Džiugas Baltrūnas Simula Research Laboratory 2014- 04-04

1. WLAN communication: an overview of 802.11 Džiugas Baltrūnas SimulaResearch Laboratory 2014-04-04 [flickr.com/photos/scoobyfoo]

2. References [1] IEEE 802.11 Tutorial. J. Zyren and A. Petrick, 1999 (?) [2] IEEE 802.11 Wireless Local Area Networks. Crow, B.P.; Widjaja, I.; Kim, L.G.; Sakai, P.T. Communications Magazine, IEEE , Volume: 35 , Issue: 9, Sept. 1997. [3] A Survey on Emerging Broadband Wireless Access Technologies. M. Kuran and T. Tugcu. Computer Networks, 2007.

3. WLAN: Wireless Local Area Networking PHYsical layer Medium Access Control layer Security

4. What is WiFi? Wireless Fidelity (Wi-Fi) is used to define IEEE 802.11 wireless standards. The term Wi-Fi was created by the Wireless Ethernet Compatibility Alliance (WECA). Products certified as Wi-Fi compliant are interoperable with each other. [microsoft.com] A Basic Service Set (BSS) consists of of two or more wireless nodes, or stations (STAs). If STAs communicate directly with each other, they form an ad hoc network or an Independent BSS (IBSS). Otherwise, a BSS contains an Access Point (AP) to form a bridge between wireless and wired LANs. A BSS in this configuration is in the infrastructure mode. Two or more overlapping BSSs (each containing an AP) connected together by means of a Distribution System (DS) form an Extended Service Set (ESS).

5. Standards IEEE 802.11 - The original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard (1999) IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001) IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999) IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard (2001) IEEE 802.11d - International (country-to-country) roaming extensions (2001) IEEE 802.11e - Enhancements: QoS, including packet bursting (2005) IEEE 802.11F - Inter-Access Point Protocol (2003) Withdrawn February 2006 IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003) IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility (2004) IEEE 802.11i - Enhanced security (2004) IEEE 802.11j - Extensions for Japan (2004) IEEE 802.11k - Radio resource measurement enhancements IEEE 802.11m - Maintenance of the standard; odds and ends. IEEE 802.11n - Higher throughput improvements (2008) IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment IEEE 802.11r - Fast roaming IEEE 802.11s - ESS Mesh Networking IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics IEEE 802.11u - Interworking with non-802 networks (for example, cellular) IEEE 802.11v - Wireless network management IEEE 802.11w - Protected Management Frames IEEE 802.11y - 3650-3700 Operation in USA

6. 802.11 standards MIMO – multiple-input multiple-output IR – infrared FHSS – frequency hopping spread spectrum DSSS – direct-sequence spread spectrum OFDM – orthogonal frequency-division multiplexing

7. 802.11 standards [http://wits.ice.nsysu.edu.tw/course/pdfdownload/95ADV/lect10-WLAN-MAC.pdf]

8. 802.11 PHY: bandwidth [wikipedia.org] • 2.4 GHz (2.414-2.484 GHz) frequency band with 14 distinct channels or 5 GHz (5.15-5.825 GHz) frequency band with 36-161 distinct channels • The center frequency of a channel must be at least 5 MHz apart from each other • A typical AP signal does not extend more than 22 MHz from the center frequency of the selected frequency • There are 3 non-overlapping channels in 2.4 GHz frequency band and 19 (12 in the US) in 5 GHz band

9. 802.11 PHY: bandwidth [http://www.connect802.com/80211n_channels.htm]

10. Example: a failure to use 40 Mhz bandwidth on 802.11n 1396534865.534465: wlan0: Event SCAN_RESULTS (3) received 1396534865.534482: Start SYNO ACS: 1396534865.535013: nl80211: Received scan results (16 BSSes) 1396534865.535113: channel: 1, noise: 395.000000 1396534865.535254: channel: 2, noise: 292.333313 1396534865.535281: channel: 3, noise: 235.666656 1396534865.535305: channel: 4, noise: 179.000000 1396534865.535328: channel: 5, noise: 196.000000 1396534865.535350: channel: 6, noise: 175.333313 1396534865.535373: channel: 7, noise: 154.666656 1396534865.535395: channel: 8, noise: 163.000000 1396534865.535417: channel: 9, noise: 272.999969 1396534865.535440: channel: 10, noise: 383.000000 1396534865.535462: channel: 11, noise: 493.000000 1396534865.535492: ACS channel (7) 1396534865.535557: ACS sec channel (3) 1396534865.535893: nl80211: Received scan results (16 BSSes) 1396534865.535958: 40 MHz affected channel range: [2407,2457] MHz 1396534865.535981: Neighboring BSS: cc:5d:4e:8c:93:84 freq=2412 pri=1 sec=0 1396534865.536000: Neighboring BSS: 34:21:09:02:40:dc freq=2412 pri=1 sec=0 1396534865.536018: Neighboring BSS: 84:1b:5e:7e:ab:69 freq=2432 pri=5 sec=0 1396534865.536036: Neighboring BSS: 50:e5:49:8e:50:b8 freq=2432 pri=5 sec=0 1396534865.536052: Neighboring BSS: 84:1b:5e:e8:53:91 freq=2412 pri=1 sec=0 1396534865.536068: Neighboring BSS: 20:c9:d0:ac:c6:c6 freq=2412 pri=1 sec=0 1396534865.536086: Neighboring BSS: 00:24:36:aa:57:d5 freq=2447 pri=8 sec=0 1396534865.536103: Neighboring BSS: ee:43:f6:97:22:bc freq=2412 pri=1 sec=0 1396534865.536122: Neighboring BSS: cc:5d:4e:19:82:1c freq=2427 pri=4 sec=8 1396534865.536137: 40 MHz pri/sec mismatch with BSS cc:5d:4e:19:82:1c <2427,2447> (chan=4+) vs. <2442,2422> 1396534865.536165: 20/40 MHz operation not permitted on channel pri=7 sec=3 based on overlapping BSSes 1396534865.536193: Completing interface initialization 1396534865.536211: Mode: IEEE 802.11g Channel: 7 Frequency: 2442 MHz 1396534865.536228: nl80211: Set freq 2442 (ht_enabled=1 sec_channel_offset=0)

