CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

Authentication The 802.11 authentication process is where an access point accepts
or rejects the identity of a wireless adapter. A wireless adapter
begins the process by sending an Authentication frame that contains
its identity to the access point. For open authentication, the access
point responds with an Authentication frame as a response to
indicate the acceptance or rejection; while for shared-key
authentication, the access point responds with an Authentication
frame containing challenge text, which the wireless client must
response with an Authentication frame containing the encrypted
version of the challenge text using the shared-key for the access
point to verify its identity. WLAN authentication occurs at L2 and
is authenticating devices instead of users. The authentication and
association processes are occurred in sequence.
Note: Authentication occurs first and then followed by association.
Deauthentication A wireless station sends a Deauthentication frame to another
wireless station in order to terminate a secure connection.
- Below lists the IEEE 802.11 control frames that assist the delivery of data frames between
wireless stations:
Request to Send (RTS) A station sends a RTS frame to another station as the 1st phase of
the necessary 2-way handshake before transmitting a data frame.
Clear to Send (CTS) A station response to a RTS frame with the CTS frame to provide
the clearance for the source station to transmit a data frame.
The CTS frame contains a time value which would cause all
nearby stations (including hidden stations) to hold off data
transmission for a certain period of time necessary for the source
station to transmit its frames.
Acknowledgement (ACK) A destination station would run an error checking process to
detect the presence of errors upon received a data frame.
The destination station would send an ACK frame to the source
station if no errors are found. The source station will retransmit
the frame if it doesn’t receive an ACK for the frame for a certain
period of time.
Note: Kindly refer to Page 174 for the discussion of the CSMA/CA and RTS/CTS mechanisms.
- Finally, data frames are used to carry upper layers data – packets.
- Below shows the wireless client association process:
i) Access points send out beacons announcing the SSID and supported data rates.
ii) A wireless client scans all changes and sends out Probe Request frames to all access
points within range.
iii) All access points within range reply with a Probe Response frame, and the wireless client
listens for the responses from the access points.
iv) The wireless client associates with the access point with the strongest signal.
Authentication and other security information are sent to the access point.
v) The access point accepts the association request and associated with the wireless client.
Note: 802.1X authentication could occur straight after the association process is completed.
- The maximum Ethernet frame size is 1518 bytes whereas a wireless frame could be as large as
2346 bytes. Usually the WLAN frame size is limited to 1518 bytes as WLANs are often
connected to and communicating with wired Ethernet networks.
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Copyright © 2008 Yap Chin Hoong
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