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Improvements in throughput in 802.11n

Improvements in throughput in 802.11n. The design goal of the 802.11n is “HT” for High Throughput. The throughput is high indeed: up to 600 Mbps in raw bit-rate. We intend to start with the maximum throughput of 802.11g (54 Mbps), and see what techniques 802.11n applies to boost it to 600 Mbps:

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Improvements in throughput in 802.11n

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  1. Improvements in throughput in 802.11n The design goal of the 802.11n is “HT” for High Throughput. The throughput is high indeed: up to 600 Mbps in raw bit-rate. We intend to start with the maximum throughput of 802.11g (54 Mbps), and see what techniques 802.11n applies to boost it to 600 Mbps: I. Physical Layer Improvements (mbojombojo) 1. More subcarriers 2. FEC (Forward Error Correction) 3. Guard Interval 4. MIMO (Beamforming and Multipath) 5. Channel Bonding MAC Layer Improvements (Boris Todorov F38692 ) 1. MSDU Aggregation 2. MPDU Aggregation 3. Block Acknowledgement 4. Lower Overhead

  2. MAC Enhancements There is only so much improvement that can be obtained in 802.11 by increasing the data rate of the radio. There is a significant amount of fixed overhead in the MAC layer protocol, and in the interframe spaces and acknowledgements of each frame transmitted, in particular. At the highest of data rates, this overhead alone can be longer than the entire data frame. In addition, contention for the air and collisions also reduce the maximum effective throughput of 802.11. 802.11n addresses these issues by making changes in the MAC layer to improve on the inefficiencies imposed by this fixed overhead and by contention losses.

  3. Aggregation Every frame transmitted by an 802.11 device has fixed overhead associated with the radio preamble and MAC frame fields that limit the effective throughput, even if the actual data rate was Infinite.

  4. To reduce this overhead, 802.11n introduces frame aggregation. Frame aggregation is essentially putting two or more frames together into a single transmission. 802.11n introduces two methodsfor frame aggregation: Mac Service Data Units (MSDU) aggregation and Message Protocol Data Unit (MPDU) aggregation. Both aggregation methods reduce the overhead to only a single radiopreamble for each frame transmission.

  5. MSDU (Mac Service Data Units) Aggregation • Aggregation of multiple frames => improve system efficiency • A-MSDU is aggregated with multiple MSDU – separate by a sub-frame header • Only aggregation with other frame of the same address type • Should not aggregate MSDUs of different values of priority

  6. MPDU (Mac Protocol Data Units) Aggregation • MPDU aggregation translates each Ethernet frame to 802.11 format • MPDU aggregation requires that all frames that constitute the aggregation frame have the same destination address. • With MPDU aggregation, it is possible to encrypt constituent frame independently • MPDU aggregation requires that all of the constituent frames be the same QoS level

  7. Block Acknowledgement Block acknowledgement compiles all the acknowledgements of the individual constituent frames produced by MPDU aggregation into a single frame returned by the recipient to the sender. This allows a compact and rapid mechanism to implement selective retransmission of only those constituent frames that are not acknowledged. In environments with high error rates, this selective retransmission mechanism can provide some improvement in the effective throughput of a WLAN using MPDU aggregation over that of one using MSDU aggregation, because much less is retransmitted when an error affects some of the constituent frames of an MPDU aggregated frame as compared to an MSDU aggregated frame.

  8. Lower Overhead: Reduced Interframe Space • When aggregation of frames is not possible, 802.11n provide a mechanism to reduce the overhead • The 802.11e extension for quality of service added the ability for a single transmitter to send a burst of frames durring a single, timed transmit opportunity. • Reduced interframe space (RIFS)

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