Review of Last Lecture! Wireless! P2P Tasks! Wireless Networks! Wireless Communication Standards! Wireless Link Characteristics!

Transcription

1 Wireless! EE : Intro to ommunication Networks Fall 00 (MW -:0 in 0 arker) cott henker Ts: ameer garwal, ara lspaugh, Igor Ganichev, Prayag Narula Materials with thanks to Jennifer exford, Ion toica, Vern Paxson and other colleagues at Princeton and U erkeley eview of Last Lecture! Peer-to-peer: esign paradigm: No central contol Economic model: leverage user nodes s an economic model: Latest step in war over control Grassroots way to build large-scale system ut need good coordination mechanism PP Tasks! earch: general keyword/wildcard search ontrolled flooding Lookup: given name, get IP address of location HT or similar structure Wireless Networks! ownload: hunking is used because of asymmetric bandwidth elf-scaling depends on an incentive mechanism o Prevent free-riders Wireless ommunication tandards! ellular G: GM (Global ystem for Mobile communication), M (ode ivision Multiple ccess) G: M000 IEEE 0. :.0Ghz band, Mbps ( Mbps operating rate) :.Ghz band, Mbps (. Mbps operating rate) G:.Ghz, Mbps ( Mbps operating rate) N:./Ghz, 0Mbps IEEE 0. lower power wireless 0..:.Ghz,. Mbps (luetooth) 0..:.Ghz, 0 Kbps (ensor Networks) Wireless Link haracteristics! (Figure ourtesy of Kurose and oss)

2 What Makes Wireless ifferent?! ignals sent by sender don t always end up at receiver intact omplicated physics involved, which we won t discuss ut what can go wrong? Two quantities we care about: ignal-to-noise ratio (N) o ignal Power/Noise Power it-error ate (E) Path Loss! ignal power attenuates by about ~r factor for omni-directional antennas in free space r is the distance between the sender and the receiver The exponent depends on placement of antennas Less than for directional antennas Greater than when antennas are placed on the ground ignal bounces off the ground and reduces the power of the signal Multipath Effects! ignals bounce off surface and interfere with one another elf-interference Floor eiling Interference from Other ources! External Interference Microwave is turned on and blocks your signal Internal Interference Hosts within range of each other collide with one another s transmission (remember loha) We have to tolerate path loss, multipath, etc., but we can try to avoid internal interference This is what makes 0. interesting!. 0 Ideal adios (courtesy of Gilman Tolle and Jonathan Hui, rchock)! eal adios (courtesy of Gilman Tolle and Jonathan Hui, rchock)!

3 The moeboed cell (courtesy of avid uller, U)! Wireless it Errors! ignal Noise istance The lower the N (ignal/noise) the higher the it Error ate (E) We could make the signal stronger! Why is this not always a good idea? Increased signal strength requires more power Increases the interference range of the sender, so you interfere with more nodes around you o nd then they increase their power!!. Error orrection schemes can correct some problems 0. rchitecture! 0.! 0. frames exchanges esigned for limited area 0. (Ethernet) frames exchanged P s (ccess Points) set to specific channel roadcast beacon messages with I (ervice et Identifier) and M ddress periodically Hosts scan all the channels to discover the P s Host associates with P (actively or passively) Wireless Multiple ccess Technique?! arrier ense? ender can listen before sending What does that tell the sender? ollision etection? Where do collisions occur? How can you detect them? Hidden Terminals! transmit range and can both send to but can t hear each other is a hidden terminal for and vice versa arrier ense will be ineffective need to sense at receiver

