Prof. Msanjila, S.S. Computer Networks

Course outline Introduction to computer networks Layers of Computer Network Physical layer Data link layer Medium access control sub-layer Network layer Transport layer Application layer

Lecture 1: Introduction to computer networks

Definitions Network: Example of networks: 1.Human networks a.Students associations b.Political parties c.Religions 2.Organizational networks a.Supply chains b.Tanzania commission for universities 3.Road networks a.Connected highways b.Connected town roads 4.Computer networks refers to connected objects/events that inter-operate or communication based on certain established rules and policies

Computer networks - History Business Motivated Technological Development 18 th century: Mechanical systems which led to industry evolutions Increase of production industries and agricultural equipments 19 th century: Steam engines which ease the transportation system Entering 20 th century was challenged with collecting and disseminating information as fast as possible Technologies for gathering, processing and distribution information: Telephone networks Radio communication (radio and television) Computer technologies Communication satellites

Computer network - Definition Refers to a collection of computers that are connected by a certain media and communicate using certain defined set of policies and rules. Type of computer networkConnection mediaPolicies and rules Internet, phones, faxWire, wirelessProtocols Local area networkWireFile sharing policies/rules Region networkwire, wirelessVirtual network protocols

Uses of Computer Networks Business Applications Home Applications Mobile Users Social Issues

Business Applications of Networks A network with two clients and one server. To enhance resource sharing by making all programs, equipment, and data available to any authorized person on the network irrespective of the location of the resource or the user e.g. access, websites

Business Applications of Networks (2) The client-server model involves requests and response e.g. web service technologies, grid computing To enhance resource sharing by making all programs, equipment, and data available to any authorized person on the network irrespective of the location of the resource or the user

Home Network Applications Access remote information –Accessing information stored in work server Person-to-person communication –Personal chatting Interactive entertainment –P2p networking e.g. live football Electronic commerce –Electronic payment, ATM machines

Home Network Applications (2) In peer-to-peer system there are no fixed clients and servers.

Home Network Applications (3) Some forms of e-commerce.

Mobile Network Users Combinations of wireless networks and mobile computing. To enhance access of information from changing remote locations

Social issues for computer networks Problems: Breach of privacy Child abuse Theft Unauthorized copying Breach of regulations ….

Characterization of computer networks 1.Based on hardware How nodes within the networks are connected 2.Based on software How information moves from one node to another and what formats does it change 3.Based on information delivery How packets are sent and received (one node or many nodes)

Classification based on network hardware Local Area Networks Metropolitan Area Networks Wide Area Networks Wireless Networks Home Networks Internetworks/inter connected networks (internet)

Broadcast Networks Types of transmission technology Broadcast links Point-to-point links

Broadcast Networks (2) Classification of interconnected processors by scale.

Local Area Network - LAN Two broadcast networks (a) Bus (b) Ring Private network within a short distance such as one building connecting computers in a company to support resource sharing and exchange information

LAN - differences LANs can be distinguished from other types of networks considering: Size : small in size, transmission time is known, easy to manage Transmission technologies : mainly cables, has speed between 10 – 100Mbps, low packet delay, very few errors Topologies : Cable connection: Bus Ring Channel allocation: static dynamic

Metropolitan Area Network - MAN A metropolitan area network based on cable TV A wireless internet access in the city Covers a relatively larger network mainly connecting different nodes in one city. E.g. cable television network available in many European cities

Wide Area Network - WAN Relation between hosts on LANs and the subnet. Cover a very large geographic area, namely a country or continent. Can consist of a set of LANs and MANs. Consists of two main parts: Hosts: machines which perform the computation Communication subnet: carrying message between hosts

WAN subnet - Components Two distinguishable components: Transmission lines: Responsible for moving bits and packets between machines/nodes Switching elements : specialized computers controlling the flow of data and connection of transmission lines Once a packet is received a switching element selects best outgoing lines based on certain indicators such as time, cost and distance Switching elements are commonly referred to as router

Wide Area Networks (2) A stream of packets from sender to receiver. Sometimes one routers may need to communicate although they do not share transmission lines. Communication shall be facilitated via other intermediate routers (indirect communication) Sometimes lines might be busy and thus received packet shall be stored until free line is available This principle is called store-and-forward or packet-switched

Wireless Networks Categories of wireless networks: System interconnection Wireless LANs Wireless WANs

System interconnection (a) Bluetooth configuration Uses short-range-radio technologies to interconnect a number of computers It is a breakthrough technology to plugging components to computers Commonly known as Bluetooth Allows digital cameras, headsets, scanners, … without wires in short ranges An approach to system inter-connection networks is master-slave paradigm

