wi-fi technical aspects

8/9/2019 Wi-Fi Technical Aspects

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Bharat Sanchar Nigam Limited, India

ALTTC, Ghaziabad

Wi-Fi




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Wireless Networks

Evolution of Wireless Networks:

Portable Machines or Mobile Stations

Popularity of Internet

Trend towards Central Server Webbased Applications

Trends towards Mobility Quick Deployment and modifications




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Wireless Networking Trends

Higher Data Rates

More Complex Multiple-access and

Modulation techniques Higher Frequencies

Single Device for Networking

Integrating wireless networking radio

modules into the existing devices Standards to be transparent to end-

users




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Wireless Networks

WPANAn alternative to cable

Coverage only few metersData rate few Mbps

WLANWireless Ethernet

WWANLarge Cells

High Mobility

User fee required




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Wireless Networking StandardsWLAN WWAN WPAN

Coverage

Area

WithinBuilding

National Few Meters

TypicalThroughput

1 to 11 Mbps 10K to 2Mbps

0.1 to4Mbps

Application Extension oralterative towired LAN

Extensionto wiredLAN

Cablereplacement

User Fee NO Yes No

Example WiFi Cellular Bluetooth




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WWAN<15 km

802.20 (proposed)

MAN<5 km

75 Mbit/s802.16d/e

WiMAXNew standard for

fixed broadbandwireless. Doing for MAN what Wi-Fi didfor LAN.

WLAN<100 m

11-54 Mbit/s

802.11a/b/ g

Wi-Fi ®

Includes 802.11a/b/g.Products must beapproved for interoperability by theWi-Fi Alliance.

PAN<10 m

802.15.1 (Bluetooth)*

802.15.3 (UWB) **

802.15.4 (ZigBee)***




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Wireless Network - WLAN

Ethernet without cabling

Carrier high-speed traffic

Does not support real time traffic

Coverage area is in order of 100 meters

Low-speed mobility

No user fee required




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>What is WiFi ?

Inter operability standard for Wireless Fidelity usingIEEE 802.11b/802.11a/802.11g standards.




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WiFi

>Wi-Fi Alliance www.wi-f i.org

In 1990, the IEEE 802 Committee formed a new working group, IEEE802.11, specifically devoted to wireless LANs with the initially interest towork in ISM Band.

The first 802.11 standard to gain broad industry acceptance was 802.11b.Although 802.11b products are all based on the same standard, there isalways a concern, whethr products from different vendors will inter-work with each other. To meet this concern, the Wireless EthernetCompatibility Alliance (WECA), an industry consortium, was formed in

1999. This organization, subsequently renamed the Wi-Fi (WirelessFidelity) Alliance, created a test suite to certify interoperability for802.11b products.




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WLAN is taking over

Home, Office and City!

Everything shipping withWLAN, laptops, PDAs,

home appliances

Standards driven

802.11 standardscommunity is as big as

cellular standards

Part of ongoing trend toportable computing

Consumer driven ±Centrino effect

Consistent userexperience to LAN

Unlicensed, simple,competitive marketplace

Inexpensive

Why WLAN ?




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WLAN Topologies

Ad Hoc Network

Administered Network

Multiple Access points and roaming




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Adhoc Networking: It is a peer-to-peer network (no

centralized server) set up temporarily to meet some immediate

need. For example, a group of employees, each with a laptop

or palmtop computer, may convene in a conference room for a

business or classroom meeting. The employees link their

computers in a temporary network just for the duration of the

meeting.




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928MHz900MHz

Radio Frequency Spectrum

5.15 5.25 5.35 5.725 5.8252.4835GHz2.4000GHz

900 MHz 2.4GHz

5.7 GHz

802.11, 11b, 11g, Bluetooth

ISM

Industrial Scientific and Medical (ISM)

Unlicensed National Information Infrastructure (U-NII)802.11a

U-NII ISM

50mW* 250mW 1000mW




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Wi-Fi Standards

IEEE 802.11b Data rate of 11Mbps

Operates at 2.4 Ghz (Industrial Scientific andMedical (ISM)

DSSS(Direct Sequence Spread Spectrum)

128 bit WEP

Currently majority of wireless products adhere tothis standard




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Wi-Fi Standards

IEEE 802.11a Data rate of 22Mbps

Operates at 5 Ghz

OFDM(Orthogonal Frequency DivisionMultiplexing)

152 bit WEP

But 5 GHz frequency was reserved for militaryuse in Europe and many countries.




