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 "start-

stop" 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)

• DTE--Data Terminal Equipment (not in 8th 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.