Wireless and Mobile Networks Slides originally from Carey
Wireless and Mobile Networks Slides originally from Carey Williamson Notes derived from “Computer Networking: A Top Down Approach”, by Jim Kurose and Keith Ross, Addison-Wesley. Slides are adapted from the book’s companion Web site, with changes by Anirban Mahanti and Carey Williamson. CPSC 441: Wireless 1
Outline r Introduction r Standards and Link Characteristics r IEEE 802. 11 Wireless LANS r Mobility r Wireless/Mobility Performance Issues r Summary CPSC 441: Wireless 2
What is Wireless Networking? r The use of infra-red (IR) or radio frequency (RF) signals to share information and resources between devices r Promises anytime, anywhere connectivity m laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access r Two important (but different) challenges m communication over wireless link m handling mobile user who changes point of attachment to network r Lots of media buzzwords! m Mobile Internet, Pervasive Computing, Nomadic Computing, M-Commerce, Ubiquitous Computing … CPSC 441: Wireless 3
Wireless Networking Technologies r Mobile devices – laptop, PDA, cellular phone, wearable computer, … r Operating modes m Infrastructure mode (uses Access Point (AP)) m Ad hoc mode r Access technology m Bluetooth (1 Mbps, up to 3 meters) m IEEE 802. 11 (up to 54 Mbps, 20 – 100 meters) CPSC 441: Wireless 4
Infrastructure Mode network infrastructure mode r base station connects mobiles into wired network r handoff: mobile changes base station providing connection into wired network CPSC 441: Wireless 5
Ad hoc Mode Ad hoc mode r no base stations r nodes can only transmit to other nodes within link coverage r nodes organize themselves into a network: route among themselves CPSC 441: Wireless 6
Outline r Introduction r Standards and Link Characteristics r IEEE 802. 11 Wireless LANS r Mobility r Wireless/Mobility Performance Issues r Summary CPSC 441: Wireless 7
Wireless link standards 54 Mbps 5 -11 Mbps 802. 11{a, g} 802. 11 b Wi. MAX 802. 15 3 G UMTS/WCDMA, CDMA 2000 384 Kbps 2 G IS-95 CDMA, GSM 56 Kbps Indoor Outdoor Mid range outdoor Long range outdoor 10 – 30 m 50 – 200 m – 4 km 5 km – 20 km CPSC 441: Wireless 8
Two Popular 2. 4 GHz Standards: r IEEE 802. 11 m Fast (11 Mbps) m High Power m Long range m Single-purpose m Ethernet replacement m Easily Available r Bluetooth m Slow (1 Mbps) m Low Power m Short range m Flexible m Cable replacement • Apple Airport, i. Book • Cisco Aironet 350 CPSC 441: Wireless 9
Wireless Link Characteristics Differences from wired link …. m m m Decreasing 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 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 Hidden terminal problem r A and B can hear each other r B and C can hear each other r A and C can’t hear each other r thus A and C are unaware of their interference at B C C’s signal strength A’s signal strength space Signal fading: r A and B hear each other r B and C hear each other r A and C can’t hear each other interfering at B CPSC 441: Wireless 11
Outline r Introduction r Standards and Link Characteristics r IEEE 802. 11 Wireless LANS r Mobility r Wireless/Mobility Performance Issues r Summary CPSC 441: Wireless 12
IEEE 802. 11 Organization Tree: PHYS: physical FHSS: Frequency-hopping spread spectrum DSSS: Direct-sequence spread specturm OFDM: Orthogonal frequency-division multiplexing CPSC 441: Wireless 13
IEEE 802. 11 Wireless LAN r 802. 11 b m 2. 4 GHz unlicensed radio spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code m widely deployed, using base stations r 802. 11 a m 5 GHz range m up to 54 Mbps r 802. 11 g m 2. 4 -5 GHz range m up to 54 Mbps r All use CSMA/CA for multiple access r All have infrastructure and ad hoc modes CPSC 441: Wireless 14
802. 11 LAN architecture r wireless host communicates Internet AP hub, switch or router BSS 1 with base station m base station = access point (AP) r Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: m wireless hosts m access point (AP) AP m BSS 2 ad hoc mode: hosts only CPSC 441: Wireless 15
Wireless Cells Channel 11 AP AP Channel 6 Channel 1 AP AP Channel 1 AP r 802. 11 has 11 channels r Channels 1, 6, and 11 are non-overlapping r Each AP coverage area is called a “cell” r Wireless nodes can roam between cells AP Channel 6 CPSC 441: Wireless 16
