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Chapter 1 Part 2 Network Core

Comp 365 Computer Networks Fall 2014. Chapter 1 Part 2 Network Core. These slides derived from Computer Networking: A Top Down Approach , 6 th edition. Jim Kurose, Keith Ross Addison-Wesley, March 2012. Chapter 1: roadmap. 1.1 What is the Internet? 1.2 Network edge

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Chapter 1 Part 2 Network Core

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  1. Comp 365 Computer Networks Fall 2014 Chapter 1Part 2 Network Core These slides derived from Computer Networking: A Top Down Approach ,6th edition. Jim Kurose, Keith RossAddison-Wesley, March 2012. Introduction

  2. Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge • end systems, access networks, links 1.3 Network core • circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History Introduction

  3. mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks” The Network Core Resources reserved Resources allocated on demand. Core Resources: buffers, link transmission rate Introduction

  4. The internet is packet switched Telephone network is circuit switched We need to understand circuit switching to know why it’s not used for the internet The Network Core Introduction

  5. End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required State maintained Data transferred at a guaranteed rate Network Core: Circuit Switching Introduction

  6. End-end resources reserved for “call” Links between circuit switches Each link can support n circuits There can be n simultaneous connections. Each circuit thus gets 1/n of the link’s bandwidth. Network Core: Circuit Switching Introduction

  7. network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) When two hosts want to communicate network establishes a dedicated end-to-end connection between the hosts. Network Core: Circuit Switching • dividing link bandwidth into “pieces” • frequency division • time division Introduction

  8. Alternative core: circuit switching end-end resources allocated to, reserved for “call” between source & dest: In diagram, each link has four circuits. call gets 2nd circuit in top link and 1st circuit in right link. dedicated resources: no sharing circuit-like (guaranteed) performance circuit segment idle if not used by call (no sharing) Commonly used in traditional telephone networks Introduction

  9. Example: 4 users FDM frequency time TDM frequency time Circuit Switching: FDM and TDM Each connection gets one band of frequencies FDM: frequency-division multiplexing Each connection gets one time slot TDM: time-division multiplexing Introduction

  10. Example: telephone networks 4 users FDM frequency time Circuit Switching: FDM and TDM 4kHz 4kHz 4kHz 4kHz 4kHz = 4,000 hertz = 4,000 cycles per second FDM: frequency-division multiplexing Radio stations also use FDM to share the frequency spectrum between 88 MHz and 108 MHz Introduction

  11. Example: 4 users TDM frequency time Circuit Switching: FDM and TDM TDM: time-division multiplexing Circuit gets same time slot in every frame One time slot One frame Transmission rate of a circuit: frame rate multiplied by the number of bits in a slot. Example: 8,000 frames per sec, slot has 8 bits, then what is the transmission rate? 64 kbps Introduction

  12. Numerical example • How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? • All links have transmission rate of 1.536 Mbps • Each link uses TDM with 24 slots/sec, 24 slots/frame. • 500 msec to establish end-to-end circuit Let’s work it out! Introduction

  13. Numerical example • How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? • All links have transmission rate of 1.536 Mbps • Each link uses TDM with 24 slots/sec, 24 slots/frame. • 500 msec to establish end-to-end circuit • Soln: • determine how many slots we need. To do this must figure out how many bits/slot: (1.536 x 106 bits/sec) / (24 slots/sec) = 64,000 bits / slot • Now can determine how many slots we need: 640,000 bits / (64,000 bits/slot) = 10 slots. • So need 10 seconds + 500 msec = 10.5 seconds. • This does not account for propagation delays. Introduction

  14. Circuit switching: analysis • Disadvantages: • Network resources are wasted during “silent” times (when no one is talking but still connected) • Establishing end-to-end circuits and reserving end-to-end bandwidth is complicated and requires complex signaling software. • Advantage: • Guaranteed bandwith and transmission time • Necessary for some applications (streamed music/video) Introduction

  15. Network Core: Packet Switching transmission: • Each end-end data stream divided into packets • Each packet travels through communication links • Links connected by packet switches • Routers • Or link-level switches. • Packet Switches use store-and-forward transmission • receives entire packet before it transmits any of it again Introduction

