Topic on WLANS• IEEE-802.11 (Hao Lian)• Analysis of campus wireless

network(Ao Shen)• Comparison between 3G and

Wi-Fi(Bichen Wang, Chen Chen)

Standardization of Wireless Networks

• Wireless networks are standardized by IEEE• Under 802 LAN MAN standards committee.

ApplicationPresentation

SessionTransportNetworkData Link

Physical

ISOOSI

7-layermodel Logical Link Control

Medium Access (MAC)

Physical (PHY)

IEEE 802standards

IEEE 802.11 Overview

• Adopted in 1997.Defines;• MAC sublayer • MAC management

protocols and services• Physical (PHY) layers

– IR – FHSS– DSSS

Goals• To deliver services in wired networks• To achieve high throughput• To achieve highly reliable data delivery• To achieve continuous network connection.

Components• Station• BSS - Basic Service Set

– IBSS : Infrastructure BSS : QBSS• ESS - Extended Service Set

– A set of infrastrucute BSSs.– Connection of APs– Tracking of mobility

• DS – Distribution System– AP communicates with another

WLAN 802.11 network

BSS

DS (usually Ethernet)

ESS

Wireless connection

BSS

AP

STASTA

STA

STA

AP

STASTA

STA

Services

• Station services: – authentication, – de-authentication, – privacy, – delivery of data

• Distribution Services ( A thin layer between MAC and LLC sublayer)– association– disassociation– reassociation– distribution– Integration

A station maintain two variables:• authentication state (=> 1)• association state (<= 1)

IEEE 802.11 overview

Services example : Roaming

AP1 AP2 AP3

Laptop com puter

1

1- Authenticate and associate2 – Laptop roaming3 – Authenticate (if needed) and (re)associate4 – Notify the new location of the laptop (disassociation of AP1)

Laptop com puter2

3

4

IEEE 802.11 overview

Old BSS

AP1 AP2 AP3

NewBSSNew

BSS

AP2 is out of servic

e

Laptop com puter

Laptop com puter

Services example : “Out of service”

Laptop com puter

Medium Access Control

Deals:• Noisy and unreliable medium• Frame exchange protocol - ACK• Overhead to IEEE 802.3 - • Hidden Node Problem – RTS/CTS• Participation of all stations• Reaction to every frame

IEEE 802.11 overview

MAC functionalities• Reliability of data delivery service• Control of shared WL network

– Distributed Coordination Function (DCF)– Point Coordination Function (PCF)

• Frame Types (informational section)• Management• Privacy service (Wired Equivalent Privacy -

WEP)

IEEE 802.11 overview

DCF Operation• Carrier Sense Multiple Access Collision

Avoidance (CSMA/CA), uses binary exponential back off (Same as in IEEE 802.3)

• IEEE 802.3 use collision detection algorithm.• IEEE 802.11 use collision avoidance (CA) algorithm

• Listen Before Talk – LBT (don’t transmit while others transmit to avoid collision)

• Network Allocation Vector (NAV) – the time till the network will be cleared from any transmitting.

• The NAV with the LBT assist to avoid collisions (CA)

DCF Operation

PCF Operation• Poll – eliminates contention• PC – Point Coordinator

– Polling List– Over DCF– PIFS

• CFP – Contention Free Period– Alternate with DCF

• Periodic Beacon – contains length of CFP• CF-Poll – Contention Free Poll• NAV prevents during CFP• CF-End – resets NAV

Other MAC Operations• Fragmentation

– Sequence control field– In burst– Medium is reserved– NAV is updated by ACK

Privacy WEP bit set when

encrypted. Only the frame body. Medium is reserved NAV is updated by ACK Symmetric variable key

WEP Details Two mechanism

Default keys Key mapping

WEP header and trailer KEYID in header ICV in trailer

dot11UndecryptableCount

Indicates an attack. dot11ICVErrorCount

Attack to determine a key is in progress.

MAC Management

• Interference by users that have no concept of data communication. Ex: Microwave

• Interference by other WLANs

• Security of data

• Mobility

• Power Management

Analysis of a Campus wideWireless Network

• Introduction• Trace Collection• Results

Introduction

• Research time: 2001 Fall• Research place: Dartmouth(161

buldings, 476 APs)• Research Target: Wireless network

analysis in Dartmouth.• Note: The paper only applied to

Dartmouth 2001.

Trace Collection• Syslog• SNMP• Sniffers• Other important definitions

Syslog

• Contains: ★AP name ★MAC address of card ★Message time ★Message type

Message Type of Syslog

• AssociatedA card selects one AP

• RoamedA card changes its current AP to another

• DeassociatedA card disconnects with one AP

SNMP

• A kind of heart-beat message, poll per 5 minutes

• Contains:★MAC address of card★Inbound bytes★Outbound bytes

Sniffers

• An application which collects packets and by extracting packets’ header, we can analyze more detailed information about users, such as types of packets, application-layer protocol used.

