Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Power over Ethernet wikipedia , lookup
Policies promoting wireless broadband in the United States wikipedia , lookup
Wake-on-LAN wikipedia , lookup
Wireless security wikipedia , lookup
Cracking of wireless networks wikipedia , lookup
Piggybacking (Internet access) wikipedia , lookup
Introduction to Wireless LAN Background Media access principle Architecture MAC control MAC management PHY layer 802.11 Wireless LAN: Background (15 min) Wireless LANs Wireless networks today Features and benefits Mobility Flexibility Scalability Wireless LANs: ready now and ready for the future Standards Security Service Roaming Wireless LAN Basic Service Set (BSS): a single cell controlled by a Base Station (also called Access Point or AP) Distribution System: the interconnection network of base stations Extended Service Set (ESS): the whole interconnected Wireless LAN (seen as a single 802 network), including the different cells, their respective Access Points, and the Distribution System Radio Frequency In Wireless Networks Radio spectrum Narrowband interference Spread spectrum FHSS – Frequency Hopping Spread Spectrum DSSS – Direct Sequence Spread Spectrum Multi-path interference IEEE 802.11 series standard 802.11: 2M 802.11b: 1M, 2M, 5.4M, 11M 802.11a: 6M, 9M, 12M, 18M, 24M, 36M, 48M, 54M 802.11g: compatible with 802.11b and 802.11a Other standards: 802.11e: provides Quality of Service (QoS) 802.11h: supplementary to comply with European regulations 802.11i: improved WLAN security Multipath Effect Multipath radio effect. Transmitter signals are reflected or diffracted by structures, changing the signals’ timing, strength, and quality. IEEE 802.11b Standard 802.11b allows unconnected client devices to communicate with an Ethernet network through an RF (Radio Frequency) transmitter that is physically connected to the wired network. Deploying Wireless LAN Ad-hoc network Association and roaming Deploying access point and wireless LANs Deploying access point Load balancing Channel selection for neighboring wireless LANs Multiple channel rate in a wireless LAN US (FCC)/Canada (IC) channel 1 2400 2412 channel 6 channel 11 2437 2462 22 MHz 2483.5 [MHz] 802.11 Wireless LAN: Media Access Principle (20 min) Media Access Can we apply media access methods from fixed networks? Example CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks signal strength decreases proportional to the square of the distance the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot “hear” the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is “hidden” Motivation - hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C Exposed terminals A B C B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address receiver address packet size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC) MACA examples MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B RTS CTS A CTS B C MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS from A RTS RTS CTS A B C Polling mechanisms If one terminal can be heard by all others, this “central” terminal (a.k.a. base station) can poll all other terminals according to a certain scheme now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) Example: Randomly Addressed Polling base station signals readiness to all mobile terminals terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address) the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminal this cycle starts again after polling all terminals of the list 802.11 Wireless LAN: Architecture (15 min) Comparison: infrastructure vs. ad-hoc networks infrastructure network AP AP ad-hoc network wired network AP: Access Point AP 802.11 - Architecture of an infrastructure network Station (STA) 802.11 LAN STA1 802.x LAN Basic Service Set (BSS) BSS1 Portal Access Point Access Point ESS group of stations using the same radio frequency Access Point Distribution System station integrated into the wireless LAN and the distribution system Portal BSS2 bridge to other (wired) networks Distribution System STA2 terminal with access mechanisms to the wireless medium and radio contact to the access point 802.11 LAN STA3 interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 802.11 - Architecture of an ad-hoc network Direct communication within a limited range 802.11 LAN Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency STA1 STA3 IBSS1 STA2 IBSS2 STA5 STA4 802.11 LAN IEEE standard 802.11 fixed terminal mobile terminal infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY 802.11 - Layers and functions MAC PLCP Physical Layer Convergence Protocol MAC Management access mechanisms, fragmentation, encryption clear channel assessment signal (carrier sense) PMD Physical Medium Dependent synchronization, roaming, MIB, power management modulation, coding PHY Management channel selection, MIB Station Management LLC MAC MAC Management PLCP PHY Management PMD Station Management PHY DLC coordination of all management functions 802.11 Wireless LANs: MAC Control (25 min) 802.11 - MAC layer I - DFWMAC Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on “best-effort” support of broadcast and multicast Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods DFWMAC-DCF CSMA/CA (mandatory) collision avoidance via randomized „back-off“ mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) DFWMAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem