Mobile devices for wireless

Transcription

1 Mobile devices for wireless Wireless sensors Limited proc. power Small battery Laptop functionally eq. to desktop standard applications TDTS04/09 Föreläsning 0/6: WLAN och TCP i trådlösa nät RFID tag A few thousands of logical gates Responds only to the RFID reader requests (no battery) Pager receive only tiny displays simple text messages Mobile phones voice, data simple text displays PDA simple graphical displays character recognition simplified WWW performance Slides are modified from J.F Kurose and K.W. Ross Wireless systems: development over last 25 years What is missing? 98: NMT : NMT 900 cellular phones 992: GSM analogue digital 994: DCS : CDMA 2000: GPRS 983: AMPS 99: D-AMPS 993: PDC 200: UMTS/IMT-2000 CDMA-2000 (USA) 4G fourth generation - and IEEE 802.n; WiMax... satellites 982: Inmarsat-A 988: Inmarsat-C 992: Inmarsat-B Inmarsat-M 998: Iridium cordless phones 980: CT0 984: CT 987: CT+ 989: CT : Fourth Generation (Internet based) wireless LAN 99: DECT 99x: proprietary 997: IEEE : 802.b; Bluetooth 2000: IEEE 802.a Communication Frequency Spectrum Electromagnetic spectrum and applications (Tanenbaum 2003) NMT: Nordic Mobile Telephone AMPS: Advanced Mobile Phone System (USA) CT: Cordless Telephone UMTS: Universal Mobile Telecom. System DECT: Digital Enhanced Cordless Telecom. DCS: Digital Cellular System PDC: Pacific Digital Cellular PAN: Personal Area Network 6-3

2 Elements of a wireless network Elements of a wireless network network wireless hosts laptop, PDA, IP phone run applications may be stationary (non-mobile) or mobile wireless does not always mean mobility network base station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its area e.g., cell towers, 802. access points Elements of a wireless network network wireless link typically used to connect mobile(s) to base station also used as backbone link multiple access protocol coordinates link access various data rates, transmission distance Data rate (Mbps) Characteristics of selected wireless link standards n a,g 802.b 802.a,g point-to-point IS-95, CDMA, GSM (WiMAX) UMTS/WCDMA-HSPDA, CDMA2000-xEVDO UMTS/WCDMA, CDMA2000 data 3G cellular enhanced 3G 2G Indoor 0-30m Outdoor m Mid-range outdoor 200m 4 Km Long-range outdoor 5Km 20 Km

3 Elements of a wireless network Elements of a wireless network network mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves Wireless network taxonomy Wireless Link Characteristics () single hop multiple hops Differences from wired link. (e.g., APs) no host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times. make communication across (even a point to point) wireless link much more difficult

4 Wireless Link Characteristics (2) SNR: signal-to-noise ratio larger SNR easier to extract signal from noise (a good thing ) SNR versus BER tradeoffs given physical layer: increase power -> increase SNR->decrease BER given SNR: choose physical layer that meets BER requirement, giving highest throughput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) BER SNR(dB) QAM256 (8 Mbps) QAM6 (4 Mbps) BPSK ( Mbps) 6-3 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other means A, C unaware of their interference at B C B A B C A s signal strength space C s signal strength Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other interfering at B 6-4 Stallings (2005), Wireless communications & Networks. 2nd Ed. Prentice Hall. IEEE 802. Standards 802. Physical Layer: Spread Spectrum Example, 802.b s DSSS (Direct Sequence Spread Spectrum) for each bit, send XOR of that bit and n random bits random sequence known to both sender and receiver called n-bit chipping code IEEE 802. defines an -bit chipping code. Data stream: 00 Random sequence: XOR of the two:

5 IEEE 802. Architecture Station: a device containing 802. equipment Basic Service Set (BSS): set of stations controlled by a coordination function Coordination function: logical function determining when a station can receive and send data in a BSS Distribution System (DS): a system connecting a set of BSS and integrated LANs to create an extended service set (ESS) Extended Service Set: a set of BSS and LANs appearing as a single unit to the LLC layer of the component stations Access point (AP) : entity providing access to the distribution system Figure from Stallings (2005), Wireless communications & Networks. 2nd Ed. Prentice Hall. IEEE 802. Services Figure from Stallings (2005), Wireless communications & Networks. 2nd Ed. Prentice Hall. 802.: Channels, association 802.: passive/active scanning 802.b: 2.4GHz-2.485GHz spectrum divided into channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing AP s name (SSID) and MAC selects AP to associate with may perform authentication will typically run DHCP to get IP in AP s subnet AP BBS H BBS AP 2 Passive Scanning: () beacon frames sent from APs (2) association Request frame sent: H to selected AP (3) association Response frame sent: H to selected AP AP BBS 2 H BBS AP 2 Active Scanning: () Probe Request frame broadcast from H (2) Probes response frame sent from APs (3) Association Request frame sent: H to selected AP (4) Association Response frame sent: H to selected AP

