Why we need computer networks

Computer networks help users on the network to share the resources and in communication. Can you imagine a world now without emails, online news papers, blogs, chat and the other services offered by the internet?

The following are the important benefits of a computer network.

File sharing:

 Networking of computers helps the users to share data files.

Hardware sharing:

 Users can share devices such as printers, scanners, CD-ROM drives, hard drives etc.

Application sharing: 

Applications can be shared over the network, and this allows to implement client/server applications

User communication: 

Networks allow users to communicate using e-mail, newsgroups, and video conferencing etc.

Network gaming:

 Lot of games are available, which are supports multi-users.

Network Topologies

 Network Topologies:- 

A network topology is the physical layout of computers, cables, and other components on a network.

 There are a number of different network topologies, and a network may be built using multiple topologies.

 The different types of network layouts are Bus topology, Star topology, Mesh topology, Ringtopology, Hybrid topology and Wireless topology. 

Bus Topology

Bus Topology

A bus topology consists of a main run of cable with a terminator at each end. All nodes like workstations, printers, laptops servers etc., are connected to the linear cable. The terminator is used to absorb the signal when the signal reaches the end, preventing signal bounce. When using bus topology, when a computer sends out a signal, the signaltravels the cable length in both directions from the sending computer. When the signalreaches the end of the cable length, it bounces back and returns in the direction it came from. This is known as signal bounce. Signal bounce will create problem in network, because if another signal is sent on the cable length at the same time, the two signals will collide. 

Advantages of Bus Topology

• Easy to connect a computer or peripheral to a linear bus.

• Requires less cable length than a star topology.

Disadvantages of Bus Topology

• Entire network shuts down if there is a break in the main cable.

• Terminators are required at both ends of the backbone cable.

• Difficult to identify the problem if the entire network shuts down.

• Not meant to be used as a stand-alone solution.

Star Topology

A star topology is designed with each node (like workstations, printers, laptops servers etc.,) connected directly to a central network hub/switch. Each workstation has a cable that goes from the network card to network hub or switch.

Advantages of Star Topology

• Easy to install and wire.

• No disruptions to the network then connecting or removing devices.

• Easy to detect faults and to remove parts.

Disadvantages of Star Topology

• Requires more cable length than a linear topology.

• If the hub fails, nodes attached are disabled.

• More expensive than linear bus topologies because of the cost of the devices.

Mesh, Ring and Hybrid Topologies

Mesh Topology

Mesh Topology is not commonly used these days. In Mesh topology, every network device is connected to other network devices. Mesh topology is costly because of the extra cables needed and it is very complex and difficult to manage.

Ring Topology

In a ring topology, all computers are connected via a cable that loops in a ring or circle. A ring topology is a circle that has no start and no end and terminators are not necessary in a ring topology. Signals travel in one direction on a ring while they are passed from one computer to the next, with each computer regenerating the signal so that it may travel the distance required. Main advantage of Ring topology is that the signal degeneration is low since each workstation participating in the network is responsible for regenerating the weak signal. Disadvantage is if one workstation fails, entire network will fail.

Hybrid Topology

Hybrid topology is a mixture of different topologies. Example is star-bus topology.

Network Infrastructure Devices

Hub

Hubs were the common network infrastructure devices used for LAN connectivity but switches are rapidly replacing hubs. Hubs function as the central connection point for LANs. Hubs are designed to work with Twisted pair cabling and normally use RJ45 jack to connect the devices. Network devices (Servers, Workstations, Printers, Scanners etc) are attached to the hub by individual network cables. Hubs usually come in different shapes and different numbers of ports.

When a hub receives a packet of data (an Ethernet frame) at one of its ports from a network device, it transmits (repeats) the packet to all of its ports to all of the other network devices.  If two network devices on the same network try to send packets at the same time a collision is said to occur.

 Bridges and Switchs

A bridge is a network device that operates at the Data Link layer (Layer 2) of OSI model. There are many different types of bridges and include Transparent bridges, Encapsulation bridges, Source-route bridges. Source-route bridges are for Token Ring network. Bridges allow segmenting a Local Network into multiple segments, thus reducing the network traffic. A bridge performs the segmenting function by examining the Data Link Layer (Layer 2) data packet (Ethernet Frame) and forwarding the packet to other physical segments only if necessary. Both swiches and bridges function using Data Link Layer (Layer 2) addressing system, also known as MAC addresses.

