Computer Networks

1. ワラ　ハッサン 岡山大学大学院自然科学研究科 Computer Networks

2. Computer Networks, Chapter 1 Chapter 1: Introduction outlines: • Definition of Computer networks • Uses of Computer Networks • Network Hardware - Local Area Networks - Metropolitan Area Network - Wide Area Network -Wireless Network -Home Networks -Internetworks

3. Computer Networks, Chapter 1 1-Def. Computer networks A System that contains a large number of separate but interconnected computers do a job is said to computer networks (interconnected by a single technology). Two computers are said to be interconnected if they are able to exchange information.

4. Computer Networks, Chapter 1 The Difference between a computer network and a distributed system A distributed system is a collection of independent computers appears to its users as a single coherent system. A well known example of a distributed system is the WWW. In a computer network users are exposed to the actual machines, without any attempt by the system to make the machines look and act in coherent way.

5. Uses of Computer Networks • Business Applications • Client-Server Model Computer Networks, Chapter 1

6. Home Applications • Access to remote information. • Person-to-person communication. • Interactive entertainments. • Electronic commerce. Some of these applications used the client-server model while others used peer-to-peer communication. Or combination between them. Computer Networks, Chapter 1

7. Mobile Users Wireless networks and mobile computing are often related , but they are not identical. A whole different application area for wireless network such as cell phones and PADs that could display stripped down Web pages on their tinier screen. WAP1.0 (Wireless Application Protocol), WAP2.0 • Social issues(???) Computer Networks, Chapter 1

8. Computer Networks, Chapter 1 2-Network Hardware There is no generally accepted classifications in which all computer networks are fit., But two dimensions stand out as important: transmission technology and scale There are two widespread types of transmission technology 1-Broadcast links. 2-point-to-point links.

9. Computer Networks, Chapter 1 1- Broadcast networks -A single communication channel that is shared by all the machines on the network. -Packets in a certain contexts, sent by any machine are received by all the other. ignore ignore ignore Packet A B received ignore ignore ignore Address Broadcasting Multicasting (question)

10. Computer Networks, Chapter 1 2-Point-to-point or unicasting Point-to-point networks consisting of many connections between individual pairs of machine. ☆As a general rule, smaller, geographically localized networks tend to use broadcasting, whereas larger networks usually are unicasting.

11. Scale, physical size Computer Networks, Chapter 1

12. Computer Networks, Chapter 1 Local Area Networks LANs are privately-owned networks within a single building or campus of up to few kilometers in size. LANs are distinguish from other kind of networks by three characteristics: Their sizes. Their transmission technology. Their topology .

13. Topology: the shape of the LAN Two broadcast networks (a) Bus (b) Ring • Broadcast networks can be divided into static and dynamic. Computer Networks, Chapter 1

14. Computer Networks, Chapter 1 Static allocation A typical static allocation would be to divide time into discrete intervals and use a round-robin algorithm(Question), allowing each machine to broadcast only when its time slot comes up. Dynamic allocation Dynamic allocation method for a common channel either centralized or decentralized. -In the centralized channel allocation method, there is a single entity. For example, bus arbitration unite -In the decentralization channel allocation, there is no certain entity. Each machine must decide for itself whether to transmit.

15. Computer Networks, Chapter 1 Metropolitan Area Network “MAN”be back in chapter2

16. Computer Networks, Chapter 1 Wide Area Network “WAN” It contains a collection of hosts connected by a subnet The relation between hosts in LANs and the subnet The subnet consists of two distinct components, The transmission lines and switching elements.

17. Computer Networks, Chapter 1 Continue…. WAN (2) Message sent, store-and-forward

18. Continue…Networks hardwareWireless Networks • Wireless networks can be divided into three main categories: 1- System interconnection 2- Wireless LANs 3- Wireless WANs Computer Networks, Chapter 1

19. Continue….Wireless Networks(2) 1- System interconnection System interconnection is all about interconnecting the components of a computer using short-range radio waves or Bluetooth 2- Wireless LANs, the wireless LANs are systems in which every computer has a radio modem and antenna with which can communicate with other systems IEEE802.11 details will be discussed in chap. 4 Computer Networks, Chapter 1

20. Continue….Wireless Networks(3) 3- Wireless WANs, the third kind of wireless networks is used in wide area systems (a) Individual mobile computers (b) A flying LAN Computer Networks, Chapter 1

21. Computer Networks, Chapter 1 Home Networks, Home networking has some fundamentally different properties than the other network types, The network and devices have to be easy to install The network and devices have to foolproof in operation. Low price is essential for success The main applications is likely to involve multimedia, so the network needs sufficient capacity. It must be possible to start out with one or two devices and expand the reach of the network gradually Security and reliability will be very important.

