802.11n (Wi-Fi 4)

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IEEE 802.11n standard introduced several improvements to increase bandwidth.

• can work over both 2.4 GHz and 5 GHz
• Each band is implemented by a separate radio
• An access point or adapter that can support simultaneous 2.4 GHz and 5 GHz operation is referred to as dual band
  • Cheaper client adapters and many smartphone adapters support only a 2.4 GHz radio
• 802.11n can work in 2 modes:
  • high throughput (HT)/greenfield mode
    • for maximum performance
    • likely to cause substantial interference if there are legacy WLANs nearby on the same channel
  • mixed mode
    • for compatibility with older standards
    • reduces overall WLAN performance
• In recent years, Wi-Fi standards have been renamed with simpler digit numbers
  • 802.11n is now officially designated as Wi-Fi 4

Channel Bonding

• 802.11n standard allows two adjacent 20 MHz channels to be combined into a single 40 MHz channel
  • referred to as channel bonding
    • is only practical option in 5 GHz band on a network with multiple APs
      • Due to the restricted channel layout of 2.4 GHz
    • 5 GHz band has wider frequency range
      • can provide up to 25 non-overlapping channels
      • are not necessarily contiguous
        • slightly reduces options for bonded channels
      • DFS and regulatory requirements can impact channel availability
        • use of some channels may be blocked if the access point detects a radar signal

MIMO

Multiple input multiple output (MIMO) is the use of multiple reception and transmission antennae to boost bandwidth via spatial multiplexing and to boost range and signal reliability via spatial diversity.

• introduced with 802.11n
• increases reliability and bandwidth by multiplexing signal streams from 2–4 separate antennas (radio chain)
  • configuration of an 802.11n radio chain is identified by AxB:C notation
    • is the number of transmit antennae
    • is the number of receive antennae
    • is the number of simultaneous transmit and receive streams
  • maximum possible is 4x4:4
    • 2x2:2 and 3x2:2 are common
  • e.g., 4x4:4 access point could allocate two streams carrying different data to a 2x2:2 client, increasing bandwidth
    • referred to as spatial multiplexing
  • spatial diversity
    • if the same data stream is sent by two or three transmit antennae
      • receiver can combine them to derive a stronger signal and increase range at a given data rate
    • multiple receive antennae can derive a stronger signal
      • even if there is only one transmit stream
    • e.g., 2x2:2 and 2x3:2 radio chains have the same throughput
      • but the 2x3:2 chain could make more use of spatial diversity to increase range

Data Rate

• nominal data rate for 802.11n is
  • 72 Mbps per stream
  • or 150 Mbps per stream for a 40 MHz bonded channel
• theoretical maximum nominal data rate:
  • 600 Mbps for a 40 MHz bonded channel
    • assumes 4 spatial streams and optimal conditions
• 802.11n access points are marketed using Nxxx designations
  • where xxx is the nominal bandwidth
  • E.g., an N600 2x2 access point can allocate a bonded channel two streams for a data rate of 300 Mbps
    • if it does this simultaneously on both its 2.4 GHz and 5 GHz radios, the bandwidth of the access point could be described as 600 Mbps