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Ch. 6: Wireless and Mobile Networks

Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to-1)! # wireless Internet-connected devices equals # wireline Internet-connected devices laptops, Internet-enabled phones promise anytime untethered Internet access

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Ch. 6: Wireless and Mobile Networks

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  1. Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to-1)! # 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 Ch. 6: Wireless and Mobile Networks Wireless, Mobile Networks

  2. 6.1 Introduction Wireless 6.2Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4Cellular Internet Access Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9Summary Chapter 6 outline Wireless, Mobile Networks

  3. network infrastructure Elements of a wireless network cell tower in cellular network (e.g. 3G or 4G) access point in 802.11 wireless LAN Wireless, Mobile Networks

  4. network infrastructure Elements of a wireless network wireless hosts • laptop, smartphone • run applications • may be stationary (non-mobile) or mobile • wireless does not always mean mobility Wireless, Mobile Networks

  5. network infrastructure Elements of a wireless network base station • typically connected to wired network • responsible for sending packets between wired network and wireless host(s) in its “area” • e.g., cell towers, 802.11 access points Wireless, Mobile Networks

  6. network infrastructure Elements of a wireless network wireless link • typically used to connect mobile(s) to base station • multiple access protocol coordinates link access • various data rates, transmission distance Wireless, Mobile Networks

  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 Data rate (Mbps) 1 802.15 .384 2.5G: UMTS/WCDMA, CDMA2000 .056 2G: IS-95, CDMA, GSM Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km Wireless, Mobile Networks

  8. hosts “operate in infrastructure mode” • all services via base station (connects mobiles into wired network) • handoff: mobile changes base station providing connection into wired network network infrastructure Elements of a wireless network Wireless, Mobile Networks

  9. 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, Mobile Networks

  10. Wireless network taxonomy multiple hops single hop host may have to relay through several wireless nodes to connect to larger Internet: mesh net host connects to base station (WiFi, cellular) which connects to larger Internet infrastructure (e.g., APs) no base station, no connection to larger Internet. May have to relay to reach other wireless node MANET(mobile ad hoc netw) no infrastructure no base station, no connection to larger Internet (e.g. Bluetooth) Wireless, Mobile Networks

  11. 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4Cellular Internet Access Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9Summary Chapter 6 outline Wireless, Mobile Networks

  12. Wireless Link Characteristics (1) important differences from wired link …. • decreased signal strength: radio signal strength decreases 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 (e.g. microwave) interfere as well • multipath propagation: radiosignals reaching the receiving antenna by two or more paths because radio signal reflects off objects and the ground …. make communication across (even a point to point) wireless link much more “difficult” => powerful CRC error detection & reliable data transfer protocols Wireless, Mobile Networks

  13. Wireless Link Characteristics (2) 10-1 • SNR: signal-to-noise ratio • measures signal strength relative to noise => larger SNR gives easier to extract signal from background noise • three different modulation schemes: • amplitude modulation of a signal: change the amplitude of periodic waveform to encode information (QAM = Quadrature amplitude modulation) • SNR versus Bit-Error-Rate (BER) • for given modulation scheme: increase transmission power -> increase SNR->decrease BER but: little practical gain beyond certain threshold, stronger signal costs more energy (bad for battery-powered users) 10-2 10-3 10-4 BER 10-5 10-6 10-7 10 20 30 40 SNR(decibels) QAM256 (8) QAM16 (4) BPSK (1) Wireless, Mobile Networks

  14. Wireless Link Characteristics (2) 10-1 • SNR versus Bit-Error-Rate (BER) • for given SNR: choose modulation scheme that meets BER requirement and gives highest throughput • SNR may change with mobility: dynamically adapt modulation technique and rate to keep BER low! (e.g. with SNR of 10 dB we have to choose BPSK to avoid high BER) • when sending at high bit rate, differences in amplitude are low => errors while we sample the signal 10-2 10-3 10-4 BER 10-5 10-6 10-7 10 20 30 40 SNR(decibels) QAM256 (8) QAM16 (4) BPSK (1) Wireless, Mobile Networks

