Communication system

SourceSource

encoding

Channel

encoding

Line

encoding

Modulation

Channel

ReceiverSource

decoding

Channel

decoding

Line

decoding

Demodulation

Distortion

Amplitude distortion Phase distortion

Amplitude response of a telephone channel

Insertion loss=10log10P0/P2

P2=Power delivered to a load by the channel

P0=Power delivered to the same load without channel

Envelope delay and Phase response of a telephone channel

Envelope delay is the derivative of the phase shift

Tx

Rx

Equalizer

Threshold detector

Data

Clock (Trig ขาข��น)

Tx Channel Equalizer ThresholdDetector

RetimingCircuit

ClockExtraction

AB C D

E

F

Communication system

SourceSource

encoding

Channel

encoding

Line

encoding

Modulation

Channel

ReceiverSource

decoding

Channel

decoding

Line

decoding

Demodulation

Data transmission

Communication modes

Simplex Half-Duplex Duplex or Full-duplex

Transmission modesAsynchronous transmission Feature : unnecessary to perfectly

synchronized Encoding Technique Used : NRZ Components : Start Bit (1 : transition from

1 to 0), Data (5-8), stop bit(1, 1.5, 2 : binary 1)

Merit : Easy-to-build Demerit : High overhead e.g. start, stop bit

1 bit, data 8 bit -> overhead = 2/10 = 20 %START BIT

5-8 BITS STOP BIT

START BIT

idle0

1

Transmission modesSynchronous Transmission Feature : synchronized by sending timing

information via separate clock line or embedding in data signal

Line coding: Manchester, AMI Component : preamble (8-bit flag), Control, Data,

Control, postamble (8-bit flag) Merit : Low overhead comparing to Asynchronous

transmission. Example : preamble + control + postamble 48 bits data 1000 bits -> overhead = 48/1048 = 4.6 % data 8000 bits -> overhead = 48/8048 = 0.6 %

8-bitflag

8-bitflag

Control ControlData

Transmission media

Transmission medium is the physical path between transmitter and receiver in a data transmission system.

Guided vs unguided (wireless transmission)

Characteristics and quality of data communication depend on characteristics of both media and signal.

Key concern in design is data rate and distance.

Guided Transmission media Two-wire open Twisted pair Coaxial cable Fiber optic

Twisted pair

&

Coaxial cable & connectors

&

&

Guided transmission media

Data rate Distance Typical use

Two-wire open 19.2 kbps 50 m Telephone

Twisted pair Category1-2

<2 Mbps 2-3 miles Telephone

Twisted pair Category3-6

200 Mbps 100 m LANs

Coaxial cable, ThinBaseband Single Ch.

10 Mbps 100 m LANs

Coaxial cable, ThickBroadband Multi Ch.

10 Mbps 2-3 miles LANs, Cable TV

Fiber optic 10 Gbps 100 miles Data, video, LANs, WANs

Unguided transmission media

Transport electromagnetic waves without using a physical conductor

Geostationary satellites The satellite must move at the same speed as

the earth so that it seems to be fixed above a certain spot.

Orbital speed is based on distance from the planet.

This orbit occurs at the equatorial plane and is approximately 22,000 miles from the surface of the earth.

At least 3 Satellite to cover all areas.

Satellite frequency bands

Band DownlinkSat. to earth

UplinkEarth to Sat.

C 3.7-4.2 GHz 5.925-6.425 GHz

Ku 11.7-12.2 GHz 14-14.5 GHz

Ka 17.7-21 GHz 27.5-31 GHz

Unguided transmission media

Data rate

Distance Typical use

Microwave

n-Gbps 20-30 miles

Building to building

Satellite n-Gbps Worldwide

Long distance

Infrared 16 Mbps 1.5 Miles Short distance

ModulationsAmplitude Shift Keying ASK (OOK)

BW for ASK

Nbaud is baud rate

Ex. Given a BW of 10,000 Hz (1,000-11,000) for a full-duplex ASK. Find the carriers and the BW in each direction. Assume there is no gap between the bands in two directions. What is the maximum baud rate and bit rate?

Frequency Shift Keying (FSK)

BW for FSK

EXample: Find the maximum bit rates for an FSK signal if the bandwidth of the medium is 12,000 Hz, and the difference between the two carriers must be at least 2000 Hz. Transmission is in full-duplex mode.

Phase Shift Keying (PSK)

PSK Constellation diagram

4-PSK

4-PSK Constellation diagram

8-PSK

BW for PSK

The same as ASK Ex. Given a bandwidth of 5000 Hz

for an 8-PSK signal, what are the baud rate and bit rate?

PSK is not susceptible to the noise degradation that affects ASK, nor the BW limitations of FSK.

Quadrature amplitude modulation (QAM)

BW for QAM

The same as ASK and PSK.

Multiplexing

multiple links on 1 physical line common on long-haul, high capacity,

links have FDM and TDM alternatives

Frequency Division Multiplexing

FDMSystem Overview

FDM Voiceband Example

Wavelength Division Multiplexing

FDM with multiple beams of light at different freq

carried over optical fiber links

Synchronous Time Division Multiplexing

TDM SystemOverview

Data communication interfacing

DTE : Data Terminal Equipment is any device that is a source of or destination for binary digital data (PC)

DCE : Data Circuit-terminating Equipment is any device that transmits or receives data in the form of an analog or digital signal through a network (Modem)

DTE DCE PSTN DTEDCE

Data communication interfacing DTE -DCE InterfacingRequirement : Use the same encoding

scheme and have common standards Mechanical characteristics : plugs,

circuits Electrical characteristics : voltage-level,

timing Functional characteristics : meaning of

each interchange circuits เช่�นขา 7 คือ Ground

V.24/EIA-232 (RS 232)

Electronic Industries Association (EIA)

(ISO 2110) 25-pin connector (but can be substituted by 9-pin (DB-9) : 8, 3, 2, 20, 7, 6, 4, 5, 22)

Mechanical characteristics

Electrical characteristics -3 to -15 V=binary 1 or OFF condition +3 to 15 V =binary 0 or ON condition Maximum bit rate 20 kbps

0

1 1

Functional specification

Control SignalName To Function

Request to send

DCE DTE wishes to transmit

Clear to send DTE DCE is ready to receive

DCE ready DTE DCE is ready to operate

DTE ready DCE DTE is ready to operate

Ring indicator DTE DCE is receiving a ringing signal

Received line signal detector

DTE DCE is receiving a signal(carrier)

Null modem

PSTN

MODEMS MOdulator/DEModulator

Bandwidth 600 Hz to 3000 Hz

Dial-up operation

20

DTE