Tag Archives: csma/ca

Seg 5: wifi association and authentication

  1. Radio hosts connect to AP
  • Listening beacon frame sent by AP
    • AP send periodically beacon frame
    • Including the SSID and MAC of AP
  • Scanning 14 channels, and select an AP to associate by 802.11 association protocol
    • After association, means that radio hosts join the sub networks of the chosen AP
    • By possibly utilizing DHCP, radio host rents IP address from AP
    • Generally speaking, radio hosts choose an AP having the strongest signal beacon frame to associate.
  1. Scanning methods
  • Passive scanning, shown in Figure 1-7.
    • AP(s) periodically send beacon frame , the host scans 14 channels to find the all the possible beacon frames from APs at the location;
    • The host sends the request frame to the selected AP ②;
    • AP send the response frame to the host ③;

Figure 1-7 Passive scanning

  • Initiative scanning, shown in Figure 1-8.
    • The radio host broadcasts the request frame to AP ①;
    • APs send probe response ②;
    • The radio host sends the association request frame to the chosen AP ③;
    • The chosen AP sends the association response frame .

Figure 1-8 Imitative scanning

  1. Authentication
  • When Associated, the radio host may need to authenticate itself by authentication/identification mechanism;
  • By MAC address to authenticate
  • By user name/password to authenticate
  1. CSMA/CA and CSMA/CD
  • Base station should be full duplex for the ability of collision detection, which requires it have the ability to receive and send at the same time. However, the received signal strength is far less than the sent signal strength for WLAN adapter, which would consume too much energy to realize the collision detection. Therefore, most of wireless devices are generally half duplex.
  • Not all WLAN devices can detect the collision because of hidden terminals and attenuation, etc.
  • During the sending frame, if collision is detected, then base station don’t give up the sending frame, but finish sending the whole frame. Thus, if collision happens, in this time, the channels would be wasted.
  • Because the high bit error rate (BER), the sender would firstly transmit a frame to the receiver. After received it, the receiver would transmit a confirm frame to the sender in the SIFS period; then, after the sender receive this confirm frame, it would transmit the next data frame to receiver. If the sender don’t receive the confirm frame, it would retransmit the frame for several times.