OVERVIEW Lecture 2 Wireless Networks Lecture 2: Wireless Networks 1

Elements of a wireless network network infrastructure wireless hosts r laptop, PDA, IP phone r run applications r may be stationary (non-mobile) or mobile base station r typically connected to wired network r relay - responsible for sending packets between wired network and wireless host(s) in its “area” wireless link r multiple access protocol coordinates link access r various data rates, transmission distance 2

Lecture 2: Wireless Networks Elements of a wireless network network infrastructure infrastructure mode r base station connects mobiles into wired network r handoff: mobile changes base station providing connection into wired network 3

Lecture 2: Wireless Networks Elements of a wireless network ad hoc mode r no base stations r nodes can only transmit to other nodes within link coverage r nodes organize themselves into a network: route among themselves 4

Lecture 2: Wireless Networks Wireless network taxonomy single hop multiple hops infrastructure (e.g., APs) no infrastructure 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 5

Lecture 2: Wireless Networks Wireless Link Characteristics (1) Differences from wired link …. m decreased signal strength: radio signal attenuates as it propagates through matter (path loss) m 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 m 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” 6

Lecture 2: Wireless Networks Wireless Link Characteristics (2) r SNR: signal-to-noise ratio m larger SNR – easier to extract signal from noise (a “good thing”) r SNR versus BER tradeoffs m given physical layer: increase power -> increase SNR->decrease BER m given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) SNR(dB) BER

Lecture 3 Wireless Networks (cont) CPE 401/601 Computer Network Systems slides are modified from Jim Kurose & Keith Ross All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

Lecture 2: Wireless Networks Code Division Multiple Access (CDMA) r used in several wireless broadcast channels (cellular, satellite, etc) standards r unique “code” assigned to each user; i.e., code set partitioning r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data r encoded signal = (original data) X (chipping sequence) r decoding: inner-product of encoded signal and chipping sequence r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 9

Lecture 2: Wireless Networks CDMA Encode/Decode slot 1 slot 0 d 1 = Z i,m = d i. c m d 0 = slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M 10

Lecture 2: Wireless Networks CDMA: two-sender interference 11

Lecture 3 outline 6.1 Introduction Wireless r 6.2 Wireless links, characteristics m CDMA r 6.3 IEEE wireless LANs (“wi-fi”) r 6.4 cellular Internet access m architecture m standards (e.g., GSM) Lecture 2: Wireless Networks 12

Lecture 2: Wireless Networks IEEE Wireless LAN r b m GHz unlicensed spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code r a m 5-6 GHz range m up to 54 Mbps r g m GHz range m up to 54 Mbps r n: multiple antennae m GHz range m up to 200 Mbps r all use CSMA/CA for multiple access r all have base-station and ad-hoc network versions 13

Lecture 2: Wireless Networks LAN architecture r wireless host communicates with base station m base station = access point (AP) r Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: m wireless hosts m access point (AP): base station m ad hoc mode: hosts only BSS 1 BSS 2 Internet hub, switch or router AP 14

Lecture 2: Wireless Networks : Channels, association r b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies m AP admin chooses frequency for AP m interference possible: channel can be same as that chosen by neighboring AP! r host: must associate with an AP m scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address m selects AP to associate with m may perform authentication [Chapter 8] m will typically run DHCP to get IP address in AP’s subnet 15

Lecture 2: Wireless Networks : passive/active scanning AP 2 AP 1 H1 BBS 2 BBS Active Scanning : (1)Probe Request frame broadcast from H1 (2)Probes response frame sent from APs (3)Association Request frame sent: H1 to selected AP (4)Association Response frame sent: H1 to selected AP AP 2 AP 1 H1 BBS 2 BBS Passive Scanning: (1)beacon frames sent from APs (2)association Request frame sent: H1 to selected AP (3)association Response frame sent: H1 to selected AP 16

Lecture 2: Wireless Networks IEEE : multiple access r avoid collisions: 2 + nodes transmitting at same time r : CSMA - sense before transmitting m don’t collide with ongoing transmission by other node r : no collision detection! m difficult to receive (sense collisions) when transmitting due to weak received signals (fading) m can’t sense all collisions in any case: hidden terminal, fading m goal: avoid collisions: CSMA/C(ollision)A(voidance) A B C A B C A’s signal strength space C’s signal strength 17

Lecture 2: Wireless Networks IEEE MAC Protocol: CSMA/CA sender 1 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) sender receiver DIFS data SIFS ACK 18

Lecture 2: Wireless Networks Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames r sender first transmits small request-to-send (RTS) packets to BS using CSMA m RTSs may still collide with each other (but they’re short) r BS broadcasts clear-to-send CTS in response to RTS r CTS heard by all nodes m sender transmits data frame m other stations defer transmissions avoid data frame collisions completely using small reservation packets! 19

Lecture 2: Wireless Networks Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer 20

Lecture 2: Wireless Networks frame control duration address 1 address 2 address 4 address 3 payloadCRC seq control frame: addressing Address 2: MAC address of wireless host or AP transmitting this frame Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode 21

Lecture 2: Wireless Networks Internet router AP H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame R1 MAC addr H1 MAC addr dest. address source address frame frame: addressing 22

Lecture 2: Wireless Networks frame control duration address 1 address 2 address 4 address 3 payloadCRC seq control Type From AP Subtype To AP More frag WEP More data Power mgt RetryRsvd Protocol version frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for RDT) frame type (RTS, CTS, ACK, data) 23

Lecture 2: Wireless Networks hub or switch AP 2 AP 1 H1 BBS 2 BBS : mobility within same subnet router r H1 remains in same IP subnet: IP address can remain same r switch: which AP is associated with H1? m self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 24

Lecture 2: Wireless Networks : advanced capabilities Rate Adaptation r base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) SNR(dB) BER operating point 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 25

Lecture 2: Wireless Networks : advanced capabilities Power Management r node-to-AP: “I am going to sleep until next beacon frame” m AP knows not to transmit frames to this node m node wakes up before next beacon frame r beacon frame: contains list of mobiles with AP- to-mobile frames waiting to be sent m node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame 26

Lecture 2: Wireless Networks M radius of coverage S S S P P P P M S Master device Slave device Parked device (inactive) P : personal area network r less than 10 m diameter r replacement for cables (mouse, keyboard, headphones) r ad hoc: no infrastructure r master/slaves: m slaves request permission to send (to master) m master grants requests r : evolved from Bluetooth specification m GHz radio band m up to 721 kbps 27

Lecture 2: Wireless Networks : WiMAX r like & cellular: base station model m transmissions to/from base station by hosts with omnidirectional antenna m base station-to-base station backhaul with point-to-point antenna r unlike : m range ~ 6 miles (“city rather than coffee shop”) m ~14 Mbps point-to-multipoint point-to-point 28

Lecture 2: Wireless Networks : WiMAX: downlink, uplink scheduling r transmission frame m down-link subframe: base station to node m uplink subframe: node to base station pream. DL- MAP UL- MAP DL burst 1 SS #1 DL burst 2 DL burst n Initial maint. request conn. downlink subframe SS #2 SS #k uplink subframe … … … … base station tells nodes who will get to receive (DL map) and who will get to send (UL map), and when r WiMAX standard provide mechanism for scheduling, but not scheduling algorithm 29