Maulana Azad National Institute of Technology,Bhopal
Department of Computer Science & Engineering
IEEE 802.11 WLAN
Presented By:
Manisha Bhadoria
082112111
Guided By:
Mrs. NAMITA TIWARI
Technology Tree for Wireless LAN
HomeRF
Bluetooth
What is unique about wireless?
 Difficult media
 interference and noise
 quality varies over space and time
 shared with Unwanted 802.11 devices
 shared with non-802 devices (unlicensed spectrum:
microwave ovens, bluetooth, etc.,)
 Full connectivity cannot be assumed
 Hidden node problem
 Multiple international regulatory requirements
Uniqueness of Wireless (continued)
 Mobility
 variation in link reliability
 battery usage: requires power management
 want seamless connections
 Security
 no physical boundaries
 overlapping LANs
Wireless LANs: Characteristics
 Types
 Infrastructure based
 Ad-hoc
 Advantages
 Flexible deployment
 Minimal wiring difficulties
 More robust against disasters (earthquake etc)
 Historic buildings, conferences, trade shows,…
 Disadvantages
 Low bandwidth compared to wired networks (1-10 Mbit/s)
 Proprietary solutions
 Need to follow wireless spectrum regulations
IEEE 802 Protocol Layers
Protocol Architecture
 Functions of media access control (MAC) layer:
 On transmission, assemble data into a frame with address
and error detection fields
 On reception, disassemble frame and perform address
recognition and error detection
 Govern access to the LAN transmission medium
 Functions of logical link control (LLC) Layer:
 Provide an interface to higher layers and perform flow and
error control
Components/Architecture
 Station (STA) - Mobile node
 Access Point (AP) - Stations are connected to access
points.
 Basic Service Set (BSS) - Stations and the AP with in
the same radio coverage form a BSS.
 Extended Service Set (ESS) - Several BSSs connected
through APs form an ESS.
infrastructure
network
AP
AP
wired network
AP: Access Point
AP
ad-hoc network
Source: Schiller
IEEE 802.11 Architecture (model)
 Distribution system (DS) – the network backbone
 Access point (AP) – a bridge or relay
 Basic service set (BSS)
 Stations competing for access to shared wireless medium
 Isolated or connected to backbone DS through AP
 The entity in which the stations are within range of each
other although BSSs can easily overlap
 Extended service set (ESS)
 Two or more basic service sets interconnected by DS
usually a wired LAN
802.11 Architecture Model
ESS
DS
Wireless LAN: Motivation
 Can we apply media access methods from fixed
networks?
 Example CSMA/CD
 Carrier Sense Multiple Access with Collision Detection
 send as soon as the medium is free, listen into the
medium if a collision occurs (original method in IEEE
802.3)
 Medium access problems in wireless networks
 signal strength decreases proportional to the square of
the distance
 sender would apply CS and CD, but the collisions
happen at the receiver
 sender may not “hear” the collision, i.e., CD does not
work
 CS might not work, e.g. if a terminal is “hidden”
Difference Between Wired and
Wireless
Ethernet LAN
Wireless LAN
B
A
B
C
A
C
 If both A and C sense the channel to be idle at the
same time, they send at the same time.
 Collision can be detected at sender in Ethernet.
 Half-duplex radios in wireless cannot detect collision
at sender.
Fragmentation
 Wireless LANs have high bit error rates.
 The probability of erroneous frame is much higher for
wireless links
 802.11 uses fragmentation to reduce the frame error
rate.
MAC management
 Synchronization
- finding and staying with a WLAN
- synchronization functions
 Power Management
- sleeping without missing any messages
- power management functions
 Roaming
- functions for joining a network
- changing access points
- scanning for access points
 Management information base
Scanning
 Scanning is required for many functions
- finding and joining a network
- finding a new access point during
roaming
 Passive scanning
- find networks simply by listening for
beacons
 Active scanning
- on each channel send a probe and wait
for probe response
802.11 Protocol Architecture
 MAC Entity
 basic access mechanism
 fragmentation/defragmentation
 encryption/decryption
 MAC Layer Management Entity
 synchronization
 power management
 roaming
 MAC MIB (management information base)
 Physical Layer Convergence Protocol (PLCP)
 PHY-specific, supports common PHY SAP
 provides Clear Channel Assessment signal (carrier sense)
