1. IE 419/519 Wireless Networks Lecture Notes #4 IEEE 802.11 Wireless LAN Standard Part #2

2. 2 IEEE 802.11 MAC Layer Key to the 802.11 specification It “rides” on every PHY layer and controls the transmission of user data into the air Provides core framing operations Provides interaction with a wired network backbone Covers three functional areas Reliable data delivery Medium access control Security

3. 3 MAC – Reliable Data Delivery WLAN using the IEEE 802.11 PHY and MAC layers is subject to considerable unreliability Even with error-correction codes, a number of MAC frames may not be received successfully More efficient to deal with errors at the MAC level than higher layer (e.g., TCP)

4. 4 MAC – Reliable Data Delivery (cont.) 802.11 incorporates positive acknowledgement Frame exchange protocol Source station transmits data Destination responds with acknowledgment (ACK) If source does not receive ACK, it retransmits frame Four frame exchange Source issues request-to-send (RTS) Destination responds with clear-to-send (CTS) Source transmits data Destination responds with ACK

5. 5 MAC – Reliable Data Delivery (cont.) Atomic Operation 802.11 allows stations to lock out contention during atomic operations so that atomic sequences are not interrupted by other stations attempting to use the transmission medium

6. MAC – Medium Access Control The 802.11 working group considered two types of proposals for a MAC algorithm Distributed (like Ethernet), or Centralized The end result is a MAC algorithm called Distributed Foundation Wireless MAC (DFWMAC) The DFWMAC architecture provides a distributed access control mechanism with an optional centralized control built on top of that 6

7. MAC – Medium Access Control (cont.) DFWMAC architecture 7

8. 8 802.11 MAC Frame Format

9. 9 MAC Frame – Address Fields FunctionToDSFromDSAddress 1Address 2Address 3Address 4 IBSS FunctionToDSFromDSAddress 1Address 2Address 3Address 4 To AP (infra.) AP Client Server 802.11 DS

10. 10 MAC Frame – Address Fields (cont.) FunctionToDSFromDSAddress 1Address 2Address 3Address 4 From AP (infra.) WDS (bridge) AP Client Server 802.11 DS 802.11 Server AP Client

11. 11 MAC – Security In wireless networks, the word “broadcast” takes on an entirely new meaning Original 802.11 standard Privacy Wired Equivalent Privacy (WEP) algorithm RC4 algorithm using a 40-bit key (  104-bit key later) Authentication Shared-key authentication For more information, go to http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html

12. 12 MAC – Security (cont.) WEP only addressed protection for the radio link Nothing beyond the AP Did not include a framework for authentication & authorization Employed a pre-shared key for encryption Suffered from severe weaknesses Key had to be manually entered/changed on the APs and all the stations Used CRC for data integrity

13. 13 MAC – Security (cont.) Types of Attacks Unauthorized association with the AP Man-in-the-middle Rogue AP Eavesdropping MAC Spoofing Denial of Service

14. 14 MAC – Security (cont.) The 802.11i task group developed a set of security mechanisms that eliminates most 802.11 security issues 802.11i addresses several security areas Access Control Authentication Authorization Confidentiality Data Integrity Key management Protection against known attacks

15. 15 MAC – Security (cont.) Security for WLANs focuses on Access Control (i.e., authentication) To prevent unauthorized users from communicating with APs To ensure that legitimate client units associate only with trusted APs (not rogue or unauthorized APs) Privacy Only intended audience understands transmitted data Encryption is key

16. 16 MAC – Security (cont.) Four distinct WLAN security solutions exist Open Access Basic Security Enhanced Security Requires a Remote Authentication Dial-In User Service (RADIUS) server Also known as an Authentication, Authorization and Accounting (AAA) server Remote Access Security Uses a VPN to allow access to corporate network and access business applications

17. 17 MAC – Security (cont.) Basic Security SSID “Sniffing” is a problem Open or Shared-Key Static WEP keys 40 or 128 bits Very time consuming process, especially if they change Stolen devices are a problem MAC Authentication Optional APs have access to a list MACs can be forged

18. 18 MAC – Security (cont.) Basic Security II WPA or WPA2 Pre-Shared Key (PSK) Uses a password or identification code Passphrase Network TypeWPAWPA2 Enterprise mode (business, government, education) Authentication: IEEE 802.1x/EAP Encryption: TKIP/MIC Authentication: IEEE 802.1x/EAP Encryption: AES-CCMP Personal mode (SOHO, home/personal) Authentication: PSK Encryption: TKIP/MIC Authentication: PSK Encryption: AES-CCMP

19. 19 IEEE 802.11 PHY Layer PHY media defined by original 802.11 standard Direct-sequence spread spectrum Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps 11 channels in the US, 13 in Europe, 1 in Japan Frequency-hopping spread spectrum Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps 70 channels in the US, 23 in Japan Infrared 1 and 2 Mbps Wavelength between 850 and 950 nm

20. 20 IEEE 802.11 PHY Layer (cont.)

21. 21 IEEE 802.11 PHY Layer (cont.)

22. 22 IEEE 802.11a Channel structure Makes use of the U-NII frequency bands Standard specifies a transmit spectrum mask Purpose is to constrain the spectral properties of the transmitted signal such that signals in adjacent channels do not interfere with one another

23. 23 IEEE 802.11a (cont.) Channel structure Available channels

24. 24 IEEE 802.11a (cont.) Channel structure A f f f FDM OFDM

25. 25 IEEE 802.11a (cont.) Coding and Modulation Uses Orthogonal Frequency Division Multiplexing (OFDM) Also called multicarrier modulation Uses multiple carrier signals at different frequencies, sending some of the bits in each channel Subcarrier modulated using BPSK, QPSK, 16-QAM or 64- QAM

26. 26 IEEE 802.11a (cont.) Coding and Modulation

27. 27 IEEE 802.11b Extension of the 802.11 DSSS scheme Provides data rates of 5.5 and 11 Mbps in the ISM band Uses chipping rate of 11 MHz thus occupying the same bandwidth as original DSSS scheme Higher data rate is achieved by using complementary code keying (CCK) as modulation scheme

28. 28 IEEE 802.11b (cont.) Channel structure 123456879 10 11121314123456879 10 11121314

29. 29 IEEE 802.11g Extension of 802.11b Achieves data rates above 20 Mbps up to 54 Mbps Operates in the 2.45 GHz range Compatible with 802.11b

30. 30 Other IEEE 802.11 Standards 802.11f Multi-vendor AP interoperability (IAPP) 802.11i Security and authentication mechanisms at the MAC layer 802.11n Range of enhancements to both PHY and MAC layers to improve throughput Multiple antennas Smart antennas Changes to MAC access protocols

31. 31 References The following references were used to complement the material presented in this module: Gast, M.S., 802.11 Wireless Networks: The Definitive Guide, 1 st Edition, O’Reilly, 2002 Rivero, J., Porter, J.D., Puthpongsiriporn, T., Lemhachheche, R., Layton, W.T., Campus Wireless Environment Deployment Guide, 2005.