lesson5 - transmission aspects

7/27/2019 Lesson5 - Transmission Aspects

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Transmission Aspects

for Telephony

Engr. Edelito A. Handig


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Introduction

The IEEE defines Transmission as the

propagation of a signal, message, or other

form of intelligence by any means such asoptical fiber, wire, or visual means.

It provides the transport of a signal from an

end-user source to the destination such that

the signal quality at the destination meets

certain performance criteria.


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Introduction

The basic building block for transmission is

the telephone channel or voice channel.

CCITT voice channel: 300 and 3400 Hz. Bell Laboratories states that the optimum

trade-off between economics and quality of

transmission occurs when the telephonespeech signal is band-limited to the range

from about200 to 3200 Hz.


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QoS

The transmission engineer calls QoS as the

customer satisfaction.

Unit of Measurement:

Loudness Rating

Grade of Service


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QoS

Some elements to be listed under QoS are:

1. Can connectivity be achieved?

2. Dial tone delay and Post dialing delay3. Availability of service tones.

4. Correctness of billing.

5. Reasonable cost of service to the customer.

6. Responsiveness to servicing requests.

7. Responsiveness and courtesy of operators.


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Impairments to Voice channel

Basic impairments:

Attenuation distortion

Phase distortionNoise

Two additional impairments are:

Echo and Singing


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Attenuation distortion

The IEEE defines attenuation distortion

(amplitude distortion) as the change in

attenuation at any frequency with respect tothat of a reference frequency.

Attenuation distortion can be avoided if all

frequencies within the pass-band are

subjected to the same loss (or gain).


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Phase distortion

A voice channel acts as a band-pass filter.

If the phasefrequency relationship over a

pass-band is not linear, phase distortion willoccur in the transmitted signal.

It is often measured by a parameter called

envelope delay distortion. It is the derivativeof the phase shift with respect to frequency.

Propagation:Delay.

Velocity: 16,000 km/sec to 297,600 km/sec.


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Typical differential delay


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Noise

Four categories:

1. Thermal noise

2. Intermodulation noise3. Crosstalk

4. Impulse noise

Solution: Increase S/N


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Signal-to-Noise Ratio

S/N ratio expresses in decibels the amount by

which a signal level exceeds the noise within

a specified bandwidth. Voice S/N : 40 dB to 45 dBbased on

customer satisfaction.

Ex:


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Echo

In telephone systems, it is the return of a

talkers voice.

It affects the talker more than the listener. The cause of echo is impedance mismatches.

Two factors determine the degree of

annoyance of echo:its loudness and its length of delay


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Singing

Singing is the result of sustained oscillations

due to positive feedback in telephone

amplifiers or amplifying circuits.

Echo and singing can generally be attributed

to the mismatch between the balancingnetwork of the hybrid and its two-wire

connection associated with the subscriber

loop.


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2 wire/4 wire transmission


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2W vs 4W


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lesson5 - transmission aspects

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