Management Frames

Management Frames
The purpose of management frames is to establish initial communications between stations and
access points. Thus, management frames provide such services as association and authentication.
Figure 4.9 depicts the common format of all management frames.  The Duration field within all management frames during the contention-free period (as defined
by the point coordination function) is set to decimal 32,768 (hexadecimal value of 8000), giving
management frames plenty of time to establish communications before other stations have
the capability to access the medium.
During the contention period (as defined by the CSMA-based distributed coordination function),
all management frames have the Duration field set as follows:
• If the destination address is a group address, the Duration field is set to 0.
• If the More Frag bit is set to 0 and the destination address is an individual address, then
the Duration field contains the number of microseconds required to transmit one ACK
frame plus one short interframe space. (The section “Access Spacing,” earlier in this
chapter, defines the interframe space.) • If the More Frag bit is set to 1 and the destination address is an individual address, then
the Duration field contains the number of microseconds required to transmit the next
fragment, plus two ACK frames, plus three short interframe spaces.
A station receiving a management frame performs address matching for receive decisions
based on the contents of the Address 1 field of the MAC frame, which is the destination
address (DA). If the address matches the station, then that station completes the reception of
the frame and hands it off to the LLC layer. If a match does not occur, the station ignores the
rest of the frame.
The following defines each of the management frame subtypes:
• Association request frame A station will send this frame to an access point if it wants
to associate with that access point. A station becomes associated with an access point
after the access point grants permission.
• Association response frame After an access point receives an association request
frame, the access point will send an association response frame to indicate whether or
not it is accepting the association with the sending station.
• Reassociation request frame A station will send this frame to an access point if it
wants to reassociate with that access point. A reassociation may occur if a station moves
out of range from one access point and within range of another access point. The station
will need to reassociate (not merely associate) with the new access point so that the new
access point knows that it will need to negotiate the forwarding of data frames from the
old access point.
• Reassociation response frame After an access point receives a reassociation request
frame, the access point will send a reassociation response frame to indicate whether or
not it is accepting the reassociation with the sending station.
• Probe request frame A station sends a probe request frame to obtain information from
another station or access point. For example, a station may send a probe request frame to
determine whether a certain access point is available.
• Probe response frame If a station or access point receives a probe request frame, the
station will respond to the sending station with a probe response frame containing specific
parameters about itself (such as parameter sets for the frequency hopping and direct
sequence PHYs).
• Beacon frame In an infrastructure network, an access point periodically sends a beacon
(according to the aBeaconPeriod parameter in the MIB) that provides synchronization
among stations utilizing the same PHY. The beacon includes a timestamp that all
stations use to update what 802.11 defines as a timing synchronization function (TSF)
timer. If the access point supports the point coordination function, then it uses a beacon frame
to announce the beginning of a contention-free period. If the network is an independent
BSS (that is, it has no access points), all stations periodically send beacons for synchronization
purposes.
• ATIM frame A station with frames buffered for other stations sends an announcement
traffic indication message (ATIM) frame to each of these stations during the ATIM window,
which immediately follows a beacon transmission. The station then transmits these
frames to the applicable recipients. The transmission of the ATIM frame alerts stations in
sleep state to stay awake long enough to receive their respective frames.
• Disassociation frame If a station or access point wants to terminate an association, it
will send a disassociation frame to the opposite station. A single disassociation frame can
terminate associations with more than one station through the broadcast address of all
ones.
• Authentication frame A station sends an authentication frame to a station or access
point that it wants to authenticate with. The authentication sequence consists of the transmission
of one or more authentication frames, depending on the type of authentication
being implemented (open system or shared key). Refer to the section “Providing
Authentication and Privacy,” earlier in this chapter.
• Deauthentication frame A station sends a deauthentication frame to a station or
access point with which it wants to terminate secure communications.
The content of the Frame Body field of management frames depends on the type of management
frame being sent. Figure 4.10 identifies the Frame Body contents of each management
frame subtype. The 802.11 standard describes the Frame Body elements of the management frame subtypes.
Refer to the standard if you need detailed information, such as field formats. The following,
however, summarizes each of the elements:
• Authentication Algorithm Number This field specifies the authentication algorithm
that the authenticated stations and access points are to use. The value is either 0 for open
system authentication or 1 for shared key authentication.
• Authentication Transaction Sequence Number This field indicates the state of
progress of the authentication process.
• Beacon Interval This value is the number of time units between beacon transmission
times.
• Capability Information This field announces capability information about a particular
station. For example, a station can identify its desire to be polled in this element.
• Current AP Address This field indicates the address of the access point that the station
is currently associated with.
• Listen Interval This value identifies, in units of Beacon Interval, how often a station
will wake to listen to beacon management frames.
• Reason Code This field indicates (via a numbered code) why a station is generating an
unsolicited disassociation or deauthentication. Examples of the reasons are as follows:
• Previous authentication no longer valid.
• Disassociated due to inactivity.
• Station requesting association is not authenticated with responding station.
• Association ID (AID) This ID, which is assigned by an access point during
association, is the 16-bit identification of a station corresponding to a particular association.
• Status Code This code indicates the status of a particular operation. Examples of status
are as follows:
• Successful.
• Unspecified failure.
• Association denied because the access point is unable to handle additional associated
stations.
• Authentication rejected due to timeout waiting for next frame in sequence.
• Timestamp This field contains the timer value at the sending station when it transmits
the frame.
• Service Set Identify (SSID) This field contains the identity of the extended service
set (ESS). • Supported Rates This field identifies all data rates a particular station can accept. This
value represents the data rate in 500Kbps increments. The MAC coordination has the
capability to change data rates to optimize performance of frame transmissions.
• FH Parameter Set This field indicates the dwell time and hopping pattern needed to
synchronize two stations using the frequency-hopping PHY.
• DS Parameter Set This field identifies the channel number that stations are using with
the direct sequence PHY.
• CF Parameter Set This field consists of a series of parameters that support the point
coordination function (PCF).
• TIM The traffic indication map (TIM) element specifies the stations having MSDUs
buffered at the access point.
• IBSS Parameter Set This field contains parameters that support the Independent Basic
Service Set (IBSS) networks.
• Challenge Text This field contains the challenge text of a shared key authentication
sequence. If you’re using access points from multiple vendors, you should disable the transmission
of the extensions.