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  • CWNP

Transmit Beamforming with Implicit Feedback

- Introduction -

Transmit Beamforming (TxBF) is a method of using a set of separate antennas as a virtual array to form high-gain beams focused at client stations.  It is important that each transmitter (called the beamformer) understand the characteristics of the MIMO channel between itself and the receiver (called the beamformee) since transmissions will be tightly focused on a specific area.

 

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  • CWNP

Link Adaptation

The complexity of 802.11n rate adaptation has given birth to the concept of Modulation Coding Scheme (MCS).  MCS includes variables such as the number of spatial streams, modulation, and the data rate on each stream.  Radios establishing and maintaining a link must automatically negotiate the optimum MCS based on channel conditions and then continuously adjust the selection of MCS as conditions change due to interference, motion, fading, and other events.  There are 77 MCSs each for 20 MHz and 40 MHz channels in the current 802.11n draft, with eight of them (all for 20 MHz channels) being mandatory for 802.11n compliance (Section 20.6, Table n82).  Link Adaptation is the 802.11n methodology of a transmitter and receiver working together to help each other understand which modulation and code scheme (MCS) is optimal given the current environment.

 

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  • CWNP

U-PSMP and U-APSD

Unscheduled PSMP (U-PSMP) extends the U-APSD (also known as WMM-Power Save or WMM-PS) mechanism.  

In a U-PSMP enabled WLAN, if there is no unscheduled service period (SP) in progress, the unscheduled SP begins when the AP receives a trigger frame from a station, which is a QoS data or QoS Null frame associated with an AC the station has configured to be trigger-enabled.  An unscheduled SP ends after the AP has attempted to transmit at least one MSDU, A-MSDU, or MMPDU associated with a delivery-enabled AC and destined for the station, but no more than the number indicated in the Max SP Length field of the QoS Info field in the trigger frame (if the field has a non-zero value).  The Max SP Length subfield is 2 bits in length and indicates the maximum number of total buffered MSDUs and MMPDUs the AP may deliver to a station during any SP triggered by the station.

 

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  • CWNP

Power Save Multi-Poll (PSMP)

- Introduction -

In the legacy power save polling method, the station awakes from dozing and sends a PS-Poll frame to the AP that has been queueing its data traffic while it dozed.  For each PS-Poll frame the station sends to the AP, the AP responds with one data frame.  Additionally, stations have to wait for the DTIM Beacon to know whether or not it has queued traffic.  It's a high-overhead, high-latency protocol that only allows stations to doze a moderate amount - not good enough for wVoIP.

 

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  • CWNP

Channel Planning with 40 MHz Channels

Let's not even consider 40 MHz channels in the 2.4 GHz ISM band.  There can only be two non-overlapping HT MIMO channels in the 2.4 GHz band.  Considering the mess of protection mechanisms, competing 20 MHz channels, and the dozens of interfering non-WiFi devices in that band, 802.11n in 2.4 GHz is a lost cause from day 1.  Instead, let's consider the only reasonable (IMHO) enterprise-class 802.11n deployment methodology: 5 GHz.

 

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  • CWNP

Is 802.11n Worth the Money?

I'm thinking that most 802.11n deployments will initially be in the 5 GHz UNII band using 40 MHz channels.  Otherwise, why would anyone roll it out - right?  There are a few enhancements other than just throughput with 802.11n equipment, but you should consider if they will be enough to justify the costs.  Throughput enhancements will likely have to be a large part of a WLAN upgrade to 802.11n to get organizations to spend the money.  Starting with an assumption that using 40 MHz channels in 5 GHz bands will be the norm due to more available channels and less congestion in each channel, let's consider the ramifications of backwards compatibility with 802.11a devices. 

 

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  • CWNP

802.11w - Management Frame Protection

The 802.11w amendment offers three new security pieces: Data Origin Authenticity, Replay Detection, and Robust Management Frame Protection.  

The data origin authenticity mechanism defines a means by which a station that receives a data or robust management frame can determine which station transmitted the data or management frame.  This feature is required in an RSNA to prevent one station from masquerading as a different station.  Data origin authenticity is only applicable to unicast data frames, or unicast Robust Management frames, and Deauthenticate or Disassociate frames with Robust Management protection.  The protocols do not guarantee data origin authenticity for broadcast/multicast (bc/mc) data frames or broadcast/multicast Robust Management frames.

 

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  • CWNP

40 MHz Channelization

When 40 MHz channels are used in 802.11n networks, two 20 MHz channels are bonded together.  The two 20 Mhz channels are designated as primary and secondary and are designated by two fields: (Primary, Secondary) where the Primary is the number of the primary channel and the Secondary is a positive or negative integer indicating whether the secondary channel is one channel above or one channel below the primary channel).  40 MHz channels MUST consist of immediately adjacent 20 MHz channels allowed within the regulatory domain.

 

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PCO Operation Overview

Phased Coexistence Operation (PCO) is an optional coexistence mechanism in which an AP divides time into alternating 20 MHz and 40 MHz phases.   Although PCO improves throughput in some circumstances, PCO might also introduce jitter.

 

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802.11n Guard Intervals (GI)

The 802.11n draft specifies two guard intervals: 400ns (short) and 800ns (long).  Support of the 400ns GI is optional for transmit and receive.  The purpose of a guard interval is to introduce immunity to propagation delays, echoes, and reflections to which digital data is normally very sensitive.

 

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