MSDU MPDU AMSDU AMPDU PPDU

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我是靠谱客的博主无限小甜瓜，最近开发中收集的这篇文章主要介绍MSDU MPDU AMSDU AMPDU PPDU，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

MSDU MPDU AMSDU AMPDU PPDU

MSDU - MACService Data Unit，MAC服务数据单元

在无线网络安全中，MSDU经过添加完整性校验MIC、分帧、添加IV、加密、添加MAC头部后，成为MPDU（MAC ProtocolData Unit，MAC协议数据单元）

MSDU可以认为是Ethernet报文，而MPDU指经过802.11封装过的数据帧。

原文地址：http://ergodicthoughts.blogspot.com/2012/02/difference-between-mpdu-msdu-ampdu-and.html

Difference in implementation:
If we see the packet being handed from the IP layer to the MAClayer the following sequence of processing is seen. The packet goes from the IPlayer to the MAC-llc (logical link control layer) or the upper MAC. This isalso called as the MAC service access point (MAC-SAP).

MSDU: Is the MAC service data unit. This is the unit oftransmission used at the MAC layer which is received from the upper layer.
MSDU是个以太网帧，由IP层下发到MAC层，MAC层加入MACheader后，再

作为MPDU下发到PHY层。MPDU=(MACheader + MSDU)

AMSDU: Aggregation of the MSDUs directly performed at theMAC layer is called an AMSDU. Such AMSDU's are now passed to the lower PHYlayer, where they are dealt with as MPDUs. Multiple MSDUs are aggregated at theMAC layer and pushed into a single MPDU (which is pushed to the PHY). They havea single frame header with multiple frames, and they are destined for the sameclient and the same service class. (Basically they all have the same TiD).

MSDU聚合：将多个MSDUs聚合在一起形成AMSDU，AMSDU+MACheadr之后形成MPDU发送到PHY层。
* The main motivation for aggregation at the MSDU layer is that: (1) Ethernetis the native frame format for most clients, (2) since the ethernet header ismuch smaller than the 802.11 header, we can combine multiple ethernet frames toform a single A-MSDU.
AMSDU目的：1、对于大多数client来说原始帧格式是以太网帧，2、以太网帧头比802.11帧头小很多，可以将多个以太网帧结合在一起使用一个802.11帧头封装成一个MPDU。
MPDU: MAC protocol data units are the frames passed from the MAC layers intothe PHY layer.
MAC协议数据单元：从MAC层传给PHY层，MPDU=(MACheader + MSDU/AMSDU)
AMPDU: These are are the aggregated MPDU units which are pushed into a singlePPDU (physical protocol data unit). All frames will have a single PLCPheader and preamble.
多个MPDU聚合成一个AMPDU，AMPDU=(MPDU1+...+MPDUn)

，PPDU=（PLCPheader preamble + MPDU/AMPDU）

** The 802.11n system was designed so that either AMPDU, AMSDUor both aggregation algorithms could be used.
AMSDU和AMPDU可以同时使用

When do we want to usean A-MPDU and when do we want to use an A-MSDU:
Rather , the topic of this discussion should be why is AMPDU aggregationpreferred over AMSDU aggregation most of the time. Or why most systems useAMPDU aggregation and not AMSDU aggregation.

A-MSDU increases the maximum frame transmission size from 2,304 bytes to almost8k bytes (7935 to be exact) while A-MPDU allows up to 64k bytes.

However, the main problem with AMSDUs is that the entire blob becomes one MACframe (or protocol data unit - PDU) and hence has only one CRC check. So as theframe size increase the probability of error increases. Since we have a singleCRC check, we cannot retransmit a part of the AMSDU and in most cases thisleads to re-transmission at lower rates which nullifies the benefitof aggregation. An AMPDU on the other hand consists of multiple PDUs each withtheir own CRCs. Hence, in the event of a failure, part of these can beretransmitted resulting in higher efficiency. However, this performance gaincomes at a cost since now with every aggregate AMPDU we are sendingMAC headers for all of the subframes.

Hence decision of using AMSDU versus AMPDU is a tradeoff between probability oferror and retransmission costs in an AMSDU versus MAC frame headeroverheads in an aggregate with AMPDU. In most real world systems, the laterwins, and hence most systems implement AMPDUs.