Why air-blown fibre beats pulling in dense urban deployments.
The received wisdom says pulling is faster. In our experience across retrofit urban FTTH in Germany and Poland, it is not.
The received wisdom in telecoms training material says cable pulling is the faster method for shorter runs. In our experience deploying FTTH across retrofit urban environments in DE and PL, the wisdom was wrong often enough that we now default to air-blown fibre (ABF) on projects that would have been pulling jobs five years ago.
This note is about why, and when.
The scenario
Most of our FTTH work is in buildings that already have conduit or microduct installed — sometimes during the initial build, more often retrofitted during a previous "we'll put fibre in later" upgrade cycle. The ducts are there. What's not there: straight runs, clean access, consistent diameters, or accurate as-built drawings.
In greenfield deployment — new build, straight conduit, good access — pulling works fine. Our guidance applies to brownfield retrofit in dense urban multi-tenant buildings. This is most of the work.
Why pulling struggles here
The three failures we see most often:
- Friction at bends. Every 90° bend in a pull run adds friction proportional to the fibre's tension. In retrofit buildings, you don't get to choose your bends — you inherit them. Three sharp bends in a 40-metre run can put you over the fibre's max tension rating before you're halfway in.
- Hidden obstructions. Ducts accumulate: construction debris, previous cable stubs, water, cured expanding foam. You discover these when the pulling head hits one. On a pull you then typically have to rod the duct, which means opening access points, which means scheduling with the building manager, which means a half-day delay per obstruction.
- Cumulative damage on retries. A fibre that has been pulled, stopped, reversed, and re-pulled is a fibre with microbends. It'll work today. It may not work in two years when thermal cycling catches up with it.
Why ABF works better in these conditions
Air-blown fibre deposits the cable as a stream of compressed air carries it through the microduct. Key differences from pulling:
- The fibre is supported along its entire length by the air stream. No point loads, no tension peaks at bends.
- Obstructions are detected early — the blow simply stops. You clear the obstruction, resume. No retries required, no cumulative damage.
- Runs up to 1 kilometre in a single blow are routine. Pulling maxes out at a few hundred metres before tension forces you to open a mid-run access.
- Significantly less micro-bending. The fibre arrives in its relaxed state.
Where ABF doesn't win
Two cases where pulling is still the right choice:
- Runs under 20 metres with clear geometry. ABF needs setup time — pressurising the duct, priming the feeder, verifying seal integrity. For short runs, the setup dominates the job.
- Damaged or leaky microducts. ABF requires a sealed duct to build pressure. If the duct has been compromised — common after ten years underground — you can't blow until you've repaired. Sometimes repair isn't economic and pulling is the pragmatic fallback.
On our last six deployments across Warsaw and Leipzig, ABF completed 87% of runs on first attempt. Pulling on comparable sites completes 54% on first attempt. The difference is mostly in the "we hit something in the duct" scenario — where ABF stops, you clear, you resume; pulling stops, you damage, you plan a return visit.
Equipment and craft
The gap between a good ABF install and a bad one isn't the blow machine — it's the joint preparation and the duct seal. A standard worth aiming for:
- Clean and inspect the microduct for at least 2 metres from each access before priming.
- Use mechanical seals rated for the working pressure of your specific duct; not all are equal.
- Log blow pressure, blow speed, and stop points for every run. If a run stops, you need to know where.
- OTDR test every completed run. Every run. Splicing loss under 0.05 dB per splice is a reasonable target; any higher warrants a redo.
None of this is secret. What's striking is how often we arrive at a site where previous work has been done without any of it.
Summary
For retrofit urban FTTH — where the ducts are where they are and the geometry is what it is — air-blown fibre finishes faster, damages less, and leaves behind a network that ages better. Pulling still has a place, but defaulting to it out of habit costs money.