Chapter 4 Medium Access Control Sublayer PART 2

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Ryerson University
Information Technology Management
ITM 301
Franklyn Prescod

444 The 80211 Frame Structure The 80211 standard defines three different classes of frames in the air data control and management Each of these has a header with a variety of fields used within the MAC sublayerData Frame first comes the Frame control field which is made up of 11 subfields1 Frame Control FieldBits Protocol version which is 00 to allow future versions of 80211 to operateType data control or management and Subtype fields RTS or CTSThe To DS and From DS bits are set to indicate whether the frame is going to or coming from the network connected to the APsdistribution systemMore fragments bit means more fragments will follow The Retry marks a retransmission of a frame sent earlier Power Management indicates that the sender is going into power save made Moredata bit indicates that the sender has additional frames for the receiver Protected demonstrates that the frame body has been encrypted for security Finally Order tells the receiver that the higher layer expects the sequence of frames to arrive strictly in order2 Duration Field tells how long the frame and its acknowledgement will occupy the channel measured in microseconds 3 Addresses Data frames sent to or from an AP has three addresses all in standard IEEE 802 format First address is the receiver the second is the transmitter the third gives distant endpoint 4 Sequence numbers frames so that duplicates can be detected5 Data Field contains the payload up to 2312 bytes The first bytes of this payload are in a formal known as Logical Link Control LLC which is the glue that identifies the higher layer protocol to which the payloads should be passed 6 Frame Check Sequence same 32 bit CRC445 Services Association Service used by mobile stations to connect themselves to APs It is used just after a station moves within radio range of the AP Upon arrival the station learns the identity and capabilities of the AP either from the beacon frames or by directly asking the AP Reassociation lets a station change its preferred AP Useful for mobile stations moving from one AP to another AP in the same extended 80211 LAN like a handoverDisassociate either the station or the AP can break their relationship station uses this service before shutting down or leaving the network AP may use this when going for maintenance Authenticate for security reasons before they can send frames via the APOpen 80211 anyone is allowed to use itWPA2 Wifi Protected Access 2 the AP can talk to an authentication server that has a username and password database to determine if the station is allowed to access the network Several frames are exchanged between the station and the AP with a challenge and response that lets the station prove it has the right credentials this happens after association Distribution Service once frames reach the AP this determines how to route them If the destination is local to the AP the frames can be sent out directly over the air Otherwise they will have to be forwarded over the wired network Integration Service handles any translation that is needed for a frame to be sent outside the 80211 LAN
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