Wireless Ethernet Wireless LANs: Design Goals
Transcrição
Wireless Ethernet Wireless LANs: Design Goals
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Wireless Ethernet Wireless LANs: Design Goals • Wireless equivalent to Ethernet: “Wireless LAN” (WLAN) • Exclusively data-oriented, wide-band Internet access solution • Standardized by the IEEE as IEEE 802.11 IEEE 802.11 (data rate of 2 MBit/s), standardised in 1997 IEEE 802.11a with 54 MBit/s, use of a 5 GHz frequency band IEEE 802.11b with 11 MBit/s in a 2.4 GHz frequency range IEEE 802.11g: enhancement of 802.11b with up to 54 MBit/s IEEE 802.11n: data rates up to several hundreds of MBit/s (not finished) … • • • • • • • • Global, seamless operation Low power for battery use No special permissions or licenses needed to use the LAN Robust transmission technology Simplified spontaneous cooperation at meetings Easy to use for everyone, simple management Protection of investment in wired networks Security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) • Transparency concerning applications and higher layer protocols, but also location awareness if necessary 802.11a 802.11 • 1 or 2 MBit/s • 2.4 GHz • FHSS, DSSS • 54 MBit/s • 5 GHz • OFDM 802.11b 802.11g • 11 MBit/s • 2.4 GHz • DSSS • 54 MBit/s • 2.4 GHz • OFDM, DSSS Page 1 Chapter 3.2: WLAN Page 2 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Structure of a WLAN Infrastructure Network 1. Infrastructure network • Access Points (APs) are attached to an existing AP fixed network (Ethernet, Satellites, …) Fixed network • Each AP manages all communication AP AP in its reception range • APs using the same frequency range must have enough distance to avoid disturbances • Control functionality (medium access, mobility management, authentication, …) are realized within the infrastructure, wireless devices only need a minimum of functionality 802.11 LAN 802.x LAN L a p to p L a p to p L a p to p L a p to p 2. Ad-hoc Network • If no AP is available, stations also can build up an own LAN • The transmission now takes place directly between the stations • Higher complexity needed within the stations (control functionality) STA1 BSS1 L a p to p Distribution System Chapter 3.2: WLAN BSS2 • Portal Gateway to another fixed network Laptop STA2 802.11 LAN Page 3 Chapter 3.2: WLAN • Access Point (AP) Station which is integrated both in the radio and the wired network (distribution system) • Basic Service Set (BSS) Group of stations incl. the AP within an AP transmission range Access Point ESS Laptop Laptop Portal Access Point • Station (STA) Computer with access mechanism to the wireless medium and by this radio connection to the AP STA3 • Distribution system Connection of different AP areas to one logical network (EES: Extended service set). Simplest principle: switch Page 4 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Ad-hoc Network 802.11 Protocols 802.11 LAN Direct communication within limited range STA1 STA3 IBSS1 STA2 IBSS2 STA5 STA4 • Station (STA) Computer with access mechanism to the wireless medium • Independent Basic Service Set (IBSS) Group of stations which use the same carrier frequency within a transmission range Different IBSS are possible by spatial separation or by using different carrier frequencies No designated stations for the forwarding of data, routing,… … 802.11 LAN Chapter 3.2: WLAN Page 5 Applications should not be aware of the existence of the wireless network (except capacity, longer access times) Medium Access Control • Access mechanism, fragmenting, encryption • MAC management: synchronization, roaming between APs, power management Physical layer • Channel selection, modulation, coding Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme IEEE 802.11 Variants IEEE 802.11 Variants 802.11a 54 MBit/s WLAN in the 5 GHz band 802.11b 11 MBit/s WLAN in the 2,4 GHz band 802.11c Wireless Bridging between Access Points 802.11d "World Mode", Adaptation to regional regulations (e.g. used frequency ranges) 802.11e QoS und streaming enhancement for 802.11a/g/h 802.11f Inter Access Point Protocol (IAPP), allows communication between Access Points of different vendors, e.g. for exchanging roaming information 802.11g 54 MBit/s WLAN in the 2,4 GHz band 802.11h 54 MBit/s WLAN in the 5 GHz band with dynamic adaptation of channel and frequency choice as well as automatic adaptation of transmission power (enhancement of IEEE 802.11a for Europe) 802.11m Summary