802.11n Features

                       As we are using more applications on the Clients like Laptops ,Smart phones and other wi-fi enabled devices with Voice and Video. The demand for Higher throughput is going on. Its like seeing some movie on these devices by connecting through the Wi-Fi. So they come up with new ammendment 802.11n to get higher throughputs to overcome growing demand of Higher Throughput.

Below are some of the main Modifications in 802.11n

1. MIMO(multiple input, multiple output antennas)
2. Frame aggregation A-MPDU & A-MSDU
3. Channel bonding (40 Mhz channels)
4. Block Acknowledgments (BLOCK ACK)
5. Modulation & Coding scheme (MCS)
6. Short Guard Interval (short GI)
7. 802.11n interoperability (HT mode)
8. RIFS
9. HT power management


Below are the ways  throughput Improvements are happened in 11n 

•     The number of OFDM data sub-carriers is increased from 48 to 52 which improves the maximum throughput  from 54 to 58.5 Mbps
•    Forward Error Correction (FEC) is a system of error control whereby the sender adds redundant data to allow the receiver to detect and correct errors. 3/4 coding rate is improved with 5/6 boosting the link rate from 58.5 to 65 Mbps
•   The Shorter Guard Interval(GI) between OFDM symbols is reduced from 800ns to 400ns and increases throughput from 65 to 72.2 Mbps Channel
•   Doubling channel bandwidth from 20 to 40 MHz slightly more than doubles rate from 72.2 to 150 Mbps.
•   Support of up to four spatial streams (MIMO) increases throughput up to 4 times 150 to 600 Mbps
•    802.11n maintain backward compatibility with existing IEEE 802.11a/b/g.

1.MIMO(multiple input, multiple output antennas)

  802.11n has ability to receive and/or transmit simultaneously through multiple antennas. 802.11n defines many "M x N" antenna configurations, ranging from "1 x 1" to "4 x 4".

MIMO antennas example

   This refers to the number of transmit (M) and receive (N) antennas – for example, an AP with two transmit and   three receive antennas is a "2 x 3" MIMO device.

 The more antennas an 802.11n device uses simultaneously, the higher its maximum data rate. 

 802.11n uses advanced signal processing techniques

  

a. Spatial Multiplexing (SM)

•      Spatial Multiplexing (SM) subdivides an outgoing signal stream into multiple pieces, transmitted through different antennas. 
•      Because each transmission propagates along a different path, those pieces – called spatial streams – arrive with  different strengths and delays.
•      Multiplexing two spatial streams onto a single channel effectively doubles capacity and thus maximizes data rate.
•      All 802.11n APs must implement at least two spatial streams,up to a maximum of four.
•      802.11n stations can implement as few as one spatial stream.

  b. Space-Time Block Coding (STBC)

•     Space-Time Block Coding (STBC) sends an outgoing signal stream redundantly,using up to four differently-coded spatial streams, each transmitted through a different antenna. 
•   By comparing arriving spatial streams, the receiver has a better chance of accurately determining the original signal stream in the presence of RF interference and distortion.
•   That is, STBC improves reliability by reducing the error rate experienced at a given Signal to Noise Ratio (SNR). This optional 802.11n feature may be combined with SM.

  c. Transmit Beamforming (TxBF) 

•     Transmit Beam-forming (TxBF)  steers an outgoing signal stream towards the intended receiver by concentrating transmitted RF energy in a given direction.
•   This technique leverage additive and destructive environmental impacts
•   This optional 802.11n feature is not yet widely implemented.

2. Frame aggregation A-MPDU & A-MSDU

•     Frame Aggregation increases the payload that can be conveyed by each 802.11 frame, reducing MAC layer overhead from a whopping 83 % to as little as 58 % (using A-MSDU) and 14 % (when using A-MPDU).
•   Legacy 802.11a/g devices can send no more than 2304 payload bytes per frame.
•   But new 802.11n devices have the option of bundling frames together for transmission, increasing payload size to reduce the significance of the fixed overhead caused by inter-frame spacing and preamble. 

  

  There are two aggregation options:

 MAC Service Data Unit Aggregation (A-MSDU):

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•  Groups logical link control packets (MSDUs) with the same 802.11e Quality of Service,independent of source or destination.
•     The resulting MAC frame contains one MAC header, followed by up to 7935 MSDU bytes.
•     Whole frame must be retransmitted if no acknowledge .

MAC Protocol Data Unit Aggregation (A-MPDU):

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•  Multiple Ethernet frames for a common destination are translated to 802.11 format and sent as burst.
•  Complete MAC frames (MPDUs) are then grouped into PHY payloads up to 65535 bytes.
•  Elements of an A-MPDUs burst can be acknowledged individually with one single Block-Acknowledge
•  Only not-acknowledged A-MPDUs are retransmitted

 3. Channel bonding (40 Mhz channels)

•    Legacy 802.11 products use channels that are approximately 20 MHz wide.
•    New 802.11n products can use 20 or 40 MHz wide channels in either the ISM or UNII band.
•    802.11n WLANs will uses 40 MHz channels mainly in the 5 GHz UNII band.

