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Chapter 6 Wireless and Mobile Networks. Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!

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Chapter 6 Wireless and Mobile Networks


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    1. Chapter 6Wireless and Mobile Networks Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

    2. Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! # wireless Internet-connected devices soon to exceed # 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 Chapter 6: Wireless and Mobile Networks Wireless, Mobile Networks

    3. The Wireless Spectrum

    4. The Wireless Spectrum • Continuum of electromagnetic waves • Data, voice communication • Arranged by frequencies • Lowest to highest • Spans 9 KHz and 300 GHz • Wireless services associated with one area • FCC oversees United States frequencies • ITU oversees international frequencies • Air signals propagate across borders

    5. The wireless spectrum The Wireless Spectrum (cont’d.)

    6. Characteristics of Wireless Transmission

    7. Characteristics of Wireless Transmission • Similarities with wired • Layer 3 and higher protocols • Signal origination • From electrical current, travel along conductor • Differences from wired • Signal transmission • No fixed path, guidance • Antenna • Signal transmission and reception • Same frequency required on each antenna • Share same channel

    8. Wireless transmission and reception Characteristics of Wireless Transmission (cont’d.)

    9. Antennas • Radiation pattern • Relative strength over three-dimensional area • All electromagnetic energy antenna sends, receives • Directional antenna • Issues wireless signals along single direction • Omnidirectional antenna • Issues, receives wireless signals • Equal strength, clarity • All directions • Range • Reachable geographical area

    10. Signal Propagation • LOS (line-of-sight) • Signal travels • In straight line, directly from transmitter to receiver • Obstacles affect signal travel • Pass through them • Absorb into them • Subject signal to three phenomena • Reflection: bounce back to source • Diffraction: splits into secondary waves • Scattering: diffusion in multiple different directions

    11. Signal Propagation (cont’d.) • Multipath signals • Wireless signals follow different paths to destination • Caused by reflection, diffraction, scattering • Advantage • Better chance of reaching destination • Disadvantage • Signal delay

    12. Multipath signal propagation

    13. Signal Degradation • Fading • Change in signal strength • Electromagnetic energy scattered, reflected, diffracted • Attenuation • Signal weakens • Moving away from transmission antenna • Correcting signal attenuation • Amplify (analog), repeat (digital) • Noise • Usually the worst problem • No wireless conduit, shielding

    14. Frequency Ranges • 2.4-GHz band (older) • Frequency range: 2.4–2.4835 GHz • 11 unlicensed communications channels • Susceptible to interference • Unlicensed • No FCC registration required • 5-GHz band (newer) • Frequency bands • 5.1 GHz, 5.3 GHz, 5.4 GHz, 5.8 GHz • 24 unlicensed bands, each 20 MHz wide • Used by weather, military radar communications

    15. Narrowband, Broadband, and Spread Spectrum Signals • Defines wireless spectrum use: • Narrowband • Transmitter concentrates signal energy at single frequency, very small frequency range • Broadband • Relatively wide wireless spectrum band • Higher throughputs than narrowband • Spread-spectrum • Multiple frequencies used to transmit signal • Offers security

    16. Narrowband, Broadband, and Spread Spectrum Signals • FHSS (frequency hopping spread spectrum) • Signal jumps between several different frequencies within band • Synchronization pattern known only to channel’s receiver, transmitter • DSSS (direct-sequence spread spectrum) • Signal’s bits distributed over entire frequency band at once • Each bit coded • Receiver reassembles original signal upon receiving bits

    17. Fixed versus Mobile • Fixed communications wireless systems • Transmitter, receiver locations do not move • Transmitting antenna focuses energy directly toward receiving antenna • Point-to-point link results • Advantage • No wasted energy issuing signals • More energy used for signal itself • Mobile communications wireless systems • Receiver located anywhere within transmitter’s range • Receiver can roam

    18. 6.1 Introduction Wireless 6.2Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4Cellular 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 networks 6.8 Mobility and higher-layer protocols 6.9Summary Chapter 6 outline Wireless, Mobile Networks

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

    20. base station • 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 network infrastructure Elements of a wireless network Wireless, Mobile Networks

    21. network infrastructure Elements of a wireless network wireless link • 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, Mobile Networks

    22. Characteristics of selected wireless link standards 200 802.11n 54 802.11a,g 802.11a,g point-to-point data 5-11 802.11b 802.16 (WiMAX) 3G cellular enhanced 4 UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO Data rate (Mbps) 1 802.15 .384 3G UMTS/WCDMA, CDMA2000 2G .056 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

    23. infrastructure mode • 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

    24. 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

    25. 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, WiMAX, 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 a given wireless node MANET, VANET no infrastructure no base station, no connection to larger Internet (Bluetooth, ad hoc nets) Wireless, Mobile Networks