22. Computer Networks, Chapter 1 Continue ……Home Networks (2) An interesting question is weather home networks will be wired or wireless. Most of the home networks are related to the need to be easy to manage, dependable, and secure, especially in the hands of non-technical users, while at the same time delivering high performance with low cost, Wireless is cost favors network Wired is security favors network

23. Computer Networks, Chapter 1 Internetworks An internetwork or internet, is a collection of interconnected networks. (How??) Different and frequently incompatible networks can be connected ,sometimes, by means of a machines is called gateways to make the necessary translation both in terms of hardware and software. Subnet, networks, and internetnetwork !!!!

24. Computer Networks, Chapter 1 3-Network software The following outlines describes the network software structuring technique in some details - Protocol Hierarchies - Design Issues for Layers - Connection oriented and connectionless services -Service Primitives -The relationship of services to protocols

25. Protocol Hierarchies • Most networks are organized as a stack of layers or levels. • Each layer is a kind of virtual machine, offering certain services to the layer above it. • A protocol is an agreement between the communication parties on how the communication is to proceed Layers, protocols, and interfaces Computer Networks, Chapter 1

26. Computer Networks, Chapter 1 Continue……..Protocol Hierarchies (2) The interface defines which primitive operations and services the lower layer makes available to the upper one. An important consideration in designing network is defining clean interface between layers, Why?? -minimizing the information -replace implementation Network architecture, is a set of layers and protocols

27. Computer Networks, Chapter 1 Continue……..Protocol Hierarchies (3) A list of protocols used by a certain system, one protocol per layer, is called a protocol stack. Q1 Q2: protocol and interface Example information flow supporting virtual communication in layer 5.

28. Computer Networks, Chapter 1 Design Issues for the Layers Addressing Rules for data transfer Error control Message order Flow control Long messages and small massages Multiplexing and demultiplexing Routing

29. Computer Networks, Chapter 1 Connection-Oriented and Connectionless Services Layers can offer two different types of services to the layer above: Connection-Oriented and Connectionless. Connection-Oriented, it is the service that the layer establish a connection, uses the connection, and then releases the connection. In contrast, Connectionless in this service each message carries the full destination address, and each one is routed through the system independent of all the others.

30. Computer Networks, Chapter 1 Continue……Connection-Oriented and Connectionless Services (2) A quality of the service, Some services are reliable in the sense that they never loss data. By using Ack. Reliable connection-oriented service has two minor variations: message sequence and byte stream. The acknowledged datagram service Request-replay-service

31. Computer Networks, Chapter 1 Continue……Connection-Oriented and Connectionless Services (3) Six different types of service. Why would anyone actually prefer unreliable communication to reliable one?

32. Computer Networks, Chapter 1 Service Primitives A service is specified by a set of primitives (operations) available to a user process to access the service. Five service primitives for implementing a simple connection-oriented service.

33. Computer Networks, Chapter 1 Continue……Service Primitives(2) Packets sent in a simple client-server interaction on a connection-oriented network.

34. Computer Networks, Chapter 1 The Relationship of Services to Protocols The relationship between a service and a protocol.

