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8/7/20151 Mobile Computing COE 446 Wireless Multiple Access Tarek Sheltami KFUPM CCSE COE ht Principles.

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Presentation on theme: "8/7/20151 Mobile Computing COE 446 Wireless Multiple Access Tarek Sheltami KFUPM CCSE COE ht Principles."— Presentation transcript:

1 8/7/20151 Mobile Computing COE 446 Wireless Multiple Access Tarek Sheltami KFUPM CCSE COE ht Principles of Wireless Networks K. Pahlavan and P. Krishnamurth

2 8/7/20152 Outline CSMA/CA BTMA DSMA

3 8/7/20153 IEEE 802.11 Wireless LAN 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

4 8/7/20154 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 BSS 1 BSS 2 Internet hub, switch or router AP

5 8/7/20155 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 8/7/20156 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 IEEE 802.11 MAC Protocol CSMA/CA

7 8/7/20157 CSMA..

8 8/7/20158 CSMA.. Several variations of CSMA have been employed in LANs, however, ALOHA is still preferred in WANs CSMA a) Persistent b) Non-Persistent After sensing the busy channel: a)The MT continues sensing the channel until the channel becomes free b)The MT attempts to another sensing after a random wait period

9 8/7/20159 CSMA.. a) 1-Persistent b) P-Persistent After the channel becomes free: a)The MT transmits its packet immediately b)The MT runs a random number generator and based on the outcome, transmits its packet with probability p Persistent

10 8/7/201510 CSMA..

11 8/7/201511 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 8/7/2015128/7/201512 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)

13 8/7/2015138/7/201513 Busy Tone Multiple Access (BTMA) Receiver transmits a busy tone in another channel (control channel) Nodes sense the control channel before transmitting

14 8/7/2015148/7/201514 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)

15 8/7/201515 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 8/7/201516 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 8/7/201517 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) A B C A B C A’s signal strength space C’s signal strength

18 8/7/2015188/7/201518 Carrier Sense Multiple Access Collision Avoidance (CSMA/CA)

19 8/7/201519 CSMA/CA.. 802.11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense 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) sender receiver DIFS data SIFS ACK

20 8/7/201520 CSMA/CA..

21 8/7/201521 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 8/7/201522 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 8/7/201523 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

24 8/7/201524 Performance Analysis

25 8/7/201525 Performance Analysis..

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