Certified Wireless Network Administrator (CWNA-109)

Notes for the CWNP Certified Wireless Network Administrator certification.

802.11 Standards

Most common PHYs

Notes for 802.11n:

• Typically no channel 146 (introduced in 802.11ac)
• 4 spatials streams in the standard, but only 3x3:3 in reality
• Frame Aggregation (A-MSDU, A-MPDU)
• TxBF introduction
• Falls back to 54 Mbps if WPA is used

Notes for 802.11ac:

• ONLY in 5 GHz band (other bands may have b/g/n)
• TxBF standardized to an option
• TxOP (transmit opportunity) with EDCA queues
• Falls back to 54 Mbps if WPA is used

Less common PHYs

Non-PHY

802.11 Frames

PHY Preamble

General format

Format of the MPDU that will be transmitted to the Physical Layer (where it name changes to PSDU).

  ◄──────────────────────────MPDU/PSDU───────────────────────────► 
  ┌─────────────────────────┌─────────────────┌──────────────────┐ 
  │ MAC Header              │ Payload (MSDU)  │ Footer           │ 
  │ 802.11 Information      │ User Data       │ Error Correction │ 
  └─────────────────────────└─────────────────└──────────────────┘ 

Several MSDU can be aggregated in in the same MPDU if they share the same MAC Header and are then called A-MSDU.

 ◄─────────────────────────────────────────────────────PPDU──────────────────────────────────────────────────► 
           ◄─────────────────────────────────────────────MPDU────────────────────────────────────────────────► 
 ┌─────────┌─────────────────────────┐─────────────────┌──────────────────┌─────────────────┌────────────────┐ 
 │ PLCP    │ MAC Header              │ Payload (A-MSDU)│ Payload (A-MSDU) │Payload (A-MSDU) │Footer          │ 
 │ Header  │ 802.11 Information      │ User Data       │ User Data        │User Data        │Error Correction│ 
 └─────────└─────────────────────────┘─────────────────└──────────────────└─────────────────└────────────────┘ 

Several MPDU can be aggregated in the same by the frame before beeing sent on the air by the PHY layer. They are then called A-MPDU. Each MPDU has its own MAC Header.

  ◄──────────────────────────────────────────────PPDU───────────────────────────────────────────────►  
 ┌────────┌───────────────────────┌────────────────────────┌───────────────────────┌─────────────────┐ 
 │ PLCP   │ A-MPDU                │ A-MPDU                 │ A-MPDU                │ Footer          │ 
 │ Header │ MAC Header + User Data│ MAC Header + User Data │ MAC Header + User Data│ Error Correction│ 
 └────────└───────────────────────└────────────────────────└───────────────────────└─────────────────┘ 

MAC Header

Detail of the MAC Header (field sizes are in bytes).

 ┌────────────┌──────────┌──────┌─────────┌─────────┌──────────────┌─────────┌─────────┌────────┐  
 │Frame       │Duration/ │Addr1 │Addr2    │Addr3    │Sequence      │Addr4    │QoS Ctrl │HT Ctrl │  
 │Control (2) │ID (2)    │(6)   │(0 or 6) │(0 or 6) │Ctrl (0 or 2) │(0 or 6) │(0 or 2) │(0 or 2)│  
 └────────────└──────────└──────└─────────└─────────└──────────────└─────────└─────────└────────┘  

Detail of the MAC Header Frame Control field (sizes are in bits).

 ┌────────────┌─────┌────────┌──────┌────────┌───────────────┌──────┌───────────┌──────────┌──────────┌──────────┐
 │Protocol    │Type │Subtype │To DS │From DS │More Fragments │Retry │Power Mgmt │More Data │Protected │+HTC/Order│
 │Version (2) │(2)  │(2)     │(1)   │(2)     │(1)            │(1)   │(1)        │(1)       │Frame (1) │(1)       │
 └────────────└─────└────────└──────└────────└───────────────└──────└───────────└──────────└──────────└──────────┘

Notes about fields

• Protocol Version : always 00.
• Power management : STA sets it to 1 if it sleeps after this frame.
• More Data : I have more data for you after this frame, do not sleep.