IEEE 802.11

Presented By:

Abhishek Pachisia

B.Tech – IT

Foreword - 1• Belong to 802.x LAN standards.• Primary goal

– Simple and Robust WLAN– Time bound and Asynchronous services

• MAC Layer– Should be able to operate with multiple

physical layers (different medium sense & transmission characteristic.

• Physical layer.– Infra red & Spread spectrum radio

transmission techniques.

Foreword - 2• Additional features of WLAN

– Support of power management– Handling of hidden nodes– Ability to operate world wide.

• 2.4 GHz ISM band – original standard.– 1 Mbits/s – Mandatory– 2 Mbits/s – Optional.

System Architecture - 1• Basic System Architecture

– Infrastructure based– Ad-hoc.

• Components– Station (STA)– Access Points (AP)– Basic Service Set (BSS)– Extended Service Set (ESS)– Portal

Architecture of an Infrastructure based IEEE 802.11

System Architecture - 2• Stations

– Access mechanism to wireless medium– Radio contact to the AP.

• BSS– BSSi are connected via Distributed System.

• ESS– Form Single network, extends wireless

coverage area.– ESSID : Name of network; Separate different

network

• Portal– Internetworking unit to other LANs.

System Architecture - 3• AP’s support roaming.• Distributed system – Transfer between

AP’s.• Infrastructure based network allow ad-hoc

network between stations -> IBSS.• IEEE 802.11 does not specify

– Routing– Forwarding of data– Exchange of topology Information

Architecture of IEEE 802.11 ad-hoc wireless LANs

Protocol Architecture - 1• 802.11 fits seamlessly into 802.x wired

networks.• Most common scenario – 802.11 & 802.3

via bridge.• Difference in bandwidth is noticed.• Upper part of logical link layer and data

link control layer covers MAC differences.

IEEE 802.11 – Protocol architecture and belonging

Protocol Architecture - 2• Physical Layer(PHY)

– Physical Layer Convergence Protocol (PLCP)– Physical Medium Dependent sublayer (PMD)

• Medium Access Control Layer (MAC)– To access medium.– To fragment user data.– To encrypt.

• PLCP – Carrier sense signal (CSS)– Common Service Access Point(SAP)

• PMD– Encoding/Decoding Signals.

Detailed IEEE 802.11 protocol architecture and management

LLC

MAC

PLCP

PMD

MAC Management

PHY Management Sta

tion

Man

agem

ent

DL

CP

HY

Physical Layer• Three Layers

– Infra red (1)– Radio transmission (2)

• Clear Channel Assessment signal (CCA).– MAC mechanism controlling medium access.– Indicates state of medium.

• Service Access Point (SAP)• Versions of PHY layer

– FHSS– DSSS– Infra red

FHSS• Allows coexistence of multiple networks.• Original Standard

– 79 hopping channel North America.– 23 hopping channel Japan

• Pseudo random hoping pattern.• Standard – GFSK as modulation for FHSS

PHY.• 1Mbits/s is mandatory & 2Mbit/s is

optional.• MAC data is scrambled using

s(z)=z7+z4+1

Format of an IEEE 802.11 PHY frames using FHSS

Synchronization SFD PLW PSF HEC Payload

80 16 12 4 16 Variable

PLCP Preamble PLCP Header

Bits

DSSS - 1• Separated by code.• Achieved using 11-chip Barker sequence.• Key characteristics

– Robustness against interference– Insensitivity to multipath propagation.

• Implementation - Complex than FHSS.• Uses

– Differential Binary Phase Shift Keying (DBPSK)

– Differential Quadrature Phase Shift Keying (DQPSK)

DSSS - 2• Maximum transmit power

– US => 1W– Europe => 100mW EIRP– Japan => 10mW/MHz

• Bits are scrambled by s(z)=z7+z4+1

• Two basic parts– PLCP part (Preamble and header)– Payload part.

Format of an IEEE 802.11 PHY frame using DSSS

Synchronization SFD Signal Service Length HEC Payload

128 16 8 8 16 16 Variable

PLCP Preamble PLCP Header

Bits

Infra red• Uses visible light => 850-950nm.• No line of sight required.• Maximum range =>10m (no interference).• Today no product offer communication

based on infra red.• Proprietary products offer up to 4Mbits/s

using diffuse infra red light.• Directed infra red communication based

on IrDA can be used (IrDA,2002).

Thank You