- 5.1 Understand 802.11 contention algorithms in-depth and know how they impact WLANs
- 5.2 Analyze QoS configuration and operations
5.1
5.1.1 Distributed Coordination Function (DCF)
- DCF uses CSMA/CA when EDCA is not used
Carrier Sense and Energy Detect
- CS is the process of checking the medium
- 2 kinds of VS
- Virtual
- Uses NAV
- Physical
- Uses CCA
- Virtual
- Both must show the medium is avaliable
Network Allocation Vector (NAV)
- Provided by the MAC and not the PHY
- Timer used to determine if the station can access the medium
- If then NAV is 0 the station may contend for the medium
- Station waits till timer is 0 to check medium
- CCA must apply when the timer reaches 0 or the timer is recalculated
Contention Window (CW) and random backoff
- Choose a integer at random
- Multiplies by the slot time
Interframe Spacing
- SIFS timing varies by PHY
- FHSS – 28 us
- DSS – 10
- OFDM – 16
- HR/DSSS – 10
- ERP – 10
- SIFS/RIFS
- Shortest IFS
- RIFS is used in greenfield and there for not used any more
- PIFS
- Used for PCF
- Greater priority than DIFS
- DIFS
- Data IFS for DCF
- EIFS
- Used to send frames that were previously corrupted
- AIFS
- QOS IFS
5.1.2 Enhanced Distributed Channel Access (EDCA)
EDCA Function (EDCAF)
- Responsibility of the AP or WLC to convert 802.11 QOS to 802.1p/DSCP for wired side
- AC_BK
- Background
- Lowest priority
- 1,2 UP Priority
- CWmin=15
- CWmax=1023
- AIFS Number = 7
- DSCP – 0-7 and 8-15
- AC_BE
- Best effort
- Lowest priority
- 0,3 UP Priority
- CWmin=15
- CWmax=1023
- AIFS Number = 3
- DSCP – 16-23 and 24-31
- AC_VI
- Video
- Middle priority
- 4,5 UP Priority
- CWmin=7
- CWmax=15
- AIFS Number = 2
- DSCP – 32-39 and 40-47
- AC_VO
- Voice
- Highest priority
- 6,7 UP Priority
- CWmin=3
- CWmax=8
- AIFS Number = 2
- DSCP – 40-55 and 65-63
Access Categories and Queues
- PHY CWmin
- DSSS – 31
- HR/DSSS – 31
- OFDM – 15
- ERP – 31 OR 15 DEPENDING ON CAPATABILITY
- HT – 15
- VHT – 15
- AC_XX – CWmin
- STA will choose a number between 0 and CWmin
- If retry is required it will double the CWmin
MOS – mean opinion score – subjective test of perceived quality
r-factor – standard calculation of overall quality
5.2
5.2.1 Verify QoS parameters in capture files
5.2.2 Ensure QoS is implemented end-to-end