CPON Concept and Performance Analysis

Given the characteristics of heavy downstream traffic in the access network environment, upgrading the existing PON with a composite PON (abbreviated as CPON) that combines TDM and WDM technologies is a good transitional solution. CPON adopts WDM technology in the downstream direction, where the OLT uses multi-wavelength light sources to assign each ONU a wavelength in the 1550nm band, with wavelength spacing typically ranging from 0.8nm to 2nm, or even less than 0.8nm. The light source transmits different users' information on different wavelengths, and the optical wave demultiplexer at the remote node separates each wavelength and delivers it to the respective ONU. The upstream direction still uses the original TDMA technology.

The downstream application of WDM technology increases user bandwidth, making it suitable for access network environments with large downstream data volumes and relatively smaller upstream data volumes. The remote node of the composite PON must simultaneously provide wavelength window routing and power coupling functions. Using a separate wavelength router and passive star coupler requires two optical fibers to transmit upstream and downstream signals separately. Currently, if downstream bandwidth demand increases, CPON is the simplest way to introduce WDM technology into PON.

CPON System Performance Analysis:

(1) Bandwidth: In TDM-PON, since bandwidth can be dynamically allocated, it can be utilized effectively and flexibly, meaning users can share bandwidth. The cost of this flexibility is that the electrical and optoelectronic devices in the OLT and ONU must operate at the aggregate bit rate. Thus, a 622Mbit/s line rate provides an average of 39Mbit/s per user in a 1×16 PS-PON. However, while maintaining a relatively low line rate, WDM technology can provide each user with a larger bandwidth, with electrical and optoelectronic devices operating at each ONU's bit rate rather than the aggregate bit rate.

(2) Power Budget: Introducing WDM devices in the downstream direction offers advantages in terms of power budget. First, WDM receivers operate at lower bit rates and therefore have higher sensitivity. Second, for larger splitting ratios, the insertion loss of the demultiplexer is lower than that of the splitter. The upstream signal power budget is the same as that of TDM-PON.

(3) Scalability: Without modifying the network's physical topology or optical cable infrastructure, the network can be upgraded simply by replacing passive splitters with passive WDM multiplexers at tree branch nodes and employing multi-wavelength optical transceivers at the OLT. As long as the maximum loss budget of the PON is not exceeded, transmitters and receivers do not need to be redesigned for different splitting ratios. The aggregate bit rate also remains unchanged.

(4) Security: CPON adopts DWDM technology in the downstream direction, where signals destined for different ONUs are separated by wavelength. As long as the crosstalk performance of the demultiplexer is sufficiently good, encryption technology can be avoided, ensuring the security of user information.

(5) Maintainability: For the upstream direction, since backscattering and reflections from all branches beyond the splitter are superimposed, it is very difficult to detect fiber breaks using an optical time-domain reflectometer (OTDR). The solution is to store OTDR traces for comparison with signals after a fault occurs. For the downstream direction, fiber branches beyond the demultiplexer can be detected using a wavelength-tunable OTDR.

(6) Cost: The ODN still uses low-cost passive splitters, eliminating the need for expensive optoelectronic devices used in full WDM-PON, such as AWG, thereby avoiding the need for wavelength and temperature monitoring.