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ECE 4450:427/527 - Computer Networks Spring 2014

ECE 4450:427/527 - Computer Networks Spring 2014. Dr. Nghi Tran Department of Electrical & Computer Engineering. Lecture 2: Overview of Computer Network. Network Definition. Definition: A system that carries a commodity between 2 or more entities via connections

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ECE 4450:427/527 - Computer Networks Spring 2014

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  1. ECE 4450:427/527 - Computer NetworksSpring 2014 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 2: Overview of Computer Network ECE 4450:427/527

  2. Network Definition • Definition: A system that carries a commodity between 2 or more entities via connections • Entities: People, Computers, Cities etc. • Connections: Relationships, Wires, Road etc. • Examples of Network: • Transportation • Computer • Many others ECE 4450:427/527

  3. Computer Network • Definition: A system that carries information between 2 or more entities, in the form of electric signals • Commodity? • Entities? • Connections? ECE 4450:427/527

  4. Example: Transportation Vs Computer Network ECE 4450:427/527

  5. Common Networks • Ethernet (LAN) • WiFi, WiMAX, 3G/4G: Mobile wireless network • The Internet: • Internet: Internetwork - interconnecting computer networks with special gateways or routers • Largest communication network adopting IP technology ECE 4450:427/527

  6. How Large is the Internet? ECE 4450:427/527

  7. How Popular is the Internet? ECE 4450:427/527

  8. Traffic of the Internet ECE 4450:427/527

  9. Users of the Internet ECE 4450:427/527

  10. Internet: Further Trend ECE 4450:427/527

  11. Internet: Challenges • Internet has grown rapidly and evolved to unprecedented size • With a significant increase in the number of users, providers, and services, the Internet of the future is facing many problems: • How to manage/design a very large system with limited resources • Yet still provide a guaranteed performance ECE 4450:427/527

  12. Objectives Not too ambitious in this course!!! • We take things step by step to help you uunderstand communication network design principles • And to pave an initial road for you to: • Design/operate Comm. Network • Design client/server software • Design equipments • Doing research ECE 4450:427/527

  13. Basic Elements of Network Divided into two main categories: • Nodes: • Links: ECE 4450:427/527

  14. What is Network Design • A task to make connections among nodes via links so that packages can be exchanged: • Timely, reliably • Using as low resources as possible • etc • To this end, a huge of work required: We need to define network connectivity, network architecture, protocols, applications, interfaces, policies, usages. • As said, we shall take the issues step by step. ECE 4450:427/527

  15. Main Factors in Network Design What are the most important factors that drive Network Design? ECE 4450:427/527

  16. Applications • Most people know about the Internet (a computer network) through applications • World Wide Web • Email • Online Social Network • Streaming Audio Video • File Sharing • Instant Messaging • … ECE 4450:427/527

  17. Example of Application A multimedia application including video-conferencing ECE 4450:427/527

  18. Network Performance When designing a network, we also need some tools to evaluate the design: Network Performance Metrics ECE 4450:427/527

  19. Network Design • A task to make connections among nodes via links so that packages can be exchanged: • Timely, reliably • Using as low resources as possible • etc • To this end, a huge of work required: We need to define network connectivity, network architecture, protocols, applications, interfaces, policies, usages. • As said, we shall take the issues step by step. • We now start with some Network Connectivity ECE 4450:427/527

  20. Network Connectivity We have the following three main categories • Directly-connected Network • Circuit-switched Network • Package-switched Network ECE 4450:427/527

  21. Directly-connected Network Nodes are directly connected (a) Point-to-Point: Each node is directly connected to all others via a link (b) Multiple-Access: All nodes share the same physical medium ECE 4450:427/527

  22. Disadvantages What are the main disadvantages of a network in which all nodes are directly connected? Given N nodes, how many connections do we need? ECE 4450:427/527

  23. Switched Network • Fortunately, connection between 2 nodes does not necessary imply a physical connection: connection is achieved among a set of cooperating nodes ECE 4450:427/527

  24. Interconnection of Networks • Two nodes can also be connected by using a set of networks (or clouds): Form an internetwork or internet ECE 4450:427/527

  25. Switched Network • Circuit-Switched • A dedicated circuit is established between two communicating nodes • Example: ? • Packet-Switched • Data is split into blocks called packets or messages. • Store-and-forward strategy: Switches to store and forward packets • Example: ? ECE 4450:427/527

  26. Circuit-Switched Network • Designed in 1878: reserve a dedicated channel for entire communication • No need for a destination address since a path is already established • Once communication is complete, connection is ended and links are released • How many switches do we need? ECE 4450:427/527

  27. Circuit-Switched: Pros and Con? Interesting note: Circuit switching currently makes a comeback in optical networking ECE 4450:427/527

  28. Packet-Switched Network • Data is divided into packets • Each packet can be delivered independently over the network • Each packet contains identification info (source/destination address seq. number) • Store-and-forward: Key strategy • Each node receive complete package • Store in memory • Forward package to the next node ECE 4450:427/527

  29. Advantages of Packet-Switched • Efficiency in using resources, which is a key requirement of computer network • Flexible multiplexing methods • What is multiplexing? • Service • More connections of lesser quality • No blocking of users • Adaptation • Can adapt to network congestion and failures ECE 4450:427/527

  30. Disadvantages of Packet-Switched ECE 4450:427/527

  31. Multiplexing • Fundamental concept: resources are shared among user, e.g., data of users be multiplexed over links that make up network • Two common methods: Time and Frequency Division Multiplexing ECE 4450:427/527

  32. Statistical Multiplexing • Shortcomings of TDM/FDM: • If no data to send: Link is idle – wasteful • Maximum number of flows is fixed and known ahead of time – not practical to add additional quanta/frequencies • Statistical multiplexing: Division of the communication medium into channels of variable bandwidth: Resource allocated on demand rather than pre-allocated ECE 4450:427/527

  33. Comparison Q: how did we get value 0.0004? Q: what happens if > 35 users ? Packet switching allows more users to use network! Example: • 1 Mb/s link • each user: • 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 N users ….. 1 Mbps link ECE 4450:427/527

  34. More Detail Packet switching allows more users to use network! • Assume there are total N users • The probability for each user to be active is p • Now we need to calculate two probabilities: • What is the probability that we exactly n active users • What is the probability that we have more than n active users • Let start with some simple cases: N=3 and n=1 ECE 4450:427/527

  35. Comparison Packet switching allows more users to use network! Example: • 1 Mb/s link • each user: • 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 N users ….. 1 Mbps link • More connections of lesser quality ECE 4450:427/527

  36. Recap • We have taken a overview about Network • We also define CONNECTIVITY in a Network: • Packet switching with statistical multiplexing • Next step: We shall look at NETWORK ARCHITECTURE • Layering • Protocols • Internet Architecture ECE 4450:427/527

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