IEEE 802. 11: multiple access r avoid collisions: > 1 nodes transmitting at same time r 802. 11: CSMA – (Carrier Sense Multiple Access / Collision Detection) sense before transmitting m don’t collide with ongoing transmission by other node r 802. 11: no collision detection! m half-duplex: antenna can’t receive (sense collisions) when transmitting due to weak received signals (fading) m can’t sense all collisions in any case: hidden terminal, fading m goal: avoid collisions: CSMA/CA (Collision Avoidance) A C A B B C C’s signal strength A’s signal strength space CPSC 441: Wireless 17
IEEE 802. 11 MAC Protocol: CSMA/CA If the medium is continuously idle for DCF Interframe Space (DIFS) duration then only it is allowed to transmit a frame. 802. 11 sender DIFS = SIFS + (2 * Slot time) 1 if sense channel idle for DIFS then sender transmit entire frame 2 if sense channel busy then start random backoff timer counts down while channel idle transmit when timer expires 3 if no ACK then increase random backoff interval, repeat step 2 receiver DIFS 802. 11 receiver - if frame received OK data SIFS ACK return ACK after SIFS (service model is connectionless, acked) Short Interframe Space (SIFS), is the small time interval between the data frame and its acknowledgment. SIFS are found in IEEE 802. 11 networks. CPSC 441: Wireless 18
Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames r sender first transmits small request-to-send (RTS) packets to base station using CSMA m RTS may still collide with each other (but they’re short) r BS broadcasts clear-to-send CTS to host in response to RTS r RTS heard by all nodes because of broadcast property m sender transmits (large) data frame m other stations defer transmissions until it is done Avoid data frame collisions completely using small reservation packets! CPSC 441: Wireless 19
Collision Avoidance: RTS-CTS exchange A B AP RTS(B) RTS(A) reservation collision RTS(A) CTS(A) DATA (A) time ACK(A) defer ACK(A) CPSC 441: Wireless 20
802. 11 frame: addressing 2 2 6 6 6 frame address duration control 1 2 3 Address 1: MAC address of wireless host or AP to receive this frame 2 6 seq address 4 control 0 - 2312 4 payload CRC Address 3: used only in ad hoc mode Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame CPSC 441: Wireless 21
802. 11 frame: addressing R 1 router H 1 Internet AP R 1 MAC addr AP MAC addr dest. address source address 802. 3 frame AP MAC addr H 1 MAC addr R 1 MAC address 1 address 2 address 3 802. 11 frame CPSC 441: Wireless 22
802. 11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) 2 2 6 6 6 frame address duration control 1 2 3 2 Protocol version 2 4 1 Type Subtype To AP 6 2 1 seq address 4 control 1 From More AP frag 1 Retry 1 0 - 2312 4 payload CRC 1 Power More mgt data 1 1 WEP Rsvd frame type (RTS, CTS, ACK, data) CPSC 441: Wireless 23
Outline r Introduction r Standards and Link Characteristics r IEEE 802. 11 Wireless LANS r Mobility r Wireless/Mobility Performance Issues r Summary CPSC 441: Wireless 24
What is mobility? r spectrum of mobility, from the network perspective: no mobility mobile wireless user, mobile user, using same access connecting/ point disconnecting from network using DHCP. high mobility mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) CPSC 441: Wireless 25
802. 11: mobility within same subnet r H 1 remains in same IP subnet: IP address can remain same r switch: which AP is associated with H 1? m self-learning: switch will see frame from H 1 and “remember” which switch port can be used to reach H 1 router hub or switch BBS 1 AP 2 H 1 BBS 2 CPSC 441: Wireless 26
Mobility: Vocabulary home network: permanent “home” of mobile (e. g. , 128. 119. 40/24) permanent address: address in home network, can always be used to reach mobile e. g. , 128. 119. 40. 186 home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote wide area network correspondent CPSC 441: Wireless 27
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 correspondent: wants to communicate with mobile home agent: entity in visited network 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? I wonder where Alice moved to? r search all phone books? r call her parents? r expect her to let you know where he/she is? CPSC 441: Wireless 29
