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.The primary sources of interference with802.11 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 andramp-down, so microwaves emit interference half the time.[2][2]In the US, appliances are powered by 60-Hz alternating current, somicrowaves 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 takesplace for 10 ms out of the 20-ms cycle.Wireless LAN stations may attempt to fragment transmissions so that interference affectsonly small fragments, not large frames.By immediately reducing the amount of data thatcan be corrupted by interference, fragmentation may result in a higher effectivethroughput.Fragmentation takes place when a higher-level packet's length exceeds the fragmentationthreshold configured by the network administrator.Fragments all have the same framesequence number but have ascending fragment numbers to aid in reassembly.Framecontrol information also indicates whether more fragments are coming.All of thefragments that comprise a frame are normally sent in a fragmentation burst, which isshown in Figure 3-8.This figure also incorporates an RTS/CTS exchange, because it iscommon for the fragmentation and RTS/CTS thresholds to be set to the same value.Thefigure also shows how the NAV and SIFS are used in combination to control access tothe medium.Figure 3-8.Fragmentation burstFragments and their acknowledgments are separated by the SIFS, so a station retainscontrol of the channel during a fragmentation burst.The NAV is also used to ensure thatother stations don't use the channel during the fragmentation burst.As with any RTS/CTSexchange, the RTS and CTS both set the NAV from the expected time to the end of thefirst fragments in the air.Subsequent fragments then form a chain.Each fragment sets theNAV 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 tohold 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 thefragmentation burst completes.3.5 Frame FormatTo meet the challenges posed by a wireless data link, the MAC was forced to adoptseveral unique features, not the least of which was the use of four address fields.Not allframes use all the address fields, and the values assigned to the address fields may changedepending on the type of MAC frame being transmitted.Details on the use of addressfields in different frame types are presented in Chapter 4.Figure 3-9 shows the generic 802.11 MAC frame.All diagrams in this section follow theIEEE conventions in 802.11.Fields are transmitted from left to right, and the mostsignificant bits appear last.Figure 3-9.Generic 802.11 MAC frame802.11 MAC frames do not include some of the classic Ethernet frame features, mostnotably the type/length field and the preamble.The preamble is part of the physical layer,and encapsulation details such as type and length are present in the header on the datacarried in the 802.11 frame.3.5.1 Frame ControlEach frame starts with a two-byte Frame Control subfield, shown in Figure 3-10.Thecomponents of the Frame Control subfield are:Protocol versionTwo bits indicate which version of the 802.11 MAC is contained in the rest of theframe.At present, only one version of the 802.11 MAC has been developed; it isassigned the protocol number 0.Other values will appear when the IEEEstandardizes changes to the MAC that render it incompatible with the initialspecification.Type and subtype fieldsType and subtype fields identify the type of frame used.To cope with noise andunreliability, a number of management functions are incorporated into the 802.11MAC.Some, such as the RTS/CTS operations and the acknowledgments, havealready been discussed.Table 3-1 shows how the type and subtype identifiers areused to create the different classes of frames. Figure 3-10.Frame control fieldIn Table 3-1, bit strings are written most-significant bit first, which is the reverse of theorder used in Figure 3-10.Therefore, the frame type is the third bit in the frame controlfield followed by the second bit (b3 b2), and the subtype is the seventh bit, followed bythe sixth, fifth, and fourth bits (b7 b6 b5 b4).Table 3-1.Type and subtype identifiersSubtype value Subtype nameManagement frames (type=00)[a]0000 Association request0001 Association response0010 Reassociation request0011 Reassociation response0100 Probe request0101 Probe response1000 Beacon1001 Announcement traffic indication message (ATIM)1010 Disassociation1011 Authentication1100 DeauthenticationControl frames (type=01)[b]1010 Power Save (PS)-Poll1011 RTS1100 CTS1101 Acknowledgment (ACK)1110 Contention-Free (CF)-End1111 CF-End+CF-AckData frames (type=10)[c]0000 Data0001 Data+CF-Ack0010 Data+CF-Poll0011 Data+CF-Ack+CF-Poll Table 3-1.Type and subtype identifiersSubtype value Subtype name0100 Null data (no data transmitted)0101 CF-Ack (no data transmitted)0110 CF-Poll (no data transmitted)0111 Data+CF-Ack+CF-Poll(Frame type 11 is reserved)[a]Management subtypes 0110-0111 and 1101-1111 are reserved and notcurrently used.[b]Control subtypes 0000-1001 are reserved and not currently used.[c]Data subtypes 1000-1111 are reserved and not currently used.ToDS and FromDS bitsThese bits indicate whether a frame is destined for the distribution system.Allframes on infrastructure networks will have one of the distribution system's bitsset.Table 3-2 shows how these bits are interpreted [ Pobierz całość w formacie PDF ]