Листовка для Cisco Aironet 2702i AIR-CAP2702I-E-K9

 

 

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Figure 10.    Forms of Aggregation Introduced by 802.11n 

 

802.11n addressed this concern using two aggregation techniques: the “intuitively” named A-MSDU and 

A-

MPDU, which can also be combined together, as in “A-MPDU of A-MSDU.” With aggregation, the data is 

packed together in a single unit that is sent with one preamble and acknowledged in one transmission. 

A-MSDU aggregates MSDUs (for example, LLC + IP + TCP + data) at the top of the MAC transmission 

path, so an individual MSDU in an A-MSDU lacks a MAC header/footer, such as a sequence number or 

frame check sequence. This is good for efficiency yet makes retries at the individual MSDU level 

impossible. Meanwhile, A-MPDU aggregates MPDUs at the bottom of the MAC, so each MPDU in an 

A-MDPU contains its own MAC header. Efficiency is not quite as good, especially for short MSDUs, but if a 

packet fails to get through the wireless link - for example, from a single isolated bit error - the other MDPUs 

can still be received correctly, and only the erroneous packet needs to be retried. This is shown in 

.