CS6: Introduction to Computer Networks

Computer Networks Unit Objectives To be able to know the:

• Features, advantages & disadvantages of the various types of Networks
• Features of Intranets, Extranets and Internet.

Networking hardware Networking hardware includes all computers, peripherals and Communications devices that enable two or more computers to exchange items such as data, instructions, and information with each other. Examples include: a server computer, clients/work stations, network interface card, modems, Hub/Switch, repeater, Router, etc. 1. SERVER A server is the host or central computer that manages the resources on a network. A server provides a centralized storage area for programs, data, and information. A dedicated server is a server that performs a specific task. Examples of dedicated Servers include: file server, print server, database server, and a network server. Roles of Dedicated Servers

• A file server stores and manages files on a network
• A print server manages printers and print jobs.
• A database server stores and provides access to a database
• A network server (e.g., a DNS) manages network traffic.

Requirements of a server computer

• It needs a computer with very high processing speed
• It needs large amounts of RAM
• It needs a very big storage capacity
• It needs a very fast Network interface card
• It needs network operating system such as Novell Netware, Windows NT Server or Apple Share

2. NETWORK INTERFACE CARD A network card, also called network interface card (NIC), is a device that enables the computer or device that does not have built-in networking capability to access a network.

• Examples include; adapter card, PC Card, USB network adapter, flash card e.t.c

3 MODEMS (signal converters) The modem, is a device which Modulates a digital signal from computers into an analog one to send data out over the phone line. Then for an incoming signal it Demodulates, the analog signal into a digital one. 4. HUBS and SWITCHES A hub, (also called a multi-station access unit (MAU)) is a device that provides a central point for cables in a network. Unlike the hubs, a switch does not broadcast the data to all the computers, it sends the data packets only to the destined computer. 5. REPEATER A repeater is a device that accepts a signal from a transmission medium, amplifies it, and retransmits it over the medium. As a signal travels over a long distance, it undergoes a reduction in strength, an occurrence called attenuation. 7. NETWORK BRIDGE A bridge connects two pieces of land together offering a path from one to another. A network bridge is device that connects two networks making each accessible to the other. A bridge knows all of the addresses on each side of the bridge and can send information accordingly. 8. MULTIPLEXER A multiplexer is a device that combines two or more input signals from various devices into a single stream of data and then transmits it over a single transmission medium. By combining the separate data streams into one, a multiplexer increases the efficiency of communications and reduces the need for using multiple separate transmission media.

Types of computer networks

A network can be relatively small or extensively large. The most common types of computer networks include

• Local Area Network (LAN)
• Peer-to-peer network
• Client/server network
• Personal area network (PAN)
• Wide area network (WAN)
• Metropolitan area networks (MANs)
• Etc.

Local Area Network (LAN)

A local area network (LAN) is a network that connects computers in a small geographic area such as a building like a computer laboratory, or an office. The nodes are connected to the LAN via cables. A wireless LAN (WLAN) is a LAN that does not use physical wires, but uses wireless media such as radio waves. The two kinds of LAN are peer-to-peer network and client/server network. Peer-To-Peer Network This is a type of network where each computer can share the hardware, data, or information located on any other computer on the network. Each computer stores files on its own storage devices. Each computer on the network contains both the network operating system and application software. https://youtu.be/ie-qRQIQT4I Advantages of a Peer To Peer Network (P2P)

• A peer-to-peer network is simple to setup i.e. does not require too much configuring
• It is not expensive to set up
• It does not require a dedicated server to control the network
• It is perfect for home and small business users.

Disadvantages of a Peer to Peer Network

• The system is not centralized, making administration difficult .
• Lack of security i.e. files can be accessed by any one on the network

Client/Server Network

A client/server network has one or more computers acting as a server while the other computers (i.e., clients) on the network can request services from the server. A client computer is a computer that can access the resources on a network. A server provides a centralized storage area for programs, data, and information. Most client/server networks have a network administrator who is in charge of the network. Advantages of a Client/Server Network

• All Resources are centralized and easier to access.
• Easy management and administration of the network.
• More data security since all network access is controlled through the server.
• The network is flexible, because changes and new technology can be easily included into system.
• Client /Server network is faster than P2P since data and resources are handled by a dedicated machine
• It is to Backup all data stored centrally on the server.
• Client Server network can support many computers as compared to a P2P network.

Disadvantages of a Client /Server Network

• It is expensive to set up as compared to a P2P network.
• It requires an extra computer to serve as a dedicated server.
• Maintenance – large networks will require an administrator staff to ensure efficient operation
• Dependence – When the server goes down, operations will cease across the network
• Server can get overloaded since all the processing is controlled at one point.

