WiFi Networks KIT Seminar Nayan

8/3/2019 WiFi Networks KIT Seminar Nayan

http://slidepdf.com/reader/full/wifi-networks-kit-seminar-nayan 1/34

Wi-Fi NetworksChallenges & Solutions

by

Nayan Prajapati

10MEEC13

KIT&RC, Kalol.



Characteristics :

Types

Infrastructure based

Ad-hoc

Advantages Flexible deployment

Minimal wiring difficulties

More robust against disasters (earthquake etc)

Disadvantages

Low bandwidth compared to wired networks

Need to follow wireless spectrum regulations



Infrastructure

network :

Ad-hoc network :

APAP

AP

wired network

AP: Access Point

Types :

Fig 1 : Types of WiFi Networks



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.



AP

30 client systems located within a radius of 103 m.

Depends on such variables as indoor oroutdoor placement, height above ground,nearby obstructions, other electronic devicesthat might actively interfere with the signal

by broadcasting on the same frequency, typeof antenna, the current weather, the poweroutput of devices.



Protocol Architecture

MAC sublayer

MAC layer

management

PLCP sublayer

PMD sublayer

Physical layermanagement

Table 1 : Protocol Architecture



802.11 Physical Layer

There are two sub layers in physical layer:

• Direct Sequence Spread Spectrum (DSSS)• Frequency Hoping Spread Spectrum (FHSS)



DSSS

• Direct sequence signaling technique divides the 2.4 GHz band into 11 22-MHz channels. Adjacent channels overlap one another partially, withthree of the 11 being completely non-overlapping. Data is sent acrossone of these 22 MHz channels without hopping to other channels.

Fi 2 : DSSS



Medium Access Control layer

Asynchronous data service (DCF)

- CSMA/CA - RTS/CTS

Time bounded service (PCF)- Polling



DCF

- CSMA/CA is a DCF

- Medium sharing through CSMA/CA

- Allows one STA to use the medium

- Random Back off interval before retransmission



IEEE 802.11 MAC Protocol

• Also known as CSMA/CA

• 802.11 CSMA (no collision detection)

Sender

If the channel is idle for DIFS seconds

- Transmit DATA packet

If the channel is busy

- Backoff

Receiver

If DATA packet received

- Return ACK after SIFS seconds

Source Destination

DIFSDATA

SIFS

ACK

Fig 3 : CSMA/CA



Wireless Media Disperse Energy

A BC D

Distance

Signalpower

SINR threshold

Signal not same at different locatio

Fig 4: Wireless Media Disperse Energy



Collision Detection Difficulty

Signal reception based on SINR

Transmitter can only hear itself

Cannot determine signal quality at receiver

A CD

B

Fig 5 : Collision Detection Difficulty



IEEE 802.11 MAC Protocol

Source Destination

DIFS

SIFS

RTS

CTS

DATA

SIFS

ACK

CSMA/CA (Collision Avoidance) - RTS/CTS Implementation

• Sender transmits an RTS packet

• Receiver responds with a CTS packet

• Purpose of CTS packet

- Reserves channel for sender

- Notify other stations

Goal – Avoid “Hidden Node” collisions

SIFS

Fig 6 : RTS/CTS -1



CTS = Clear

To Send

RTS = Request

To Send

RTS-CTS

D

Y

S

M

K

RTS

CTS

X

Fig 7 : RTS/CTS -2



MATLAB Simulation

• Includes RTS/CTS exchange

•

Simulates CSMA/CA



Analysis

Fig 8 : Simulation result of CSMA and CSMA/CA



Exposed Node Problem :

Fig 9 : Exposed Node



Virtual Sieve : Distributed

channel assignment[3]

In [1], the authors proposed a modified transmission power threshold based on a multi-

channel medium access control protocol toimprove the hidden node avoidance.

In [2], a multi-channel MAC protocol wasproposed and nodes can dynamically switch toother channels to avoid collisions.