  16. Network Core: Packet Switching transmission: Introduction

  17. each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation Network Core: Packet Switching Introduction

  18. Switching delays Assume packet has L bits Assume there are Q switches Assume a switch transmits at rate of R Takes each switch how long to transmit the packet? L/R Total delay is: QL/R Network Core: Packet Switching Introduction

  19. This Figure: takes L/R seconds to transmit (push out) packet of L bits on to link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link delay = 3L/R (assuming zero propagation delay) Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay = ?? 15 sec Packet-switching: store-and-forward L R R R more on delay shortly … Introduction

  20. Network Core: Packet Switching Switching delays: • Each switch has multiple links • Each link has buffer • If packet arrives and another packet is already being transmitted on that link, must wait in queue • Called queuing delay. • Varies depending on network congestion • Packet loss: queue is full when packet arrives. Introduction

  21. Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand  statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. D E Packet Switching: Statistical Multiplexing 100 Mb/s Ethernet C A statistical multiplexing 1.5 Mb/s B On-demand sharing of resources is called statistical multiplexing queue of packets waiting for output link Introduction

  22. Two key network-core functions routing:determines source-destination route taken by packets • routing algorithms forwarding:move packets from router’s input to appropriate router output routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 1 0111 2 3 dest address in arriving packet’s header Network Layer

  23. 1 Mb/s link For each user assume: 100 kb/s when “active” active 10% of time circuit-switching: 10 users packet switching: with 35 users, probability > 10 active at same time is less than .0004 Packet switching allows more users to use network! Packet switching versus circuit switching N users 1 Mbps link Q: how did we get value 0.0004? A: see this pdf: https://147.129.181.14/~barr/classes/comp365/lectures/Prob10orMoreUsers.pdf Introduction

  24. packet switching: with 35 users, probability > 10 active at same time is less than .0004 With 10 or fewer simultaneously active users, aggregate arrival rate of data is less than or equal to 1 Mbps. No loss of time Allows essentially same performance as circuit switching But allows > 3 times number of users Packet switching allows more users to use network! Packet switching versus circuit switching Introduction

  25. great for bursty data resource sharing simpler, no call setup excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 7) Is packet switching a “slam dunk winner?” Packet switching versus circuit switching Most networks use packet-switching; even telephone networks are going to this! Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Introduction

  26. Routing • How do packets make their way through packet-switched Networks? Introduction

  27. Internet structure: network of networks • End systems connect to Internet via access ISPs (Internet Service Providers) • Residential, company and university ISPs • Access ISPs in turn must be interconnected. • So that any two hosts can send packets to each other • Resulting network of networks is very complex • Evolution was driven by economics and national policies • Let’s take a stepwise approach to describe current Internet structure

  28. Internet structure: network of networks Question: given millions of access ISPs, how to connect them together? … … … … access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … …

  29. Internet structure: network of networks Option: connect each access ISP to every other access ISP? … … … … connecting each access ISP to each other directly doesn’t scale: O(N2) connections. … … … … access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … …

  30. Internet structure: network of networks Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement. … … … … globalISP access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … …

  31. Internet structure: network of networks But if one global ISP is viable business, there will be competitors …. … … ISP B ISP A ISP C … … access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … …

  32. Internet structure: network of networks But if one global ISP is viable business, there will be competitors …. which must be interconnected Internet exchange point … … ISP B ISP C ISP A IXP IXP … … access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net peering link … …

  33. Internet structure: network of networks … and regional networks may arise to connect access nets to ISPS … … ISP B ISP C ISP A IXP IXP … … access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net regional net … …

  34. Internet structure: network of networks … and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users … … ISP B ISP B ISP A IXP IXP … … Content provider network access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net regional net … …

  35. Internet structure: network of networks at center: small # of well-connected large networks “tier-1” commercial ISPs(e.g., Level 3, Sprint, AT&T, NTT), national & international coverage content provider network (e.g, Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs Tier 1 ISP Tier 1 ISP Google IXP IXP IXP Regional ISP Regional ISP access ISP access ISP access ISP access ISP access ISP access ISP access ISP access ISP Introduction

  36. Tier-1 ISP: e.g., Sprint POP: point-of-presence to/from backbone peering … … … … … to/from customers Introduction

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