Telecommunications Switching Systems 5

7/29/2019 Telecommunications Switching Systems 5

1/19

Wideband Transmission Media

Coaxial Cable

Microwave Radio

Frequency Diversity

Space Diversity

Satellites


2/19

Co-axial cable

Radio-grade flexiblecoaxial cable.

A: outer plastic sheath

B: copper screen

C: inner dielectricinsulator

D: copper core

Coaxial MUX


3/19

Microwave Radio

Freq Diversity

The signal is transferred using severalfrequency channels or spread over a widespectrum that is affected by frequency-selective fading.

Middle 20th century microwave radio relaylines often used several regular wideband

radio channels, and one protectionchannel for automatic use by any fadedchannel.


4/19

Space Diversity

The signal is transferred over several differentpropagation paths. In the case of wired transmission, thiscan be achieved by transmitting via multiple wires. In thecase of wireless transmission, it can be achieved byantenna diversity using multiple transmitter antennas(transmit diversity) and/or multiple receiving antennas(diversity reception).

Transmission Impairments

Signal Attenuation

Interference

Noise

Distortion

Echoes and Singing


5/19

Signal Attenuation

Subjective listening tests have shown thatthe preferred acoustic to acoustic loss in atelephone connection should be inneighborhood of 8 dB.

Signal Attenuation

A typical local call had only 0.6 dB moreloss than ideal.

The average analog toll connection had anadditional 6.7 dB loss.

The standard deviation of loss in toll

connections was 4 dB. Trunks within the toll network used

amplifiers to offset transmission losses.


6/19

Interference

Interference is more structured than noisesince it arises as unwanted coupling from

just a few signals in the network.

Interference if intelligible, is referred to ascrosstalk.

Sources of crosstalk

Coupling between wire pairs in cable

Inadequate filtering or carrier offsets inolder FDM equipment, and the effects ofnon-linear components on FDM signals.


7/19

Forms of Crosstalk

Near-end Crosstalk (NEXT)

Far-end Crosstalk (FEXT)

Which form of crosstalk is more troublesome?

Noise

White Noise : easy to analyze, easy to find,arises as thermal noise, truly random,

uncorrelated, quantified in terms of average

power.

Impulse noise: occur from switching transients inolder equipment, measured in terms of impulses

per second.

Impulse noise is usually of less concern to voice

quality than background white noise. Impulse noise is a problem in data

communication.

Quantization Noise


8/19

Weighting Curves

Disturbances at some frequencies withinthe passband of a voice signal aresubjectively more annoying than others.

Thus, more useful measurements of noiseor interference power in a speech networktake into account the subjective effects ofthe noise as well as the power level.

C-message and Psophometric Weighting


9/19

Weighting Curves

These curves essentially represent filtersthat weight the frequency spectrum ofnoise according to its annoyance effect toa listener.

C-message is a north American standardwhile psophometric weighting is aEuropean (ITU-T) Standard.

Relationships between various noisemeasurements


10/19

Distortion

Due to Internal characteristics of channel,deterministic in nature.

Can be compensated or controlled, once known.

Some distortions arises from non-linearities (e.g.saturated VF amplifiers) in the network while

other are linear in nature.

Amplitude and Phase distortion.

Amplitude Distortion

Attenuation of some frequencies in thevoice spectrum more than others.

Introduced by spectrum limiting filters inFDM eqpt.

Loading coils eliminate Amp distortion.


11/19

IL vs fof typical toll connection

Phase Distortion

Delay characteristics of transmission medium.

Individual frequency components experience differentdelays.

Delay of Individual Freq. component is referred to as itsenvelope delay

Uniform envelope delay linear phase systems.

Any deviation from a linear phase characteristic is

referred to as phase distortion. Perceptual effects of phase distortion to a voice signal

are small. Thus requires minimal attention.


12/19

Envelope delay and phase response


13/19

In addition to amp and phase

distortion, other frequency relateddistortions are:

frequency offsets,jitter, phase hits and signal dropouts.

Echoes & Singing

Transmitted energy coupled to return path.

Most common cause impedancemismatch at a 2-to-4-wire hybrid.


14/19

Echoes & Singing

Talker echo 1st reflection

Listener echo 2nd reflection

Singing repeated coupling to forwardpath leads to oscillations (if loop gain at afrequency 1).

Echoes & Singing

Talker echo is usually most noticeable andtroublesome.

The degree of echo annoyance isdependent on magnitude of returning

signal and the amount of delay involved.


15/19

Echo Suppressor and Canceller

Power Levels

ITU-T zero-relative level point

North American zero-transmission-levelpoint (0-TLP).


16/19

TLP

Transmission Level Point, or TLP, issimply one way of representing gain orloss in a channel in dB.

loss

0 TLP -3 TLP3 dB

loss

-16 TLP -19 TLP3 dB

0 TLP, Zero Transmission Level Point Reference Point

1.5 dB 3 dB 4.5 dB dBm

0 TLP -1.5 TLP +1.5 TLP -3 TLP

loss lossgain

1.5 dB 3 dB 4.5 dB dBm

0 TLP -1.5 TLP +1.5 TLP -3 TLP

loss lossgain

-10 dBm0-10 dBm

-10 dBm0-11.5 dBm

-10 dBm0-8.5 dBm

-10 dBm0-13 dBm


17/19

TLP = dBm dBm0

dBm0 means power at 0 TLP.

TLP does not describe the (absolute)power present at a point.

To know the power present at a givenTLP, it is necessary to know the power

present at some other TLP in the channel.

Example 1: If a signal is specified as -13 dBm0 at aparticular point and -6 dBm is measured at that point,the TLP is

TLP = -6 dBm ( - 13 dBm0) = +7 TLP

Example 2: If a signal is -13 dBm at the 0 TLP, thenat the +7 TLP the signal level in dBm is

(+7 TLP) + ( - 13 dBm0) = -6 dBm

Example 3: A -13 dBm0 signal measured at -16 TLP is

(-16 TLP) + ( - 13 dBm0) = -29 dBm


18/19

TLP

TLP applies to noise levels as well assignal levels.

When referred to the 0 TLP, noise is givenin dBrnC0.

TLP = dBrnC dBrnC0

Example 4: A 71 dBrnC0 signal measured at -3 TLP is

(-3 TLP) + ( 71 dBrnC0) = -68 dBrnC

Example 5 : A 71 dBrnC0 signal measured at +7 TLP is

(+7 TLP) + ( 71 dBrnC0)= 78 dBrnC

0/0 TLP means a circuit has zero gain (loss)-16/+7 TLP > Rx/Tx TLP


19/19

Example 1.1 (Text book)

Date post:	04-Apr-2018
Category:	Documents
Upload:	ramcharan-buddi
View:	220 times
Download:	0 times

Telecommunications Switching Systems 5

Documents