6 IEEE 802.: multiple access IEEE 802. MAC Protocol: CSMA/CA avoid collisions: 2 + nodes transmitting at same time 802.: CSMA - sense before transmitting don t collide with ongoing transmission by other node 802.: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) A C B A B C A s signal strength space C s signal strength sender if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS sender data ACK receiver SIFS 6-22 IEEE 802. MAC: Priority-based scheme Use 3 values for IFS (Inter-Frame Space): SIFS (short IFS): shortest IFS used for immediate responses such as ACK, CTS, poll response PIFS (point coordination function IFS): middle length IFS used for issuing polls by a centralized controller DIFS (distributed coordination function IFS): longest IFS used for regular asynchronous frames Avoiding collisions idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets! Figure from Stallings (2005), Wireless communications & Networks. 2nd Ed. Prentice Hall

7 Collision Avoidance: RTS-CTS exchange 802. frame: ing A AP B reservation collision frame control duration 2 3 seq control 4 payload CRC time DATA (A) defer Address : MAC of wireless host or AP to receive this frame Address 2: MAC of wireless host or AP transmitting this frame Address 3: MAC of router interface to which AP is attached Address 4: used only when the DS is wireless frame: ing 802. frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for reliable ARQ) H AP R router Internet frame control duration 2 3 seq control 4 payload CRC R MAC addr AP MAC addr dest. source frame AP MAC addr H MAC addr R MAC addr frame Protocol version Type Subtype To AP From AP frame type (RTS, CTS, ACK, data) More frag Power Retry mgt More data WEP Rsvd

8 IEEE 802. MAC Frame Types Six types of control frames Power save - poll (PS-poll) Request to send (RTS) Clear to send (CTS) Acknowledgment (ACK) Contention-free (CF)-end CF-end + CF-Ack Management frames association request and association response reassociation request and reassociation response probe request and probe response beacon announcement traffic indication message disassociation Eight types of data frames Carry user data Data Data + CF-Ack Data + CF-poll Data + CF-Ack + CF-poll Do not carry user data Null Function CF-Ack CF-Poll CF-Ack + CF-Poll 802.: mobility within same IP subnet Organizations extend WLAN with multiple BSSs H remains in same IP subnet: IP can remain same Problem w/switch: which AP is associated with H? authentication and deauthentication 6-30 BBS AP router hub or switch AP 2 H BBS : advanced capabilities 802.: advanced capabilities Rate Adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM256 (8 Mbps) QAM6 (4 Mbps) BPSK ( Mbps) operating point BER SNR(dB). SNR decreases, BER increase as node moves away from base station 2. When BER becomes too high, switch to lower transmission rate but with lower BER Power Management node-to-ap: I am going to sleep until next beacon frame AP knows not to transmit frames to this node node wakes up before next beacon frame beacon frame: contains list of mobiles with APto-mobile frames waiting to be sent node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame

9 What is mobility? spectrum of mobility, from the network perspective: no mobility mobile wireless user, using same access point mobile user, connecting/ disconnecting from network using DHCP. high mobility mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) Wireless, mobility: impact on higher layer protocols logically, impact should be minimal best effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile but performance-wise: packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff TCP interprets loss as congestion, will decrease congestion window un-necessarily delay impairments for real-time traffic limited bandwidth of wireless links TCP i trådlösa nät Snooping TCP a) Från fast nät till mobil station indirect (split) TCP snooping TCP TCP i radionät: trådlös profil 9

10 Snooping TCP, forts. b) Från mobil station till fast nät TCP:s trådlösa profil stora fönsterstorlekar (fönsterskalning) stor MTU selektiva ACKar explicit stockningsmeddelande (ECN) tidsstämpel TCP: Fönsterskalning Källa: Halsall (2005), Computer networking and the Internet. 5th Ed. Addison-Wesley. 0