Overview of IEEE 802.11 Wireless LAN Standards: MAC Layer and Data Delivery Mechanisms

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

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

8. 802.11 MAC Frame Format

9. MAC Frame – Address Fields 802.11 DS Server AP Client

10. MAC Frame – Address Fields (cont.) 802.11 DS Server AP Client 802.11 AP AP Client Server

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

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. 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. 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. 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. MAC – Security (cont.) • Basic Security II • WPA or WPA2 Pre-Shared Key (PSK) • Uses a password or identification code • Passphrase

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. IEEE 802.11 PHY Layer (cont.)

21. IEEE 802.11 PHY Layer (cont.)

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

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

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

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. 10 1 2 3 4 5 6 7 8 9 11 12 13 14 10 1 2 3 4 5 6 7 8 9 11 12 13 14 IEEE 802.11b (cont.) • Channel structure

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