1. School of Engineering NTM, WLAN, 1 IEEE 802.11 WLAN: Short Introduction References [1]J. Schiller, „Mobile Communications“, 2nd Ed., Pearson, 2003. [2]M. Sauter, „Grundkurs Mobile Kommunikationssyteme“, 4. Auflage, Vieweg+Teubner, 2011. [3]wiki to WLAN: http://de.wikipedia.org/wiki/IEEE_802.11http://de.wikipedia.org/wiki/IEEE_802.11 IEEE standard 802.11 is the most famous WLAN standard belongs to the 802.x LAN standards specifies PHY and MAC layer adapted to special requirements of wireless LANs standardization is ongoing 802.11b (1999) - 802.11n (2009), …

2. School of Engineering ESS extended service set BSS2 BSS1 basic service set STA1 station Portal 802.X LAN Internet The Distribution System connects several BSS to form a single network (ESS) with extended coverage. 1 AP and some STAs in the same radio coverage form a BSS. AP periodically broadcasts the SSID in beacon frames. Radio range is 30 – 300 m. Distribution System Access Point System Architecture NTM, WLAN, 2

3. School of Engineering System Architecture [2] Conditions for problem-free intra-ESS roaming the APs must belong to the same IP-subnet size of an ESS is limited (e.g. to the size of a building) all AP have the same BSS-ID (SSID) APs transmit on different frequencies the APs of an ESS should come from the same supplier IEEE has not specified the distribution system yet (cf. IEEE 802.11f), but specified distribution system services. overlapping radio coverage of the APs channel 6 channel 11channel 1 frequency gap of at least 5x5 = 25 MHz! AP NTM, WLAN, 3

4. School of Engineering System Architecture independent BSS STA5 can communicate directly with STA4, but not with STA3 STAs have to agree on some parameters: SSID, channel, key, IP addresses ad-hoc mode is not used often because of complex configuration no routing! NTM, WLAN, 4

5. School of Engineering Protocol Architecture [1], Figure 7.5, p. 210 IEEE 802.11 fits seamlessly into other 802.x standards for wired LANs WLAN behaves like a slow wired LAN Application should not notice „anything“ NTM, WLAN, 5

6. School of Engineering IEEE 802.11 Standards 1- 6- (6-600 Mbps) 5 / for other 802.11 standards, e.g. 802.11p, please cf. to [4] MIMO, OFDMOFDM DPSK/DQPSK DSSS CCK NTM, WLAN, 6

7. School of Engineering Management Operations [2] Scanning and Beacon Frames AP periodically broadcasts (e.g. every 100 ms) beacon frames with SSID, capability information, supported data rates, … STAs perform passive scanning or active scanning with probe requests Authentication and Association or Shared Key Authentication with challenge-response-procedure open or WEP protected or better, WPA / WPA2 protected NTM, WLAN, 7

8. School of Engineering MAC Coordination Functions Distributed Coordination Function (DCF) mandatory, for asynchronous data service packet exchange on best effort (no delay bounds can be given) based on a version of CSMA/CA SIFSshort interframe space (highest priority for ACK, CTS, …) PIFSPCF interframe space (medium priority) DIFSDCF interframe space (lowest priority for asynchronous data) Point Coordination Function (PCF) optional, for time-bounded services, contention free polling method NTM, WLAN, 8

9. School of Engineering MAC - CSMA Carrier Sense Multiple Access = 20 us NTM, WLAN, 9

10. School of Engineering MAC - CSMA Carrier Sense Multiple Access on the air interface (physical carrier sensing, via RSSI) RSSI: Received Signal Strength Indicator on the MAC-layer (virtual carrier sensing, via NAV-timer) NAV: Network Allocation Vector (virtual reservation scheme) Collision Avoidance (CA) with random backoff-procedure 802.11b and g first Tx attempt: random slot <= CWmin = 31 slots further Tx attempts: random slot <= 63, 127, … CWmax CWmax = 1023 slots (20 ms), then the frame is discarded 802.11n CWmin = 15 slots (0.3 ms) NTM, WLAN, 10

11. School of Engineering MAC - CSMA ACK Data Packet data transmission without RTS/CTS Source Destination DIFS SIFS DIFS Other NAV defer accessbackoff after defer DIFSDCF Interframe Space SIFSShort Interframe Space NAVNetwork Allocation Vector CWContention Window CW NTM, WLAN, 11

12. School of Engineering MAC - CSMA CTS RTS Packet data transmission with RTS/CTS (optional) CA for long packets avoiding the hidden terminal problem Source Destination DIFS SIFS DIFS Other NAV (CTS) defer accessbackoff started RTSRequest To Send CTSClear To Send CW SIFS Data ACK SIFS NAV (RTS) NAV (Data) NTM, WLAN, 12

13. School of Engineering MAC – HCF (IEEE 802.11e) [2] Hybrid Coordination Function (HCF) DCF (QoS) extension in IEEE 802.11e WiFi Multi-Media (WMM specification) different CWmin and CWmax for 4 different QoS-classes NTM, WLAN, 13

14. School of Engineering MAC frames [1] Interpretation of 48 bit MAC addresses SA: Source Address, DA: Destination Address user data, management or control frame IP frames do not usually exceed 1500 bytes NTM, WLAN, 14

15. School of Engineering PHY – Operating Channels for 802.11 for 802.11b and g at least 5 channels spacing between AP only ETSI/Japan non-overlapping channel selection [1], Figure 7.23 NTM, WLAN, 15 P max = 100 mW (20 dBm) ETSI 5-GHz range in Europe 5.170-5.350 GHz 5.470-5.725 GHz Total 435 MHz, 18 indep. networks (only 3 in 2.4 GHz ISM-band)

16. School of Engineering PHY – 802.11b * symbol spread with 11 Chip Barker Code (DSSS) * * ** header always with 1 Mbps ** NTM, WLAN, 16 *** 11 Mbit/s can only be achieved over short distances of a few meters ***

17. School of Engineering PHY – 802.11b Energy Spread Sequence by spreading with 11 Mchip/s (robustness) 802.11b channel bandwidth 22 MHz NTM, WLAN, 17

18. School of Engineering PHY – 802.11g Extended-Rate PHY (ERP) OFDM with 52 subchannels (4 pilot channels and 48 data channels) symbol rate = 250 kSps (symbol period = 4us) total bandwidth 16-20 MHz (same channel use as 802.11b) Data Rates = 48 channel · 6 bit / channel · 3/4 (conv. code rate) / 4 us (symbol time) NTM, WLAN, 18

19. School of Engineering PHY – 802.11g Backward compatible with 802.11b protection measures CTS before packet transmission to set NAV-timer of 802.11b terminal PHY header with 1 Mbps 40% performance loss G-only option to avoid 802.11b/g interworking overhead Speed Comparison 802.11g (optimal condition): 20 Mbps (terminal-terminal 10 Mbps) 802.11b (optimal condition): 5 Mb/s (terminal-terminal 2.5 Mbps) 802.11a PHY almost identical with 802.11g, but allocated in 5 GHz band but no backward compatibility required (higher throughput) NTM, WLAN, 19

20. School of Engineering PHY – 802.11n High Throughput (HT) PHY 40 MHz channels frame aggregation (1 ACK for many packets) 400 ns instead of 800 ns OFDM guard interval 5/6 FEC (convolutional coding) MIMO Data Rate Comparison NTM, WLAN, 20