GPON Optical Signal Levels and Singtel BRAS Topology
If you’ve ever logged into your Singtel ONT (typically ZTE F660/F620 or Nokia ONT) and seen a row of numbers under “Optical Signal Information” — RX power around -19 dBm, TX power around +3 dBm — you probably wondered: is that good? What do these numbers actually mean?
This post maps the full path an optical signal takes from your home to the internet, with the signal levels (dBm) at every hop, and what they tell you about your connection health.
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The Full Path
Where the Light Levels Matter
The optical signal only exists in the Passive Optical Network (PON) segment — ONT to Splitter to OLT. Everything after the OLT is electrical (Ethernet).
Practical signal level guide
| RX Power (dBm) | Status |
|---|---|
| -15 to -22 | Green / Ideal |
| -22 to -25 | Yellow / Acceptable |
| -25 to -28 | Yellow / Marginal (inspect) |
| Below -28 | Red / Fail |
| Above -8 | Red / Saturated (add attenuator) |
Singtel typically aims for -18 to -22 dBm RX at the ONT for GPON B+ optics. If you see -25 or below, your fibre termination point (FTP) may have a dirty connector or excessive bend loss.
Singtel BRAS Topology
After the OLT terminates the GPON signal, traffic enters the Ethernet/IP domain. Here’s how Singtel’s network is structured:
VLAN architecture (QinQ)
Reading Your ONT’s Optical Levels
ZTE F660 / F620
- Browse to
http://192.168.1.1 - Login (default:
admin/adminor on sticker on ONT) - Go to Status > ONT Info > Optical Signal Information
You’ll see:
RX Power: -19.2 dBm
TX Power: +3.5 dBm
Temperature: 42°C
Voltage: 3.31 V
Bias Current: 12.5 mANokia ONR (XGS-PON for 3Gbps / 10Gbps plans)
- Browse to
http://192.168.1.1 - Login (credentials on sticker)
- Go to Diagnostics > Optical Module Info
What to look for
- RX Power should be between -18 and -22 dBm for a healthy connection
- TX Power should be between +0.5 and +5 dBm
- If RX drops below -25 dBm, check:
- Dust cap on the fibre connector at the FTP (wall point)
- Fibre bend radius (sharp kinks cause loss)
- Dirty patch cord end at the ONT (use a one-click cleaner)
- If RX rises above -8 dBm, the signal is saturating the receiver — you need an optical attenuator
Quick Reference: GPON Optical Classes
| Class | OLT Tx | OLT Rx Sens. | ONT Tx | ONT Rx Sens. | Max Budget |
|---|---|---|---|---|---|
| B+ | +1.5 to +5 dBm | -28 dBm | +0.5 to +5 dBm | -28 dBm | 28 dB |
| C+ | +3 to +7 dBm | -32 dBm | +0.5 to +5 dBm | -32 dBm | 32 dB |
| C++ | +4.5 to +10 dBm | -33 dBm | +0.5 to +5 dBm | -33 dBm | 33 dB |
Singtel’s 1Gbps / 2Gbps plans primarily use GPON B+ optics. XGS-PON (3Gbps and 10Gbps plans) uses N1 / N2 class optics with a different wavelength (1270 nm upstream, 1577 nm downstream).
Reference: Typical Loss Budget Calculation
For a Singtel GPON connection with B+ optics:
| Component | Loss |
|---|---|
| 5 km fibre at 1490 nm (0.3 dB/km) | -1.5 dB |
| 1:32 splitter | -17 dB |
| Connectors (4 × 0.5 dB) | -2 dB |
| Splices (3 × 0.1 dB) | -0.3 dB |
| Safety margin | -3 dB |
| Total loss | -23.8 dB |
With an OLT transmitting at +3 dBm, the ONT receives: +3 - 23.8 = -20.8 dBm — well within the B+ limit of -28 dBm.
GPON signal levels follow ITU-T G.984.2. Singtel network topology based on public documentation, job listings, and ZTE/Nokia OLT deployment info. Singtel currently pilots 50G XGS-PON (January 2026) while maintaining GPON B+ for legacy plans and XGS-PON for 3Gbps/10Gbps enhanced plans.