Table of Contents:-
- What is computer network?
- Components of Computer Network
- Computer Network Architecture
- Network Topology
- Types of Topology
- Basic Network Topology
- Advantages of Computer Network
What is computer network?
A computer network can be defined as a group of interconnected computers that are very useful when users want to share resources such as printers, databases, electronic mail, bulletin boards, etc.
Each station is known as a node, which may include peripherals, computer terminals, and various communication devices.
Computer networks can be considered as information highways for data.
Computer networks are typically categorized based on their structure and geographic location into the following types:
• Local Area Network (LAN): A network that spans a relatively small area, such as a single building or campus, is known as a LAN.
• Metropolitan Area Network (MAN): Designed for a city or town, a MAN is a type of computer network.
• Wide Area Network (WAN): Encompassing a large geographical area, WANs cover different cities, states, and sometimes even countries.
Additional characteristics used to categorize various types of networks include:
• Topology: Referring to the graphical arrangement of computer systems in a network, common topologies include star, bus, ring, and mesh.
• Protocol: Protocols define a standard set of rules used by computers on the network to facilitate communication between hardware and software entities. One of the most popular LAN protocols is Ethernet, while another commonly used protocol for PCs is the token-ring network.
• Architecture: Networks can be broadly classified based on their peer-to-peer or client/server architecture use.
Components of Computer Network
There are various essential components in a network. The following are some crucial components of a network:
1) Clients: In a network, an individual computer is referred to as a client, capable of accessing servers and shared resources.
2) Modem: A modem (modulator-demodulator) is a device that converts incoming analog signals into digital signals and vice versa. They come in two types:
- Internal Modem: Built inside the computer and is more cost-effective.
- External Modem: An external device that costs more than internal modems.
3) Channels: Channels, also known as network circuits, provide a path for information to travel between clients and servers in the network. Channels are selected based on their speeds and capabilities as transmission media.
4) Servers: Also known as host computers, these powerful devices connect to various resources shared by network users and can store applications and data.
5) Routers: These dedicated hardware devices forward data packets between computer networks. Routers can be either wireless or wired. Users need to set a password to secure a wireless router from unauthorized access.
Computer Network Architecture
The communication subsystem comprises a complex piece of hardware and software. Implementing the software for such systems was based on a single, complex, and unstructured program with several components. As such, the resultant software became challenging to test and modify.
To address this problem, the ISO adopted a layered approach for the reference model where the communication system is broken into layers, each performing a well-defined function. Thus, the layers perform two generic functions: network-dependent and application-oriented.
The two essential network architectures are:
1) OSI Reference Model
As defined by the ISO (International Organization for Standardization), computer-to-computer communications can be divided into seven connected layers known as a protocol stack. Every higher layer builds on the functions of the layer just below.
Open Systems Interconnection (OSI) is a reference model that defines how messages will be transmitted between any two points in the network.
The two endpoints in the OSI model are divided into layers. Data flows through every layer at one end and down through the layers at the other end. Once the message is received, data flows through the layers at the receiving endpoint and finally to the end-user or program.
The different layers of OSI are as follows:
i) Presentation Layer
The presentation layer functions as the data translator within a network. It is usually part of an operating system and converts incoming and outgoing data from one presentation format to another. Syntax layer is another name for this layer.
ii) Physical Layer
The physical layer defines the cable or physical medium, e.g., thinnet, thicknet, and Unshielded Twisted Pairs (UTP). All media are functionally equivalent. The main difference is in the convenience and cost of installation and maintenance. Converters from one media to another operate at this level. The physical layer is responsible for transmitting and packaging data on the physical media. This layer conveys the bit stream through the network at the electrical and mechanical levels.
iii) Transport Layer
The transport layer ensures that messages are delivered in the order they are sent and that there is no loss or duplication. It provides complete data transfer. The transport layer subdivides the user buffer into network buffer-sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sit at the transport layer.
iv) Data-Link Layer
The data-link layer is responsible for the error-free, accurate and reliable transfer of data frames. This layer provides synchronization at the physical level. The Data Link layer defines the format of data on the network.
v) Application Layer
The application layer provides a window for users and application processes to access network services. It handles issues such as resource allocation, network transparency, etc. This type of layer provides network services to the end users. Network applications include Mail, FTP, Telnet, DNS, NIS, and NFS.
vi) Session Layer
The session layer establishes a communication session between processes running on different communication entities in a network and can support message-mode data transfer. It deals with connection coordination and sessions.
vii) Network Layer
It determines the data’s physical path based on network conditions, service priority, and other factors. The network layer is responsible for forwarding data packets and routing.