Mobility: approaches r Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. m routing tables indicate where each mobile located m no changes to end-systems r Let end-systems handle it: m indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote m direct routing: correspondent gets foreign address of mobile, sends directly to mobile CPSC 441: Wireless 30
Mobility: approaches r Let routing handle it: routers advertise permanent not address of mobile-nodes-in-residence via usual scalable routing table exchange. to millions of m routing tables indicate mobiles where each mobile located m no changes to end-systems r let end-systems handle it: m indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote m direct routing: correspondent gets foreign address of mobile, sends directly to mobile CPSC 441: Wireless 31
Mobility: registration visited network home network 2 1 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: r Foreign Agent (FA) knows about mobile r Home Agent (HA) knows location of mobile CPSC 441: Wireless 32
Mobility via Indirect Routing foreign agent receives packets, forwards to mobile home agent intercepts packets, forwards to foreign agent home network visited network 3 wide area network correspondent addresses packets using home address of mobile 1 2 4 mobile replies directly to correspondent CPSC 441: Wireless 33
Indirect Routing: comments r Mobile uses two addresses: m permanent address: used by correspondent (hence mobile location is transparent to correspondent) m care-of-address: used by home agent to forward datagrams to mobile r foreign agent functions may be done by mobile itself r triangle routing: correspondent-home-networkmobile m inefficient when correspondent, mobile are in same network CPSC 441: Wireless 34
Indirect Routing: moving between networks r suppose mobile user moves to another network m registers with new foreign agent m new foreign agent registers with home agent m home agent update care-of-address for mobile m packets continue to be forwarded to mobile (but with new care-of-address) r mobility, changing foreign networks transparent: ongoing connections can be maintained! CPSC 441: Wireless 35
Mobility via Direct Routing correspondent forwards to foreign agent receives packets, forwards to mobile home network 4 wide area network 2 correspondent requests, receives foreign address of mobile visited network 1 3 4 mobile replies directly to correspondent CPSC 441: Wireless 36
Mobility via Direct Routing: comments r overcome triangle routing problem r non-transparent to correspondent: correspondent must get care-of-address from home agent m what if mobile changes visited network? CPSC 441: Wireless 37
Accommodating mobility with direct routing r anchor foreign agent: FA in first visited network r data always routed first to anchor FA r when mobile moves: new FA arranges to have data forwarded from old FA (chaining) foreign net visited at session start wide area network anchor foreign agent 1 2 4 5 correspondent agent 3 new foreign agent new foreign network CPSC 441: Wireless 38
Mobile IP r RFC 3220 r has many features we’ve seen: m home agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) r three components to standard: m indirect routing of datagrams m agent discovery m registration with home agent CPSC 441: Wireless 39
Outline r Introduction r Standards and Link Characteristics r IEEE 802. 11 Wireless LANS r Mobility r Wireless/Mobility Performance Issues r Summary CPSC 441: Wireless 40
Wireless, mobility: impact on higher layer protocols r logically, impact should be minimal … m best effort service model remains unchanged m TCP and UDP can (and do) run over wireless, mobile r … but performance-wise: m packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoffs from mobility and transient connectivity m TCP interprets loss as congestion, will decrease congestion window un-necessarily m delay impairments for real-time traffic m limited bandwidth of wireless links CPSC 441: Wireless 41
Summary Wireless r wireless links: m m m capacity, distance channel impairments CDMA r IEEE 802. 11 (“Wi. Fi”) m CSMA/CA reflects wireless channel characteristics Mobility r principles: addressing, routing to mobile users m m m home, visited networks direct, indirect routing care-of-addresses r Mobile IP r impact on higher-layer protocols CPSC 441: Wireless 42
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