1 / 29

Special Topics in Computer Engineering Wireless and Mobile Networks: I

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. Background:

season
Download Presentation

Special Topics in Computer Engineering Wireless and Mobile Networks: I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Special Topics in Computer EngineeringWireless 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 CPE 0907532:Wireless and Mobile Networks I

  2. Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! computer nets: laptops, palmtops, PDAs, Internet-enabled phones promise anytime untethered Internet access two important (but different) challenges communication over wireless link handling mobile user who changes point of attachment to network Chapter 6: Wireless and Mobile Networks CPE 0907532:Wireless and Mobile Networks I

  3. 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9Summary Chapter 6 outline CPE 0907532:Wireless and Mobile Networks I

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

  5. base station • typically connected to wired network • relay - responsible for sending packets between wired network and wireless host(s) in its “area” • e.g., cell towers, 802.11 access points network infrastructure Elements of a wireless network CPE 0907532:Wireless and Mobile Networks I

  6. network infrastructure Elements of a wireless network wireless link • typically used to connect mobile(s) to base station • also used as backbone link • multiple access protocol coordinates link access • various data rates, transmission distance CPE 0907532:Wireless and Mobile Networks I

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

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

  9. Elements of a wireless network Ad hoc mode • no base stations • nodes can only transmit to other nodes within link coverage • nodes organize themselves into a network: route among themselves CPE 0907532:Wireless and Mobile Networks I

  10. Wireless Link Characteristics Differences from wired link …. • decreased 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 • 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” CPE 0907532:Wireless and Mobile Networks I

  11. B A C C C’s signal strength A’s signal strength B A space Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): 1. Hidden terminal problem • B, A hear each other • B, C hear each other • A, C can not hear each other means A, C unaware of their interference at B 2. Signal fading: • B, A hear each other • B, C hear each other • A, C can not hear each other interferring at B CPE 0907532:Wireless and Mobile Networks I

  12. Code Division Multiple Access (CDMA) • used in several wireless broadcast channels (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”) CPE 0907532:Wireless and Mobile Networks I

  13. d0 = 1 1 1 1 1 1 1 d1 = -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M Di = SZi,m.cm m=1 M d0 = 1 d1 = -1 CDMA Encode/Decode channel output Zi,m Zi,m= di.cm data bits sender slot 0 channel output slot 1 channel output code slot 1 slot 0 received input slot 0 channel output slot 1 channel output code receiver slot 1 slot 0 CPE 0907532:Wireless and Mobile Networks I

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

  15. 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9Summary Chapter 6 outline CPE 0907532:Wireless and Mobile Networks I

  16. 802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc network versions IEEE 802.11 Wireless LAN CPE 0907532:Wireless and Mobile Networks I

  17. AP AP Internet 802.11 LAN architecture • wireless host communicates with base station • base station = access point (AP) • Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: • wireless hosts • access point (AP): base station • ad hoc mode: hosts only hub, switch or router BSS 1 BSS 2

  18. 802.11: Channels, association • 802.11b: 2.4GHz-2.485GHz frequency spectrum (85 MHz) divided into 11 channels at different frequencies • AP admin chooses frequency for AP • interference possible: channel can be same as that chosen by neighboring AP! • A WiFi jungle • any physical location where a wireless station receives a sufficiently strong signal from two or more APs • Each AP would • Be located in a different subnet • Have been independently assigned a channel CPE 0907532:Wireless and Mobile Networks I

  19. 802.11: Channels, association (cont. I) • host: must associate with an AP • scans channels, listening for beacon frames containing AP’s name (Service Set Identifier, SSID) and MAC address • 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 • may perform authentication • will typically run DHCP to get IP address in AP’s subnet CPE 0907532:Wireless and Mobile Networks I

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

  21. B A C C C’s signal strength A’s signal strength B A space IEEE 802.11: multiple access • avoid collisions: 2+ nodes transmitting at same time • 802.11: CSMA - sense before transmitting • don’t collide with ongoing transmission by other node • 802.11: no collision detection! • difficult to receive (sense collisions) when transmitting due to weak received signals (fading) • can’t sense all collisions in any case: hidden terminal, fading • goal: avoid collisions: CSMA/C(ollision)A(voidance) CPE 0907532:Wireless and Mobile Networks I

  22. DIFS data SIFS ACK 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 ifsense channel busy then • start random backoff time • timer counts down when channel is idle • When channel is busy. counter value remains frozen. • transmit when timer expires • if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS {Short Inter-frame Spacing} • ACK needed due to hidden terminal problem sender receiver 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 • sender first transmits small request-to-send (RTS) packets to AP using CSMA • RTSs may still collide with each other (but they’re short) • AP broadcasts clear-to-send CTS in response to RTS • RTS heard by all nodes • sender transmits data frame • other stations defer transmissions Avoid data frame collisions completely using small reservation packets! CPE 0907532:Wireless and Mobile Networks I

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

  25. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 802.11 frame: addressing Address 4: used only in ad hoc mode 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 2: MAC address of wireless host or AP transmitting this frame CPE 0907532:Wireless and Mobile Networks I

  26. router AP Internet R1 MAC addr AP MAC addr source address dest. address 802.3frame AP MAC addr H1 MAC addr R1 MAC addr address 3 address 2 address 1 802.11 frame 802.11 frame: addressing H1 R1 CPE 0907532:Wireless and Mobile Networks I

  27. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To AP From AP More frag Retry Power mgt More data WEP Rsvd 802.11 frame: more frame seq # (for reliable ARQ) duration of channel reserved transmission time (for data & RTS/CTS frames) frame type (RTS, CTS, ACK, data) CPE 0907532:Wireless and Mobile Networks I

  28. H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning: switch will see frame from H1 and “remember” which switch port can be used to reach H1 router 802.11: mobility within same subnet hub or switch BBS 1 AP 1 AP 2 H1 BBS 2 CPE 0907532:Wireless and Mobile Networks I

  29. P P P P P M M Master device Slave device Parked device (inactive) S S S S 802.15: WPAN, wireless personal area network • less than 10 m diameter • replacement for cables (mouse, keyboard, headphones) • ad hoc: no infrastructure • master/slaves: • slaves request permission to send (to master) • master grants requests • 802.15: evolved from Bluetooth specification • 2.4-2.5 GHz radio band • up to 721 kbps radius of coverage CPE 0907532:Wireless and Mobile Networks I

More Related