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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
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

mac enhancements
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.

aggregation
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.

slide4

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.

msdu mac service data units aggregation
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
mpdu mac protocol data units aggregation
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
block acknowledgement
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.

lower overhead reduced interframe space
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)