Topic on WLANS

1. Topic on WLANS • IEEE-802.11 (HaoLian) • Analysis of campus wireless network(AoShen) • Comparison between 3G and Wi-Fi(Bichen Wang, Chen Chen)

2. Application Presentation ISO OSI 7-layer model Session IEEE 802 standards Transport Network Logical Link Control Data Link Medium Access (MAC) Physical Physical (PHY) Standardization of Wireless Networks • Wireless networks are standardized by IEEE • Under 802 LAN MAN standards committee.

3. IEEE 802.11 Overview • Goals • To deliver services in wired networks • To achieve high throughput • To achieve highly reliable data delivery • To achieve continuous network connection. • Adopted in 1997. Defines; • MAC sublayer • MAC management protocols and services • Physical (PHY) layers • IR • FHSS • DSSS

4. 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

5. DS (usually Ethernet) STA AP AP STA STA STA STA STA STA BSS BSS ESS WLAN 802.11 network Wireless connection

6. 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)

7. 2 Services example : Roaming 1- Authenticate and associate 2 – Laptop roaming 3 – Authenticate (if needed) and (re)associate 4 – Notify the new location of the laptop (disassociation of AP1) 1 3 4 AP3 AP1 AP2 IEEE 802.11 overview

8. Old BSS New BSS New BSS AP2 is out of service Services example : “Out of service” AP3 AP1 AP2 IEEE 802.11 overview

9. 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

10. 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

11. 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) IEEE 802.11 overview

12. DCF Operation

13. 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

14. Other MAC Operations • 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. • 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

15. MAC Management • Interference by users that have no concept of data communication. Ex: Microwave • Interference by other WLANs • Security of data • Mobility • Power Management

16. Analysis of a Campus wideWireless Network • Introduction • Trace Collection • Results

17. 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.

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

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

20. Message Type of Syslog • Associated A card selects one AP • Roamed A card changes its current AP to another • Deassociated A card disconnects with one AP

21. SNMP • A kind of heart-beat message, poll per 5 minutes • Contains: ★MAC address of card ★Inbound bytes ★Outbound bytes

22. 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.

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

24. Other Important Definition • Roamer Card • Mobile Card • Session

25. Session • Starts when a card associates with an AP. • Ends: ★Changes one AP to another ★Network Problem: Power Off .etc

26. Results & Analysis • Traffic • Card • Session • AP • Protocol

27. 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.

28. Traffic • 77 days’ traffic overview • Weekly pattern • Reduction in Thanksgiving • Reduction in the end • Holes

29. Traffic Based on Week Graph • 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

30. Traffic Day Pattern Graph • Around 10:00 AM, busiest • In the afternoon, very steady • After 12:00 AM, is declining

31. Card • Activity varies significantlyfrom active only once to active all 77 days(1706 cards) • Median activity days: 28 days

32. Card • 77 days’ overviewof the number of active cards • Follow the pattern of Traffic

33. Card • Daily Card Activity • Most active in the afternoon, very steady in the afternoon • From 12AM, huge reduction

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

35. Roamed Session • 18% of all the sessions are roamed session • 60% of the roaming sessions roamed only within only one subnet

36. AP • 476 APs, more than 20APs not found in trace • Each day between 171 and 352 APs are used

37. AP • AP traffic: • Busiest AP: 2GB per day • Median: 39 MB

38. 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%) …

39. Protocol • All traffic is concerned with web browsing, email, backup, file transfer, and file sharing • Inbound traffic is more than outbound traffic

40. 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

41. 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

42. 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

43. PROBLEM • Can WiFi be used effectively in outdoors and mobile scenarios to reduce the load on the expensive 3G networks?

44. 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

45. 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.

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

47. ComparingWiFi and 3G Throughputs • CDF of instantaneous TCP throughputs for WiFi and 3G

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

49. Correlation with Vehicle Speed Short Drive Long Drive

50. 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.