3 Subscriber Local Loop Designss

8/3/2019 3 Subscriber Local Loop Designss

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SUBSCRIBER LOCAL LOOP DESIGN


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In comparison to the long-distance sector, the

per capita invested, but there would be no

national network without it.


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to build the most

assuming an established quality

of service.

The design of a network involves a number of limiting factors, the


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Intentionally Left Blank


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Signaling Limit is the function of the IR drop of the line , conductivity ofthe loop conductor and its diameter or gauge.

Attenuation limits are controlled by the same parameters.

Seizure of a circuit in switch is caused by the flow of current in loop .Certain amount of current is required to operate the relay for seizure.

If the current flow is insufficient the relay will close and open

intermittently.


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Quality of a Telephone Speech Connection

Loudness Rating

A group of people selected randomly to judge the subjective quality of a

telephone connection

Customer opinion, as a function of loudness loss, can vary with the test

group and particular test design.


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Overall Loudness Rating, Opinion Results


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Telephone sensitivity includes both microphone and earpiece sensitivity.

Overall loudness rating (OLR) is then calculated using the following

formula

OLR = SLR + CLR + RLR

is defined as the loudness loss between the speaking subscribers

mouth and the listening subscribers ear via a connection.

is defined as the loudness loss between the speaking subscribers

mouth and an electric interface in the network.


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is the loudness loss between an electric interface in the network and

the listening subscribers ear.

is the loudness loss between two electrical interfaces in a

connection or circuit, each interface terminated by its nominal

impedance, which may complex.


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Subscriber Loop Design Techniques

Simplified subscriber loop:

The distance D, the loop length, is a critical parameter.

The greater the value of D, the greater the attenuation that the loop

suffers, and signal level drops as a result.

Greater the wire diameter of the loop pair, the less resistance there is

per unit length

Also, the less attenuation there is per unit length.


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When designing a subscriber loop , the interesting parameter is the

maximum length required.


There are two variables that must be established:

1.The maximum loop resistance

This value is a function of the circuit in the switch where the loop

terminates.

2.The maximum loss or attenuation on the loop.

In Europe, 6 dB is commonly used for this value. This is 6 dB at the

reference frequency of 800 Hz.

In North America the reference frequency is 1000 Hz. The loss value

may be as high as 9 dB.


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Calculating the Loss Limit


The attenuation of a wire pair varies with frequency, resistance,

inductance, capacitance, and leakage conductance.

Also, resistance of the line will depend on temperature. For open-wire

lines, attenuation may vary by 12% between winter and summer

conditions.

If we are limited to 6 dB (loss) on a subscriber loop, then by simple

division we can derive the maximum loop length permissible for

transmission design considerations for the wire gauges .


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Loss and Resistance per 1000 ft of Subscriber Cable


Then by simple division we can derive the maximum loop length permissible

for transmission design considerations for the wire gauges


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Loading

In many situations it is desirable to extend subscriber loop lengths

beyond the limits

Common methods to attain longer loops without exceeding loss

limits are :

1. Increase conductor diameter2. Use amplifiers

3.Range extender

4. Use inductive loading

Inductive loading tends to reduce transmission loss on subscriber loopsand on other types of voice pairs at the expense of good attenuation-

frequency response beyond 3000 Hz.

A range extender is a device that increases battery voltage on a loop. This

extends its signaling range.

b b h


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Loading

Loading a particular voice-pair loop consists ofinserting inductances

in series (loading coils) into the loop at fixed intervals.

Adding load coils tends to decrease the velocity of propagation

and increase the impedance.



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S b ib L D i T h i


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Loading

Transmission Loss (Attenuation) Values with and without Load Coils for the

More Popular Wire Gauges Found in the Subscriber Plant



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S b ib L D i T h i


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Both limiting factors can be extended, but that extension costs money,

particularly when there may be many thousands of pairs involved. The

decision boils down to the following:


1. If the pairs to be extended are few, they should be extended.

2. If the pairs to be extended are many, it probably is worthwhile to

set up a new exchange area or a satellite exchange or to use an

outside plant module in the area.

These economies are linked to the cost of copper. The current tendency

is to reduce the wire gauge wherever possible or even resort to the use

of aluminum as the pair conductor.


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Si f h b d b


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Size of an exchange area based on number

Of subscribers served

The size of an exchange area will depend largely on:

Exchange sizes are often in units of 10,000 lines.

For the subscriber number there are 10,000 number combination

possibilities, from 0000 to 9999.

SHAPE OF A SERVING AREA


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SHAPE OF A SERVING AREA

If an entire local area is to be covered, fully circular exchange serving

areas are impractical.

Either the circles will overlap or uncovered spaces will result, neither

of which is desirable.

There are then two possibilities:

1. square serving areas 2.hexagonal serving areas.

EXCHANGE LOCATION


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EXCHANGE LOCATION

Bounded exchange area (exchange area boundaries are known)

EXCHANGE LOCATION


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Using a map to scale, a defined area is divided into small squares of 100

500 m on a side.

One guide for determination of side length would be to use a standard length of

the side of a standard city block in the serving area of interest.

EXCHANGE LOCATION

1

The next step is to write in the total number of subscribers in each of the blocks.

This total is the sum of three figures:(1) existing subscribers

(2) waiting list

(3) forecast of subscribers for 15 or 20 years into the future

2

The third step is to trace two lines over the subscriber area. Oneis a horizontal line that has approximately the same number of total subscribers

above the line as below. The second is a vertical line where the number of

subscribers to the left of the line is the same as that to the right.

3

The point of intersection of these two lines is the theoretical optimum center or

exchange location.

Steps required for finding exchange location:

EXCHANGE LOCATION


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EXCHANGE LOCATION

Availability of buildings and land.

Existing and potential cable or feeder runs.

Trunk pull

Layout of streets, roads, and highways.

Trunk pull refers to the tendency to place a new exchange near

the one or several other exchanges with which it will be

interconnected by trunks ( junctions).

But the actual location may depends on secondary parameters

such as:

EXCHANGE LOCATION


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Assume that exchange locations are known.

To determine boundaries of serving areas we use ratio technique

EXCHANGE LOCATION

Outer boundary will be the signaling limits of loops

.

These limits have been basically determined by the type of exchange and the

copper wire gauge utilized for subscriber loops.

Maximum permissible distance from exchange A to B is 3300/270 = 12.2 km

The distance from exchange A be DA DA = 1700 12.2/(1700 + 1600) 6.28 km

The total distance from exchange B will be the difference 12.2 6.28 = 5.92 KM

EXCHANGE LOCATION


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EXCHANGE LOCATION

Determining serving area boundaries with the ratio method

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3 Subscriber Local Loop Designss