35. Computer Networks, Chapter 1 Continue…. Computer Networks Chapter 1: Introduction, Section 1.4: Reference Models -OSI -TCP/IP - OSI vs. TCP/IP - OSI critiques - TCP/IP critiques

36. 4-Reference Models -OSI -TCP/IP • The OSI -Open System Interconnection- Reference Model -A layer should be created where a different abstraction is needed -Each layer should perform a well-defined functions -The function of each layer should be chosen toward defining ISO protocols -The layer boundaries should be chosen to minimize the information across the interface. -The no. of layers should be large enough that distinct functions. Computer Networks, Chapter 1

37. Computer Networks, Chapter 1 The OSI reference model 6-syntax and semantics of the information transmitted 5-establish session between different users 4-accept the data from above split it pass it to network layer ensure arrived correctly ? 3-Controls the operation of the subnet 2-Transform a row transmission facility 1-Transmitting row bits

38. Computer Networks, Chapter 1 The TCP/IP Reference Model • The TCP/IP model was used in the grandparent of all computer networks. The major design goals of TCP/IP are: • The ability to connect multiple networks together in a seamless way. • The ability to survive loss of subnet hardware, with conversations not being broken off. • A flexible architecture for supporting applications with divergent requirements, ranging from transferring files to real-time speech transmission

39. Computer Networks, Chapter 1 The TCP/IP Reference Model …..Continue(2) TCP (Transmission Control Protocol): a reliable connection-oriented protocol. UDP (User Datagram Protocol): an unreliable connectionless protocol Its job is to inject IP packets into any network and have them travel independently to the destination The TCP/IP model just points out that the host has to connect to the network using some protocol so it can send IP packets over it. This protocol is not defined and varies from host and network to network

40. Computer Networks, Chapter 1 The TCP/IP Reference Model …..Continue(3) Protocols and networks in the TCP/IP model initially.

41. Computer Networks, Chapter 1 A comparison of the OSI and TCP/IP reference models Fundamental similarities: The same concept of a stack of independent protocols. Similar functionality of the layers. The three central concepts to the OSI model: Services: which tell what the layer does, not how entities above it access it or how the layer works. Interfaces: which tell the processes above it how to access it (i.e., what the parameters and results are), not how the layer works inside. Protocols: which are used between peer entities to implement the offered services.

42. Computer Networks, Chapter 1 A comparison of the OSI and TCP/IP reference models . ….Continue (2) major differences OSI makes the distinction between services, interfaces, and protocol clearly, however the TCP/IP model did not clearly distinguish them. The OSI model was devised before the protocols were invented, but the reserve was true with the TCP/IP model. The OSI model has seven layers, but the TCP/IP model has only four layers. The OSI model supports both connection-oriented and connectionless communication in the network layer, but only connection-oriented communication in the transport layer.

43. Computer Networks, Chapter 1 A comparison of the OSI and TCP/IP reference models . ….Continue (3)

44. A critique of the OSI model and protocols • Bad timing. • Bad technology. • Bad implementation. • Bad politics. Bad Timing Computer Networks, Chapter 1

45. Computer Networks, Chapter 1 A critique of the TCP/IP reference model Service, interface, and protocol not distinguished Not a general model Host-to-network ‘layer’ not really a layer No mention of physical and data link layers Protocols other than the IP and TCP were ad hoc.

46. Computer Networks, Chapter 1 A critique of the TCP/IP reference model ……. Continue (2) As a summary The OSI model minus the session and presentation layers) is exceptionally useful for discussing computer networks, but the OSI protocols have not become popular. The TCP/IP model is practically nonexistent, but the protocols are widely used.

47. Computer Networks, Chapter 1 Chapter 1: Introduction, Section 1.5 Example Network - Internet -ARPANET -NSFNET Continue…. Computer Networks

48. 5-Example Network • The ARPANET “Review” • It is the creation of ARPA (later DARPA, now ARPA), the (periodically Defense) Advanced Research Projects Agency of the US Dept. of Defense. • ARPANET (Datagram subnet ) technologies : -IMP (Interface Message Processor) minicomputers connected by 56kbps transmission lines. -Protocols (subnet and host protocols) The TCP/IP model and protocols were specifically designed to handle the interconnection of the vast number of WANs and LANs comprising the ARPA internet. Computer Networks, Chapter 1

49. Computer Networks, Chapter 1 Example Network: The ARPANET….. continue(2) The original ARPANET design.

50. Computer Networks, Chapter 1 Example Network: NSFNET By the late 1970s, the NSF (National Science Foundation, USA) set up CSNET to provide networking facilities to the computer science community in USA as a whole (particularly those without access to ARPANET). By 1984, NSF began designing a high-speed network, called NSFNET, that would be open to all university research groups. NSFNET consists of a backbone network connecting six supercomputer centers, and about 20 regional networks.