When the standard needs to specify "one" it uses the more clumsy term "infrastructure BSS".


Hi Rico:

Not a typo. But on checking the standard for confirmation I find the following.

A STA starting a new BSS has to internally specify one of the following strings: INDEPENDENT or INFRASTRUCTURE.

A STA scanning for an existing BSS to join has to internally specify one of the following strings: INDEPENDENT or INFRASTRUCTURE or ANY_BSS.

So one has to look elsewhere in the standard for confirmation that "One Infrastructure BSS = ESS"

Of the more than one hundred references to "ESS" this one is typical:

8.2.2.1 Overview In an ESS, a non-AP STA and an AP must both complete an IEEE 802.11 authentication exchange prior to association. Such an exchange is optional in an independent BSS network.

I hope this helps. Thanks. /criss

Hi Chris,

Thanks for your time and patience. I am newby to this i appreciate your advice and that of anyone else on this forum on my road to clarity, and hope i can in some way contribute back.

I see from how your example that IBSS and BSS are both Basic Service Sets so the terminiology can get tricky. ESS does seem like a better way to describe the infrastructure set.

I was reading in one implementation that in the capability information field there is an ESS field which can be set to 1 to distinguish ESS and 0 to distinguish IBSS. The station in IBSS gets the 0 and an AP in BSS gets the 1.
In this implementation i would imagine they could run code from these values to adhere to the standards you mentioned and to assist with roaming and such by identifing machines with this value set. Those APs which had this value as 1 could then be grouped as an ESS. Its the BSSID that determines their uniqueness as a IBSS or BSS. This is not a subcategory of ESS

So by using what you so kindly shared i then maybe a can deduce

1. A STA could see this field in the probe response or association response to see if it was a BSS or IBSS.

2. For an AP or STA this field is set initially when they enter
and can check this field with other APS and Sta to determine a functional relationship.

Thanks again in this humbling learning experience. :-)

Hi Rico:

Congratulations. After posting to you yesterday I also considered the Capability Information field as evidence for the general proposition that a single infrastructure BSS is one example of an ESS and is generally referred to as an ESS in the standard.

From IEEE 802.11m Draft 5.2:
"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."

"APs set the ESS subfield to 1 and the IBSS subfield to 0 within transmitted Beacon or Probe Response management frames. STAs within an IBSS set the ESS subfield to 0 and the IBSS subfield to 1 in transmitted Beacon or Probe Response management frames."

I hope this helps. Thanks. /criss