802.11 WLAN

Department of Computer Science

DCSCOMSATS Institute of Information Technology

1

802.11 (Wireless LAN)

Presented by:

Rab Nawaz &

Shahzad Ali

Department of Computer Science 2 2

Presentation Overview

Introduction Architecture Types of wireless LANs Protocols Standard and

amendments 802.11 Layers

A Compaq 802.11b PCI card.

A Linksys Residential gateway with an 802.11b radio and a 4-port Ethernet switch.

Department of Computer Science 3

Introduction IEEE 802.11 is a set of standards for wireless

local area network (WLAN) computer communication.

It is developed by the IEEE LAN/MAN Standards Committee (IEEE 802) in the 5 GHz and 2.4 GHz public spectrum bands.

802.11-1997 was the first wireless networking standard, but 802.11b was the first widely accepted one, followed by 802.11g and 802.11n.


Wireless LAN

A wireless LAN or WLAN is a wireless local area network, which is the linking of two or more computers or devices without using wires.

WLAN uses spread-spectrum or OFDM modulation technology.

It is based on radio waves to enable communication between devices in a limited area, also known as the basic service set.

Notebook is connected to the wireless access point using a PC card wireless card.


Benefits of WLAN Mobility With the emergence of public wireless networks,

users can access the internet even outside their normal work environment.

Productivity Users connected to a wireless network can

maintain a nearly constant affiliation with their desired network as they move from place to place.

Deployment Initial setup of an infrastructure-based wireless

network requires little more than a single access point. Wired networks on the other hand, have the additional cost and complexity of actual physical cables being run to numerous locations.


Benefits of WLAN

ExpandabilityWireless networks can serve a suddenly-increased number of clients with the existing equipments. In a wired network, additional clients would require additional wiring.

Cost Wireless networking hardware is at worst a

modest increase from wired networks.


Limitations of WLAN

Some of the limitations of the WLAN are!

Security Less secure due to wireless environment. Range

The typical range of a common 802.11g network with standard equipment is of tens of metres.

Speed The speed on most wireless networks (typically 1-108 Mbit/s) is

reasonably slow compared to the slowest common wired networks (100 Mbit/s up to several Gbit/s).

ReliabilityLike any radio frequency transmission, wireless networking signals are subject to a wide variety of interference.


Architecture of WLAN Stations (STA)

All components that can connect into a wireless medium in a network are referred to as stations.

Wireless stations fall into one of two categories: access points, and clients.

BSS The basic service set (BSS) is a set of all stations that

can communicate with each other. Independent BSS ( IBSS ) Ad-hoc network that

contains no access points. infrastructure BSS An infrastructure BSS can

communicate with other stations of other BSS through Access point.


Independent BSS & Infrastructure BSS


Architecture

ESS An extended service set (ESS) is a set of connected

BSSes. Access points in an ESS are connected by a

distribution system. Each ESS has an ID called the SSID which is a 32-byte

(maximum) character string. Distribution System

A distribution system (DS) connects access points in

an extended service set. The concept of a DS can be to increase network

coverage through roaming between cells.


Extended Service Set


Distribution System

Access points in an ESS are connected by a distribution system.


Types of wireless LANs Peer to Peer

An ad-hoc network is a network where stations communicate only peer to peer (P2P).

There is no Access point. This is accomplished using the Independent

Basic Service Set (IBSS).


Types of wireless LANs Bridge

A bridge can be used to connect networks, typically of different types.

A wireless Ethernet bridge allows the connection of devices on a wired Ethernet network to a wireless network.

The bridge acts as the connection point to the Wireless LAN.

Wireless distribution system When it is difficult to connect all of the APs in

a network by wires, it is also possible to put up access points as repeaters.


Station Types

802.11 defines three types of stations. No-Transition Mobility

Station is either stationary or moving inside BSS

BSS Transition Mobility Station can move from one BSS to other

but the movement is confined within one ESS

ESS Transition Mobility Station can move from one ESS to other


Protocols

802.11-1997 (802.11 legacy) 802.11a 802.11b 802.11g 802.11-2007 802.11n


802.11-1997 (802.11 legacy)

The original version of the standard IEEE 802.11, released in 1997 and clarified in 1999.

Operating frequency is 2.4 GHz. Bit rate is 1 or 2 Mbps. Forward error correction code Legacy 802.11 was rapidly supplemented

by 802.11b.


Summary of 802.11 standards

802.11Protocol

ReleaseFreq.(GHz)

Typ throughput

(Mbit/s)

Max net bitrate(Mbit/s)

Mod.