Virtual Sieve : Distributed

channel assignment[3]

Fig 10 : Distributed Channel Assignment



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



Synchronization

Timing synchronization function (TSF)

Used for power management- beacons sent at well known intervals - all station timers in BSS are synchronized



Power Management

Mobile devices are battery powered- power management is important formobility

802.11 power management protocol- allows transceiver to be off as much as

possible

- is transparent to existing protocols



Power management approach

Allow idle stations to go to sleep.

Periodically listen to Traffic Indication Map(TIM).

APs buffer packets for sleeping stations. OFPM protocol : variably adjust LI(listen

interval) [4]



Roaming

Mobile stations may move beyond the coverage area of their AP but within range of another AP

Re association allows station to continue operation.

performing authentication at the new access point. In [5], authors proposed an innovative lightweight

authentication scheme called SFRIC (secure fast roamingusing ID-based cryptography).

SFRIC employs ID-based cryptography to simplify theauthentication process. It performs mutual authenticationfor the mobile client and AP with a 3-way handshake.

It does not require contacting an authentication server orexchanging certificates.



Roaming approach

Station decides that link to its current AP is poor.

Station uses scanning function to find another AP

Station sends Re-association Request to new AP

If Re-association Response is successful- then station has roamed to the new AP- else station scans for another AP

If AP accepts Re-association Request- AP indicates Re-association to thedistributed system

- Distributed system information isupdated



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 waitfor probe response



Problems in Wireless

Signal strength decreases proportional to thesquare of the distance

Sender would apply CS and CD, but the collisionshappen at the receiver

Sender may not “hear” the collision, i.e., CD doesnot work

CS might not work, e.g. if a terminal is “hidden”



Net Stress

Fig 11 : Net Stress Software



Wi-Fi Security Threats

• Wireless technology doesn’t remove any oldsecurity issues, but introduces new ones

• Eavesdropping

• Man-in-the-middle attacks

• Denial of Service



Eavesdropping

• Easy to perform, almost impossible to detect

• By default, everything is transmitted in clear text

• Usernames, passwords, content ...

• No security offered by the transmission medium

• Different tools available on the internet

• Network sniffers, protocol analysers . . .

• Password collectors

• With the right equipment, it’s possible toeavesdrop traffic.



MITM Attack

1. Attacker spoofes adisassociate messagefrom the victim

2. The victim starts to lookfor a new access point,and the attackeradvertises his own APon a different channel,using the real AP’s MACaddress

3. The attacker connectsto the real AP usingvictim’s MAC address

Fig 13 : MITM Attack



Denial of Service

• Attack on transmission frequecy used

• Frequency jamming

• Not very technical, but works

• Attack on MAC layer• Spoofed deauthentication / disassociation messages

• can target one specific user

• Attacks on higher layer protocol (TCP/IP protocol)

• SYN Flooding



References

[1] T. B. Manjukumar, “Performance degradation of IEEE 802.15.4slotted CSMA/CA due to hidden nodes,” 32nd IEEE Conferenceon Local Computer Networks, 2007.

[2] S. Wiwatthanasaranrom and A.honphoem, “Multichannel macprotocol for ad-hoc wireless networks,” 7th National ComputerScience and Engineering Conference (NCSEC 2003), 2003.

[3] Zhe Yu,”Virtual Sieve: A Distributed Channel AssignmentAlgorithm Resolving the Hidden Node Problem “.

[4] Zi-Tsan Chou,”OFMP for power management in wirelessneetworks”,ICWCS,2008.

[5] Yoohwan Kim,”SFRIC authentication method”,IEEE

international conference,2007. [5],Brian P Crow,IEEE 802.11 Wireless Local Area Networks. IEEE

Communications Magazine

[6]www.breezecom.com

WiFi Networks KIT Seminar Nayan

Documents

Transcript of WiFi Networks KIT Seminar Nayan