CS 640: Introduction to Computer NetworksScenarios and RoadmapIEEE 802.11 Wireless LANSlide 4Ad Hoc NetworksCSMA/CD Does Not WorkHidden Terminal EffectIEEE 802.11 MAC Protocol: CSMA/CACollision Avoidance MechanismsCollision Avoidance: RTS-CTS ExchangeIEEE 802.11 MAC ProtocolAd Hoc RoutingTraditional Routing vs Ad HocProblems using DV or LSProposed protocolsDSRDSR ComponentsDSR Route DiscoveryC Broadcasts Route Request to FSlide 20H Responds to Route RequestC Transmits a Packet to FForwarding Route RequestsRoute CacheSending DataDiscussionCapacity of multi-hop networkSensor Networks - smart devicesSensor System Types – Smart-Dust/MotesSensors and power and radiosSensor nets goalsPowerExample: AggregationCS 640: Introduction to Computer NetworksAditya AkellaLecture 23 -CSMA/CA, Ad Hoc and Sensor Networks2Scenarios 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.3IEEE 802.11 Wireless LAN•802.11b–2.4-2.5 GHz unlicensed radio spectrum–up to 11 Mbps–direct sequence spread spectrum (DSSS) in physical layer•all hosts use same chipping code–widely deployed, using base stations•802.11a –5-6 GHz range–up to 54 Mbps•802.11g –2.4-2.5 GHz range–up to 54 Mbps•All use CSMA/CA for multiple access•All have base-station and ad-hoc network versions4IEEE 802.11 Wireless LAN•Wireless host communicates with a base station–Base station = access point (AP)•Basic Service Set (BSS) (a.k.a. “cell”) contains:–Wireless hosts–Access point (AP): base station•BSS’s combined to form distribution system5•Ad hoc network: IEEE 802.11 stations can dynamically form network without AP•Applications:–Laptops meeting in conference room, car–Interconnection of “personal” devicesAd Hoc Networks6CSMA/CD Does Not Work•Collision detection problems–Relevant contention at the receiver, not sender•Hidden terminal•Exposed terminal–Hard to build a radio that can transmit and receive at same timeABCABCDHidden Exposed7Hidden Terminal Effect•Hidden terminals: A, C cannot hear each other–Obstacles, signal attenuation–Collisions at B –Collision if 2 or more nodes transmit at same time•CSMA makes sense:–Get all the bandwidth if you’re the only one transmitting–Shouldn’t cause a collision if you sense another transmission•Collision detection doesn’t work•CSMA/CA: CSMA with Collision Avoidance8IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender•If sense channel idle for DIFS (Distributed Inter Frame Space) then transmit entire frame (no collision detection)•If sense channel busythen binary backoff802.11 CSMA: receiver•If received OKreturn ACK after SIFS --Short IFS (ACK is needed due to hidden terminal problem)9Collision Avoidance Mechanisms•Problem: –Two nodes, hidden from each other, transmit complete frames to base station–Wasted bandwidth for long duration!•Solution: –Small reservation packets–Nodes track reservation interval with internal “network allocation vector” (NAV)10Collision Avoidance: RTS-CTS Exchange•Explicit channel reservation–Sender: send short RTS: request to send–Receiver: reply with short CTS: clear to send–CTS reserves channel for sender, notifying (possibly hidden) stations•RTS and CTS short:–collisions less likely, of shorter duration–end result similar to collision detection•Avoid hidden station collisions•Not widely used/implemented–Consider typical traffic patterns11IEEE 802.11 MAC Protocol802.11 CSMA Protocol: others•NAV: Network Allocation Vector; maintained by each node•802.11 RTS frame has transmission time field•Others (hearing CTS) defer access for NAV time units•Reserve bandwidth for NAV time units12Ad 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 changes13Traditional 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 frequent14Problems 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 convergence15Proposed protocols•Destination-Sequenced Distance Vector (DSDV)•Dynamic Source Routing (DSR)•Ad Hoc On-Demand Distance Vector (AODV)•Let’s look at DSR16DSR•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…)17DSR 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 valid18DSR 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 found19C Broadcasts Route Request to FASourceCGHDestinationFEDBRoute Request20C Broadcasts Route Request to FASourceCGHDestinationFEDBRoute Request21H Responds to Route RequestASourceCGHDestinationFEDBG,H,F22C Transmits a Packet to FASourceCGHDestinationFEDBFH,FG,H,F23Forwarding Route Requests•A request is forwarded if:–Node is not the destination–Node not already listed in recorded source route–Node has not seen request with same sequence number–IP TTL field may be used to limit scope•Destination copies route into a Route-reply packet and sends it back to Source24Route Cache•All source routes learned by a node are kept in Route Cache–Reduces cost of route
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