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Wi-Fi Standards

IEEE 802.11g Data rate of 54Mbps

Operates at 2.4 Ghz

OFDM

Backward compatible with 802.11b

Latest protocol in market




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Wi-Fi (IEEE 802.11) Terminology

Access Point (AP): Any entity that has stationfunctionality and provides access to the distribution systemvia the wireless medium for associated stations.

Basic Service Set (BSS): A set of stations controlled by asingle coordination function.

Distribution System (DS): A system used to interconnecta set of BSSs and integrated LANs to create an ESS

Extended Service Set (ESS): A set of one or moreinterconnected BSSs and integrated LANs that appear as a

single BSS to the LLC layer at any station associated withone of the these BSSs

Station: Any device that contains an IEEE 802.11conformant MAC and physical layer




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IEEE 802.x LANExtended

Service Set

Distribution system

Basic

Service set

BasicService set

STA3STA2

STA4

STA1

AP

STA3STA4

AP




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Figure illustrate the model developed by the 802.11working group. The smallest building block of wireless LAN is aBasic Service Set (BSS), which consists of some number of stations executing the same MAC protocol and competing for

access to the same shared wireless medium. A BSS may beisolated or it may connect to a backbone distribution system (DS)through an access point (AP). The AP functions as a bridge and arelay point.

In a BSS, client stations do not communicate with another

station in the same BSS, the MAC frame is first sent from theorigination station to the AP, and then relayed by the AP to thedestination station. The BSS generally, corresponds to what isreferred to as a cell in the literature. The DS can be a switch, awired network, or a wireless network.




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When all the stations in the BSS have no connection toother BSSs, the BSS is called an independent BSS (IBSS). AnIBSS is typically an ad hoc network.

An Extended Service Set (ESS) consists of two ormore basic service sets interconnected by a distributionsystem. Typically, the distribution system is a wired backboneLAN but can be any communications network. The extendedservice set appears as a single logical LAN to the logical linkcontrol (LLC) level.




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IEEE 802.11 Architecture ± Protocol

Layers

Physical Layer

MAC

LLC

U pper layer Protocols

Wireless LANFunctions

Data Link Layer




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IEEE 802.11 ± Physical Layer Technologies

Spread Spectrum Technologies

FHSS

DSSS

Orthogonal Frequencies DivisionMultiplexing




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Spread Spectrum Technology

Uses a modulation Technique that requiresmore bandwidth than needed

Uses a code in the transmitter that must beknown to receiver




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Why do we spread the signal?

Low power spectral density

Protection against multiple paths

Interference resistant

Better security and privacy




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Direct Sequence Spread Spectrum

Source Information

BitsX Bits to I-Q X

Code Generator

Bit Stream

IQ Modulator

Carrier

Transmit

DSSS




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Frequency Hoping Spread Spectrum

Source Information

BitsX

Carrier Frequency

Synthesizer

IQ ModulatorTransmitFHSS

Code Generator

Bit Stream




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Data Link Layer

Media Access Control Layer

Logical Link Control Layer

The IEEE 802.11 MAC layer covers three functional areas:

Reliable Data Delivery, Medium Access Control, and Security




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MAC Layer

Core Mechanism is

Carrier Sense Multiple Access/Collision Avoidance




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CSMA/CA

CSMA/CA uses additional techniques toimprove performance

Acknowledgement/ Retransmission

RTS/CTS




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R eliable Data delivery

As with any wireless network, a wireless LAN using the IEEE

802.11 physical and MAC layers is subject to considerable

unreliability. Noise, interference, and other propagation effectsresult in the loss of a significant number of frames. Even with

error-correction codes, a number of MAC frames may not

successfully be received.

This situation can be dealt with by reliability mechanisms at ahigher layer, such as TCP.




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IEEE 802.11 includes a frame exchange protocol. When a

station receives a data frame from another station, it returns an

acknowledgement (ACK) frame to the source station. This

exchange is treated as an atomic unit, not to be interrupted by atransmission from any other station. If the source does not

receive n ACK within a short period of time, either because its

data frame was damaged or because the returning ACK was

damage, the source retransmits the frame.

