Wireless and Mobile Networks By Behzad Akbari Fall

Wireless and Mobile Networks By Behzad Akbari Fall 2008 These slides are based on the slides of J. Kurose (UMASS) and Keith Ross (Poly) , the authors of book “Computer Networking: A Top Down Approach 4 th edition. Addison-Wesley, July 2007” 1

Chapter 6: Wireless and Mobile Networks Background: r # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! r computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access r two important (but different) challenges m m wireless: communication over wireless link mobility: handling the mobile user who changes point of attachment to network 2

Chapter 6 outline 6. 1 Introduction Wireless r 6. 2 Wireless links, characteristics m CDMA r 6. 3 IEEE 802. 11 wireless LANs (“wi-fi”) r 6. 4 Cellular Internet Access m m architecture standards (e. g. , GSM) Mobility r 6. 5 Principles: addressing and routing to mobile users r 6. 6 Mobile IP r 6. 7 Handling mobility in cellular networks r 6. 8 Mobility and higherlayer protocols 6. 9 Summary 3

Elements of a wireless network infrastructure wireless hosts r laptop, PDA, IP phone r run applications r may be stationary (nonmobile) or mobile m wireless does not always mean mobility 4

Elements of a wireless network infrastructure base station r typically connected to wired network r relay - responsible for sending packets between wired network and wireless host(s) in its “area” m e. g. , cell towers, 802. 11 access points 5

Elements of a wireless network infrastructure wireless link r typically used to connect mobile(s) to base station r also used as backbone link r multiple access protocol coordinates link access r various data rates, transmission distance 6

Characteristics of selected wireless link standards Data rate (Mbps) 200 54 5 -11 802. 11 n 802. 11 a, g 802. 11 b 4 1 802. 11 a, g point-to-point data 802. 16 (Wi. MAX) UMTS/WCDMA-HSPDA, CDMA 2000 -1 x. EVDO 3 G cellular enhanced 802. 15 . 384 3 G UMTS/WCDMA, CDMA 2000 . 056 2 G IS-95, CDMA, GSM Indoor Outdoor 10 -30 m 50 -200 m Mid-range outdoor Long-range outdoor 200 m – 4 Km 5 Km – 20 Km 7

Elements of a wireless network infrastructure mode r base station connects mobiles into wired network r handoff: mobile changes base station providing connection into wired network 8

Elements of a wireless network 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 9

Wireless network taxonomy single hop infrastructure (e. g. , APs) no infrastructure host connects to base station (Wi. Fi, Wi. MAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) multiple hops host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET 10

Wireless Link Characteristics (1) Differences from wired link …. m decreased signal strength: radio signal attenuates as it propagates through matter (path loss) m 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 m 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” 11

Wireless Link Characteristics (2) r SNR: signal-to-noise ratio larger SNR – easier to extract signal from noise (a “good thing”) r SNR versus BER tradeoffs m given physical layer: increase power -> increase SNR->decrease BER m given SNR: choose physical layer that meets BER requirement, giving highest thruput 10 -1 m • SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) 10 -2 BER 10 -3 10 -4 10 -5 10 -6 10 -7 10 20 30 40 SNR(d. B) QAM 256 (8 Mbps) QAM 16 (4 Mbps) BPSK (1 Mbps) 12

Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B Hidden terminal problem C C’s signal strength A’s signal strength space r B, A hear each other Signal attenuation: r A, C can not hear each other r B, C hear each other r B, A hear each other means A, C unaware of their interference at B r A, C can not hear each other interfering at B 13

Code Division Multiple Access (CDMA) r used in several wireless broadcast channels r r r (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”) 14

CDMA Encode/Decode sender data bits code Zi, m= di. cm d 0 = 1 -1 -1 -1 1 1 1 -1 -1 -1 slot 1 channel output 1 -1 1 1 1 d 1 = -1 1 channel output Zi, m -1 -1 -1 slot 0 channel output M Di = S Zi, m. cm m=1 received input code receiver 1 1 1 1 -1 -1 -1 1 -1 -1 -1 slot 1 M 1 1 -1 -1 slot 0 d 0 = 1 d 1 = -1 slot 1 channel output slot 0 channel output 15

