Today CMPE 257 Wireless and Mobile Networking n n n Satellite MANETs MAC Spring 2003 Lecture 2 CMPE 257 Spring 2003 1 Satellite Communications n n n n n 2 Satellite Communications Satellite based antenna e in stable orbit above earth Two or more earth stations communicate via one or more satellites serving as relay s in space Uplink earth satellite Downlink satellite earth Transponder satellite electronics converting uplink signal to downlink CMPE 257 Spring 2003 CMPE 257 Spring 2003 3 SAT ground stations CMPE 257 Spring 2003 4 Orbits n n n GEO Satellites Shape circular elliptical Plane equatorial polar Altitude geostationary GEO medium earth MEO low earth LEO CMPE 257 Spring 2003 n n n n CMPE 257 Spring 2003 n Stationarity no frequency changes due to movement Tracking by earth stations simplified At that altitude provides good coverage of the earth n n n Minus s n n n n Weakening of signal Polar regions poorly served Delay Spectral waste for point to point communications CMPE 257 Spring 2003 Most common type Orbit at 35 863 Km above earth and rotates in equatorial plane Many GEO satellites up there 6 LEO Satellites Plus s n n 5 GEO Plus s and minus s n n 7 n Circular or slightly eliptical orbit under 2 000 Km Orbit period 1 5 to 2 hours Coverage diameter 8 000 Km RTT propagation delay 20ms compared to 300ms for GEOs Subject to large frequency changes and gradual orbit deterioration CMPE 257 Spring 2003 8 LEOs LEO Constellations n Advantages over GEOs n n n constellation SAT Lower delay stronger signal more localized coverage SAT SAT But for broad coverage many satellites needed Example Iridium 66 satellites ground stations CMPE 257 Spring 2003 9 In Summary n n n Long delay 250 300 ms LEOs n n Relatively low delay 40 200 ms Large variations in delay multiple hops route changes relative motion of satellites queuing n n n n CMPE 257 Spring 2003 10 Satellite Bandwidths GEOs n CMPE 257 Spring 2003 11 Typical satellite has 12 20 transponders each ranging from 36 50 Mbps Computer Networks 3rd Edition A Tanenbaum T1 1 54 Mbps T2 6 312 Mbps T3 44 736 Mbps T4 274 176 Mbps CMPE 257 Spring 2003 12 MANETs n n n n n Multi hop Mobile wireless multi hop ad hoc networks Formed by wireless hosts which may be mobile Without necessarily using a pre existing infrastructure Routes between nodes may potentially contain multiple hops Mobilitty cause routes to change CMPE 257 Spring 2003 n 13 Why MANETs n n n May need to traverse multiple hops to reach destination CMPE 257 Spring 2003 Many Applications Ease of deployment Speed of deployment Decreased dependence on infrastructure n Personal area networking n Military environments n Civilian environments n Emergency operations n n n n n n CMPE 257 Spring 2003 14 15 Cell phone laptop ear phone wrist watch Soldiers tanks planes Smart environments Search and rescue Policing and fire fighting Monitoring and surveillance CMPE 257 Spring 2003 16 Many Variations Many Variations cont d Fully Symmetric Environment n n n All nodes have identical capabilities and responsibilities n Asymmetric Capabilities n n n Transmission ranges battery life processing capacity and speed of movement may vary n Asymmetric Responsibilities n n n n Only some nodes may route packets Some nodes may act as leaders of nearby nodes e g cluster head CMPE 257 Spring 2003 n n n n CMPE 257 Spring 2003 n People sitting at an airport lounge New York taxi cabs Students moving on campus Military movements Personal area network CMPE 257 Spring 2003 May co exist and co operate with an infrastructure based network 18 Many Variations cont d Mobility patterns may be different n n Bit rate Timeliness constraints Reliability requirements Unicast multicast geocast 17 Many Variations cont d n Traffic characteristics may differ in different ad hoc networks Mobility characteristics n n Speed Predictability n n n 19 direction of movement pattern of movement uniformity or lack thereof of mobility characteristics among different nodes CMPE 257 Spring 2003 20 Challenges n n n n n n n n n Hidden Terminal Problem Limited wireless transmission range Broadcast nature of the wireless medium Hidden terminal problem Packet losses due to transmission errors Mobility induced route changes Mobility induced packet losses Battery constraints Potentially frequent topology changes Ease of snooping on wireless transmissions CMPE 257 Spring 2003 21 Carrier Sense Does Not Work n n Station avoids collisions by sensing carrier before transmitting Relevant contention at the receiver not sender n n Collision happens when multiple signals interfere at receiver CS cannot avoid collisions at receiver CMPE 257 Spring 2003 CMPE 257 Spring 2003 Hidden terminals n A n B n C 23 22 B can hear both A and C But A and C cannot hear each other If A is transmitting to B and if C starts to transmit hidden terminal scenario at B CMPE 257 Spring 2003 A B C 24 Exposed terminals n n n B is sending to A C is ready to transmit but detects carrier and defers transmission But A is out of C s range CMPE 257 Spring 2003 Research on MANETs Variations in capabilities responsibilities Variations in traffic characteristics mobility models etc Performance criteria e g optimize throughput reduce energy consumption Increased research funding Significant research activity A B C 25 One size fits all n n n 26 References Perhaps using an adaptive hybrid approach that can adapt to situation at hand Difficult problem Solutions usually try to address a subspace of the problem domain CMPE 257 Spring 2003 CMPE 257 Spring 2003 27 n Nitin Vaidya s tutorials www crhc uiuc edu nhv presentations html n n Stalling s Wireless Communications and Networks Rappaport s Wireless Communications Principles and Practice CMPE 257 Spring 2003 28 Frequency Division Multiplexing The Multiplexing Problem frequency frequency user 1 A wireless channel user 2 how to divide resource among multiple recipients user 3 user 4 guard band Analogy a highway shared by many users time Analogy a highway has multiple lanes CMPE 257 Spring 2003 29 Time Division Multiplexing time CMPE 257 Spring 2003 30 Frequency Time Division frequency frequency user 1 user 2 user 3 user 4 user 1 user 2 time slot usually of the same size guard band Requirement precise time coordination CMPE 257 Spring 2003 time time 31 CMPE 257 Spring 2003 32 Medium Access Control Protocols APPLICATION Coordinate competing requests for medium Medium Access for Wireless n MAC from wired medium unsuitable n Special features of wireless