2) TCP/IP Reference Model
Computer communication became problematic as the number of networks connected to the ARPAnet increased. Common standards were required for communication because the software and hardware used were vendor-specific. A standard protocol is required for communication between the computers. This led to the creation of IP and TCP. With the increase in requirements, several protocols were created to address all the requirements, creating a new reference model called the TCP/IP architecture. TCP/IP comprises four layers: Application, Transport, Internet, and Network Interface.
TCP/IP consists of four layers:
i) Network Access Layer
The network access layer is responsible for exchanging data between a host and the network and delivering data between two devices on the same network. Node physical addresses are used to accomplish delivery on the local network.
Functions performed at this level include encapsulation of IP datagram (i.e., the packet format defined by Internet Protocol.) into the frames transmitted by the network and mapping IP addresses into the physical addresses used by the network.
The TCP/IP network access layer can encompass the function of all three lower layers of the OSI reference model (Data Link Layer, Network Layer, and Physical Layer.)
ii) Internet Layer
The internet layer is responsible for sending source packets from any network on the internet work and having them arrive at their destination regardless of their path.
This layer uses Internet Protocol (IP) to provide the packet delivery service on which the TCP/IP is based. IP protocol implements a system of logical host addresses known as IP addresses. The internet and higher layers use the IP addresses to identify devices and perform internetwork routing.
iii) Transport Layer
The transport layer is responsible for the reliability, flow control, and error correction of data sent across the network. Its primary protocol is called the transmission control protocol (TCP). TCP provides reliable data delivery service with end-to-end detection and correction of errors.
User Datagram Protocol (UDP) is another protocol that provides slow-overhead, connectionless datagram delivery service. UDP is unreliable but enhances network throughput when error correction is not required at the host-to-host layer. Both protocols deliver data between the Internet Layer and the Application Layer.
iv) Application Layer
The application layer handles high-level protocols, representation issues, encoding, and dialogue control. This layer is broadly equivalent to the OSI model’s application, presentation, and session layers. It provides an application with access to the communication environment.
Examples of protocols found at this layer are HTTP (Hyper Text Transfer Protocol), Telnet, SNMP (Simple Network Management Protocol), FTP (File Transfer Protocol), and SMTP (Simple Mail Transfer Protocol).
Network topology is the pattern used to arrange the nodes or stations of a network, either physically or logically. It defines the path a pair of stations uses for communication within the network. Topology refers to the shape or layout of a network and establishes the arrangement of various nodes governing their communication. These topologies can be classified as either physical or logical.
The criteria to be considered when selecting a physical topology include:
- Ease of installation.
- Ease of reconfiguration.
- Ease of troubleshooting.
Types of Topology
There are two types of topology as given below:
1) Physical Topology: It is the actual geometric configuration of nodes interconnected via cables in a network.
2) Logical Topology: Logical topology refers to how information is passed between two nodes in a network. This topology is bound to the network protocols and explains how data is moved throughout the network.
Basic Network Topology
There are five types of network topologies:
- Bus topology
- Ring topology
- Star topology
- Mesh topology
- Tree topology
It is the simplest physical network. In this type of topology, all the computers, including servers, are connected by a single cable with the help of interface connectors.
The cable is known as the bus and acts as the network’s backbone, joining every computer and peripheral in the network.
In Bus topology, all devices are connected to a central cable, the bus or backbone. This topology connects workstations using a single cable, with each workstation linked to the next in a point-to-point fashion. All workstations share the same cable. In this type of topology, if one workstation malfunctions, all workstations may be affected, as they share the same cable for sending and receiving information. The cabling cost of bus systems is the lowest among different topologies, and each cable end is terminated using a special terminator.