of earlier enhancements, correction of errors in former specifications (maintenance) 802.11n Enhancement for a future, faster WLAN with data rate of 100 - 600 MBit/s 802.11p WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) 802.11q Support of Virtual WLANs 802.11r Fast roaming between APs to avoid gaps in Voice over WLAN audio 802.11s ESS Mesh Networking 802.11t Wireless Performance Prediction (WPP) - test methods and metrics 802.11u Interworking with non-802 networks (for example, cellular) 802.11i Authentication/encryption for 802.11a/b/g, e.g. WPA 802.11v Wireless network management 802.11j Japanese variant of 802.11a for the frequency range of 4,9 GHz - 5 GHz 802.11w Protection of Management Frames 802.11k Improved measurement/evaluation/management of radio parameters (e.g. signal strength), e.g. for enabling location based services 802.11y 3650-3700 MHz Operation in the U.S. Chapter 3.2: WLAN Page 6 Page 7 Chapter 3.2: WLAN Page 8 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 802.11 – Physical Layer IEEE 802.11b Variants for transmission: 2 using radio (in the 2.4 GHz band), 1 using infrared • FHSS (Frequency Hopping Spread Spectrum) – 79 different channels with 1 MHz bandwidth each – Hopping between 2 channels for 1 MBit/s, between 4 channels for 2 MBit/s – Min. 2.5 hops/sec – GFSK modulation – Max. transmission power: 1 W (USA)/100 mW (EU), min. 1 mW • DSSS (Direct Sequence Spread Spectrum) – DBPSK modulation for 1 MBit/s (Differential Binary Phase Shift Keying), DQPSK for 2 MBit/s (Differential Quadrature PSK) – Chipping sequence: (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1), a Barker-Code – Max. transmission power: 1 W (USA)/100 mW (EU), min. 1 mW • Infrared – 850-950nm, diffuse light, typically 10 m range Page 9 Chapter 3.2: WLAN • Data rate – 1, 2, 5.5, 11 MBit/s, depending on SNR – User throughput max. approx. 6 MBit/s • Transmission range – 100m outdoor, 30m indoor (directed links: several km) – Max. data rate ~ 10m (indoor) • Frequency range – Unlicensed 2.4 GHz ISM band • Security – SSID, WPA2 • Connection setup time – Connectionless, „always on“ • QoS – Best effort, no guarantees (some defined in “bad” way, later on much better standardized in 802.11e) • Manageability – Limited (no automatic key distribution, symmetrical encryption) • Special advantages/disadvantages – Advantages: free ISM band, many vendors, simple system – Disadvantage: heavy interferences on the ISM band, no QoS, relatively low data rates • Usage – Preferred version in Europe Page 10 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Channels in IEEE 802.11b Channels in IEEE 802.11b • Two APs using the same frequency would have interferences in the overlapping area – thus: divide the whole frequency range in channels • Each channel in IEEE 802.11b has a bandwidth of 22 MHz • 13 channels in Germany (2412, 2417, 2422, …, 2472 MHz), 11 in USA/Canada • Channels overlap! Non-overlapping choice of channels: Available in the ISM band (most of Europe): 2400 – 2483,5 MHz Channel 1 Channel 6 Channel 1 2401 2412 2423 USA/Canada: channel 1 - 11 Carrier frequency Channel 1 2401 2412 2423 Channel 6 2426 2437 2448 Channel 2 2406 2417 2428 Channel 11 Channel 11 2451 2462 2473 Channel 7 2431 2442 2453 Channel 3 2411 2422 2433 Channel 12 2456 2467 2478 Channel 8 2436 2447 2458 Channel 13 2461 2472 2483 [MHz] 2400 2412 2437 22 MHz 2462 Channel 4 2416 2427 2438 2483.5 11 Channel 5 2421 2432 2443 6 • Ideal case: only use channels 1, 6 und 11: 1 Channel 9 2441 2452 2463 Channel 10 2446 2457 2468 Channel 14 2473 2484 2495 11 6 Chapter 3.2: WLAN Japan ( 1 – 14) 1 Page 11 2400 2410 Chapter 3.2: WLAN 2420 2430 2440 2450 2460 2470 2480 MHz Page 12 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Dynamic Rate Shifting Channels Adjustment of the data rate to the transmission quality: The whole 2.4GHz ISM band is divided into 11 resp. 13 overlapping channels. On each channel, DSSS is used for signal spreading: Data Rate Code length 1 Mbit/s Modulation Used Symbol Rate Bits/Symbol DSSS/PSK 11 (barker code) 2 Mbit/s 1 1 MS/s DSSS/QPSK 5,5 Mbit/s 8 (CCK) 11 Mbit/s Modified DSSS/QPSK 2 1,375 MS/s 4 8 CCK: Complementary Code Keying • Use of an 8-chip spreading sequence where each chip is modulated with QPSK • QPSK has 4 states, chipping sequence has length 8 → 48 resulting states • Select 64 (for 11 Mbit/s) resp. 4 (for 5,5 Mbit/s) of the states which have as good cross correlation