Virtual channel sense or virtual carrier sense is a mechanism to predict future traffic in wireless networks that uses carrier sense multiple access with collision avoidance (CSMA/CA). It is implemented in wireless network protocols, IEEE 802.11 and IEEE 802.16, which operates in the medium access control (MAC) layer.In virtual channel sensing, a timer mechanism is used that is based upon information of durations of previous frame transmission in order to predict future traffic in the channel. It uses network allocation vector (NAV), which can be considered as a counter that counts down to zero.TechniqueVirtual channel sensing mechanism initialises a NAV ... Read More

The network allocation vector (NAV) is a virtual carrier sensing mechanism that forms an important part of the carrier sense multiple access with collision avoidance (CSMA/CA). It is implemented in wireless network protocols, IEEE 802.11 and IEEE 802.16, which operates in the medium access control (MAC) layer.TechniqueThe network allocation vector can be considered as a counter that counts down to zero. The maximum NAV duration is the transmission time required by frame, which is the the time for which the channel will be busy. At the start of transmission of a frame, the NAV value is set to its maximum. ... Read More

Point coordination function (PCF) is an optional technique used to prevent collisions in IEEE 802.11-based WLAN standard including Wi-Fi. It is a medium access control (MAC) sublayer technique used in areas where carrier-sense multiple access with collision avoidance (CSMA/CA) is used.PCF is used additionally along with the mandatory distributed coordination function (DCF). It is used in centralised control system, and is present in the access point (AP) of the wireless network. An AP is generally a wireless router that coordinates network communication.Features of PCFIt is an optional function that resides on the top of the mandatory DCF. Both PCF and ... Read More

Distributed coordination function (DCF) is a mandatory technique used to prevent collisions in IEEE 802.11-based WLAN standard (Wi-Fi). It is a medium access control (MAC) sublayer technique used in areas where carrier-sense multiple access with collision avoidance (CSMA/CA) is used.TechniqueThe basic technique of DCF is as follows −When a station has a frame to transmit, it waits for a random backoff time. The random backoff time is defined by a contention window having a random number of time slots. The backoff time is given by the following equation −Timebackoff = random()×TimeslotsHere, the function generates a random number and is the ... Read More

IEEE 802.11 standard, popularly known as WiFi, lays down the architecture and specifications of wireless LANs (WLANs). WiFi or WLAN uses high frequency radio waves instead of cables for connecting the devices in LAN. Users connected by WLANs can move around within the area of network coverage.The 802.11 MAC sublayer provides an abstraction of the physical layer to the logical link control sublayer and upper layers of the OSI network. It is responsible for encapsulating frames and describing frame formats.MAC Sublayer frame of IEEE 802.11The main fields of a frame of wireless LANs as laid down by IEEE 802.11 are ... Read More

In wireless communications, when multiple antennas are used both at the transmitting end as well as the receiving end, the configuration is said to be multiple input, multiple output or MIMO. MIMO helps in sending and receiving multiple data signals simultaneously over the same radio channel. MIMO is a smart antenna technology, the other popular technology being multiple input, single output (MISO) and single input, multiple output (SIMO).The following diagram shows MIMO configuration −Advantage of MIMO over Traditional Wireless SystemsIn traditional wireless communication systems, a single antenna is used at the transmitter site, and another single antenna is used at ... Read More

Setting up a wireless internet connection without router is usually done using adhoc network. It is a latin phrase sometimes used to describe a quick fix which can be temporary. Small businesses generally establish ad-hoc computer to computer network, if they do not have access to a router. They use a wireless internet session to do different works like checking email, attending a web-based meeting or browsing the web for a short period of time.Ad-hoc NetworkMany are unaware that, if they have Windows Vista, Windows 7 or Windows XP installed in their system, they can build their own wireless network ... Read More

In data communications and networking, orthogonal frequency-division multiplexing (OFDM) is a method of digital data modulation, whereby a single stream of data is divided into several separate sub-streams for transmission via multiple channels.OFDM uses the principle of frequency division multiplexing (FDM), where the available bandwidth is divided into a set of sub-streams having separate frequency bands. OFDM was introduced in 1966 by Chang at Bell Labs and was improved by Weinstein and Ebert in 1971.Working Principle of OFDMOFDM is a specialised FDM having the constraint that the sub-streams in which the main signal is divided, are orthogonal to each other. ... Read More

Complementary Code Keying (CCK) is a modulation technique deployed in wireless local area networks (WLANs) that follow IEEE 802.11b specification. CCK came into use in 1999 whereby it replaced the Barker code in wireless networks. CCK helps in attaining data rates higher than 2 Mbps, though at the cost of shorter ranges.Features of CCKCCK is used in wireless LANs to attain theoretical maximum data rates of 11 Mbps.CCK is implemented for transmission in the radio frequency range (RF band) of 2.4GHz – 2.4835GHz.CCK includes a pair of codes called chipping sequences which are complementary to each other. In complementary codes, ... Read More

Rate adaptation is a criteria that determines the performance of IEEE 802.11 wireless networks, or WiFi. Rate adaptation allows transmission to be done at different rates within the wireless network, depending upon the network conditions.In wireless networks, the signal may be strong or weak. Through rate adaptation technique, the data transfer date can be changed depending upon the signal strength, i.e. when the signal is strong, high data rates are adopted, while low data rates are adopted during weak signals.Rate Adaptation AlgorithmsIn traditional rate adaptation algorithms, the lowest permissible data rate was selected for data transfer. Though this ensured that ... Read More