Wireless network throughput is usually influenced by interference caused by multiple nodes concurrent activity. IEEE 802.11 MAC protocol utilizes different methods to maintain low interference and avoid packet collisions. These methods aim at “one transmitter at a time” on a given channel to avoid concurrency. The section belowreviews two competing interference avoidance methods.

a)      Clear Channel Assessment

IEEE 802.11 (WiFi) implements CarrierSense Multiple Access with Collision Avoidance (CSMA/CA). Because of CSMA/CA, 802.11 compliant devices usually listen to/sense the medium to check the status and see if the medium is idle before transmitting. This notion is often referred to as “listen-before-talk”. WiFi carrier sense is composed of two functions: clear channel assessment (CCA) and Network Allocation Vector (NAV). CCA provides information about the current state of the medium i.e. the medium is currently busy.  NAV indicates how many future frames the medium is expected to be busy following the current frame. In order to detect the current state of the medium, CCA utilizes the following techniques:

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1)     Carrier sensing (CS)

2)     Energy detection (ED)

CS is the receiver ability to detect and decode incoming WiFi preamble. It is also used as a signal to synchronize transmission timing in network, and when detected, the channel is considered busy for the duration of time dictated by PLCP length.  ED function indicates the presence of energy on the current frequency range above a certain predefined threshold (CCA threshold). It should be noted that CS is envisioned to avoid interference from other 802.11 stations while ED capability of avoiding interference with other non-802.11 devices as well as 802.11 compliant devices (using a reasonable threshold which exceeds the 802.11 signal energy). Note that CCA mechanism shall be performed by all the nodes in the network and different nodes might have different understanding of the network.

b)     Dynamic Frequency Selection

Dynamic Frequency Selection (DFS) is a mechanism which allows the radio system operating on specific spectrum to detect, and take action to avoid interference with devices that are considered primary-use or mission-critical. DFS avoids interference by selecting a frequency that is spatially separated (proven not to interfere with primary users). Devices should be equipped with means and procedures to identify designated primary-users and stop transmissions on spatially close frequencies, which could potentially interfere with primary users. Note that, DFS could be performed by the network master detectors only and associated slave devices would be informed by the master about the presence of primary users.

One major difference between CCA and DFS is the fact that CCA requires the station to postpone the transmission for duration of time called backoff while DFS requires the station to vacate the frequency and move the operation to an alternative channel or shut down the operation. Another difference is that CCA detects presence of other users and acts based upon the detection, while DFS detects designated primary licensed users.


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