Transcript class1
Chapter 1 Introduction Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. Adapted from original book supplement slides All material copyright 1996-2005 J.F Kurose and K.W. Ross, All Rights Reserved Introduction 1-1 What’s a network A group of devices that can communicate with one another - hosts Physical connections between devices are called links workstation workstation mobile workstation server Introduction 1-2 What’s a network A group of devices that can communicate with one another - hosts Physical connections between devices are called links workstation workstation router mobile router workstation server Introduction 1-3 Who can talk on a network? Desktops Laptops Routers Cell phones Printers Scanners Cameras Cars Toasters Fridges Microwaves TVs TiVos Phones Picture frames Watches Satellites Sensors and more! Introduction 1-4 What we couldn’t do without networks? Introduction 1-5 What’s the Internet A worldwide, public network connecting millions of devices Internet isworkstation a network of networks! workstation workstation workstation workstation workstation workstation workstation workstation workstation workstation router workstation workstation router router workstation workstation router router workstation router router router router router router mobile workstation router router mobile workstation router router mobile workstation router mobile workstation server mobile workstation server mobile workstation server mobile workstation server mobile workstation server server server server Introduction 1-6 What’s the Internet There is a hierarchy of networks: At the edge: company or university networks and end hosts Middle: local and regional Internet Service Providers (ISPs) TMobile, AOL Tier 3 and Tier 2 ISPs Top: national/international ISPs AT&T, Sprint Tier 1 ISPs router server local ISP company network workstation mobile regional ISP Introduction 1-7 How to talk on the Internet? Must speak the same language! Accessing the physical media of the link Physical protocol Accessing the devices on the same link Link protocol Traveling to a host at the other end via routers on the path Internet Protocol (IP) – just making sure messages travel from source to destination host, like post office Delivering message to the right application, handling loss Transport protocols, some of them are: Transport Control Protocol (TCP) User Datagram Protocol (UDP) Making sure the application understands what we want Application protocols Introduction 1-8 What’s a protocol? Human protocols: “What’s the time?” “I have a question” Introductions … specific messages sent … specific actions taken when messages are received, or other events occur Network protocols: Machines rather than humans All communication activity in Internet governed by protocols Protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt Pretty much like game rules! Introduction 1-9 What’s a protocol? A human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 <file> time Q: Can you think of other human protocols? Introduction 1-10 Internet standards Internet standard body IETF (Internet Engineering Task Force) Define Internet protocols and best practices Standard proposals – Internet drafts Accepted standards – Requests for comments (RFCs) http://www.ietf.org Introduction 1-11 Network structure Network edge: generates data for end users Applications Hosts (end systems) Client/server model Peer-to-peer model Network core: makes sure messages arrive to their destination Routers Introduction 1-12 How do end-hosts talk? Connectionless service I only care that some data gets to the destination Best-effort Connection-oriented service I would like to have some guarantees Delay Loss Introduction 1-13 Connectionless service Goal: Data transfer between end systems Simply send data from local application to a remote one UDP – User Datagram Protocol [RFC 768] Doesn’t care if messages are lost or reordered No speed control Introduction 1-14 Connection-oriented service Goal: Reliable data transfer between end systems Handshaking: connection establishment Setup (prepare for) data transfer ahead of time Hello, hello back human protocol Set up “state” in two communicating hosts TCP - Transmission Control Protocol [RFC 793] Transmit and receive messages in order Detect message loss and retransmit Control the speed of the transfer so that the receiver does not get overwhelmed Reduce the speed if there is network congestion Introduction 1-15 Connections: yes or no? Applications using TCP Web applications Telnet File transfer E-mail Applications using UDP Streaming video Teleconferencing Internet telephony Introduction 1-16 The Network Core Mesh of interconnected routers Links have limited resource Bandwidth – how much data can be transferred in time unit The fundamental question: how is data transferred through a network? circuit switching: dedicated circuit per call, reserved resources packet-switching: data sent through network in “chunks”, may need to wait or get dropped Introduction 1-17 Network Core: circuit switching Call setup required Network resources (bandwidth) are divided into pieces Pieces reserved for the “call” on the path Dedicated resources: no sharing, idle if not used Guaranteed performance Bandwidth is: Frequency range Data rate Pieces can be divided by: Time Frequency Introduction 1-18 Circuit switching: FDM and TDM Example: FDM (Frequency division multiplexing) 4 users frequency frequency band time TDM (Time division multiplexing) frame frequency slot time Introduction 1-19 Exercise 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 are 1.536 Mbps (in the whole freq. range) Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit Introduction 1-20 Exercise 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 are 1.536 Mbps (in the whole freq. range) Each link uses FDM with 24 channels/frequency band 500 msec to establish end-to-end circuit Introduction 1-21