characteristics as possible (i.e. are as different as possible) • That means: make use of 4 resp. 16 code words which can be transferred instead of only 1 as with the barker code (i.e. skip some robustness) Page 13 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme → One sub-band has a bandwidth of 22 MHz. The sent data are spread to those bandwidth to avoid environmental disturbances → The chips of the barker code resp. CCK are sent in sequence – this increases the number of symbols per second compared with “pure” sending of the data, thus a larger bandwidth is needed → Purpose: even if the frequency range Channel n is disturbed partly, enough of the signal power reaches the receiver on the rest of the channel; if a non-spread transmission would take place, the whole data would be lost in case of 22 MHz narrowband interference → If CCK is used, we use “several codes” instead of the same chipping sequence everytime - the transmission becomes more susceptible for disturbances than with use of the barker code, if we have a distortion (maybe caused by an overlapping channel)! Chapter 3.2: WLAN Page 14 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Range of IEEE 802.11b Range of 802.11b Due to “abused” spreading in case of CCK, the higher data transmission rates are more susceptible for disturbances. Thus, a smaller range results: Data rate 10 Mbit/s 8 6 802.11b 4 2 802.11 0 10 Chapter 3.2: WLAN 30 60 100 m Distance Page 15 Chapter 3.2: WLAN Page 16 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme IEEE 802.11a Channels in IEEE 802.11a • Data rates – 6, 9, 12, 18, 24, 36, 48, 54 MBit/s, depending on SNR – User Throughput: max. 32 MBit/s – 6, 12, 24 MBit/s mandatory • Transmission range – 100m outdoor, 10m indoor (e.g. 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30 m, 18 up to 40 m, 12 up to 60 m) • Frequency range – Free 5.15-5.35 + 5.725-5.825 GHz ISM band • Security – SSID, WPA2 • Connection setup time – Connectionless, „always on“ • QoS – Best effort, no guarantees (same as for 802.11b) • Manageability – Limited (same as for 802.11b) • Special advantages/disadvantages – Advantages: uses less crowded free ISM band, available worldwide, simple system, many vendors – Disadvantages: strong shading due to high frequencies, no QoS • Usage – Preferred version in USA Page 17 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 312,5 kHz phase reference (pilot) And: IEEE 802.11g simply is introducing OFDM on the existing 802.11b system, i.e. replacing of DSSS by OFDM for higher data rates (while keeping the ability to switch to DSSS for interworking with 802.11b) -7 -1 1 7 channel center frequency 5150 40 44 48 52 56 60 channel-no. 64 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz] 16,6 MHz 149 153 157 161 channel-no. center frequency = 5000 + 5·channel-no. [MHz] 5725 5745 5765 5785 5805 5825 [MHz] 16,6 MHz Chapter 3.2: WLAN Page 18 Medium Access Control • OFDM with 52 subcarriers (64 in total, 6 as guard space on each side) • Subcarriers overlap with 312,5 kHz spacing, but orthogonality of chosen frequencies allows for clear separation • 48 data subchannels + 4 subchannels for phase reference (pilot) • Pilots are used by the receiver to deal with multipath propagation: phase references for the whole band are sent here, the receiver can interpolate phase shifts for the data carriers -26 -21 36 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Modulation in 802.11a: OFDM Chapter 3.2: WLAN Channels are also overlapping, as in 802.11b: 21 26 subcarrier number Page 19 We can assign one channel with an AP – but then we have to coordinate all mobile stations in their communication with the AP. Chosen for IEEE 802.11a/b/g/…: „Wireless Ethernet“ – MAC protocol is oriented at CSMA/CD • Hidden Station Problem • Exposed Station Problem Solution of the problems, especially Hidden Station CSMA/CA – CSMA with Collision Avoidance Types of traffic • Asynchronous data service (standard) Exchange of data by „best effort“ Support of broadcast and multicast • Time-bound services (optional) Implementation of some degree of QoS Only for infrastructure networks Chapter 3.2: WLAN Page 20 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 802.11 – MAC Layer: DFWMAC 802.11 – MAC Layer Priorities for medium access • defined through different timing intervals • no guaranteed priorities Access strategies • DFWMAC-DCF CSMA/CA (standard) DFWMAC: Distributed Foundation Wireless