Subscriber loop and cable hierarchy

Subscriber loop

The subscriber loop is the dedicated pair of wires (local loop) that connects a subscriber's telephone to the local exchange, forming a closed circuit through the telephone when it is off-hook.

Subscriber loop system and cable hierarchy

 Exchange MDF
      │
      │ Feeder / main cable (e.g. 1200 pairs)
      v
 ┌─────────────┐
 │Cross-connect│ (cabinet / pillar)
 │ point (PCP) │
 └──┬───────┬──┘
    │       │  Distribution cables (e.g. 100 pairs)
    v       v
  [DP]     [DP]   Distribution points (10-20 pairs)
   │ │      │
   Drop wires (1-2 pairs)
   │ │      │
  Tel Tel  Tel   Subscribers

1. Main (feeder) cable: runs from the MDF of the exchange; large pair count; laid in ducts.
2. Cross-connection point (cabinet/pillar): jumpers connect any feeder pair to any distribution pair, giving flexibility and better use of pairs.
3. Distribution (branch) cables: smaller cables running along streets, underground or aerial.
4. Distribution point (DP): pole-mounted or wall-mounted terminal block.
5. Drop/service wire: individual pair from the DP to the subscriber's socket.

Loop limits: loop resistance is kept below about 1200–1800 Ω and loss below about 7–8 dB at 800 Hz, so that −48 V feeding, ringing and dial signalling work; thicker conductors or loading coils are used on long loops. The loop carries DC feed, ringing, dialling and two-way speech on the same two wires.