15-441: Computer NetworkingScenarios and RoadmapWireless Challenges (review)Wireless Bit-ErrorsTCP Problems Over Noisy LinksPerformance DegradationPerformance Degredation 2Proposed SolutionsApproach Styles (Link Layer)Approach Styles (End-to-End)Ad Hoc NetworksAd Hoc RoutingTraditional Routing vs Ad HocProblems using DV or LSProposed protocolsDSRDSR ComponentsDSR Route DiscoveryC Broadcasts Route Request to FSlide 22H Responds to Route RequestC Transmits a Packet to FForwarding Route RequestsRoute CacheSending DataDiscussionForwarding Packets is expensiveETXCapacity of multi-hop networkSensor Networks - smart devicesSensor System Types – Smart-Dust/MotesSensors and power and radiosSensor nets goalsPowerExample: Aggregation15-441: Computer NetworkingLecture 25: Wireless, Ad-Hoc Networks, Sensor NetworksLecture 25:11-30-2006 2Scenarios and Roadmap•Point to point wireless networks•Example: Your laptop to CMU wireless•Challenges:•Poor and variable link quality (makes TCP unhappy)•Many people can hear when you talk•Pretty well defined.•Ad hoc networks (wireless++)•Rooftop networks (multi-hop, fixed position)•Mobile ad hoc networks•Adds challenges: routing, mobility•Some deployment + some research•Sensor networks (ad hoc++)•Scatter 100s of nodes in a field / bridge / etc.•Adds challenge: Serious resource constraints•Current, popular, research.Lecture 25:11-30-2006 3Wireless Challenges (review)•Need to share airwaves rather than wire•Don’t know what hosts are involved•Host may not be using same link technology•No fixed topology of interconnection•Interference•Other hosts: collisions, capture, interference•The environment (e.g., microwaves + 802.11)•Mobility -> Things change often•Environmental changes do too•How do microwaves work? Relate to 802.11 absorption.•Other characteristics of wireless•Noisy lots of losses•Slow•Multipath interferenceLecture 25:11-30-2006 4Wireless Bit-ErrorsRouterComputer 2Computer 12322Loss Congestion210Loss CongestionWirelessLecture 25:11-30-2006 5TCP Problems Over Noisy Links•Wireless links are inherently error-prone•Fading, interference, attenuation -> Loss & errors•Errors often happen in bursts•TCP cannot distinguish between corruption and congestion•TCP unnecessarily reduces window, resulting in low throughput and high latency•Burst losses often result in timeouts•What does fast retransmit need?•Sender retransmission is the only option•Inefficient use of bandwidthLecture 25:11-30-2006Performance Degradation0.0E+005.0E+051.0E+061.5E+062.0E+060 10 20 30 40 50 60Time (s)Sequence number (bytes)TCP Reno(280 Kbps)Best possible TCP with no errors(1.30 Mbps)2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLANLecture 25:11-30-2006 7Performance Degredation 2•Recall TCP throughput / loss / RTT rel:•BW = MSS / (rtt * sqrt(2p/3))•= proportional to 1 / rtt * sqrt(p)•== ouch!•Normal TCP operating range: < 2% lossInternet loss usually < 1%Lecture 25:11-30-2006 8Proposed Solutions•Incremental deployment•Solution should not require modifications to fixed hosts•If possible, avoid modifying mobile hosts •Reliable link-layer protocols•Error-correcting codes (or just send data twice)•Local retransmission•End-to-end protocols•Selective ACKs, Explicit loss notification•Split-connection protocols•Separate connections for wired path and wireless hopLecture 25:11-30-2006 9Approach Styles (Link Layer)•More aggressive local rexmit than TCP•802.11 protocols all do this. Receiver sends ACK after last bit of data.•Faster; Bandwidth not wasted on wired links. Recover in a few milliseconds.•Possible adverse interactions with transport layer•Interactions with TCP retransmission•Large end-to-end round-trip time variation•Recall TCP RTO estimation. What does this do?•FEC used in some networks (e.g., 802.11a)•But does not work well with burst lossesWired link Wireless linkARQ/FECLecture 25:11-30-2006 10Approach Styles (End-to-End)•Improve TCP implementations•Not incrementally deployable•Improve loss recovery (SACK, NewReno)•Help it identify congestion•Explicit Loss/Congestion Notification (ELN, ECN), •ACKs include flag indicating wireless loss•Trick TCP into doing right thing E.g. send extra dupacks if you know the network just burped (e.g., if you moved)Wired link Wireless linkLecture 25:11-30-2006 13Ad Hoc Networks•All the challenges of wireless, plus some of:•No fixed infrastructure•Mobility (on short time scales)•Chaotically decentralized (:-)•Multi-hop!•Nodes are both traffic sources/sinks and forwarders•The big challenge: RoutingLecture 25:11-30-2006 14Ad Hoc Routing•Find multi-hop paths through network•Adapt to new routes and movement / environment changes•Deal with interference and power issues•Scale well with # of nodes•Localize effects of link changesLecture 25:11-30-2006 15Traditional Routing vs Ad Hoc•Traditional network:•Well-structured•~O(N) nodes & links•All links work ~= well•Ad Hoc network•N^2 links - but many stink!•Topology may be really weird•Reflections & multipath cause strange interference•Change is frequentLecture 25:11-30-2006 16Problems using DV or LS•DV loops are very expensive•Wireless bandwidth << fiber bandwidth…•LS protocols have high overhead•N^2 links cause very high cost•Periodic updates waste power•Need fast, frequent convergenceLecture 25:11-30-2006 17Proposed protocols•Destination-Sequenced Distance Vector (DSDV)•Dynamic Source Routing (DSR)•Ad Hoc On-Demand Distance Vector (AODV)•Let’s look at DSRLecture 25:11-30-2006 18DSR•Source routing•Intermediate nodes can be out of date•On-demand route discovery•Don’t need periodic route advertisements•(Design point: on-demand may be better or worse depending on traffic patterns…)Lecture 25:11-30-2006 19DSR Components•Route discovery•The mechanism by which a sending node obtains a route to destination•Route maintenance•The mechanism by which a sending node detects that the network topology has changed and its route to destination is no longer validLecture 25:11-30-2006 20DSR Route Discovery•Route discovery - basic idea•Source broadcasts route-request to Destination•Each node forwards request by adding own address and re-broadcasting•Requests propagate outward until:•Target is found, or•A node that has a route to Destination is foundLecture
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