MAC DCF: Distributed Coordination Function collision avoidance by random access with backoff mechanism Minimum time between two frames ACKs for acknowledging correct receipt (not for broadcast) • SIFS (Short Inter Frame Spacing) – 10µs – highest priority, used for ACK, CTS, polling response • PIFS (PCF IFS) – 30µs – medium priority, for time-bounded services using PCF • DFWMAC-DCF with RTS/CTS (optional) Avoidance of Hidden Stations MACA variant (Multiple Access with Collision Avoidance) • DIFS (DCF IFS) – 50µs – lowest priority, für asynchronous data service DIFS • DFWMAC-PCF (optional) PCF: Point Coordination Function Collision-free, centralized Polling strategy where the AP has a list of all connected stations Medium busy Page 21 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Medium busy next frame t direct access, if time the medium is free ≥ DIFS Page 22 Example - Backoff DIFS PIFS SIFS contention window (randomized backoff mechanism) B1 = 25 B1 = 5 wait next frame data t waiting time time slot (20 µs) • • • • Mandatory for all implementations Before sending, a station performs carrier sense If the medium is free for at least the duration of a DIFS, the station may send If the medium is occupied, when becoming free the station waits for one DIFS and then randomly chooses a backoff time (collision avoidance, in multiples of a slot time). The station continues to listen to the medium • If the medium is occupied by another station during the backoff time, the backoff timer stops. In the next try, no new backoff time is chosen randomly, but the old timer is gone on with. • Also usable for broadcast Chapter 3.2: WLAN contention Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 802.11 - CSMA/CA Method DIFS Chapter 3.2: WLAN DIFS PIFS SIFS Page 23 data B2 = 20 wait B2 = 15 B2 = 10 B1 and B2 are backoff intervals at nodes 1 and 2 Chapter 3.2: WLAN Page 24 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Competing Stations DIFS 802.11 - CSMA/CA Method DIFS DIFS boe bobo e r Station1 boe Station2 DIFS bobo e bo e r boe boe busy boe bor boe boe Unicast transmission: the receipt is additionally confirmed, since collisions possibly are not detected by the transmitter • Data can be sent after waiting for DIFS • Receivers answer immediately (after SIFS, without additional backoff time), if the frame arrived correctly (CRC) • In case of an error the frame is repeated automatically. No special treatment of a transmission repetition, same access mechanism as before busy busy busy Station3 Station4 bobo e bo e r Station5 busy DIFS bor Data sender t SIFS busy Medium busy (Frame, ACK, etc.) boe elapsed backoff time Sending request bor remaining backoff time receiver DIFS The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 7 and is doubled with each collision up to CW max = 255 Page 25 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme SIFS DIFS Station1 DIFS bo11 bo11 bo41 busy bo42 bo51 busy bo52 busy bo21 busy Station2 busy Station3 ACK Station4 bo51 Station5 Medium occupied (Frame, ACK, etc.) Sending request Page 26 Optional extension for the avoidance of the hidden station problem: • RTS with holding time as parameter can be sent after waiting for DIFS (plus backoff time) • Confirmation of the receiver by CTS after SIFS (also containing holding time) • Immediate sending of the data is possible, confirmation by ACK • Other stations store the holding time, which were sent in the RTS and CTS, in their NAV (Network Allocation Vector) • Collisions are only possible with RTS/CTS messages, but substantial overhead through RTS/CTS messages DIFS RTS data SIFS boij jth backoff time of station i CTS SIFS SIFS ACK receiver contention ACK Acknowledgement The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 8 and is doubled with each collision up to CW max = 256 Chapter 3.2: WLAN t contention Chapter 3.2: WLAN sender t busy waiting time 802.11 – DFWMAC with RTS/CTS DIFS bo11 Data other stations Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Competing Stations (with ACK) DIFS ACK Page 27 other stations Chapter 3.2: WLAN NAV (RTS) NAV (CTS) waiting time DIFS data t Page 28 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 802.11 – DFWMAC with RTS/CTS DFWMAC-PCF • Fragmenting data can decrease the damage caused by transfer errors • Special mechanism: adapt size of the fragments to current error rate of the medium • First: normal reservation with RTS/CTS • Fragments