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...
I was a little perturbed at 802.11n when I first started testing it, and now, after discovering the truth, I'm just saddened.Continue reading...
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.
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).
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.
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
The 802.11n amendment allows for up to 4x4 MIMO. That's 4 transmitters and 4 receivers working at the same time. Due to cost, nobody is building a 4x4 system. Instead, we're seeing 2x2 on the cheap stuff, 2x3 on the higher-end client radios and the lower-end APs, and 3x3 on the higher-end APs. More transmitters and receivers means better quality: better reception, higher throughput, the works. Always look for 2x3 stations and 3x3 APs whenever possible. Continue reading...
Earlier this year Nokia released an updated version of their popular Nokia N770 Internet Tablet. The N800 excels at being a small, light-weight device capable of WiFi as well as Bluetooth access to the Internet. THis is a user-based review of this great little device.