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INTRODUCTION TO COMPUTER NETWORKS, OSI MODEL COM211 Communications and Networks CDA College Theodoros Christophides Email: [email protected] www.cdacollege.ac.cy/site/info-com-technology-ll/ Course Content • Lecture 1: Introduction to Computer Networks, OSI Model • Lecture 2: Hardware building blocks and encoding • Lecture 3: Physical Media and Cabling • Lecture 4: Protocols • Lecture 5: LAN and WAN Technologies • Lecture 6: Data Link Layer and Ethernet • Lecture 7: Midterm • Lecture 8: Network Layer of OSI • Lecture 9: Transport Layer of OSI • Lecture 10: Application Services Course Content • Lecture 11: Security in Computer Networks • Lecture 12: Wireless Networks • Lecture 13: Revision for the Final examinations References Andrew S. Tanenbaum, Computer Network, Prentice-Hall Doughlas E. Comer, Computer Networks and Internet Larry L. Peterson and Bruce S. Davie, Computer Networks: A Systems Approach Computer Networks A collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information. Connectivity Interconnect machines. Maintain data confidentiality, data integrity, and system accessibility. Support growth by allowing more and more computers, or nodes, to join in (scalability). Support increases in geographical coverage. Internet Is not a single network but a network of networks Links Each node needs one interface for each link. point-to-point(a) multiple-access(b) Geographical coverage and scalability are limited. Network Topology The network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions. Mesh Topology This topology connects all devices to each other for redundancy and fault tolerance. It is used in WANs to interconnect LANs and for mission critical networks (banks, financial institutions, etc.) Implementing the mesh topology is expensive and difficult. Bus Topology All the devices on a bus topology are connected by one single cable. Star Topology The star topology is the most commonly used architecture in Ethernet LANs. Tree Topology Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host. Ring Topology A frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame. The frame then continues around the ring until it finds the destination node, which takes the data out of the frame. Types: ring – All the devices share a single cable Dual ring – Allows data to be sent in both directions and provides redundancy Single Network Components Physical Media Network Devices Computers Protocols Services Networking Media Twisted Pair Coaxial Fiber Optics Wireless Transmissions Network Devices Hub Switches ( Level 2 and 3 ) Routers Wireless Access Points Modems NIC’s Computers End Devices: Acts as a source/destination. For message transmitting or receiving. Server: In a client/server network environment, network services are located in a dedicated computer whose only function is to respond to the requests of clients. The server contains file sharing, http and other services that are continuously available to respond to client requests. Client: Our computers. We request a service from a server (ex. We log into gmail to check our email ) Protocols A protocol, in contrast, is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entities within a layer. Entities use protocols to implement their service definitions. They are free to change their protocols at will, provided they do not change the service visible to their users. TCP, UDP, IP, X.25, ICMP, IPSec Services DHCP - Dynamic Host Configuration Protocol DNS – Domain Name System HTTP - Hypertext Transfer Protocol SSH – Secure Shell Telnet SNMP - Simple Network Management Protocol SMTP - Simple Mail Transfer Protocol FTP – File Transfer Protocol IRC – Internet Relay Chat POP – Post Office Protocol Internetworking To interconnect two or more networks, one needs a gateway or router. Host-tohost connectivity is only possible if there’s a uniform addressing scheme and a routing mechanism. Messages can be sent to a single destination (unicast), to multiple destinations (multicast), or to all possible destinations (broadcast). Synchronous Time Division Multiplexing (STDM) Divide time into equal-sized quanta and assign each them to flows on the physical link in round-robin fashion. Frequency-Division Multiplexing (FDM) Flows are transmitted simultaneously on the link, but each one uses a different frequency. Code Division Multiplexing (CDM) Flows are transmitted simultaneously on the link, but each one uses a different coding scheme. For a chosen group of nodes, a unique coding scheme can be used. Each bit is encoded in multiple pulses. Multiple senders may use the same time slots with different coding. Statistical Multiplexing Flow is broken into packets and sent to a switch, which can deal with the arriving packets according to the switch policy (FIFO, round-robin, etc). Computer A queue Computer B switch Computer C Range of Coverage Computer networks can be classified according to their geographical coverage: • LAN: local area network • WLAN: wireless local area network • MAN: metropolitan area network • WAN: wide area network In Interconnecting multiple networks (internetworking), we are interested in the seamless integration of all these levels. Have in mind that different levels use different technologies! ISO: International Standards Organization OSI: Open Systems Interconnection Application The protocol stack: The idea behind the model: Break up the design to make implementation simpler. Each layer has a well-defined function. Layers pass to one another only the information that is relevant at each level. Communication happens only between adjacent layers. Presentation Session Transport Network Data link Physical Layers of the OSI model • Physical: Transmit raw bits over the medium. • Data Link: Implements the abstraction of an error free medium (handle losses, duplication, errors, flow control). • Network: Routing and Addressing. IP • Transport: Break up data into chunks, send them down the protocol stack, receive chunks, put them in the right order, pass them up. • Session: Establish connections between different users and different hosts. • Presentation: Handle syntax and semantics of the info, such as encoding, encrypting. • Application: Protocols commonly needed by applications (cddb, http, ftp, telnet, etc). Communication Between Layers sender receiver data data AH Application PH Presentation data Application data Presentation Session SH data Session Transport TH data Transport Network NH data Network Data link Physical DH data BITS DT Data link Physical Layers in TCP Application Presentation FTP HTTP DNS NFS … Session Transport ICMP Network TCP UDP IGMP IP ARP Data link Physical RARP TCP/IP and OSI Thank you for your time and patience! Questions??