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Ch. 6 Digital Data
Communication Techniques
6.1Asynchronous & Synchronous
• Asynchronous Transmission: transmission in
which each information character is individually
synchronized (usually by the use of start and stop
• 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"
• 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
Appendix G: Interfacing
• DTE--Data Terminal Equipment (not in 8
– 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
– 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
• 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
• 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.