DATA TRANSMISSION METHOD
DATA TRANSMISSION
A terminal or computer produces digital signals, which is simply the presence or absence of an electric pulse. The state of being on or off represents the binary number 1 or 0, respectively. Some communication lines accept digital transmission directly, and the trend in the communications industry is toward digital signals. However, most telephone lines through which these digital signals are sent were originally built for voice transmission, and voice transmission required analog signals. We will look at these two types of transmission and then study modems, which translate between them.
- DIGITAL AND ANALOG TRANSMISSION
Digital transmission sends data as distinct pulses, either on or off, in much the same way that data travels through the computer. However most communications media are not digital. Communications devices such as telephone lines, coaxial cables, and microwave circuits are already in place for voice transmission. The easiest choice for most users is to piggyback on one of these. thus, the most common communications device all use analog transmission, a continuous electric signal in the form of a wave.
To be sent over analog lines, a digital signal must first be converted to an analog form. It is converted by altering an analog signal, called a Carrier wave, which has alterable characteristics. One such characteristic is the amplitude, or height of the wave, which can be increased to represent the binary number one. Another characteristic that can be altered is the frequency, or number of times a wave repeat during a specific time interval; frequency can be increased to represent a 1.
Conversion from digital to analog signals is called modulation, and the reserve process-reconstructing the original digital message at the other end of the transmission-is called demodulation. (you probably know amplitude and frequency modulation by their abbreviations, AM and FM, the methods used for radio transmission). An extra device is needed to make the conversions: a modem.
- Modems
A modem is a device that converts a digital signal to an analog signal and vice versa. Modem is sort for modulate/demodulate.
- Type of Modems
Modems vary in the way they connect to the telephone line. There are two main type: acoustic coupler modems and direct-connect modems. Acoustic coupler modems include a cradle to hold the telephone handset. most modems today, however, are directly connected to the phone system.
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| Modems convert-modulate-digital data signal |
A direct connected to the telephone line by means of a telephone jack. An external modem is separate from the computer. Its main advantage is that it can be used with a variety of computers. If you buy a new personal computer, for example you can probably keep the same external modem. For those personal computer users who regard external modem as one more item taking up desk space, new modem-on-a-chip designs have produced a modem that is so small you will hardly notice it. For a modem that is our of sight-liter-ally-an internal modem board can be inserted into the computer as standard equipment. Hayes is the brand name that is the standard for modems: most modems are Hayes compatible. As we will discuss shortly, most modems today also have fax capability.
- Modem Data Speeds
Users who connect their computers via communications services may pay charger based on the time the computers are connected. Thus, there is a strong incentive to transmit as quickly as possible. The old standard modem speeds of 1200, 2400, and 9600 bits per second (bps) have now been superseded by modems that transmit an astonishing 14,400 or 28,800 bps. The speedsters using these rates are usually corporations sending data from one office to another. If you buy a modem today, you will probably find only modems with the 14,400 bps or higher for sale. However, transmission from one modem to another can be no faster than the speed of the slower modem, and many services to which you may be connected still receive and transmit at the lower rates. Still, there is no harm having the faster rate in anticipation of the day when all services will also transmit at that rate.
- ASYNCHRONOUS AND SYNCHRONOUS TRANSMISSION
Sending data off to a far destination work only if the receiving device is ready to accept it. By ready we mean more than just available; The receiving device must be able to keep in step with the sending device. Two techniques commonly used to keep the sending and receiving units dancing to the same tune are asynchronous and synchronous transmission.
when asynchronous transmission (also called start/stop transmission) is used, a special start signal is transmitted at the beginning of each group, of message bits-a group is usually just a single character. Likewise, a stop signal is sent at the end of the group of message bits. When the receiving device gets the start signal, it set up a timing mechanism to accept the group of message bits.
Data
trasfer fates compared
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Data Transfer rate (bps)
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Time to trasmit a 20-page single-spaced report
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1,200
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10 min
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2,400
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5 min
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9,600
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1,25 min
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14,400
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50 sec.
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28,800
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20 sec.
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Synchronous transmission is a little trickier because characters are transmitted together in a continuous stream. There are no call-to-action signals for each character. Instead, the sending and receiving devices are synchronized by having their internal clocks put in time with each other by a bit pattern transmitted at the beginning of the message. Further more, error check bits are transmitted at the end of each message to make sure all characters were received properly. Synchronous transmission equipment is more complex and more expensive but, without all the start/stop bits, transmission is much faster.
- SIMPLEX, HALF-DUPLEX, AND FULL-DUPLEX TRANSMISSION
Data transmission can be characterized as simplex, half duplex, or full duplex, depending on permissible directions of traffic flow. Simplex transmission sends data in one direction only; everyday examples are television broadcasting and arrival/departure screens at airports. Half-duplex transmission allows transmission in either direction, but only one way at a time.
An analogy is tall on a CB radio. In a bank a teller using half-duplex transmission can send the data about a deposit and, after it is received, the computer can send a confirmation reply. Full-duplex transmission allows transmission in both directions at once. An analogy is a telephone conversation in which, good manners aside, both parties can talk at the same time. We have discussed data transmission at some length. Now it is time to turn to the actual media that transmit the data.
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| Transmission direction |
An analogy is tall on a CB radio. In a bank a teller using half-duplex transmission can send the data about a deposit and, after it is received, the computer can send a confirmation reply. Full-duplex transmission allows transmission in both directions at once. An analogy is a telephone conversation in which, good manners aside, both parties can talk at the same time. We have discussed data transmission at some length. Now it is time to turn to the actual media that transmit the data.
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