The broadband industry is becoming highly competitive. Many small and medium ISPs are upgrading and replacing their original GPON OLT with XGSPON OLT to enhance user’s bandwidth and improve service experiences. However, many Isps are working in this way, they do not understand the technical principles, they often have concerns over FTTH networking compatibility, fiber link loss, and residential ONU terminal compatibility, and making it difficult to determine implementation plans.
This article will adopts a Q&A format to analyze the key technical questions arising from upgrading existing FTTH networks to XGSPON. It will analyze technical aspects such as the coexistence of old and new optical modems, matching OLT-side Combo optical modules, and planning upstream bandwidth for broadband services. It will analyze issues like the incompatibility of old access devices and wasteful costs in network renovation, providing practical upgrade references for small and medium-sized ISPs
Q: Currently we have all GPON devices. Do we need to replace all the fiber and splitters when upgrading to XGSPON OL?
A: The core principle of PON technology iteration is to maximize the use of existing in-home wiring and ODN networks. Every fiber and every splitter you have installed can be used for the next 5-10 years, preserving the value of your investments. The only thing required is to replace the OLT with a Combo device, allowing the simultaneous connection of the old GPON ONU and the new 10G XGSPON ONU under the same PON port. No need to rewire, and existing user services will not be interrupted.
Q: If the XGSPON OLT connects both GPON ONU and XGS ONU, will the total bandwidth be affected?
A: According to our understanding, the mainstream combo OLT chips physically integrate two independent MAC channels:
GPON Channel: Downstream 2.488Gbps / Upstream 1.244Gbps;
XGS-PON Channel: Downstream 9.953Gbps / Upstream 9.953Gbps.
Both channels are completely isolated using built-in WDM wave division technology, resulting in a total bandwidth: Downstream 12.5Gbps and Upstream 11.25Gbps. If connecting traditional GPON ONU, it will only occupy GPON channel bandwidth, and if connecting the 10G XGS-PON ONU will only occupy XGS-PON channel bandwidth, and they do not interfere with each other.
Q: Can the OLT of XGSPON work normally with both GPON ONU and XGS ONU connected, ensuring QOS?
A: It is necessary to confirm whether the purchased PON access product supports dual MAC hard isolation: that is, each port has independent GPON and XGSPON MACs, each running its own DBA scheduling algorithm. One is dedicated to the GPON protocol, and the other is dedicated to the XGS-PON protocol. The core processing chip has two sets of independent circuits, each with its own registers, caches, queues, and clock domains, and they do not share hardware resources.
| Comparison Item | Hard Isolation | Soft Isolation |
| Resource Usage | Two sets of independent hardware circuits, no CPU overhead | Shared single MAC, relying on CPU software for time division processing |
| Bandwidth Isolation | 100% physical isolation, no preemption | Bandwidth sharing, mutual influence under high load |
| Latency | Fixed low latency (<1ms) | Latency fluctuations can reach tens of ms when the CPU is busy |
| Reliability | A failure in one MAC does not affect another | A single MAC failure causes a total service interruption |
Q: What should be noted when calculating link attenuation for XGS ONU and GPON ONU under the same PON?
A: Typical optical fiber attenuation parameters are as follows:
| Direction | Wavelength | Optical Fiber Attenuation Coefficient (G.652D/G.657A) |
| Uplink | 1270nm | 0.37~0.40 dB/km |
| Uplink | 1310nm | 0.34~0.36 dB/km |
| Downlink | 1490nm | 0.22~0.23 dB/km |
| Downlink | 1577nm | 0.20~0.21 dB/km |
Normally XGSPON upstream 1270nm attenuation is the maximum, becoming the bottleneck of the entire link. Many projects only calculate the downstream, resulting in frequent disconnections of XGS ONU while GPON operates normally.
Other relevant considerations are below:
First, PLC splitter: nominally 1260-1650nm full spectrum, but in practice, there is a ±0.5dB difference in insertion loss at different wavelengths, for example, 1270nm is usually slightly higher.
Second, it is recommended to use the worst-case calculation method, taking the maximum value for all parameters (e.g., fiber attenuation should take 0.4dB/km@1270nm, connector loss should take 0.5dB per connector), and providing sufficient safety margin: suggested ≥3dB, to address long-term factors such as connector aging, temperature changes, and fiber bending.
Q: Will there be any issues with XGS ONU and GPON ONU under the same PON when upgrading through the OLT?
A: In theory, there should be no issues. The OLT can automatically recognize the ONU type (GPON/XGSPON) through the standard OMCI protocol and push the corresponding compatible version firmware. However, the following risks may exist:
First, upgrading a large number of ONUs simultaneously may exhaust the bandwidth of the channel, leading to a poor user experience. It is recommended to choose low-traffic time for batch upgrades; second,after the upgrade, the ONU may fail to register, as the new firmware may not be compatible with the OLT equipment, resulting in the ONU being offline after the upgrade.
Summary
Rayin Technology has over 10 years of development experience in 10G optical access PON technology, creating Combo OLT products based on mainstream commercial chips. Each port is equipped with hardware-level independent dual MAC hardware, completely isolating GPON and XGS-PON bandwidth without interference, with a maximum support of 1:256 split ratio per port. It perfectly fits the mainstream optical network renovation scenarios for small to medium-sized broadband operators, parks, hotels, and high-definition monitoring. It can not only meet current mainstream broadband service demands for high-definition video, Wi-Fi 6, and high-bandwidth internet, but also build a solid foundation for a smooth transition to 50G PON in the future.
If you are unsure whether your existing OLT supports Gigabit and 10-Gigabit hybrid access, Rayin Technology engineers can provide you with a free on-site network assessment and customized upgrade plan. Please visit www.szrayin.com for more information.
