Shortest to Longest: RIFS, SIFS, PIFS, DIFS, AIFS, EIFS
Mnemonic to help remember: Really Super Powerful Dog Ate Everything ***EXAM*** The above (Shortest to longest) could be on the exam. The below are notes from my CWDP notes
SIFS (Shortest Inter Frame Space) - Used with all of the coordination functions. SIFS is the shortest of the IFS for 802.11-2007. Used prior to ACK and CTS frames. As well as in between MPDU's of a fragment burst. For 802.11n a shorter IFS (RIFS) was introduced.
RIFS (Reduced Inter Frame Space) - Introduced with 802.11n to help improve efficiency for transmissions that do not require a SIFS to a single receiver. Such as a transmission burst (CFB-Contention Free Burst.) 802.11n uses RIFS and Block ACK. RIFS is *only* used when Block ACK is enabled. When Block ACK are used data frames of a CFB can be sent continuously without stopping for an ACK. At the end of the CFB, the TX STA will send a BAR (Block ACK Request) and will/should receive a single Block ACK (BA)
DIFS (Distributed Inter Frame Space) - When a STA wants to transmit a data frame (MPDU) or a management frame (MMPDU) for the first time in a DCF (Distributed Coordination Function) network, the duration of the DIFS must be observed after the previous frames completion. DIFS are longer than SIFS and PIFS.
DIFS = SIFS + 2x SlotTime
SlotTime for 802.11a/n/ac (5 GHz) = 9μS
SlotTime for 802.11g/n (2.4 GHz – HT or ERP) = 9μS with short preamble
SlotTime for 802.11g/n (2.4 GHz – HT or ERP) = 20μS with long preamble
SlotTime for 802.11b/g/n (2.4 GHz – DSS ) = 20μS
EIFS (Extended Inter Frame Space) - EIFS are used by STA's that have received a frame that contained errors. By using the longer IFS, the transmitting station will have enough time to recognize the frame was no received correctly before the receiving station commences transmission. If, during the EIFS duration the STA receives a frame correctly (regardless of intended recipient), it will resume using DIFS or AIFS, as appropriate.
- EIFS does Have a drawback. STA's near to the AP can cause problems for STA's further away from the AP. This is because STA's close to the AP are using higher data rates, and as such higher modulation mechanisms. The STA's further away cannot demodulate these, and due to this interpret it as a corrupted frame. Making it stay quiet for the EIFS. Providing the near STA's to use DIFS or AIFS and giving it priority and getting more opportunity to transmit while the far station will remain quiet.
EIFS (in DCF) = SIFS + DIFS + ACK_Tx_Time
EIFS 802.11b/g/n devices using DSS = 364μS
EIFS 802.11g/n devices using OFDM = 160μS
EIFS 802.11a/n devices (5GHz) = 160μS
EIFS (in EDCA) = SIFS + AIFS[AC] + ACK_Tx_Time
AIFS (Arbitration Inter Frame Space) - The AIFS shall be used by QoS STAs to transmit all data frames (MPDUs), all management frames (MMPDUs), and the following control frames: PS-Poll, RTS, CTS (when not transmitted as a response to the RTS), BlockAckReq, and BlockAck (when not transmitted as a response to the BlockAckReq).
The number of slot times used in the AIFS is called the Arbitration Inter Frame Space Number (AIFSN). 802.11e specifies 4 access categories (AV_VO : Voice, AC_VI : Video, AC_BE : Best Effort & AC_BK : Background). Voice & Videocategory use 2 slottimes by default. Best Effort category use 3 slottimes where as Background traffic use 7 slottimes by default.
Below is the formula to calcluate AIFS for a given Access Category (AC)
AIFS[AC] = AIFSN[AC] × SlotTime + SIFSTime
Chapter 3 - Security Communications Brief March 2, 2018
WPA and WPA2
It's important to remember that these are certifications by the WiFi Alliance and not from the 802.11 standard. This means that they validate that a device uses portions of the security that 802.11 provides. They both come in two forms, Personal and Enterprise. Personal is known as Pre Shared Key because it uses a PSK.
WPA has been depreciated and as such its use should be as well. It used TKIP/RC4 and again, as such, TKIP/RC4 should no longer be used either.
The Enterprise version of both WPA and WPA2 both use the 802.1x framework for authentication and key management. This framework has three primary components.
1.) Supplicant (Client STA)
2.) Authenticator (AP or Controller)
3.) Authentication Server (This is normally your RADIUS server)
The EAPoL protocol is used for communication between the Supplicant and Authenticator, and RADIUS is used between the Authenticator and the Authentication Server.
The process looks something like this
Client Authenticates and Associates to an AP
Open System Authentication takes place
EAP Authentication using the RADIUS server
4-way handshake generates encryption keys for STA and AP
Encrypted communications commence.
Carpenter, Tom. CWAP: Certified Wireless Analysis Professional: Official study guide: Edition CWAP-402. Certitrek Publishing, 2016.
RADIUS - Remote Authentication Dial-In User Service