Wireless LAN Technology Chapter 13

Wireless LAN  Wireless LAN is one that make use of a wireless transmission medium.  Wireless LAN use infrared or radio signals to share information and resources between devices.  They promise to provide connectivity anytime, anywhere.

Wireless LAN Applications LAN Extension - Large open areas, warehouses, historical buildings, small offices. Cross-building interconnect - To connect LANs between two buildings a point-to-point wireless link is used (bridge or router). Nomadic Access - Between a LAN hub and a mobile data terminal such as a laptop. Ad hoc networking - Temporary Peer-to-Peer network, e.g. conference room, classroom meeting.

Wireless LAN Configurations Simple (single-cell) wireless LAN configuration - Wired backbone LAN to support servers, workstations, and one or more bridges or routers to link with other networks. - A control module (CM) acts as an interface to a wireless LAN (bridge or router to link to the backbone, also it regulates the access from the end systems). - Hubs or other user modules (UMs). Multiple-cell wireless LAN - Multiple control modules interconnected by a wired LAN. - Each control module supports a number of wireless end systems within its transmission range. (figure 13.2)

Multiple-cell Wireless LAN

Wireless LAN Requirements Throughput (efficient use of wireless medium) Number of nodes (hundreds of nodes) Connection to backbone LAN (via a control module) Service area (100m – 300m) Battery power consumption (use of sleep mode) Transmission robustness and security Collocated network operation (2 or more WLANs) License-free operation (user’s preference) Handoff/roaming ( move from one cell to another) Dynamic configuration (addition, deletion, relocation of end systems)

Wireless LAN Categories Infrared (IR) LANs Spread spectrum LANs Narrowband microwave

Spread spectrum LANs In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but the goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy. In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but the goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy.

Spread spectrum

Spread Spectrum Input is fed into a channel encoder Produces analog signal with narrow bandwidth Signal is further modulated using sequence of digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number generator Effect of modulation is to increase bandwidth of signal to be transmitted

Spread Spectrum

What can be gained from apparent waste of spectrum? Immunity from various kinds of noise Can be used for hiding and encrypting signals Several users can independently use the same higher bandwidth with very little interference

Spread spectrum LANs There are two techniques to spread the bandwidth:. Frequency Hopping Spread Spectrum (FHSS). Direct Sequence Spread Spectrum (DSSS).

Frequency Hoping Spread Spectrum (FHSS) Signal is broadcast over seemingly random series of radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input signal Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is selected

Frequency hopping spread spectrum (FHSS)

Frequency Hoping Spread Spectrum Channel sequence dictated by spreading code Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only at knocking out a few bits

Frequency selection in FHSS

FHSS cycles

Direct Sequence Spread Spectrum (DSSS) Each bit in original signal is represented by multiple bits in the transmitted signal Spreading code spreads signal across a wider frequency band Spread is in direct proportion to number of bits used One technique combines digital information stream with the spreading code bit stream using exclusive-OR.

DSSS

DSSS example