The 802.11 MAC layer provides reliable data delivery over the wireless medium using CSMA/CA access method. It also provides security services like authentication and WEP encryption. The physical layer transmits and receives frames using modulation like FHSS or DSSS and provides carrier sensing to the MAC. It operates in the 2.4GHz band and FHSS uses frequency hopping while DSSS spreads the signal across a wider frequency band.

1. 802.11 Media Access Control
 The 802.11 MAC layer provides functionality to allow reliable data
delivery for the upper layers over the wireless PHY media.
 The data delivery itself is based on an asynchronous, best-effort,
connectionless delivery of MAC layer data.
 There is no guarantee that the frames will be delivered successfully.
 The 802.11 MAC provides a controlled access method to the shared
wireless media called Carrier-Sense Multiple Access with Collision
Avoidance (CSMA/CA).
 CSMA/CA is similar to the collision detection access method
deployed by 802.3 Ethernet LANs.

2. 802.11 Media Access Control cntd
Another function of the 802.11 MAC is to
protect the data being delivered by providing
security and privacy services.
 Security is provided by the authentication
services and by Wireless Equivalent Privacy
(WEP), which is an encryption service for data
delivered on the WLAN.

3. 802.11 Physical Layer (PHY)
The 802.11 physical layer (PHY) is the interface
between the MAC and the wireless media where
frames are transmitted and received.
 The PHY provides three functions.
 First, the PHY provides an interface to exchange
frames with the upper MAC layer for transmission and
reception of data.
Secondly, the PHY uses signal carrier and spread
spectrum modulation to transmit data frames over
the media.
 Thirdly, the PHY provides a carrier sense indication
back to the MAC to verify activity on the media.

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5. Physical Layer
Frequency Hopping Spread Spectrum (FHSS)
 Direct Sequence Spread Spectrum (DSSS)

6. Frequency-hopping spread spectrum
– Operating in 2.4 GHz ISM band ,.Lower cost,
power consumption, Most tolerant to signal
interference.
– Signal is broadcast over seemingly random series
of radio frequencies
– Signal hops from frequency to frequency at fixed
intervals
– Receiver, hopping between frequencies in
synchronization with transmitter, picks up
message

7. Direct-sequence spread spectrum
– Operating in 2.4 GHz ISM band ,Supports higher data
rates, More range than FH or IR physical layers
– Each bit in original signal is represented by multiple
bits in the transmitted signal
– Spreading code spreads signal across a wider
frequency band
– DSSS is the only physical layer specified for the
802.11b specification 802.11a and 802.11b differ in
use of chipping method.
– 802.11a uses 11-bit barker chip
– 802.11b uses 8-bit complimentary code keying (CCK)
algorithm