Chapter -2
Data Communication & Computer Network
Definition of communication system: A system that exchange data between persons and equipment’s or device is called communication system. A communication system is made up of device that employee one of two communication method (Wireless or wired), different types of equipment (portable radios, mobile radios, base or fixed station and repeaters) and various accessories (speaker, microphone, battery eliminators) to meet the user needs.
Another definition of communication system: A system that exchange data between persons and equipment or device is called communication system
Data communication system: Data communication system is the function of transporting data from one point another.
Elements of data communication system: A data communication system has five fundamental elements, such as
i)Data source: Source is the computer where available data are obtained to send; examples are telephone and computers.
ii)Transmitter: The transmitter is the device that transmits the source data in the communication medium; example modem.
iii)Medium: The transmission medium is the physical path which a message travels from transmitter to receiver . It could be twisted pair wire, coaxial cable , fiber optic cable or radio waves.
iv)Receiver: It is the modem located at destination side that converts received data into computer readable form or in digital form.
v)Destination: The computer that finally store the incoming message or data is known as destination.
Fig: General Block diagram of Data communication Data transmission Speed
A term used to describe the data-handling capacity of a communications system is bandwidth. Bandwidth is the range of frequencies that is available for the transmission of data. The wider the bandwidth of a communications system, the more data it can transmit in a given period of time. Depending on the transmission speed, communication channels are grouped into three basic components-
A. Narrow Band: Narrow band channels range is speed from 45 to 300 bps. They are used for handling low data volumes and are fit for low-speed devices.
Example- Telegraph and low speed terminals.
B. Voice Band: Voice band channels can transmit data at speeds up to 9600 bps. They are so called because their major application is for ordinary telephone voice communication.
Example: Data transmission from card reader to CPU or from CPU to line printer.
C. Broad Band: Broad band or wide band channels are used when large volumes of data is to be transmitted at high speed. They system provide data transmission rate of 1 million bps or more.
Example: Use Optical fiber for data transmission.
2.3 Methods of data transmission: Data transmission method means how data is being transmitted from source to destination. Data transmission on a line is normally carried out in three different modes
i)Asynchronous Transmission:
ii) Synchronous Transmission:
iii) Isochronous Transmission
i)Asynchronous Transmission: Asynchronous transmission is often referred to as start-stop transmission because of its nature, that is, the sender can send a character at any time convenient and the receiver will accept it
Characteristics of Asynchronous transmission:
1. The sender can transmit the data any time and the receiver can receive the data at any time.
2. There is no necessity of primary storage device in transmission station.
3. Interval between characters may be uneven.
4. Less efficient and low speed.
5. A start bit and a stop bit is to send with each character.
6. The medium remains idle when there is no transmission of data.
Advantage of Asynchronous transmission:
The advantages of Asynchronous transmission is given below-
1. It is suitable for few amount of data transmission.
2. It does not requires any local storage at the terminal.
3. The cost is comparatively less then synchronous transmission
4. The sender can transmit the data any time and the receiver can receive the data at any time.
Disadvantage of Asynchronous transmission:
The disadvantage of Asynchronous transmission is given below-
1.The efficiency and speed is less then synchronous transmission.
2.It is does not use when there is no data transmission.
3. The cost of transmission is increased for microwave or satellite wave if transmission is closed.
4. Large overhead for start and stop bits.
5.Error handling usually more complex.
6. It does not support full-duplex data transmission.
ii) Synchronous Transmission: Synchronous transmission means that the bit stream is combined into longer “frames” which contain multiple bytes. Each byte is introduced onto the transmission link without a gap between it and the next one .Each frame contains 80 and 132 bytes data.
Characteristics of Synchronous Transmission:
1.There are no gaps between character being transmitted.
2.Efficiency is more.
3.Transmission speed is more.
4.Needs primary storage at sender station.
Advantage of Synchronous Transmission:
The advantages of Synchronous transmission is given below-
1.Lower overhead and thus, greater throughput.
2.Higher speeds then Asynchronous.
3.Error handling usually easy.
4.Support full-duplex data transmission.
5.It is suitable for large amount data transmission.
6.It is suitable for long distance data transmission. Disadvantage of Synchronous Transmission:
The disadvantages of Synchronous transmission is given below-
1.Slightly more complex.
2.Hardware is more expensive.
3.Service must be up and early
4.For correct operation the receiver must start to sample the line at the correct instant.
5.It needs a primary storage device in transmitter station.
