Computer Networks Overview PDF

Imagine being the desk officer of KWASU CODeL(congregational delegation led by a member of congress) and you need to monitor the activities of other representative of the unit in various part of the country so as to know the details of the number of people who came to inquire about the activities of...

Imagine being the desk officer of KWASU CODeL(congregational delegation led by a member of congress) and you need to monitor the activities of other representative of the unit in various part of the country so as to know the details of the number of people who came to inquire about the activities of the unit, those who show interest in one activity or the other and the number of people who have officially registered as students in a program undertaken by the unit, by the various representatives of the unit. A possible solution is for the desk officer to visit the various representatives of the units at their various locations so as to obtain that information which may be tasking, tedious and time consuming. An alternative solution is to connect the various representatives' computers at different locations together through a network which allows them to communicate, share resources and information. This will facilitate you (the desk officer) to have access to all those information from your seat enhancing your productivity. Wow! Imagine how great that would be. You are worried that what is a network and how can it be established. You will find out in a short while. **What is a Network?** A network is a group of computers connected together\--wired or wireless\--by some special protocols so that they can communicate, share resources and enhance productivity. In a typical network, there are usually one or more dedicated computers that work as servers in storing, processing and routing data/information to other computers in the network. I.e. it provides services to other computers or clients in the network. Figure 1 illustrates the structure and interconnection of computers to form a network. It comprises of three computers named computer A, B and C respectively which are connected together via wired to a central server so as to communicate, exchange file, video, voice and data and also shared resources like the central copier and printer which any computer in the network can send documents to print. Figure1: An Illustration of a Typical Network **1.2 Types of Networks** Networks can be classified into four (4) main types based on their geographical coverage areas, equipment installation cost, as well as support tools and cost. The four types of networks are: i. **Local Area Network:** LAN is a computer network widely used for local communication. LAN connects computers in a small area like a room, building, office or a campus spread up to a few kilometers. They are privately owned networks, with a purpose to share resources and to exchange information. The computers in a LAN are generally connected using cables. LAN is different from other types of network since they share the network. The different computers connected to a LAN take turns to send data packets over the cables connecting them. This requires coordination of the use of the network. Some of the transmission protocols used in LAN are Ethernet, Token bus, and FDDI (Fiber distributed data interface) ring Star, Bus, and Ring are some of the common LAN networking topologies. LAN runs at a speed of 10 Mbps to 100 Mbps and has low delays. A LAN based on WiFi wireless network technology is called Wireless Local Area Network (WLAN). ii. **Campus Area Network:** this is a network that connects multiple local area networks. This type of network is larger than the LANs but smaller than the metropolitan area network. Example of such network is the type of network found in a university (KWASU ) which connects many LANs found in various schools and units together. It is abbreviated as CANs and requires more sophisticated equipment and cost to set up and maintain when compared to LAN. iii. **Metropolitan Area Network:** this is abbreviated as MAN, it is a high speed network that is larger than CAN and connects many LANs in a large and densely populated urban area to the internet. iv. **Wide Area Network:** this is a type of network that connects many LANs over very large group of cities through the internet or over the public switched telephone network. The MANs and WANs required more sophisticated networking equipment and cost to set-up and maintain. In addition, it requires on-site administrator and support staff. Having discussed what a network is and the different types of network, I know you kept wondering that what are the importance or benefits of this so called networks and what an internet means. You need not worry, sit back and continue to read through this unit as you will find out soon yourself. **NETWORK TOPOLOGIES** There are different types of network topologies that are used in a network. The network topologies in the structure or the layout of the different devices and computers connected to the network. The topologies commonly used in LAN are---Bus topology, Star topology, and Ring topology. **Bus Topology** C:\\Users\\user\\Pictures\\bus topology.png - All devices on the network are connected through a central cable called a **Bus**. - The data signal is available to all computers connected to the bus. - The point where a device connects to the central communication channel is referred to as **Node** - The data signal carries the address of the destination computer. - Each computer on the network checks the destination address as the data signal travels through the bus. The computer whose address matches makes a copy of the signal and converts it into data. The data signal on the bus does not get destroyed and still transmits along the bus, and is finally absorbed by the terminator attached to the end of the network. - Bus topology commonly makes use of Coaxial cable. - It is good for connecting 15--20 computers. - A single coaxial cable is generally used in bus topology, to which the computers or devices are connected. - Ethernet is a commonly used protocol in networks connected by bus topology. - Termination in bus topology simply means matching impedance at the end of the cable, which means two ends, the section which is also referred to has **segments** - Terminator is used to prevent signal reflection in bus topology which is found at both ends. - Hub helps to manage data flow in bus topology. **Advantages of Bus Topology** - Easy to implement(computers are connected linearly through the cable) - Easily extendable(new devices can be