CDMA: two-sender interference 16

Chapter 6 outline 6. 1 Introduction Wireless r 6. 2 Wireless links, characteristics m CDMA r 6. 3 IEEE 802. 11 wireless LANs (“wi-fi”) r 6. 4 cellular Internet access m m architecture standards (e. g. , GSM) Mobility r 6. 5 Principles: addressing and routing to mobile users r 6. 6 Mobile IP r 6. 7 Handling mobility in cellular networks r 6. 8 Mobility and higherlayer protocols 6. 9 Summary 17

IEEE 802. 11 Wireless LAN r 802. 11 a r 802. 11 b m 5 -6 GHz range m 2. 4 -5 GHz unlicensed spectrum m up to 54 Mbps m up to 11 Mbps r 802. 11 g m direct sequence spread spectrum (DSSS) in physical m 2. 4 -5 GHz range layer m up to 54 Mbps • all hosts use same chipping r 802. 11 n: multiple antennae code m 2. 4 -5 GHz range m up to 200 Mbps r all use CSMA/CA for multiple access r all have base-station and ad-hoc network versions 18

802. 11 LAN architecture r wireless host communicates Internet AP hub, switch or router BSS 1 AP BSS 2 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): base station m ad hoc mode: hosts only 19

802. 11: Channels, association r 802. 11 b: 2. 4 GHz-2. 485 GHz spectrum divided into 11 channels at different frequencies m AP admin chooses frequency for AP m interference possible: channel can be same as that chosen by neighboring AP! r host: must associate with an AP m scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address m selects AP to associate with m may perform authentication [Chapter 8] m will typically run DHCP to get IP address in AP’s subnet 20

802. 11: passive/active scanning BBS 1 AP 1 BBS 2 1 1 2 AP 2 BBS 1 AP 1 BBS 2 1 2 3 (1) beacon frames sent from APs (2) association Request frame sent: H 1 to selected AP (3) association Response frame sent: H 1 to selected AP AP 2 4 H 1 Passive Scanning: 2 3 Active Scanning: (1) Probe Request frame broadcast from H 1 (2) Probes response frame sent from APs (3) Association Request frame sent: H 1 to selected AP (4) Association Response frame sent: H 1 to selected AP 21

IEEE 802. 11: multiple access r avoid collisions: 2+ nodes transmitting at same time r 802. 11: CSMA - sense before transmitting m don’t collide with ongoing transmission by other node r 802. 11: no collision detection! m difficult to 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/C(ollision)A(voidance) A C A B B C C’s signal strength A’s signal strength space 22

IEEE 802. 11 MAC Protocol: CSMA/CA 802. 11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 sender receiver DIFS 802. 11 receiver - if frame received OK data SIFS ACK return ACK after SIFS (ACK needed due to hidden terminal problem) 23

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 BS using CSMA m RTSs may still collide with each other (but they’re short) r BS broadcasts clear-to-send CTS in response to RTS r CTS heard by all nodes m sender transmits data frame m other stations defer transmissions avoid data frame collisions completely using small reservation packets! 24

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) 25

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 4: 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 26

802. 11 frame: addressing R 1 router H 1 Internet AP R 1 MAC addr H 1 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 27

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) 28

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 (Ch. 5): 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 29

802. 11: advanced capabilities QAM 256 (8 Mbps) QAM 16 (4 Mbps) BPSK (1 Mbps) operating point 10 -1 10 -2 10 -3 BER Rate Adaptation r base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies 10 -4 10 -5 10 -6 10 -7 10 20 30 SNR(d. B) 40 1. SNR decreases, BER increase as node moves away from base station 2. When BER becomes too high, switch to lower transmission rate but with lower BER 30

802. 11: advanced capabilities Power Management r node-to-AP: “I am going to sleep until next beacon frame” m. AP knows not to transmit frames to this node mnode wakes up before next beacon frame r beacon frame: contains list of mobiles with APto-mobile frames waiting to be sent mnode will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame 31

