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The subscriber loop:
1. Sketch a typical subscriber loop, showing the handset, dialer, switch hook, ringer,
Tip and Ring leads, the LO, the office battery connection etc.
2. What prevents the d.c current from flowing into the Ringer circuit. ___________
3. What limits the maximum usable distance of a subscriber loop. _______________
4. Draw a D.C equivalent circuit of a subscriber loop and calculate the voltage
across the telephone if the resistance of the wire in the loop is 600 ohms. Assume the
total D.C resistance of the LO battery supply is 600 ohms and the internal resistance
of the Telephone is 500 Ohms.
5. What is the typical a.c talking current? _________________
6. What is the typical d.c loop current? _____________________
7. The characteristics of #26 AWG Pulp telephone cable at 2kHz are as shown
R = 440 Ohms /km,
L = 0.995 mH/km,
C = 0.084F/km.
Draw a simplified series circuit indicating the total resistance and Inductive and
Capacitive reactances.
Hyder Khoja
Page 1
8. Sketch a hybrid and describe how it connects the transmitter and receiver to
2Wire line.
9. How does the telephone office know that a subscriber is ON-HOOK or OFFHOOK? __________________________________________________________
Describe two types of dialing. _______________________________________.
Calculate the time required to dial 416-675-6622 using
(a) Pulse Dialing. ___________________________________________________
(b) Touch tone Dialing: ______________________________________________
10. Calculate the number of trunks required to provide a dedicated connection
between 500 customers.
11. How many customers a LO can serve? ________________________________
12. List two call progress signals: ______________________________________
13. Name two supervisory signals: _____________________________________
14. What is the amplitude and frequency of the ringing signal: _______________
15. What is the difference between a switched line ( Dialed-up line) and a Leased
16. What constitutes an office address? ______________________________________
17. How many Local offices can exist in a an area.? ___________________________
18. What is the purpose of office hierarchy in North America?
19. What is blocking? ____________________________________________________
20. Can a Leased line be blocked?____________Explain? ______________________
21. Define a foreign Exchange line: _________________________________________
22. What is the Standard North American Voice grade Channel BW: ___________
23. Describe a trunk ____________________________________________________
24. What is tandem office ? _____________________________________________
Hyder Khoja
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25. What is the function of a signaling system?
26. What is the most commonly used signaling system in North America. _________.
27. Differentiate between a simplex, half duplex and full duplex transmission
system. Give one example of each.
28. Describe two major problems in Telephone system and explain how these
problems are tackled.
29. What is an acceptable S/N Ratio for voice communications? ___________ For
Video communications? ____________________
30. A meter calibrated to read directly in dBm across a 600 Ohm resistor gives a
reading of +6.5 dB across a 1200 Ohm resistor. What is the actual power in dBm?
( ANS: 3.5 dBm
31. Draw a diagram showing 7 layer OSI Model for Data communication
32. Define the functions of physical , Data Link and Network Layers
Physical: _______________________________________________________
Data Link:
Hyder Khoja
Page 3
33. What are the normal busy signal frequencies and duration?
34. What does the acronym PBX stands for? ___________________________ What
is its function. __________________________________________________________
35. What are the color codes for telephone wiring?
36. For the Telephone transmission system shown, calculate the output voltage and
express the input and out power in dBm
23dB gain
0.548 V
Hyder Khoja
cable loss 6dB
Page 4