Telephone Systems
Telephone Systems
This domain tests the telephone set, local loop and call signalling in a PSTN or PBX environment. Expect questions on battery feed, hook states, dialling, ringing, hybrid circuits and basic fault symptoms.
Core rules
- The exchange battery is typically −48 V DC; the local loop carries DC current when the handset is off-hook. On-hook means no loop current.
- Ringing is a high AC voltage, often around 75–90 V RMS at 20–25 Hz, superimposed on the line; it is not the same as battery voltage.
- Pulse dialling sends interrupted loop current; DTMF sends two simultaneous audio tones, one low-group and one high-group, so each key has a unique pair.
- A hybrid converts two-wire local loop to four-wire transmit and receive paths; sidetone lets the speaker hear their own voice at reduced level.
- A PBX provides internal switching and trunk access; the PSTN provides public switching and numbering.
Worked example
A loop has 48 V battery and 2000 Ω total resistance including the telephone. Off-hook current I = 48/2000 = 24 mA. Power dissipated in the loop is 48 × 0.024 = 1.15 W. If the loop resistance rises to 3000 Ω, current falls to 16 mA, which may be too low for a satisfactory speech path. This shows why long loops and poor joints cause weak transmission.
Traps that cost marks
Calling the ringing voltage DC; confusing on-hook with off-hook; assuming DTMF uses one frequency; forgetting that a PBX extension may not have direct outside line access; and blaming the handset when the fault is in the line cord or jack.