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SOFTWARE MODEM

M.SASIKALA

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What is modem?

A modem is a device or program

that enables a computer to transmit

data over, for example, telephone orcable lines.

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Difference

A cable modem is a rather broad

term and may vary in technical

specification depending on supplier.In general cables modems connect to

a dedicated cable and offer internet

services, possibly along side a cable

TV service.

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Definition:

Software modem:

A modem implemented entirely

in software. Software modems rely on

the computer's processor to modulate

and demodulate signals.

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Theory of operation

FSK technique is used in 1200 band

modems.

It transmits sinusoidal tones

- 0s and 1s of different frequencies.

The 01 bit pattern creates thechirping sound characteristic of

modems. 2008 Wayne Overheads for

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Theory of operation

Higher-speed modems are backward

compatible with the 1200 band FSK

scheme and begin a transmission witha protocol.

2008 Wayne Overheads for

time

0 1

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FSK encoding

Generate waveforms based on current

bit:

bit-controlledwaveformgenerator

0110101

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FSK detection scheme

A/Dc

onve

rter

zero filter

one filter

detector0 bit

detector1 bit

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FSK detection scheme

The analog input is sampled and the

resulting bit stream is sent to two

digital FSK filters

- one passes frequencies in the range of0 and rejects 1-band.

-the other one does the reverse.

The output of the filter is sent to

the detectors.

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FSK detection scheme

It computes the average value of the

signal over the past n samples.

When the energy goes above the

threshold value, the appropriate bit

is detected.

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Transmission scheme

Send data in 8-bit bytes. Arbitrary

spacing between bytes.

Byte starts with 0 start bit.

Receiver measures length of start bit to

synchronize itself to remaining 8 bits.start (0) bit 1 bit 2 bit 3 bit 8...

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Requirements

Inputs Analog sound input, reset button.

Outputs Analog sound output, LED bit display.

Functions Transmitter: Sends data from memoryin 8-bit bytes plus start bit.

Receiver: Automatically detects bytesand reads bits. Displays current bit onLED.

Performance 1200 baud.

Manufacturing cost Dominated by microprocessor andanalog I/O

Power Powered by AC.

Physicalsize/weight

Small desktop object.

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Specification

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Receiver

sample-in()

bit-out()

1 1

Transmitter

bit-in()sample-out()

Line-out*

output()

1 1

Line-in*

input()

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System architecture

It consists of one small

subsystem(interrupt handler for

input and output) And two major

subsystem(transmitter andreceiver).

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Transmitter

Waveform generation by table

lookup.

float sine_wave[N_SAMP] = { 0.0, 0.5,0.866, 1, 0.866, 0.5, 0.0, -0.5, -0.866,

-1.0, -0.866, -0.5, 0};

2008Wayne Wolf

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time

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Receiver

Filters (FIR for simplicity) use circular

buffers to hold data.

Timer measures bit length.

State machine recognizes start bits,

data bits.

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Hardware platform

Hardwar architecture is relatively

simple.

-A/D converter.-D/A converter.

-Timer.

The amount of memory required to

implement algorithm is relatively very

small.

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Component design and

testing

Easy to test transmitter and receiver on

host.

Transmitter can be verified withspeaker outputs.

Receiver verification tasks: start bit recognition;

data bit recognition.

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System integration and

testing

Two ways to test modem system:-by having the modems transmitter send

bits to its receiver.

-by connecting two different modems. Use loopback mode to test components

against each other. Loopback in software or by connecting D/A

and A/D converters.

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THANK YOU!!!