Wireless and Mobile Networks. Outline. What is Wireless Networking?

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1 Wireless and Mobile Networks Instructor: Anirban Mahanti Office: ICT Class Location: ICT 121 Lectures: MWF 12:00 12:50 hours Notes derived from Computer Networking: A Top Down Approach Featuring the Internet, 2005, 3 rd edition, Jim Kurose, Keith Ross, Addison-Wesley. Slides are adapted from the companion web site of the book, as modified by Anirban Mahanti (and Carey Williamson). CPSC 441: Wireless 1 Outline Introduction Standards and Link Characteristics IEEE Wireless LANS Mobility Wireless/Mobility Performance Issues Summary CPSC 441: Wireless 2 What is Wireless Networking? The use of infra-red (IR) or radio frequency (RF) signals to share information and resources between devices Promises anytime, anywhere connectivity laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access Two important (but different) challenges communication over wireless link handling mobile user who changes point of attachment to Buzz words! Mobile Internet, Pervasive Computing, Nomadic Computing, M-Commerce, Ubiquitous Computing CPSC 441: Wireless 3 1

2 Wireless Networking Technologies Mobile devices laptop, PDA, cellular phone, wearable computer, Operating modes Infrastructure mode (Access Point) Ad hoc mode Access technology Bluetooth (1 Mbps, up to 3 meters) IEEE (up to 55 Mbps, meters) CPSC 441: Wireless 4 Infrastructure Mode infrastructure infrastructure mode base station connects mobiles into wired handoff: mobile changes base station providing connection into wired CPSC 441: Wireless 5 Ad hoc Mode Ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a : route among themselves CPSC 441: Wireless 6 2

3 Outline Introduction Standards and Link Characteristics IEEE Wireless LANS Mobility Wireless/Mobility Performance Issues Summary CPSC 441: Wireless 7 Wireless link standards 54 Mbps 5-11 Mbps 1 Mbps {a,g} b.11 p-to-p link 384 Kbps 56 Kbps UMTS/WCDMA, CDMA2000 IS-95 CDMA, GSM 3G 2G Indoor Outdoor Mid range outdoor Long range outdoor 10 30m m 200m 4Km 5Km 20Km CPSC 441: Wireless 8 Two Popular 2.4 GHz Standards: IEEE Fast (11b) High Power Long range Single-purpose Ethernet replacement Easily Available Apple Airport, ibook, G4 Cisco Aironet 350 Bluetooth Slow Low Power Short range Flexible Cable replacement CPSC 441: Wireless 9 3

4 Wireless Link Characteristics Differences from wired link. Decreasing signal strength: radio signal attenuates as it propagates through matter (path loss) Interference from other sources: standardized wireless frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well Multi-path 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 CPSC 441: Wireless 10 Wireless Network Characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B C A B A s signal strength C s signal strength 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 space Signal fading: B, A hear each other B, C hear each other A, C can not hear each other interfering at B CPSC 441: Wireless 11 Outline Introduction Standards and Link Characteristics IEEE Wireless LANS Mobility Wireless/Mobility Performance Issues Summary CPSC 441: Wireless 12 4

5 IEEE Organization Tree: IEEE Working Group PHYS Layer MAC Layer Infra-Red (IR) 2.4 GHz (FHSS) 2.4 GHz (DSSS) 5 GHz (OFDM) MAC IR FHSS DSSS a e 1 / 2 Mbit/s 1 / 2 Mbit/s 1 / 2 Mbit/s 6 / 12 / 24 Mbit/s MAC Enhancements Optional 9/18/36/54 Mbit/s b High Data Rate Extension Security 5.5 / 11 Mbit/s QOS g Data Rates > 20 Mbit/s CPSC 441: Wireless 13 IEEE Wireless LAN b GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations a 5-6 GHz range up to 54 Mbps g GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc versions CPSC 441: Wireless LAN architecture BSS 1 Internet hub, switch or router wireless host communicates with base station base station = access point () Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts access point (): base station ad hoc mode: hosts only BSS 2 CPSC 441: Wireless 15 5

6 Wireless Cells Channel 1 Channel 11 Channel 1 Channel 1 Channel 6 Channel has 11 channels Channels 1, 6, and 11 are non-overlapping Each coverage area is called a cell Wireless nodes can roam between cells Q. Suppose an administrator setups three b s at the same physical location. What is the maximum possible aggregate throughput of this WLAN? CPSC 441: Wireless 16 IEEE : multiple access avoid collisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting don t collide with ongoing transmission by other node : 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) C A B C A B A s signal strength C s signal strength space CPSC 441: Wireless 17 IEEE MAC Protocol: CSMA/CA sender 1 if sense channel idle for DIFS then 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 3 if no ACK, increase random backoff interval, repeat receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS sender data ACK receiver SIFS CPSC 441: Wireless 18 6

