Ad-Hoc Networks Seminar
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- Patience Barber
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1 ellular Network Topology hu backbone network, e.g. leased line d-hoc Networks Seminar Introduction SoSe Terminals communicate only with base stations. ssociation between terminal and base station. Handoff = process where terminal associates with another base station. May ase station coordinates access to wireless medium. ase stations connected to high-capacity backbone (e.g. optical fiber), deployment is well planned. Not a true wireless network - only last hop is wireless. (d-hoc) Wireless Mesh Network Topology ad hoc ad hoc [æd 'hɒk] adjective, adverb(ial) formed or used for specific or immediate problems or needs. example: ad hoc solution fashioned from whatever is immediately available. example: ad hoc decision No base stations. Terminal communicates with neighboring terminals. Packets forwarded from terminal to terminal (multi-hop) until final destination. Terminals have to magically coordinate access to wireless medium. May Terminal deployment is not usually planned d-hoc deployment. Dynamic topology (i.e. location of terminals changes) May True wireless network. 3 4
2 dvantages of Wireless Mesh Networks Disadvantages of wireless ad-hoc networks andwidth limitations: - Professionally deployed, d fiber-based b backbone carries ~10 Gbits/sec - Wireless ad-hoc backbone has unpredictable capacity, ~10 Mbit/sec obstacle (wall) No capacity planning possible; no guaranties. Works around shadowing obstacles Potential for multiple concurrent communications Fast deployment (good for rescue operations) - ackbone is created on the fly Very resilient (useful in aggressive environment) - network still functions even if individual node fails May How to encourage/enforce cooperation amongst terminals? - To preserve its battery, a terminal may shutdown if it has no own traffic. - terminal may act selfish (not forward other node s traffic) or even destructive (confuse routing protocol) May heap - no base station - no backbone infrastructure - easy adaptation to changing requirements 6 Hybrid Networks ommunity Mesh Networks backbone network, e.g. leased line WiMax 7 Network contains both, ad-hoc and cellular parts. 4-G Networks will likely have this topology. Our understanding of these networks is still limited. May e.g coordination - 7 of access to wireless medium seems difficult. 8 May Mesh Network Internet DSL Mesh node (RN) lient station (ST) Wireless mesh networks have the potential to extend broadband connectivity
3 ommunity Network Formation Q.: How many homes in the neighborhood have to sign up before a viable mesh forms? nswer depends on - Definition of viable - Wireless range (50, 100, 200 and 1000m) - Neighborhood topology (North Seattle, 4km x 4km) - Probability of participation by a given household May Indoor Testbed 40 to 50 nodes Netgear WGT634u router Purpose: Test of new technology (after NS2/Jist simulation) - Link quality (measurement, prediction) - Topology creation - New routing protocols - Software distribution - Security issues - Stress tests (TP with iperf) - Distributed onfiguration May P. ahl, Opportunities and hallenges of ommunity Mesh Networking, MIS Workshop ETH Zurich, July 6, Visualization Tools Outdoor Testbed ommunity network ( ) > 500 nodes Linksys router, Mesh-cube Purpose Test of new routing protocols/services in real world Permanent use May May
4 History Research started in 70 s. - RP project initiated at Stanford University in alled packet radio networks Interest cooled off in the 80 s. - alled multihop wireless networks Renewed interest in the 90 s. - alled wireless ad-hoc networks Discovery of fundamental capacity limitations in 2000 s. ooming interest in sensor networks in 2000 s. May mateur packet radio network (1970 s) May luetooth (1990 s) Interference fundamental difference between wired and wireless 2 wired links (no interference) In wired networks links are independent. 2 wireless links (with interference) In wireless networks, links are coupled by interference. Wired networks are modeled with graphs. May Wireless networks are modeled in terms of n(n-1) power gains. at receiver: interference < signal Interesting questions Power control - Transmitter power Medium access control - Who can send when to avoid collisions Routing - Finding path to distant destination apacity - How much information can the network carry? Security - What is a node malfunctions / is malicious? May
5 Power ontrol Power ontrol P 1 P 2 γ γ2 P3 γ 1 γ 3 transmissions at powers P 1 P 3 3 simultaneous 17 Each node receives noise, resulting from transmissions of other nodes and general background noise. Signal / Noise ratio (SNR) decides if a node can recover a received signal. May Each node has a SNR requirement γ i To enable simultaneous transmission: - re there powers P i that achieve γ i? - an they be found in a distributed manner? Each node makes its own decision when to send. If a node receives two signals of similar strength, it will not be able to recover either one. May Two transmissions may start at same time (collision) There is no fixed duration of a transmission. 18 Routing apacity The capacity is the theoretical limit on the performance of the wireless network. There are n users, each with n-1 potential destinations. apacity is not a number, but a n(n-1) dimensional region. alculating is a very hard, not completely solved problem. Each node must find a route to every destination Nobody has clear picture of network s topology Topology may change, even as a packet is forwarded (nodes are moving) May How to avoid loops in forwarding path? Traffic aggregation is bad idea - Depletes energy of involved nodes - auses massive interference at involved nodes May
