Steps to WLAN Connection
Steps to WLAN Connection There are 3 basic steps that must occur in order to connect to a WLAN and access network resources: 1. Discovery – the process whereby the station learns of available WLANs so that one can be chosen for connection. There are two types of Discovery: a. Passive – An access point normally transmits a Beacon frame every 100ms. The Beacon frame contains information such as SSID, channel, security features and other information about the WLAN. Stations scan for Beacon frames on all available channels continuously. i. If an SSID is discovered that the station has previously associated with it will be on the “Preferred Networks” list. By default, Windows client software will connect to either the top-most listed SSID or the SSID with the strongest signal. Which one depends on the wireless NIC drivers. ii. All SSIDs discovered by the station are listed on the “Available Wireless Networks” so that the user may initiate a manual connection. b. Active – The station seeks out SSIDs from the “Preferred Networks” list by transmitting a Probe Request frame to that SSID on all channels. Optionally, a station may transmit a Probe Request with the SSID field left blank, which is called a “Null” Probe Request. i. When an access point receives a Probe Request that contains a valid SSID used by that access point, it will respond with a Probe Response. The Probe Response has nearly identical information as a Beacon frame. An access point, by default, will also respond to a “Null” Probe ii. Optionally, an access point may be configured to suppress the response to Probe Requests of either variety. iii. Stations continuously probe for all SSIDs on their “Preferred Networks” list on all channels even while they are connected to an active access point. Once the station has selected a WLAN to connect with then step two is taken. 2. Authentication – By rule, all stations must authenticate to an access point prior to Association. There are two forms of Authentication used at this point: a. Open Authentication – this is the only form of 802.11 authentication actually used in step two. The station transmits an Authentication Request frame and the access point responds with an Authentication Response frame granting access. b. Shared Key Authentication – this form is never used. The station transmits the WEP encryption key, in clear text, to the access point and the access point then verifies that it matches the key it holds. The access point would then transmit an Authentication Response frame to the station. Once open authentication is achieved, it is on to step 3. 3. Association – the station joins the Basic Service Set by transmitting an Association Request frame to the access point. The access point responds by transmitting an Association Response frame to the station. The Association Response frame includes an Association Identifier (AID) which is a short code that is used during Power Management. Additional security, such as 802.11i authentication and encryption would happen after Association. Otherwise, network traffic commences, typically with the station transmitting a DHCP Discovery frame.
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