STA Access
- Scanning
- Link authentication
- Association
Scanning
A STA can actively or passively scan wireless networks.
Active Scanning
Probes containing an SSID: The STA sends a Probe Request frame containing an SSID in each channel to search for the AP with the same SSID. Only the AP with the same SSID will respond to the STA. For example, in Figure 4-10, the STA sends a Probe Request frame containing the SSID huawei to search for an AP with the SSID huawei.
This method applies to the scenario where a STA actively scans wireless networks to access a specified wireless network.
Probes that do not contain an SSID: The STA periodically broadcasts a Probe Request frame that does not contain an SSID in the supported channels as shown in Figure 4-11. The APs return Probe Response frames to notify the STA of the wireless services they can provide.
This method applies to the scenario where a STA actively scans wireless networks to determine whether wireless services are available.
Passive Scanning
When passive scanning is enabled, a STA listens on the Beacon frames that an AP periodically sends in each channel to obtain AP information, as shown in Figure 4-12. A Beacon frame contains information including the SSID and supported rate.
To converse power, enable the STA to passively scan wireless networks. In most cases, VoIP terminals passively scan wireless networks.
Link Authentication
- Open system authentication requires no authentication. STAs that attempt to access the AP are successfully authenticated as long as the AP supports this mode. An illustration of the open system authentication procedure is shown in Figure 4-13.
- Shared key authentication requires that the STA and AP have the same shared key preconfigured. The AP checks whether the STA has the same shared key to determine whether the STA can be authenticated. If the STA has the same shared key as the AP, the STA is authenticated. Otherwise, STA authentication fails. Figure 4-14 shows the shared key authentication process.The shared key authentication process consists of the following steps:
- The STA sends an Authentication Request packet to the AP.
- The AP generates a challenge and sends it to the STA.
- The STA uses the preconfigured key to encrypt the challenge and sends it to the AP.
- The AP uses the preconfigured key to decrypt the encrypted challenge and compares the decrypted challenge with the challenge sent to the STA. If the two challenges are the same, the STA is authenticated. Otherwise, STA authentication fails.
Association
STA association is also known as to link negotiation. After link authentication is complete, a STA initiates link negotiation using Association packets. Figure 4-15 shows the association process in the Fit AP architecture.
- The STA association process in the Fit AP architecture consists of
the following steps:
- The STA sends an Association Request packet to the AP. The Association Request packet carries the STA's parameters and the parameters that the STA selects according to the service configuration, including the transmission rate, channel, QoS capabilities, access authentication algorithm, and encryption algorithm.
- The AP receives the Association Request packet, encapsulates the packet into a CAPWAP packet, and sends the CAPWAP packet to the AC.
- The AC determines whether to authenticate the STA and replies with an Association Response packet.
- The AP decapsulates the received Association Response packet and sends it to the STA.
- If the STA does not need to be authenticated, the STA can access the wireless network.
- If the STA needs to be authenticated, the STA initiates user access authentication. After authentication, the STA can access the wireless network. For details about user access authentication, see NAC in Feature Description - Security.
