IE 419/519 Wireless Networks Lecture Notes #4 IEEE Wireless LAN Standard Part #2
2 IEEE MAC Layer Key to the 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 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.) 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 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
MAC – Medium Access Control The 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
MAC – Medium Access Control (cont.) DFWMAC architecture 7
MAC Frame Format
9 MAC Frame – Address Fields FunctionToDSFromDSAddress 1Address 2Address 3Address 4 IBSS FunctionToDSFromDSAddress 1Address 2Address 3Address 4 To AP (infra.) AP Client Server DS
10 MAC Frame – Address Fields (cont.) FunctionToDSFromDSAddress 1Address 2Address 3Address 4 From AP (infra.) WDS (bridge) AP Client Server DS Server AP Client
11 MAC – Security In wireless networks, the word “broadcast” takes on an entirely new meaning Original 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
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 i task group developed a set of security mechanisms that eliminates most security issues i 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 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 IEEE PHY Layer PHY media defined by original 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 PHY Layer (cont.)
21 IEEE PHY Layer (cont.)
22 IEEE a 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 a (cont.) Channel structure Available channels
24 IEEE a (cont.) Channel structure A f f f FDM OFDM
25 IEEE a (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 a (cont.) Coding and Modulation
27 IEEE b Extension of the 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 IEEE b (cont.) Channel structure
29 IEEE g Extension of b Achieves data rates above 20 Mbps up to 54 Mbps Operates in the 2.45 GHz range Compatible with b
30 Other IEEE Standards f Multi-vendor AP interoperability (IAPP) i Security and authentication mechanisms at the MAC layer n 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., 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.