WLAN: Wireless LAN by IEEE 802.11, 802.11a, 802.11b, 802.11g, 802.11n

The Wireless Local Area Networl (WLAN) technology is defined by the IEEE 802.11 family of specifications. There are currently four specifications in the family: 802.11, 802.11a, 802.11b, and 802.11g. All four use the Ethernet protocol and CSMA/CA (carrier sense multiple access with collision avoidance instead of CSMA/CD) for path sharing.

• 802.11 -- applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).
• 802.11a -- an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps in the 5GHz band. 802.11a uses an orthogonal frequency division multiplexing (OFDM) encoding scheme rather than FHSS or DSSS. The 802.11a specification applies to wireless ATM systems and is used in access hubs.
• 802.11b (also referred to as 802.11 High Rate or Wi-Fi) -- an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.
• 802.11g -- offers wireless transmission over relatively short distances at 20 - 54 Mbps in the 2.4 GHz band. The 802.11g also uses the OFDM encoding scheme.
• 802.11n - builds upon previous 802.11 standards by adding MIMO (multiple-input multiple-output). IEEE 802.11n offers high throughput wireless transmission at 100Mbps – 200 Mbps.

The modulation used in 802.11 has historically been phase-shift keying (PSK). The modulation method selected for 802.11b is known as complementary code keying (CCK), which allows higher data speeds and is less susceptible to multipath-propagation interference. 802.11a uses a modulation scheme known as orthogonal frequency-division multiplexing (OFDM) that makes possible data speeds as high as 54 Mbps, but most commonly, communications takes place at 6 Mbps, 12 Mbps, or 24 Mbps. For short range and low power wireless (less than 10 meters) communications among personal devices such as PDA, Bluetooth and subsequent IEEE standards (802.15) are taking effects. For long range wireless communications in the metropolitan areas, WiMax as defined in the IEEE 802.16 is  the standard.

The 802.11 stack structure is as follows:

Protocol Structure - WLAN: Wireless LAN by IEEE 802.11, 802.11a, 802.11b,802.11g, 802.11n801.11 protocol family MAC frame structure:

2	2	6	6	6	2	6	0-2312	4
Frame Control	Duration	Address 1	Address 2	Address 3	Seq	Address 4	Data	Check sum

• Frame Control Structure:

2	2	4	1	1	1	1	1	1	1	1
Version	Type	Subtype	To DS	From DS	MF	Retry	Pwr	More	W	O

• Protocol Version - indicates the version of IEEE 802.11 standard.
• Type - Frame type: Management, Control and Data.
• Subtype - Frame subtype: Authentication frame, Deauthentication frame; Association request frame; Association response frame; Reassociation request frame; Reassociation response frame; Disassociation frame; Beacon frame; Probe frame; Probe request frame and Probe response frame.
• To DS - is set to 1 when the frame is sent to Distribution System (DS)
• From DS - is set to 1 when the frame is received from the Distribution System (DS)
• MF- More Fragment is set to 1 when there are more fragments belonging to the same frame following the current fragment
• Retry indicates that this fragment is a retransmission of a previously transmitted fragment. (For receiver to recognize duplicate transmissions of frames)
• Pwr - Power Management indicates the power management mode that the station will be in after the transmission of the frame.
• More - More Data indicates that there are more frames buffered to this station.
• W - WEP indicates that the frame body is encrypted according to the WEP (wired equivalent privacy) algorithm.
• O - Order indicates that the frame is being sent using the Strictly-Ordered service class.
• Duration/ID (ID) -
• Station ID is used for Power-Save poll message frame type.
• The duration value is used for the Network Allocation Vector (NAV) calculation.
• Address fields (1-4) - contain up to 4 addresses (source, destination, transmittion and receiver addresses) depending on the frame control field (the ToDS and FromDS bits).
• Sequence Control - consists of fragment number and sequence number. It is used to represent the order of different fragments belonging to the same frame and to recognize packet duplications.
• Data - is information that is transmitted or received.
• CRC - contains a 32-bit Cyclic Redundancy Check (CRC).