Subscriber loop or line plant

The subscriber loop (line plant or outside plant) is the network of cables and wires that connects each subscriber's telephone to the local exchange. It is usually a twisted copper pair per subscriber (two wires forming a "loop"), and it is the most expensive part of a local network because it is not shared.

Structure

 Exchange
 ┌──────────┐
 │ Switch ─ │ MDF
 └──────┬───┘
        │ Main (feeder) cable, 600-2400 pairs
        v
   ┌──────────┐
   │ Cabinet/ │  primary cross-connection
   │ pillar   │  point (flexibility)
   └──┬───┬───┘
      │   │ Distribution (branch) cables, 50-200 pairs
      v   v
     [DP] [DP]  distribution points (pole/wall box)
      │ │  │
     drop wires (1-2 pairs)
      │ │  │
     subscribers' premises

1. Main Distribution Frame (MDF) at the exchange – outside cables end here and are cross-connected to the exchange line equipment; protection devices fitted.
2. Feeder (main/primary) cable – large multi-pair cable, usually underground in ducts, from MDF to cross-connect points.
3. Cross-connection cabinet (pillar) – lets any feeder pair be jumpered to any distribution pair, giving flexibility when demand changes.
4. Distribution (secondary) cable – smaller cables, underground or aerial, running along streets.
5. Distribution point (DP) – a terminal box on a pole or wall serving a few buildings.
6. Drop wire / service wire – from DP to the subscriber, ending at the subscriber's terminal/socket.

Electrical features

• Loop resistance limited (about 1200–1800 Ω) so that DC signalling and −48 V feeding work; loading coils used on long analogue loops.
• Carries DC power, ringing (75 V, 25 Hz), dial signals and speech in both directions on two wires (hybrid at each end).
• Modern loops are being replaced by fibre (FTTC/FTTH) and wireless access.