•1CS640: Introduction to Computer NetworksAditya AkellaLecture 22 -Wireless Networking2Wireless Challenges• Force us to rethink many assumptions• Need to share airwaves rather than wire• Mobility• Other characteristics of wireless– Noisy lots of losses– Slow– Interaction of multiple transmitters at receiver• Collisions, capture, interference– Multipath interference3IEEE 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 versions•24IEEE 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 system5CSMA/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 Exposed6Hidden 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 Avoidance•37IEEE 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)8Collision Avoidance Mechanisms• Problem:– Two nodes, hidden from each other, transmit complete frames to base station– Wasted bandwidth for long duration!• Solution: – Small reservation packets: RTS+CTS– Nodes track reservation interval with internal “network allocation vector” (NAV)9Collision 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 patterns•410IEEE 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 units11Wireless Bit-ErrorsRouterComputer 2Computer 12322Loss Congestion210Burst losses lead to coarse-grained timeoutsResult: Low throughputLoss CongestionWireless12TCP Problems Over Noisy Links• Wireless links are inherently error-prone– Fades, interference, attenuation– 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• Sender retransmission is the only option– Inefficient use of bandwidth•5Performance 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 WaveLAN14Proposed Solutions• Incremental deployment– Solution should not require modifications to fixed hosts– If possible, avoid modifying mobile hosts • End-to-end protocols– Selective ACKs, Explicit loss notification• Split-connection protocols– Separate connections for wired path and wireless hop• Reliable link-layer protocols– Error-correcting codes– Local retransmission15Approach Styles (Link Layer)• More aggressive local rexmit than TCP– Bandwidth not wasted on wired links• Possible interactions with transport layer– Interactions with TCP retransmission– Large end-to-end round-trip time variation• FEC does not work well with burst lossesWired link Wireless linkARQ/FEC•616Approach Styles (End-to-End)• Improve TCP implementations– Not incrementally deployable– Improve loss recovery (SACK, NewReno)– Help it identify congestion (ELN, ECN)• ACKs include flag indicating wireless loss– Trick TCP into doing right thing E.g. send extra dupacksWired link Wireless
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