and ACKs (except the last for each case) contain reservation durations DIFS RTS frag1 sender SIFS CTS SIFS SIFS ACK1 SIFS PCF for guarantees concerning bandwidth and access delay AP controls medium access and cyclic queries all stations (Polling) Super-frames with competition-free period and competition period (like before) If the medium gets free (t1) after the begin of the super-frame (t0), the coordinator cyclic asks all stations x (Dx) for sending needs. If necessary, they answer with Ux (the data to be sent) • If the phase is ended earlier than planned (t2 instead of t3), more time remains for the competition phase (end is announced by a control frame CFend) t0 t1 frag2 receiver • • • • SIFS ACK2 PIFS NAV (RTS) NAV (CTS) other stations SIFS D1 DIFS SIFS D2 SIFS NAV (frag1) NAV (ACK1) Chapter 3.2: WLAN PIFS U1 SIFS D4 t4 CFend SIFS U2 U4 data Page 29 NAV contention-free period Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme contention Page 30 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme What is implemented? Frame Format Any vendor has to implement the standard CSMA/CA variant, the other two are optional • RTS/CTS very often is implemented by AP manufacturers, but: disabled! • Usual method: A frame size threshold is defined, and only frames longer than the threshold are sent with RTS/CTS (to avoid overhead for small frames) The threshold value in basic configuration is sent to maximum allowed frame length… Changing the threshold value allows you to enable the RTS/CTS Only possibility to really avoid collisions • PCF mechanism usually is not implemented Not needed in many cases, and not possible in ad-hoc networks Would allow for real-time data transmission, but is not good in it, thus it doesn’t became prominent – instead, a QoS enhancement for real-time transmission was defined (IEEE 802.11e) Chapter 3.2: WLAN D3 SIFS t contention t2 t3 super-frame Page 31 • Types Control frames, administrative frames, data frames • Sequence numbers For detecting duplicated frames due to lost ACKs • Addresses Receiver, transmitter (physical), sender (logical), BSS identifier • Misc Duration of transmission, data bytes 2 2 6 6 6 2 6 Frame Duration/ Address Address Address Sequence Address Control ID 1 2 3 Control 4 bits 2 2 4 1 1 1 1 1 1 1 0-2312 4 Data CRC 1 Protocol To From More Power More Type Subtype Retry WEP Order version DS DS Frag Mgmt Data Chapter 3.2: WLAN Page 32 t Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Frame Format MAC Address Format Frame Control • Protocol version, frame type (administration, control, data), fragmenting, encryption information, meaning of the following address fields Duration ID • Sent along with RTC, CTS for setting the NAV Sequence Control • Recognition of duplicated frames by sequence numbers CRC • Checksum for detecting transmission errors Addresses • Each field contains a 48-Bit MAC address. MAC frames can be transferred between two stations, between station and AP or between two APs within the distribution system. In the field Frame Control, two bits are determining the current meaning of the addresses. Addresses can be: Final destination, source address, BSS Identifier, intermediate sender address, intermediate receiver address Page 33 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 2 2 6 Frame Receiver Duration Control Address 4 CRC bytes 2 2 6 6 Frame Receiver Transmitter Duration Control Address Address bytes Clear to Send, CTS Chapter 3.2: WLAN address 3 BSSID SA address 4 - SA DA - TA DA SA Distribution System Access Point Destination Address Source Address Basic Service Set Identifier Receiver Address Transmitter Address Page 34 Chapter 3.2: WLAN FHSS Frame Format (PHY) bytes Request to Send, RTS DS: AP: DA: SA: BSSID: RA: TA: address 2 SA BSSID Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Special Frames Acknowledgement, ACK Scenario to DS from DS address 1 ad-hoc network 0 0 DA infrastructure 0 1 DA network, from AP infrastructure 1 0 BSSID network, to AP infrastructure 1 1 RA network, within DS 2 2 6 Frame Receiver Duration Control Address 4 CRC • Synchronization – Synchronization of receivers by the pattern 010101... • SFD (Start Frame Delimiter) – 0000110010111101 to announce start of frame • PLW (PLCP_PDU Length Word) – Length of payload including the 32 Bit CRC (at the end of the payload). Allowed values are between 0 and 4095 • PSF (PLCP Signaling Field) – Data rate of payload (1 or 2 