Combo port resource usage status
With the development of fixed gigabit services, XG-PON is gradually taking a dominant position in broadband access networks. In order to share ODN with GPON, XG-PON usually uses Combo optical modules (commonly known as “Combo ports”). Whether users use GPON ONU or XG-PON ONU, they can directly connect to the optical splitter in the fiber distribution box through the entry optical cable to achieve business access.
In existing networks, ODN usually uses two-level splitting, with a total splitting ratio of 1:64. That is, a Combo port can access up to 64 users. For example, when the first-level splitter is 1×8 splitter and the second-level splitter is 1×8 splitter, each fiber distribution box can accommodate up to 8 users. When the ports of the fiber splitter in the fiber distribution box are full, a new fiber optic splitter needs to be added to the fiber distribution box to expand the capacity. If the first-level splitter still has free ports, you need to add an ODN link from cross cabinet to the fiber distribution box; if the first-level splitter has no free ports, you need to add a Combo port and add an ODN link from the OLT to the fiber distribution box, as shown in Figure 1.
Since the port utilization rate of the plc splitter at the fiber distribution box is only about 50%, and the port utilization rate of the plc optical splitter at the cross cabinet is only about 80%, so the average number of users actually installing a Combo port is less than 30 households. The average number of installed users of PON ports in some provinces of a certain operator is shown in Figure 2 (the abscissa in the figure represents different provinces, and the province with the serial number 1 represents the average of each province).
On the one hand, because the number of users connected to each PON port is too small, on the other hand, users’ demand for bandwidth is not that great, which results in very low bandwidth utilization of the Combo port. For example, as of June 2024, the maximum bandwidth utilization (second level, GPON channel or XG-PON channel) of more than 99.9% of Combo interfaces in a certain first-tier city is less than 10%. The resource utilization rate of the combo port needs to be improved urgently.
Ideas for improving combo port resource utilization
The article “What is XGS-PON, GPON and XG-PON?” introduces how XG-PON coexists with GPON. Logically speaking, Combo only combines XG-PON and GPON into the ODN. The maximum number of users connected to a Combo port should be the sum of XG-PON and GPON channels. Why is the number of users connected to Combo still only 64 instead of 128?
Let’s look at the BOSA (Optical Transmitter and Receiver Assembly) principle of the optical module in the ONU. Figure 3 shows the BOSA of the optical module in the XG-PON onu. In the downstream direction, the 1490nm and 1577nm wavelength optical signals downstream from the Combo are filtered out at the 1490nm wavelength in BOSA. In this way, the onu can only receive the XG-PON optical signal. The same is true for the optical module of GPON onu.
Therefore, before the downlink optical signal reaches the onu, as long as the GPON and XG-PON channels are separated by a multiplexer and demultiplexer, and the XG-PON onu and GPON onu are connected respectively, the number of users connected to the Combo port can be doubled.
Composition of capacity-doubling optical splitter
In ODN, the fiber distribution box is the dividing point between construction and installation. The multiplexer and demultiplexer used for GPON and XG-PON multiplexing and demultiplexing can be set up at the fiber splitter box. The combiner path of the multiplexer and splitter is connected to the combiner side fiber of the original optical splitter. The two branches of the multiplexer and splitter are each connected to a splitter with the same splitting ratio as the original optical splitter in the fiber splitter box, as shown in Figure 4.
In this way, two optical splitters (optical splitter 1 and optical splitter 2 in Figure 4b) can accommodate GPON and XG-PON users respectively.
The combiner, demultiplexer and two optical splitters in Figure 4b are packaged into one to form a new type of optical splitter, as shown in Figure 5. It can be used in place of the original optical splitter in an ODN that originally used Combo optical modules, thereby doubling the number of Combo port users.
Figure 6 is a physical diagram of the new optical splitter, which mainly includes 4+4 type, 8+8 type and 64+64 type. Figure 6a shows the 4+4 type, which can access 4 GPON and 4 XG-PON users. The GPON and XG-PON ports are distinguished by different colors; Figure 6b shows the 8+8 type. The 4+4 type and 8+8 type are both lgx type splitters and are suitable for installation in fiber distribution boxes. Figure 6c shows the 64+64 type, which is suitable for installation in the ODF unit. In the following, different models of capacity doubling optical splitters are collectively referred to as new optical splitters.
The following takes the 4+4 type splitter as an example to introduce the technical principles of the new optical splitter. The input port of the new optical splitter is connected to the Combo port of the XG-PON device through the combining side optical fiber. The combining side optical fiber contains 4 wavelengths, including 1577nm and 1490nm for the downlink and 1310nm and 1270nm for the uplink, as shown in Figure 7.
The above four wavelengths are divided into two channels after passing through the multiplexer and splitter: one is a GPON channel with a down/uplink wavelength of 1490nm/1310nm; the other is an XG-PON channel with a down/uplink wavelength of 1577nm/1270nm. The GPON channel is divided into 4 branches after passing through the PLC optical splitting device, corresponding to 4 GPON user ports. Each port can accommodate a GPON user through the entrance optical cable. The XG-PON channel is divided into 4 branches after passing through the PLC optical splitting device, corresponding to 4 XG-PON user ports. Each port can accommodate one XG-PON user through the entrance optical cable.
In this way, the 4+4 capacity-doubling optical splitter can accommodate 4 GPON users and 4 XG-PON users for a total of 8 users, while the ordinary 1×4 optical splitter can only accommodate 4 users (GPON users or XG-PON users). Compared with ordinary 1×4 optical splitters, due to the addition of a multiplexer and splitter in the link, the insertion loss of the new optical splitter will increase by about 0.5dB. Since the attenuation of ODN optical fiber links generally has a maintenance margin of more than 2.0dB, the use of new optical splitters will basically not affect the transmission indicators of the ODN link.
Using the new optical splitter will double the number of users connected to the Combo port at most. Since the bandwidth utilization (second level) of the existing Combo port is generally very low, the bandwidth utilization of the Combo port will increase significantly as the number of access users increases, but it will not exceed the access capability of the Combo port and can ensure that the needs of business development are met within a certain period of time.
Conclusion
The use of new optical splitters can double the number of fiber access users at the Combo port and the combining side, thereby saving investment in Combo ports and optical cables. The usage and investment analysis of the new optical splitter are detailed below ”How new optical splitters can significantly reduce ODN expansion costs”.
