CWAP – WLAN Medium Access

• 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
• 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