  15. Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): B A C C C’s signal strength A’s signal strength B A space Hidden terminal problem • B, A hear each other • B, C hear each other • A, C can not hear each other => A, C unaware of their interference at B Fading problem: • B, A hear each other • B, C hear each other • A, C can not hear each other as signal strength decreases => interference at B Wireless, Mobile Networks

  16. Code Division Multiple Access (CDMA) • uses: 3G mobile networks (UMTS, etc.) • channel partitioning protocol • unique “code” assigned to each user • all users share same frequency, but each user uses his own code to encode data => allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) • encoded signal= (original data) x (code) • decoding:sum of product of encoded signal and code • divide time slot for transmitting a bit into many “mini slots” • view “0” are “-1” Wireless, Mobile Networks

  17. d0 = 1 1 1 1 1 1 1 d1 = -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M Di = SZi,m.cm m=1 M d0 = 1 d1 = -1 CDMA encode/decode channel output Zi,m Zi,m= di.cm data bits sender slot 0 channel output slot 1 channel output code slot 1 slot 0 8x1/8 -8x1/8 received input slot 0 channel output slot 1 channel output code receiver slot 1 slot 0 Wireless, Mobile Networks

  18. CDMA: two-sender interference channel sums together transmissions by sender 1 and 2 Sender 1 Sender 2 (-2-2-2-2)/8 (2+2+2+2)/8 using same code as sender 1, receiver recovers sender 1’s original data from summed channel data! Wireless, Mobile Networks

  19. CDMA: Code Division MA • Drawbacks: • Synchronization of all users is required • Multipath propagation: delayed copies of signal may be received which are not orthogonal any longer! • Near-far problem: nearby users will completely swamp far away users (signal of users far away cannot be received correctly) => accurate power control needed • having J more codes, the bandwidth must J times larger • Main advantage: • - when using FDMA or TDMA for cellular systems, frequencies used in a cell cannot be reused in adjacent cells (to avoid interference); in CDMA one can reuse the frequencies • more details: • Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison Wesley, 1995. Wireless, Mobile Networks

  20. 6.1 Introduction Wireless 6.2Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4Cellular Internet Access Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7Summary Chapter 6 outline Wireless, Mobile Networks

  21. IEEE 802.11 Wireless LAN • different standards (802.11a/b/g/n) • differ in modulation techniques used (=> different maximal transmission rate) and in frequency range (2.4 or 5 GHz) • all use CSMA/CA (carrier sense multiple access with collision avoidance) • all have base-station and ad-hoc network versions Wireless, Mobile Networks

  22. Internet 802.11 LAN architecture • wireless host communicates with base station • base station = access point (AP) • Basic Service Set (BSS) contains: • wireless hosts • access point (AP): base station • ad hoc mode: hosts only hub, switch or router BSS 1 BSS 2 Wireless, Mobile Networks

  23. 802.11: Channels, association • 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different (partly overlapping) frequencies • admin chooses frequency for AP • interference possible: channels can be same or overlapping • host: must associatewith an AP • scans channels, listening for beacon frames (beacon = “Blinklicht”) containing AP’s name (service set identifier = SSID) and MAC address • selects AP to associate with • may perform authentication • will typically run DHCP to get IP address in AP’s subnet Wireless, Mobile Networks

  24. 3 2 4 2 3 2 1 1 1 802.11: passive/active scanning BSS 1 BSS 1 BSS 2 BSS 2 AP 2 AP 2 AP 1 AP 1 H1 H1 • active scanning: • Probe Request frame broadcast from H1 • Probe Response frames sent from APs • Association Request frame sent: H1 to selected AP • Association Response frame sent from selected AP to H1 • passive scanning: • beacon frames sent from APs • association Request frame sent: H1 to selected AP • association Response frame sent from selected AP to H1 Wireless, Mobile Networks

