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Lecture 14 Networking - Administrivia Project 3 due...

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Administrivia Project 3 due Thursday - As usual, due at 4:15pm - Extension to midnight if you come to class - For longer extensions, please email cs140-staff Project 4 goes out at end of week This Friday will have section on project 4 1/36
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Lab 1 grades 0 5 10 15 20 25 0 20 40 60 80 100 120 140 160 180 200 ’-’ using 1:2 Mean: 149 2/36
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Lab 2 grades 0 5 10 15 20 25 0 50 100 150 200 ’-’ using 1:2 Mean: 162 3/36
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Networks What is a network? - A system of lines/channels that interconnect - E.g., railroad, highway, plumbing, communication, telephone, computer What is a computer network? - A form of communication network—moves information - Nodes are general-purpose computers Computer networks are particularly interesting - You can program the nodes - Very easy to innovate and develop new uses of network - Contrast: Telephone network—can’t program most phones, need FCC approval for new devices, etc. 4/36
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Physical connectivity Computers send bits over physical links - E.g., Coax, twisted pair, fiber, radio, . . . - Bits may be encoded as multiple lower-level “chips” Two categories of physical links - Point-to-point networks (e.g., fiber, twisted pair): - Shared transmission medium networks (e.g., coax, radio): Any message can be seen by all nodes Allows broadcast/multicast, but introduces contention One implication: speed of light matters! - 300, 000 km/sec in a vacum, slower in fiber SF ≥ ∼ 15 msec -------------→ NYC Moore’s law does not apply! 5/36
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Bandwidth-delay Network delay over WAN will never improve much But throughput (bits/sec) is constantly improving Can view network as a pipe Bandwidth Delay - For full utilization want # bytes in flight bandwidth × delay (But don’t want to overload the network, either) What if protocol doesn’t involve bulk transfer? - E.g., ping-pong protocol will have poor throughput Another implication: Concurrency is critical for good network utilization 6/36
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Indirect connectivity Rarely have direct physical connection to destination Instead, communications usually “hop” through multiple devices source ---→ switch ----→ destination - Allows links and devices to be shared for multiple purposes - Must determine which bits are part of which messages intended for which destinations Circuit switched networks provide virtual links - E.g., old telephone network when numbers dialed Packet switched networks - Pack a bunch of bytes together intended for same destination - Slap a header on packet describing where it should go - Most networks today are packet switched 7/36
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Example: Ethernet Originally designed for shared medium (coax) - Medium Access Control ( MAC ) protocol governs access to coax - E.g., don’t transmit when someone else is - If you collide, used randomized backoff and try again Vendors give each device a unique 48-bit MAC address - Specifies which node should receive a packet Packet format: Dest addr 64 48 32 CRC Preamble Src addr Type Body 16 48 - Preamble helps device recognize start of packet - CRC allows card to ignore corrupted packets - Body up to 1,500 bytes for same destination -
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