802.11 Physical Layer
Frequency Allocation 802.11
Direct-Sequence (DS) spread-spectrum radio
Frequency-hopping(FH) spread-spectrum radio
Infrared Light (IR)
Image acquired from
79 channels from 2.402 to 2.480 GHz with 1
MHz channel space
78 hopping sequences with minimum 6 MHz
hopping space, each sequence uses every 79
frequency elements once
Direct Sequence Spread Spectrum
Spreads signal by expanding the signal over a broad portion of the radio band.
DBPSK Differential Binary Phase Shift Keying
Ideal for high bandwidth apps.
Uses higher power to transmit and more expensive to implement compared to FHSS.
Image acquired from http://www.wireless-telecom.com/images/dsss_pic.gif
Orthogonal Frequency Division
OFDM – Uses all 48 channels.
Each Parallel bit stream is modulated differently; BPSK QPSK
Multipath propagation environment.
mage acquired from http://iaf-bs.de/img/con/projects/3glte-mimo-ofdm-testbed/3GLTE_Testbed_fading_adapt.jpg
MAC Access Modes
DCF – Distributed coordination function.
PCF – Point coordination function.
Image acquired from www.cisco.com/.../ 119001-120000/119161.jpg
rich phelps 802.11 framing 6
Data Link Layer 802.11
Two sub layers: Logical Link Control (LLC), Media Access Control (MAC)
Same 802.2 LLC and 48-bit MAC addressing as for all other 802 LANs
MAC layer similar but different to Ethernet 802.3
◦ Medium is free for all
◦ A node senses the free medium and occupies it as long as data packet
Differences: CSMA/CD vs CSMA/CA, CRC, packet fragmentation
Ethernet 802.3 - CSMA/CD (Carrier Sense Multiple Access / Collision Detect)
◦ Collision detection possible because can send/receive simultaneously
Ethernet 802.11 - CSMA/CA (Carrier Sense Multiple Access / Collision
◦ Collision detection not possible because of near/far problem for radios
◦ Steps: sense the air, wait, send, wait for ACK, retransmit if needed
◦ Optional RTS/CTS feature to solve “hidden node” problem
“Hidden node”: where two stations on either side of an AP can’t hear each
CRC – error checking at layer 2 (Ethernet assumes this is layer 3 task)
Packet fragmentation: breaking large packets into smaller ones at MAC layer
Overhead of MAC layer reduces throughput by 40-50%
802.11 MAC Frame
NOTE: This frame structure is common for all data sent by a 802.11 station
control info (WEP, data type as management, control, data ...)
next frame duration info for RX
-Basic service identification frame specific,
-source/destination address variable length
Wired Equivalent Privacy
Implemented at the MAC layer.
RC4 Stream Cipher
Provides encryption to data frames only