7 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 base station using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes sender transmits data frame other stations defer transmissions Avoid data frame collisions completely using small reservation packets! CPSC 441: Wireless 19 Collision Avoidance: RTS-CTS exchange A B RTS(A) RTS(B) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) CPSC 441: Wireless frame: addressing frame duration address address address seq address payload CRC control control 4 Address 1: MAC address of wireless host or to receive this frame Address 2: MAC address of wireless host or transmitting this frame Address 3: MAC address of router interface to which is attached Address 3: used only in ad hoc mode CPSC 441: Wireless 21 7

8 frame: addressing H1 R1 router Internet R1 MAC addr MAC addr dest. address source address frame MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame CPSC 441: Wireless frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for reliable ARQ) frame duration address address address seq address payload CRC control control 4 Protocol version Type Subtype To From More frag 1 1 Power Retry mgt More data WEP Rsvd frame type (RTS, CTS, ACK, data) CPSC 441: Wireless 23 Outline Introduction Standards and Link Characteristics IEEE Wireless LANS Mobility Wireless/Mobility Performance Issues Summary CPSC 441: Wireless 24 8

9 What is mobility? spectrum of mobility, from the perspective: no mobility high mobility mobile wireless user, using same access point mobile user, connecting/ disconnecting from using DHCP. mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) CPSC 441: Wireless : mobility within same subnet H1 remains in same IP subnet: IP address can remain same switch: which 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 BBS 1 1 router hub or switch 2 H1 BBS 2 CPSC 441: Wireless 26 Mobility: Vocabulary home : permanent home of mobile (e.g., /24) home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote Permanent address: address in home, can always be used to reach mobile e.g., wide area correspondent CPSC 441: Wireless 27 9

10 Mobility: more vocabulary Permanent address: remains constant (e.g., ) visited : in which mobile currently resides (e.g., /24) Care-of-address: address in visited. (e.g., 79, ) wide area correspondent: wants to communicate with mobile home agent: entity in visited that performs mobility functions on behalf of mobile. CPSC 441: Wireless 28 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? CPSC 441: Wireless 29 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 CPSC 441: Wireless 30 10

11 Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence not via usual routing table exchange. scalable to millions of routing tables indicate mobileswhere 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 CPSC 441: Wireless 31 Mobility: registration home visited 2 wide area foreign agent contacts home agent home: this mobile is resident in my 1 mobile contacts foreign agent on entering visited End result: Foreign agent knows about mobile Home agent knows location of mobile CPSC 441: Wireless 32 Mobility via Indirect Routing home correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area 2 foreign agent receives packets, forwards to mobile 4 3 visited mobile replies directly to correspondent CPSC 441: Wireless 33 11

12 Indirect Routing: comments 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 foreign agent functions may be done by mobile itself triangle routing: correspondent-home-mobile inefficient when correspondent, mobile are in same CPSC 441: Wireless 34 Indirect Routing: moving between s suppose mobile user moves to another registers with new foreign agent new foreign agent registers with home agent home agent update care-of-address for mobile packets continue to be forwarded to mobile (but with new care-of-address) mobility, changing foreign s transparent: on going connections can be maintained! CPSC 441: Wireless 35 Mobility via Direct Routing home correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile 4 visited correspondent requests, receives foreign address of mobile 2 1 wide area 3 4 mobile replies directly to correspondent CPSC 441: Wireless 36 12

13 Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to correspondent: correspondent must get care-of-address from home agent what if mobile changes visited? CPSC 441: Wireless 37 Accommodating mobility with direct routing anchor foreign agent: FA in first visited data always routed first to anchor FA when mobile moves: new FA arranges to have data forwarded from old FA (chaining) correspondent wide area 1 correspondent agent anchor foreign agent 5 4 new foreign agent 3 foreign net visited at session start 2 new foreign CPSC 441: Wireless 38 Mobile IP RFC 3220 has many features we ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) three components to standard: indirect routing of datagrams agent discovery registration with home agent CPSC 441: Wireless 39 13

14 Outline Introduction Standards and Link Characteristics IEEE Wireless LANS Mobility Wireless/Mobility Performance Issues Summary CPSC 441: Wireless 40 Wireless, mobility: impact on higher layer protocols logically, impact should be minimal best effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile but performance-wise: packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff TCP interprets loss as congestion, will decrease congestion window un-necessarily delay impairments for real-time traffic limited bandwidth of wireless links CPSC 441: Wireless 41 Summary Wireless wireless links: capacity, distance channel impairments CDMA IEEE ( wi-fi ) CSMA/CA reflects wireless channel characteristics Mobility principles: addressing, routing to mobile users home, visited s direct, indirect routing care-of-addresses Mobile IP impact on higher-layer protocols CPSC 441: Wireless 42 14

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