IEEE 802.11 standard
802.11 defines a Basic Service Set (BSS) comprising of two or more fixed, portable, and/or moving nodes or stations that can communicate with each other in a limited geographical area. There are two configurations:- Infrastructure mode: at least one central access point (AP) connected to the wired network
- Ad hoc or peer-to-peer mode: a set of wireless stations communicate directly with one another without requiring a central access point or a wired connection
802.11 define the physical layer and medium access (MAC) layer. There are three radio frequency (RF) physical layers that operate at the 2.4 GHz ISM band, namely frequency hopping spread spectrum (FHSS), digital sequence spread spectrum (DSSS) and infrared (IR).
In WLAN, CSMA/CD is unsuitable due to the hidden station problem. Refering to the figure below, let's say A and C are not within transmission range of each other. C is currently transmitting to B. Using CSMA/CD, when A wants to transmit to B, it has to listen to determine if there is currently any ongoing transmission. Because C is not within A's transmission range, it is unable to detect C's transmission. When A starts transmitting to B, it results in a collision at B.
In order to overcome this problem, 802.11 specifies a distributed coordination function (DCF) known as carrier sense multiple access with collision avoidance (CSMA/CA) as an access protocol at the MAC layer. DCF works as follows.
It senses the channels. If an idle channel is detected, the station continues sensing the channel for an additional duration, termed DCF Interframe Space (DIFS), to avoid a collision in case two stations senses an idle channel at the same time. If the channel is still idle after a period of DIFS, the station may start transmission.
If the channel is busy, it defers transmission and enters a backoff state. The time period following DIFS is called a contention window (CW), consisting of a pre-determined number of slots. Upon entering a backoff state, the station randomly selects a slot in the contention window and continuously senses the channel for a period up to the selected slot. If no transmission is detected, it captures the medium by starting to transmit. If it detects a transmission, it enters a backoff state again and repeat the whole process.
IEEE 802.11b standard
This is more popularly known as the Wi-Fi (Wireless Fidelity). It ensures compatibility with legacy 802.11 products by operating at the same 2.4 GHz frequency band and using the DSSS with either complementary code keying (CCK) or packet binary convolutional coding (PBCC). It offers a maximum bit rate of 11 Mbps with fallback rates of 5 Mbps, 2 Mbps and 1 Mbps, which provides for smooth transition to faster WLAN systems. The access protocol is similar to 802.11.A Wi-Fi device typically consumes about 100 – 350 mA power. The power management strategy specifies that a Wi-Fi device may be either in awake state or doze state (the station cannot transmit nor receive). There are 2 power management modes: active mode (AM) and power save mode (PS). A station that wishes to go into PS must inform the AP by setting the power management bit in the packet header.
In an infrastructured network, the AP stores all traffic addressed to stations that are in PS. When the AP transmits the periodic beacon, it checks the stations in PS to see if there is pending traffic for them. Stations in PS switch to AM at regular intervals to receive the beacon. If there is pending traffic for them, they receive the traffic and then return to PS.
In an ad hoc network, traffic destined for stations in PS is stored in a distributed fashion among active stations since there is no AP. All stations in PS switch to awake state in a temporal window, called Announcement Traffic Indication Message (ATIM) window. During ATIM, stations that store pending traffic for other nodes send special frames called ATIM frames. If a station receives an ATIM frame, it remains awake to receive the traffic, otherwise it returns to PS until the next ATIM window.
Even though Wi-Fi quickly became popular due to its high bit rate, interference in the 2.4 GHz ISM band is a serious problem. To overcome this problem, IEEE released the 802.11a standard that operates in the 5 GHz frequency band.
Reference
IEEE 802.11: Wireless LANs
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