Amity School of Engineering & Technology

WLANs IEEE 802.11

standard

Dr. Manoj Kumar Shukla

Associate Professor
Department of Computer Science & Engineering
ASET, Amity University, Noida
E-mail:– mkshukla@amity.edu

1
 Wireless?
• A wireless LAN or WLAN is a wireless local
area network that uses radio waves as its
carrier.
• The last link with the users is wireless, to give
a network connection to all users in a building
or campus.
• The backbone network usually uses cables
 Common Topologies
The wireless LAN connects to a wired LAN

• There is a need of an access point that bridges wireless LAN traffic into the wired
LAN.
• The access point (AP) can also act as a repeater for wireless nodes, effectively
doubling the maximum possible distance between nodes.
 Integration With Existing Networks

• Wireless Access Points (APs) - a small device

that bridges wireless traffic to your network.
• Most access points bridge wireless LANs into
Ethernet networks, but Token-Ring options are
available as well.
 Advantages of WLANs

• Flexibility
• Planning
• Design
• Robustness
• Cost
 Disadvantages

• Quality of service
• Proprietary solutions
• Restrictions
• Safety and security
 WLANs to ensure their commercial
success
• Global operation
• Low power
• License-free operation
• Robust transmission technology
• Implified spontaneous cooperation
• Easy to use
• Protection of investment
• Safety and security
• Transparency for applications
 How are WLANs Different?
• They use specialized physical and data link protocols
• They integrate into existing networks through access
points which provide a bridging function
• They let you stay connected as you roam from one
coverage area to another
• They have unique security considerations
• They have specific interoperability requirements
• They require different hardware
• They offer performance that differs from wired LANs.
 Physical and Data Link Layers
Physical Layer:
• The wireless NIC takes frames of data from
the link layer, scrambles the data in a
predetermined way, then uses the modified
data stream to modulate a radio carrier
signal.
Data Link Layer:
• Uses Carriers-Sense-Multiple-Access with
Collision Avoidance (CSMA/CA).
 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:

26 MHz 83.5 MHz 200 MHz 255 MHz

902 928 2400 2484 5150 5350 5470 5725 f/MHz

EIRP power 100 mW 200 mW 1W
in Finland indoors only

EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna
Equipment technical requirements for radio frequency usage defined in ETS 300 328
 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
 The IEEE 802.11 and
supporting LAN Standards
IEEE 802.2
Logical Link Control (LLC) OSI Layer 2
(data link)

MAC
IEEE 802.3 IEEE 802.4 IEEE 802.5
IEEE 802.11
Carrier Token Token Wireless
Sense Bus Ring OSI Layer 1
PHY
(physical)
a b g

bus star ring

• See also IEEE LAN/MAN Standards Committee
Web site
www.manta.ieee.org/groups/802/
 Basic service sets (BSSs)

14.13
 Extended service sets (ESSs)

14.14
 IEEE 802.11 Architecture
• IEEE 802.11 defines the physical (PHY), logical link (LLC) and media access
control (MAC) layers for a wireless local area network
• 802.11 networks can work as Network
LLC

802.11
– basic service set (BSS)
MAC
– extended service set (ESS)
FHSS DSSS IR PHY
• BSS can also be used in ad-hoc
networking
LLC: Logical Link Control Layer DS,
MAC: Medium Access Control Layer ESS
PHY: Physical Layer
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
SS: Spread spectrum
IR: Infrared light
BSS: Basic Service Set
ESS: Extended Service Set
AP: Access Point
DS: Distribution System ad-hoc network
 BSS and ESS

Basic (independent) service set (BSS) Extended service set (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 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) LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
– DSSS: Direct Sequence SS PHY: Physical Layer
– IR: Infrared transmission FH: Frequency hopping
DS: Direct sequence
IR: Infrared light
*MSDU: MAC service data unit
** with an access point in ESS or BSS
 802.11 LAN architecture
r wireless host communicates
with base station
Internet
m base station = access point
(AP)
r Basic Service Set (BSS) (aka
“cell”) in infrastructure mode
hub, switch contains:
AP or router m wireless hosts
m access point (AP): base
BSS station
1 AP m ad hoc mode: hosts only

