1 DT 066 Distributed Information Systems Chapter 6

1 DT 066 Distributed Information Systems Chapter 6 Wireless, Wi. Fi and mobility Adapted from: Computer Networking, Kurose/Ross

Elements of a wireless network wireless hosts v v v network infrastructure Adapted from: Computer Networking, Kurose/Ross pp 539 -544 v laptop, smartphone run applications may be stationary (nonmobile) or mobile May roam or migrate Wireless, Mobile Networks 6 -2

Elements of a wireless network base station v v network infrastructure Adapted from: Computer Networking, Kurose/Ross pp 539 -544 connected to wired network relaying- responsible for sending packets between wired network and wireless host(s) in its “radio area” (blue circle) § e. g. , 3 G/cell towers, 802. 11 access points Wireless, Mobile Networks 6 -3

Infrastructure mode infrastructure mode v v network infrastructure Adapted from: Computer Networking, Kurose/Ross pp 539 -544 base station connects mobiles into wired network handoff: mobile changes base station Wireless, Mobile Networks 6 -4

Ad hoc mode ad hoc mode v No (wired) base stations v nodes can only transmit to other nodes within radio reach v nodes organize themselves into a network: route only among themselves Adapted from: Computer Networking, Kurose/Ross pp 539 -544 Wireless, Mobile Networks 6 -5

Wireless Link Characteristics important 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) 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 Wireless, point. Mobile to Networks point)6 -6 wireless link much more “difficult” compared to a Adapted from: Computer Networking, Kurose/Ross pp 545

Characteristics of selected wireless links Data rate (Mbps) 200 54 5 -11 802. 11 n 802. 11 a, g point-to-point 802. 11 b 4 4 G: LTWE WIMAX 3 G: UMTS/WCDMA-HSPDA, CDMA 2000 -1 x. EVDO 1 802. 15 . 384 2. 5 G: UMTS/WCDMA, CDMA 2000 . 056 2 G: IS-95, CDMA, GSM Indoor Outdoor 10 -30 m 50 -200 m Adapted from: Computer Networking, Kurose/Ross pp 543 Mid-range outdoor Long-range outdoor 200 m – 4 Km 5 Km – 20 Km Wireless, Mobile Networks 6 -7

IEEE 802. 11 Wireless LAN 802. 11 b v 2. 4 GHz unlicensed spectrum v up to 11 Mbps 802. 11 a § 5 -6 GHz range § up to 54 Mbps v v 802. 11 g § 2. 4 -5 GHz range § up to 54 Mbps 802. 11 n: multiple antenna § 2. 4 -5 GHz range § up to 200 Mbps all use the radio sharing protocol CSMA/CA for multiple access all have base-station and an ad-hoc network configuration mode Adapted from: Computer Networking, Kurose/Ross pp 552 Wireless, Mobile Networks 6 -8

802. 11 LAN architecture v Internet wireless host communicates with base station § base station = access point (AP) hub, switch or router BSS 1 v Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: § wireless hosts § access point (AP): base station BSS 2 Adapted from: Computer Networking, Kurose/Ross pp 553 -554 Wireless, Mobile Networks 6 -9

802. 11: Host association to AP v host: must associate with 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 Adapted from: Computer Networking, Kurose/Ross pp 555 -557 Wireless, Mobile Networks 6 -10

IEEE 802. 11: Sharing the radio channel v v 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 v 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, B A C C § goal: avoid collisions: CSMA/C(ollision)A(voidance) A C’s signal strength A’s signal strength B space Adapted from: Computer Networking, Kurose/Ross pp 557 -560 Wireless, Mobile Networks 6 -11

IEEE 802. 11 MAC Protocol: CSMA/CA 802. 11 sender 1 if sense channel idle for DIFS then sender transmit entire frame (no CD) 2 if sense channel busy then DIFS start random backoff timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, Draw this figure! repeat 2 802. 11 receiver - if frame received OK receiver data SIFS ACK return ACK after SIFS (ACK needed due to hidden terminal problem) Adapted from: Computer Networking, Kurose/Ross pp 557 -560 Wireless, Mobile Networks 6 -12

What is mobility? v spectrum of mobility, from the network perspective: no mobility mobile wireless user, using same access point Roaming - high mobility mobile user, connecting/ disconnecting from network using DHCP. mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) Adapted from: Computer Networking, Kurose/Ross pp 581 Wireless, Mobile Networks 6 -13

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 Adapted from: Computer Networking, Kurose/Ross pp 582 Wireless, Mobile Networks 6 -14

Mobility: more vocabulary permanent address: remains constant (e. g. , 128. 119. 40. 186) visited network: network in which mobile currently resides (e. g. , 79. 129. 13/24) 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 Adapted from: Computer Networking, Kurose/Ross pp 582 Wireless, Mobile Networks 6 -15

Mobility: registration visited network home network 1 2 wide area network foreign agent contacts home agent home: “this mobile is resident in my network” mobile contacts foreign agent on entering visited network end result: v foreign agent knows about visiting mobile v home agent knows location of mobile Adapted from: Computer Networking, Kurose/Ross pp 583 -588 Wireless, Mobile Networks 6 -16

Mobility via indirect routing home agent intercepts packets, forwards to foreign agent receives packets, forwards to mobile home network visited network 3 1 wide area network 2 4 correspondent addresses packets using home address of mobile Adapted from: Computer Networking, Kurose/Ross mobile replies directly to correspondent pp 583 -588 Wireless, Mobile Networks 6 -17

Indirect Routing: comments v mobile uses two addresses: § permanent address: used by correspondent (hence mobile location is transparent to correspondent) § care-of-address: used by home agent to forward datagrams to mobile v triangle routing: correspondent 2 home 2 network 2 mobile § inefficient when Correspondent and Mobile are in same network. Adapted from: Computer Networking, Kurose/Ross pp 583 -588 Wireless, Mobile Networks 6 -18