Fragmentation
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Fragmentation
- Fragmentation breaks an 802.11 frame into smaller pieces known as
fragments, adds header information to each fragment, and transmits
each fragment individually. - Although the same amount of actual data is being transmitted, each fragment requires its own header, and the transmission of each fragment is followed by a SIFS and an ACK.
- In a properly functioning 802.11 network, smaller fragments will actually decrease data throughput because of the MAC sublayer overhead of the additional header, SIFS, and ACK of each fragment.
- On the other hand, if the network is experiencing a large amount of data corruption, lowering the 802.11 fragmentation setting may improve data throughput.
- Fragmentation was sometimes used in legacy 802.11b/g networks but is no longer needed with 802.11n/ac networks that support frame aggregation.
- If an 802.11 frame is corrupted and needs to be retransmitted, the entire frame must be sent again.
- When the 802.11 frame is broken into multiple fragments, each fragment is smaller and transmits for a shorter period of time.
- If interference occurs, instead of an entire large frame becoming corrupted, it is likely that only one of the small fragments will become corrupted and only this one fragment will need to be retransmitted.
- Retransmitting the small fragment will take much less time than retransmitting the larger frame.
- If fragmentation is implemented, retransmission overhead may be reduced.
- Figure 9.9 illustrates how smaller fragments reduce retransmission overhead. This illustration shows the transmission and retransmission of a large 1,500-byte frame above and the transmission and retransmission of smaller 500-byte fragments below.
- If there was no RF interference, only the solid-lined rectangles would need to be transmitted.
- Because of the additional headers (H) and the time between the fragments for each SIFS and ACK, the smaller fragments would take longer to transmit.
- However, if RF interference occurred, it would take less time to retransmit the smaller fragment than it would to retransmit the larger frame.
- The transmission of a fragment is treated the same way as the transmission of a frame. Therefore, every fragment must participate in the CSMA/CA medium access and must be followed by an ACK.
- If a fragment is not followed by an ACK, it will be retransmitted.
- Not all wireless LAN adapters allow you to adjust the fragmentation settings.
- If you do set your wireless LAN adapter to use a smaller fragment size, you must realize that as you roam between APs and as you move between networks, all of your 802.11 frames will be fragmented using the setting you have configured.
- This means that if you roam to a location where there is no interference, your station will still be using the smaller frame fragments and will actually perform worse than if you had left the fragmentation value at its largest setting.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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