Transcript Fixed Telephony Basics

BASES OF NETWORK
CONFIGURATIONS
• The design of such a network involves a number of limiting factors, the
most important of which is economic.
• Considering both quality of service and economy, certain restraints will
have to be placed on the design. We will want to know:
1. Geographic extension of the local area of interest.
2.
Number of inhabitants and existing telephone density.
3. Calling habits.
4.
Percentage of business telephones.
5.
Location of existing telephone exchanges and extension of their
serving areas.
6.
Trunking scheme.
7.
Present signaling and transmission characteristics.
SUBSCRIBER LOOP DESIGN
It is a dc loop in that it is a wire pair supplying a metallic path for the following:
1. Talk battery for the telephone transmitter
2. An ac ringing voltage supplied from a special ringing source voltage.
3. Current to flow through the loop when the telephone instrument is taken
out of its cradle.
4. Unique pairs of audio tones, representing touch-tone pad with digit
buttons, transmitted to the serving exchange switching equipment.
battery and ground are
fed through inductors LB
and LA
Quality of a telephone speech
connection: Loudness Rating (LR)
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Loudness rating is the principal parameter for measuring the quality of a
speech connection. It is a measure of speech level (volume).
To determine LR telephone sensitivity must be measured.
Overall loudness rating (OLR) is then calculatedusing the following formula:
OLR = SLR + CLR + RLR
Subscriber Loop Design Techniques
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•
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The distance D, the loop length, is a critical parameter if D ↑ then attenuation ↑
then signal level drops as a result. must be enough current flow in the loop to
actuate the local switch where the loop terminates.
When designing a subscriber loop, we would be vitally interested on what its
maximum length would be. two variables:
(1) The maximum loop resistance (resistance limit): This value is a function
of the circuit in the switch where the loop terminates. One current value that
comes to mind is 2400 .
(2) The maximum loss or attenuation on the loop (loss limit): This will be
taken from the national transmission plan.
Subscriber Loop Design Techniques
• 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.
• When budgeting parameter values for the resistance limit on a subscriber
loop, we must budget something for the telephone subset itself. Use 300
for this value.
• The maximum loop resistance is set by the local serving switch design. in
the United States, the value was 1300, allowing 300 for the telephone
subset leaving only 1000 for the loop itself. Some earlier digital switches
advanced this value to 1800 ; some Northern Telecom switches provide
2400 ohms.
Calculating the Resistance Limit.
• To calculate the dc loop resistance for copper conductors, the following
formula is applicable:
Rdc = 0.1095 / d²
where Rdc is the loop resistance in ohms per mile (statute) and d is the
diameter of the conductor (in inches).
For example: If we wish a 10-mile loop and allow 100Ω per mile of loop
(for the stated 1000- limit), what diameter of copper wire would be
needed?
100 = 0.1095/d²
d² = 0.1095/ 100
d = 0.033 in. or 0.76 mm (rounded off to 0.80 mm)
Calculating the Loss Limit.
• The attenuation of a wire pair varies with frequency, resistance, inductance,
capacitance, and leakage conductance.
• we can compute maximum loop lengths for 1000- signaling resistance.
Use a 26-gauge loop. We then have 1000 / 83.5 = 11.97 kilofeet
• Also, resistance of the line will depend on temperature. For open-wire
lines, attenuation may vary by ±12% between winter and summer
conditions.
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. Increasing conductor diameter,
2. Using amplifiers and/or range extenders (A range extender is a device that
increases battery voltage on a loop).
3. Using inductive loading.
• Inductive loading reduce transmission loss on subscriber loops. Loading a
particular voice-pair loop consists of inserting 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. Loaded
cables are coded according to the spacing of the load coils.
Loading
Loaded cables are typically designated as 19H-44, 24-B-88, and so forth. The first number
indicates the wire gauge (AWG); the letter is
taken from Table 2.5 and is indicative of the
spacing, and the third set of digits is the
inductance of the coil in millihenries (mH).
For 19-H-66 cable the
attenuation/Km is 0.29 dB.
Thus, for our 6-dB loop
loss limit, we have
6.0/0.29, limiting the loop
length to 21 km.