Thus, the basic data transfer mechanism in IEEE 802.11

involves an exchange of two frames.




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To further enhance reliability, a four-frame exchange may be

used. In this scheme, a source first issues a request to send (RTS)

frame to the destination. The RTS alerts all station that are within

reception range of the source that an exchange is under way;these stations refrain from transmission in order to avoid a

collision between two frames transmitted at the same time.

Similarly, the CTS alerts all stations that are within reception

range of the destination that an exchange is under way. The

RTS/CTS portion of the exchange is a required function of theMAC but may be disabled.




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CSMA/CA

Packet R eady?

Channel Idle?

DIFS Delay

Channel Idle?

Transmit Packet

SISF Delay

ACK ?

+

+

Continue to sense channel

DISF Delay

Channel Idle?

Store R andom Backoff time

Decrement Backoff Counter when channel Idle

Is Backoff Timer Zero

N

N

N

N

N

N

Y

Y

YY

Y

Y




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Other MAC Layer Functions

Polling(active and passive)

Synchronization(beacons)

Authentication(open and sharedkey)

Frame Fragmentation

Security and Privacy




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Logical Link Layer

Controls exchange of data betweenusers

Specifies mechanism for addressing of source and destination




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Wireless Security

Wired Vs Wireless

No Physical Barrier as Signal is available

all around A Big Issue in WiFi




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What is being protected?

802.11 specif ication addresses protection for the

radio link layer only; the communications link

between the mobile station and the AP.

Types of Attacks Unauthorized association to the AP

Unauthorized APs

Man-in-the-middle

Denial of Service




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Security Technologies Background

Authentication

Verification of credentials

Author ization

Allowing access to specific networkresources

Conf identialityof information exchanged between twoor more units




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Security Technologies Background

Data Integr ity

Receipt of data should have a method of

knowing the tamperingKey Management

automate the management of

Authentication key

Encryption key

Protection against well known attacks:




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Security Option

WEP (wire Equivalent Privacy) ±Original Security Mechanism given by

IEEE 802.11 standards 802.11i New Security Mechanism ±RSN (Robust Secure Network)

More Complex Encryption Algorithm

Automated Key Management WiFi Protected Access(WPA) is subset

of 802.11i




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RSN 802.11i

Key Management : 802.1X

Encryption: AEN Advanced Encryption Standard




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The 802.11i task group has developed a set capabilities to

address the WLAN security issues. In order to accelerate the

introduction of strong security into WLANs, the Wi-Fi

Alliance promulgated Wi-Fi Protected Access (WPA) as aWi-Fi standard. WPA is a set of security mechanisms that

eliminates most 802.11 security issues and was based on the

current state of the 802.11i standard. As 802.11i evolves,

WPA will evolve to maintain compatibility.

RSN 802.11i




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IEEE 802.11i addresses three main security areas:

Authentication

Key Management

Data Transfer Privacy

To improve authentication, 802.11i requires the use of an

authentication server (AS) and defines a more robust

authentication protocol. The AS also plays a role in keydistribution. For privacy, 802.11i provides three different

encryption schemes. The scheme that provides a long-term

solution makes use of the Advanced Encryption Standard

(AES) with 128 ± bit keys.

RSN 802.11i




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Station Access Point Authentication Server

Security capabilities discovery

Authentication

Key Management Key distribution

Data protection




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Component of HotSpot

Mobile Station(s)

Access Point(s)

Switches, Routers, Network Access Controller

Web Server

AAA Server

High speed Internet connection such as DSL (WAN

backhaul)




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HotSpot Environment

Physical size

Number of users

Type of users




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Usage Models

Coffee shop Small

The number of simultaneous users is also going to besmall: probably fewer than five. In this environment,

a single AP would be sufficient to provide adequatecoverage and service

Usages would include e-mail, web surfing, and on-linechat; all of which have relatively low bandwidthrequirements

An Internet connection with a bandwidth of ~500Kbps(e.g. a DSL line) would be sufficient .




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Usage Models

H otel ³What is the coverage area?´.