PERSONAL AREA NETWORK (PAN)

A personal area network (PAN) is the interconnection of computer devices within the range of an individual person, typically within a range of 10 meters.

• For example, a person traveling with a laptop, a personal digital assistant (PDA), and a portable printer could interconnect them without having to plug anything in, using some form of wireless technology. Typically, this kind of personal area network could also be interconnected without wires to the Internet or other networks.

Wide area network (WAN)

A wide Area Network (WAN) is a network that covers a large geographic area. An example of a WAN is a network that connects the district office computers of a company across the country or across several counties in the world. Computers are often connected to a WAN via public networks such as the telephone system or by dedicated lines or satellites. The Internet is the world’s largest WAN.

Metropolitan area networks (MANs)

A metropolitan area network (MAN) is a large computer network that usually spans a city or a large campus. A MAN usually interconnects two or more LANs using a high-capacity backbone technology, such as fiber-optical links or other digital media. A MAN covers a smaller geographic area than a WAN. A virtual private network (VPN) extends a private network across a public network, and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. Applications running across the VPN may therefore benefit from the functionality, security, and management of the private network. VPNs may allow employees to securely access a corporate intranet while located outside the office. They are used to securely connect geographically separated offices of an organization, creating one cohesive network. Intranet, Extranet and Internet

• Individual Computer networks such LANS and PANs can be interconnected to form extended networks.

QN: What is the difference between Intranet, Extranet and Internet?

• Intranet is used within an organization;
• Extranet is and extension of an intranet - used even out side the organization.
• Internet is global.

Intranet

• Intranet refers to a connection of private computer networks within an organization.
• An intranet has tools to facilitate communication between organization's employees or workgroups to improve the knowledge and data sharing capability.
• Many schools and non-profit groups have deployed intranets
• A simple intranet consists of an internal email system.
• More complicated intranets include Web sites and databases containing company news, forms, and personnel information.
• An example of an intranet is a school network.

Advantages of Installing a School Network

• Speed. Networks provide a very rapid method for sharing and transferring files.
• Cost. Networkable versions of many popular software programs are available at considerable savings when compared to buying individually licensed copies.
• Security. Files and programs on a network can be safe i.e. passwords can be established for specific directories to restrict access to authorized users.
• Sharing resources such as laser printers, fax machines, modems, scanners, etc. is simplified
• Electronic Mail. Electronic mail on a LAN can enable students to communicate with teachers and peers at their own school.
• Flexible Access. School networks allow students to access their files from computers throughout the school. Students can also work cooperatively through the network.

Disadvantages of Installing a School Network

• Expensive to Install. Although a network will generally save money over time, the initial costs of installation can be prohibitive.
• Requires Administrative Time. Proper maintenance of a network requires considerable time and expertise.
• Must Monitor Security Issues. Wireless networks are becoming increasingly common; however, security can be an issue with wireless networks

EXTRANET An extranet is a computer network that allows controlled access from the outside for specific business or educational purposes. Extranets are extensions to, or segments of, private intranet networks that have been built in many corporations for information sharing. Most extranets use the internet as the entry point for outsiders, a firewall configuration to limit access and a secure protocol for authenticating users Advantages of extranet

• Exchange large volumes of data using Electronic Data Interchange (EDI)
• Share product catalogs exclusively with trade partners
• Collaborate with other companies on joint development efforts
• Jointly develop and use training programs with other companies
• Provide or access services provided by one company to a group of other companies, such as an online banking application managed by one company on behalf of affiliated banks.
• Share news of common interest exclusively

Disadvantages of extranet

• Extranets can be expensive to implement and maintain within an organization (e.g., hardware, software, employee training costs)
• Security of extranets can be a concern when hosting valuable or proprietary information.

https://youtu.be/5boN5SrrlWQ NETWORK TOPOLOGIES A network topology is a description of the possible physical connections within a network. In other words, a topology is the physical arrangement of the devices in a communications network. Three commonly used network topologies are bus, ring, and star. However, Most computer networks are hybrids—combinations of these topologies. In a network topology, any network hardware component is also called a node. Bus topology A bus or linear network topology consists of a single central cable that connects all computers and devices together. The physical cable that connects the computers and other devices is known as the bus or the backbone. Merits of BUS Topology

• Easy to implement and extend (quick setup)
• Cheaper than other topologies.
• Computers and devices can be attached and detached at any point on the bus without disturbing the rest of the network.
• Failure of one device usually does not affect the rest of the bus network.
• Data, instructions, and information in a bus network can be transmitted in both directions.
• Cable faults are easily identified.
• Weight reduction due to less wires

Demerits of  BUS Topology

• If there is a problem with the cable, the entire network goes down.
• There is no central host computer to control the network.
• Only one device can transfer items at a time.
• If many computers are attached, the amount of data flowing along the cable increases, data collisions occur and the network slows down.
• Limited cable length and number of stations.
• Performance degrades as additional computers are added or on heavy traffic.(shared bandwidth)
• It is slower than the other topologies.

