CIS 585 Communication and Information Services
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1 California State University Los Angeles CIS 585 Communication and Information Services Lecture 4: Wireless LAN, PhD California State University, Los Angeles Computer Department Computer 1 Contents Characteristics IEEE PHY MAC Roaming.11a, b, g, h, i HIPERLAN Standards overview HiperLAN2 QoS Bluetooth Comparison Computer 2 1
2 Characteristics of wireless LANs Advantages very flexible within the reception area Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings, firewalls) more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... Disadvantages typically very low bandwidth compared to wired networks (1-10 Mbit/s) higher error rates (10-4 )due to interference delay/jitter due to extensive error correction and detection many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE ) products have to follow many national restrictions Safety and security Computer 3 Design goals for wireless LANs global, seamless operation low power for battery use no special permissions or licenses needed to use the LAN 2.4 GHz ISM band 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 Computer 4 2
3 Comparison: infrared vs. radio transmission Infrared 900 nm wave length uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible no interference from electrical devices Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth (max 4 Mbps) Radio typically using the license free ISM band at 2.4 GHz Advantages experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license free frequency bands shielding more difficult, interference with other electrical devices Example WaveLAN, HIPERLAN, Bluetooth Computer 5 Comparison: infrastructure / ad-hoc networks infrastructure network AP AP wired network AP: Access Point AP ad-hoc network Computer 6 3
4 Comparison: infrastructure / ad-hoc networks Infrastructure networks Communications take place between the wireless nodes and AP, not directly between nodes. can use different access schemes collision wireless nodes and AP are not coordinated collision free AP controls medium access (QoS support) Ad-hoc networks medium access: collision wireless node may transfer data (router) , HiperLAN2 : Infrastructure-based (ad-hoc support) Bluetooth: ad-hoc based Computer 7 IEEE 802 Network IEEE 802 standard series LLC 802 OUTLINE & ARCHITECTURE MANAGEMENT MAC PHY MAC PHY FHSS PHY MAC DSSS PHY a OFDM PHY b HR/DSSS PHY LLC(Logical Link Control), FHSS(Frequency Hopping Spread Spectrum), IR(infrared) DSSS(Direct Sequence Spread Spectrum), HR/DSSS(High Rate DSSS) OFDM(Orthogonal Frequency-Division Multiplexing) IEEE Wired Ethernet LAN, IEEE Token Bus LAN, IEEE Token Ring LAN Computer 8 4
5 IEEE Standard Standard process MAC(1997) a PHY, 54Mbps, 5Ghz b PHY, 11Mbps, 2.4Ghz g PHY, 54Mbps, 2.4Ghz(Super set of b PHY) e for QoS support f for IAPP(Inter-Access Point Protocol) h for spectrum and transmit power management i for security enhancement k for radio resource management enhancement n for higher throughput Computer Architecture of an infrastructure STA LAN BSS 1 ESS Extended Service Set Access Point BSS 2 Distribution System Access Point 802.x LAN Portal STA LAN STA 3 Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) group of stations using the same radio frequency Access Point station integrated into the wireless LAN and the distribution system Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (EES: Extended Service Set) based on several BSS Computer 10 5
6 Architecture of an ad-hoc network LAN Direct communication within a STA 1 IBSS 1 STA 2 STA 3 limited range Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency IBSS 2 STA 5 STA LAN Computer 11 IEEE standard mobile terminal fixed terminal application TCP IP LLC MAC PHY access point LLC MAC MAC PHY PHY infrastructure network application TCP IP LLC MAC PHY Computer 12 6
7 Layers and functions PHY DLC MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management LLC MAC PLCP PMD MAC Management PHY Management PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) PMD Physical Medium Dependent modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions Station Management Computer Physical layer 3 versions: 2 radio (typ. 2.4 GHz), 1 IR Data rate: 1 or 2 Mbps (basic version) FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s 79 hopping channels (23 in JAPAN) 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 nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchonization Computer 14 7
8 Synchronization FHSS PHY packet format synch with pattern : signal synchronization SFD (Start Frame Delimiter) start pattern : frame synchronization PLW (PLCP_PDU Length Word) length of payload incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) data rate of payload: 0000 (1Mbps) 0010 (2 Mbps) HEC (Header Error Check) CRC with x 16 +x 12 +x variable bits synchronization SFD PLW PSF HEC Payload (MAC) PLCP preamble PLCP header Computer 15 Synchronization DSSS PHY packet format synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter): Signal data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service: future use, 00: compliant Length: length of the payload HEC (Header Error Check) protection of signal, service and length, x 16 +x 12 +x variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header Computer 16 8
