Lecture 3 - Sept 10 - 1 Mobile Communications Chapter 3 :...

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Unformatted text preview: 1 Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha, reservation schemes Collision avoidance, MACA Polling 2 About CSMA/CD Can we apply media access methods from fixed networks? Example of CSMA/CD C arrier S ense M ultiple A ccess with C ollision D etection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks a radio can usually not transmit and receive at the same time signal strength decreases proportionally to the square of the distance or even more the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot “hear” the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is “hidden” 3 Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B Hidden and exposed terminals B A C 4 Access methods SDMA/TDMA/FDMA/CDMA SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) CDMA (Code Division Multiple Access) assign an appropriate code to each transmission channel (DSSS, Direct Sequency Spread Spectrum) frequency hopping over separate channels (FHSS, Frequency Hopping Spread Spectrum) 5 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA • Used in GSM Slotted Non-persistent p-persistent CSMA/CA • Copes with hidden and exposed terminal • RTS/CTS • Used in 802.11 (optional) MACAW MACA-BI FAMA CARMA • Used in 802.11 (mandatory) • Used in 802.11 (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple FHSS DSSS • Used in GSM Fixed • Used in Bluetooth • Used in UMTS • Used in GSM 6 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations...
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This note was uploaded on 11/03/2009 for the course COMPUTERS CS537 taught by Professor Salman during the Spring '09 term at Texas A&M University–Commerce.

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Lecture 3 - Sept 10 - 1 Mobile Communications Chapter 3 :...

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