Elements of a wireless network

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. Outdoor 50 – 200m Mid range outdoor 200m – 4Km Long range outdoor 5Km – 20Km Indoor 10 – 30m Characteristics of selected wireless link standards mesh 54 Mbps 802.11{a,g} 802.16e or Mobile WiMAX 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 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 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. CDMA example 6: Wireless and Mobile Networks

27. 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

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