Chapter 6 Wireless and Mobile Networks Computer Networking: A Top Down Approach th 6 edition Jim Kurose, Keith Ross Addison-Wesley March 2012 All material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved Wireless, Mobile Networks 6-1 Ch. 6: Wireless and Mobile Networks Background: v # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to-1)! v # wireless Internet-connected devices exceeds # wireline Internet-connected devices § laptops, Internet-enabled phones promise anytime untethered Internet access v two important (but different) challenges § wireless: communication over wireless link § mobility: handling the mobile user who changes point of attachment to network Wireless, Mobile Networks 6-2 Chapter 6 outline 6.1 Introduction Mobility Wireless 6.5 Principles: addressing and routing to mobile users 6.2 Wireless links, characteristics 6.6 Mobile IP § CDMA 6.7 Handling mobility in 6.3 IEEE 802.11 wireless cellular networks LANs (“Wi-Fi”) 6.8 Mobility and higher-layer 6.4 Cellular Internet Access protocols § architecture § standards (e.g., GSM) 6.9 Summary Wireless, Mobile Networks 6-3 Elements of a wireless network network infrastructure Wireless, Mobile Networks 6-4 Elements of a wireless network wireless hosts v laptop, smartphone v run applications v may be stationary (non- mobile) or mobile network infrastructure § wireless does not always mean mobility Wireless, Mobile Networks 6-5 Elements of a wireless network base station v typically connected to wired network v relay - responsible for sending packets between network wired network and infrastructure wireless host(s) in its “area” § e.g., cell towers, 802.11 access points Wireless, Mobile Networks 6-6 Elements of a wireless network wireless link v typically used to connect mobile(s) to base station v also used as backbone link network v multiple access protocol infrastructure coordinates link access v various data rates, transmission distance Wireless, Mobile Networks 6-7 Characteristics of selected wireless links 200 802.11n 54 802.11a,g 802.11a,g point-to-point 5-11 802.11b 4G: LTWE WIMAX 4 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 1 802.15 Data rate(Mbps) .384 2.5G: UMTS/WCDMA, CDMA2000 .056 2G: IS-95, CDMA, GSM Indoor Outdoor Mid-range Long-range 10-30m 50-200m outdoor outdoor 200m – 4 Km 5Km – 20 Km Wireless, Mobile Networks 6-8 Wireless, Mobile Networks 6-9 Elements of a wireless network infrastructure mode v base station connects mobiles into wired network v handoff: mobile changes network base station providing infrastructure connection into wired network Wireless, Mobile Networks 6-10 Elements of a wireless network ad hoc mode v no base stations v nodes can only transmit to other nodes within link coverage v nodes organize themselves into a network: route among themselves Wireless, Mobile Networks 6-11 Wireless network taxonomy single hop multiple hops host connects to host may have to infrastructure base station (WiFi, relay through several (e.g., APs) WiMAX, cellular) wireless nodes to which connects to connect to larger larger Internet Internet: mesh net no base station, no connection to larger no base station, no no Internet. May have to connection to larger infrastructure relay to reach other Internet (Bluetooth, a given wireless node ad hoc nets) MANET, VANET Wireless, Mobile Networks 6-12 Chapter 6 outline 6.1 Introduction Mobility Wireless 6.5 Principles: addressing and routing to mobile users 6.2 Wireless links, characteristics 6.6 Mobile IP § CDMA 6.7 Handling mobility in 6.3 IEEE 802.11 wireless cellular networks LANs (“Wi-Fi”) 6.8 Mobility and higher-layer 6.4 Cellular Internet Access protocols § architecture § standards (e.g., GSM) 6.9 Summary Wireless, Mobile Networks 6-13 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 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, reaching destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” Wireless, Mobile Networks 6-14 Wireless Link Characteristics (2) -1 v SNR: signal-to-noise ratio 10 § larger SNR – easier to 10-2 extract signal from noise (a 10-3 “good thing”) 10-4 v SNR versus BER tradeoffs BER § given physical layer: increase 10-5 power