easily added) - Not very expensive **Disadvantages of Bus Topology** - If the cable gets damaged, the whole network collapses - A computer can transmit data only if network is not been utilized - Network slows down if additional computers are connected to the network. - Single point of failure. **Ring Topology**  - All devices in the network are connected in the form of a ring. - Each device has a receiver and transmitter to receive the data signals and to send them to the next computer, respectively. - Ring network does not have terminated ends, thus data signals travel in a circle. - Ring topology uses token passing method to provide access to the devices in the network. - The computers or devices are connected to the ring using fiber optic cable. - The protocols used to implement ring topology are Token passing and Fiber Distributed Data Interface (FDDI)s **Advantages of Ring Topology** - All computers in the ring have equal access to the ring - Each computer in the ring gets an opportunity to transmit data **Disadvantages of Ring Topology** - Adding or removing devices is difficult and affects the complete network - Failure in a node or the cable breaks down the ring and thus the network - The length of the ring and the number of nodes are limited. **Star Topology** C:\\Users\\user\\Pictures\\Star topology.png All devices are connected through a central link forming a star-like structure. The central link is a hub or switch. The computers commonly makes use of twisted pair cables for connection to the hub or switch. Star topology is the most popular topology to connect computer and devices in network, which is suitable for large networks with high traffic and data transmission requirement. The data signal is transmitted from the source computer to the destination computer via the hub or switch. i.e. it controls data flow. The common protocols used in star topology are Ethernet, Token Ring, and Local Talk. In addition to the bus, ring, and star topologies, there are complex topologies like the tree topology, and the mesh topology used for networking in LAN. - Note: when more devices are added to the network it decreases its performance. **Advantages of Star Topology** - Failure of a device attached to the network does not halt the complete network; only that device is down - Easily extendable -- by attaching a new device to the hub or switch - No disturbance when a new device is added or removed. - Easy to troubleshoot the network. - It is more resilient to cable fault and provides better reliability. **Disadvantages of Star Topology** - It is costly(higher cost) since each device on the network is attached by a single cable to the central link - Failure of the hub or switch breaks the complete network The International Standards Organization (ISO)in year 1984 has developed a seven-layer reference model for data networks, known as Open System Interconnection (OSI) model. The OSI model specifies the functions of each layer. It does not specify how the protocol needs to be implemented. It is independent of the underlying architecture of the system and is thus an open system. The seven layers of the OSI model are---(1) Physical layer, (2) Data link layer, (3) Network layer, (4) Transport layer, (5) Session layer, (6) Presentation layer, and (7) Application layer. The functions of the different layers are as follow Highest level s Lowest level **Physical Layer**---This layer specifies the basic network hardware. Some of the characteristics defined in the specification are---interface between transmission media and device, encoding of bits, bit rate, error detection parameters, network topology, and the mode of transmission (duplex, half-duplex or simplex). **Data Link Layer**---This layer specifies the functions required for node-to-node transmission without errors. It specifies the organization of data into frames, error detection in frames during transmission, and how to transmit frames over a network. **Network Layer**---The network layer specifies the assignment of addresses (address structure, length of address etc.) to the packets and forwarding of packets to the destination i.e. routing. **Transport Layer**---It specifies the details to handle reliable transfer of data. It handles end-to-end error control and flow control, breaking up data into frames and reassembling the frames. **Session Layer**---The session layer maintains a session between the communicating devices. It includes specifications for password and authentication, and maintaining synchronization between the sender and the receiver. **Presentation Layer---**This layer specifies the presentation and representation of data. Its functions include translation of the representation of the data into an identifiable format at the receiver end, encryption, and decryption of data etc. **Application Layer**---This layer specifies how an application uses a network. It deals with the services attached to the data. It contains the protocols used by users like HTTP, protocol for file transfer and electronic mail. **1.3 Importance of Networking** i. Facilitate data resource sharing: with the advent of computer networks, it makes information sharing between computers users on the network easy. Hence, making working together easy for network users and enhancing personal and collective productivity. ii. Enable users to share hardware resources: networking computers enables users to share hardware resources such as printers, copier and scanners. All users within the network can access these resources. iii. Sharing a high-speed Internet connection: all computers connected in a network can share high speed internet in communicating with other computers within the network and also connect to other outside networks via the internet. iv. Networks serve as a communication and support tools: computer networks serve as a communication tool in exchanging files and other information via the network. An example of such is the electronic mail (e-mail) **1.0 What is the Internet?** The internet can simply be defined as a network of networks using a common set of rules that govern the transfer of information and communication over the network known as Internet protocol (IP). The internet give you access to numerous information, files, voice and videos all over the world. A good illustration of the importance of the internet is you accessing this course material titled \"Introduction to Computing\" written for CODeL online irrespective of your location or receiving lecture online just by logging in to the internet and accessing the unit website wherein you then log in with your username and password.