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Ch. 6 Digital Data Communication Techniques 6.1Asynchronous & Synchronous Transmission • Asynchronous Transmission: transmission in which each information character is individually synchronized (usually by the use of start and stop elements). • Synchronous Transmission: transmission in which the time of occurrence of each signal representing a bit is related to a fixed time frame. 6.1 Asynchronous Transmission • Also known as character transmission or "startstop" transmission. • One character at a time is transmitted. – – – – – The line usually idles at a logic 1 Each character has a start bit (logic 0) . The start bit is followed by 5-8 data bits. A single party bit can be generated, but it is optional. 1, 1.5, or 2 stop bits (logic 1) finish the "framing" of the character. 6.1 Asynchronous Transmission (Fig. 6.1) • The efficiency E = # of inf. bits/ total # of bits. • Example: ASCII code, odd parity, 2 stop bits. – # of inf. bits= 7 – Total =1 start + 7 data + 1 parity + 2 stop = 11 – Efficiency = 7/11= .64 or 64%. • Transmitter and receiver have a "shift-register" structure. • A separate clock exists at each end. • UART--Integrated circuit implementation. 6.1 Asynchronous Transmission • Timing Requirements (Fig. 6.1) – Consider a 10 kbps transmitter clock. – Each bit will be 100 microseconds. – Assume the receiver is faster by 6%, or 6 microseconds during each bit time. – The transmitter sends 1 start bit and 7 data bits in 800 microseconds. – The receiver looks for the 8th data bit after 8.5x94=799 microseconds. 6.1 Synchronous Transmission (Fig. 6-2) • • • • • • Also known as block transmission. Clock is transmitted along with the info. bits. Higher data rates can be obtained. Overhead bytes are transmitted. Can be character-oriented or bit-oriented. Large information fields relative to total overhead can provide high throughput (sometimes.) Appendix G: Interfacing (Fig.G.1) th • DTE--Data Terminal Equipment (not in 8 Edition) – Equipment consisting of digital end instruments that convert the user information into data signals for transmission, or reconvert the received data signals into user information. • DCE--Data Circuit-terminating Equipment – In a data station, the equipment that provides the signal conversion and coding between the data terminal equipment (DTE) and the line. – DCE may be separate equipment or an integral part of the DTE or intermediate equipment. G.1 Interfacing (cont.) • Interchange Circuits – The connection between the DTE and DCE. • Standards--Physical Layer of the OSI Model – V.24/EIA-232-F (RS-232--1962) – X.21--15 wire interface for public switched network interfacing. – ISDN Physical Interface (8 wire interface). G.1 Four Characteristics • Mechanical – Pertain to the actual physical connection of the DTE and DCE (the terminator plugs and sockets). • Electrical – The voltage levels and timing of voltage changes. • Functional – The functions performed by various interchange circuits: data, control, timing and ground. • Procedural – The sequence of events for transmitting data. G.1 EIA-232-F • Mechanical (ISO 2110) – DB-25 connector (a 25 pin connector) – Fig. G.2. • Electrical(V.28) – – – – Digital signaling; up to 20 kbps; up to 15m. Logic 1 and OFF : less than -3 volts Logic 0 and ON : greater than +3 volts And more (C, R, short circuit current, max voltages, slew rate, etc.) G.1 EIA-232-F (p.2) • Functional (V.24) – Table G-1--Interchange Circuits • Procedural (V.24) – Fig. G.4 G.1 Loopback Testing • EIA-232-F control circuits assist in loopback testing and fault isolation. – Local loopback tests are used to check the functioning of the local interface and the local DCE. – Remote loopback tests are used to check the transmission channel and the remote DCE. • Figure G.3 Local and remote loopback. G.1 The Null Modem • Used to connect two DTEs directly (no DCEs used). • It is not a real modem, but simply a cable that rewires the circuits to trick the DTEs into thinking that they are talking with DCEs. • Fig. G.5 illustrates the null modem wiring. G.2 ISDN Physical Interface • X.21--15 pin connection for digital interface to public switched networks. • ISDN--ISO 8877 specifies an 8 pin connector. • The reduction of interface circuits forced greater complexity in the logic circuits at each end of the cable, but integrated circuits have become cheap whereas wire remains relatively expensive. • Fig. G.6 shows the ISDN Interface.