9 MAC layer - 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) Distributed Foundation Wireless MAC 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 Contention-Free PCF PCF < MAC Extent > DCF DCF General Computer 17 DCF DCF(Distributed Coordination Function) Channel access methods can prevent collisions Carrier sense multiple access with collision avoidance (CSMA/CA) Based on CSMA/CD Dsitributed Contention-based channel access Based CSMA/CA Asynchronous data transport IBSS, Infrastructure BSS FIFO transmission queue Backoff Computer 18 9
10 IEEE Standard (continued) Computer 19 IEEE Standard (continued) Distributed coordination function (continued) CSMA/CD is designed to handle collisions CSMA/CA attempts to avoid collisions altogether With contention-based channel access methods Most collisions occur after a device completes its transmission CSMA/CA makes all devices wait a random amount of time CSMA/CA also reduces collisions by using explicit packet acknowledgment (ACK) Computer 20 10
11 IEEE b Standard (continued) Distributed coordination function (continued) Additional mechanisms to reduce collisions Request to Send/Clear to Send (RTS/CTS) protocol Fragmentation Polling Another type of channel access method Each computer is asked if it wants to transmit Polling effectively eliminates collisions Point coordination function (PCF) AP serves as the polling device Computer 21 Priorities MAC layer defined through different inter frame spaces no guaranteed, hard priorities SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response 10usec (DSSS) PIFS (PCF IFS) medium priority, for time-bounded service using PCF SIFS + 1time slot (20 usec) DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service SIFS + 2 time slots DIFS DIFS PIFS medium busy SIFS contention next frame direct access if medium is free DIFS t Computer 22 11
12 CSMA/CA access method DIFS DIFS contention window (randomized back-off mechanism) medium busy direct access if medium is free DIFS slot time next frame t Station ready to send starts sensing the medium If the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) typically DIFS If the medium is busy, the station has to wait for a free IFS (DIFS), 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) Computer competing stations - simple version DIFS DIFS bo e bo r DIFS bo e bo r DIFS bo e busy station 1 bo e busy station 2 station 3 busy bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t busy medium not idle (frame, ack etc.) bo e elapsed backoff time packet arrival at MAC bo r residual backoff time Computer 24 12
13 2 n 1 Contention Window CW(Contention Window) Contention window or backoff window Station selects random slot(probability same)_[0,cw] Cwmin = 7 Each time a collision occurs, the window doubles up to a maximum Cwmax=255 Exponential backoff Computer CSMA/CA access method (ACK) Computer 26 13
14 CSMA/CA access method (ACK) Cont d 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 But the sender has to wait again and compete for the access right. sender receiver other stations DIFS data SIFS ACK DIFS waiting time contention data t Computer 27 RTS / CTS Concept CTS(to A) B A RTS(to B) With NAV C NAV Update Frame A ACK B C NAV Update Computer 28 14
15 IEEE b Standard (continued) Computer DFWMAC Sending unicast packets Station can send RTS with reservation parameter (duration) 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 NAV (Net Allocation Vector) sender DIFS RTS data receiver SIFS CTS SIFS SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t Computer 30 15
16 Fragmentation sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t Computer 31 PCF Computer 32 16
17 PCF (Cont d) PCF(Point Coordination Function) PC(Point Coordinator) in AP Centralized Contention-Free channel access Delay-sensitive traffic Based poll-and-response mechanism Beacon The AP generates beacon frames at regular beacon frame intervals Computer 33 DFWMAC-PCF PC-to-Station Frame materiality Data, CF-ACK, CF-Poll, Data+CF-ACK, Data+CF-Poll CF-ACK+CF-Poll, Data+CF-ACK+CF-Poll, CF-End CF-End+CF-ACK, Any management frame Computer 34 17
18 Types Frame format 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 bytes sending time, checksum, frame control, data Duration/ ID Address 1 Address 2 Address 3 Sequence Control Frame Control Protocol version To Type Subtype DS More Frag Power Retry Mgmt Address Data 4 bits From DS More Data WEP Order CRC Computer 35 MAC format Frame control contain several sub-fields Duration/ID If value is less than 32, 768 (bit 15:0) duration : NAV setting (in usec) CFP frame (Bit 0-13, 14 : 0, 15 : 1) Address (48 bits each) Two type Individual address Group address Multicast-group address Broadcast address BSSID(Indicating BSS Identifier) DA(Destination Address), SA(Source Address) RA(Receiver Address), TA(Transmitter Address) SA/TA RA(BSSID) DS DA Computer 36 18