Mbit/s) • HEC (Header Error Check) – CRC with x16+x12+x5+1 4 80 Synchronization CRC Page 35 Chapter 3.2: WLAN Preamble transmission with 1 Mbit/s 16 12 4 16 variable SFD PLW PSF HEC Payload Header transmission with 1 or 2 Mbit/s Bits Page 36 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme DSSS Frame Format (PHY) IEEE 802.11b – Frame Format (PHY) • Synchronization – Synchronization, gain setting, energy detection, frequency offset compensation • SFD (Start Frame Delimiter) – 1111001110100000 as start pattern • Signal – Data rate of payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) • Service – Reserved for future use, standard: 00 for 802.11 frames • Length (length of payload) and HEC (CRC) as for FHSS Long frame format: 128 16 Synchronization 8 8 16 16 SFD Signal Service Length HEC Preamble transmission with 1 Mbit/s variable Bits 6 16 tail service 12 Signal 1 6 Mbit/s Chapter 3.2: WLAN payload 1, 2, 5.5 or 11 Mbit/s short synch. 16 SFD 8 8 16 16 variable signal service length HEC Bits Payload Header (2 Mbit/s, DQPSK) 2, 5.5 or 11 Mbit/s Chapter 3.2: WLAN Page 38 802.11 - MAC Management variable 6 variable payload tail pad Bits • Synchronization Find a LAN, try to remain in the LAN Synchronization of internal clocks (e.g. FHSS, PCF, power saving mechanisms) Timer etc. • Power management Sleep mode without missing a message Periodic sleeping, frame buffering, traffic monitoring PLCP-Header Preamble, SFD Bits variable Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme IEEE 802.11a – Frame Format (PHY) 1 16 Short frame format, optional: 96 µs Page 37 12 16 192 µs at 1 Mbit/s DBPSK transmission with 1 or 2 Mbit/s Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 1 8 Header Preamble (1 Mbit/s, DBPSK) Chapter 3.2: WLAN 4 8 signal service length HEC Payload Header rate reserved length parity SFD Preamble 56 128 16 synchronization Data variable Symbols • Association/Re-association Integration into a LAN Roaming, i.e. moving between networks from one Access Point to another Scanning, i.e. active search for a network • MIB - Management Information Base Managing, read and write of management attributed and state variables inside APs, the distribution system, etc 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s Page 39 Chapter 3.2: WLAN Page 40 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Synchronization using a Beacon Synchronization using a Beacon (Ad-hoc) • Beacon frame contains time stamps and administrative information for power saving mechanisms and roaming • All stations try to send a Beacon frame in fixed intervals • Varying times between beacon frames, since the medium can be occupied • One station wins and sends a beacon frame at first. All other stations synchronize to this frame. • In infrastructure networks: AP takes over the sending of the beacons beacon interval Interval of the periodic radio signal (beacon): 20ms - 1s B B busy B1 Station1 B busy busy B2 busy Medium busy value of the timestamp Chapter 3.2: WLAN busy busy value of the timestamp beacon frame Page 41 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme busy B random backoff beacon frame Page 42 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Power Management with Wake-up Patterns (Infrastructure) Power Management • Idea: Switch off the sending/receiving device when not needed • Timing Synchronization Function Regular activation of all stations. Transmissions for sleeping stations are buffered; when waking up, the stations receive the transmission • Infrastructure: AP can store all pending frameworks for sleeping stations With each beacon frame, a Traffic Indication Map (TIM) is sent along which indicates, for which stations frames are buffered. Additionally: List for broadcast/multicast receivers (Delivery Traffic Indication Map, DTIM) • Ad-hoc Similar to the infrastructure mod, an aA-hoc Traffic Indication Map (ATIM) is defined Stations, which have data to send, announce the receivers of stored packages More complex, no central AP: all stations have to temporarily store frames Collisions of ATIMs possible (scalability?) Chapter 3.2: WLAN B2 t t B B1 Station2 B AP Medium • Standard access procedure with backoff Page 43 TIM interval AP DTIM interval D B T busy Medium busy T d D B busy busy p Station d t Chapter 3.2: WLAN T TIM D B Broadcast/Multicast DTIM awake p PS Poll d Data transmission from/to the station Page 44 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Power