•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 interference3The Road Ahead• Internet mobility• TCP over noisy links• Link layer challenges•24Routing to Mobile Nodes• Obvious solution: have mobile nodes advertise route to mobile address/32– Should work!!!• Why is this bad?– Consider routing tables on backbone routers• Would have an entry for each mobile host– No aggregation• Not very scalable• What are some possible solutions?5Handling Mobile Nodes: Addressing• Dynamic Host Configuration (DHCP)– Host gets new IP address in new locations– Problems• Host does not have constant name/address how do others contact host• What happens to active transport connections?6Handling Mobile Nodes: Naming• Naming– Use DHCP and update name-address mapping whenever host changes address– Fixes contact problem but not broken transport connections•37Handling Mobile Nodes: Transport• TCP currently uses 4 tuple to describe connection– <Src Addr, Src port, Dst addr, Dst port> • Modify TCP to allow peer’s address to be changed during connection• Security issues– Can someone easily hijack connection?• Difficult deployment both ends must support mobility8Handle Mobile Nodes: Link Layer• Link layer mobility– Learning bridges can handle mobility– Encapsulated PPP (PPTP) Have mobile host act like he is connected to original LAN• Works for IP AND other network protocols9Handling Mobile Nodes: Routing• Allow mobile node to keep same address and name• How do we deliver IP packets when the endpoint moves?– Can’t just have nodes advertise route to their address• What about packets from the mobile host?– Routing not a problem– What source address on packet? this can cause problems• Key design considerations– Scale– Incremental deployment•410Basic Solution to Mobile Routing • Same as other problems in computer science– Add a level of indirection• Keep some part of the network fixed, and informed about current location of mobile node– Need technique to route packets through this location (interception)• Need to forward packets from this location to mobile host (delivery)11Interception• Somewhere along normal forwarding path– At source– Any router along path– Router to home network– Machine on home network (masquerading as mobile host)• Clever tricks to force packet to particular destination– “Mobile subnet” – assign mobiles a special address range and have special node advertise route12Delivery• Need to get packet to mobile’s current location• Tunnels– Tunnel endpoint = current location– Tunnel contents = original packets• Source routing– Loose source route through mobile current location•513Mobile IP (RFC 2290)• Interception– Typically home agent – a host on home network• Delivery– Typically IP-in-IP tunneling– Endpoint – either temporary mobile address or foreign agent• Terminology– Mobile host (MH), correspondent host (CH), home agent (HA), foreign agent (FA)– Care-of-address, home address14Mobile IP (MH at Home)Mobile Host (MH)Visiting LocationHomeInternetCorrespondent Host (CH)Packet15Mobile IP (MH Moving)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Mobile Host (MH)I am here•616Mobile IP (MH Away – FA)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Foreign Agent (FA)EncapsulatedMobile Host (MH)17Mobile IP (MH Away - Collocated)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Mobile Host (MH)Encapsulated18Other Mobile IP Issues• Route optimality– Resulting paths can be sub-optimal– Can be improved with route optimization• Unsolicited binding cache update to sender (direct routing)• Authentication– Registration messages• Must send updates across network– Handoffs can be slow• Problems with basic solution– Triangle routing– Reverse path check for security•719Wireless Bit-ErrorsRouterComputer 2Computer 12322Loss Congestion210Burst losses lead to coarse-grained timeoutsResult: Low throughputLoss CongestionWireless20TCP 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 bandwidthPerformance 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 WaveLAN•822Proposed 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 retransmission23Approach 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/FEC24Approach 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 link•925IEEE 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
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