Wi-Fi (802.11abgn) - 2

Well. In last article we saw a simple introduction to wireless networks and simple connection protocol in a BSS.

Introduction to Wi-Fi

Let us march further little towards physical layer elements.

Channels:

Channels are nothing but the frequencies which are used to transmit the information over the air. As of now we got two spectrum for operation of Wi-Fi. One is 2.4 GHz Spectrum and other is 5 GHz Spectrum. The number of channels supported for a country is variable and guided by strict Regulatory domain.
Both the spectra are unlicensed spectra and can be used by non WLAN devices (Microwave oven, bluetooth for 2.4 GHz, Cordless phone and baby monitor for both 2.4 and 5 GHz) as well.

2.4 GHz Spectrum:

This spectrum starts from frequency 2.412 GHz. This 2.412 is the centre frequency for the channel 1. Every channel has bandwidth of 20 MHz out of that only 5 MHz is dedicated. The other bandwidth will be shared with neighboring channels. Hence every channel will have dedicated 5 MHz BW, 5 MHz BW shared by one neighboring channel, 5 MHz BW shared by two neighboring channel and 5 MHz BW shared by three neighboring channel.

In Asian Counties, Europe, the usage of channel 12 (2.467) and 13 (2.472) is also permitted. For 802.11b only operation channel 14 (2.477) is also allowed.
In 2.4 GHz spectrum, the standards 802.11b,g are specified specific to 2.4 GHz radio.

5 GHz Spectrum:

Unlike 2.4 GHz spectrum, 5 GHz spectrum has dedicated 20 MHz Bandwidth for every channel. The 5 GHz spectrum starts with channel 36 (5.18 GHz). We can split 5 GHz radio into three, viz. a. Lower 5 GHz (channels 36,40,44,48), b. Upper 5 GHz (channels 149, 153, 157, 161, 165), c. DFS channels (channels from 52 to 64 and 100 to 140).

There are strict regulatory domains which will guide the usage of DFS channel and 5 GHz spectrum for WLAN purpose. 802.11a standard is defined specific to 5 GHz radio.

40 MHz Bandwidth in 802.11n:

The default bandwidth for 802.11 is 20 MHz. But with developments in 802.11n standard the usage of additional bandwidth 40 MHz is also defined. In 40 MHz Channel Width Mode (CWM), the 20 MHz of the neighboring channel is also used. Under this case the neighboring channel will be termed as secondary channel or extension channel or data channel. The actual channel will be called as primary channel or control channel. The usage of 40 CWM is also referred as channel bonding.

In 2.4 GHz radio, for channel 1 the secondary channel will be 5 under extension channel spacing of 20 (default). Extension channel spacing should have a minimum value of 20 MHz.

For understanding more on extension channel let us assume the primary channel to be 1. Channel 1 shares its bandwidth with channel 2(15 MHz shared), 3(10 MHz shared) and 4(5 MHz shared) (Refer table for clean understanding).

If the Extension channel spacing is 15 MHz (less than 20 MHz), then the secondary channel for channel 1 will be channel 4

2.412(channel 1) + 0.015(extn channel spacing) = 2.427(channel 4)

Our frequency range will be from 2.402 (Starting of channel 1) to 2.437 (Ending of channel 4). Hence Bandwidth will be

2.437 - 2.402 = 0.035 GHz or 35 MHz.

In other words, Channel 4 already shares 5 MHz bandwidth with channel 1. Hence we will get only 35 MHz for our operation. As per standards two extension channel spacing is defined 20 MHz and 25 MHz. In 20 MHz ECS mode, extension channel will be primary channel (Plus or Minus) 4 and in 25 MHz ECS mode, extension channel will be primary channel (Plus or Minus) 5.

In case of 5 GHz, channel 36 will be bonded with channel 40 and vice versa. channel 44 and 48 will be bonded. 40 should not be bonded with 44 which will lead to violation of regulatory domain.

Recently, Wi-Fi organisation along with Wi-Gi is working on evolution of new standard which will operate on 60 GHz spectrum. Once the standard got defined, we may achieve a faster WLAN.