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1DT066 Distributed Information Systems Chapter 6 Wireless, WiFi and mobility

1DT066 Distributed Information Systems Chapter 6 Wireless, WiFi and mobility. network infrastructure. Elements of a wireless network. wireless hosts laptop, smartphone run applications may be stationary (non-mobile) or mobile May roam or migrate. pp 539-544. network

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1DT066 Distributed Information Systems Chapter 6 Wireless, WiFi and mobility

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  1. 1DT066Distributed Information SystemsChapter 6 Wireless, WiFi and mobility

  2. network infrastructure Elements of a wireless network wireless hosts • laptop, smartphone • run applications • may be stationary (non-mobile) or mobile • May roam or migrate Wireless, Mobile Networks pp 539-544

  3. network infrastructure Elements of a wireless network base station • connected to wired network • relaying- responsible for sending packets between wired network and wireless host(s) in its “radio area”(blue circle) • e.g., 3G/cell towers, 802.11 access points Wireless, Mobile Networks pp 539-544

  4. infrastructure mode • base station connects mobiles into wired network • handoff: mobile changes base station network infrastructure Infrastructure mode Wireless, Mobile Networks pp 539-544

  5. Ad hoc mode ad hoc mode • No (wired) base stations • nodes can only transmit to other nodes within radio reach • nodes organize themselves into a network: route only among themselves Wireless, Mobile Networks pp 539-544

  6. Wireless Link Characteristics important differences from wired link …. • decreased signal strength: radio signal attenuatesas it propagates through matter (path loss) • interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) are shared by other devices (e.g., microwave oven). Electrical devices, such as electrical motors, interfere as well. • multipath propagation: radio signal reflects off objects, the ground, atmosphere, etc. Reflections arrive at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” compared to a wire. Wireless, Mobile Networks pp 545

  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 pp 543

  8. 802.11b 2.4 GHz unlicensed spectrum up to 11 Mbps 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps 802.11n: multiple antenna 2.4-5 GHz range up to 200 Mbps IEEE 802.11 Wireless LAN • all use the radio sharing protocol CSMA/CA for multiple access • all have base-station and an ad-hoc network configuration mode Wireless, Mobile Networks pp 552

  9. 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 hub, switch or router BSS 1 BSS 2 Wireless, Mobile Networks pp 553-554

  10. 802.11: Host association to AP • host: must associatewith an Access Point • Host scans radio channels, listening for beacon frames containing AP’s name (SSID) and MAC address • Host selects (the best) AP to associate with • may perform authentication to get access • will typically run DHCP to get IP address in AP’s subnet Wireless, Mobile Networks pp 555-557

  11. IEEE 802.11: Sharing the radio channel • Many nodes can independently chose to send at the same time • 802.11: Carrier Sense Multiple Access – host senses (listen) radio channel if busy before transmitting • Don’t transmit and collide with ongoing transmission by other node • 802.11: difficult to detect a collision! • difficult to receive (sense collisions) when transmitting due to weak received signals • can’t sense all collisions in any case: hidden terminal, • goal: avoid collisions: CSMA/C(ollision)A(voidance) C B A C C’s signal strength B A’s signal strength A space Wireless, Mobile Networks pp 557-560

  12. 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 Draw this figure! pp 557-560 Wireless, Mobile Networks

  13. no mobility Roaming - high mobility What is mobility? • spectrum of mobility, from thenetworkperspective: mobile wireless user, using same access point mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) mobile user, connecting/ disconnecting from network using DHCP. Wireless, Mobile Networks pp 581

  14. 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 pp 582

  15. 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 pp 582

  16. mobile contacts foreign agent on entering visited network foreign agent contacts home agent home: “this mobile is resident in my network” 1 2 Mobility: registration visited network home network end result: • foreign agent knows about visiting mobile • home agent knows locationof mobile wide area network Wireless, Mobile Networks pp 583-588

  17. foreign agent receives packets, forwards to mobile home agent intercepts packets, forwards to foreign agent correspondent addresses packets using home address of mobile mobile replies directly to correspondent 3 2 4 1 Mobility via indirect routing visited network home network wide area network Wireless, Mobile Networks pp 583-588

  18. Indirect Routing: comments • mobile uses two addresses: • permanent address: used by correspondent (hence mobile location is transparentto correspondent) • care-of-address: used by home agent to forward datagrams to mobile • triangle routing:correspondent2home2network2mobile • inefficient when Correspondent and Mobile are in same network. Wireless, Mobile Networks pp 583-588

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