1. 6: Wireless and Mobile Networks 6-1 Chapter 6 Wireless and Mobile Networks All material copyright 1996-2011 J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.

2. 6: Wireless and Mobile Networks 6-2 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 architecture m 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 higher- layer protocols 6.9 Summary

3. 6: Wireless and Mobile Networks 6-3 Chapter 6: Wireless and Mobile Networks Background: r # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! m 34 million in 1993 m 4 billion by the end of 2008 r Advantages of computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise untethered Internet access m anywhere m anytime

4. 6: Wireless and Mobile Networks 6-4 Chapter 6: Wireless and Mobile Networks Background: r two important (but different) challenges m wireless: communication over wireless link m mobility: handling the mobile user who changes point of attachment to network r Wireless but not mobile m Wireless home or office networks r Mobility that do not require wireless links m Wired laptop at home, the same laptop attached company’s wired network r Both wireless and mobile m VoIP call while racing down the autobahn m The most interesting technical challenges!

5. 6: Wireless and Mobile Networks 6-5 Wireless - History r 1880: Hertz discovered electromagnetic waves r 1898: First commercial radio data service r 1921: First Mobile Radio: m Wireless dispatch system for Detroit Police (one way) r 1946: First Mobile Telephone Service: m In St. Louis by AT&T. Half-duplex ⇒ Push to talk. r 1970: First Cellular Phone Service: AT&T Chicago r 1971: First Wireless Data Network: m Aloha at University of Hawaii r 1990: First Commercial WLAN Product m AT&T WaveLAN r 1997: First WLAN Standard - IEEE 802.11

6. 6: Wireless and Mobile Networks 6-6 Elements of a wireless network network infrastructure wireless hosts r End system devices that run applications r laptop, desktop computer, PDA, IP phone r may be stationary (non-mobile) or mobile m wireless does not always mean mobility Including both wireless LANs (802.11), cellular networks (3G)

7. 6: Wireless and Mobile Networks 6-7 Elements of a wireless network network infrastructure base station r typically connected to wired network r responsible for sending packets between wired network and wireless host(s) in its “area” m e.g., cell towers, 802.11 access points

8. 6: Wireless and Mobile Networks 6-8 Elements of a wireless network network infrastructure base station r wireless host associated with a base station m the host is within the distance of the base station m the host uses that base station to relay data r figure represents a base station connected to the larger network m functioning as a link- layer relay

9. 6: Wireless and Mobile Networks 6-9 Elements of a wireless network network infrastructure wireless link r typically used to connect mobile(s) to base station or to another host r also used as backbone link r multiple access protocol coordinates link access r various data rates, transmission distance

10. 6: Wireless and Mobile Networks 6-10 Characteristics of selected wireless link standards Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km.056.384 1 4 5-11 54 IS-95, CDMA, GSM 2G UMTS/WCDMA, CDMA2000 3G 802.15 802.11b 802.11a,g UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellular enhanced 802.16 (WiMAX) 802.11a,g point-to-point 200 802.11n Data rate (Mbps) data

11. 6: Wireless and Mobile Networks 6-11 Elements of a wireless network network infrastructure infrastructure mode r base station connects mobiles into wired network r address assignment, routing are provided larger network which a host may communicate with it

12. 6: Wireless and Mobile Networks 6-12 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, address assignment

13. 6: Wireless and Mobile Networks 6-13 Elements of a wireless network handoff  mobile changes base station providing connection into wired network m finding the mobile host’s current location? m Data routing provides that the connection continues uninterrupted?

14. 6: Wireless and Mobile Networks 6-14 Wireless network taxonomy single hop multiple hops infrastructure (e.g., APs) no infrastructure host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet – cafe, library no base station, no connection to larger Internet (Bluetooth, ad hoc nets) host may have to relay through several wireless nodes to connect to larger Internet: mesh net, wsn no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET

15. 6: Wireless and Mobile Networks 6-15 Wireless Link Characteristics (1) Differences from wired link …. virtually no changes needed at the network layer or above 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”

