Data Transmission:

Data transmission is the movement of data from one place to another using some form of representation appropriate to the transmission medium within a network, Data Transmission system caninclude electrical signals carried along a conductor, optical signalscarried along an optical fiber and electromagnetic waves, e.g. radio,or infrared signals transmitted through space.

Internal Data Transmission:
Internal Data Transmission refers to the data within a computer,between its different operational units e.g., between CPU andmemory. This takes place through internal data paths known asBuses.
External Data Transmission:
External Data Transmission refers to the transfer of data toeither local equipment (e.g. printers) or remote computers.
Here are some of the types of data transmission.

Analog Transmission: Analog Transmission has dominated all communication for thelast few decades. In analog transmission, signals are transmitted byvarying one of the physical characteristics continuously as a functionof time.Normally, continuously varying voltage is used for such atransmission (e.g. telephone line).Analog Transmission is generally used for the transmission ofvoice signals or TV signals, In terms of voltage, voice can be representedas follows

Digital Transmission:
In digital (binary) transmission the strings of 0,s and 1's aretransmitted. These bit streams consist of data, which might consist ofcharacters or numbers digitally, e.g., equivalent of 25 can be representedas 11001. This binary bit string can then be transmitted.
Communication Channel:
• Narrow bands handle low data volume. Data transmission ratesare from 45 to 300 baud. The low speed devices might usenarrow band communication.
• Voice bands handles moderate data transmission volumebetween 300 and 9600 baud. They are used for applicationranging from operating CRT to running a line printer. Theremajor application is for telephone voice communication, hencethe term voice band.
• Broadband handles very large volume of data. These systemsprovide data transmission rates of 1 million bytes or more.High-speed data analysis and satellite communication areexamples of broadband communication system.
Simplex, Half-duplex, Communication:
The direction in which information can flow over a transmissionpath is determined by the properties of both the transmitting and thereceiving devices. There are three basic options.
In Simplex mode, the communication channel is used in onedirection. The receiver receives the signals from the transmittingdevice. A typical use is to gather data from a monitoring device at aregular interval. The simplex mode is rarely used for data communication
In Half-duplex mode, the communication channel is used inboth directions, but only in one direction at a time. This requires thereceiving and transmitting devices to switch between sent andreceive modes after each transmission. The analogous example ofthis mode is the old wireless system (walkie-talkie), which is used ineither transmit mode or receive mode.
Parallel Transmission:
In parallel data transmission, there are multiple parallel linesconnecting the transmitting and receiving units.Each wire carries a bit of information. Normally, one characteris transferred in one go.
Serial Transmission:
In serial data transmission, each bit is sent sequentially oneafter another and it requires only one pair of wire conductors forconnecting the receiving and transmitting units.Serial transmission is slower than parallel transmission, whichis used primarily for transferring data between devices at the samesite; most common example is Personal Computer to Printer. Communicationb between computers is almost always serial.
Communication protocols/ Standard:
There are several manufacturers of computer hardware andsoftware across the, globe. For successful data communication theseproducts should be compatibles with each other or they should compatiblewith each other or they should conform to certain set of rulesso that any one can use them. These sets of rules are known ascommunication protocols or communication standards.
In other words, protocols are technical customs or guides thatgovern the exchange of signal transmission and reception betweenequipment.
Some of the functions that communication protocol regulates are:
•Control of information transfer
• Structure and formats of data
• Error recoveries• Re-transmission control
• Interface management-only communication devices using sameprotocols can communicate with each other.
• To make sure that the bits (or characters) are transmittedthrough the channel or line, we need a link level protocolbetween two computers.
Synchronous transmission:
The mode of transmission is the way in which coded charactersare assembled for the process of transmission and permits thereceiving devices to” identify where the coding for each characterbegins and ends within the torrent of bits. When two computerscommunicate, they must have a way to synchronize the flow of dataso that the receiving computer can read at the same speed at whichthe sending computer transmits.
Asynchronous Transmission:
In asynchronous transmission, each character is transmittedseparately, that is, one character at a time. Each character beginswith a start bit, which tells the receiving device where the charactercoding begins and ends with a stop bit, which tells the receivingdevice where the character coding ends. Then, the next character issent, with start and stops bits. The start and stop bits and the intervalof time between consecutive characters allow the receiving andsending computers to synchronize the transmission. The parity bit isused for error checking while transmission of data.
Wide Area Network:
A WAN spans a large geographic area, such as a state,province or country.WANs often connect multiple smaller networks, such as localarea networks (LANs) or metro area networks (MANs)
The world's most popular WAN is the Internet. Some segmentsof the Internet, like VPN-based extranets, are also WANs inthemselves. Finally, many WANs are corporate or research networksthat utilize leased lines.
Subnet is a logical grouping of connected network devices.Nodes on a subnet tend to be located in close physical proximity toeach other on a LAN.
Network designers employ subnets as a way to partitionnetworks into logical segments for greater ease of administration.When subnets are properly implemented, both the performance andsecurity of networks can be improved.

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