Wireless LANs (b) Wireless LAN  Every computer has a radio modem and antenna with which it can communicate with other systems  If computers are too close a peer – to – peer network can be established  Current wireless LANs are based on IEEE standard  Wireless networks connect to wide area networks to access files, databases and the internet

Home Network Categories Computers (desktop PC, PDA, shared peripherals) Entertainment (TV, DVD, VCR, camera, stereo, MP3) Telecomm (telephone, cell phone, intercom, fax) Appliances (microwave, fridge, clock, furnace, airco) Telemetry (utility meter, burglar alarm, babycam). Focus on enhancing the connectedness of required equipment at home to outside environments

Internet The need for inter-connecting one network to another led to the birth of internet The need to connect different and incompatible networks Common form of internet is the collection of LAN connected by WAN Connection makes use of Gateways Internet stands for: Internetworks Internetworking Inter connected networks

Network Software Lecture 2

Network Software Networking ideas first focused on connecting hardware Making cables that can transfer data Solving the incompatibility of hardware No concern with software Many problems with this approach: Difficult to troubleshoot Difficult to ensure security Difficulty to ensure privacy.….

Network Software Addresses Protocol Hierarchies Design Issues for the Layers Connection-Oriented and Connectionless Services Service Primitives The Relationship of Services to Protocols

Protocol Hierarchies Layers, protocols, and interfaces. Networks are designed into layers to reduce design complexity Networks differ by: Number of layers Name of layers Content of layers Function of layers

Protocol Hierarchies (2) The philosopher-translator-secretary architecture.

Design Issues for the Layers Addressing: Mechanisms to identify senders and receivers Error Control: Mechanisms to detect and correct errors caused by physical communication circuit are needed Flow Control: Controlling the process of sending data and receiving notification Multiplexing: Mechanisms for many users to sending data at the same time using one channel Routing: Finding the right and optimal path for sending data

Connection-Oriented and Connectionless Services Six different types of service.

Service Primitives Five service primitives for implementing a simple connection- oriented service. Primitives are operations that allow a user to access the service Primitives for client-server environments

Service Primitives (2) Packets sent in a simple client-server interaction on a connection-oriented network.

Services to Protocols Relationship The relationship between a service and a protocol.

Reference models

Reference Models The OSI Reference Model The TCP/IP Reference Model A Comparison of OSI and TCP/IP A Critique of the OSI Model and Protocols A Critique of the TCP/IP Reference Model

ISO - Open systems interconnection - OSI Developed by the International Standard Organization –ISO Has seven layers: Physical layer : Transmitting raw bits over communication channels Data link layer : transform raw transmission facility into a line that appears free of undetected transmission errors to the network layer Data frames and acknowledgement frame Network layer : Control operation of subnet Define how packets are routed from source to destination Transport layers : ensures safer sending of data Accept data, break into small piece, pass to network layer and ensure all pieces arrive correctly Session layer : define mechanisms for using channels by multiple partners Dialog control, token control, synchronization Presentation layer : controlling semantics and syntax of information. Transforms Bits to text Application layer : Defines various protocols that support interaction between systems, network and human users

ISO - Open systems interconnection - OSI The OSI reference model.

Transfer control protocol / internet protocol The TCP/IP reference model.

Comparing OSI and TCP/IP Models Concepts central to the OSI model Services Interfaces Protocols

A Critique of the OSI Model and Protocols Why OSI did not take over the world Bad timing Bad technology Bad implementations Bad politics

Bad Timing The apocalypse of the two elephants.

A Critique of the TCP/IP Reference Model Problems: 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 Minor protocols deeply entrenched, hard to replace

Hybrid Model Will be the focus of this course

Internet Usage Traditional applications (1970 – 1990) News Remote login File transfer

Architecture of the Internet Overview of the Internet.

ATM Virtual Circuits A virtual circuit. Asynchronous Transfer Mode: Transfer of data over the internet which is not tied to time

ATM Virtual Circuits (2) An ATM cell.

The ATM Reference Model The ATM reference model.

The ATM Reference Model (2) The ATM layers and sublayers and their functions.

Ethernet Architecture of the original Ethernet.

Wireless LANs (a) Wireless networking with a base station. (b) Ad hoc networking.

Wireless LANs (2) The range of a single radio may not cover the entire system.

Wireless LANs (3) A multicell network.

Network Standardization Who’s Who in the Telecommunications World Who’s Who in the International Standards World Who’s Who in the Internet Standards World