Single AP may be sufficient or we may require 20 or more APs.

The user base will be larger than a coffee shop, on the order of

10-50 simultaneous users, depending on the hotel size

The user density would be very small if 10 or more APs were

deployed

User density in the conference rooms might be high and could

potentially warrant increasing the number of APs located there.

Typical users are likely business travelers who would befocused on email, access to corporate intranets, web-surfing

and file downloads. 2 Mbps or higher bandwidth Internet

connection will most likely be required.




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Usage Models

Convention Center A convention center environment combines a large space

with a large user population with the possibility of high user

density in the conference sessions.

Many APs, possibly 50 or more, would be required, both tocover the physical space and to provide the performance

needed to support the large number of users.

Usage would be similar to the hotel environment: email,

corporate intranet access, web-surfing and file downloads

along with the potential for event-specific content. 10s to 100s of Mbps of bandwidth would be required to

support this usage scenario.




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Wireless Network




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HotSpot Functionality

Enabling access to the wireless link

1) Providing the mobile station withinformation about the wireless network

2) Creating an association with the mobilestation

3) Providing access to the local network

4) Providing data packet transfer services

5) Disassociation from with the mobilestation

h S h i i i d di




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HotSpot Functionality «Contd

Providing access to LAN

Providing access to the WAN

Protecting user data privacy

Provide accounting information

(keep track of user network usage)

Bh S h Ni Li i d I di




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The Access Point

Wireless access to the HotSpot network.

The AP is typically tasked with securing

access to the network. However, due to known vulnerabilities with the current security

standard, Wired Equivalent Privacy (WEP), and lack of key

management, most deployed APs should not be relied upon to

secure network access. Those that are, have done so by

including non-standard enhancements made by manufacturers.

Bh S h Ni Li i d I di




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Important AP features & Functionality

1. Providing the mobile device with

information about the wireless network O

n a regular basis, APs will broadcast the wireless networkparameters through a message called the beacon. The purpose

of this message is to make it easier for end-users to determine

what networks they can connect to and to let the mobile device

know about the wireless network characteristics (channel in use,

frequency hopping information, etc.). Beacons are always sent

by the AP and cannot be disabled in any way.

Bh t S h Ni Li it d I di




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Important AP features & functionality..contd

2 Responding to mobile station requests for informationThe mobile station does not have to wait for the AP to advertise

its presence. As a matter of fact, if it did, the mobile stationwould never be able to connect to networks that don¶t advertise.The mobile station can proactively get information from an AP inone of two ways:

it can send a probe request to discover nearby

802.11 networksor it can send a request for association to a

specific AP.In either case, the AP includes the necessary information about the wirelessnetwork in its response to the mobile station.

Bh t S h Ni Li it d I di




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Important AP features andfunctionality ..contd

3 Privacy and security considerations

The data exchanged between two or

more clients should be known only tothem



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Important access point features and

functionality ..contd

5 Device Authorization

Authorizing the mobile station (MS) to connect to thenetwork means that the MS is able to associate withthe AP and send and receive packets through thatassociation.

In a WEP-enabled wireless network, authorization isthe job of the AP.

Many Hot-Spots do not use WEP and therefore only

useO

pen Authentication. When Hot-Spots use WPA or RSN, authorization

comes from an AAA server.





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Important access point features and

functionality ..contd

6 Providing access to the local network

Once the MS has an association to the AP, it can

send and receive data frames on the local HotSpotnetwork. The AP serves as a bridge between the

wireless and wired networks, providing access to the

HotSpot wired network.





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Choosing your AP

The AP is the direct means of communication between the

LAN and the user¶s device.

The quality of the AP and its feature set is a determining

factor in the success of HotSpot deployment.

Consistent interoperability of the AP with 802.11 wireless

network cards from diverse vendors is the most critical AP

feature. It is important to remember that it is unlikely that

two Access Points from different manufacturers will work together .





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Choosing your AP

802.11 protocol

Security Compliance

RF Power Adjustable (5-100 mW)

Multiple Antenna Type

Roaming




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PCMCIA wireless card

Used For Portables:





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PCI and USB adapters

Used For Desktops

PCI adapter

(inside)

USB adapter

(outside)





Thanks!

wi-fi technical aspects

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