CWDM PON Network construction and maintenance

The TDM-PON technology is not an effective solution to support over 100M bit rate FTTx access. DWDM PON could do that but it is not mature, probably not cost effective either. A Hybrid solution of CWDM-TDM-PON may be the best solution, and support up to 1x256 splitting ratio, while still offer up to 155M bit rate to premises sites.

Passive optical network (PON) is the vital solution to FTTx. It has three key elements: OLT (Optical Line Terminal), ODN (Optical Distributing Network) and ONU (Optical Network Unit). The ONUs of remote node, office or residence, share one OLT. The EPON ONU and EPON OLT are connected via ODN, which is passive.

The term PON (Passive Optical Network) emphasizes on the passive feature of the ODN. That passive structure offers low cost, and ease to maintenance to the FTTx provider. The popular TDM-PON solutions (GPON, GePON) use an optical power splitter to implement the passive ODNs. A popular implementation of this power splitter is based on PLC (Planar Light Circuit) technology. So, it is referred to as a PLC splitter, or a shorter jargon PLC. In fact, PLC has much wider applications than optical power splitter.

Once the TDM-PON (GPON, GePON) technology has reached its limitations, leading suppliers began to explore the next generation of PON infrastructure. The WDM-PON leverages the WDM (Wavelength Division Multiplex) technologies to implement the ODN. One of the decent designs is Nortel-LG (now LG-Ericsson) injection-locking approach (www.lgericsson.com).

Arguably, a switch to WDM-PON is inevitable, if network providers want to deliver higher bandwidth to the FTTx remote nodes. The GPON Fiber Access with 1x32 splitter configurations only deliver 300M bit rate to the premier site. To deliver the 1G bit rate to the remote note, one 10GPON can only support eight (8) remote notes. That is not cost effective.

So, the more pra ctical question is how to leverage the WDM feature of the ODN. Currently, the well discussed solution is a DWDM, pure WDM-PON. It has many benefits. However, the technology is not mature, leading to higher costs. Besides, it gives up the benefit of the existing TDM-PON solutions.

If we combine the mature, low cost CWDM and TDM-PON technologies together, we may have an affordable solution. The CWDM transceivers can support 2.5G bit rate with very competitive price. The 16-channel CWDM mux/demux module are cost effective. With 1x16 splitters, the CWDM-TDM ODN can support 1x256 splitting ratio, much higher than current 1x64 TDM-PON. With that splitting ratio, the premises sites can still have about 155M bit rate.