How to Test FTTH Signal Levels with a PON Power Meter (GPON Guide)
When a new fibre-to-the-home (FTTH) connection will not come up, or a customer complains that the internet drops every evening, the first question an installer asks is simple: how much light is reaching the ONT? The answer comes from a PON power meter. This guide explains what a PON power meter measures, which signal levels are good for GPON, how to test step by step, and how to troubleshoot readings that are too low or too high.
A quick refresher: how a PON works
A passive optical network (PON) carries signals from one port on an OLT (optical line terminal) at the exchange or hub, through one or more passive splitters, to up to 64 or more customer ONTs (optical network terminals, often called ONUs). Everything in between is passive glass: feeder cable, distribution cable, splitter boxes and the drop cable to the home. Because nothing in the field is powered, the only thing that tells you whether the network is healthy is the optical power at each point.
GPON (ITU-T G.984) sends three signals on the same single fibre, separated by wavelength:
| Signal | Wavelength | Direction | How it behaves |
|---|---|---|---|
| GPON data downstream | 1490 nm | OLT to ONT | Continuous, shared by all ONTs on the splitter |
| RF video overlay (optional) | 1550 nm | OLT side to ONT | Continuous, only on networks that carry cable TV over fibre |
| GPON data upstream | 1310 nm | ONT to OLT | Short bursts: each ONT transmits only in its own time slot |
| XGS-PON downstream | 1577 nm | OLT to ONT | Continuous, can share the same fibre as GPON |
| XGS-PON upstream | 1270 nm | ONT to OLT | Bursts, like GPON upstream |
Why an ordinary power meter is not enough
A standard optical power meter, such as the EXFO FPM-600, is ideal for loss testing with a light source. On a live PON it has two limits. First, its detector sees all wavelengths at once, so on a network with video it adds the 1490 nm and 1550 nm signals together and gives one misleading number. Second, it cannot read the burst-mode upstream signal from the ONT correctly, because the ONT only transmits for tiny slices of time.
A PON power meter solves both problems. It has wavelength filters for each PON signal and a burst-capture circuit for the upstream. In pass-through mode it sits between the drop fibre and the ONT, so the ONT keeps working while the meter reads downstream and upstream power at the same time. That is how you prove the link in both directions, not just one.
What signal levels are good for GPON?
GPON equipment is sold in optical classes. The class sets how strong the OLT transmitter is and how weak a signal the ONT can still receive. The two classes you will meet most often are B+ and C+ (ITU-T G.984.2):
| Parameter | Class B+ | Class C+ |
|---|---|---|
| OLT transmit power (1490 nm) | +1.5 to +5 dBm | +3 to +7 dBm |
| ONT receiver sensitivity (weakest usable signal) | −27 dBm | −30 dBm |
| ONT receiver overload (strongest safe signal) | −8 dBm | −8 dBm |
| ONT transmit power (1310 nm) | +0.5 to +5 dBm | +0.5 to +5 dBm |
| Allowed loss between OLT and ONT | 13 to 28 dB | 17 to 32 dB |
In practice, installers use a simple rule for the downstream (1490 nm) reading at the ONT:
- About −15 to −24 dBm: healthy. The link has margin for ageing, extra splices and connector wear.
- About −25 to −27 dBm (B+): the connection will probably work, but there is little margin. Find and fix the extra loss before you leave the site.
- Weaker than −27 dBm (B+) or −30 dBm (C+): fail. Expect dropouts, errors or no sync at all.
- Stronger than −8 dBm: too much light. This is rare on a split network, but it can damage or saturate the ONT receiver.
Always check the actual figures on your OLT and ONT datasheets and your operator's acceptance limits, because some networks set stricter targets than the standard.
Worked example: what reading should you expect?
You can estimate the expected reading before you go to site. Take a class B+ OLT port transmitting +3 dBm, a single 1:32 splitter, 3 km of fibre and a few joints:
| Item | Typical loss |
|---|---|
| 1:32 PLC splitter | About 17 dB |
| 3 km of single-mode fibre at 1490 nm (about 0.25 dB/km) | About 0.75 dB |
| 4 connector pairs at about 0.3 dB each | About 1.2 dB |
| 6 fusion splices at about 0.1 dB each | About 0.6 dB |
| Total link loss | About 19.6 dB |
Expected power at the ONT = +3 dBm − 19.6 dB = about −16.6 dBm. If your meter shows −24 dBm on this link, around 7 dB is missing, which points to a dirty connector, a sharp bend or a bad splice rather than a design problem. Our guide to calculating a fibre optic loss budget explains these values in more detail.
How to test an FTTH connection step by step
- Inspect and clean every connector first. Dirt on an SC/APC end face is the most common cause of low power. Use a probe such as the EXFO FIP-400B Wireless and follow our connector inspection and cleaning guide. Never look into a live fibre.
- Measure at the splitter or distribution box. Connect the meter to the customer's splitter output port and record the 1490 nm (and 1550 nm, if used) level. This tells you what the network delivers before the drop.