Fig: Asynchronous and Synchronous transmission method
iii) Isochronous Transmission: An isochronous data transfer systems combines the feature of an asynchronous and synchronous data transfer system. An isochronous data transfer system sends blocks of data asynchronously, in other words the data stream can be transferred at random interval.
Each transmission begins with a start packet. Once the start packet is transmitted, the data must be delivered with a guaranteed bandwidth. Isochronous data transfer is commonly used for where data must be delivered within certain time constraints, like streaming video.
Transmission Modes in Computer Networks Transmission mode means transferring of data between two devices. It is also called communication mode. These modes direct the direction of flow of information. There are three types of transmission mode. They are : Simplex Mode Half duplex Mode Full duplex Mode
SIMPLEX Mode In this type of transmission mode data can be sent only through one direction i.e. communication is unidirectional. We cannot send a message back to the sender. Unidirectional communication is done in Simplex Systems. Examples of simplex Mode is loudspeaker, television broadcasting, television and remote, keyboard and monitor etc.
HALF DUPLEX Mode In half duplex system we can send data in both directions but it is done one at a time that is when the sender is sending the data then at that time we can’t send the sender our message. The data is sent in one direction. Example of half duplex is a walkie- talkie in which message is sent one at a time and messages are sent in both the directions.
FULL DUPLEX Mode In full duplex system we can send data in both directions as it is bidirectional. Data can be sent in both directions simultaneously. We can send as well as we receive the data. Example of Full Duplex is a Telephone Network in which there is communication between two persons by a telephone line, through which both can talk and listen at the same time. In full duplex system there can be two lines one for sending the data and the other for receiving data.
According to selection of path in network routing, data transmission mode can be divided into many ways:
Unicast
Broadcast
Multicast
Anycast
Geocast
Medium Of Data Communication
There are two categories of transmission media
Bounded/ Guided media
Unbounded/ Unguided
Bounded/Guided Transmission Media It is the transmission media in which signals are confined to a specific path using wire or cable. The types of Bounded/ Guided are discussed below.
Twisted Pair Cable This cable is the most commonly used and is cheaper than others. It is lightweight, cheap, can be installed easily, and they support many different types of network. Some important points : Its frequency range is 0 to 3.5 kHz. Typical attenuation is 0.2 dB/Km @ 1kHz. Typical delay is 50 μs/km. Repeater spacing is 2km. Twisted Pair is of two types : Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP)
Unshielded Twisted Pair Cable It is the most common type of telecommunication when compared with Shielded Twisted Pair Cable which consists of two conductors usually copper, each with its own color plastic insulator. Identification is the reason behind colored plastic insulation. UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4 pair cable useRJ-45 connector. Advantages : Installation is easy Flexible Cheap It has high speed capacity, 100 meter limit Higher grades of UTP are used in LAN technologies like Ethernet. It consists of two insulating copper wires (1mm thick). The wires are twisted together in a helical form to reduce electrical interference from similar pair. Disadvantages : Bandwidth is low when compared with Coaxial Cable Provides less protection from interference.
Shielded Twisted Pair Cable This cable has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Electromagnetic noise penetration is prevented by metal casing. Shielding also eliminates crosstalk (explained in KEY TERMS Chapter). It has same attenuation as unshielded twisted pair. It is faster the unshielded and coaxial cable. It is more expensive than coaxial and unshielded twisted pair. Advantages : Easy to install Performance is adequate Can be used for Analog or Digital transmission Increases the signalling rate Higher capacity than unshielded twisted pair Eliminates crosstalk Disadvantages : Difficult to manufacture Heavy
Coaxial Cable Coaxial is called by this name because it contains two conductors that are parallel to each other. Copper is used in this as centre conductor which can be a solid wire or a standard one. It is surrounded by PVC installation, a sheath which is encased in an outer conductor of metal foil, barid or both. Outer metallic wrapping is used as a shield against noise and as the second conductor which completes the circuit. The outer conductor is also encased in an insulating sheath. The outermost part is the plastic cover which protects the whole cable.
Advantages : Bandwidth is high Used in long distance telephone lines. Transmits digital signals at a very high rate of 10Mbps. Much higher noise immunity Data transmission without distortion.
The can span to longer distance at higher speeds as they have better shielding when compared to twisted pair cable Disadvantages : Single cable failure can fail the entire network. Difficult to install and expensive when compared with twisted pair. If the shield is imperfect, it can lead to grounded loop.