Summary of Limiting Conditions
• the physical size of an exchange serving area is limited by factors of
economy involving signaling and transmission:
• Signaling limitations are a function of the type of exchange and the
diameter of the subscriber pairs and their conductivity.
• transmission is influenced by pair characteristics.
• Both limiting factors can be extended, 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.
SIZE OF AN EXCHANGE AREA BASED ON
NUMBER OF SUBSCRIBERS
• The size of an exchange area (also called a serving area) obviously will
depend largely on subscriber density and distribution. Subscriber traffic
is another factor to be considered.
• Exchange sizes are often in units of 10,000 lines. This is important in
telephone numbering.
• If statistics on subscriber traffic intensity are not available the table below
can be used.
PABX: Private Automatic Branch Exchange.
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. Solution : hexagonal serving areas.
hexagonal serving areas
EXCHANGE LOCATION
• A fairly simple, straightforward method for determination of the theoretical
optimum exchange location is by determining the center of subscriber
density in much the same way the center of gravity would be calculated. In
fact, other publications call it the center-of gravity method.
• Now that the ideal location is known, where will the real optimum location
be? This will depend considerably on secondary parameters such as
availability of buildings and land; existing and potential cable or feeder
runs; the so-called trunk pull; and layout of streets, roads, and highways.
EXCHANGE LOCATION
Types of practical exchanges
The ratio technique
• The ratio technique may be described using the following example. Given
an existing exchange A and a new exchange B that will be installed on a
cable route from A, let’s assume that A is a DMS-100 with 1900 Ω
resistance and B is a 5ESS extended to 2000 Ω resistance capability. The
maximum distance along the cable route from A to B can be calculated by
equating the distance to the sum of the resistances of (A-300 Ω ) and (B300 Ω) (or 1700 + 1600 Ω or 3300 Ω ). What we are doing here is
subtracting 300 from each value for the telephone subset terminating each
cable end.
• Use 26-gauge cable in this case. From Table 2.4, no loading, the resistance
can be equated to 270 Ω/km. Use the sum of the net resistance limits or
3300 Ω and divide by 270. Thus the maximum permissible distance from
exchange A to B is 12.2 km (3300/270 = 12.2 km). Let the distance from
exchange A be DA:
DA = 1700 × 12.2/(1700 + 1600) ≈ 6.28 km
The ratio technique
• The total distance from exchange B will be the difference or 12.2 − 6.28 =
5.92 km. The total distance from B to the boundary line will be 5.92 km;
the total distance from A to the boundary line will be 6.28 km. These
distances might be apportioned as shown in Figure below.
Tandem routing
DIMENSIONING OF TRUNKS
• A primary effort for the system engineer in the design of the local trunk
network is the dimensioning of the trunks of that network. Which is the
economic optimum number of trunk circuits between exchanges X and Y.
• the trunk traffic intensities used in the design of trunk routes should allow
for growth—that is, the increase of traffic from the present with the passage
of time. This increase is attributable to several factors:
(1) the increase in the number of telephones in the area that generate traffic
(2) the probable increase in telephone usage
(3) the possibility of a change in character of the area in question, such as
from rural to suburban or from residential to commercial.
Suppose we use a sample local area with five exchanges: A, B, C, D, and
E. For an 8-year forecast period we could possibly come up with the
traffic matrix like that shown in the following Table.
DIMENSIONING OF TRUNKS
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Thus from the traffic matrix in the direction of exchange C to exchange B there is a
traffic intensity of 11 erlangs. Applying the 11 erlangs, we see that 23 circuits would
be required. If the traffic intensity were 13 erlangs, 26 trunks would be required.
These circuit figures suppose a grade of service of p = 0.001. For a grade of service
of p = 0.01, 19 and 22 circuits would be required, respectively.
COMMUNITY OF INTEREST
• The community-of-interest factor K is defined as follows:
• This is the expected proportion of all A traffic that is directed to B.
• The K factor is a useful reference for the installation of a new exchange
where the serving area of the new exchange will be cut out from serving
areas of other exchanges. The community-of-interest factors may then be
taken from traffic data from the old exchanges, and the values averaged and
then applied to the new exchange.