Ring Topology Ring network consists of a cable forming a closed ring, or loop, with all the computers and devices in a network. A ring network links all nodes together in a circular chain. The node examines any data that passes by to see if it is the addressee; if not, the data is passed on to the next node in the ring. Advantages of  Ring Topology

• Ring topology Can cover a larger distance as compared to a bus network and is commonly used in wide area networks (WAN)
• No collisions occur because data takes one direction only
• Very orderly network where every device has access to the token and the opportunity to transmit
• The speed of data transmission is faster than in a bus topology.

Demerits of a Ring Topology

• Ring Topology Network is More difficult to establish.
• If the cable fails, the whole network goes down.
• Data messages travel in only one direction from device to device around the entire ring
• If a node on a ring network fails, all nodes after the failed nodes cannot function.
• There is no central host computer to control the network.
• Moves, adds and changes of devices can affect the network

Star Topology On a star network, all of the computers and devices (nodes) on the network connect to a central hub or switch. All data that is transferred from one computer to another passes through the hub. Merits of a Star Topology

• Easy to install and maintain.
• Better performance: The star topology prevents the passing of data packets through an excessive number of nodes.
• Computers and devices can be added to or removed from the network with little or no disruption to the network.
• Reliable because each device connects directly to the hub, if one device fails, only that device is affected.

Demerits of a Star Topology

• If the hub fails, the entire network fails
• Lots of cable required so that the installation cost is expensive.
• Network size is limited by the number of connections that can be made to the hub.
• Performance for the entire network depends on the capabilities of the hub.
• Set up of the system can be very complex.

Mesh Topology This is the type of network topology in which each of the nodes of the network is connected to each of the other nodes in the network. Fully connected Mesh topology makes it possible for data to be simultaneously transmitted from any single node to all of the other nodes. Merits of Mesh Topology

• Data will always be delivered.
• All of the data that is transmitted between nodes in the network takes the shortest path between nodes.
• In the case of a failure or break in one of the links, the data takes an alternate path to the destination.

Demerits of Mesh Topology

• Mesh topology is generally too costly and complex for practical networks, and very hard to setup.
• Lots of cable required so that the installation cost is expensive.
• Network size is limited by the number of interconnections that can be made between the computers.
• It requires that the nodes of the network possess some type of logical 'routing' algorithm to determine the correct path to use at any particular time.

Tree Topology Tree network topology is also known as a the hierarchical network topology. This is because it contains different levels of hierarchy. The type of network topology in which a central 'root' node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level), Each of the second level nodes will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it. The hierarchy of the tree is symmetrical - Each node in the network having a specific fixed number, of nodes connected to it at the next lower level in the hierarchy. It usually has three layers: the core layer, the distribution layer and the Access layer.

Factors to Consider When Choosing a Topology:

1.   Budget

A rule of thumb is to never make technology procurement decisions based on price alone. There’s no denying though that you can only cut your coat according to your cloth. If a topology is unaffordable, it’s off the table no matter how perfectly suited it might be for your situation. In any case, irrespective of what your preferred topology is, there’ll almost always be a lower-priced alternative that’s nearly as effective. On pricing matters, bus and ring topologies are quite cost-effective while star, mesh, tree and hybrid topologies are expensive.

2.   Hardware Resources

Certain network topologies work best with certain hardware. And vise-versa. So before you make a decision on the topology to adopt, perform an inventory of your current hardware. You may also already have the hardware needed to implement a certain type of topology. So as opposed to buying everything from scratch, such existing resources give you a head start. For instance, you may have hardware limitations such as the length of the network cable. In that case, you’d go for a topology that requires the least amount of cable for connecting nodes. Bus and star topologies perform pretty well in this regard.