Fiber Optic Cable These are similar to coaxial cable. It uses electric signals to transmit data. At the centre is the glass core through which light propagates. In multimode fibres, the core is 50microns, and In single mode fibres, the thickness is 8 to 10 microns. The core in fiber optic cable is surrounded by glass cladding with lower index of refraction as compared to core to keep all the light in core. This is covered with a thin plastic jacket to protect the cladding. The fibers are grouped together in bundles protected by an outer shield. Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second) Advantages :
Provides high quality transmission of signals at very high speed. These are not affected by electromagnetic interference, so noise and distortion is very less. Used for both analog and digital signals. Disadvantages : It is expensive Difficult to install. Maintenance is expensive and difficult. Do not allow complete routing of light signals. UnBounded/UnGuided Transmission Media Unguided or wireless media sends the data through air (or water), which is available to anyone who has a device capable of receiving them. Types of unguided/ unbounded media are discussed below : Radio Transmission MicroWave Transmission
Radio Transmission Its frequency is between 10 kHz to 1GHz. It is simple to install and has high attenuation. These waves are used for multicast communications. Types of Propogation Radio Transmission utilizes different types of propogation : Troposphere : The lowest portion of earth’s atmosphere extending outward approximately 30 miles from the earth’s surface. Clouds, jet planes, wind is found here.
Ionosphere : The layer of the atmosphere above troposphere, but below space. Contains electrically charged particles.
Microwave Transmission It travels at high frequency than the radio waves. It requires the sender to be inside of the receiver. It operates in a system with a low gigahertz range. It is mostly used for unicast communication. There are 2 types of Microwave Transmission : 1. Terrestrial Microwave 2. Satellite Microwave Advantages of Microwave Transmission Used for long distance telephone communication Carries 1000’s of voice channels at the same time Disadvantages of Microwave Transmission It is Very costly
Terrestrial Microwave For increasing the distance served by terrestrial microwave, repeaters can be installed with each antenna .The signal received by an antenna can be converted into transmittable form and relayed to next antenna as shown in below figure. It is an example of telephone systems all over the world
Satellite Microwave This is a microwave relay station which is placed in outer space. The satellites are launched either by rockets or space shuttles carry them. These are positioned 3600KM above the equator with an orbit speed that exactly matches the rotation speed of the earth. As the satellite is positioned in a geo-synchronous orbit, it is stationery relative to earth and always stays over the same point on the ground. This is usually done to allow ground stations to aim antenna at a fixed point in the sky. Features of Satellite Microwave :
Bandwidth capacity depends on the frequency used. Satellite microwave deployment for orbiting satellite is difficult. Advantages of Satellite Microwave : Transmitting station can receive back its own transmission and check whether the satellite has transmitted information correctly. A single microwave relay station which is visible from any point. Disadvantages of Satellite Microwave : Satellite manufacturing cost is very high Cost of launching satellite is very expensive Transmission highly depends on whether conditions, it can go down in bad weather Types of Communication Networks
Local Area Network (LAN) It is also called LAN and designed for small physical areas such as an office, group of buildings or a factory. LANs are used widely as it is easy to design and to troubleshoot.
Personal computers and workstations are connected to each other through LANs. We can use different types of topologies through LAN, these are Star, Ring, Bus, Tree etc. LAN can be a simple network like connecting two computers, to share files and network among each other while it can also be as complex as interconnecting an entire building. LAN networks are also widely used to share resources like printers, shared hard-drive etc. Applications of LAN One of the computer in a network can become a server serving all the remaining computers called clients. Software can be stored on the server and it can be used by the remaining clients. Connecting Locally all the workstations in a building to let them communicate with each other locally without any internet access. Sharing common resources like printers etc are some common applications of LAN.
Metropolitan Area Network (MAN) It is basically a bigger version of LAN. It is also called MAN and uses the similar technology as LAN. It is designed to extend over the entire city. It can be means to
connecting a number of LANs into a larger network or it can be a single cable. It is mainly hold and operated by single private company or a public company.
Wide Area Network (WAN) It is also called WAN. WAN can be private or it can be public leased network. It is used for the network that covers large distance such as cover states of a country. It is not easy to design and maintain. Communication medium used by WAN are PSTN or Satellite links. WAN operates on low data rates.
Wireless Network It is the fastest growing segment of computer. They are becoming very important in our daily life because wind connections are not possible in cars or aeroplane. We can access Internet at any place avoiding wire related troubles.. These can be used also when the telephone systems gets destroyed due to some calamity/disaster. WANs are really important now-a-days.
Types of Network Topology
Network Topology is the schematic description of a network arrangement, connecting various nodes(sender and receiver) through lines of connection.
