Wireless LAN.pptx

Wireless Local Area Networks Ch. 6: Wireless and Mobile Networks Background:  # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to1)!  # wireless Internet-connected devices equals # wireline Internet-connected devices • laptops, Internet-enabled phones promise anyti...

Wireless Local Area Networks Ch. 6: Wireless and Mobile Networks Background:  # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to1)!  # wireless Internet-connected devices equals # wireline Internet-connected devices • laptops, Internet-enabled phones promise anytime untethered Internet access  two important (but different) challenges • wireless: communication over wireless link • mobility: handling the mobile user who changes point of attachment to network Wireless and Mobile Networks 7-2 Elements of a wireless network network infrastructure Wireless and Mobile Networks 7-3 Elements of a wireless network wireless hosts network infrastructure  laptop, smartphone  run applications  may be stationary (non-mobile) or mobile • wireless does not always mean mobility Wireless and Mobile Networks 7-4 Elements of a wireless network base station network infrastructure  typically connected to wired network  relay - responsible for sending packets between wired network and wireless host(s) in its “area” • e.g., cell towers, 802.11 access points Wireless and Mobile Networks 7-5 Elements of a wireless network wireless link network infrastructure  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 Wireless and Mobile Networks 7-6 Characteristics of selected wireless links 1300 Data rate (Mbps) 450 54 5-11 802.11 ac 802.11n 802.11a,g 802.11b 4 1 802.11a,g point-to-point 4G: LTWE WIMAX 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 802.15 .384 2.5G: UMTS/WCDMA, CDMA2000 .056 2G: IS-95, CDMA, GSM Indoor Outdoor 10-30m 50-200m Mid-range outdoor Long-range outdoor 200m – 4 Km 5Km – 20 Km Wireless and Mobile Networks 7-7 Elements of a wireless network infrastructure mode network infrastructure  base station connects mobiles into wired network  handoff: mobile changes base station providing connection into wired network Wireless and Mobile Networks 7-8 Elements of a wireless 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 and Mobile Networks 7-9 Wireless network taxonomy single hop infrastructure (e.g., APs) host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no no infrastructure connection to larger Internet (Bluetooth, ad hoc nets) multiple hops 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 Wireless and Mobile Networks 7-10 Chapter 7 outline 7.1 Introduction Mobility Wireless 7.5 Principles: addressing and routing to mobile users 7.6 Mobile IP 7.7 Handling mobility in cellular networks 7.8 Mobility and higherlayer protocols 7.2 Wireless links, characteristics • CDMA 7.3 IEEE 802.11 wireless LANs (“WiFi”) 7.4 Cellular Internet Access • architecture • standards (e.g., 3G, LTE) Wireless and Mobile Networks 7-11 Wireless Link Characteristics (1) important differences from wired link ….  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 and Mobile Networks 7-12 point) wireless link much more Wireless “difficult” Wireless Link Characteristics (2)  SNR: signal-to-noise ratio  SNR versus BER tradeoffs • given physical layer: increase power -> increase SNR->decrease BER • given SNR: choose physical layer that meets BER requirement, giving highest thruput • SNR may change with mobility: dynamically adapt physical layer 10-2 10-3 BER • larger SNR – easier to extract signal from noise (a “good thing”) 10-1 10-4 10-5 10-6 10-7 10 20 30 40 SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) Wireless and Mobile Networks 7-13 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): B A C C A B 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’s signal strength A’s signal strength space Signal attenuation:  B, A hear each other  B, C hear each other  A, C can not hear each other interfering at B Wireless and Mobile Networks 7-14 Code Division Multiple Access (CDMA)  unique “code” assigned to each user; i.e., code set partitioning • all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)  encoded signal = (original data) X (chipping sequence)  decoding: inner-product of encoded signal and chipping sequence Wireless and Mobile Networks 7-15 Chapter 7 outline 7.1 Introduction Mobility Wireless 7.5 Principles: addressing and routing to mobile users 7.6 Mobile IP 7.7 Handling mobility in cellular networks 7.8 Mobility and higherlayer protocols 7.2 Wireless links, characteristics • CDMA 7.3 IEEE 802.11 wireless LANs (“WiFi”) 7.4 Cellular Internet Access • architecture • standards (e.g., 3G, LTE) Wireless