Wireless LAN - WLAN

1. Wireless LAN - WLAN Mohamed Mokdad Ecole d’Ingénieurs de Bienne

2. Agenda • WLAN Basics • WLAN Configurations • Customer considerations • Components • Frequencies • Frames

3. First approach • (Wired) LAN is a data network • Wireless LAN is an RF extension to LAN • Data over the air interface • No wires • i.e. Connectivity & Mobility

4. Why WLAN? • Mobility for data • As for voice, i.e. GSM (PWLAN) • Installation's speed and simplicity • No wiring needed • Installation flexibility • Easy reconfiguration • Scalability • WLAN can expand easily (Roaming)

5. The air interface technologies • Narrowband • Classical RF transmission • Spread spectrum (DSSS) • Sounds like noise if not tuned • Reliability, Integrity & Security • Infrared • Very high frequencies just below visible spectrum (Not very used in WLAN)

6. Spread Spectrum Technology • Direct-Sequence - DSSS • Robust to noise & interference • Resistant to multipath effects • More secure, harder to detect • Higher throughput • Frequency-Hopping - FHSS • Simple implementation • Less power consumption • Can add overlapping capacity with orthogonal hopping sequences

7. Typical configuration • An Access Point - AP - connects • The LAN to • Wireless LAN • An AP can support a small group of users in a range of 100 to 300 m. • The antenna is attached to the AP and defines the coverage area. • Isotropic or directional antennas

8. Peer to peer network - This configuration needs no AP and no network in order to connect the 2 PCs - The existing PCMCIA Cards generally allow this configuration

9. Clients and Access Points LAN Typical configuration

10. Multiple access points & roaming LAN Roaming or Handover? This means mobiles can move between different coverage areas

11. Use of an extension point LAN The extension point extends the coverage area of the AP without adding any complexity to the AP architecture. E.g. Hot Spot in Airports

12. The use of directional antennas LAN LAN Does this make sense?

13. Customer considerations • Range and coverage • Distance over which equipment can communicate • Radiated power in the coverage area - CA • Throughput • The mean bit rate (bps) a user can expect • N users need less bandwidth than N x times • Integrity and reliability • Protect your WLAN from taping - Small CAs • Protect your AP - Encrypted access & data

14. Customer considerations 2 • Compatibility with the existing network • Many different backbones supported • WLAN should be multivendor compatible • Interoperability of wireless devices • Air transmission technology (FHSS, DSSS) • Channel allocation (not all channels available) • Interference and Coexistence • Spectrum overlap • Different vendors can interfere

15. Customer considerations 3 • WLAN Licensing issues • Free spectrum or licensed - Hyperlan? ISM? • Simplicity/Ease of Use • A configured Wireless LAN can be moved with little or no modification at all • Security • WLANs are less secure than LANs • Cost • Main factors: Coverage area  # of APs

16. Customer considerations 4 • Scalability • Extension of coverage area • Support of additional APs at no costs • Battery Life for Mobile Platforms • This is rather a hand held computer issue • End user wireless products with less power • Safety • Much less than hand held cellular phones • Safety deals with human safety

17. ISM • Industrial/Scientific/Medical bands • 902-928 MHz • Crowded: Cordless phone, wireless speaker, garage door (telemetry) • 2400-2483.5 MHz • Medium use, Microwave oven • 5725-5850 MHz • US & Canada only • Light use, some radar • Expensive

18. OSI Layers

19. Configurations • Wireless LAN • Specified in IEEE 802.11 • Specify Physical and MAC layers • Stations • Portable: fixed when used • Mobile: mobile during usage • Emitted Power • 100 mW (1 W in US and 10 mW in Japan) • Impacts directly coverage area (bandwidth)

20. BSS & BSA BSS: Basic Service Set This figures out a coverage area STA1: Station 1 BSA: Basic Service Area  BSS Dynamic association between BSS and STAx

21. DS & ESS DS: Distribution System AP: Access Point LAN 2 BSSs can overlap – Co-located coverage areas

22. 11b Operating Channels Overlapping - close coverage areas should not overlap Non overlapping

23. 11b Channels sets for Europe HR&DSSS: High Rate / Direct Sequence Spread Spectrum

24. 11b Regulation agencies Code point Regulatory agency Region X'10' FCC United States X'20' IC Canada X'30' ETSI Most of Europe X'31' Spain Spain X'32' France France X'40' MKK Japan

25. Channels allocation Regulatory domains X'10' X'20' X'30' X'31' X'32' X'40' CHNL_ID Frequency FCC IC ETSI Spain France MKK 1 2412 MHz X X X — — — 2 2417 MHz X X X — — — 3 2422 MHz X X X — — — 4 2427 MHz X X X — — — 5 2432 MHz X X X — — — 6 2437 MHz X X X — — — 7 2442 MHz X X X — — — 8 2447 MHz X X X — — — 9 2452 MHz X X X — — — 10 2457 MHz X X X X X — 11 2462 MHz X X X X X — 12 2467 MHz — — X — X — 13 2472 MHz — — X — X — 14 2484 MHz — — — — — X