802. 15: personal area network r less than 10 m diameter r replacement for cables (mouse, keyboard, headphones) r ad hoc: no infrastructure r master/slaves: m m slaves request permission to send (to master) master grants requests r 802. 15: evolved from Bluetooth specification m m 2. 4 -2. 5 GHz radio band up to 721 kbps P S P radius of coverage M S P M Master device S Slave device P Parked device (inactive) 32

802. 16: Wi. MAX r like 802. 11 & cellular: point-to-point base station model m transmissions to/from base station by hosts with omnidirectional antenna m base station-to-base station backhaul with point-to-point antenna point-to-multipoint r unlike 802. 11: m range ~ 6 miles (“city rather than coffee shop”) m ~14 Mbps 33

802. 16: Wi. MAX: downlink, uplink scheduling r transmission frame m down-link subframe: base station to node m uplink subframe: node to base station pream. … DL- ULMAP DL burst 1 DL burst 2 downlink subframe … … DL burst n Initial request SS #1 SS #2 maint. conn. … SS #k uplink subframe base station tells nodes who will get to receive (DL map) and who will get to send (UL map), and when r Wi. MAX standard provide mechanism for scheduling, but not scheduling algorithm 34

Chapter 6 outline 6. 1 Introduction Wireless r 6. 2 Wireless links, characteristics m CDMA r 6. 3 IEEE 802. 11 wireless LANs (“wi-fi”) r 6. 4 Cellular Internet Access m m architecture standards (e. g. , GSM) Mobility r 6. 5 Principles: addressing and routing to mobile users r 6. 6 Mobile IP r 6. 7 Handling mobility in cellular networks r 6. 8 Mobility and higherlayer protocols 6. 9 Summary 35

Components of cellular network architecture MSC cell q connects cells to wide area net q manages call setup (more later!) q handles mobility (more later!) q covers geographical region q base station (BS) analogous to 802. 11 AP q mobile users attach to network through BS q air-interface: physical and link layer protocol between mobile and BS Mobile Switching Center Public telephone network, and Internet Mobile Switching Center wired network 36

Cellular networks: the first hop Two techniques for sharing mobile-to-BS radio spectrum r combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots frequency bands r CDMA: code division multiple access time slots 37

Cellular standards: brief survey 2 G systems: voice channels r IS-136 TDMA: combined FDMA/TDMA (north america) r GSM (global system for mobile communications): combined FDMA/TDMA m most widely deployed r IS-95 CDMA: code division multiple access TDMA/FDMA 0 CDMA-200 GPRS EDGE UMTS IS-136 GSM IS-95 Don’t drown in a bowl of alphabet soup: use this for reference only 38

Cellular standards: brief survey 2. 5 G systems: voice and data channels r for those who can’t wait for 3 G service: 2 G extensions r general packet radio service (GPRS) m evolved from GSM m data sent on multiple channels (if available) r enhanced data rates for global evolution (EDGE) m also evolved from GSM, using enhanced modulation m data rates up to 384 K r CDMA-2000 (phase 1) m data rates up to 144 K m evolved from IS-95 39

Cellular standards: brief survey 3 G systems: voice/data r Universal Mobile Telecommunications Service (UMTS) m data service: High Speed Uplink/Downlink packet Access (HSDPA/HSUPA): 3 Mbps r CDMA-2000: CDMA in TDMA slots m data service: 1 x. Evlution Data Optimized (1 x. EVDO) up to 14 Mbps …. . more (and more interesting) cellular topics due to mobility (stay tuned for details) 40

Chapter 6 outline 6. 1 Introduction Wireless r 6. 2 Wireless links, characteristics m CDMA r 6. 3 IEEE 802. 11 wireless LANs (“wi-fi”) r 6. 4 Cellular Internet Access m m architecture standards (e. g. , GSM) Mobility r 6. 5 Principles: addressing and routing to mobile users r 6. 6 Mobile IP r 6. 7 Handling mobility in cellular networks r 6. 8 Mobility and higherlayer protocols 6. 9 Summary 41

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) 42

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 43

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 foreign agent: entity in visited network that performs mobility functions on behalf of mobile. 44

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? 45

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 46

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 47

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 knows about mobile r Home agent knows location of mobile 48

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 49

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 50

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: on going connections can be maintained! 51