and Mobile Networks 7-16 IEEE 802.11 Wireless LAN 802.11b  2.4-5 GHz unlicensed spectrum  up to 11 Mbps  direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code 802.11a  5-6 GHz range  up to 54 Mbps 802.11g  2.4-5 GHz range  up to 54 Mbps 802.11n: multiple antennae  2.4-5 GHz range  up to 200 Mbps  all use CSMA/CA for multiple access  all have base-station and ad-hoc network versions Wireless and Mobile Networks 7-17 802.11 LAN architecture Internet hub, switch or router BSS 1 BSS 2  wireless host communicates with base station • base station = access point (AP)  Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: • wireless hosts • access point (AP): base station • ad hoc mode: hosts only Wireless and Mobile Networks 7-18 802.11: Channels, association  802.11b: 2.4GHz-2.485GHz spectrum divided into 11 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 address • selects AP to associate with • may perform authentication [Chapter 8] • will typically run DHCP to get IP address in AP’s subnet Wireless and Mobile Networks 7-19 802.11: passive/active scanning BBS 1 AP 1 BBS 1 BBS 2 BBS 2 1 1 1 2 AP 2 AP 1 2 3 2 3 H1 AP 2 4 H1 passive scanning: active scanning: (1)beacon frames sent from APs (2)association Request frame sent: H1 to selected AP (3)association Response frame sent from selected AP to H1 (1) Probe Request frame broadcast from H1 (2) Probe Response frames sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent from selected AP to H1 Wireless and Mobile Networks 7-20 IEEE 802.11: multiple access  avoid collisions: 2+ nodes transmitting at same time  802.11: CSMA - sense before transmitting • don’t collide with ongoing transmission by other node  802.11: 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) B A C A B C C’s signal strength A’s signal strength space Wireless and Mobile Networks 7-21 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then sender transmit entire frame (no CD) 2 if sense channel busy then DIFS start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) receiver data SIFS ACK Wireless and Mobile Networks 7-22 Avoiding collisions (more) 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  CTS heard by all nodes • sender transmits data frame • other stations defer transmissions avoid data frame collisions completely using small reservation packets! Wireless and Mobile Networks 7-23 Collision Avoidance: RTS-CTS exchange A B AP RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) Wireless and Mobile Networks 7-24 802.11 frame: addressing 2 2 6 6 6 frame address address address duration control 1 2 3 Address 1: MAC address of wireless host or AP to receive this frame 2 6 seq address 4 control 0 - 2312 4 payload CRC Address 4: used only in ad hoc mode Address 3: MAC address of router interface to Address 2: MAC addresswhich AP is attached of wireless host or AP transmitting this frame Wireless and Mobile Networks 7-25 802.11 frame: addressing Internet R1 router H1 R1 MAC addr H1 MAC addr dest. address source address 802.3 frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame Wireless and Mobile Networks 7-26 802.11 frame: more frame seq # (for RDT) duration of reserved transmission time (RTS/CTS) 2 2 6 6 6 frame address address address duration control 1 2 3 2 Protocol version 2 4 1 Type Subtype To AP 6 2 1 seq address 4 control 1 From More AP frag 1 1 Retry 0 - 2312 4 payload CRC 1 Power More mgt data 1 1 WEP Rsvd frame type (RTS, CTS, ACK, data) Wireless and Mobile Networks 7-27 802.11: mobility within same subnet  H1 remains in same IP subnet: IP address can remain same  switch: which AP is associated with H1? • self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 BBS 1 H1 BBS 2 Wireless and Mobile Networks 7-28 802.11: advanced capabilities Rate adaptation QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point 10-2 BER  base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies 10-1 10-3 10-4 10-5 10-6 10-7 10 20 30 SNR(dB) 40 1. 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 Wireless and Mobile Networks 7-29 802.11: advanced capabilities 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 AP-to-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 Wireless and Mobile Networks 7-30 802.15: personal area network  less than 10 m diameter  replacement for cables (mouse, keyboard, headphones)  ad hoc: no infrastructure  master/slaves: • slaves request permission to send (to master) • master grants requests  802.15: evolved from Bluetooth specification • 2.4-2.5 GHz radio band • up to 721 kbps P S P radius of coverage M S P S P M Master device S Slave device P Parked device (inactive) Wireless and Mobile Networks 7-31