3.   Ease of Implementation

If you’ll contract a third party to install and/or maintain your network, then the complexity of the network topology you choose is perhaps a non-issue. A competent networking professional will have the education and experience needed to comprehend what each topology entails and implement it accordingly. However, if you expect to leave network implementation in the hands of novices or individuals without the requisite IT training, then the ease of the topology should be a major factor in your choice. In this case, the bus and star topologies score pretty well. The mesh, tree and hybrid, on the other hand, are complex and difficult for a layman to install or understand.

4.   Size of Network

How many devices are going to be on your network? How geographically dispersed are they? How far from the ‘center’ is the furthest device? Some topologies are inadequate or expensive when applied to large networks. A topology that works perfectly for a 5-device network may prove a disaster when applied to a 10,000-device organization. Part of the inventorying process we referred to in point 2 should include determining the total number of devices to be interconnected. Armed with this information, you can choose the topology that would best serve the purpose. The tree topology works well with large networks. The bus topology is best suited for small organizations.

5.   Reliability

When it comes to reliability, network topologies aren’t created equal. If you are looking for high reliability because you are in an industry where even brief downtime and delays are frowned upon (e.g. banking), then network reliability is a fundamental consideration. Choose the topology that delivers the highest reliability. Ring topology performs pretty well under heavy loads but is prone to a single point of failure. Star topology doesn’t depend on any node but the network will collapse if the hub fails. Mesh and hybrid topologies score highest on the reliability front.

6.   Future Expansion

If you expect your organization to grow in size in the medium to long-term, opt for a network topology that’s readily scalable. Identify the topology that’s easy to add new nodes to, without negatively affecting network performance or the user experience of other devices on the network. The tree topology is perhaps the most compatible with future expansion requirements as it’s fairly easy to extend or shrink the network. The bus topology is also easy to expand but only to a certain extent which is why it would only work for small networks. https://youtu.be/ambOI3n9lX0 Choosing a network topology is one of the most important decisions you’ll make for your technology infrastructure and will have far-reaching ramifications over the long-term. A wrong choice can prove to be an expensive mistake. It’s a decision that requires careful thought in order to get it right from the start.

NETWORK PROTOCOLS

• PROTOCOLS are rules governing the transmissions of that on a network known as TCP/IP

(Transmission Control Protocol/Internet Protocol) WHAT BRINGS COMMUNICATION IN A NETWORK? NB: 1,2&3 layers are physical and deal in transferring data from one device to another e.g. how one configures a switch, router TRANSPORT LAYER It links the two sub groups i.e. the upper (software) and the lower (hardware). Carry voice and data and It uses protocols i.e. UDP and TCP.

• UDP – USER DATAGRAM PROTOCOL
• RAP – RSOLUTION ADDRESS PROCOTOL
• RARP-RESERVSE ADDRESS RESOLUTION PROTOCOL
• ICMP- INTERNET CONTROL MESSAGE PROTOCOL
• TFT – TRIVIAL FILE TRANSFER PROTOCOL
• NFS -  NETWORK FILE SYSTEM

APPLICATION LAYER RULES and Regulation FOR APPLICATION LAYER

• File transfer e.g. FTP
• Send and Receive emails e.g. SMTP
• Remote login e.g. (TELNET)

In the lower 3 are physical devices. PHYSICAL LAYER DEVICES (1)

• Computer
• Server
• Physical and wireless media

DATA LINK LAYER DEVICES (2)

• Switches
• Bridge

NETWORK LAYER DEVICES (3)

• Router
• Switches (higher series - 4000-5000) very tall

TRANSPORT LAYER DEVICES Fibre optic cables that are normally used in backbones. What is an IP address and why is it important? IP stands for Internet Protocol. An IP address is a series of binary numbers that provide information about the network and the host (the computer or other device). These numbers are typically written as four numbers separated by dots in the older, IP Version 4 (IPv4) address numbering that is most common. In other words, An IP address is a set of 4 numbers assigned to each device on a computer network. When we apply this definition for the internet, the IP address can be considered a numerical representation of a website address. For example, the domain google.com would go to the IP address 73.14.213.99. IP address contains 4 octets, each octet can be represented by number 0-255 and separated by periods. For example, 192.168.100.2 is an IP address. Why is it important?

• It is used as an interface identification for a network of machines
• It also serves to provide a location of that machine, much like a physical address for a home or business.
• Because an IP address is a unique identifier, it allows computers to send and receive information to and from specific computers in a given network.
• IP address is very important in TCP/IP networking. It’s the address that recognized and understood by computers and networking devices, so that they can communicate with each other.

https://youtu.be/zFyl_zaRYuQ