DFWMAC- PCF (optional) access point polls terminals according to a list 802.11 - MAC layer II Priorities defined through different inter frame spaces no guaranteed, hard priorities SIFS (Short Inter Frame Spacing) PIFS (PCF IFS) highest priority, for ACK, CTS, polling response medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS direct access if medium is free DIFS contention next frame t 802.11 - CSMA/CA access method I DIFS DIFS medium busy direct access if medium is free DIFS contention window (randomized back-off mechanism) next frame t slot time station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) 802.11 - competing stations - simple version DIFS DIFS station1 station2 DIFS boe bor boe busy DIFS boe bor boe busy boe busy boe bor boe boe busy station3 station4 boe bor station5 busy bor t busy medium not idle (frame, ack etc.) boe elapsed backoff time packet arrival at MAC bor residual backoff time 802.11 - CSMA/CA access method II Sending unicast packets station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) automatic retransmission of data packets in case of transmission errors DIFS sender data SIFS receiver ACK DIFS other stations waiting time data t contention 802.11 - DFWMAC Sending unicast packets station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS DIFS sender RTS data SIFS receiver other stations CTS SIFS SIFS NAV (RTS) NAV (CTS) defer access ACK DIFS data t contention Fragmentation DIFS sender RTS frag1 SIFS receiver CTS SIFS frag2 SIFS ACK1 SIFS SIFS ACK2 NAV (RTS) NAV (CTS) other stations NAV (frag1) NAV (ACK1) DIFS contention data t DFWMAC-PCF I t0 t1 medium busy PIFS point coordinator wireless stations stations‘ NAV SuperFrame SIFS D1 SIFS SIFS D2 SIFS U1 U2 NAV DFWMAC-PCF II t2 point coordinator wireless stations stations‘ NAV D3 PIFS SIFS D4 t3 t4 CFend SIFS U4 NAV contention free period contention period t 802.11 - Frame format Types control frames, management frames, data frames Sequence numbers important against duplicated frames due to lost ACKs Addresses receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous sending time, checksum, frame control, 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 MAC address format scenario ad-hoc network infrastructure network, from AP infrastructure network, to AP infrastructure network, within DS to DS from DS 0 0 0 1 address 1 address 2 address 3 address 4 DA DA SA BSSID BSSID SA - 1 0 BSSID SA DA - 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address Special Frames: ACK, RTS, CTS Acknowledgement bytes ACK 2 2 6 Frame Receiver Duration Control Address 4 CRC Request To Send bytes RTS 2 2 6 6 Frame Receiver Transmitter Duration Control Address Address Clear To Send bytes CTS 2 2 6 Frame Receiver Duration Control Address 4 CRC 4 CRC 802.11 Wireless LANs: MAC Management 802.11 - MAC management Synchronization try to find a LAN, try to stay within a LAN timer etc. Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network MIB - Management Information Base managing, read, write Synchronization using a Beacon (infrastructure) beacon interval access point medium B B busy busy B busy B busy t value of the timestamp B beacon frame Synchronization using a Beacon (ad-hoc) beacon interval station1 B1 B1 B2 station2 medium busy busy B2 busy busy t value of the timestamp B beacon frame random delay Power management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) stations wake up at the same time Infrastructure Traffic Indication Map (TIM) list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP Ad-hoc Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?) Power saving with wake-up patterns (infrastructure) TIM interval access point DTIM interval D B T busy medium busy T d D B busy busy p station d t T TIM D B broadcast/multicast DTIM awake p PS poll d data transmission to/from the station Power saving with wake-up patterns (ad-hoc) ATIM window station1 beacon interval B1 station2 A B2 B2 D a B1 d t B beacon frame awake random delay a acknowledge ATIM A transmit ATIM D transmit data d acknowledge data Power Mode of Wireless NIC Transmit mode Used during data transmission (sending) Power consumption: high (e.g., 450mA) Receive mode Default mode for both data receiving and listening Power consumption: medium (e.g., 270mA) Sleep mode Power consumption: low (e.g., 15mA) Power Saving Mechanism Frame types Data Frames: used for data transmission Control Frames: used to control access to the medium Management Frames: such as Beacon Frame (for synchronization) The Access Point maintains a continually updated record of the stations currently working in Power Saving mode buffers the packets addressed to these stations periodically transmits information (as part of its Beacon Frames) about which Power Saving Stations have frames buffered at the AP The Power Saving Station wake up periodically (100ms) in order to receive the Beacon Frame if there are frames stored at the AP waiting for delivery, the station stays awake and sends a Polling message to the AP to get these frames otherwise goes back sleep Power Consumed during PS Mode Power consumed by Orinoco Gold NIC during Power Save Mode Power consumed by Cisco AIR-PCM350 NIC during Power Save Mode Ecycle (n,t) = 0.060nt + 3300, 0 =< n =< 65535 Ecycle (n,t) = 0.060nt + 3300, 0 =< n =< 65535 Problems Energy consumption Wireless networking card consumes a great amount of energy in mobile devices Over 50% total energy of handheld PC Up to 10% total energy of laptop PC Networking performance Highly depends on the signal strength and transmit power signal attenuation: distance, obstacles, and environment (humidity, temperature, etc) transmit power levels: 1mW, 5mW, 20mW, 30mW, 50mW, and 100mW for Cisco Aironet 350 NIC Latency Data transmission time: effective bandwidth, loss rate Mode transition time: power saving mode – normal mode AP handoff time 802.11 - Roaming No or bad connection? