Chapter 6 Wireless and Mobile Networks Computer Networking: Top Down pproach 5 th edition. Jim Kurose, Keith Ross ddison-wesley, pril 009. Chapter 6: Wireless and Mobile Networks ackground: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! computer nets: laptops, palmtops, PDs, Internet-enabled phone 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 6: Wireless and Mobile Networks 6 6: Wireless and Mobile Networks 6- Chapter 6 outline Elements of a wireless 6. Introduction Wireless 6. Wireless links, characteristics CDM 6.3 IEEE 80. wireless LNs ( wi-fi ) 6.4 Cellular Internet ccess architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular s 6.8 Mobility and higherlayer protocols 6.9 Summary wireless hosts laptop, PD, IP phone run applications may be stationary (non-mobile) or mobile does not always mean mobility 6: Wireless and Mobile Networks 6-3 6: Wireless and Mobile Networks 6-4 Elements of a wireless Elements of a wireless base station typically connected to wired - responsible for sending packets between wired and wireless host(s) in its area e.g., cell towers, 80. access points wireless link typically used to connect mobile(s) to base station also used as link multiple access protocol coordinates link access various rates, transmission distance 6: Wireless and Mobile Networks 6-5 6: Wireless and Mobile Networks 6-6
Data rate (Mbps) 54 5 4.384.056 Characteristics of selected wireless link standards 00 80.n 80.5 80.a,g 80.b 80.a,g point-to-point UMTS/WCDM-HSPD, CDM000xEVDO UMTS/WCDM, CDM000 IS-95, CDM, GSM 80.6 (WiMX) 3G cellular enhanced 3G G Elements of a wireless mode base station connects mobiles into wired : mobile changes base station providing connection into wired Indoor 0-30m Outdoor 50-00m Mid-range outdoor 00m 4 Km Long-range outdoor 5Km 0 Km 6: Wireless and Mobile Networks 6-7 6: Wireless and Mobile Networks 6-8 Elements of a wireless Wireless taxonomy mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a : route among themselves (e.g., s) no single hop host connects to base station (, WiMX, cellular) which connects to larger Internet no base station, no connection to larger Internet (luetooth, 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 MNET, VNET 6: Wireless and Mobile Networks 6-9 6: Wireless and Mobile Networks 60 Wireless Link Characteristics () Wireless Link Characteristics () Differences from wired link. decreased signal : radio signal as it propagates through matter (path loss) interference from other sources: standardized wireless frequencies (e.g.,.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times. make communication across (even a point to point) wireless link much more difficult 6: Wireless and Mobile Networks 6 SNR: signal-to-noise ratio larger SNR easier to extract signal from noise (a good thing ) SNR versus ER tradeoffs given physical layer: increase power -> increase SNR->decrease ER given SNR: choose physical layer that meets ER requirement, giving highest thruput SNR may change with mobility: adapt physical layer (modulation technique, rate) ER 0 0-0 -3 0-4 0-5 0-6 0-7 0 0 30 40 SNR(d) QM56 (8 Mbps) QM6 (4 Mbps) PSK ( Mbps) 6: Wireless and Mobile Networks 6
Wireless characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): terminal problem, hear each other, C hear each other, C can not hear each other means, C unaware of their interference at C C s signal space C s signal Signal attenuation:, hear each other, C hear each other, C can not hear each other interfering at 6: Wireless and Mobile Networks 63 Code Division Multiple ccess (CDM) used in several wireless broadcast s (cellular, satellite, etc) standards unique assigned to each user; i.e., set partitioning all users share same frequency, but each user has own chipping sequence (i.e., ) to en end signal = (original ) X (chipping sequence) decoding: inner-product of end signal and chipping sequence allows multiple users to coexist and transmit simultaneously with minimal interference (if s are ) 6: Wireless and Mobile Networks 64 CDM En/De CDM: two-sender interference sender bits d = - - - d 0 = slot slot 0 Z i,m = d i. cm Z i,m - slot slot 0 received input receiver - - - - - - - slot slot 0 M D i = Σ Z. i,m cm m= M d = slot d 0 = slot 0 6: Wireless