The standard implementation of this topology is Ethernet, where all other workstations hear a message transmitted by one workstation.
Advantages of Bus Topology
- Installation is easy and cost-effective compared to other topologies.
- Connections are simple, and this topology is user-friendly.
- Requires less cabling.
Disadvantages of Bus Topology
- Suitable only for comparatively small networks.
- The network performance deteriorates as the number of computers increases beyond a specific limit, as they all share the same bus.
- Fault identification is difficult.
- A single fault in the cable can halt all transmissions.
All the computers (nodes) are connected in a closed loop in a ring topology. This topology operates on a token-based system, where the token travels in the loop. If the token is free, the node can capture it, attach the data and destination address to it, and then release it.
When the token reaches the destination node, the destination node removes the data, and the token is free to carry the next set of data. If another node wants to send data, it can capture the free token. Each node or computer acts as a repeater in this type of topology.
The main drawback of ring topology is that the complete network will go down if one node fails.
In Ring Topology, all devices are interconnected in a closed loop, forming a configuration where each device is directly linked to two other devices on either side. In other words, the ring topology connects workstations in a closed loop, with each terminal connected to two others (the next and the previous) and the last terminal linked to the first. Data circulates the ring in one direction only, with each station passing the data to the next station until it reaches its destination.
Information travels sequentially around the ring from one workstation to the next. Each data packet sent on the ring is prefixed with the address of the station to which it is intended. Upon receiving a data packet, the workstation checks if the packet’s address matches its own. If it does, the workstation retrieves the data in the packet. If the packet does not belong to it, it forwards it to the next workstation in the ring.
Faulty workstations can be isolated from the ring. When a workstation is powered on, it connects itself to the ring. When powered off, it disconnects itself from the ring, allowing information to bypass the workstation.
The standard implementation of this topology is the token ring. A break in the ring can lead to a complete network failure, and individual workstations can be isolated from the ring.
Advantages of Ring Topology
- Easy installation and modification of the network.
- Simplified fault isolation.
- Unlike Bus topology, there is no signal loss in Ring topology because the tokens are in the form of data packets that are re-generated at each node.
Disadvantages of Ring Topology
- Adding or removing computers disrupts the whole network.
- A break in the ring can halt transmission throughout the entire network.
- Locating faults is challenging.
- Relatively expensive compared to other topologies.
This is the most popular topology for creating a network. In this topology, nodes are connected to a centrally located device known as a hub with UTP (Unshielded Twisted-Pair) wire. Data are transferred from one node to another via the hub.
In star topology, each computer (node) has a distinct connection to the hub, making it easy to maintain and troubleshoot.
Star topology utilizes a central hub through which all components are connected. In a star topology, the central hub functions as the host computer, and a terminal is at the end of each connection.
Nodes communicate across the network by transmitting data through the hub. A star network requires significant cable, as each terminal is wired back to the central hub. This holds even if two terminals are located side by side but several hundred meters from the host. The central hub is responsible for all routing decisions; all other workstations can be relatively simple.
One advantage of the star topology is that the failure of one terminal does not affect any other terminal. However, the failure of the central hub impacts all terminals. This type of topology is commonly employed to connect terminals to a large time-sharing host computer.
Advantages of Star Topology
- Installation and configuration of the network are easy.
- Less expensive when compared to mesh topology.
- Faults in the network can be easily identified.
- Expansion and modification of the star network is straightforward.
- If a single computer fails it does not affect the entire network.
- Supports multiple cable types such as shielded twisted pair cable, unshielded twisted pair cable, ordinary telephone cable, etc.
Disadvantages of Star Topology
- Failure in the central hub brings the whole network to a halt.
- More cabling is required than tree or bus topology because each node is connected to the central hub.
In a mesh topology, all the computers are interconnected through various redundant connections, providing multiple paths for data delivery from one computer to another.
Mesh topology provides two different types of connection management:
1) Full Mesh Topology: In this topology, a computer or device is connected to all other computers or devices in a network.
2) Partial Mesh Topology: Not all in this topology, but only specific computers or devices are connected to those they communicate frequently. The remaining computers (nodes) are connected to all computers (nodes).