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Reassociation Request station sends a request to one or several AP(s) Reassociation Response success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request signal the new station to the distribution system the distribution system updates its data base (i.e., location information) typically, the distribution system now informs the old AP so it can release resources 802.11 Wireless LANs: PHY Layer 802.11 - Physical layer 3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiated power 1 W (USA), 100 mW (EU), min. 1mW Infrared 850-950 nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchonization FHSS PHY packet format Synchronization synch with 010101... pattern SFD (Start Frame Delimiter) 0000110010111101 start pattern PLW (PLCP_PDU Length Word) length of payload incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) data of payload (1 or 2 Mbit/s) HEC (Header Error Check) CRC with x16+x12+x5+1 80 synchronization 16 12 4 16 variable SFD PLW PSF HEC payload PLCP preamble PLCP header bits DSSS PHY packet format Synchronization synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) 1111001110100000 Signal data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: 802.11 compliant length of the payload HEC (Header Error Check) protection of signal, service and length, x16+x12+x5+1 128 synchronization 16 SFD PLCP preamble 8 8 16 16 signal service length HEC PLCP header variable payload bits WLAN: IEEE 802.11b Data rate 1, 2, 5.5, 11 Mbit/s, depending on SNR User data rate max. approx. 6 Mbit/s Transmission range 300m outdoor, 30m indoor Max. data rate ~10m indoor Frequency Free 2.4 GHz ISM-band Security Limited, WEP insecure, SSID Cost 100€ adapter, 250€ base station, dropping Availability Many products, many vendors Connection set-up time Connectionless/always on Quality of Service Typ. Best effort, no guarantees (unless polling is used, limited support in products) Manageability Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only IEEE 802.11b – PHY frame formats Long PLCP PPDU format 128 16 synchronization SFD 8 8 16 16 signal service length HEC PLCP preamble bits variable payload PLCP header 192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s Short PLCP PPDU format (optional) 56 short synch. 16 SFD 8 8 16 16 signal service length HEC PLCP preamble (1 Mbit/s, DBPSK) variable payload PLCP header (2 Mbit/s, DQPSK) 96 µs 2, 5.5 or 11 Mbit/s bits Channel selection (non-overlapping) Europe (ETSI) channel 1 2400 2412 channel 7 channel 13 2442 2472 22 MHz 2483.5 [MHz] US (FCC)/Canada (IC) channel 1 2400 2412 channel 6 channel 11 2437 2462 22 MHz 2483.5 [MHz] WLAN: IEEE 802.11a Data rate Connection set-up time 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) 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 30m, 18 up to 40 m, 12 up to 60 m Frequency Limited, WEP insecure, SSID 280€ adapter, 500€ base station Cost Availability Some products, some vendors Typ. best effort, no guarantees (same as all 802.11 products) Manageability Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages Connectionless/always on Quality of Service Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band Security Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band Disadvantage: stronger shading due to higher frequency, no QoS IEEE 802.11a – PHY frame format 4 1 12 1 rate reserved length parity 6 16 tail service variable 6 variable payload tail pad bits PLCP header PLCP preamble 12 signal 1 6 Mbit/s data variable 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s symbols Operating channels for 802.11a / US U-NII 36 5150 40 44 48 52 56 60 64 5180 5200 5220 5240 5260 5280 5300 5320 channel 5350 [MHz] 16.6 MHz 149 153 157 161 channel 5725 5745 5765 5785 5805 5825 [MHz] 16.6 MHz center frequency = 5000 + 5*channel number [MHz] OFDM in IEEE 802.11a (and HiperLAN2) OFDM with 52 used subcarriers (64 in total) 48 data + 4 pilot (plus 12 virtual subcarriers) 312.5 kHz spacing 312.5 kHz pilot -26 -21 -7 -1 1 7 channel center frequency 21 26 subcarrier number WLAN: IEEE 802.11 – future developments (08/2002) 802.11d: Regulatory Domain Update – completed 802.11e: MAC Enhancements – QoS – ongoing Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol. 802.11f: Inter-Access Point Protocol – ongoing Establish an Inter-Access Point Protocol for data exchange via the distribution system. 802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM – ongoing 802.11h: Spectrum Managed 802.11a (DCS, TPC) – ongoing 802.11i: Enhanced Security Mechanisms – ongoing Enhance the current 802.11 MAC to provide improvements in security. Study Groups 5 GHz (harmonization ETSI/IEEE) – closed Radio Resource Measurements – started High Throughput – started