  25. IEEE 802.11: multiple access • random access protocol as for Ethernet (carrier sense MAC) • sense before transmitting • refrain from transmitting when channel is sensed busy • no collision detection during transmission! • difficult to receive (sense collisions) when transmitting due to weak received signals that gets overwhelmed by sending signal • can’t sense all collisions in any case: hidden terminal, fading • => once transmission is running, it is never aborted (as opposed to Ethernet when collision is detected) • optional feature: try to avoid collisions when several nodes are transmitting at the same time • goal: avoid collisions: CSMA/C(ollision)A(voidance) Wireless, Mobile Networks

  26. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA • SIFS= Short Inter-frame Spacing • Time required for a receiving station to sense the end of a frame and start transmitting an ACK (compute CRC etc.) • DIFS= Distributed Inter-frame Spacing • sender waits before transmission • If the medium is continuously idle for this duration, only then a node is supposed to transmit a frame. • DIFS = SIFS + (2 * Slot time) where slot time is 20μs in 802.11b sender receiver Wireless, Mobile Networks

  27. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idlefor time period DIFSthentransmit entire frame (no CD) 2 if sense channel busy then • start random backofftime • timer counts down while channel idle • transmit when timer expires • if no ACK, increase random backoffinterval (binary exponential), repeat step 2 802.11 receiver - if frame received OK return ACK after time period SIFS (ACK needed due to hidden terminal problem) sender receiver Wireless, Mobile Networks

  28. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA • Why not send directly when channel is sensed idle? Assume three senders, one is transmitting and finishes => other senders both start sending => collision • With random backoff time, one of the senders might send directly while the other waits • When do collisions occur: same backoff time chosen or senders are hidden from each other (can’t sense whether channel idle) sender receiver Wireless, Mobile Networks

  29. decrement by one slot time Wireless, Mobile Networks

  30. IEEE 802.11 MAC Protocol: CSMA source: IEEE Std 802.11TM-2012 (Revision of IEEE Std 802.11-2007) Wireless, Mobile Networks

  31. Optional avoidance mechanism 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, Mobile Networks

  32. RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) Collision Avoidance: RTS-CTS exchange B A AP defer time Wireless, Mobile Networks

  33. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 802.11 frame: four address fields! bytes Address 4: used only in ad hoc mode Address 1: MAC address of wireless host or AP to receive this frame (destination within subnet) Address 3: MAC address of router interface to which AP is attached (=> send to Internet; see example on next slide) Address 2: MAC address of wireless host or AP transmitting this frame (sender: wireless host or AP) Wireless, Mobile Networks

  34. router Internet H1 MAC addr R1 MAC addr source address dest. address 802.3Ethernetframe H1 MAC addr AP MAC addr R1 MAC addr address 3 address 2 address 1 802.11frame 802.11 frame: addressing H1 R1 from the router’s perspective, the AP is invisible (ethernet subnet!) now, H1 knows MAC adress of source router Wireless, Mobile Networks

  35. router Internet R1 MAC addr H1 MAC addr source address dest. address 802.3Ethernetframe AP MAC addr H1 MAC addr R1 MAC addr address 3 address 2 address 1 802.11frame 802.11 frame: addressing H1 R1 address 3 allows the AP to determine correct destination MAC address Wireless, Mobile Networks

  36. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To AP From AP More frag Retry Power mgt More data WEP Rsvd 802.11 frame: more frame seq # (since frames may be retransmitted if not ACKed) expected duration of planned transmission (including ACKs) when RTS, CTS or data is sent frame type (RTS, CTS, ACK, data) frame sent by access point detailed specification: http://standards.ieee.org/about/get/802/802.11.html

  37. How does switch know 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 802.11: mobility within same subnet • H1 detects weakening signal from AP1 • scans and finds beacon frames of AP2 • dissociates with AP1, associates with AP2 • remains in same IP subnet: keep IP address and all ongoing TCP connections BSS 2 H1 BSS 1 • better solutions are currently being developed (so that switch is informed when H1 disassociates/associates) Wireless, Mobile Networks