16. 6: Wireless and Mobile Networks 6-16 Wireless Link Characteristics (2) r SNR: signal-to-noise ratio m larger SNR – easier to extract signal from noise (a “good thing”), dB, base 10 log r SNR versus BER tradeoffs m given physical layer: increase power -> increase SNR->decrease BER m disadvantages of inceasing transmission power: more energy expended by the sender senders transmissions may interfere 10 20 30 40 QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) SNR(dB) BER 10 -1 10 -2 10 -3 10 -5 10 -6 10 -7 10 -4

17. 6: Wireless and Mobile Networks 6-17 Wireless Link Characteristics (3) r SNR: signal-to-noise ratio m larger SNR – easier to extract signal from noise (a “good thing”), dB, base 10 log r SNR versus BER tradeoffs m given SNR: choose physical layer that meets BER requirement, giving highest throughput SNR may change with mobility or environment: dynamically adapt physical layer (modulation technique, rate) 10 20 30 40 QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) SNR(dB) BER 10 -1 10 -2 10 -3 10 -5 10 -6 10 -7 10 -4

18. 6: Wireless and Mobile Networks 6-18 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C Hidden terminal problem (physical obstructions) r B, A hear each other r B, C hear each other r A, C can not hear each other means A, C unaware of their interference at B A B C A’s signal strength space C’s signal strength Signal attenuation: r B, A hear each other r B, C hear each other r A, C can not hear each other interfering at B

19. 6: Wireless and Mobile Networks 6-19 Code Division Multiple Access (CDMA) r three classes of medium access protocols: m channel partitioning m random access m taking turns r used in several wireless broadcast channels (cellular, satellite, etc) standards r unique “code” assigned to each user; i.e., code set partitioning r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data r encoded signal = (original data) X (chipping sequence) r decoding: inner-product of encoded signal and chipping sequence r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

20. 6: Wireless and Mobile Networks 6-20 CDMA Encode/Decode r each original data bit to be transmitted requires a one-bit slot time r d i be the value of the data bit for the ith bit slot r each bit slot is subdivided into M mini slots r CDMA code consists of a sequence of M values r figure illustrates how CDMA works

21. 6: Wireless and Mobile Networks 6-21 CDMA Encode/Decode slot 1 slot 0 d 1 = -1 111 1 1 - 1 - 1 -1 - Z i,m = d i. c m d 0 = 1 111 1 1 - 1 - 1 - 1 - 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 -1 - slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = 1 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 - 1 - 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 -1 - slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M

22. 6: Wireless and Mobile Networks 6-22 CDMA: more than one sender interference r CDMA receiver recovers the original data bit when more than one sender interfere r example: m 4 senders: 1, 1, 1, -1 m received signal during that mini slot: 2 (for all receivers) r if the senders’ codes are chosen carefully each receiver can recover

23. 6: Wireless and Mobile Networks 6-23 CDMA: two-sender interference

24. 6: Wireless and Mobile Networks 6-24 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 architecture m 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 higher- layer protocols 6.9 Summary

25. 6: Wireless and Mobile Networks 6-25 Wireless LAN r Pervasive in the home, school, etc. r IEEE 802.11 Wireless LAN m WiFi r frame structure r medium access protocol r internetworking

26. 6: Wireless and Mobile Networks 6-26 IEEE 802.11 Wireless LAN r 802.11b m 2.4-5 GHz unlicensed spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code r 802.11a m 5-6 GHz range m up to 54 Mbps r 802.11g m 2.4-5 GHz range m up to 54 Mbps r 802.11n: multiple antennae m 2.4-5 GHz range m up to 200 Mbps

27. 6: Wireless and Mobile Networks 6-27 IEEE 802.11 Wireless LAN r similarities(a,b,g): m all use CSMA/CA for medium access protocol m all have base-station and ad-hoc network versions m all use the same frame structure for link layer m all have the ability to reduce transmission rate to reach greater distances r differences: m 802.11a bitrate > 802.11b bitrate but shorter transmission distance, suffer from multipath propogation m 802.11g bitrate > 802.11b, lower frequency r Pros of 802.11n m fastest maximum speed and best signal range; less radio interference as compared to 2.4 GHz band r Cons of 802.11n m standard is not yet finalized; costs more than 802.11g; the use of multiple signals may greatly interfere with nearby 802.11b/g based networks.

