Build a link budget and decide go or no-go

applied · 40 min · Objective 1.5

Task

Calculate whether a proposed fibre run will actually work, using the loss budget method: transmit power, minus every source of loss along the path, against the receiver's sensitivity. This is the calculation that decides whether a link is a purchase order or a support ticket.

Steps

  1. Take the link: 1000BASE-LX single-mode, 8 km, with two patch panels at each end and one fusion splice in the middle.
  2. Write down the budget inputs. Typical 1000BASE-LX: transmit power minus 9.5 dBm minimum, receiver sensitivity minus 20 dBm. That gives you 10.5 dB to spend.
  3. Count the losses. Single-mode fibre attenuates about 0.4 dB/km at 1310 nm. Each mated connector pair costs about 0.75 dB, and you have four. A fusion splice costs about 0.1 dB.
  4. Total the losses, subtract from the budget, and state the margin. Anything under about 3 dB of margin is a link that works on day one and fails after somebody cleans a connector badly.
  5. Re-run it for 20 km and state what you would change. Then take the optic you would change to -- a typical 1000BASE-ZX, rated to about 70 km, which transmits up to +5 dBm -- and run it over a 50 m link with the same four connector pairs. Receivers overload above about -3 dBm: work out the power that arrives and what you must add to the link. Do the same sum for the LX optic, whose transmit power tops out at -3 dBm, and note why it cannot overload a receiver even back to back.

Verify

python3 -c "
tx, rx = -9.5, -20.0
budget = tx - rx
for km, conns, splices, label in ((8,4,1,'8 km as specified'), (20,4,1,'20 km')):
    loss = km*0.4 + conns*0.75 + splices*0.1
    margin = budget - loss
    verdict = 'OK' if margin >= 3 else 'MARGINAL or FAIL'
    print(label, 'loss', round(loss,2), 'dB   margin', round(margin,2), 'dB  ', verdict)
loss = 0.05*0.4 + 4*0.75
for optic, tx_max in (('LX', -3.0), ('ZX', 5.0)):
    arrives = tx_max - loss
    over = arrives - -3.0
    print('50 m with', optic, 'arrives at up to', round(arrives,2), 'dBm  ',
          'OVERLOAD: add at least %.0f dB of attenuation' % (over + 0.5) if over > 0 else 'within the receiver range')
"

The 8 km link comes out at about 4.2 dB of margin — above the 3 dB floor, with little to spare. The 20 km link fails, and the answer is a longer-reach optic, not more patching. At 50 m the LX optic is fine; the ZX optic you would fit for distance delivers about +2 dBm into a receiver that overloads above -3 dBm, and the fix is an in-line attenuator of at least 5 dB, not a longer fibre.

Notes

That last case is the one field engineers actually hit: a long-reach optic on a short link overdrives the receiver. The symptom is errors rather than a dead link, which sends people hunting for a cable fault that does not exist. The fix is a fixed attenuator, and lesson 40's signal-strength readings are how you spot it — received power above the maximum rather than below the minimum.

Real installers use a certifier's measured loss rather than a table of typicals. The table is how you decide what to buy; the certifier is how you prove what you installed.

This is an independent study companion for CompTIA Network+ and is not produced by or endorsed by CompTIA.