BUS Topology
Bus topology is a network type in which every computer and network device is connected to single cable. When it has exactly two endpoints, then it is called Linear Bus topology.
Features of Bus Topology
1. It transmits data only in one direction.
2. Every device is connected to a single cable Advantages of Bus Topology
1. It is cost effective.
2. Cable required is least compared to other network topology.
3. Used in small networks.
4. It is easy to understand.
5. Easy to expand joining two cables together. Disadvantages of Bus Topology
1. Cables fails then whole network fails.
2. If network traffic is heavy or nodes are more the performance of the network decreases.
3. Cable has a limited length.
4. It is slower than the ring topology.
RING Topology
It is called ring topology because it forms a ring as each computer is connected to another computer, with the last one connected to the first. Exactly two neighbours for each device.
Features of Ring Topology
1. A number of repeaters are used for Ring topology with large number of nodes, because if someone wants to send some data to the last node in the ring topology with 100 nodes, then the data will have to pass through 99 nodes to reach the 100th node. Hence to prevent data loss repeaters are used in the network.
2. The transmission is unidirectional, but it can be made bidirectional by having 2 connections between each Network Node, it is called Dual Ring Topology.
3. In Dual Ring Topology, two ring networks are formed, and data flow is in opposite direction in them. Also, if one ring fails, the second ring can act as a backup, to keep the network up.
4. Data is transferred in a sequential manner that is bit by bit. Data transmitted, has to pass through each node of the network, till the destination node. Advantages of Ring Topology
1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having tokens can transmit data.
2. Cheap to install and expand Disadvantages of Ring Topology
1. Troubleshooting is difficult in ring topology.
2. Adding or deleting the computers disturbs the network activity.
3. Failure of one computer disturbs the whole network.
STAR Topology
In this type of topology all the computers are connected to a single hub through a cable. This hub is the central node and all others nodes are connected to the central node.
Features of Star Topology
1. Every node has its own dedicated connection to the hub.
2. Hub acts as a repeater for data flow.
3. Can be used with twisted pair, Optical Fiber or coaxial cable. Advantages of Star Topology
1. Fast performance with few nodes and low network traffic.
2. Hub can be upgraded easily.
3. Easy to troubleshoot.
4. Easy to setup and modify.
5. Only that node is affected which has failed, rest of the nodes can work smoothly. Disadvantages of Star Topology
1. Cost of installation is high.
2. Expensive to use.
3. If the hub fails then the whole network is stopped because all the nodes depend on the hub.
4. Performance is based on the hub that is it depends on its capacity
MESH Topology
It is a point-to-point connection to other nodes or devices. All the network nodes are connected to each other. Mesh has n(n-2)/2 physical channels to link n devices.
There are two techniques to transmit data over the Mesh topology, they are :
1. Routing
2. Flooding
Types of Mesh Topology
1. Partial Mesh Topology : In this topology some of the systems are connected in the same fashion as mesh topology but some devices are only connected to two or three devices.
2. Full Mesh Topology : Each and every nodes or devices are connected to each other. Features of Mesh Topology
1. Fully connected.
2. Robust.
3. Not flexible. Advantages of Mesh Topology
1. Each connection can carry its own data load.
2. It is robust.
3. Fault is diagnosed easily.
4. Provides security and privacy. Disadvantages of Mesh Topology
1. Installation and configuration is difficult.
2. Cabling cost is more.
3. Bulk wiring is required.
TREE Topology
It has a root node and all other nodes are connected to it forming a hierarchy. It is also called hierarchical topology. It should at least have three levels to the hierarchy.
Features of Tree Topology
1. Ideal if workstations are located in groups.
2. Used in Wide Area Network. Advantages of Tree Topology
1. Extension of bus and star topologies.
2. Expansion of nodes is possible and easy.
3. Easily managed and maintained.
4. Error detection is easily done.
Disadvantages of Tree Topology
1. Heavily cabled.
2. Costly.
3. If more nodes are added maintenance is difficult.
4. Central hub fails, network fails.
HYBRID Topology
It is two different types of topologies which is a mixture of two or more topologies. For example if in an office in one department ring topology is used and in another star topology is used, connecting these topologies will result in Hybrid Topology (ring topology and star topology).
Features of Hybrid Topology
1. It is a combination of two or topologies
2. Inherits the advantages and disadvantages of the topologies included
Advantages of Hybrid Topology
1. Reliable as Error detecting and trouble shooting is easy.
2. Effective.
3. Scalable as size can be increased easily.
4. Flexible. Disadvantages of Hybrid Topology
1. Complex in design.
2. Costly.
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