Management with Wake-up Patterns (Ad-hoc) ATIM window Station1 Bad or even no connection? B1 beacon frame awake • Scanning – Scanning of environment (listen for beacons of APs or send a probe and wait for a response) beacon interval A B2 Station2 B 802.11 - Roaming random backoff a ACK for ATIM B2 D a B1 • Reassociation Request – Station requests joining the network to AP(s) d t A ATIM transmission D data transmission d ACK for data Chapter 3.2: WLAN Page 45 • Reassociation Response – If an AP responds, the station takes part in the network – Otherwise, go on scanning • AP accepts Reassociation Request – Announce new station to the Distribution System – Distribution System updates its databases (location information) – The old AP is informed by the Distribution System Page 46 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Quality of Service – IEEE 802.11e Extended Distributed Channel Access The PCF variant of CSMA/CA should allow some quality in data transmission: • By polling at certain times, allow for deterministic delay of information • Also, guarantee a certain data rate to each participant • But…frames in polling can be between 0 and 2304 bytes… and the data rate on physical layer can change due to channel conditions… → no way to calculate transmission time of a frame in advance, thus the above quality cannot be given The scheme from before (all stations use the DIFS time interval) is refined: • Assign different priorities to different data streams (traffic classes, TC) • As before, priority is given by waiting times: the Arbitration Inter-Frame space (AIFS) Solution: define additional CSMA/CA variants which can give priority to real-time data (defined in IEEE 802.11e) • Only an add-on the IEEE 802.11a/b/g, not a stand-alone WLAN standard • Definition of Extended Distributed Channel Access (EDCA) as better version of DCF using several classes of access priority by refining the inter-frame gaps and introducing so-called Transmission Opportunities (TXOP) Hybrid Coordination Function Controlled Channel Access (HCCA) as better version of PCF also using TXOP Chapter 3.2: WLAN Page 47 AIFS[TC0] AIFS[TC6] DIFS = AIFS[TC7] PIFS busy SIFS RTS contention window t TC Access Category (AC) Purpose 0 1 2 3 4 5 6 7 0 1 1 2 2 2 3 3 Best Effort Background Background Video Probe Video Video Voice Voice • Classify all data streams in traffic classes regarding their QoS • 8 priority classes, TC 7 has highest priority • Give longer waiting times to lower priority – thus higher priority streams can start sending earlier •Chapter Fairness is given – even high priority senders can draw a large backoff number Page 48 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme EDCF Implementation HCCA With EDCF, each station has to handle up to 8 queues performing the same access procedure as “plain” DCF with backoff counter (BC) and contention window (CW): As in PCF, HCCA is a combination of a contention-free period and a contention period • In the contention-free period the AP polls the stations Difference to PCF: stations can place reservations for the polling phase The AP polls stations by granting a TXOP oriented at reservation wishes and current traffic load • In the contention period, EDCF is used Question: why giving QoS? Why not overprovisioning, i.e. only increase the data rate? One more enhancement: each class also a TXOP is assigned, which is a maximum sending duration – after getting medium access, for time of TXOP several frames can be sent (Contention Free Burst) Chapter 3.2: WLAN Page 49 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Faster! MIMO Not an end with 802.11a/g – go on with 802.11n • up to 600 MBit/s! • over 70 – 250m! How to achieve such a data rate while keeping compatibility to 802.11a/b/g? • Applied to 2.4 as well as 5 GHz ISM band to only have a single variant for the future • Modify OFDM with increasing symbol rate and slightly enlarge the bandwidth: → increase data rate from 54 MBit/s to 65 MBit/s • Optional: Greenfield mode, i.e. skip support for 802.11a/b/g (an increasing number of legacy devices reduces the average throughput in the whole network) • Optional: increase a channel’s bandwidth to 40 MHz (dynamic adaptation to other WLANs in the environment necessary!) • Use MIMO – multiple input multiple output Chapter 3.2: WLAN Page 50 Page 51 MIMO means: use several antennas in parallel to send data to one receiver • Apply Space Division