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 52

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? 53

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 54

Chapter 6 outline 6. 1 Introduction Wireless r 6. 2 Wireless links, characteristics m CDMA r 6. 3 IEEE 802. 11 wireless LANs (“wi-fi”) r 6. 4 Cellular Internet Access m m architecture standards (e. g. , GSM) Mobility r 6. 5 Principles: addressing and routing to mobile users r 6. 6 Mobile IP r 6. 7 Handling mobility in cellular networks r 6. 8 Mobility and higherlayer protocols 6. 9 Summary 55

Mobile IP r RFC 3344 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 56

Mobile IP: indirect routing foreign-agent-to-mobile packet sent by home agent to foreign agent: a packet within a packet dest: 79. 129. 13. 2 dest: 128. 119. 40. 186 Permanent address: 128. 119. 40. 186 dest: 128. 119. 40. 186 Care-of address: 79. 129. 13. 2 packet sent by correspondent 57

Mobile IP: agent discovery r agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) H, F bits: home and/or foreign agent R bit: registration required 58

Mobile IP: registration example 59

Components of cellular network architecture recall: correspondent wired public telephone network MSC MSC MSC different cellular networks, operated by different providers 60

Handling mobility in cellular networks r home network: network of cellular provider you subscribe to (e. g. , Sprint PCS, Verizon) m home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network) r visited network: network in which mobile currently resides m visitor location register (VLR): database with entry for each user currently in network m could be home network 61

GSM: indirect routing to mobile home network HLR 2 home MSC consults HLR, gets roaming number of mobile in visited network correspondent home Mobile Switching Center 1 VLR 3 Mobile Switching Center 4 Public switched telephone network call routed to home network home MSC sets up 2 nd leg of call to MSC in visited network mobile user visited network MSC in visited network completes call through base station to mobile 62

GSM: handoff with common MSC r Handoff goal: route call via new base station (without interruption) r reasons for handoff: VLR Mobile Switching Center old routing old BSS m new routing m new BSS m stronger signal to/from new BSS (continuing connectivity, less battery drain) load balance: free up channel in current BSS GSM doesn’t mandate why to perform handoff (policy), only how (mechanism) r handoff initiated by old BSS 63

GSM: handoff with common MSC VLR Mobile Switching Center 2 4 1 8 old BSS 5 7 3 6 new BSS 1. old BSS informs MSC of impending handoff, provides list of 1+ new BSSs 2. MSC sets up path (allocates resources) to new BSS 3. new BSS allocates radio channel for use by mobile 4. new BSS signals MSC, old BSS: ready 5. old BSS tells mobile: perform handoff to new BSS 6. mobile, new BSS signal to activate new channel 7. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call 8 MSC-old-BSS resources released 64

GSM: handoff between MSCs r anchor MSC: first MSC visited during cal home network correspondent Home MSC anchor MSC m call remains routed through anchor MSC r new MSCs add on to end PSTN MSC (a) before handoff of MSC chain as mobile moves to new MSC r IS-41 allows optional path minimization step to shorten multi-MSC chain 65

GSM: handoff between MSCs r anchor MSC: first MSC visited during cal home network correspondent Home MSC anchor MSC m call remains routed through anchor MSC r new MSCs add on to end PSTN MSC (b) after handoff of MSC chain as mobile moves to new MSC r IS-41 allows optional path minimization step to shorten multi-MSC chain 66

Mobility: GSM versus Mobile IP GSM element Comment on GSM element Mobile IP element Home system Network to which mobile user’s permanent phone number belongs Home network Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR) Home MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information Home agent Visited System Network other than home system where mobile user is currently residing Visited network Visited Mobile services Switching Center. Visitor Location Record (VLR) Visited MSC: responsible for setting up calls Foreign agent to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user Mobile Station Roaming Number (MSRN), or “roaming number” Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent. Care-ofaddress 67

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 handoff 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 68

Chapter 6 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 r cellular access m architecture m standards (e. g. , GSM, CDMA-2000, UMTS) Mobility r principles: addressing, routing to mobile users m m m home, visited networks direct, indirect routing care-of-addresses r case studies m mobile IP m mobility in GSM r impact on higher-layer protocols 69