28. 6: Wireless and Mobile Networks 6-28 802.11 LAN architecture r wireless host communicates 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 r two BSSs connected to a router leads to the Internet r Each AP has a unique MAC address BSS 1 BSS 2 Internet hub, switch or router AP

29. 6: Wireless and Mobile Networks 6-29 802.11 LAN architecture  ad hoc mode: m wireless hosts only m laptops get together m exchange data in the absence of an AP

30. 6: Wireless and Mobile Networks 6-30 802.11: Channels r 802.11b: 2.4GHz-2.485GHz spectrum divided into (partially overlapping) 11 channels at different frequencies m two channels are non-overlaping ⇔ they are separated by four or more channels (1, 6, 11) m AP admin chooses frequency for AP m interference possible: channel can be same as that chosen by neighboring AP!

31. 802.11: Channels 6: Wireless and Mobile Networks 6-31

32. 6: Wireless and Mobile Networks 6-32 802.11: Association r host: must associate with an AP m scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address m may receive signal from two or more APs m selects AP to associate with m may perform authentication [Chapter 8] m association is a virtual wire m will typically run DHCP (Dynamic Host Configuration Protocol) to get IP address in AP’s subnet

33. 6: Wireless and Mobile Networks 6-33 802.11: passive/active scanning AP 2 AP 1 H1 BBS 2 BBS 1 1 2 2 3 4 Active Scanning : (1)Probe Request frame broadcast from H1 (2)Probes response frame sent from APs (3)Association Request frame sent: H1 to selected AP (4)Association Response frame sent: H1 to selected AP AP 2 AP 1 H1 BBS 2 BBS 1 1 2 3 1 Passive Scanning: (1)beacon frames sent from APs (2)association Request frame sent: H1 to selected AP (3)association Response frame sent: selected AP to H1

34. 6: Wireless and Mobile Networks 6-34 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 m can’t sense all collisions in any case: hidden terminal, fading A B C A B C A’s signal strength space C’s signal strength

35. 6: Wireless and Mobile Networks 6-35 IEEE 802.11: multiple access r station transmits a frame in its entirety r no turning back! r should be prevent to transmit long frames more than once m degradation in protocol performance r goal: avoid collisions: CSMA/C(ollision)A(voidance) r link layer acknowledgement/retransmission (ARQ)

36. 6: Wireless and Mobile Networks 6-36 IEEE 802.11 MAC Protocol: CSMA/CA r Link layer acknowledgement scheme m frame may not reach the destination r SIFS: short interframe spacing m The small time interval between data frame and its acknowledgement m A SIFS duration is a constant value and it depends on the amendments r DIFS: distributed interframe space m to sense the status of wireless medium

37. 6: Wireless and Mobile Networks 6-37 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 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 DIFS data SIFS ACK

38. 6: Wireless and Mobile Networks 6-38 IEEE 802.11 MAC Protocol: CSMA/CA r if sender does not receive an ACK m retransmit it r after some fixed number of retransmissions m sender discards the frame r Why do CSMA/CD (ethernet) and CSMA/CA take different approaches? m an ethernet station transmits immediately m a wireless station waits for a random time r Collusions may still occur m hidden terminal m same random backoff values

39. 6: Wireless and Mobile Networks 6-39 Hidden terminal r H1 wants to transmit to AP r H2 does not hear the transmission r After DIFS interval transmits the frame H1 H2 AP

40. 6: Wireless and Mobile Networks 6-40 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 AP using CSMA m RTSs may still collide with each other (but they’re short) m indicates total time required to transmit the data frame and ACK r AP 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 r disadvantages: m introduces delay and consumes channel resources avoid data frame collisions completely using small reservation packets!

