No internet in a new apartment: a short, moved pairs and the wrong details

The call

A tenant had just moved into a unit in a large apartment complex in Chiswick. The router was plugged in and there was no internet.

In a fibre to the building unit block, that usually means one thing. The service is live at the building’s main distribution frame (the MDF, the point in the basement where every line for the building arrives), and nobody has run the short wire, the jumper, that connects it through to the unit. Running a jumper, the everyday work of MDF jumpering, takes about fifteen minutes. That is what we expected to do.

The first test changed the job

We started in the unit, putting a tone on the unit’s cable so it could be followed back through the building. The tone showed a short circuit on the cable straight away, and there was a bridge tap inside the unit: an extra socket still connected to the line. This was not only a missing jumper. The building’s wiring had faults of its own.

Three problems on one line

1. 1

   A short circuit and a bridge tap in the unit's wiring

   The first pair of wires running to the unit was shorted, and an extra socket was still connected to the line inside the unit. A unit's cable has spare pairs, so we moved to the second pair, which was clear, and sent the tone down that to the frame on the unit's floor, the IDF.

2. 2

   Pairs that did not match between the frames

   From the floor frame the line should have gone to the same pair number on the main frame: pair 91 to pair 91. The tone came up on pair 73. At some point the pairs had been moved between the two frames and the records had not followed.

3. 3

   The wrong pair details

   The details the customer had been given for where the service sat on the frame were wrong, a string of numbers that matched nothing on it. It took half an hour on the phone, with the customer and the provider's second-level support, to get the right ones.

Follow the line, one problem at a time

1The unit's wiring 2The pairs don't match 3The wrong details 4Fixed

Swipe the path sideways to follow it to the main frame.

Your unit's cable Floor frame (IDF) Riser Main frame (MDF) Provider's service 1234 91 73 91 Short on pair 1 and a bridge tap; the tone goes down pair 2 Leaves on pair 91 Turns up on pair 73 The pair details given: no match on the frame Jumper in at the main frame

The first pair on the unit's cable was shorted, and an extra socket was still connected inside the unit, a bridge tap. The tone went down the second pair instead, to the floor frame.

The line left the floor frame on pair 91 and should have reached pair 91 on the main frame. The tone came up on pair 73.

The pair details the customer had been given matched nothing on the frame. It took half an hour on the phone to get the right ones.

The unit on the working pair, the path between the frames corrected, and the jumper run at the main frame from the provider's service. The service came up.

The same job, by the clock

Expected: about fifteen minutes

Took: two hours

The fix

With the right details in hand and every section of the path traced, the repair itself was straightforward.

1. The bridge tap was taken off, and the unit was moved onto the working pair, away from the short circuit.
2. The path between the floor frame and the main frame was corrected, so the line ran where it was meant to.
3. The jumper was run at the main frame from the provider’s service to the corrected pair.

The service came up. What had looked like fifteen minutes took two hours.

What this job shows

• Start with the simple explanation and test it. The jumper was missing, as expected. The first test showed that was not the whole story.
• Trace the whole path. In a building, a line passes through the unit’s cable, the floor frame, the riser and the main frame. A fault can sit in any of them, and here it sat in more than one. Tracing it end to end is how we find a fault on a building’s own cabling.
• Finding one fault can hide the next. Fixing the short circuit did not bring the service up. Neither would the jumper alone have.
• Records drift. Frames get worked on by many people over many years. What the paperwork says and what the wire does are not always the same.

If your provider says the service is live and your unit still has nothing, an internet technician can trace the line from your socket to the frame.

For how the frame and the jumper work, see what MDF jumpering is. For how this kind of connection is built, see what fibre to the building is. Another unit block job, with a different fault entirely: a jitter noise in a Rosehill frame room.