A Study on Frame Aggregation in Wireless Networks.ppt

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A Study on Frame Aggregation in

Wireless Networks

Submitted By

Mrs. Raule Megha B .

M.E(CNE)

Under the Guidance Of

Dr. Kasabegoudar V.G.



Abstract

• The timing and headers overheads of IEEE 802.11 PHY and MAC layers

consume a large part of the channel time leading to performance

degradation especially at higher data rates.

• A key enhancement is frame aggregation in which the timing and headers

overheads are reduced by aggregating multiple frames into a single largeframe and then transmit it in a single channel access.

• This paper addresses the frame aggregation techniques that have been

proposed for the next generation wireless networks.



Frame Aggregation

It has many benefits: first, transmitting large frame leads to higher throughput than

transmitting small frames; second and most important benefit is the reduction of timing and header overheads that are required to transmit a frame by the MAC

distributed coordination function.

Figure :The saved time when aggregation three frames



Frame Aggregation Approaches

•Aggregation can be performed either at the packet

level or at the frame level.

•It is called packet aggregation if it is performed at

the higher layers such as IP and application layers.

• However, it is called frame aggregation if it is performed at the lower layers, PHY and MAC layers.



Aggregation at the MAC Layer

•Aggregation at the MAC level is the widely usedaggregation where the MAC headers overhead can besqueezed or even removed. Moreover, the channel accesscan be optimized by reducing the timing overhead such as

backoff and message exchange overhead such as ACKs.

•Yang [11] proposed several MAC strategies to improve thethroughput and reduce the overhead of the next generationwireless networks. The author proposed many techniquessuch as packing, concatenation and multiple frametransmission which adopt the idea of sending multipleframes in a single transmission.

http://www.tr.ietejournals.org/article.asp?issn=0256-4602;year=2011;volume=28;issue=4;spage=336;epage=350;aulast=Saif






A-MSDU

In this aggregation scheme, several MSDUs destined to the same receiver are

concatenated in a single MPDU. This operation is performed at the top of theMAC layer where the incoming MSDUs are buffered and then concatenatedin order to form the A-MSDU. The aggregation process is ended when thesize of the buffered MSDUs reaches the maximum A-MSDU frame size or the delay of the oldest MSDU reaches the maximum delay limit.

F igure : A-MSDU fr ame structure.



A-MPDU

The aggregate MPDU combines multiple MPDUs in a single PHY protocol

data unit (PPDU) frame. The A-MPDU frame consists of n subframes eachsubframe has a MAC header, delimiter of 4 bytes, variable-size MPDU,

padding and frame check sequence.

F igure : A-MPDU frame structure.



Aggregation at the PHY Layer

•The data units that are passed from the MAC layer to the PHY layer arecalled physical service data unit (PSDU) while the physical protocol dataunit (PPDU) is a PSDU attached with PHY layer headers.

• Aggregation at the PHY layer can be performed over the PSDUs (A-

PSDU) or PPDUs (A-PPDU) and would support transmission of multiple frames having different rates in the same burst.



A-PSDU

• This scheme involves high overhead (4 μs for each signaling filed) and the

corruption in one bit of the signaling field will prevent the demodulation o f the

remaining frame.

• In this scheme several PSDUs are aggregated and delimited by a physical signaling

filed that indicates the duration of the next PSDU and the modulation coding

scheme (MCS).

Figure : A-PSDU aggregation scheme



A-PPDU •A-PPDU aggregation consists of sequence of frames transmitted by a single stationwithout intervening frames by any other station.

• The consecutive PPDUs are separated by interframe spacing.• In this scheme, retaining both the MAC and PHY headers, makes the addressing todifferent destinations possible.

Figure : A-PPDU scheme.



Aggregation Schemes Comparison



Conclusions

•Frame aggregation improves the throughput of the IEEE 802.11 wireless networks by

concatenating multiple frames into a single frame and then transmits it.

•Frame aggregation can be categorized based on the place of aggregation into MAC level

and PHY level aggregations.

•The A-MSDU and A-MPDU aggregations are preformed at the MAC layer whereas the A-

PSDU and A-PPDU are performed at the PHY layer.

•These issues include the performance under unsaturated channels, the performance with

TCP traffic, and the tradeoff between the efficiency and robustness in error-prone channels.



References

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A Study on Frame Aggregation in Wireless Networks.ppt

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