- Otherwise it is noise or nonterrestrial data.. - Any meaningful information can be represented into Electrical,. - Electronic Data can be of two kind:. - Digital Data - take on discrete values, switching ON/OFF, text, integers, etc.. - Can be grouped into. - Normally we mean digital data processed by a computer.. - A signal is a series of analogue or digital data. - Definitions of Data and Signals:. - Analog Data - Data represented by a continuous physical quantity whose magnitude is proportional to a suitable function of the data (information). - It can have more than two discrete values in contrast to digital data.. - Digital Data - Data represented by discrete values or conditions.. - A series of digital data or bits.. - Characters can be encoded according to variety standards:. - Digital Encoding of Digital Data: (p. - Biphase (all of the biphase techniques require at least one transition per bit time and may have as many as two transitions.. - 0 - A transition at the beginning of the bit period represents 0, and. - 1 - an absence of a transition at the beginning of the bit period represents 1.. - Data transmission is the transfer of data between two electronic system on a suitable media.. - Both analog and digital data can be represented and propagated by either analog or digital signals.. - Both analog and digital signals can be transmitted on suitable media.. - Analog transmission - is the transmission of analog signals.. - Digital transmission - is the transmission of digital signals.. - Four possible combination of data and transmission:. - Digital data, digital signal.. - In general, the equipment for encoding digital data into a digital signal is less complex than digital-to-analog equipment.. - Analog data, digital signal.. - Digital signal can be propagated only a limited distance before attenuation endangers the integrity of data.. - Analog signal can be transmitted to longer distances.. - Analog transmission is affected by noise.. - Analog signal can be transmitted through Copper, Fiber and Satellite.. - Serial Transmission:. - Parallel transmission is primarily limited to transmission of data within a computer, between computers and between a computer and a printer.. - It is fast compared to serial transmission but limited to shorter distances.. - It is slower but can travel longer distances and is widely used.. - Serial Transmission is the predominant method of transferring information in data communications.. - It is the mode of communication used on a PC’s serial port as well as on local area networks.. - It is not used very often in data communications but is used extensively for computer busses and for printer connections.. - Reception of Digital data:. - The reception of digital data involves sampling the incoming signal once per bit time to determine the binary value (0 or 1).. - One of the reasons is that all the digital system operates in steps with a stepper (also called a clock, which creates a stream of pulses).. - 148) The timing difficulties in data transmission can be avoided as follows:. - The first is called the asynchronous transmission and the second one is the synchronous transmission.. - Asynchronous transmission:. - Asynchronous transmission is achieved by stoping the transmission before the error occurs because of timing drifts. - In Asynchronous transmission a maximum of 9 bits are transmitted between each pair of start and stop signals.. - Asynchronous transmission requires 20% or more overhead (a waste of 2 bits for every 10 bits of transmission (1 start + 8 data + 1 stop). - The solution is the synchronous transmission.. - Asynchronous Transmission involves sending each character (or byte) with its own start and stop bits.. - It is also called “character-framed” data.. - It is inexpensive and often used for low speed connections of PCs or terminals to mainframe computers or other full duplex point-to-point circuits.. - When the sender is waiting for the next character to be sent, the line is idle and remains in its default state which is the same as the stop bit state (binary 1 or -12v in case of serial port. - Synchronous transmission:. - This can be accomplished with Manchester data encoding.. - The block synchronization is maintained with a start/stop preamble bit patterns, just like the character synchronization achieved with start/stop bits in asynchronous transmission.. - The start and end of the entire block of data must be marked with a header at the beginning and a trailer at the end which may include an error-checking code and an ending flag but individual. - It is more efficient than asynchronous transmission because more information can be sent in fewer bits.. - You can think of Synchronous transmission as sending large blocks of data in an envelope as one unit and Asynchronous transmission as sending each character in its own envelope.. - It is sometimes called two-way alternate.. - Most digital data processing devices generate simple digital signals (e.g., NRZL) are limited of data transmission capability.. - Consequently, it is rare for such a device (terminal, computer) to attach directly to a transmission or networking facility.. - Rather, a DTE makes use of the transmission system through the mediation of Data Circuit- Terminating Equipment (DCE).. - To ease the process for the manufacturers and the users standards have been developed that specify the exact nature of the interface between the DTE and the DCE.. - The ITU-T standard corresponds to it is as follows:. - To avoid this problem is the biphase.. - In biphase for a series of 0's the phase changes every bit.
Xem thử không khả dụng, vui lòng xem tại trang nguồn hoặc xem
Tóm tắt