In Mesh topology devices are connected with numerous redundant interconnections between network nodes. In a well-connected topology, every node connects to every other node in the network. While the cable requirements are high, redundant paths are built in. The failure of one computer does not cause the network to break down, as alternative paths exist for other computers.
Mesh topologies are employed in critical connections of host computers, typically in telephone exchanges. Alternate paths enable each computer to balance the load to other computer systems in the network by utilizing more than one of the available connection paths. Therefore, a fully connected mesh network has n (n-1)/two physical channels to link n devices. To accommodate these, every device on the network must have (n-1) input/output ports.
Advantages of Mesh Topology
- The use of dedicated links removes traffic problems.
- Failure in one of the computers does not affect the entire network.
- Point-to-point links make fault isolation easy.
- It is robust.
- Privacy between computers is maintained as messages travel along dedicated paths.
Disadvantages of Mesh Topology
- The amount of cabling required is high.
- Many I/O (input/output) ports are required.
In a tree topology, all the computers are connected hierarchically. The topmost node of the network is known as the root node. Except for the root node, all other nodes have exactly one parent node, while all the nodes in the tree are descendants of the root node.
Therefore, only one path exists for data transmission from one node to another node in the tree topology.
Tree topology is a LAN configuration where only one route exists between any two nodes on the network. The connection pattern resembles a tree, with all branches stemming from one root.
Tree topology is a hybrid configuration, similar to the star topology, but with nodes connected to a secondary hub, which is, in turn, linked to the central hub. In bus topology, groups of star-configured networks are connected to a linear bus backbone.
Advantages of Tree Topology
- Installation and configuration of the network are straightforward.
- Less expensive compared to mesh topology.
- Faults in the network can be easily detected and traced.
- The addition of the secondary hub allows one or more devices to be attached to the main hub.
- Supports multiple cable types like ordinary telephone cable, shielded twisted pair cable, unshielded twisted pair cable, etc.
Disadvantages of Tree Topology
- Failure in the main hub brings the whole network to a halt.
- More cabling is required compared to bus topology because each node is connected to the central hub.
Advantages of Computer Network
Computers in a networked environment offer numerous advantages compared to computers in a standalone environment. The significant benefits provided by computer networks include excellent sharing of computational resources, computational load distribution, increased reliability, cost-effectiveness, and efficient person-to-person communication.
Here are some significant advantages of using computer networks:
1. High Reliability
Computer networks provide high reliability by incorporating alternative sources of supply. For example, files can be replicated on two or three machines, allowing continued access if one machine becomes unavailable due to hardware failure. Additionally, multiple CPUs enable other machines to take over tasks in the event of a failure, albeit at reduced performance. This high reliability is crucial for applications in the military, banking, air traffic control, nuclear reactor safety, and more.
2. Communication Medium
A computer network is a powerful communication medium for widely separated users. It facilitates collaboration among individuals working on the same project, even if they are geographically distant. Changes to an online document are immediately visible to all collaborators, eliminating the need for prolonged waiting times associated with traditional communication methods.
Computer networks offer scalability, allowing for a gradual increase in system performance as the workload grows. Unlike centralized mainframes that require replacement with larger systems when complete, the client-server model permits adding new clients and servers as needed, providing flexibility and adaptability.
4. Increased Productivity
Networks enhance productivity by allowing multiple users to input data simultaneously and collaborate on shared data processing. For example, one person can handle accounts receivable while another processes profit-and-loss statements, streamlining tasks and improving overall efficiency.
5. Resource Sharing
The primary objective of a computer network is to make all programs, equipment, and data available to anyone on the network, irrespective of the physical location of the resource and the user. Besides sharing files, users can share other resources like printers. For instance, printers, utilized only a small percentage of the time, can be efficiently shared among users, eliminating the need for a dedicated printer for each computer.
6. Cost Savings
Smaller computers offer a better price/performance ratio than larger ones. Although mainframes may be roughly ten times faster than personal computers, they come at a significantly higher cost. This cost imbalance has led to the adoption of a client-server model, where individual users have personal computers connected to shared file servers. This arrangement is cost-effective and provides efficient performance.