1270nm vs 1310nm

1270 nm and 1310 nm are the two upstream wavelengths used in passive optical networks. 1270 nm carries upstream traffic in XGS-PON, the 10 Gbps symmetrical PON generation. 1310 nm carries upstream traffic in GPON, the 2.5 Gbps downstream / 1.25 Gbps upstream generation that has been the workhorse of FTTH for nearly two decades.

Both wavelengths are in the O-band and are transmitted from the Optical Network Terminal (ONT) at the subscriber premises to the Optical Line Terminal (OLT) at the central office. They travel in the opposite direction from the downstream wavelengths—1490 nm for GPON and 1577 nm for XGS-PON—and are separated by wavelength division multiplexing at both ends of the link.

The differences between 1270 nm and 1310 nm affect fiber attenuation, chromatic dispersion, laser technology, burst-mode receiver design, power budget, and coexistence planning. These differences explain why XGS-PON selected 1270 nm instead of reusing the 1310 nm upstream of GPON, and how operators manage the coexistence of GPON and XGS-PON on the same fiber plant.

1. Wavelength Fundamentals

1270 nm and 1310 nm are both in the O-band, the original wavelength band used for single-mode fiber communications. 1310 nm is at the zero-dispersion wavelength of standard single-mode fiber. 1270 nm is on the shorter-wavelength side of the zero-dispersion point, where chromatic dispersion is negative but relatively low.

2. PON Generation and Upstream Role

1270 nm and 1310 nm serve the same function—upstream transmission from the ONT to the OLT—but they belong to different PON generations and support different data rates.

2.1 1310 nm in GPON

GPON uses 1310 nm for upstream transmission at 1.25 Gbps. Each ONT transmits at 1310 nm in burst mode, and the OLT schedules upstream transmission using time-division multiple access (TDMA). The OLT receiver must handle bursts from different ONTs at different power levels and different arrival times, which requires a burst-mode receiver with fast threshold adjustment and clock recovery.

The 1310 nm wavelength was chosen for GPON upstream because it has near-zero chromatic dispersion, which simplifies burst-mode receiver design, and because low-cost uncooled DFB and DML lasers are available at this wavelength.

2.2 1270 nm in XGS-PON

XGS-PON uses 1270 nm for upstream transmission at 10 Gbps. Like GPON, XGS-PON uses TDMA for upstream scheduling, and the OLT receiver must handle bursts from multiple ONTs. The higher data rate requires a higher-bandwidth laser and a faster burst-mode receiver.

The 1270 nm wavelength was chosen for XGS-PON upstream to coexist with GPON's 1310 nm upstream. By using a different wavelength, XGS-PON can share the same fiber plant with GPON, and a coexistence element can combine and separate the two upstream wavelengths.

3. Attenuation and Dispersion

Attenuation and dispersion determine the reach and power budget of the upstream direction.

3.1 Attenuation

Standard single-mode fiber attenuation is approximately 0.40 dB/km at 1270 nm and approximately 0.35 dB/km at 1310 nm. The higher attenuation at 1270 nm is because the wavelength is further from the 1550 nm minimum-loss window and closer to the water-peak absorption region around 1383 nm. The difference is modest—about 1 dB over a 20 km link—but it must be accounted for in the power budget.

3.2 Chromatic Dispersion

Chromatic dispersion is near zero at 1310 nm and negative but low in magnitude at 1270 nm. The low dispersion at both wavelengths simplifies burst-mode receiver design, because the pulses do not spread significantly as they travel through the fiber. The negative dispersion at 1270 nm is small enough that it does not require compensation in PON upstream links.

4. Laser Sources and Transmitter Design

The laser sources differ between the two wavelengths because of the different data rates and the different requirements of GPON and XGS-PON upstream transmitters.

4.1 1310 nm Laser Sources

GPON ONT transmitters at 1310 nm use low-cost uncooled DFB or directly modulated lasers (DMLs) operating at 1.25 Gbps. These lasers are mature, high-volume products with competitive pricing. The lower data rate allows the use of simpler and less expensive laser designs.

4.2 1270 nm Laser Sources

XGS-PON ONT transmitters at 1270 nm use DFB or DML lasers operating at 10 Gbps. The higher data rate requires a laser with higher modulation bandwidth and lower chirp. The 1270 nm wavelength is close to the water-peak absorption region, which requires careful design to maintain output power and wavelength stability over temperature.

5. Burst-Mode Operation and Receiver Design

Both GPON and XGS-PON use burst-mode upstream transmission, in which each ONT transmits in its assigned time slots. The OLT receiver must recover the clock and adjust the decision threshold for each burst, because the bursts arrive from different ONTs at different power levels and with different phase alignment.

5.1 Burst-Mode at 1.25 Gbps

At 1.25 Gbps, the burst-mode receiver has a relatively generous timing budget. The preamble and delimiter at the start of each burst provide enough time for the receiver to lock onto the incoming signal and set the threshold. The receiver sensitivity is typically -28 dBm to -8 dBm, with a dynamic range that accommodates the different power levels from different ONTs.

5.2 Burst-Mode at 10 Gbps

At 10 Gbps, the burst-mode receiver has a much tighter timing budget. The preamble and delimiter are shorter in time, and the receiver must lock onto the incoming signal and set the threshold within a few nanoseconds. This requires a faster and more complex burst-mode receiver, which increases cost and power consumption. The receiver sensitivity is similar to GPON, but the dynamic range and the tolerance to timing errors are tighter.

