CWNP Blog

CWNP
August 21, 2007

802.11 Fast BSS Transition (FT) Part 1 of 2

The 802.11i amendment gave us Preauthentication and Pairwise Master Key (PMK) Caching. Nothing fancy, just the basics. Preauthentication enables supplicants (stations) to authenticate with authenticators (APs or WLAN controllers) to which they may roam. Preauthentication always happens through the AP to which the station is currently associated – over the distribution system (typically an Ethernet network).

CWNP
August 20, 2007

Opportunistic PMK Caching - Complaints

Having just written a whitepaper on Fast BSS Transition, I decided to look into the nuances of configuring supplicants for Opportunistic PMK Caching (OPC). Holy smokes batman - what a pain it is to find documentation on this. Microsoft says that OPC is supported when you have the KB893357 update loaded: http://support.microsoft.com/kb/893357

CWNP
August 17, 2007

802.11n Primary and Secondary Channels

The 802.11n draft gives us 20 and 40 MHz wide channels. This is no secret and has been widely publicized for months. What hasn't been publicized is how 40 MHz channels work. The draft explains the concepts of "primary" and "secondary" channels - each 20 MHz wide using OFDM modulation. A Secondary Channel is defined as a 20 MHz channel associated with a primary channel used by HT stations for the purpose of creating a 40 MHz channel. Continue reading...

CWNP
August 16, 2007

802.11n Throughput Testing Methodology: Hopeless with Existing Equipment

I was a little perturbed at 802.11n when I first started testing it, and now, after discovering the truth, I'm just saddened.

CWNP
August 15, 2007

Another Interframe Space: RIFS

RIFS (Reduced Interframe Spacing) is a means of reducing overhead and thereby increasing network efficiency. RIFS may be used in place of SIFS to separate multiple HT format transmissions from a single transmitter when no SIFS-separated response transmission (like an ACK) is expected. The RIFS is the time from the end of the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the air interface. The value of RIFS for the MIMO PHY is 2us.

CWNP
August 14, 2007

Ramblings on Infrastructure Design

You hear all the time about how 802.11n is going to change everything - especially the design infrastructure. I thought I'd ramble about that a bit.

CWNP
August 13, 2007

802.11n Protection Mechanisms: Part 2

Dual CTS bit in the HT IE:

When this feature is used, beacons in a BSS have the "Dual CTS Protection subfield" set to 1. Stations will then start every TXOP with an RTS frame addressed to the AP. The AP responds to this RTS with two CTS frames. If the RTS is an STBC frame, then the first CTS is an STBC frame back to the station and the second CTS is a non-STBC frame back to the station. This assures that all STBC and non-STBC stations receive the CTS and set their NAVs accordingly. NAVs are set to cover the entire transmission process (as always), including both CTS transmissions (which is new).

CWNP
August 10, 2007

802.11n Protection Mechanisms: Part 1

When 802.11g was introduced, we had RTS/CTS and CTS-to-Self protection mechanisms. What do we get with 802.11n so that it's backwards compatible with 802.11a and 802.1b/g? First, there's a couple of new things I'd like to introduce, and then we'll get to the protection rules.

In an ERP Beacon, ERP stations look at the "ERP Information Element" to determine whether or not protection is necessary in the BSS. See my whitepaper here:
http://www.cwnp.com/learning_center/search_details.php?doc_id=l7w2 for further information.

In an HT Beacon, HT stations use the "Operating Mode" and "Non-greenfield STAs Present" fields in the HT Information Element to determine whether or not to use protection.

CWNP
August 09, 2007

Say hello to my little friends!

Now we have two new 802.11 friends called A-MSDU and A-MPDU. Their job: to make the protocol analyst's job more difficult.

A-MSDU = Aggregate MSDU
A-MPDU = Aggregate MPDU

Both can be used together, with restrictions (of course, because it's not as much fun unless you have extra rules to follow).

You can string MSDUs together - called subframes when aggregated - in the payload of a single QoS MPDU. An A-MSDU subframe structure consists of:

DA (6), SA (6), Length (2), MSDU (0-2304), Padding (0-3) -- all of the numbers are in octets

CWNP
August 09, 2007

Evaluating Readiness to Support Wireless LANs

Make certain you have solid support plans, including knowledgeable staff and the correct tools, before you give a thumbs up for that new wireless network.

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