Wireless Ad Hoc & Sensor Networks Wireless Ad Hoc & Sensor - PowerPoint PPT Presentation

Outline Wireless Ad Hoc & Sensor Networks Wireless Ad Hoc & Sensor Networks • Multiple Access Technique Medium Access Control Application • Designing Issues of MAC protocols • Classification of MAC protocols Classification of MAC protocols Transport Protocol • Protocols examples • Characteristics of Link layer protocols • Characteristics of Link layer protocols N Network Protocol k P l • The lower layers in detail WS 2010/2011 Media Access Protocol Media Access Protocol • Summary Prof. Dr. Dieter Hogrefe Physical Channel (Radio) Physical Channel (Radio) Dr. Omar Alfandi Dr. Omar Alfandi 2 Media Access Control (Intro.) Multiple Access Technique • Wireless medium is shared • Reservation-based ( Recall: mobile communication 1 ) – FDMA : Frequency Division Multiple Access • Many nodes may need to access the wireless medium to – TDMA : Time Division Multiple Access send or receive messages – CDMA : Code Division Multiple Access • Concurrent message transmissions may interfere with – SDMA : Space Division Multiple Access SDMA : Space Division Multiple Access each other  collisions  message drops • Random – ALOHA : University of Hawaii Protocol ALOHA : University of Hawaii Protocol – CSMA : Carrier Sense Multiple Access – MACA : Multiple Access with Collision Avoidance • Random with reservation – DAMA : Demand Assigned Multiple Access – PRMA : Packet Reservation Multiple Access 3 4

Reservation-based FDD and TDD • FDMA (Frequency Division Multiple Access) • In case of tow communicating parties sharing the medium: – assign a certain frequency to a transmission channel – permanent (radio broadcast), slow hopping (GSM), fast hopping – Simplex : one way communication from sender to receiver (FHSS, Frequency Hopping Spread Spectrum) – Duplex : two way communication between two parties • TDMA (Time Division Multiple Access) • TDMA (Time Division Multiple Access) – assign a fixed sending frequency for a certain amount of time – Frequency division duplex (FDD) • Combination of two simplex channels with different carrier • CDMA (Code Division Multiple Access) CDMA (Code Division Multiple Access) frequencies • SDMA (Space Division Multiple Access) – Time division duplex (TDD) – segment space into sectors, use directed antennas g p , • Time sharing of a single channel achieves quasi-simultaneous Time sharing of a single channel achieves quasi simultaneous duplex transmission – Use cells to reuse frequencies • Combinations 5 6 Random Access Multiple Access • However, wireless communication is often much more Characteristics: ad-hoc • Shared medium : radio channel is shared by an priori – New terminals have to register with the network unknown number of stations – Terminals request access to the medium spontaneously • Broadcast medium: all stations within transmission range – In many cases there is no central control In many cases there is no central control of a sender receive the signal Challenge: Other access methods such as distributed and • Wireless communication channel is prone to errors and non arbitrated = random access non-arbitrated = random access problems, e.g., hidden/exposed node problems & signal bl hidd / d d bl & i l attenuation 7 8

Wired vs. Wireless Outline • Ethernet uses 1-persistent CSMA/CD • Multiple Access Technique – carrier sense multiple access with collision detection • Designing Issues of MAC protocols g g p • Sense if the medium is free and start sending as soon as it becomes free • While sending listen to the medium to detect other senders • Classification of MAC protocols Classification of MAC protocols • In case of a collision immediately stop sending and wait for the • Protocols examples random amount of time • Problems in wireless networks • Problems in wireless networks • Characteristics of Link layer protocols • Characteristics of Link layer protocols – signal strength decreases quickly with distance • The lower layers in detail – senders apply CS and CD, but the collisions happen at receivers pp y , pp • Summary – Energy efficiency: having the radio turned on costs almost as much energy as transmitting, so to seriously save energy one needs to turn the radio off! needs to turn the radio off! 9 10 Need for MAC Protocols ? Hidden Terminal Problem • Popular CSMA/CD (Carrier Sense Multiple • A sends to B, C cannot receive A Access/Collision Detection) scheme is not applicable to • C wants to send to B, C senses a “free” medium (CS wireless networks fails) • CSMA suffers hidden terminal & exposed terminal problems • collision at B, A cannot receive the collision (CD fails) • Collision Detection is impossible in wireless Collision Detection is impossible in wireless • A is “hidden” for C communication Specific MAC protocols for the access to the physical layer physical layer A B C 11 12

Exposed Terminal Problem Near and Far Terminals • B sends to A, C wants to send to D • Terminals A and B send, C receives – the signal of terminal B hides A’s signal • C has to wait, CS signals a medium in use – C cannot receive A • since A is outside the radio range of C waiting is not necessary • C is “exposed” to B A B C – This is also a severe problem for CDMA networks – precise power control required D D A A B B C C 13 14 Classification of MAC protocols Outline • Multiple Access Technique • Designing Issues of MAC protocols g g p • Classification of MAC protocols • Protocols examples • Characteristics of Link layer protocols • Characteristics of Link layer protocols • The lower layers in detail • Summary 15 16

In general (1/2) In general (2/2) • Contention-based protocols: • Contention-based with scheduling – A node does not make any resource reservation a priori. – These protocols focus on packet scheduling at nodes, and also scheduling nodes for access to the channel h d li d f t th h l – Whenever a node receives a packet to be transmitted, it contends with its neighbour nodes for access – Used for enforcing priorities among flows whose packets are q queued at nodes – Can not provide QoS (Quality of Service) guarantees to session Can not provide QoS (Quality of Service) guarantees to session since nodes not guaranteed regular access to the channel – Some of them take into consideration battery characteristics (remaining battery power) • Other protocols Oth t l • Contention-based with reservation – Wireless networks may need to support real-time traffic – Reservation mechanisms for reserving bandwidth a priori R ti h i f i b d idth i i – Such protocols can provide QoS support to time-sensitive traffic sessions 17 18 Outline Multiple Access with Collision Avoidance (MACA) A B C D • MACA uses a two step signaling • Multiple Access Technique procedure to address the hidden RTS • Designing Issues of MAC protocols g g p and exposed terminal problems d d t i l bl • Classification of MAC protocols • Use short signaling packets for CTS • Protocols examples Protocols examples collision avoidance collision avoidance – Request (or ready) to send RTS: a b u sender requests the right to send q g • Characteristics of Link layer protocols • Characteristics of Link layer protocols s s Data Data from a receiver with a short RTS b y u • The lower layers in detail packet before it sends a data packet s y y – Clear to send CTS: the receiver Clear to send CTS: the receiver • Summary grants the right to send as soon as it ACK is ready to receive 19 20