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Elements of a wireless network

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. base station typically connected to wired network

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Elements of a wireless network

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  1. 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 6: Wireless and Mobile Networks

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

  3. 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 6: Wireless and Mobile Networks

  4. Characteristics of selected wireless link standards 200 802.11n Mesh 54 802.11a,g 802.11a,g point-to-point 5-11 802.11b 4G: LTWE WIMAX 4 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO Data rate (Mbps) 1 802.15 .384 2.5G: UMTS/WCDMA, CDMA2000 .056 2G: IS-95, CDMA, GSM Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km 6: Wireless and Mobile Networks

  5. WiMax Vs. Wireless Mesh • WiMax • Similar to cellular network infrastructure • Use licensed spectrum • 10 Mbit/s at 10 km in good environment • Is under development by many companies • Wireless Mesh • Extension of 802.11 Wireless LAN • Use unlicensed public spectrum • 802.11’s access routers interconnect together • Ad Hoc (usually non-mobile) networking and routing • Currently used in some places • Town & small city’s government agents (firefighter, police) • More popular in Europe than in US • Challenges: complex routing, high error rate, QoS 6: Wireless and Mobile Networks

  6. 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 6: Wireless and Mobile Networks

  7. 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 Wireless active research area: Ad hoc network Sensor network 6: Wireless and Mobile Networks

  8. Ad Hoc Vs. Sensor Networks • Ad Hoc network • Challenge  Mobility of nodes • Good features: Plenty of power, computation resource • Applications • Mostly mobile laptops or PDAs • Vehicular network • Sensor network • Challenge  limited power, computing resource • Good features: • Usually stationary, dense network • Applications • Military battlefield, civil engineering, environmental monitoring 6: Wireless and Mobile Networks

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

  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” 6: Wireless and Mobile Networks

  11. 802.11b 2.4-2.485 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 5.1-5.8 GHz range up to 54 Mbps 802.11g 2.4-2.485 GHz range up to 54 Mbps Use OFDM in physical layer All use CSMA/CA for multiple access All have base-station and ad-hoc network versions IEEE 802.11 Wireless LAN 6: Wireless and Mobile Networks

  12. 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 6: Wireless and Mobile Networks

  13. 802.11: Channels, association • 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies • 11 channels are partial overlapping (1, 6, 11 non-overlapping) • AP admin chooses frequency for AP • interference possible: channel can be same as that chosen by neighboring AP! • host: must associate with an AP • scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address • selects AP to associate with • may perform authentication [Chapter 8] • will typically run DHCP to get IP address in AP’s subnet 6: Wireless and Mobile Networks

  14. B A C C C’s signal strength A’s signal strength B A space IEEE 802.11: multiple access • 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) 6: Wireless and Mobile Networks

  15. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFSthen transmit entire frame (no CD) 2 ifsense 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 else /* received ack */ return back to 2 (why?) to transmit next frame 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) (no ack in ethernet!!) sender receiver DIFS: distributed inter-frame spacing, SIFS: short inter-frame spacing 6: Wireless and Mobile Networks

  16. 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 BS using CSMA • RTSs may still collide with each other (but they’re short) • BS broadcasts clear-to-send CTS in response to RTS • RTS heard by all nodes • sender transmits data frame • other stations defer transmissions Avoid long data frame collisions using small reservation packets! 6: Wireless and Mobile Networks

  17. 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 DIFS CIFS CIFS defer CIFS time 6: Wireless and Mobile Networks

  18. RTS/CTS in Practice • RTS/CTS introduces delay, consume channel resource. • Benefit when the data frame is much larger than RTS/CTS. • APs set threshold of data frame length in order to use RTS/CTS • If > threshold, use RTS/CTS • Many APs skip RTS/CTS by using a threshold larger than the Max frame length 6: Wireless and Mobile Networks

  19. 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 6: Wireless and Mobile Networks

  20. 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 6: Wireless and Mobile Networks

  21. 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 duration of reserved transmission time (data, RTS/CTS) frame type (RTS, CTS, ACK, data) 6: Wireless and Mobile Networks

  22. H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 AP2 broadcast H1’s MAC to switch router 802.11: mobility within same subnet hub or switch BBS 1 AP 1 AP 2 H1 BBS 2 6: Wireless and Mobile Networks

  23. 802.15 MAC and Bluetooth • 802.11 MAC • 11 Mbps – 54 Mbps • Up to 100 meters range • 802.15 MAC • Wireless personal area network (WPAN) • < 10 meters range • Simple (cheap) device, low power assumption • Cable, wire replacement • E.g., mouse, keyboard, headphone • Example: Bluetooth 6: Wireless and Mobile Networks

  24. Bluetooth • Physical layer properties: • 2.4GHz unlicensed spectrum • Frequency-hopping spread spectrum • 79 channels with different frequencies • TDM transmit: jump among channels with preset sequences (coding) • Up to 721bps (802.11 is 11 Mbps to 54 Mbps) 6: Wireless and Mobile Networks

  25. Bluetooth • Ad hoc network structure • One master, <=7 slaves • Odd time slot: master • Even time: slaves • Parked: inactive devices • Problem: slow speed can be achieved by RF device • Much cheaper, simpler 6: Wireless and Mobile Networks

  26. CDMA Principle (6.2.1) • Code Division Multiple Access • Wide spectrum technique • All users use the full spectrum • Users with different codings not interfere • Each bit is encoded by much high rate signal (code) • Receiver can recover the bit with the corresponding code 6: Wireless and Mobile Networks

  27. CDMA example 6: Wireless and Mobile Networks

  28. Working with multiple users • How to extract data when multiple users transmit at the same time? • Assumptions: • Interfering signals are additive • Signal 1+1+1+(-1) = 2 • New signals in the air (N senders): Same decoding formula! 6: Wireless and Mobile Networks

  29. Why extract correctly By each user? A: user codes are orthogonal 6: Wireless and Mobile Networks

  30. connects cells to wired tel. net. • manages call setup (more later!) • handles mobility (more later!) Mobile Switching Center Mobile Switching Center • covers geographical region • base station(BS) analogous to 802.11 AP • mobile usersattach to network through BS • air-interface:physical and link layer protocol between mobile and BS Public telephone network MSC cell wired network Components of cellular network architecture Wireless, Mobile Networks

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

  32. Base transceiver station (BTS) Base station controller (BSC) Mobile Switching Center (MSC) Mobile subscribers 2G (voice) network architecture Base station system (BSS) MSC G BTS BSC Public telephone network Gateway MSC Legend Wireless, Mobile Networks

  33. Serving GPRS Support Node (SGSN) Gateway GPRS Support Node (GGSN) 3G (voice+data) network architecture MSC G Public telephone network radio network controller Gateway MSC G Public Internet SGSN • Key insight: new cellular data • network operates in parallel • (except at edge) with existing • cellular voice network • voice network unchanged in core • data network operates in parallel GGSN Wireless, Mobile Networks

  34. 3G (voice+data) network architecture MSC G Public telephone network radio network controller Gateway MSC G Public Internet SGSN GGSN radio interface (WCDMA, HSPA) core network General Packet Radio Service (GPRS) Core Network public Internet radio access network Universal Terrestrial Radio Access Network (UTRAN) Wireless, Mobile Networks

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