What is Unguided Media. ?
Unguided media is used for transmitting the signal without any physical media. It transports electromagnetic waves and is often called wireless communication. Signals are broadcast through air and received by all who have devices to receive them. It can be categorized as follows:
a) Radio waves
b) Micro waves
c) Infrared
a) Radio Waves:- Electromagnetic waves ranging in frequencies between 3 Kilo-Hertz and 1 Giga-Hertz are normally called radio waves. Radio waves are easy to generate and can travel long distances and can penetrate buildings easily, therefore widely used for communication. These are omni-directional which implies that these travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically. Radio signals have been used for a long time to transmit analog information. They are particularly attractive for long distance communication over difficult terrain or across the oceans, where the cost of installing cables can be too prohibitive. An increasingly-popular form of radio is cellular radio, which is currently being used by carriers for providing mobile telephone networks. These operate in the VHF (Very High Frequency) band and subdivide their coverage area into conceptual cells, where each cell represents a limited area which is served by a low-power transmitter and receiver station. As the mobile user moves from one cell area to another, its communication is handed over from one station to another. Radio waves transmitted by one antenna are susceptible to interference by another antenna due to its Omni-directional property. Radio waves can be received both inside and outside the building. Radio waves are very useful in multicasting and hence used in AM and FM radios, cordless phones and paging. You may be wondering about the term multicasting. If the communication is between single source and destination then it is called unicast; on the other hand, if one source is transmitting signal and any destination that is in the range may be able to reach it then it is called broadcast. Multicast is when a source transmits a signal for some specific group of destinations which may be more than one. Bluetooth: Bluetooth is a very popular application of short wave length radio transmission in the frequency band of 2400 to 2480 MHz. It is a proprietary wireless technology standard used for exchanging data over short distances in mobile phones and other related devices. It allows wireless devices to be connected to wireless host which may be a computer over short distances. You may have it for transferring data between a mobile phone and a computer provided both have Bluetooth technology.
b) Microwaves:- Electromagnetic waves ranging from 1 to 300 Gigahertz are called microwaves. Microwaves are unidirectional that is the sending and receiving antennas need to be aligned. Microwave is by far the most widely used form of radio transmission. It operates in the GHz range with data rates in order of hundreds 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 as shown in Figure 1.5. The signals transmitted by earth stations are received, amplified, and retransmitted to other earth stations by the satellite. Like other microwave systems, the bandwidth is subdivided into channels of 10s of MHz each, providing data rates in order of 100s of mbps. Because of their high bandwidths, satellites are capable of supporting an enormous number and variety of channels, including TV, telephone, and data. The satellite itself, however, is a major investment and typically has a limited lifetime (at most a few decades).
Unidirectional property of microwave helps in avoiding interference by a pair of aligned antenna to another. High frequency micro waves cannot be received inside the building.
c) Infrared:- Infrared signals range between 300 Giga-Hertz to 400 Tera-Hertz. These can be used for short range communication. High range infrared rays cannot be used for long range communication as it cannot penetrate walls. This also helps in avoiding interference. Infrared signals are generated and received using optical transceivers. Infrared systems represent a cheap alternative to most other methods, because there is no cabling involved and the necessary equipment is relatively cheap. Data rates similar to those of twisted pairs are easily possible. However, applications are limited because of distance limitations (of about one kilometer). One recent use of infra-red has been for interfacing hand-held and portable computing devices to Local Area Networks as shown in Figure. It cannot be used outside building as rays of sun contain infrared which leads to interference in communication. Infrared having wide bandwidth can be used to transmit digital data with a very high data rate. Infrared signals can be used for communication between keyboards, mouse and printers.
Check Your Progress 1
1. What is the need of computer networks?
Computer networks are manly used for the purpose of resource sharing which helps in reducing organizational costs. Networks are highly reliable, scalable and very powerful communication system.
2. In the context of communication system, what does a MODEM do?
MODEM is an encoder as well as decoder it converter digital signal to analog at the source and analog signal back to digital at the destination.
3. How can you improve the performance of twisted pair cables?
The performance of the twisted pair can be improved by adding a metallic shield around the wires. Shielded wires are much more resistant to thermal noise and crosstalk effects.
4. Describe the principal of optical fiber and its advantages and disadvantages?
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. The density of core and cladding must differ sufficiently to reflect the beam of light instead of refracting. The core is used for guiding a light beam, whereas the cladding (which has a different refractive index) acts as a reflector to prevent the light from escaping from the core. Optical Fiber has high bandwidth and does not suffer from noise. However, it is costly and requires experts to do the connections.
WHAT IS NETWORK TOPOLOGIES .?
Network Topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network interconnection between the nodes. It also determines the strategy for physically expanding the network, in future. Topologies can be physical or logical. Physical Topology means the physical design of a network including the devices, location and cable installation. Logical Topology refers to the fact that how data actually transfers in a network as opposed to its design. There are different types of the topologies like bus, ring, tree, mesh etc. However, we will discuss only the first three to introduce you to the concepts.
Bus Topology Bus topology is a single common communication to which all the computers are connected. It has a single length of cable with a terminator at each end as shown in the Figure. It is a passive topology which means only one computer at a time can send a message. Hence, the number of computers attached to a bus network can significantly affect the speed of the network. A computer must wait until the bus is free before it can transmit. Each node is connected to others nodes. The network operating system keeps track of a unique address of each node and manages the flow of data between machines.
The bus topology is the simplest and most widely used with local area network design. The computers on the bus keep on listening. When they hear data that belongs to them, they receive. When one device on the network wants to send a broadcast message to another device on the network, it first makes sure no one else on the bus is transmitting, and then it sends information out on the media. All other devices on the network see it, but only the intended recipient accepts and processes it. This is accomplished by using data frames which contain source and destination addresses.
Advantages
a) It is simple, reliable, and easy to be used in a small sized local area network.
b) It requires least amount of cable to connect computers together and is therefore less expensive than other cabling arrangements.
c) It is easy to implement and extend using connectors.
d) If one computer on the bus fails, it does not affect the rest of the traffic on the bus.
Disadvantages:-
a) In this topology, no two computers can transmit data at the same time.
b) It does not cope well with heavy load which can slow down a bus considerably.
c) Performance degrades as additional computers are added.
d) Terminators are required at both ends of the cable.
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