Role of exchange and rotary dial operation

Role and need of an exchange

Without an exchange, $n$ telephones would need $n(n-1)/2$ separate lines (1000 phones → 499,500 lines). An exchange (central office or branch office) connects all subscriber lines to one point and joins any two of them only when needed.

Roles/functions:

• Detect a call request (off-hook) and give dial tone.
• Receive and store the dialled digits.
• Find the called line or route to another exchange through trunks.
• Test whether the called line is busy; ring the called party and give ringback/busy tone.
• Set up the speech path, supervise the call, charge for it, and release it at the end.
• Provide shared use of costly trunks and services (operator, emergency, supplementary services).
• A branch office/PBX does the same for an organisation's internal phones, sharing a few external lines.

Rotary dial operation

  finger wheel with 10 holes (1..9, 0)
  finger stop ─┐
               v        Off-normal contacts
  Wind clockwise  ─────> spring is wound
  Release ─────> wheel returns at fixed speed
                (governor) and cam opens the
                pulsing contact n times
  Loop current:
  ──┐  ┌──┐  ┌──┐  ┌──────   (digit 3)
    └──┘  └──┘  └──┘
    break make

1. The subscriber puts a finger in the hole of a digit and turns the wheel clockwise to the finger stop, winding a spring.
2. On release, the spring returns the wheel; a centrifugal governor keeps the return speed constant (10 pulses per second).
3. A cam opens and closes the impulsing contacts in the subscriber loop, giving one break (loop open) per digit unit: digit 1 = 1 break, digit 9 = 9 breaks, digit 0 = 10 breaks. Break:make ratio is about 2:1 (66.7 ms : 33.3 ms).
4. Off-normal contacts short the receiver during dialling to avoid clicks and keep the loop steady.
5. The time taken to rotate the dial for the next digit gives the inter-digit pause (at least ~200–300 ms) that the exchange uses to separate digits.

Drawbacks

• Slow: digit 0 takes about 1 s plus rotation time.
• Mechanical wear of governor and contacts changes pulse speed/ratio, causing wrong numbers.
• Only 10 signals; no * and # for services.
• Pulses cannot pass end-to-end over a call (no IVR use).
• Pulse distortion on long lines; needs DC loop signalling, not suitable for electronic systems.
• Uncomfortable for users; more misdialling.