4 Exposed Terminals! Key Points! No concept of a global collision ifferent receivers hear different signals ifferent senders reach different receivers Exposed node: sends a packet to ; hears this and decides not to send a packet to (despite the fact that this will not cause interference)! arrier sense would prevent a successful transmission ut we do carrier sense anway (why?) ollisions are at receiver, not sender Only care if receiver can hear the sender clearly It does not matter if sender can hear someone else s long as that signal does not interfere with receiver Goal of protocol: etect if receiver can hear sender Tell senders who might interfere with receiver to shut up 0 asic ollision voidance! ince can t detect collisions, we try to avoid them arrier sense: When medium busy, choose random interval Wait that many idle timeslots to pass before sending When a collision is inferred, retransmit with binary exponential backoff (like Ethernet) Use K from receiver to infer no collision Use exponential backoff to adapt contention window How would we get Ks if we didn t do carrier sense? M with ollision voidance (M)! sender T T data K receiver other node in sender s range efore every data transmission ender sends a equest to end (T) frame containing the length of the transmission eceiver respond with a lear to end (T) frame ender sends data eceiver sends an K; now another sender can send data When sender doesn t get a T back, it assumes collision M, con t! M, con t! receiver sender T T data other node in sender s range data sender T T data receiver other node in sender s range K If other nodes hear T, but not T: send Presumably, destination for first sender is out of node s range! If other nodes hear T, but not T: send Presumably, destination for first sender is out of node s range!! an cause problems when a T is lost When you hear a T, you keep quiet until scheduled transmission is over (hear K)

5 T / T Protocols (M)! sends to T T Minute reak! Overcome hidden terminal problems with contention-free protocol. sends to equest To end (T). hears T and defers (to allow to answer). replies to with lear To end (T). hears T and defers to allow the data. sends to Questions efore We Proceed? hannelization of spectrum! Preventing ollisions ltogether! Typically, available frequency spectrum is split into multiple channels ome channels may overlap Frequency pectrum partitioned into several channels Nodes within interference range can use separate channels channels channels channels MHz 00 MHz 00 MHz 0 MHz MHz. GHz. GHz. GHz 0 MHz 00 MHz 000 MHz. GHz. GHz. GHz Now and can send without any interference! ggregate Network throughput doubles Using Multiple hannels! 0.: P s on different channels Usually manually configured by administrator utomatic onfiguration may cause problems Most cards have only transceiver Not Full uplex: annot send and receive at the same time Multichannel M Protocols utomatically have nodes negotiate channels o hannel coordination amongst nodes is necessary o Introduces negotiation and channel-switching latency that reduce throughput Wireless Multihop Networks! Vehicular Networks elay Tolerant (batch) sending over several hops carry data to a base station ommon in ensor Network for periodically transmitting data Infrastructure Monitoring o E.g., structural health monitoring of the Golden Gate ridge Multihop networking for Internet connection sharing outing traffic over several hops to base station connected to Internet E.g., Meraki Networks 0

6 Large Multihop Network (courtesy of anjit iswas, MIT)! Multi-Hop Wireless d Hoc Networks (ourtesy of Tianbo Kuang and arey Williamson University of algary)! kilometer Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! (ssume ideal world!) 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0

7 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0

8 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0

9 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! What o YOU Think eally Happens?! 0 Multi-Hop Wireless d Hoc Networks! 0 (eality check!) Problem : node can t use both of these links at the same time - shared wireless channel - transmit or receive, but not both Multi-Hop Wireless d Hoc Networks! 0 Problem : and can t use both of these links at same time - range overlap at

10 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 Problem : LOT of contention for the channel - in steady state, all want to send - need T/T to resolve contention 0 T T T: equest-to-end T: lear-to-end Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 T T Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 0

11 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0

12 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Problem : TP uses K to indicate reliable data delivery - bidirectional traffic (T, K) - even more contention!!! 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0

13 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0

14 Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! Multi-Hop Wireless d Hoc Networks! 0 0 Lesson! Multihop wireless is hard to make efficient ummary! Wireless is a tricky beast istributed multiple access problem Hidden terminals Exposed terminals urrent protocols sufficient, given overprovisioning Multihop even more complicated

15 est of ourse! Next lecture: ecurity fter that: Future of Networking Why things will look completely different in ten years eview