Fragmentation and Reassembly

Higher-level packets and some large management frames may need to be broken into smaller pieces to fit through the wireless channel. Fragmentation may also help improve reliability in the presence of interference. The primary sources of interference with 802.11 LANs are microwave ovens, with which they share the 2.4-GHz ISM band. Electromagnetic radiation is generated by the magnetron tube during its ramp-up and ramp-down, so microwaves emit interference half the time.121

[2] In the US, appliances are powered by 60-Hz alternating current, so microwaves interfere for about 8 milliseconds (ms) out of every 16-ms cycle. Much of the rest of the world uses 50-Hz current, and interference takes place for 10 ms out of the 20-ms cycle.

Wireless LAN stations may attempt to fragment transmissions so that interference affects only small fragments, not large frames. By immediately reducing the amount of data that can be corrupted by interference, fragmentation may result in a higher effective throughput.

Fragmentation takes place when a higher-level packet's length exceeds the fragmentation threshold configured by the network administrator. Fragments all have the same frame sequence number but have ascending fragment numbers to aid in reassembly. Frame control information also indicates whether more fragments are coming. All of the fragments that comprise a frame are normally sent in a fragmentation burst, which is shown in Figure 3-8. This figure also incorporates an RTS/CTS exchange, because it is common for the fragmentation and RTS/CTS thresholds to be set to the same value. The figure also shows how the NAV and SIFS are used in combination to control access to the medium.

Figure 3-8. Fragmentation burst

Figure 3-8. Fragmentation burst

Fragments and their acknowledgments are separated by the SIFS, so a station retains control of the channel during a fragmentation burst. The NAV is also used to ensure that other stations don't use the channel during the fragmentation burst. As with any RTS/CTS exchange, the RTS and CTS both set the NAV from the expected time to the end of the first fragments in the air. Subsequent fragments then form a chain. Each fragment sets the NAV to hold the medium until the end of the acknowledgment for the next frame. Fragment 0 sets the NAV to hold the medium until ACK 1, fragment 1 sets the NAV to hold the medium until ACK 2, and so on. After the last fragment and its acknowledgment have been sent, the NAV is set to 0, indicating that the medium will be released after the fragmentation burst completes.

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