Data Transmission

Data Transmission
   Data transmission, digital transmission, or digital communications is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media. The data are represented as an electromagnetic signal, such as an electrical voltage, radiowave, microwave, or infrared signal. 
   Transmission Media: Copper Wire (Coaxial Cable, Twisted Pair), Glass Fiber, Microwave, Radio, Infrared. These are the media usually used in communication channels. In considering different kinds of media we are concerned, among other things, with the amount of data that can be transmitted through the medium per unit of time, the power required for the transmission, its attenuation rate, the distorsion, the error rate, the cost, and the security.
   Simplex/Duplex: If in a channel transmission can take place in only one direction, we say that the transmission is simplex. If the transmission can take place in both directions, but not at the same time, we say that the transmission is half-duplex, and if it can take place at the same time in both directions it is full-duplex.
   Multiplexing: We often want to share a communication channel between more than two pairs of communicating entities. This can be done by Time Division Multiplexing (the channel is used in different time slots by different pairs of entities) or by Frequency Division Multiplexing (communicating pairs use different carriers that can propagate simultaneously through the channel). [There is also, among others,          Statistical Multiplexing.] We call multiplexor the device that inserts two or more signals into the communication channel, and demultiplexor the device that separates the signals out.
   Digital/Analog Signals: The signals we consider are usually electromagnetic waves. They propagate at the speed of light (300,000Km/sec) in the vacuum or slightly slower (200,000Km/sec) in materials. Signals are analog i.e. they are represented as a continuous line usually with a variety of levels. Digital signals are analog signals that can be approximated as having two levels, i.e. they are square waves. 
   Baseband Transmission takes place on a communication medium, usually a local area network, using only one communication channel. The signal is digital and directly inserted in the channel as pulses. For example the Ethernet that we use in our LANs uses baseband technology - a single communication channel is shared by all communication parties. Also our regular telephone service uses baseband communication on the line. 

   Broadband Transmission instead uses analog signals and multiple communication channels share the communication medium, usually modulating various carriers. For example the cable that you may have at home for both television and your computer uses broadband technology - television and computer use different channels in the medium. Similarly if your telephone line supports DSL it uses broadband technology with a channel for normal phone service and channels for the computer communication. A codec (coder/decoder) is a device that transforms analog signals to/from binary signals (for example, phone, where the analog signal representation of voice is transformed for transmission on the high speed digital network into a digital signal). For instance the conversion from analog to binary signal can be done using PCM (Pulse Code Modulation). See below the Nyquist Sampling Theorem. Also, codec is used for a component tha compresses/decompresses data, for example mpeg data.
   Bandwidth: If we consider the signals that propagate through a physical medium, there will be one with highest frequency and one with lowest frequency. The difference between these two frequencies is the bandwidth of the medium. [Signals with frequencies outside of this range are with negligible power, where power is voltage times intensity.] For example for phone communication we use frequencies between 300Hz and 3300Hz, for a bandwidth of 3000Hz. Normally a telephone channel is allocated 4KHz.
   Digital/Analog Data: Analog data is real (continuous) data, digital data is binary data. The conversion between binary data and analog signals is done by modems (modulator/demodulator). Some modems use 4 wires (2 to transmit/modulate and 2 to receive/demodulate) to connect two computers, each computer with its own modem. Most modems are dialup modems. A computer or terminal can use a dialup modem to connect to the phone network. This modem has only two wires and it has equipment for making and receiving phone calls. The two wires are used to transmit and to receive (using different frequencies). Cable modems are used to connect a computer to a TV cable network. Data is transmitted usually at different speeds from the cable to the computer and viceversa. Data rates in the Mbps are possible. Essentially one TV channel (6 MHz) per direction is shared by all the users connected to the cable.
   Modulation: Modulation/Demodulation is the conversion between binary data and analog signals. Usually the binary data is used to modify some characteristics of a sinusoidal signal, the carrier. We can represent the binary data by modifying the amplitude of the carrier(Amplitude Modulation), or by modifying its frequency (Frequency Modulation), or its phase (Phase Shift Modulation).