Wireless LANs

Definition of Terms

• The IEEE 802.11 standard for wireless LANs defines two services: basic service set (BSS) and extended service set (ESS). An ESS consists of two or more BSSs; each BSS must have an access point (AP).
• A BSS without an AP is called an ad hoc network; a BSS with an AP is called an infrastructure network.
• The basic service set (BSS) is the building block of a wireless LAN.
• An extended service set (ESS) is made up of two or more BSSs with APs. In this case, the BSSs are connected through a distribution system, which is usually a wired LAN.
• The physical layer methods used by wireless LANs include frequency-hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), orthogonal frequency-division multiplexing (OFDM), and high-rate direct sequence spread spectrum (HR-DSSS).
• FHSS is a signal generation method in which repeated sequences of carrier frequencies are used for protection against hackers.
• One bit is replaced by a chip code in DSSS.
• OFDM specifies that one source must use all the channels of the bandwidth.
• The orthogonal frequency-division multiplexing (OFDM) method for signal generation in a 5-GHz ISM band is similar to frequency division multiplexing (FDM), with one major difference: All the subbands are used by one source at a given time. Sources contend with one another at the data link layer for access.
• HR-DSSS is DSSS with an encoding method called complementary code keying (CCK).
• The wireless LAN access method is CSMA/CA.
• The access method used in the distributed coordination function (OCF) MAC sublayer is CSMAICA.
• The access method used in the point coordination function (PCF) MAC sublayer is polling.
• The network allocation vector (NAV) is a timer used for collision avoidance.
• Network Allocation Vector (NAV) forces other stations to defer sending their data if one station acquires access. In other words, it provides the collision avoidance aspect. When a station sends an RTS frame, it includes the duration of time that it needs to occupy the channel. The stations that are affected by this transmission create a timer called a NAV.
• The MAC layer frame has nine fields. The addressing mechanism can include up to four addresses.
• Wireless LANs use management frames, control frames, and data frames.
• IEEE 802.11 defines several physical layers, with different data rates and modulating techniques.
• The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.
• Bluetooth is a wireless LAN technology that connects devices (called gadgets) in a small area.
• A Bluetooth network is called a piconet. Multiple piconets form a network called a scatternet.
• A Bluetooth network consists of one primary device and up to seven secondary devices.
• The Bluetooth radio layer performs functions similar to those in the Internet model's physcial layer.
• The Bluetooth baseband layer performs functions similar to those in the Internet model's MAC sublayer.
• A Bluetooth network consists of one master device and up to seven slave devices.
• A Bluetooth frame consists of data as well as hopping and control mechanisms. A frame is one, three, or five slots in length with each slot equal to 625 µs.
• A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneously.
• In multiple-secondary communication, the primary sends on the even-numbered slots; the secondary sends on the oddnumbered slots.

Basic service sets (BSSs)

Extended service sets (ESSs)

CSMA/CA flowchart

Physical layers

Industrial, scientific, and medical (ISM) band

Piconet

Scatternet

Bluetooth layers

Layer comparison of Bluetooth and the Internet model

• Radio layer → Internet physical layer
• Baseband layer → MAC sublayer of Internet data link layer
• L2CAP layer → LLC sublayer of Internet data link layer

Note: You can proceed to take the multiple choice exam regarding this topic. Wireless LANs - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com

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