1. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 1 Special Topics in Computer Engineering Wireless and Mobile Networks: I Some of these Slides are Based on Slides by Kurose and Ross Prepared for Sections 6.1 – 6.3 of the Book Computer Networking: A Top Down Approach Featuring the Internet

2. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 2 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 phones 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

3. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 3 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

4. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 4 Elements of a wireless network network infrastructure wireless hosts r laptop, PDA, IP phone r run applications r may be stationary (non-mobile) or mobile m wireless does not always mean mobility

5. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 5 Elements of a wireless network 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

6. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 6 Elements of a wireless network 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

7. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 7 Characteristics of selected wireless link standards 384 Kbps 56 Kbps 54 Mbps 5-11 Mbps 1 Mbps 802.15 802.11b 802.11{a,g} IS-95 CDMAIS-95 CDMA, GSMGSM UMTS UMTS / WCDMA, CDMA2000WCDMA CDMA2000.11 p-to-p link 2G 3G Indoor 10 – 30m Outdoor 50 – 200m Mid range outdoor 200m – 4Km Long range outdoor 5Km – 20Km

8. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 8 Elements of a wireless network network infrastructure infrastructure mode r base station connects mobiles into wired network r handoff: mobile changes base station providing connection into wired network

9. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 9 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

10. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 10 Wireless Link Characteristics 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 at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”

11. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 11 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C 1. Hidden terminal problem 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 2. Signal fading: r B, A hear each other r B, C hear each other r A, C can not hear each other interferring at B

12. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 12 Code Division Multiple Access (CDMA) 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”)

13. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 13 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

14. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 14 CDMA: two-sender interference

15. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 15 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

16. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 16 IEEE 802.11 Wireless LAN r 802.11b m 2.4-5 GHz unlicensed radio spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layerDSSS all hosts use same chipping code m widely deployed, using base stations 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 All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions

17. Spring 2006 17 802.11 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 m ad hoc mode: hosts only BSS 1 BSS 2 Internet hub, switch or router AP

18. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 18 802.11: Channels, association r 802.11b: 2.4GHz-2.485GHz frequency spectrum (85 MHz) 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 A WiFi jungle m any physical location where a wireless station receives a sufficiently strong signal from two or more APs m Each AP would Be located in a different subnet Have been independently assigned a channel

19. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 19 802.11: Channels, association (cont. I) r host: must associate with an AP m scans channels, listening for beacon frames containing AP’s name (Service Set Identifier, SSID) and MAC address m selects AP to associate with creates a virtual wire between itself and the AP wireless host and AP dialogue with each other using 802.11 association protocol wireless station joins subnet to which AP belongs m may perform authentication m will typically run DHCP to get IP address in AP’s subnet

20. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 20 802.11: Channels, association (cont. II) r wireless host: may perform authentication m Companies permit access based on a wireless station’s MAC address m Internet cafés employ user names and passwords r AP communicates with an authentication server r One authentication server for many APs m Centralizing decisions of authentication m Keeping AP costs and complexity low

21. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 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 B C A B C A’s signal strength space C’s signal strength

22. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 22 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS (Distributed Inter-frame Space) then transmit entire frame (no CD) 2 if sense channel busy then a) start random backoff time b) timer counts down when channel is idle m When channel is busy. counter value remains frozen. c) transmit when timer expires d) if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS {Short Inter-frame Spacing} m ACK needed due to hidden terminal problem sender receiver DIFS data SIFS ACK

23. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 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 AP using CSMA m RTSs may still collide with each other (but they’re short) r AP broadcasts clear-to-send CTS in response to RTS r RTS heard by all nodes m sender transmits data frame m other stations defer transmissions Avoid data frame collisions completely using small reservation packets!

24. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 24 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

25. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 25 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

26. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 26 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 AP MAC addr dest. address source address 802.3 frame 802.11 frame: addressing

27. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 27 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 channel reserved transmission time (for data & RTS/CTS frames) frame seq # (for reliable ARQ) frame type (RTS, CTS, ACK, data)

28. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 28 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 switch: which AP is associated with H1? m self-learning: switch will see frame from H1 and “remember” which switch port can be used to reach H1

29. Spring 2006 CPE 0907532:Wireless and Mobile Networks I 29 M radius of coverage S S S P P P P M S Master device Slave device Parked device (inactive) P 802.15: WPAN, wireless 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 slaves request permission to send (to master) m master grants requests r 802.15: evolved from Bluetooth specification m 2.4-2.5 GHz radio band m up to 721 kbps