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Understanding 802.11 Frame Types
By Jim Geier
August 15, 2002
When analyzing or troubleshooting the operation of a wireless LAN, you'll likely
be using an 802.11 packet analyzer (e.g., AiroPeek or Sniffer Wireless)
to monitor the communications between radio network interface cards (NICs) and
access points. After capturing the packets, you need to understand the different
802.11 frame types as a basis for deciphering what the network is or isn't doing.
In this tutorial, I'll give you an overview of the more common 802.11 frames
to help you become more adept at comprehending the operation of a wireless LAN
and solving network problems.
The 802.11 standard defines various frame types that stations (NICs and access
points) use for communications, as well as managing and controlling the wireless
link. Every frame has a control field that depicts the 802.11 protocol version,
frame type, and various indicators, such as whether WEP is on, power management
is active, and so on. In addition all frames contain MAC addresses of the source
and destination station (and access point), a frame sequence number, frame body
and frame check sequence (for error detection).
802.11 data frames carry protocols and
data from higher layers within the frame body. A data frame, for example, could
be carrying the HTML code from a Web page (complete with TCP/IP headers) that
the user is viewing. Other frames that stations use for management and control
carry specific information regarding the wireless link in the frame body. For
example, a beacon's frame body contains the service set identifier (SSID), timestamp,
and other pertinent information regarding the access point.
Note: For more details regarding
802.11 frame structure and usage, refer to the 802.11 standard, which is free
for download from the 802.11
Working Group Web site.
Management Frames
802.11 management frames enable stations
to establish and maintain communications. The following are common 802.11 management
frame subtypes:
- Authentication frame: 802.11 authentication is a process whereby
the access point either accepts or rejects the identity of a radio NIC. The
NIC begins the process by sending an authentication frame containing its identity
to the access point. With open system authentication (the default), the radio
NIC sends only one authentication frame, and the access point responds with
an authentication frame as a response indicating acceptance (or rejection).
With the optional shared key authentication, the radio NIC sends an initial
authentication frame, and the access point responds with an authentication
frame containing challenge text. The radio NIC must send an encrypted version
of the challenge text (using its WEP key) in an authentication frame back
to the access point. The access point ensures that the radio NIC has the correct
WEP key (which is the basis for authentication) by seeing whether the challenge
text recovered after decryption is the same that was sent previously. Based
on the results of this comparison, the access point replies to the radio NIC
with an authentication frame signifying the result of authentication.
- Deauthentication frame: A station sends a deauthentication frame
to another station if it wishes to terminate secure communications.
- Association request frame: 802.11 association enables the access
point to allocate resources for and synchronize with a radio NIC. A NIC begins
the association process by sending an association request to an access point.
This frame carries information about the NIC (e.g., supported data rates)
and the SSID of the network it wishes to associate with. After receiving the
association request, the access point considers associating with the NIC,
and (if accepted) reserves memory space and establishes an association ID
for the NIC.
- Association response frame: An access point sends an association
response frame containing an acceptance or rejection notice to the radio NIC
requesting association. If the access point accepts the radio NIC, the frame
includes information regarding the association, such as association ID and
supported data rates. If the outcome of the association is positive, the radio
NIC can utilize the access point to communicate with other NICs on the network
and systems on the distribution (i.e., Ethernet) side of the access point.
- Reassociation request frame: If a radio NIC roams away from the currently
associated access point and finds another access point having a stronger beacon
signal, the radio NIC will send a reassociation frame to the new access point.
The new access point then coordinates the forwarding of data frames that
may still be in the buffer of the previous access point waiting for transmission
to the radio NIC.
- Reassociation response frame: An access point sends a reassociation
response frame containing an acceptance or rejection notice to the radio NIC
requesting reassociation. Similar to the association process, the frame includes
information regarding the association, such as association ID and supported
data rates.
- Disassociation frame: A station sends a disassociation frame to
another station if it wishes to terminate the association. For example, a
radio NIC that is shut down gracefully can send a disassociation frame to
alert the access point that the NIC is powering off. The access point can
then relinquish memory allocations and remove the radio NIC from the association
table.
- Beacon frame: The access point periodically sends a beacon frame
to announce its presence and relay information, such as timestamp, SSID, and
other parameters regarding the access point to radio NICs that are within
range. Radio NICs continually scan all 802.11 radio channels and listen to
beacons as the basis for choosing which access point is best to associate
with.
- Probe request frame: A station sends a probe request frame when it
needs to obtain information from another station. For example, a radio NIC
would send a probe request to determine which access points are within range.
- Probe response frame: A station will respond with a probe response
frame, containing capability information, supported data rates, etc., when
after it receives a probe request frame.
Control Frames
802.11 control frames assist in the delivery
of data frames between stations. The following are common 802.11 control frame
subtypes:
- Request to Send (RTS) frame: The RTS/CTS function is optional and
reduces frame collisions present when hidden stations have associations with
the same access point. A station sends a RTS frame to another station as
the first phase of a two-way handshake necessary before sending a data frame.
- Clear to Send (CTS) frame: A station responds to a RTS with a CTS
frame, providing clearance for the requesting station to send a data frame.
The CTS includes a time value that causes all other stations (including hidden
stations) to hold off transmission of frames for a time period necessary for
the requesting station to send its frame. This minimizes collisions among
hidden stations, which can result in higher throughput if you implement it
properly.
- Acknowledgement (ACK) frame: After receiving a data frame, the receiving
station will utilize an error checking processes to detect the presence of
errors. The receiving station will send an ACK frame to the sending station
if no errors are found. If the sending station doesn't receive an ACK after
a period of time, the sending station will retransmit the frame.
Data Frames
Of course the main purpose of having a
wireless LAN is to transport data. 802.11 defines a data frame type that carries
packets from higher layers, such as web pages, printer control data, etc., within
the body of the frame. When viewing 802.11 data frames with a packet analyzer,
you can generally observe the contents of the frame body to see what packets
that the 802.11 data frames are transporting.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
(SAMs, 2001), and regularly instructs workshops on wireless LANs.
Join Jim for discussions as he answers questions in the 802.11 Planet Forums.
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