This document discusses long-distance communication and modulation techniques used for data transmission over long distances. It covers types of modulation like amplitude modulation and frequency modulation that are used to encode data onto a carrier signal. It also discusses modems, leased telephone circuits, and multiplexing techniques like frequency division multiplexing and time division multiplexing that allow multiple logical connections to share a single physical medium.

2.  Long-Distance Communication
 Sending signals over long distances
 Types of Modulation
 Modems
 Other types of modems
 Leased serial data circuits
 Optical, radio and dialup modems
 Carrier frequencies and multiplexing
 Time division multiplexing

3. • Encoding used by RS-232 cannot work in all situations
 Over long distances
 Using existing systems like telephone
• Different encoding strategies needed
• Line coding
• Modulation

5. • Send an oscillating carrier wave and then modulate it in some
way
• Technique originated with radio and TV (stations use different
carrier frequencies)
• Transmitter generates carrier and modulates according to
data, receiver discards carrier
• Two approaches from radio are frequency modulation (FM) and
amplitude modulation (AM)

7. • Amplitude modulation – strength, or amplitude of carrier is
modulated to encode data
• Frequency modulation – frequency of carrier is modulated to
encode data
• Phase shift modulation – changes in timing, or phase shifts
encode data

8. • Change amplitude of the carrier according to the data

9. • Slightly change frequency of the carrier according to the data

10. • FM and AM require at least one wave cycle to send a bit
• Phase shift changes the timing of the carrier and can send
several bits per cycle

11. • Amount of phase shift can be measured
• How much of sine wave is "skipped"
• Example shows 1/4, 1/2 and 3/4 cycle
• Each phase shift can be used to carry more than one bit. In
example:
• 00 - no shift
• 01 - 1/4 phase
• 10 - 1/2 phase
• 11 - 3/4 phase
• Thus, each phase shift carries 2 bits

12. • Hardware that takes bits and applies modulation is a modulator
• Hardware that takes a modulated wave and extracts bits is a
demodulator
• Full duplex communication requires a combined modulator-
demodulator (MODEM) at both ends

13. • ISDN modem
• Cable modem (Front)
• Cable modem (rear) with coax connector for cable and
10Base-T connector

15. • Long distance four wire circuits can be leased from a phone
company (spare circuits are often included in trunk cables for
expansion purposes)
• Often called a serial line or serial data circuit

17. • Modems also used with optical fibre, radio and conventional
phone connections
• Dial-up modems work with the existing phone system
• mimic telephones
• use a carrier that is an audible tone
• use a single voice channel (2 wire circuit) and co-ordinate to
achieve full duplex communication

20. • Two or more signals with different carrier frequencies
transmitted over one medium
• Several logical connections share a single physical
connection - Frequency Division Multiplexing (FDM)
• Minimum frequency separation => requires high bandwidth
connection
• Spread spectrum - use of multiple carriers to improve
reliability
• Also, single logical channel may simultaneously use multiple
carriers to improve performance

21. • TDM is an alternative to FDM where the sources sharing the
medium take turns (e.g., round robin)
• Example:-
 How fast data can be sent across a voice telephone system?
 Telephone system:
 Bandwidth = 3000 Hz
 S/N = 30dB
 decibels (db) = 10log10S/N
 Maximum data rate on a noisy medium is:
 D = B log2 (1 + S/N)
 D =3000 x 10 ~30000bps

22. • Signals degrade over distance
• Oscillating waves propagate further
• Modulation - FM, AM and phase shift
• Modems, including dial-up modems
• Multiplexing - FDM and TDM