INTRODUCTION TO COMMUNICATION MEDIA AND COMMUNICATION DEVICES

A computer network is the infrastructure that allows two or more computers (called hosts) to communicate with each other. The network achieves this through two ways:

(i) by providing a set of rules for communication, called protocols, which should be observed by all participating hosts 

(ii) through communication devices which are the parts of a computer of network. 

Here we will discuss about about various types of network devices like gateway, bridges routers, hubs, switches, gateway and routers. We will also describe about different types of communication channels (media) and their advantages, disadvantages and application.

We know about various kinds of communication devices like network interface cards, hubs, bridges, repeaters switches and gateways etc. We also know about their application at different levels of networking. Each of these devices let us connect one or more computers to other computers, networked devices, or to other networks. Each has two or more connectors called ports into which you plug in the cables to make the connection.

The network interface card (NIC) provides the physical connection between the network and the computer. Network interface cards are a major factor in determining the speed and performance of a network. Ethernet cards contain connections for either coaxial or twisted pair cables. A Repeater enables signals to travel longer distances over a network. A repeater regenerates the received signals and then retransmits the regenerated (or conditioned) signals on other segments on the network. A bridge can join segments or LANs. Bridge Interconnects LAN segments at the network interface layer level and forwards data between them.

A router translates information from one network to another; it is similar to an  intelligent bridge. Routers select the best path to route a message, based on the destination address and origin. While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network. A hub is the simplest of these devices. Any data packet coming from one port is sent to all other ports. It is then up to the receiving computer to decide if the packet is for it. A switch does essentially what a hub does but more efficiently. By paying attention to the traffic that comes across it, it can "learn" where particular addresses are. A switch steps up on a bridge in that it has multiple ports.

 One should implement a switch when you have a network with 20 or more users that have bogged down the network by excess traffic. It splits the network into two or more segments with devices that normally talk with each other.

Gateway Interconnects networks at higher layers than bridges or routers. A gateway usually supports address mapping from one network to another, and may also provide transformation of the data between the environments to support end to end application connectivity. Gateways make communication possible between different architectures and environments.

In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet. A computer running Microsoft Windows however describes this standard networking feature as Internet Connection Sharing; which will act as a gateway, offering a connection between the Internet and an internal network.

We know about various kinds of communication media which includes guided and unguided media. Guided media are those that provide a conduit between two devices. Guided media includes twistedpair cable, Coaxial cable and Fiber optic cable. A twisted pair cable consists of two conductors which are normally made of copper. Each conductor has its own plastic insulation. These cables are twisted together. A twisted pair has a bandwidth to distance ratio of about 1 MHz per kilometer.

Coaxial cable has a central core conductor of a solid or stranded wire enclosed in insulating sheath. This sheath is encased in an outer conductor of metal braid and the whole is protected by a plastic cover. Coaxial cables can carry signals of higher frequency as compared to twisted pair cable. Coaxial cables are extensively used in LANs and long distance telephone trunk lines. Coaxial cables are used in cable TV networks and Ethernet LANs due to higher band width and data rate.

An optical fiber consists of two concentric cylinders: an inner core surrounded by a cladding. Both the core and the cladding are made of transparent plastic or glass material. Optical fiber use reflections to guide light through a channel. Optical fibers can provide bandwidth to distance ratios in order of 100s of MHz per kilometer. Like other cables, hundreds of optical fibers are usually housed within one cable. Cable TV companies use a combination of optical fiber and coaxial cable. Advantage of optical fiber is that it offers higher bandwidth and has less signal attenuation. Disadvantage of optical fiber is that it is unidirectional and costly.

For transmitting the signal without any physical media, unguided media is used. Unguided media transports electromagnetic waves without using a physical conductor and is often called wireless communication. It includes radio waves, microwaves and infrared. Electromagnetic waves ranging in frequencies between 3 kilohertz and 1 gigahertz are normally called radio waves. They are Omni directional that is an antenna transmits radio waves in all the directions. A minimum radio system consists of a transmitter and a receiver. Radio waves are very useful in multicasting and hence used in AM and FM radios, cordless phones and paging.

Electromagnetic waves ranging from 1 to 300 gigahertz are called micro waves. Micro waves are unidirectional that is the sending and receiving antennas need to be aligned. It operates in the GHz range with data rates in order of 100s of mbps per channel. Telecommunication carriers and TV stations are the primary users of microwave transmission. An important form of microwave system is a satellite system, which is essentially a microwave system plus a large repeater in the sky. High frequency micro waves cannot be received inside the building.

Infrared signals range between 300 gigahertz to 400 terahertz. These can be used for short range communication. High range infrared rays cannot be used for long range communication as it cannot penetrate walls. Infrared systems represent a cheap alternative to most other methods, because there is no cabling involved and the necessary equipment is relatively cheap. Infrared having wide band width can be used to transmit digital data with a very high data rate.