Wireless LANs

Question 1

WLAN

characteristics & applications

Answer 1

• basic characteristics:
• mobility
• flexibility
• hard to wire areas
• reduced cost of wireless systems
• improved performance of wireless systems
• uses Radio Frequencies instead of cables at the physical layer and MAC sublayer of the data link layer
• connect clients to a network through a wireless access point (AP) or wireless router, instead of an Ethernet switch
• applications:
• LAN extensions
• cross building interconnection
• nomadic access
• ad-hoc networks

Question 2

WLAN

technologies

Answer 2

• infrared (IR) LANs
• spread spectrum radio LANs
• narrow band microwave

Question 3

Spread Spectrum

WLAN Technology

Answer 3

• FDM based technique using multiple carriers for the same data; improving reliability
• efficient for radio transmissions & energy consumption low (ideal for RF communications)
• sender sends signal on a set of carrier frequencies, the receiver checks all carrier frequencies, so the signal is spread over a wider bandwidth

Techniques used:

1. Frequency Hopping*(FHSS):
   * signal is broadcast over a seemingly random series of RF carriers, hopping from one frequency to another, at split-seconds interval
   * the receiver, hopping between frequencies in synchronization with the sender, will pick-up the signal
2. Direct Sequence:
   * each bit in the original signal is represented by multiple bits in the transmitted signal - chipping code

Question 4

FHDS vs DSSS

Answer 4

• FH systems use a radio carrier that “hops” from frequency to frequency in a pattern known to both transmitter and receiver - easy to implement, resistance to noise, limited throughput
• DS systems use a carrier that remains fixed to a specific frequency band; the data signal is spread onto a much larger range of frequencies using a specific encoding scheme - higher throughput than FH; better range; less resistant to noise