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Medium Access Control in Wireless Networks

Medium Access Control in Wireless Networks 45
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Published Date:19-07-2017
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Medium Access Control in Wireless NetworksAcknowledgements Many of the slides are based on a tutorial by Prof. Nitin Vaidya and some other sources.Overview • Issues with spectrum sharing • Current MAC protocols • Power management • Rate control • Interaction with PHY layerMedium Access Control • Wireless channel is a shared medium • MAC coordinates transmission between users sharing the spectrum • Goals: prevent collisions while maximizing throughput and minimizing delay • Types: – Centralized – DecentralizedMAC Protocols: a taxonomy Three broad classes: • Channel Partitioning – divide channel into smaller “pieces” (time slots, frequency) – allocate piece to node for exclusive use • Random Access – allow collisions – “recover” from collisions • “Taking turns” – tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralizedChannel Partitioning MAC protocols: TDMA TDMA: time division multiple access • Access to channel in "rounds" • Each station gets fixed length slot (length = pkt trans time) in each round • Unused slots go idle Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access • Channel spectrum divided into frequency bands • Each station assigned fixed frequency band • Unused transmission time in frequency bands go idle Random Access Protocols: Unslotted ALOHA • Simpler, no synchronization • Packet needs transmission: – Send without awaiting for beginning of slot – Maximum throughput: 18.4%Slotted Aloha • time is divided into equal size slots (= pkt trans. time) • node with new arriving pkt: transmit at beginning of next slot • if collision: retransmit pkt in future slots with probability p, until successful. • Maximum throughput: 37% Success (S), Collision (C), Empty (E) slotsCarrier Sense Multiple Access (CSMA) • In some shorter distance networks, it is possible to listen to the channel before transmitting • In radio networks, this is called “sensing the carrier” • The CSMA protocol works just like Aloha except: If the channel is sensed busy, then the user waits to transmit its packet, and a collision is avoided • This really improves the performance in short distance networksHidden Terminal Problem A B C Nodes A and C cannot hear each other Transmissions by nodes A and C can collide at node B Nodes A and C are hidden from each otherBusy Tone Solutions Tobagi75 • A receiver transmits busy tone when receiving data • All nodes hearing busy tone keep silent • Avoids interference from hidden terminals • Requires a separate channel for busy toneMACA Solution for Hidden Terminal Problem Karn90 • When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to A • On receiving RTS, node A responds by sending Clear-to-Send (CTS), provided node A is able to receive the packet • When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer – Transfer duration is included in RTS and CTS both A B CReliability • Wireless links are prone to errors. High packet loss rate detrimental to transport- layer performance. • Mechanisms needed to reduce packet loss rate experienced by upper layersSimple Solution to Improve Reliability • When node B receives a data packet from node A, node B sends an Acknowledgement (Ack). This approach adopted in many protocols. • If node A fails to receive an Ack, it will retransmit the packet. A B CIEEE 802.11 Wireless MAC Distributed and centralized MAC components – Distributed Coordination Function (DCF) – Point Coordination Function (PCF) • DCF suitable for multi-hop ad hoc networking • DCF is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocolIEEE 802.11 DCF • Uses RTS-CTS exchange to avoid hidden terminal problem – Any node overhearing a CTS cannot transmit for the duration of the transfer • Uses ACK to achieve reliability • Any node receiving the RTS cannot transmit for the duration of the transfer – To prevent collision with ACK when it arrives at the sender – When B is sending data to C, node A will keep quite A B CCollision Avoidance • With half-duplex radios, collision detection is not possible • CSMA/CA: Wireless MAC protocols often use collision avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism • Carrier sense: When a node wishes to transmit a packet, it first waits until the channel is idle. • Collision avoidance: Nodes hearing RTS or CTS stay silent for the duration of the corresponding transmission. Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit.IEEE 802.11 RTS = Request-to-Send RTS A B C D E F Pretending a circular rangeIEEE 802.11 RTS = Request-to-Send RTS A B C D E F NAV = 10 NAV = remaining duration to keep quiet