1. 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)

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

3. 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.

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

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. IEEE 802.11 overview Services example : Roaming AP1AP2 AP3 1 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) 2 3 4

8. IEEE 802.11 overview Old BSS AP1AP2 AP3 New BSS New BSS AP2 is out of service Services example : “Out of service”

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. 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)

11. 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)

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 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.

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 wide Wireless 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. 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 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

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

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

32. Card 77 days’ overview of 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.

51. Temporal Correlation Autocorrelation R(k) of the instantaneous throughputs measured in 1 sec intervals

52. 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.

53. Thanks so much!