and Mobile Networks 65 6: Wireless and Mobile Networks 66 Chapter 6 outline IEEE 80. Wireless LN 6. Introduction Wireless 6. Wireless links, characteristics CDM 6.3 IEEE 80. wireless LNs ( wi-fi ) 6.4 cellular Internet access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular s 6.8 Mobility and higherlayer protocols 6.9 Summary 6: Wireless and Mobile Networks 67 80.b.4-5 GHz unlicensed spectrum up to Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping 80.a 5-6 GHz range up to 54 Mbps 80.g.4-5 GHz range up to 54 Mbps 80.n: multiple antennae.4-5 GHz range up to 00 Mbps all use for multiple access all have base-station and ad-hoc versions 6: Wireless and Mobile Networks 68 3
80. LN architecture 80.: Channels, association SS Internet hub, switch or router SS wireless host communicates with base station base station = access point () Service Set (SS) (aka cell ) in mode contains: wireless hosts access point (): base station ad hoc mode: hosts only 6: Wireless and Mobile Networks 69 80.b:.4GHz-.485GHz spectrum divided into s at different frequencies admin chooses frequency for interference possible: can be same as that chosen by neighboring! host: must associate with an scans s, listening for beacon frames containing s name ( ) and MC address selects to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in s subnet 6: Wireless and Mobile Networks 6-0 80.: passive/active scanning S H S 3 Passive Scanning: () frames sent from s () association Request frame sent: H to selected (3) association Response frame sent: H to selected S H S 3 4 ctive Scanning: () Probe Request frame broadcast from H () Probes response frame sent from s (3) ssociation Request frame sent: H to selected (4) ssociation Response frame sent: H to selected 6: Wireless and Mobile Networks 6- IEEE 80.: multiple access avoid collisions: + nodes transmitting at same time 80.: CSM - sense before transmitting don t collide with ongoing transmission by other node 80.: 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: CSM/C(ollision) C C s signal C s signal space 6: Wireless and Mobile Networks 6- IEEE 80. MC Protocol: CSM/C voiding collisions (more) 80. sender if sense idle for DIFS then transmit entire frame (no CD) if sense busy then start random time timer counts down while idle transmit when timer expires if no CK, increase random backoff interval, repeat 80. receiver - if frame received OK return CK after SIFS (CK needed due to hidden terminal problem) DIFS sender CK receiver SIFS idea: allow sender to reserve rather than random access of frames: avoid collisions of long frames sender first transmits small request-to-send (RTS) packets to S using CSM RTSs may still collide with each other (but they re short) S broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits frame other stations transmissions avoid frame collisions completely using small reservation packets! 6: Wireless and Mobile Networks 6-3 6: Wireless and Mobile Networks 6-4 4
Collision voidance: RTS-CTS exchange 80. frame: addressing reservation collision 6 6 6 6 0-3 4 frame duration address address address seq address payload CRC control 3 control 4 time DT () defer ddress : MC address of wireless host or to receive this frame ddress : MC address of wireless host or transmitting this frame ddress 3: MC address of router interface to which is attached ddress 4: used only in mode 6: Wireless and Mobile Networks 6-5 6: Wireless and Mobile Networks 6-6 80. frame: addressing 80. frame: more duration of reserved transmission time frame seq # (for RDT) H R router Internet 6 6 6 6 0-3 4 frame duration address address address seq address payload CRC control 3 control 4 R MC addr H MC addr dest. address source address 80.3 frame MC addr H MC addr R MC addr address address address 3 80. frame 6: Wireless and Mobile Networks 6-7 Protocol version 4 Type Subtype To From frame type (RTS, CTS, CK, ) More frag Power Retry mgt More WEP Rsvd 6: Wireless and Mobile Networks 6-8 80.: mobility within same subnet H remains in same IP subnet: IP address can remain same switch: which is associated with H? (Ch. 5): switch will see frame from H and remember which switch port can be used to reach H S router hub or switch H S 6: Wireless and Mobile Networks 6-9 5