Wireless Ethernet 802.11 uses either direct-sequence spread spectrum, frequency-hopping spread spectrum, or infrared light in the physical layer. It uses direct sequence spread spectrum to spread signals over a broad frequency band for high bandwidth applications, while frequency hopping spreads signals across 79 channels from 2.402 to 2.480 GHz using 78 hopping sequences. The data link layer uses the same 802.2 logical link control and 48-bit MAC addressing as other 802 LANs, but the MAC layer is similar yet different to Ethernet with CSMA/CA instead of CSMA/CD and optional RTS/CTS to address hidden nodes.

1. Wireless Ethernet
802.11 Presentation 2
1

2. 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
http://www.pulsewan.com/data101/802_11_b_basics.htm
2

3. Frequency-Hopping
 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
3

4. 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

4

5. Orthogonal Frequency Division
Multiplexing
 OFDM – Uses all 48 channels.
 Each Parallel bit stream is modulated differently; BPSK QPSK
or QAM.
 Multipath propagation environment.
mage acquired from http://iaf-bs.de/img/con/projects/3glte-mimo-ofdm-testbed/3GLTE_Testbed_fading_adapt.jpg
5

6. MAC Access Modes
 DCF – Distributed coordination function.
 PCF – Point coordination function.
 Image acquired from www.cisco.com/.../ 119001-120000/119161.jpg
 www.cisco.com/.../ 119001-120000/119161.jpg
rich phelps 802.11 framing 6

7. 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
 Similarities: Contention-based
◦ Medium is free for all
◦ A node senses the free medium and occupies it as long as data packet
requires it
 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
Avoidance)
◦ 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
other broadcast.
 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%
7

8. 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 ...)
frame ordering
next frame duration info for RX
-Basic service identification frame specific,
-source/destination address variable length
-transmitting station
-receiving station
frame check
sequence
(CRC)
8

9. Wired Equivalent Privacy
 Implemented at the MAC layer.
 RC4 Stream Cipher
 Provides encryption to data frames only
http://i.msdn.microsoft.com/Aa503279.Native_802_11_wep(en-us,MSDN.10).gif
http://www.cs.wustl.edu/~jain/cse574-06/ftp/wireless_security/fig14.gif
9