4. Block Acknowledgements (BLOCK ACK)

•   Rather than sending an individual acknowledge following each data frame, 802.11n introduces the technique of confirming a burst of up to 64 frames with a single Block ACK(BA) frame
•   The Block ACK even contains a bitmap to selectively acknowledgeindividual frames of a burst (comparable to selective acknowledges of TCP)
•   The use of combined acknowledges can be requested by sending a Block ACK Request(BAR)
•   The Block-ACK options are negotiated and confirmed with ‘Action’ framesdefined in 802.11e (WLAN QoS)

5. Modulation & Coding scheme (MCS)

•   802.11n APs and stations need to negotiate capabilities like the number of spatial streams and channel width.
•   They also must agree upon the type of RF modulation,coding rate, and guard interval to be used. 
•   The combination of all these factors determines the actual PHY data rate, ranging from a minimum 6.5 Mbps to a   maximum 600 Mbps

6. Short Guard Interval (short GI)

•    Guard Interval is the time between transmitted symbols (the smallest unit of data sent at once).
•    This Guard Interval is necessary to offset the effects of multipath that would otherwise cause Inter-Symbol Interference (ISI).
•    Legacy 802.11a/g devices use an 800 ns guard interval, but 802.11n devices have the option of pausing just 400 ns. 
•    Shorter Guard Intervals would lead to more interference and reduced throughput, while a longer Guard Interval would lead to unwanted idle time in the wireless environment. 
•    A Short Guard Interval (SGI) boosts data rate by 11 percent while maintaining symbol separation sufficient for most environments.

7. 802.11n interoperability and coexistence

  

•   Given the fact that millions of legacy 802.11a/b/g devices have been deployed to date, and that those devices operate in the same frequency bands used by 802.11n,enabling coexistence is critical.
•  802.11n deployments must therefore be able to "play nicely" with 802.11a/b/g, both by limiting 802.11n impact on nearby legacy WLANs and by enabling communication with legacy stations. 
•  These goals are accomplished using HT Protection and Coexistence mechanisms.

   a. High Throughput (Greenfield) Mode:

•         There are three 802.11n operating modes: HT, Non-HT, and HT Mixed.
•     An 802.11n AP using High Throughput (HT) mode – also known as Greenfield mode – assumes that there are no nearby legacy stations using the same frequency band.
•     If legacy stations do exist, they cannot communicate with the 802.11n AP. HT mode is optional.

      b. Non-HT (Legacy) Mode :

•    An 802.11n AP using Non-HT mode sends all frames in the old 802.11a/g format so that legacy stations can understand them. 
•    That AP must use 20 MHz channels and none of the new HT features .
•    All products must support this mode to ensure backward compatibility, but an 802.11n AP using Non-HT delivers no  better performance than 802.11a/g.

   c. HT Mixed Mode :

•    The mandatory HT Mixed mode will be the most common 802.11n AP operating mode for the next year or so. 
•    In this mode, HT enhancements can be used simultaneously with HT Protection mechanisms that permit communication  with legacy stations.
•    HT Mixed mode provides backwards compatibility, but 802.11n devices pay significant throughput penalties as compared to Greenfield mode.

8. RIFS 

•    RIFS is a means of reducing overhead and thereby increasing network efficiency.
•    RIFS may be used in place of SIFS to separate multiple transmissions from a single transmitter, when no  SIFS-separated response transmission is expected. 
•    RIFS shall not be used between frames with different RA values.
•    The duration of RIFS is defined by the aRIFS PHY characteristic .
•    A STA shall not allow the space between frames that are defined to be separated by a RIFS time, as measured on the medium, to vary from the nominal RIFS value (aRIFSTime) by more than ± 10% of aRIFSTime. 
•    Two frames separated by a RIFS shall both be HT PPDUs.

9. HT powermanagment

•  802.11n introduces two powersaving mechanisms that can be used by HT clause radios
•  802.11n radios still support legacy power save mode.
•  The first new power save management is called Spatail Multiplexing PowerSave mode (SM power save)
•  The purpose of SM powersave is powerdown the all but one of its radios
•  For example a 4*4 MIMO device with four radio chains would power down three of its four radios
•  The second Power save managment is Power save Multipoll (PSMP).
•  PSMP is extention to the automatic power save mode
•  U-PSMP is similar to U-APSD and uses trigger and deliver enabled mechanisms.
•  Scheduled PSMP is similar to the S-APSD Power save 

Hope this will help as brief introduction.