Bridge can connect only a few Networks, LANs or Hosts. A Bridge has comparatively less ports than a Switch. A Switch has usually 24 ports or 48 ports. Brides and Switches are considered to operate at Data Link Layer (Layer 2) of OSI model.

Router

A router is another network infrastructure device that directs packets through the network based on information from Network Layer (Layer 3) of OSI model. A router uses a combination of hardware and software to "route" data from its source to its destination. A router can be configured to route data packets from different network protocols, like TCP/IP, IPX/SPX, and AppleTalk.

Routers segment large networks into logical segments called subnets. The division of the network is based on the Layer 3 addressing system, like IP addresses. If the Network Layer (Layer 3) Data packet (IP Datagram) is addressed to another device on the local subnet, the packet does not cross the router and create a traffic congestion problem in another network. If data is addressed to a computer outside the subnet, the router forwards the data to the addressed network. Thus routing of network data helps conserve network bandwidth.

Common Network Cable types

Cables are commonly used to carry communication signals within LAN. There are threecommon types of cable media that can be used to connect devices to a network and they are coaxial cable, twisted-pair cable, and fiber-optic cable.

Coaxial cable

Coaxial cable looks similar to the cable used to carry TV signal. A solid-core copper wire runs down the middle of the cable. Around that solid-core copper wire is a layer of insulation, and covering that insulation is braided wire and metal foil, which shields against electromagnetic interference. A final layer of insulation covers the braided wire.

There are two types of coaxial cabling: thinnet and thicknet. Thinnet is a flexible coaxial cable about ¼ inch thick. Thinnet is used for short-distance. Thinnet connects directly to a workstation’s network adapter card using a British Naval Connector (BNC). The maximum length of thinnet is 185 meters. Thicknet coaxial is thicker cable than thinnet. Thicknet cable is about ½ inch thick and can support data transfer over longerdistances than thinnet. Thicknet has a maximum cable length of 500 meters and usually is used as a backbone to connect several smaller thinnet-based networks.

The bandwidth for coaxial cable is 10 mbps (mega bits per second).

Twisted Pair Cable

Twisted-pair cable is the most common type of cabling you can see in todays LAN networks. A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. When a wire is carrying a current, the current creates a magnetic field around the wire. This field can interfere with signals on nearby wires. To eliminate this, pairs of wires carry signals in opposite directions, so that the two magnetic fields also occur in opposite directions and cancel each other out. This process is known as cancellation. Two Types of Twisted Pairs are Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP).

Unshielded twisted-pair (UTP) cable is the most common networking media. Unshielded twisted-pair (UTP) consists of four pairs of thin, copper wires covered in color-coded plastic insulation that are twisted together. The wire pairs are then covered with a plastic outer jacket. The connector used on a UTP cable is called a Registered Jack 45 (RJ-45) connector. UTP cables are of small diameter and it doesn’t need grounding.  Since there is no shielding for UTP cabling, it relies only on the cancellation to avoid noise. 

UTP cabling has different categories. Each category of UTP cabling was designed for a specific type of communication or transfer rate. The most popular categories in use today is 5, 5e and 6, which can reach transfer rates of over 1000 Mbps (1 Gbps).

Optical Fiber Cable

Optical Fiber cables use optical fibers that carry digital data signals in the form of modulated pulses of light. An optical fiber consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, known as the cladding. There are two fibers per cable—one to transmit and one to receive. The core also can be an optical-quality clear plastic, and the cladding can be made up of gel that reflects signals back into the fiber to reduce signal loss.

There are two types of fiber optic cable: Single Mode Fibre (SMF) and Multi Mode Fibre (MMF).

1. Single Mode Fibre (SMF) uses a single ray of light to carry transmission over longdistances.

2. Multi Mode Fibre (MMF) uses multiple rays of light simultaneously with each ray of light running at a different reflection angle to carry the transmission over shortdistances

Straight-Through and Cross-Over cables

Straight-Through Cabling

CAT 5 UTP cabling usually uses only four wires when sending and receiving informationon the network. The four wires, which are used, are wires 1, 2, 3, and 6. When you configure the wire for the same pin at either end of the cable, this is known as a straight-through cable.

Cross-Over Cabling

If we use want to connect two computers together with a straight-through cable, we can see that, the transmit pins will be connected to transmit pins and receive pins will be connected to receive pins. We will not be able to connect two computers or two hubs together using straight through cables.