BSS 2 and Mobile Networks

6: Wireless 6-18
 IEEE 802.11: multiple access
• avoid collisions: 2+ nodes transmitting at same time
• 802.11: CSMA - sense before transmitting
– don’t collide with ongoing transmission by other node
• 802.11: no collision detection!
– difficult to receive (sense collisions) when transmitting due to weak
received signals (fading)
– can’t sense all collisions in any case: hidden terminal, fading
– goal: avoid collisions: CSMA/C(ollision)A(voidance)

A B C
C
A’s signal C’s signal
strength
B strength
A
space
6: Wireless and Mobile Networks 6-19
 Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than random access of
data frames: avoid collisions of long data frames
• sender first transmits small request-to-send (RTS) packets to BS using
CSMA
– RTSs may still collide with each other (but they’re short)
• BS broadcasts clear-to-send CTS in response to RTS
• CTS heard by all nodes
– sender transmits data frame
– other stations defer transmissions

avoid data frame collisions completely

using small reservation packets!
 802.11 frame: addressing

2 2 6 6 6 2 6 0 - 2312 4
frame address address address seq address
duration payload CRC
control 1 2 3 control 4

Address 4: used only in

Address 1: MAC address ad hoc mode
of wireless host or AP Address 3: MAC address
to receive this frame of router interface to which
AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame
 IEEE 802.11 Media
Access Control (MAC)

Carrier-sense multiple access protocol

with collision avoidance (CSMA/CS)

DIFS: Distributed Inter-Frame Spacing

SIFS: Short Inter-Frame Spacing
ack: Acknowledgement
Exposed station problem
 IEEE 802.11 Mobility
• Standard defines the following mobility types:
– No-transition: no movement or moving within a local BSS
– BSS-transition: station movies from one BSS in one ESS to another BSS
within the same ESS
– ESS-transition: station moves from a BSS in one ESS to a BSS in a different
ESS (continuos roaming not supported)

• Especially: 802.11 don’t support roaming

with GSM!

- Address to destination
mapping
- seamless integration ESS 1
of multiple BSS ESS 2
 Security
• In theory, spread spectrum radio signals are
inherently difficult to decipher without
knowing the exact hopping sequences or
direct sequence codes used
• The IEEE 802.11 standard specifies optional
security called "Wired Equivalent Privacy"
whose goal is that a wireless LAN offer privacy
equivalent to that offered by a wired LAN. The
standard also specifies optional authentication
measures.
 Authentication and privacy
• Goal: to prevent unauthorized access & eavesdropping
• Realized by authentication service prior access
• Open system authentication
– station wanting to authenticate sends authentication management frame -
receiving station sends back frame for successful authentication
• Shared key authentication (included in WEP*)
– Secret, shared key received by all stations by a separate, 802.11 independent
channel
– Stations authenticate by a shared knowledge of the key properties
• WEP’s privacy (blocking out eavesdropping) is based on ciphering:

*WEP: Wired Equivalent Privacy

 802.11b Security Features
• Wired Equivalent Privacy (WEP) – A protocol to
protect link-level data during wireless transmission
between clients and access points.
• Services:
– Authentication: provides access control to the network by
denying access to client stations that fail to authenticate
properly.
– Confidentiality: intends to prevent information
compromise from casual eavesdropping
– Integrity: prevents messages from being modified while in
transit between the wireless client and the access point.
 WLAN technology problems
• Date Speed
– IEEE 802.11b support up to 11 MBps, sometimes this is not enough -
far lower than 100 Mbps fast Ethernet
• Interference
– Works in ISM band, share same frequency with microwave oven,
Bluetooth, and others
• Security
– Current WEP algorithm is weak - usually not ON!
• Roaming
– No industry standard is available and propriety solution are not
interoperable - especially with GSM
• Inter-operability
– Only few basic functionality are interoperable, other vendor’s features
can’t be used in a mixed network
 WLAN implementation
problems
• Lack of wireless networking experience for most IT
engineer
• No well-recognized operation process on network
implementation
• Selecting access points with ‘Best Guess’ method
• Unaware of interference from/to other networks
• Weak security policy
• As a result, your WLAN may have
– Poor performance (coverage, throughput, capacity, security)
– Unstable service
– Customer dissatisfaction