6. Coexistence and Wavelength Plan

1270 nm and 1310 nm are designed to coexist on the same fiber plant. This coexistence is the foundation of the migration path from GPON to XGS-PON.

6.1 GPON Wavelength Plan

The GPON wavelength plan consists of 1490 nm downstream, 1310 nm upstream, and 1550 nm for RF video overlay. These three wavelengths are separated by wavelength division multiplexers at the OLT and ONT.

6.2 XGS-PON Wavelength Plan

The XGS-PON wavelength plan consists of 1577 nm downstream and 1270 nm upstream. These wavelengths are chosen to avoid overlap with the GPON wavelengths, allowing GPON and XGS-PON to coexist on the same fiber.

6.3 Coexistence Element

A coexistence element (CE) is a passive optical device that combines the GPON and XGS-PON wavelengths onto a single fiber and separates them at the subscriber end. The CE uses thin-film filters or arrayed waveguide gratings to route each wavelength to the appropriate PON interface. This allows an operator to deploy XGS-PON alongside GPON and migrate subscribers gradually.

7. Power Budget and Reach

The power budget of a PON is determined by the ONT transmitter output power, the splitter loss, the fiber attenuation, the connector losses, and the OLT receiver sensitivity. The two upstream wavelengths have different attenuation and dispersion characteristics, but the overall power budget is designed to support the same reach and split ratio for both GPON and XGS-PON.

For GPON with a 1:32 split, the power budget is typically 28 dB, supporting a reach of up to 20 km. XGS-PON has a slightly higher power budget of 29 to 33 dB, partly because the 1270 nm wavelength has higher fiber attenuation and partly because the higher data rate requires more transmitter power and receiver sensitivity. With a 1:32 split, XGS-PON can reach 20 km; with a 1:64 split, it can also reach 20 km; with a 1:128 split, the reach is typically 10 to 15 km.

8. Applications

1270 nm and 1310 nm serve the upstream direction of XGS-PON and GPON respectively. The application determines which wavelength is used.

8.1 1310 nm Applications

• GPON upstream: ONT to OLT at 1.25 Gbps.

• Residential broadband: FTTH services using GPON.

• Triple-play services: Data, voice, and video over GPON.

• Business broadband: GPON-based business services.

• Mobile backhaul: GPON-based backhaul for small cells.

8.2 1270 nm Applications

• XGS-PON upstream: ONT to OLT at 10 Gbps.

• 10G residential broadband: FTTH services using XGS-PON.

• Business services: Symmetrical 10G for enterprises.

• Mobile backhaul and fronthaul: XGS-PON for 5G transport.

• Wholesale access: XGS-PON for open access networks.

9. Comparison Summary

10. Selection Framework

11. Emerging Trends

11.1 50G-PON Wavelength Plan

50G-PON defines several wavelength options for upstream and downstream. One option uses 1310 nm for upstream, maintaining compatibility with the GPON wavelength plan. Another option uses 1270 nm or other wavelengths. The 50G-PON standard supports coexistence with GPON and XGS-PON on the same fiber plant, and the coexistence elements will need to handle the additional wavelengths.

11.2 Higher-Speed XGS-PON

XGS-PON is being deployed at scale worldwide, and the 1270 nm upstream wavelength is becoming the mainstream for new FTTH builds. As XGS-PON volumes increase, the cost of 1270 nm lasers and burst-mode receivers is declining, narrowing the cost gap with GPON.

11.3 Coexistence Element Integration

Coexistence elements that combine GPON, XGS-PON, and RF video onto the same fiber are becoming more integrated and lower cost. These elements use thin-film filters or arrayed waveguide gratings to separate and combine the wavelengths, allowing multiple PON generations to share the same optical distribution network.

11.4 Wavelength Plans for Future PON

Future PON generations beyond 50G-PON will require new wavelength plans. The ITU-T is studying higher-speed PON options that may use different wavelengths in the O-band, C-band, or L-band. The 1270 nm and 1310 nm wavelengths will remain in use for XGS-PON and GPON respectively, and coexistence with future generations will be a key requirement.

12. Summary

1270 nm and 1310 nm are the upstream wavelengths of XGS-PON and GPON respectively. 1310 nm carries 1.25 Gbps upstream from the ONT to the OLT in GPON, paired with 1490 nm downstream. 1270 nm carries 10 Gbps upstream from the ONT to the OLT in XGS-PON, paired with 1577 nm downstream.

The two wavelengths are separated by wavelength division multiplexing, allowing GPON and XGS-PON to coexist on the same fiber. This coexistence is the foundation of the migration path from GPON to XGS-PON, enabling operators to deploy XGS-PON alongside existing GPON equipment and migrate subscribers gradually without replacing the optical distribution network.

1310 nm has lower fiber attenuation and near-zero chromatic dispersion. 1270 nm has slightly higher attenuation and low negative dispersion. The lower attenuation at 1310 nm provides a small advantage for GPON upstream, while the higher data rate of XGS-PON requires a higher-performance laser and burst-mode receiver at 1270 nm.

For new FTTH deployments, 1270 nm (XGS-PON) is the mainstream choice, offering symmetrical 10 Gbps upstream and downstream. For existing GPON deployments, 1310 nm remains in use, and the migration to 1270 nm XGS-PON is enabled by coexistence elements and dual-mode ONTs.

13. Q&A