1. Ch. 6 Digital Data Communication Techniques

2. 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.

3. 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.

4. 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.

5. 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. 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.)

7. 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.

8. 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 )
X wire interface for public switched network interfacing.
ISDN Physical Interface (8 wire interface).

9. 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.

10. G.1 EIA-232-F Mechanical (ISO 2110) Electrical(V.28)
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.)

11. G.1 EIA-232-F (p.2) Functional (V.24) Procedural (V.24)
Table G-1--Interchange Circuits
Procedural (V.24)
Fig. G.4

12. 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.

13. 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.

14. G.2 ISDN Physical Interface
X 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.