11. 802.11 PHY: FHSS modulation [cdt21.com]

12. 802.11 PHY: DSSS modulation • DSSS takes the normal narrow-band transmission and spreads it over a larger, preventing interference from blocking the entire signal • Original user data is multiplied (XOR) with a pseudo random spreading (chipping) code • The resulting high speed digital stream is then modulated onto the carrier frequency using Differential Phase Shift Keying (DPSK)

13. 802.11 PHY: DSSS modulation • 802.11b and 802.11g uses Differential Binary Phase-Shift Keying (DBPSK) for 1 Mbps and Differential Quadrature Phase-Shift Keying (DQPSK) for 2 Mbps • Higher (5.5 and 11 Mbps) rates are achieved by using the same DBPSK and DQPSK modulation methods, but paired with Complementary Code Keying (CCK) encoding method

14. 802.11 PHY: OFDM modulation [ni.com] • OFDM is a subset of frequency division multiplexing in which a single channel utilizes multiple sub-carriers on adjacent frequencies • The available radio band is divided into a number of sub-channels and some of the bits are sent on each • The transmitter encodes the bit streams on the 64 subcarriers using BPSK, QPSK, or one of two levels of Quadrature Amplitude Modulation (16, or 64-QAM)

15. 802.11 PHY: OFDM modulation

16. 802.11 MAC: Distributed Coordination Function (DCF) • MAC layer of IEEE 802.11 utilizes a contention based DCF scheme • Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism is used to avoid collisions • STAs associated with the AP sense the air interface for channel availability • Random back-off procedure is used if the channel is busy and an STA wants to transmit the data

17. 802.11 MAC: Distributed Coordination Function (DCF) • Packet reception in DCF requires acknowledgement • The period between completion of packet transmission and start of the ACK frame is one Short Inter Frame Space (SIFS) • Transmissions of MPDUs (MAC Protocol Data Unit) other than ACKs must wait at least one DCF Inter Frame Space (DIFS) before transmitting the data

18. 802.11 MAC: Point Coordination Function (PCF) [http://www.iith.ac.in/~tbr/teaching/docs/Schiller-802.11.pdf] • PCF is a contention-free protocol and enables stations to transmit data frames synchronously • In PCF, the AP polls each STA in a round-robin fashion to check if they have packets to send • The AP waits for PCF Inter Frame Space (PIFS) duration • PIFS < DIFS • Any STA that is not polled till the end of the polling period, will be queued for polling during the next polling period

19. 802.11 MAC: the “hidden node” problem • CSMA/CA assumes that every station can “hear” all other stations, which not always the case • If at least one pair of STAs cannot directly communicate, the AP enables the use of Request To Send (RTS) / Clear To Send (CTS) mechanism together with DCF • For each transmission, the source STA transmits a RTS message and the destination responds with a CTS message

20. 802.11 MAC: DCF with RTS/CTS [http://www.iith.ac.in/~tbr/teaching/docs/Schiller-802.11.pdf] • Station can send RTS with reservation parameter after waiting for DIFS • When the source STA receives the CTS message, it starts its data transmission • All other STAs assume that the medium is in use for the duration given in the message when they receive RTS and/or CTS messages

21. 802.11 MAC: security • WEP was the first form of authentication used with Wi-Fi. Unfortunately it was easy to crack (static key). • WPA is a improvement over WEP (TKIP, changed key on each packet), but still proved to be insecure. • WPA2 (802.11i) is the next update to WPA and provides significant improvement in the level of security. WEP – Wired Equivalent Privacy; WPA – Wi-Fi Protected Access PSK – Pre-shared Key AES – Advanced Encryption Standard; TKIP – Temporal Key Integrity Protocol MIC – Message Integrity Code; CBC-MAC – Cipher block chaining message authentication code

22. In summary, the use WLAN communication is increasing and further standardization of IEEE 802.11 is ongoing Questions? [broadcom.com]