123f11_Lec2

123f11_Lec2 - Lecture 2 Protocols and Layering CSE 123 Computer Networks Stefan Savage Last time Bandwidth latency overhead message size error rate

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Lecture 2: Protocols and Layering CSE 123: Computer Networks Stefan Savage
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Last time z Bandwidth, latency, overhead, message size, error rate z Bandwidth-delay product z High-level run through of how Web browsing works Bandwidth Delay
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Today z Circuit Switching vs Packet Switching z Protocols and Layering
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Circuit Switching z Original phone system: continuous DC circuit from sender to receiver z Physically switch circuit z Circuit Switching: same model in digital domain ± Model: data sent continuously ± Created a session (e.g., phone call) reserves dedicated bandwidth in series of switches between caller and recipient ± Guaranteed capacity (in both directions) so long as session up 4
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Circuit Switching z Before sending, must reserve capacity for session z Success = bandwidth is guaranteed for life of session 5
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Circuit Switching z What if request fails? z Session refused (e.g., busy signal on phone) 6 Mothers Day Link full
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Packet Switching z Packet Switching ± Model: data sent opportunistically in small chunks (packets) ± No session setup; send immediately ± Each switch must know how to forward along any packet ± Use queues to buffer bursts of traffic that exceed capacity z History (mid-60s and 70s): ± Paul Baran (US), Donald Davies (UK) ± Kleinrock: queuing theory ± Bob Taylor, JCR Licklider, Lawrence Roberts et al. ARPAnet 7
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Packet Switching z Break data into “packets”; send when they are ready z Try to use bandwidth only when it is needed
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Packet Switching z What if link full? 9 Black Friday Link full
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Packet Switching z What if link full? Queue packet 10
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Packet Switching z What if the queue is full? z Drop packet 11
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Pros/cons? z Circuit switching ± Pro: If you get a circuit you have guaranteed bandwidth ± Con: latency to set up circuit (one round-trip time) ± Con: you may not get a circuit ± Con: if you get a circuit and don’t use it, bandwidth is wasted z Packet switching ± Pro: can send immediately (not required latency) ± Pro: can share bandwidth dynamically among users ( statistical multiplexing ) ± Con: available bandwidth per user can fluctuate, packets can be dropped 12
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Why do you think the Internet is based on packet switching? z Internet applications are bursty ± E-mail only consumes bandwidth when mail send/recvs ± Web browsing only consumes bandwidth when you visit site z Packet switching is a much more efficient way to support bursty applications z We will primarily focus on packet switching in this class since this is most of modern data networking
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Protocols and Layering z What’s a protocol?
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This note was uploaded on 11/30/2011 for the course CSE cse123 taught by Professor Stefansavage during the Fall '11 term at UCSD.

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123f11_Lec2 - Lecture 2 Protocols and Layering CSE 123 Computer Networks Stefan Savage Last time Bandwidth latency overhead message size error rate

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