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Figure 4-7.Data frame of subtype NullSeveral other frame types exist for use within the contention-free period.However,contention-free service is not widely implemented, so the discussion of the contention-free frames (Data+CF-Ack, Data+CF-Poll, Data+CF-Ack+CF-Poll, CF-Ack, CF-Poll,and CF-Ack+CF-Poll) can be found in Chapter 8.4.1.5 Applied Data FramingThe form of a data frame can depend on the type of network.The actual subtype of theframe is determined solely by the subtype field, not by the presence or absence of otherfields in the frame.4.1.5.1 IBSS framesIn an IBSS, three address fields are used, as shown in Figure 4-8.The first addressidentifies the receiver, which is also the destination address in an IBSS.The secondaddress is the source address.After the source and destination addresses, data frames inan IBSS are labeled with the BSSID.When the wireless MAC receives a frame, it checksthe BSSID and passes only frames in the station's current BSSID to higher protocollayers.Figure 4-8.IBSS data frame IBSS data frames have the subtype data or Null; the latter is used only to communicatepower management state.4.1.5.2 Frames from the APFigure 4-9 shows the format of a frame sent from an access point to a mobile station.Asin all data frames, the first address field indicates the receiver of the frame on the wirelessnetwork, which is the frame's destination.The second address holds the transmitteraddress.On infrastructure networks, the transmitter address is the address of the station inthe access point, which is also the BSSID.Finally, the frame indicates the source MACaddress of the frame.The split between source and transmitter is necessary because the802.11 MAC sends acknowledgments to the frame's transmitter (the access point), buthigher layers send replies to the frame's source.Figure 4-9.Data frames from the APNothing in the 802.11 specification forbids an access point from transmitting Nullframes, but there is no reason to transmit them.Access points are forbidden from usingthe power-saving routines, and they can acknowledge Null frames from stations withoutusing Null frames in response.In practice, access points send Data frames during thecontention-based access period, and they send frames incorporating the CF-Poll featureduring the contention-free period.4.1.5.3 Frames to the APFigure 4-10 shows the format of a frame sent from a mobile station in an infrastructurenetwork to the access point currently serving it.The receiver address is the BSSID.Ininfrastructure networks, the BSSID is taken from the MAC address of the network stationin the access point.Frames destined for an access point take their source/transmitteraddress from the network interface in the wireless station.Access points do not performfiltering, but instead use the third address to forward data to the appropriate location inthe distribution system.Figure 4-10.Data frames to the AP Frames from the distribution system have the ToDS bit set, but the FromDS bit is 0.Mobile stations in an infrastructure network cannot become the point coordinator, andthus never send frames that incorporate the contention-free polling (CF-Poll) functions.4.1.5.4 Frames in a WDSWhen access points are deployed in a wireless bridge, or WDS, topology, all four addressfields are used, as shown in Figure 4-11.Like all other data frames, WDS frames use thefirst address for the receiver of the frame and the second address for the transmitter.TheMAC uses these two addresses for acknowledgments and control traffic, such as RTS,CTS, and ACK frames.Two more address fields are necessary to indicate the source anddestination of the frame and distinguish them from the addresses used on the wirelesslink.Figure 4-11.WDS framesOn a wireless bridging link, there are usually no mobile stations, and the contention-freeperiod is not used.Access points are forbidden to enter power-saving modes, so thepower management bit is always set to 0.4.1.5.5 Frames using WEPFrames protected by WEP are not new frame types.When a frame is handled by WEP,the WEP bit in the Frame Control field is set to 1, and the Frame Body field begins withthe WEP header described in Chapter 5.4.2 Control Frames Control frames assist in the delivery of data frames.They administer access to thewireless medium (but not the medium itself) and provide MAC-layer reliability functions.4.2.1 Common Frame Control FieldAll control frames use the same Frame Control field, which is shown in Figure 4-12.Figure 4-12.Frame Control field in control framesProtocol versionThe protocol version is shown as 0 in Figure 4-12 because that is currently theonly version.Other versions may exist in the future.TypeControl frames are assigned the Type identifier 01.By definition, all controlframes use this identifier.SubtypeThis field indicates the subtype of the control frame that is being transmitted.ToDS and FromDS bitsControl frames arbitrate access to the wireless medium and thus can onlyoriginate from wireless stations.The distribution system does not send or receivecontrol frames, so these bits are always 0.More Fragments bitControl frames are not fragmented, so this bit is always 0.Retry bitControl frames are not queued for retransmission like management or data frames,so this bit is always 0.Power Management bitThis bit is set to indicate the power management state of the sender afterconclusion of the current frame exchange. More Data bitThe More Data bit is used only in management and data frames, so this bit is setto 0 in control frames.WEP bitControl frames may not be encrypted by WEP, which may be used only for dataframes and association requests.Thus, for control frames, the WEP bit is always
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