19 MAC format Sequence control To filter duplicates Frame body Maximum 2,304 Bytes(If WEP is used, 2,312 Bytes) MTU (Maximum Transfer Unit) FCS (Frame Check Sequence) CRC(Cyclic Redundancy Check) : 32 bit checksum Computer 37 MAC format (frame control) Protocol version (0) Type / Subtype combination Type : Management(00), Control(01), Data(10) Subtype: Association Request (0000), beacon (1111), RTS (1011), CTS (1100), data (0000) To DS / From DS combination More fragment : if it has another fragment to follow, set to 1 Retry: if current frame is a retransmission of an earlier frame, set to 1 Power management 1: the station will to into power save mode More data Indicate receiver that the sender has more data to send. WEP(Wired Equivalent Privacy) Encryption Order Strict order (if 1) bits Protocol version Type Subtype To DS From DS More Frag Retry Power Mgmt More Data WEP Order Computer 38 19
20 MAC format (address) To DS From DS Address1(R) Address2(T) Address3 Address4 IBSS 0 0 DA SA BSSID N/A To AP 1 0 RA(BSSID) SA/TA DA N/A From AP 0 1 DA/RA TA(BSSID) SA N/A WDS 1 1 RA TA DA SA WDS(Wireless Distribution System) TA RA SA DA Computer 39 Special Frames: ACK, RTS, CTS Acknowledgement ACK bytes Frame Duration Receiver CRC Control Address Request To Send RTS bytes Frame Duration Receiver Transmitter CRC Control Address Address Clear To Send CTS bytes Frame Duration Receiver CRC Control Address Computer 40 20
21 Management Frame Beacon IBSS ATIM (Announcement Traffic Indication Message) Disassociation Association request / Association response Reassociation request / Reassociation response Probe request / Probe response Authentication / Deauthentication Computer MAC management Synchronization try to find a LAN, try to stay within a LAN synchronize the clocks of all node (timing synchronization function) 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 Computer 42 21
22 Synchronization using a Beacon (infrastructure) beacon interval access point medium B B B B busy busy busy busy value of the timestamp B beacon frame t Computer 43 Synchronization using a Beacon (ad-hoc) beacon interval station 1 B 1 B 1 station 2 B 2 B 2 medium busy busy busy busy t value of the timestamp B beacon frame random delay Computer 44 22
23 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?) Computer 45 Power management (continued) Power management Most clients in a WLAN are portable devices To conserve battery power, they can go into sleep mode When a client is part of a WLAN, it must remain fully powered Power management allows the mobile client s NIC to be off as much as possible But still not miss out on data transmissions The key to power management is synchronization Computer 46 23
24 Power management (continued) Power management (continued) When a mobile client goes into sleep mode, the AP is informed of the change AP temporarily stores the synchronized frames (this function is called buffering) At predetermined times, the AP sends out a beacon frame to all clients Containing the traffic indication map (TIM) Traffic indication map (TIM) List of clients with buffered frames waiting at the AP Computer 47 IEEE b Standard (continued) Computer 48 24
25 Power saving with wake-up patterns (infrastructure) TIM interval DTIM interval access point medium D B busy T T d D busy busy busy B station T TIM D DTIM p awake d t B broadcast/multicast p PS poll d data transmission to/from the station Computer 49 Power saving with wake-up patterns (ad-hoc) ATIM window beacon interval B station 1 A D B 1 1 station 2 B 2 B 2 a d B beacon frame random delay A transmit ATIM t D transmit data awake a acknowledge ATIM d acknowledge data Computer 50 25
26 Roaming Moving between APs 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 Computer Roaming(continued) Association and reassociation Allow a client to join a WLAN and stay connected Association begins with the client scanning the airwaves Types of scanning Passive scanning Client listening to each available channel for a set period of time Client listens for a beacon frame transmitted from all available APs Frame includes the AP s SSID Computer 52 26
27 Roaming (continued) Association and reassociation (continued) Types of scanning (continued) Active scanning Client sends a probe frame to each channel Client then waits for the probe response frame Associate request frame Includes the client s capabilities and supported rates Associate response frame Sent by the AP Contains a status code and client ID number for that client Computer Roaming (continued) Association and reassociation (continued) Reassociation Client may drop the connection with one AP and reestablish the connection with another Necessary when mobile clients roam beyond the coverage area of one AP Client sends a reassociation request frame to new AP AP sends back a reassociation response frame New AP sends a disassociation frame to old AP Computer 54 27
28 Roaming (continued) Computer 55 28
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