6 Shared hannel ccess May Wireless Medium ccess ontrol (Wireless M) Overlapping areas indicate shared communication channel ccess to communication channel is shared. ny node can transmit at any time. ollisions result from simultaneous transmission in shared channel. Random access leads to many collisions at high load poor May 2006 throughput t No central controller to coordinate access in ad-hoc networks. M protocol enables efficient, coordinated use of medium. M is concerned with per-link communication, not end-to-end Timing in M Protocols Hidden Terminal Problem Synchronous M Protocols: Timer master broadcasts beacon (e.g. from GPS satellite) ll nodes synchronize to same time Time slots may be defined ontention-free channel access possible collision synchronous M Protocols: Time intervals have different offsets at each node. Offsets may drift over time. ontention based channel access May (two nodes race for access to channel) 1. transmits to ; 2. decides to transmit to ollision at does not hear this. ( and may not even notice the collision at ) Two nodes are hidden from each other (out of signal range) May when both perceive radio silence and send data to the same receiving node, resulting in a collision at the receiver
7 Exposed Terminal Problem D transmits to D okay. overhears transmission from to D. wants to transmit to does not know where and are located could be close together; can not know that would not be intimidated by the transmission from. will not transmit to (even en though it would work) n exposed node is a node in range of the transmitter, but out of range of the receiver. Overhearing a transmission from a neighbor inhibits a node from transmitting to other nodes. May Impact: Nodes are unnecessarily blocked, reducing the system s total throughput. Solution: (1) Use directional antennas (so does not hear when transmits to D). (2) Use different channels for data and control messages (so can use - to figure out if it can transmit to without t interfering i with and D). Sender-Initiated M Sender informs receiver that it has data (). Receiver immediately polls sender for data () - same as receiver- initiated iti t M. Receiver may choose to issue at later time. Examples: - M - Multiple ccess with ollision voidance - MW - M with cknowledgement - FM - Floor cquisition Multiple ccess : Ready To Send May : lear To Send M M = Multiple ccess with ollision voidance Suggested by Phil Karn for amateur packet radio networks single-frequency ad-hoc networks --DT three-way handshake Supposed to solve hidden & exposed terminal problem llows for per-packet transmitter power control (improves network capacity) M (--DT Handshake with K) 1. transmits to 2. replies that channel is clear () oth & overhear the broadcast. K K 3. sends data to knows about this. 4. acknowledges data transfer. learns that channel is free again. May May handshake is used to resolve hidden terminal problem. = Ready to Send; transmitted by sender to inform receiver of available data = lear to Send; broadcasted by receiver to inform all neighbors that channel will be occupied K = cknowledgement; broadcasted by receiver to inform all neighbors that channel is free again 27 28
8 Shortcomings of - What happens if a neighbor of the receiver does not hear the? is sender, the receiver, the receiver s neighbor that does not hear the. Possible reason: from and from D collide at. (D can not hear or ). Result: does not hear not aware of data transmission between and. D Shortcomings of - (2) More common example where - problem materializes: Dcan t hear or. D wants to send data to transmits. from D and from collide at hears no or. D re-transmits to broadcasts collision at D node D re-transmit May suddenly, node wants to send to node () May Shortcomings of - (3) node may not hear a because it is transmitting (i.e. cannot receive) D did not hear s because it was transmitting at the time Receiver-Initiated M Why have at all? Let receiver start with. How does receiver know that sender has data? Polling: Receiver er periodically checks if sender has data for it. Received data packet piggybacks number of packets still available at sender - If more packets are available, receiver issues new poll/ immediately after receiving of current packet is complete. Performs well under high load, streaming data (audio). Increased delay under light load (packets have to wait for next poll). Examples: - M-I May May
9 M-I (y Invitation) RTR DT 1. transmits RTR to. locks s neighbors from transmitting. 2. transmits to. locks s neighbors from transmitting. M vs. M-I onstant it Rate - High efficiency of M-I (less control traffic) ursty traffic - M initiates data-forwarding as soon as data available. - Sender s queue fills up in M-I when packets arrive but receiver extends no invitation. Hybrid approach: - M-I with. - only used in rare event where sender s queue too full. 33 Proposed by Fabrizio Talucci 2-way handshake: RTR-DT - no May RTR renamed (Ready To Receive) Node cannot transmit data unless receiver announces invitation (RTR) Sender s queue length, arrival rate regulate receiver s transmission of invitations. 34 M May M-I RTR RTR RTR Time Time Media ccess with Reduced Handshake Receiver/sender initiation hybrid - Tries to work receiver-initiated most of the time Omni-directional antennas - Node can overhear neighbors control traffic Upstream node initiates based on downstream-node s activity. includes Route-ID ID. Upstream-Node knows if it will be next. D ha! PMS usy Tone PMS = Power-ware Multi-ccess with Signaling - over separate signaling channel 1. Sender transmits to receiver 2. Receiver replies with 3. Sender transmits data; Receiver transmits busy tone on signaling g channel while receiving data usy tone introduced by Fouad Tobagi (Stanford Univ.) for TM usy Tone Multiple ccess. - solves hidden terminal problem Energy conservation: - Nodes power off data channel if they hear busy tone and are not sender or receiver of this transmission. 2 May ha! May
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