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Mobile Computing COE 446 Wireless Multiple Access

Mobile Computing COE 446 Wireless Multiple Access. Tarek Sheltami KFUPM CCSE COE ht http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm. Principles of Wireless Networks K. Pahlavan and P. Krishnamurth. CSMA/CA BTMA DSMA. Outline. 802.11b 2.4-5 GHz unlicensed radio spectrum

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Mobile Computing COE 446 Wireless Multiple Access

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  1. Mobile Computing COE 446Wireless Multiple Access Tarek Sheltami KFUPM CCSE COE hthttp://faculty.kfupm.edu.sa/coe/tarek/coe446.htm Principles of Wireless Networks K. Pahlavan and P. Krishnamurth

  2. CSMA/CA BTMA DSMA Outline

  3. 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

  4. 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

  5. 802.11: Channels, association • 802.11b: 2.4GHz-2.485GHz spectrum 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! • 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 • will typically run DHCP to get IP address in AP’s subnet

  6. IEEE 802.11 MAC Protocol CSMA/CA • Listen Before Talk (LBT) • MT1 senses the channel first and then sends a packet • During the second transmission of MT1, MT2 senses the channel and discovers that another terminal is using the medium • MT2 delays the transmission using a pack-off algorithm • CSMA reduces packet collision probability significantly compared with ALOHA • The longer the propagation time, the less the efficient the CSMA

  7. CSMA..

  8. a) Persistent b) Non-Persistent CSMA.. • Several variations of CSMA have been employed in LANs, however, ALOHA is still preferred in WANs CSMA • After sensing the busy channel: • The MT continues sensing the channel until the channel becomes free • The MT attempts to another sensing after a random wait period

  9. a) 1-Persistent b) P-Persistent CSMA.. Persistent • After the channel becomes free: • The MT transmits its packet immediately • The MT runs a random number generator and based on the outcome, transmits its packet with probability p

  10. CSMA..

  11. CSMA.. • Hidden Terminal Problem: • Two terminal within the range of intended third node, but out of range of each other • Busy Tone Multiple Access (BTMA) (for Ad hoc) • BW is divided into two channels, message channel and busy channel • Whenever an MT sends signal on the message channel, it transmits a simple busy tone signal on its busy tone channel • When any MT senses the busy tone signal, it runs on its own busy tone channel • An MT with packet ready to send, first senses the busy tone channel to determine if the network is occupied

  12. It is possible that a node senses the channel idle, but should not transmit (the hidden terminal problem) Also, It is possible that a node senses the channel busy, but should transmit (In the examples, only nodes connected by a straight line can listen to each other’s transmissions) Carrier Sense Multiple Access (CSMA) 10/7/2014 12

  13. Busy Tone Multiple Access (BTMA) Receiver transmits a busy tone in another channel (control channel) Nodes sense the control channel before transmitting 10/7/2014 13

  14. Disadvantages of BTMA Some bandwidth is sacrificed, and nodes must be full-duplex (harder than half-duplex) If all receivers transmit BT, some transmitters are unnecessarily stopped If only intended receiver transmits BT, there are collisions elsewhere In first case, B can not transmit a packet to A (he should have been allowed) In second case, B transmits a packet to C and there is collision (bandwidth is wasted) 10/7/2014 14

  15. CSMA.. • Most cellular mobile data use different frequencies for downlink and uplink • The BS is not hidden from MTs, whereas MTs may be hidden from on another • In this situation, one may use the downlink channel to announce the availability of the MT • This concept is used in a protocol referred to as digital or data sense multiple access (DSMA)

  16. CSMA.. • DSMA • Widely used in mobile networks • The downlink broadcasts a periodic busy-idle bit announcing availability of downlink channel for data transmission of its packet • An MT checks the busy-idle bit prior to transmission of its packet • As soon as the MT starts the transmission of its packet, BS changes the busy-idle bit to the busy-state to prevent other MTs from transmission

  17. B A C C C’s signal strength A’s signal strength B A space CSMA/CA • 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)

  18. Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) 10/7/2014 18

  19. DIFS data SIFS ACK CSMA/CA.. sender receiver 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 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem)

  20. CSMA/CA..

  21. CSMA/CA.. • IEEE 802.11b has four types of Inter Frame Space (IFS): • Short IFS (SIFS):is the period between the completion of packet transmission and the start of the ACK frame. (The minimum IFS) • Point Coordination IFS (PIFS): is SIFS plus a Slot Time, which is optional and we do not use it in our study • Distributed IFS (DIFS): is PIFS plus a Slot Time • Extended IFS (EIFS): is a longer IFS used by a station that has received a packet that it could not understand. This is needed to prevent collisions

  22. RTS/CTS 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

  23. 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

  24. Performance Analysis

  25. Performance Analysis..

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