The demand for higher broadband speeds is driving network operators to upgrade their fiber-to-the-home (FTTH) networks. Many operators have deployed GPON networks, which provide up to 2.5 Gbps of shared bandwidth. To meet the need for 10 Gbps and beyond, operators are now deploying XGPON and 10G XGPON networks. But how do GPON and XGPON coexist in a network?
GPON uses a single wavelength for downstream and upstream traffic. The ITU-T G.984 standards define the 2.5 Gbps downstream and 1.25 Gbps upstream rates. Most GPON ONUs support 2.5G/1.25G speeds, providing sufficient bandwidth for current broadband services. However, the sharing of bandwidth across 32-64 users per PON means that individual user speeds are much lower than 2.5 Gbps.
XGPON and 10G XGPON boost the downstream and upstream rates significantly. XGPON supports 10 Gbps downstream and 2.5 Gbps upstream. 10G XGPON boosts the downstream rate to 10 Gbps. The higher bandwidth enables gigabit broadband services and supports more users per PON.
To deliver these multi-gigabit services, XGPON ONUs utilize wavelength division multiplexing (WDM) to provide dedicated 10G wavelengths. Unlike GPON, which uses a single wavelength, XGPON ONUs have tunable lasers that can transmit signals at specific wavelengths assigned by the optical line terminal (OLT).
At the optical distribution network (ODN), wavelength-division multiplexers (WDMs) combine the GPON and XGPON wavelengths onto a single fiber. So both GPON and XGPON traffic can coexist on the same fiber backbone.
The OLT manages the traffic through the use of dual-band optics. Dual-band OLTs have separate lasers, receivers and RF signal processing for GPON and XGPON. This enables the OLT to seamlessly manage and interoperate between GPON ONUs and XPON ONUs on the same port.
During initial network upgrade, the operator will install new XGPON ONUs for customers subscribing to 10G services. But many customers will still use existing GPON ONUs. The dual-band OLT allows both types of ONUs to operate simultaneously.
A key advantage of this approach is that it avoids a "forklift" upgrade, where the entire network needs to be replaced at once. Instead, XGPON can be gradually deployed while GPON services continue unchanged. Once bandwidth demands increase, customers can be upgraded to XGPON ONUs individually.
In summary, GPON and XGPON can coexist in FTTH networks by:
Using WDMs to combine GPON and XGPON wavelengths
Installing dual-band OLTs to manage both GPON and XGPON ONUs
Deploying XGPON ONUs for new 10G customers while retaining GPON ONUs for existing customers
This allows a smooth network evolution that protects the operator's GPON investment while providing an upgrade path to deliver multi-gigabit broadband speeds. The combination of GPON and XGPON gives operators the flexibility to meet customers' growing bandwidth demands both now and in the future.
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