1. Line Coding, Modem, RS232 interfacing sequences.

2. Line Coding
Process of converting binary data to a digital signal

3. DC Components
Residual direct-current (dc) components or zero frequencies are undesirable
Some systems do not allow passage of a dc component; may distort the signal and create output errors
DC component is extra energy and is useless

4. Self-Synchronization
Includes timing information in the data being transmitted to prevent misinterpretation
Lack of synchronization

5. Line Coding
Unipolar
Polar
Bipolar

6. Unipolar Simplest method; inexpensive Uses only one voltage level
Polarity is usually assigned to binary 1; a 0 is represented by zero voltage

7. Unipolar
Potential problems:
DC component
Lack of synchronization

8. Polar Uses two voltage levels, one positive and one negative
Alleviates DC component
Variations
Nonreturn to zero (NRZ)
Return to zero (RZ)
Manchester
Differential Manchester

9. Nonreturn to Zero (NRZ)
Value of signal is always positive or negative
NRZ-L
Signal level depends on bit represented; positive usually means 0, negative usually means 1
Problem: synchronization of long streams of 0s or 1s
NRZ-I (NRZ-Invert)
Inversion of voltage represents a 1 bit
0 bit represented by no change
Allows for synchronization
Transition between voltage, not voltage itself, represents a 1 bit

10. NRZ-L and NRZ-I Encoding

11. Return to Zero (RZ)
In NRZ-I, long strings of 0s may still be a problem
May include synchronization as part of the signal for both 1s and 0s
How?
Must include a signal change during each bit
Uses three values: positive, negative, and zero
1 bit represented by positive-to-zero
0 bit represented by negative-to-zero

12. RZ Encoding

13. RZ Encoding Disadvantage
Requires two signal changes to encode each bit; more bandwidth necessary

14. Manchester
Uses an inversion at the middle of each bit interval for both synchronization and bit representation
Negative-to-positive represents binary 1
Positive-to-negative represents binary 0
Achieves same level of synchronization with only 2 levels of amplitude

15. Differential Manchester
Inversion at middle of bit interval is used for synchronization
Presence or absence of additional transition at beginning of interval identifies the bit
Transition means binary 0; no transition means 1
Requires two signal changes to represent binary 0; only one to represent 1

16. Bipolar Encoding
Uses three voltage levels: positive, negative, and zero
Zero level represents binary 0; 1s are represented with alternating positive and negative voltages, even when not consecutive
Alternate mark inversion (AMI)

17. Bipolar AMI Neutral, zero voltage represents binary 0
Binary 1s represented by alternating positive and negative voltages

18. MODEM

19. Telephone Modems
A telephone line has a bandwidth of almost 2400 Hz for data transmission

20. Modem stands for modulator/demodulator.
Modulator : creates a analog signal from binary data
Demodulator : recovers the binary data from the modulated signal

21. V.32
ITU-T's V.32 standard was issued in 1989 for asynchronous, full-duplex operation at 9600 bps.
Although designed for asynchronous DTEs, two V.32 modems actually communicate synchronously.
A circuit converts the asynchronous data stream into synchronous blocks, invisible to the application.
V.32 supports modulation rates of 2400, 4800, and 9600 bps.

22. V.32bis
ITU-T's V.32 standard was issued in 1991 for asynchronous, full-duplex operation at 14.4 Kbps.
V.32bis is an extension of the V.32 technology. V.32bis supports modulation rates of 2400, 4800, 9600 bps and 14.4 Kbps.
Data compression and error correction can increase the throughput rates.

23. Traditional Modems
After modulation by the modem, an analog signal reaches the telephone company switching station where it sampled and digitized to be passed through the digital network.
Bit rate is 56,000bps.
Uploading :33.6kbps.
Downloading 56kbps.

24. Traditional Modems

25. RS232 Interface

26. Introduction Specifies the interface between DTE and DCE:
V.28 : mechanical and electrical characteristics
V.24 : functional and procedural characteristics
Even used in applications where there is no DCE
e.g. connecting computer to printer, magnetic card reader, robot, … etc.
Introduced in 1962 but is still widely used

27. DTE, DCE and RS232

28. Vocabulary DTE DCE data terminal equipment e.g. computer, terminal
data communication equipment
connects DTE to communication lines
e.g. modem

29. Mechanical Characteristics
9-pin connector
9-pin connector is more commonly found in IBM-PC but it covers signals for asynchronous serial communication only
Use male connector on DTE and female connector on DCE
N.B.: all signal names are viewed from DTE

30. 9-Pin RS232 Connector

31. Electrical Characteristics
Single-ended
one wire per signal, voltage levels are with respect to system common (i.e. signal ground)
Mark: –3V to –15V
represent Logic 1, Idle State (OFF)
Space: +3 to +15V
represent Logic 0, Active State (ON)
Usually swing between –12V to +12V
Recommended maximum cable length is 15m, at 20kbps

32. RS232 Logic Waveform

33. RS-232 Interface RS-232 is the Serial interface on the PC
Three major wires for the Serial interface:
Transmit - Pin 2
Receive - Pin 3
Ground - Pin 7 (25 pin connector) Pin 5 (9 pin connector) Tx Tx
Computer
Device Rx Rx
Gnd
Gnd
Transmit connects to Receive

34. Function of Signals TD: transmitted data RD: received data
DSR: data set ready
indicate whether DCE is powered on
DTR: data terminal ready
indicate whether DTR is powered on
turning off DTR causes modem to hang up the line
RI: ring indicator
ON when modem detects phone call

35. Function of Signals DCD: data carrier detect RTS: request to send
ON when two modems have negotiated successfully and the carrier signal is established on the phone line
RTS: request to send
ON when DTE wants to send data
Used to turn on and off modem’s carrier signal in multi-point (i.e. multi-drop) lines
Normally constantly ON in point-to-point lines
CTS: clear to send
ON when DCE is ready to receive data
SG: signal ground

36. Flow Control
Means to ask the transmitter to stop/resume sending in data
Required when:
DTE to DCE speed > DCE to DCE speed
(e.g. terminal speed = 115.2kbps and line speed = 33.6kbps, in order to benefit from modem’s data compression protocol)
without flow control, the buffer within modem will overflow – sooner or later
the receiving end takes time to process the data and thus cannot be always ready to receive