– 1997 2.4 0.9 2

a 1999 5 23 54 OFDM

b 1999 2.4 4.3 11 DSSS

g 2003 2.4 19 54 OFDM

n 2008 2.4, 5 74 248 OFDM

y 2008 3.7 23


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.


Standard and amendments

IEEE 802.11 - THE WLAN STANDARD was original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard (1997), all the others listed below are Amendments to this standard, except for Recommended Practices 802.11F and 802.11T.

IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)

IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)

IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard (2001)



IEEE 802.11d - International (country-to-country) roaming extensions (2001)

IEEE 802.11e - Enhancements: QoS, including packet bursting (2005)

IEEE 802.11F - Inter-Access Point Protocol (2003) Withdrawn February 2006

IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)

IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)



IEEE 802.11i - Enhanced security (2004) IEEE 802.11j - Extensions for Japan (2004) IEEE 802.11-2007 - A new release of the standard that

includes amendments a, b, d, e, g, h, i & j. (July 2007) IEEE 802.11k - Radio resource measurement

enhancements (2008) IEEE 802.11l - (reserved and will not be used) IEEE 802.11m - Maintenance of the standard. Recent

edits became 802.11-2007. (ongoing) IEEE 802.11n - Higher throughput improvements using

MIMO (multiple input, multiple output antennas) (November 2009)

IEEE 802.11o - (reserved and will not be used)


Standard and amendments IEEE 802.11p - WAVE - Wireless Access for the Vehicular

Environment (such as ambulances and passenger cars) (working - 2009?)

IEEE 802.11q - (reserved and will not be used, can be confused with 802.1Q VLAN trunking)

IEEE 802.11r - Fast roaming Working "Task Group r" - (2008)

IEEE 802.11s - ESS Extended Service Set Mesh Networking (working - 2008?)

IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics Recommendation (working - 2008?)

IEEE 802.11u - Interworking with non-802 networks (for example, cellular) (proposal evaluation - ?)

IEEE 802.11v - Wireless network management (early proposal stages - ?)

IEEE 802.11w - Protected Management Frames (early proposal stages - 2008?)



IEEE 802.11x - (reserved and will not be used, can be confused with 802.1x Network Access Control).

IEEE 802.11y - 3650-3700 MHz Operation in the U.S. (2008).

IEEE 802.11z - Extensions to Direct Link Setup (DLS) (Aug. 2007 - Dec. 2011).

There is no standard or task group named "802.11x.


802.11 Layers


802.11 Physical Layer

802.11 Physical layer provides An interface to exchange frames with the

upper MAC layer It uses signal carrier and spread spectrum

modulation to transmit data frames over the media

It provides a carrier sense indication back to the MAC to verify activity on the media


Freq. Hopping Spread Spectrum (FHSS)

Uses 79 separate 1 MHz channels from 2.402-2.480 GHz

Hops about every 0.1 sec (22 hop pattern, 2.5 hop/sec minimum in US)

Immune to single frequency noise Many networks can be located in the

same area Uses less power to transmit & less

expensive to build than DSSS


Direct Sequence Spread Spectrum-DSSS

It is used in for signal generation in 2.4 GHz ISM band

Each bit sent by sender is replaced by a sequence of bits called chip code.

However time needed to send on chip code must be equal to time needed to send 1 bit

The modulation scheme used is PSK at 1 Mbaud/s.


OFDM

It distributes the data over a large number of carriers that are spaced apart at precise frequencies

This spacing provides the "orthogonality" which prevents the demodulators from seeing frequencies other than their own

The benefits of OFDM are high spectral efficiency resiliency to RF interference lower multi-path distortion


802.11 MAC Layer

It provides functionality to allow reliable data delivery for the upper layers over the wireless Physical media

It provides a controlled access method to the shared wireless media

It uses Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)


CSMA/CA


Persistence Strategies

Non-persistent – immediately on a non busy medium send the frame! Persistent -- If medium is busy wait a random time (exponential)

Issues? Inefficiency due to backoff when medium is idle


Hidden Terminal Problem


Exposed Terminal Problem


Overcoming Hidden Terminal Problem


Modified CSMA/CA


IEEE 802.11 Frame Structure

Data, controlor management RTS, CTS,

ACK etc.

To, or fromthe Intercell network

RetransmissionPayload is WEP encrypted(Wired Equivalent Privacy)

PowerManagement

More Fragments

Four Addresses


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802.11 WLAN