    26. Wireless Link Characteristics (1) 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 point) wireless link much more “difficult” Wireless, Mobile Networks

    27. Wireless Link Characteristics (2) 10-1 • SNR: signal-to-noise ratio • larger SNR – easier to extract signal from noise (a “good thing”) • 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 (modulation technique, rate) 10-2 10-3 10-4 BER 10-5 10-6 10-7 10 20 30 40 SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) Wireless, Mobile Networks

    28. B A C C C’s signal strength A’s signal strength B A space Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): 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 Signal attenuation: • B, A hear each other • B, C hear each other • A, C can not hear each other interfering at B Wireless, Mobile Networks

    29. Code Division Multiple Access (CDMA) • used in several wireless broadcast channels (cellular, satellite, etc) standards • 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 • encoded signal = (original data) X (chipping sequence) • decoding: inner-product of encoded signal and chipping sequence • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) Wireless, Mobile Networks

    30. 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 received input slot 0 channel output slot 1 channel output code receiver slot 1 slot 0 Wireless, Mobile Networks

    31. CDMA: two-sender interference Wireless, Mobile Networks

    32. 6.1 Introduction Wireless 6.2Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4Cellular 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 networks 6.8 Mobility and higher-layer protocols 6.9Summary Chapter 6 outline Wireless, Mobile Networks

    33. 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 IEEE 802.11 Wireless LAN • all use CSMA/CA for multiple access • all have base-station and ad-hoc network versions Wireless, Mobile Networks

    34. AP AP Internet 802.11 LAN architecture • 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 hub, switch or router BSS 1 BSS 2 Wireless, Mobile Networks

    35. 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

    36. 4 2 2 2 3 3 1 1 1 802.11: passive/active scanning BBS 1 BBS 1 BBS 2 BBS 2 AP 1 AP 1 AP 2 AP 2 H1 H1 • Active Scanning: • Probe Request frame broadcast from H1 • Probes response frame sent from APs • Association Request frame sent: H1 to selected AP • Association Response frame sent: H1 to selected AP • Passive Scanning: • beacon frames sent from APs • association Request frame sent: H1 to selected AP • association Response frame sent: H1 to selected AP Wireless, Mobile Networks

    37. 802.11 WLANs • Wireless technology standard • Describes unique functions • Physical and Data Link layers • Differences • Specified signaling methods, geographic ranges, frequency usages • Developed by IEEE’s 802.11 committee • Wi-Fi (wireless fidelity) standards • 802.11b, 802.11a, 802.11g, 802.11n (draft) • Share characteristics • Half-duplexing, access method, frame format

    38. Access Method • 802.11 MAC services • Append 48-bit (6-byte) physical addresses to frame • Identifies source, destination • Same physical addressing scheme as 802.3 • Allows easy combination • Wireless devices • Not designed for simultaneous transmit, receive • Cannot quickly detect collisions • Use different access method

    39. Access Method (cont’d.) • CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) • Minimizes collision potential • Uses ACK packets to verify every transmission • Requires more overhead than 802.3 • Real throughput less than theoretical maximum • RTS/CTS (Request to Send/Clear to Send) protocol • Optional • Ensure packets not inhibited by other transmissions • Efficient for large transmission packets • Further decreases overall 802.11 efficiency

    40. Association • Several packet exchanged between computer, access point • Gain Internet access • Scanning • Surveying surroundings for access point • Active scanning transmits special frame • Probe • Passive scanning listens for special signal • Beacon frame

    41. Association (cont’d.) • SSID (service set identifier) • As shown, names like NETGEAR or 2WIRE619 • Unique character string identifying access point • In beacon fame information • Configured in access point • Better security, easier network management

    42. Association (cont’d.) BSS (basic service set) Station groups sharing Access Point BSSID (basic service set identifier) Station group identifier The MAC address of the Access Point

    43. Association (cont’d.) • ESS (extended service set) • Access point group connecting same LAN • Share ESSID (extended service set identifier) • Allows roaming • Station moving from one BSS to another without losing connectivity • Several access points detected • Select strongest signal, lowest error rate • Poses security risk • Powerful, rogue access point can perform a man-in-the-middle attack

    44. A network with a single BSS

    45. A network with multiple BSSs forming an ESS

    46. Association (cont’d.) • ESS with several authorized access points • Must allow station association with any access point • While maintaining network connectivity • Reassociation • Mobile user moves from one access point’s range into another’s range • Occurs by simply moving, high error rate • Stations’ scanning feature • Used to automatically balance transmission loads • Between access points

    47. B A C C C’s signal strength A’s signal strength B A space 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) Wireless, Mobile Networks

    48. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFSthen 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 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver Wireless, Mobile Networks

    49. 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, Mobile Networks