  38. Rate adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies 2 unack’d frames in a row => next lower modul. scheme 10 frames ack’d in a row => next higher modul. scheme 802.11: advanced capabilities 10-1 10-2 QAM256 10-3 QAM16 BPSK BER 10-4 operating point 10-5 10-6 10-7 10 20 30 40 SNR(dB) 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, Mobile Networks

  39. 802.11: advanced capabilities power management • node-to-AP: “I am going to sleep until next beacon frame” (power manag. bit in header) • AP knows not to transmit frames to this node (buffers frames!) • node wakes up before next beacon frame (every 100 msec) • beacon frame: contains list of mobiles with buffered AP-to-mobile frames • node will stay awake if AP-to-mobile frames to be sent (=> has to send request to AP) • otherwise sleep again until next beacon frame Wireless, Mobile Networks

  40. 6.1 Introduction Wireless 6.2Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4 Cellular Internet access Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7Summary Chapter 6 outline Wireless, Mobile Networks

  41. connects cells to wired tel. net. • manages call setup (more later!) • handles mobility (more later!) Mobile Switching Center Mobile Switching Center • covers geographical region • base station(BS) analogous to 802.11 AP • mobile usersattach to network through BS • air-interface:physical and link layer protocol between mobile and BS Public telephone network MSC cell wired network Components of cellular network architecture Wireless, Mobile Networks

  42. time slots frequency bands Cellular networks: the first hop Two techniques for sharing mobile-to-BS radio spectrum (1) combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots (used in 2G systems) (Having F different sub-bands and T time slots, how many simultaneous calls can be supported?) (2) CDMA: code division multiple access (used in 3G UMTS) Wireless, Mobile Networks

  43. 6.1 Introduction Wireless 6.2Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4 Cellular Internet Access Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7Summary Chapter 6 outline Wireless, Mobile Networks

  44. no mobility high mobility Various degrees of mobility? • spectrum of mobility, from thenetworkperspective: mobile wireless user, using same access point mobile user, passing through multiple access point while maintaining ongoing connections (e.g. cellular access in an ICE) mobile user, connecting/ disconnecting from network using DHCP (e.g. moving to different room). Wireless, Mobile Networks

  45. Mobility: vocabulary home network:permanent “home” of mobile (e.g., 128.119.40/24) home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote wide area network permanent address:address in home network, can always be used to reach mobile e.g., 128.119.40.186 Wireless, Mobile Networks

  46. Mobility: more vocabulary visited network:network in which mobile currently resides (e.g., 79.129.13/24) permanent address:remains constant (e.g., 128.119.40.186) care-of-address:address in visited network. (e.g., 79,129.13.2) wide area network foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile Wireless, Mobile Networks

  47. How do you contact a mobile friend: Consider friend frequently changing addresses, how do you find her? • search all phone books? • call her parents? • expect her to let you know where he/she is? I wonder where Alice moved to? Wireless, Mobile Networks

  48. Mobility: approaches search all phone books • let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. • routing tables indicate where each mobile located • no changes to end-systems • let end-systems handle it: • indirect routing:communication from correspondent to mobile goes through home agent, then forwarded to remote • direct routing:correspondent gets foreign address of mobile, sends directly to mobile call her parents expect her to let you know Wireless, Mobile Networks

  49. Mobility: approaches • let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. • routing tables indicate where each mobile located • no changes to end-systems • let end-systems handle it: • indirect routing:communication from correspondent to mobile goes through home agent, then forwarded to remote • direct routing: correspondent gets foreign address of mobile, sends directly to mobile not scalable to millions of mobiles Wireless, Mobile Networks

  50. mobile contacts foreign agent on entering visited network foreign agent contacts home agent: “this mobile is resident in my network” 1 2 Mobility: registration visited network home network end result: • foreign agent knows about mobile • home agent knows location of mobile wide area network Wireless, Mobile Networks

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