41. 6: Wireless and Mobile Networks 6-41 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

42. 6: Wireless and Mobile Networks 6-42 frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control 802.11 frame: addressing Address 2: MAC address of wireless host or AP transmitting this frame Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode bytes

43. 6: Wireless and Mobile Networks 6-43 802.11 frame: addressing r Payload and CRC m holding an IP datagram or ARP (Address Resolution Protocol) packet m CRC is more useful in wireless LAN than wired LAN r Sequence number, duration, frame control m distinguish between newly transmitted frame and the retransmission m time to transmit data frame + ack m frame control includes more information

44. 6: Wireless and Mobile Networks 6-44 Internet router AP H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame R1 MAC addr H1 MAC addr dest. address source address 802.3 frame 802.11 frame: addressing

45. 6: Wireless and Mobile Networks 6-45 frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control Type From AP Subtype To AP More frag WEP More data Power mgt RetryRsvd Protocol version 2 2411111111 802.11 frame: more duration of reserved transmission time (RTS/CTS) frame seq # frame type (RTS, CTS, ACK, data)

46. 6: Wireless and Mobile Networks 6-46 hub or switch AP 2 AP 1 H1 BBS 2 BBS 1 802.11: mobility within same subnet router r H1 remains in same IP subnet: IP address can remain same r H1 can keep its ongoing TCP connections r H1 detect weakining signal from AP1 r starts to scan for a stronger signal r disconnects AP1, associate with AP2

47. 6: Wireless and Mobile Networks 6-47 hub or switch AP 2 AP 1 H1 BBS 2 BBS 1 802.11: mobility within same subnet router r switch: which AP is associated with H1? m self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 r One solution: m after associating with H1, AP2 sends broadcast Ethernet frame

48. 6: Wireless and Mobile Networks 6-48 802.11: advanced capabilities Rate Adaptation r base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies r shares the same philosophy as TCP’s congestion control 1020 30 40 SNR(dB) BER 10 -1 10 -2 10 -3 10 -5 10 -6 10 -7 10 -4 QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point 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

49. 6: Wireless and Mobile Networks 6-49 802.11: 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 (buffers frames if necessary) m node wakes up before next beacon frame m AP sends a beacon frame every 100 msec r beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent m node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame m node can enter to active state in 250 microsec

50. 6: Wireless and Mobile Networks 6-50 M radius of coverage S S S P P P P M S Master device Slave device Parked device (inactive) P 802.15: personal area network r less than 10 m diameter (short range, low power, low rate) r replacement for cables (mouse, keyboard, headphones) r 802.15: evolved from Bluetooth specification m 2.4-2.5 GHz radio band r operates in TDM manner m time slots of 625 microseconds m sender transmits on one of 79 channels m frequency-hopping spread spectrum

51. 6: Wireless and Mobile Networks 6-51 M radius of coverage S S S P P P P M S Master device Slave device Parked device (inactive) P 802.15: personal area network r less than 10 m diameter (short range, low power, low rate) r replacement for cables (mouse, keyboard, headphones) r ad hoc: no infrastructure r organized into a “piconet” m up to 8 active devices m the others are inactive devices r master/slaves: m slaves request permission to send (to master) m master grants requests

52. 6: Wireless and Mobile Networks 6-52 802.16: WiMAX r like 802.11 & cellular: 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 r base station serves large number of clients (subscriber stations) point-to-multipoint point-to-point

53. 6: Wireless and Mobile Networks 6-53 802.16: WiMAX r unlike 802.11: m range ~ 6 miles (“city rather than coffee shop”) m ~14 Mbps r supports mobility at speeds of 70/80 miles per hour r TDM frame structure r MAP (Media Access Protocol) messages inform clients m modulation scheme, coding m error-correction parameters r frames consisting of bursts, bursts consisting of packets m packets within the bursts are transmitted with the same physical layer properties m these propoerties may change from one burst to another :opportunistic scheduling

54. 6: Wireless and Mobile Networks 6-54 802.16: WiMAX: downlink, uplink scheduling r transmission frame m down-link subframe: base station to node m uplink subframe: node to base station pream. DL- MAP UL- MAP DL burst 1 SS #1 DL burst 2 DL burst n Initial maint. request conn. downlink subframe SS #2 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 WiMAX standard provide mechanism for scheduling, but not scheduling algorithm r connection oriented architecture, each connection have m associated QoS m traffic parameters

55. Reading r Computer Networks – A top down approach m Chapter 6: m 802.11 m 802.15 m 802.16 6: Wireless and Mobile Networks 6-55