Multiplexing (SDM) – i.e. split the data stream into multiple parts (called spatial stream) and transmit each part with a separate antenna (for up to 4 antennas) • Necessary: power control – only use MIMO if necessary, otherwise lots of power is consumed • Apply beam-forming to focus the sender’s antennas to the receiver’s antennas • By antenna diversity, a receiver can find out the angle of incidence of certain spatial streams and thus distinguish between several streams • Optional: apply diversity on improving signal strength, i.e. improve signal by receiving the same stream with several antennas and combine the outputs (for up to 4 antennas, but only if the number of receiver antennas is larger than the number of spatial streams) Chapter 3.2: WLAN Page 52 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 802.11n – MAC Layer 802.11s – WLAN Mesh Networking Other WLAN variant: mesh networks • Classical WLAN: wired infrastructure between APs • Sometimes called “Wireless Paradox” Many improvements on PHY layer, only a few on the MAC layer: • Introduce Reduced Inter-Frame Space (RIFS) to shorten the waiting time after detecting the medium to be idle • Use frame aggregation, i.e. pack together several frames of one station and remove redundant header information Figures from: IEEE 802.11s tutorial Availability of 802.11n? • Draft version 2 finished this year • Lot of products of several vendors (compliance to a non-finished standard?) • Potential problems with a patent? • Planned release date – varies between September 2008 and March 2009… Let APs interconnect in wireless manner, also using WLAN (lower costs, simple installation, resilient, …) Page 53 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Page 54 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Mesh Topology Secure or not Secure… Mesh Point Special component, establishes peer links with neighbors Mesh Portal As mesh point, but additionally connects to some other network Figures from: IEEE 802.11s tutorial Mesh AP As mesh point, but additionally implements AP functionallity Changes in the 802.11 standard regarding: • Addresses • Security • MAC scheme (oriented at 802.11e) • And: routing (layer 3!) • Synchronization / power modes Chapter 3.2: WLAN Chapter 3.2: WLAN Page 55 Within a WLAN „data are flying free through the air“. Within WLAN everybody in transmission range can share your Access Point. Thus: security! Registration of allowed MAC addresses • But: MAC addresses can be faked, large effort for large networks Hiding of SSID • Broadcast of SSID in beacons can be switched of, thus only someone knowing the SSID can join the network (but: intuitive names? Default names?) WEP: Wired Equivalent Privacy • Authentication at the Access Point, encryption of data before transmission • Connection is only possible if knowing the WEP key • But: no key management, short keys • Thus: WPA/WPA2 (Wi-Fi Protected Access) today give much better security ... but many users are overtaxed with configuring an Access Point – even if today a good user guide to install security functions is implemented on APs, there is a lot of open networks... Page 56 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Wardriving Warchalking New kind of sports: search for open WLANs. Just take: • A notebook with WLAN card and a connector for a GPS device • A software for detcting Access Points, e.g. Network Stumbler What can be found at walls after a wardiver has passed... • A GPS receiver • Time for driving around Page 57 Chapter 3.2: WLAN Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme (separated by installation) 100 byte 802.15 79 channels SIFS ACK DIFS 500 byte SIFS ACK 100 byte 802.11b 3 channles DIFS DIFS SIFS ACK SIFS ACK DIFS 100 byte DIFS DIFS 100 byte 500 byte SIFS ACK SIFS ACK 100 byte DIFS SIFS ACK 1000 byte 500 byte DIFS DIFS DIFS f [MHz] 2480 SIFS ACK 802.11 vs. 802.15/Bluetooth (separated by hopping pattern) 2402 t • Bluetooth may act like a rogue member of a 802.11 network – does not know anything about gaps, IFS etc. • IEEE 802.15-2 discusses these problems – Proposal: Adaptive Frequency Hopping (only co-existence, no collaboration) • Real effects? Many different opinions, tests, formulae, … – Results from complete breakdown to almost no effect – Bluetooth (FHSS) seems to be more robust than 802.11b (DSSS) – Maybe Bluetooth adaptive frequency hopping has better effect Chapter 3.2: WLAN Page 59 Chapter 3.2: WLAN Page 58