- Measure at the end of the drop cable. At the customer's premises, connect the drop to the meter's network port. The difference from step 2 is the loss of the drop cable and its joints, which should normally be only a dB or two.
- Test in pass-through mode with the ONT connected. Connect the drop to the meter's network port and the ONT to the meter's ONT port with a short SC/APC patch cord. Wait for the ONT to register with the OLT, then read downstream and upstream together. A good 1310 nm reading shows the ONT is transmitting and talking to the OLT.
- Compare with your thresholds and save the result. Most PON meters give a pass, warning or fail verdict against limits you set, and store the readings for the job report.
- Prove the service. Good light does not guarantee good internet. A tester such as the EXFO EX1 runs a full-rate speed test, emulates a GPON ONT and checks Wi-Fi, so you can show the customer that the ordered speed is delivered.
Troubleshooting common readings
| What you see | Likely cause | What to do |
|---|---|---|
| Downstream low at the ONT, but good at the splitter | Problem in the drop: dirty or damaged connector, tight bend, crushed cable or poor splice | Clean and inspect connectors, check bends at clamps and entry points, re-splice if needed |
| Downstream low at the splitter port too | Problem upstream of the drop: feeder or distribution cable, splitter or OLT port | Escalate to the network team; test the feeder section with an OTDR |
| Downstream good, no upstream (1310 nm) reading | ONT not transmitting: not registered on the OLT, wrong profile or faulty ONT | Check that the ONT serial number is authorised on the OLT, then try another ONT |
| Downstream stronger than −8 dBm | Too little loss, for example a very short link or a test without a splitter | Add an attenuator of the right value at the ONT |
| Levels go up and down when the cable is moved | Cracked fibre, loose connector or a bend that changes with movement | Locate the point with a visual fault locator or OTDR and repair it |
| Good light, but slow or unstable internet | Service, router or Wi-Fi issue, not the fibre | Run a speed test on Ethernet first, then check the Wi-Fi |
When a reading shows extra loss but you cannot see where it is, an OTDR shows the distance to each splice, bend and break. The EXFO MaxTester 715B is built for last-mile and FTTH work; see our OTDR testing guide for how to read a trace. For new networks, an optical fibre multimeter such as the EXFO Optical Explorer OX1 quickly checks for continuity and loss before the customer is connected.
Five habits that keep PON levels healthy
- Use bend-insensitive drop cable for the last metres into homes and flats. G.657 fibre, as used in our outdoor FTTH drop cable, tolerates tight corners far better than standard fibre.
- Keep the bend radius gentle at clamps, windows and wall boxes, and never pull a drop tight around a sharp edge.
- Cap every unused connector and adapter so dust cannot settle on the end face.
- Record the reading for every connection at handover. A baseline makes later faults much faster to find.
- Do not mix APC and UPC connectors. Most GPON networks use green SC/APC connectors; mating an APC with a blue UPC connector causes high loss and can damage both end faces.
In short: a PON power meter in pass-through mode reads the downstream and upstream signals at the same time without interrupting service. For class B+ GPON, aim for roughly −15 to −24 dBm at the ONT, treat anything weaker than −27 dBm as a fail, and always clean connectors before you blame the network.
Frequently asked questions
What is a good GPON ONT receive power?
For class B+ equipment, a downstream level of about −15 to −24 dBm is healthy. The ONT can still work down to about −27 dBm, but with little margin. Class C+ ONTs work down to about −30 dBm.
Can I use a normal optical power meter on a live PON?
You can get a rough downstream reading at 1490 nm with the ONT disconnected, but a normal meter cannot separate the 1490 nm and 1550 nm signals or measure the burst-mode upstream from the ONT. A PON power meter measures each signal separately while the ONT stays online.
Does a PON power meter work on XGS-PON?
Only if it is designed for it. XGS-PON uses 1577 nm downstream and 1270 nm upstream. Check that the meter lists XGS-PON support; the EXFO PPM-350C, for example, covers BPON, EPON and GPON.
Why is my ONT showing "LOS"?
LOS means loss of signal: the ONT is receiving no usable light. The usual causes are a disconnected or dirty connector, a broken drop cable, a very sharp bend, or a fault further back in the network. Measure at the end of the drop and at the splitter to find which section has lost the light.
How much loss does a fibre splitter add?
Typical PLC splitter losses are about 3.5 dB for 1:2, 7 dB for 1:4, 10.5 dB for 1:8, 14 dB for 1:16, 17 dB for 1:32 and 20.5 dB for 1:64. The exact value is on the splitter's test sheet.
Need FTTH test equipment or drop cable?
Litech supplies genuine EXFO test equipment in Pakistan, including the EXFO PPM-350C PON power meter, the EX1 FTTH tester and the FIP-400B inspection probe, together with G.657 FTTH drop cable, flat drop cable and fibre patch cords. Browse the full EXFO range or message us on WhatsApp +92 313 7776766 for a quotation. Delivery is free across Pakistan.
Sources: ITU-T G.984.2 and its amendments (GPON physical media dependent layer, classes B+ and C+); ITU-T G.9807.1 (XGS-PON); EXFO PPM-350C and EX1 product specifications.
