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ECE 4450:427/527 - Computer Networks Spring 2014

ECE 4450:427/527 - Computer Networks Spring 2014. Dr. Nghi Tran Department of Electrical & Computer Engineering. Lecture 5.6: Wireless Networks - MAC. Wireless Mobile Networks. Brief Introduction Wireless Channel Characteristics WiFi : CSMA/CA - C ollision A voidance.

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ECE 4450:427/527 - Computer Networks Spring 2014

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  1. ECE 4450:427/527 - Computer NetworksSpring 2014 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 5.6: Wireless Networks - MAC ECE 4450:427/527

  2. Wireless Mobile Networks • Brief Introduction • Wireless Channel Characteristics • WiFi: CSMA/CA - Collision Avoidance ECE 4450:427/527

  3. Elements of Wireless Networks wireless hosts • laptop, PDA, IP phone • run applications • may be stationary (non-mobile) or mobile • wireless does not always mean mobility network infrastructure ECE 4450:427/527

  4. Elements of Wireless Mobile Networks 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 (AP) • Handoff: Mobile changes station network infrastructure ECE 4450:427/527

  5. Characteristics of selected wireless link standards 200 802.11n 54 802.11a,g 802.11a,g point-to-point data 5-11 802.11b 802.16 (WiMAX) 3G cellular enhanced 4 UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO Data rate (Mbps) 1 802.15 .384 3G UMTS/WCDMA, CDMA2000 2G .056 IS-95, CDMA, GSM Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km ECE 4450:427/527

  6. Wireless Mobile Networks 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 ECE 4450:427/527

  7. 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) ECE 4450:427/527

  8. Wireless Channel 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 ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” ECE 4450:427/527

  9. Multipath propagation • Constructive and destructive interference: channel strengths change randomly with time→ Fading • When channel is weak, i.e., bad quality→Low reliability ECE 4450:427/527

  10. Hidden Terminal Problem C B A Multiple wireless senders and receivers create additional problems (beyond multiple access): Hidden terminal problem: Signals blocked • 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 ECE 4450:427/527

  11. Hidden Terminal Problem B A C C’s signal strength A’s signal strength space • Hidden terminal problem due to signal attenuation: • B, A hear each other • B, C hear each other • A, C can not hear each other interfering at B ECE 4450:427/527

  12. Exposed Node Problem • Suppose B is sending to A. Node C is aware of this communication because it hears B’s transmission. • It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission. • Suppose C wants to transmit to node D. This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B. ECE 4450:427/527

  13. IEEE 802.11 Wireless LAN • 802.11a • 5-6 GHz range • up to 54 Mbps • 802.11g • 2.4-5 GHz range • up to 54 Mbps • 802.11n:multiple antenna • 2.4-5 GHz range • up to 600 Mbps • 802.11b • 2.4-5 GHz unlicensed spectrum • up to 11 Mbps • direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code • all use CSMA/CA for multiple access • all have base-station and ad-hoc network versions ECE 4450:427/527

  14. 80211. Wireless LAN Architecture AP AP Internet • 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 ECE 4450:427/527

  15. 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 as in Ethernet! • difficult to receive (sense collisions) and transmitting at the same time due to weak received signals: swamps the receiving circuitry • can’t sense all collisions in any case: hidden terminal, fading • goal: avoid collisions: CSMA/C(ollision)A(voidance) ECE 4450:427/527

  16. IEEE 802.11 MAC Protocol: CSMA/CA DIFS data SIFS ACK To deal with transmitting and sensing problems 802.11 sender 1 if sense channel idle for DIFSthen transmit entire frame (no CD) – cannot sense when transmitting 2 if sense channel busy then - start random backoff time • timer counts down while channel idle. If busy, timer frozen • transmit when timer expires. Wait for ACK - ACK: New frame, go to step 1; if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS sender receiver ECE 4450:427/527

  17. IEEE 802.11 MAC Protocol: CSMA/CA To deal with hidden terminal problem: Using RTS and CTS idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of data frames • sender first transmits smallrequest-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 • CTS heard by all nodes • sender transmits data frame • other stations defer transmissions avoid data frame collisions completely using small reservation packets! ECE 4450:427/527

  18. IEEE 802.11 MAC Protocol: CSMA/CA RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) B A AP defer time ECE 4450:427/527

  19. IEEE 802.11 MAC Protocol: CSMA/CA • Applets to check out: • http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withhidden.html • http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withouthidden.html ECE 4450:427/527

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