à increase SNR à 10-6 decrease bit-error-rate (BER) 10-7 § given SNR: choose physical layer 10 20 30 40 that meets BER requirement, SNR(dB) giving highest thruput QAM256 (8 Mbps) • SNR may change with mobility: dynamically adapt QAM16 (4 Mbps) physical layer (modulation BPSK (1 Mbps) technique, rate) Wireless, Mobile Networks 6-15 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C C A’s signal C’s signal B strength strength A Hidden terminal problem space v B, A hear each other Signal attenuation: v v B, C hear each other B, A hear each other v B, C hear each other v A, C can not hear each other means A, C unaware of their v A, C can not hear each other interference at B interfering at B Wireless, Mobile Networks 6-16 Code Division Multiple Access (CDMA) v 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”) v encoded signal = (original data) X (chipping sequence) v decoding: inner-product of encoded signal and chipping sequence Wireless, Mobile Networks 6-17 CDMA encode/decode channel output Z . i,m Zi,m= di cm d0 = 1 data 1 1 1 1 1 1 1 1 d1 = -1 bits - - -1 - - - - -1 sender 1 1 1 1 1 1 slot 1 slot 0 code 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 channel channel output output slot 1 slot 0 M D = Z .c i m=1Σ i,m m M received 1 1 1 1 1 1 1 1 d0 = 1 -1 -1 -1 -1 -1 -1 -1 -1 input d1 = -1 slot 1 slot 0 code 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 channel channel output output receiver slot 1 slot 0 Wireless, Mobile Networks 6-18 CDMA: two-sender interference channel sums together transmissions by sender 1 and 2 Sender 1 Sender 2 using same code as sender 1, receiver recovers sender 1’s original data from summed channel data! Wireless, Mobile Networks 6-19 Chapter 6 outline 6.1 Introduction Mobility Wireless 6.5 Principles: addressing and routing to mobile users 6.2 Wireless links, characteristics 6.6 Mobile IP § CDMA 6.7 Handling mobility in 6.3 IEEE 802.11 wireless cellular networks LANs (“Wi-Fi”) 6.8 Mobility and higher-layer 6.4 Cellular Internet Access protocols § architecture § standards (e.g., GSM) 6.9 Summary Wireless, Mobile Networks 6-20 IEEE 802.11 Wireless LAN 802.11b 802.11a § 5-6 GHz range v 2.4-5 GHz unlicensed spectrum § up to 54 Mbps v up to 11 Mbps 802.11g v direct sequence spread spectrum § 2.4-5 GHz range (DSSS) in physical layer § up to 54 Mbps 802.11n: multiple antennae § 2.4-5 GHz range § up to 200 Mbps v all use CSMA/CA for multiple access v all have base-station and ad-hoc network versions Wireless, Mobile Networks 6-21 802.11 LAN architecture v wireless host communicates with base Internet station § base station = access point (AP) hub, switch v Basic Service Set (BSS) (aka or router “cell”) in infrastructure mode contains: § wireless hosts BSS 1 § access point (AP): base station § ad hoc mode: hosts only BSS 2 Wireless, Mobile Networks 6-22 802.11: Channels, association v 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! v 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 subnet Wireless, Mobile Networks 6-23 802.11: passive/active scanning BBS 1 BBS 2 BBS 1 BBS 2 1 1 1 2 AP 2 AP 1 AP 2 AP 1 2 2 3 3 4 H1 H1 passive scanning: active scanning: (1) beacon frames sent from APs (1) Probe Request frame broadcast (2) association Request frame sent: H1 to from H1 selected AP (2) Probe Response frames sent (3) association Response frame sent from from APs selected AP to H1 (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent from selected AP to H1 Wireless, Mobile Networks 6-24 IEEE 802.11: multiple access v avoid collisions: 2+ nodes transmitting at same time v 802.11: CSMA - sense before transmitting § don’t collide with ongoing transmission by other node v 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) A B C C A’s signal C’s signal B strength A strength space Wireless, Mobile Networks 6-25 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then sender receiver transmit entire frame (no CD) 2 if sense channel busy then DIFS start random backoff time timer counts down while channel idle data transmit when timer expires if no ACK, increase random backoff interval, repeat 2 SIFS 802.11 receiver ACK - if frame received OK return