Sniffers

• We assemble “Sniffers” in four buildings★Sudikoff(6 APs)★Brown(2 APs)★Berry(13 APs)★Collis/Thayer(9 APs)

Other Important Definition

• Roamer Card• Mobile Card• Session

Session

• Starts when a card associates with an AP.

• Ends:★Changes one AP to another★Network Problem: Power Off .etc

Results & Analysis

• Traffic• Card • Session• AP• Protocol

Traffic• The busiest card transferred 117GB, while the median

card transferred only 350MB. On the busiest day, the traffic has the amount of 240GB, while the median daily traffic is only 53MB. Inbound traffic is greater than outbound traffic.

Traffic• 77 days’ traffic

overview• Weekly pattern• Reduction in

Thanksgiving• Reduction in the end• Holes

TrafficBased on Week• Monday is busiest,

because weekday starts with Monday

• Friday and Saturday are quietest, because students always rest on that two days

• From Sunday, the traffic is beginning to grow, because students usually start to finish homework on Sunday

Graph

Traffic

Day Pattern• Around 10:00 AM, busiest• In the afternoon, very

steady• After 12:00 AM, is declining

Graph

Card

• Activity varies significantly from active only once to active all 77 days(1706 cards)

• Median activity days: 28 days

Card• 77 days’ overview of

the number of active cards

• Follow the pattern of Traffic

Card• Daily Card Activity• Most active in the

afternoon, very steady in the afternoon

• From 12AM, huge reduction

Session• Median Session length: 16.6 min• 71% less than one hour• 27% less than one minute(Overlap AP

areas)

Roamed Session• 18% of all the

sessions are roamed session

• 60% of the roaming sessions roamed only within only one subnet

AP• 476 APs, more than 20APs not found in

trace• Each day between 171 and 352 APs are

used

AP• AP traffic:• Busiest AP: 2GB per

day• Median: 39 MB

Protocol• Get protocol by packet header and port

number• 99% are IP packets• In all of the IP packets: 99% are using TCP or

UDP• Application-layer protocol: http(53%),

dantz(15%) …

Protocol• All traffic is concerned

with web browsing, email, backup, file transfer, and file sharing

• Inbound traffic is more than outbound traffic

Protocol• Although it is

assymmetric in terms of traffic, it is symmetric when it comes to the number of TCP connections.

• Inbound connections equal to outbound connections

• Performance Comparison of 3G and Metro-Scale WiFi for Vehicular Network Access

• Pralhad Deshpande, Xiaoxiao Hou, and Samir R. Das

• Computer Science Department, Stony Brook University

• Stony Brook, NY 11794, USA

INTRODUCTION• 3G:

– licensed bands and macrocells with large coverage areas.

– base station and associated radio access network setup have significant capital and operational costs.

– ubiquitous.• WiFi:

– unlicensed spectrum. – access point (AP) coverage is relatively smaller

and typically capital and operational costs are lower.

– free or inexpensive, and would provide a significantly higher bit rate

– local area

PROBLEM• Can WiFi be used effectively in

outdoors and mobile scenarios to reduce the load on the expensive 3G networks?

MEASUREMENT SETUP• Network

– Optimum WiFi provided by Cablevision and has roughly 18,000 APs

– Verizon’s EVDO Rev 3G access• Testbed

– Dell Latitude laptop running Linux as the client in the car

– carrier-grade interface and transmit power choices + Verizon’s USB-based USB760 EVDO Rev

– DHCP– TCP maximum retransmission timer on the server side

is 1 sec.ps: measure end-to-end throughput only instead of the throughput on the

wireless hop because of lack of access to the provider network

• Driving Scenarios– Long Drive (Once):

• 500 miles• vehicle speed varied depending on the road traffic• reasonable sample of the quality of WiFi access from

moving vehicles in a metro-scale deployment scenario.

– Short Repeated Drives (10 times): • 9 mile• selected stretch where the quality of the AP coverage

is good. • experimental results.

• Log– TCP throughputs/second (in-staneous throughputs) on

both the connections along with GPS location and vehicle speed.

MEASUREMENT RESULTS• Quality of WiFi Coverage

• CDF of run lengths (consecutive 1 sec segments) with zero and non-zero throughputs seen on WiFi.

• ComparingWiFi and 3G Throughputs

• CDF of instantaneous TCP throughputs for WiFi and 3G

• CDF of relative difference of instantaneous throughputs (in Kbps) between WiFi and 3G. Plot for the long drive only.

• Correlation with Vehicle Speed

Short Drive Long Drive

• Correlation with Location

• Comparison of total and location entropies for 3G and WiFi networks

• H(X|li) is the entropy of throughput for a specific location li.

• Temporal Correlation

• Autocorrelation R(k) of the instantaneous throughputs measured in 1 sec intervals

CONCLUSION• WiFi

– frequent disconnections even in a commercially operated, metro-scale deployment;

– when connected indeed delivers high throughout even in a mobile scenario.

• 3G network – much lower throughputs– much better coverage and less throughput variability.

• A hybrid design that exploits the best properties of the two networks opportunistically can be very successful. Better throughput + lower cost for the provider by moving expensive 3G bits onto WiFi networks.

Thanks so much!