802.11 Protocol Architecture (cont.)
 Physical Medium Dependent Sublayer (PMD)
 modulation and encoding
 PHY Layer Management
 channel tuning (channel switching delay : 224us in 802.11b)
 PHY MIB
 Station Management
 interacts with both MAC Management and PHY Management
Commercial Products : WLAN Cards
 One piece
 Two pieces
Commercial Products : AP
Outdoor Application - Antenna
Outdoor Application
Wi-Fi Protected Access (WPA) - Authentication
 Open system authentication
 Exchange of identities, no security benefits
 Shared Key authentication
 Shared (secret) Key assures authentication
 WEP (wired equivalent privacy) is easily broken since key is used
to encrypt every transmission
 Immediate solution for WEP was to increase key length from
40, to 128, to 156 bits where 128 bits is the minimum used
today.
IEEE 802.11a, 802.11b and 802.11g
 IEEE 802.11a
 5-GHz band with data rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
 Uses orthogonal frequency division multiplexing (OFDM)
 Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-QAM
 IEEE 802.11b
 Provides data rates of 5.5 and 11 Mbps at 2.4 GHz
 Complementary code keying (CCK) modulation scheme
 IEEE 802.11g
 2.4 GHz, up to 54 Mbps, OFDM
 .11g and .11b can operate simultaneously but with an .11b user in the cell
the wireless network will degrade the .11g performance (AP must do
translation for .11b) but still much faster than .11b alone.
802.11a : 802.11a The 802.11a standard uses the same data link
layer protocol and frame format as the original standard, but an
OFDM based air interface (physical layer). It operates in the 5 GHz
band with a maximum net data rate of 54 Mbit/s, plus error
correction code, which yields realistic net achievable throughput in
the mid-20 Mbit/s[citation needed]. Since the 2.4 GHz band is
heavily used to the point of being crowded, using the relatively unused 5 GHz band gives 802.11a a significant advantage. However,
this high carrier frequency also brings a disadvantage: The effective
overall range of 802.11a is less than that of 802.11b/g; 802.11a
signals cannot penetrate as far as those for 802.11b because they are
absorbed more readily by walls and other solid objects in their path
due to their smaller wavelength.
802.11b : 802.11b 802.11b has a maximum raw data rate of 11
Mbit/s and uses the same media access method defined in the
original standard. 802.11b products appeared on the market in
early 2000, since 802.11b is a direct extension of the modulation
technique defined in the original standard. The dramatic increase
in throughput of 802.11b (compared to the original standard)
along with simultaneous substantial price reductions led to the
rapid acceptance of 802.11b as the definitive wireless LAN
technology. 802.11b devices suffer interference from other
products operating in the 2.4 GHz band. Devices operating in the
2.4 GHz range include: microwave ovens, Bluetooth devices,
baby monitors and cordless telephones.
802.11-2007 : 802.11-2007 In 2003, task group TGma
was authorized to "roll up" many of the amendments to the
1999 version of the 802.11 standard. REVma or 802.11ma,
as it was called, created a single document that merged 8
amendments (802.11a,b,d,e,g,h,i,j) with the base standard.
Upon approval on March 08, 2007, 802.11REVma was
renamed to the current standard IEEE 802.11-2007.[4] This
is the single most modern 802.11 document available that
contains cumulative changes from multiple sub-letter task
groups.
Useful Web Sites from Stallings
IEEE 802.11 Wireless LAN Standard
 IEEE 802.11 Wireless LAN Working Group: Contains
working group documents plus discussion archives.
 Wi-Fi Alliance: An industry group promoting the
interoperabiltiy of 802.11 wireless products with each
other.
References
 Brian P Crow, Indra Widjaja, J G Kim, Prescott T Sakai.
IEEE 802.11 Wireless Local Area Networks. IEEE
Communications Magazine
 www.breezecom.com
 Jochen H. Schiller, Mobile Communications