1. 6: Wireless and Mobile Networks6-1 19 - Wireless LANs

2. 6: Wireless and Mobile Networks6-2 Chapter 6 Wireless and Mobile Networks Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996-2004 J.F Kurose and K.W. Ross, All Rights Reserved

3. 6: Wireless and Mobile Networks6-3 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 communication over wireless link m handling mobile user who changes point of attachment to network

4. 6: Wireless and Mobile Networks6-4 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. 6: Wireless and Mobile Networks6-5 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. 6: Wireless and Mobile Networks6-6 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. 6: Wireless and Mobile Networks6-7 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 CDMA, GSM UMTS/WCDMA, 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. 6: Wireless and Mobile Networks6-8 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. 6: Wireless and Mobile Networks6-9 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. 6: Wireless and Mobile Networks6-10 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. 6: Wireless and Mobile Networks6-11 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C 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 Signal fading: r B, A hear each other r B, C hear each other r A, C can not hear each other interfering at B

12. 6: Wireless and Mobile Networks6-12 Code Division Multiple Access (CDMA) r fourth type of medium access 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. 6: Wireless and Mobile Networks6-13 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. 6: Wireless and Mobile Networks6-14 CDMA: two-sender interference