04-04-2011, 01:39 PM
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IEEE 802.11 Wireless Local Area Networks (RF-LANs)
802.11 WLANs - Outline
801.11 bands and layers
Link layer
Media access layer
frames and headers
CSMA/CD
Physical layer
frames
modulation
Frequency hopping
Direct sequence
Infrared
Security
Implementation
802.11 WLAN technologies
IEEE 802.11 standards and rates
IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band )
IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi
IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band)
IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz) backward compatible to 802.11b
IEEE 802.11 networks work on license free industrial, science, medicine (ISM) bands:
Other WLAN technologies
High performance LAN or HiperLAN (ETSI-BRAN EN 300 652) in the 5 GHz ISM
version 1 up to 24 Mbps
version 2 up to 54 Mbps
HiperLAN provides also QoS for data, video, voice and images
Bluetooth
range up to 100 meters only (cable replacement tech.)
Bluetooth Special Interest Group (SIG)
Operates at max of 740 kbps at 2.4 GHz ISM band
Applies fast frequency hopping 1600 hops/second
Can have serious interference with 802.11 2.4 GHz range network
802.11a
Operates at 5 GHz band
Supports multi-rate 6 Mbps, 9 Mbps,… up to 54 Mbps
Use Orthogonal Frequency Division Multiplexing (OFDM) with 52 subcarriers, 4 us symbols (0.8 us guard interval)
Use inverse discrete Fourier transform (IFFT) to combine multi-carrier signals to single time domain symbol
IEEE 802.11a rates and modulation formats
IEEE 802-series of LAN standards
802 standards free to
download from
http://standards.ieee.org
/getieee802/portfolio.html
The IEEE 802.11 and
supporting LAN Standards
See also IEEE LAN/MAN Standards Committee Web site
manta.ieeegroups/802/
IEEE 802.11 Architecture
IEEE 802.11 defines the physical (PHY), logical page link (LLC) and media access control (MAC) layers for a wireless local area network
802.11 networks can work as
basic service set (BSS)
extended service set (ESS)
BSS can also be used in ad-hoc
networking
BSS and ESS
In ESS multiple access points connected by access points and a distribution system as Ethernet
BSSs partially overlap
Physically disjoint BSSs
Physically collocated BSSs (several antennas)
802.11 Logical architecture
LLC provides addressing and data page link control
MAC provides
access to wireless medium
CSMA/CA
Priority based access (802.12)
joining the network
authentication & privacy
Services
Station service: Authentication, privacy, MSDU* delivery
Distributed system: Association** and participates to data distribution
Three physical layers (PHY)
FHSS: Frequency Hopping Spread
Spectrum (SS)
DSSS: Direct Sequence SS
IR: Infrared transmission
802.11 DSSS
Supports 1 and 2 Mbps data transport, uses BPSK and QPSK modulation
Uses 11 chips Barker code for spreading - 10.4 dB processing gain
Defines 14 overlapping channels, each having 22 MHz channel bandwidth, from 2.401 to 2.483 GHz
Power limits 1000mW in US, 100mW in EU, 200mW in Japan
Immune to narrow-band interference, cheaper hardware
802.11 FHSS
Supports 1 and 2 Mbps data transport and applies two level - GFSK modulation* (Gaussian Frequency Shift Keying)
79 channels from 2.402 to 2.480 GHz ( in U.S. and most of EU countries) with 1 MHz channel space
78 hopping sequences with minimum 6 MHz hopping space, each sequence uses every 79 frequency elements once
Minimum hopping rate
2.5 hops/second
Tolerance to multi-path,
narrow band interference,
security
Low speed, small range
due to FCC TX power
regulation (10mW)
How ring-network works
A node functions as a repeater
only destination copies
frame to it,
all other nodes
have to discarded
the frame
Unidirectional link
Token ring
A ring consists of a single or dual (FDDI) cable in the shape of a loop
Each station is only connected to each of its two nearest neighbors. Data in the form of packets pass around the ring from one station to another in uni-directional way.
Advantages :
(1) Access method supports heavy load without degradation of performance because the medium is not shared.
(2) Several packets can simultaneous circulate between different pairs of stations.
Disadvantages:
(1) Complex management
(2) Re-initialization of the ring whenever a failure occurs
How bus-network works
In a bus network, one node’s transmission traverses the entire network and is received and examined by every node. The access method can be :
(1) Contention scheme : multiple nodes attempt to access bus; only one node succeed at a time (e.g. CSMA/CD in Ethernet)
(2) Round robin scheme : a token is passed between nodes; node holds the token can use the bus (e.g.Token bus)
Advantages:
(1) Simple access method
(2) Easy to add or remove
stations
Disadvantages:
(1) Poor efficiency with high
network load
(2) Relatively insecure, due to
the shared medium
MAC Techniques - overview
Contention
Medium is free for all