COM211 Communications and Networks CDA College Theodoros Christophides INTRODUCTION TO COMPUTER NETWORKS, OSI MODEL

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 (a) (b) point-to-point multiple-access Geographical coverage and scalability are limited. Each node needs one interface for each link.

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:  Single ring – All the devices share a single cable  Dual ring – Allows data to be sent in both directions and provides redundancy

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 )

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-to- host 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). switch queue Computer A Computer B Computer C

Range of Coverage LAN: local area network WLAN: wireless local area network MAN: metropolitan area network WAN: wide area network Computer networks can be classified according to their geographical coverage: 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 Physical Presentation Session Transport Network Data link 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.

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 data AH data PH data SH data TH data NH data DH DT data BITS Physical Presentation Session Transport Network Data link Application Physical Presentation Session Transport Network Data link Application receiver sender

Layers in TCP Physical Data link IP ARP RARP ICMP IGMP Transport TCP UDP Network Session Presentation Application FTP HTTPDNS NFS …

TCP/IP and OSI

Questions?? Thank you for your time and patience!