26. 2 state variables & 3 logical states State 1 Unauthenticated Unassociated Class 1 Frames Deauthentication Notification Successful Authentication State 2 Authenticated Unassociated Deauthentication Notification Class 1,2 Frames Successful Association or Reassociation Deassociation Notification State 3 Authenticated Associated Class 1,2,3 Frames

27. Frame Classes • Control • Management • Data • Class 1 • Control, Management & Data • Class 2 • Management • Class 3 • Control, Management & Data

28. Control Frames • Acknowledgment (ACK) • CF-End + CF-Ack • Clear To Send (CTS) • Contention-Free (CF)-End • Power Save (PS)-Poll • Request To Send (RTS)

29. Management Frames • Announcement traffic indication message (ATIM) • Association request • Association response • Authentication • Beacon • Deauthentication • Disassociation • Probe request • Probe response • Reassociation request • Reassociation response

30. Data Frames • CF-Ack (no data) • CF-Ack + CF-Poll (no data) • CF-Poll (no data) • Data + CF-Ack • Data + CF-Ack + CF-Poll • Data + CF-Poll • Null function (no data)

31. MAC Header Frame Control Duration ID Address 1 Address 2 Address 3 Sequence Control Address 3 Frame Body FCS Protocol Version Type Sub Type To DS From DS More Fragment Retry Power Mgmt More Data WEP Order MAC frame format - General Power Management = From Station to say 1: power save or 0: Active mode Retry for corrupted frames More fragments for segmented frames Duration ID = either AID (Station Association Identity) or Frame duration Sequence Control Field = Frames (12 bits) and Fragments (4bits) numbering

32. MAC Header Frame Control Duration RA TA FCS Protocol Version Type Sub Type To DS From DS More Fragment Retry Power Mgmt More Data WEP Order MAC frame format - Control Example of RTS control frame

33. Address Fields DA Destination Address of the MSDU RA Receiver Address - Address of STA in the AP SA Source Address of the MSDU TA Transmitter Address - Address of STA in the AP BSSID BSS ID

34. Address Fields bis • To DS = 0 & From DS = 0 • A data frame direct from one STA to another STA within the same IBSS, as well as all management and control type frames. • To DS = 1 & From DS = 0 • Data frame destined for the DS. • To DS = 0 & From DS = 1 • Data frame exiting the DS. • To DS = 1 & From DS = 1 • Wireless distribution system (WDS) frame being distributed from one AP to another AP.

35. Type Coding Management

36. Type Coding Control & Data

37. Architectural services • Station services • Authentication & Deauthentication • Privacy - WEP • MSDU delivery - Data delivery • Distribution system services • Association, Disassociation & Reassociation • Distribution • Integration (to the LAN)

38. Security • Service set identifier (SSID) • SSID associated with an AP or a groupof APs. Client must know SSID • Media Access Control (MAC) address filtering • No access to client’s MAC not in this list. • Wired Equivalent Privacy (WEP) • Encryption (not strong) • Others: IP range definition (e.g. WPA1/2)

39. WEP: Wired Equivalent Privacy • WEP provides equivalent functionality to the wired LAN • It is reasonably strong: from 40 to 128 bits • It is self-synchronizing: in each packet • It is efficient: it works • It may be exportable: US & Europe issue • E.g Internet Explorer @ 128 kbps • It is optional: It’s an IEEE option

40. WEP • 40 to 256 bits • Devices can work with lower encryption level - 120 bits can work @ 64 and 40 bits • Encryption level determined by key length • Lower level with 10 Hex characters • i.e. 5 ASCII characters • Highest level with 64 Hex characters • i.e. 32 ASCII characters

41. Antenna

42. Antenna

43. Medium Access • Carrier Sense Multiple Access with Collision Avoidance CSMA/CA • Carrier Sense Multiple Access with Collision Detection CSMA/CD

44. LAN - CSMA/CD • Supports two or more nodes on common bus • Node postpones transmission of data packets until network is clear of traffic • Other nodes transmit in event of collision • Back off period before retransmission

45. LAN - CSMA/CD A B B will detect tranmission from A It stops its transmission and Send the ethernet jam sequence, i.e. 32 bits Collision detected with higher current values

46. Back-Off • Detect collision N • Start with N = 0 • After a collision • Increment N and Increment K (if K < 10) • Calculate number set and pick R (Random) • {0,1,2,3} for N = 2 (and K = 2) • {0,1,2,3,4,5,6,7} for N = 3 (and K = 3) • Items number = 2K (max 2K = 1024) • R defines the wait time, i.e. R*51.2 μs • The collision probability gets lower

47. Back-Off

48. WLAN - CSMA/CA • Cannot detect data packet collision • Detects medium busy • Node can transmit Request to Send (RTS) to destination • Destination can transmit Clear to Send (CTS) to originating node • Destination can transmit Acknowledgement when data packet has been received

49. WLAN - CSMA/CA D MPDU S A D CW MPDU Node A Node B ACK Node C D = DCF Interframe Space DCF = Distributed Coordination Function MPDU = MAC Protocol Data Unit S = Short Interframe Space CW = Contention Window A = Acknowledgment

50. Hidden Node B A B C • - A is sending to B • - C is out of range of A’s transmission • C CSs and transmits  Collision