tailieunhanh - Kỹ thuật truyền số liệu 6

Tham khảo tài liệu 'kỹ thuật truyền số liệu 6', giáo dục - đào tạo, cao đẳng - đại học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | . R6 Transmission of Digital Data Interfaces and Modems Once we have encoded our information into a format that can be transmitted the next step is to investigate the transmission process itself. Information-processing equipment generates encoded signals but ordinarily requires assistance to transmit those signals over a communication link. For example a PC generates a digital signal but needs an additional device to modulate a carrier frequency before it is sent over a telephone line. How do we relay encoded data from the generating device to the next device in the process The answer is a bundle of wires a sort of minicommunication link called an interface. Because an interface links two devices not necessarily made by the same manufacturer its characteristics must be defined and standards must be established. Characteristics of an interface include its mechanical specifications how many wires are used to transport the signal its electrical specifications the frequency amplitude and phase of the expected signal and its functional specifications if multiple wires are used what does each one do . These characteristics are all described by several popular standards and are incorporated in the physical layer of the OSI model. DIGITAL DATA TRANSMISSION Of primary concern when considering the transmission of data from one device to another is the wiring and of primary concern when considering the wiring is the data stream. Do we send one bit at a time or do we group bits into larger groups and if so how The transmission of binary data across a link can be accomplished either in parallel mode or serial mode. In parallel mode multiple bits are sent with each clock pulse. In serial mode one bit is sent with each clock pulse. While there is only one way to send parallel data there are two subclasses of serial transmission synchronous and asynchronous see Figure . 139 140 CHAPTER 6 TRANSMISSION OF DIGITAL DATA INTERFACES AND MODEMS Figure Data transmission .