Combo became popular around 2002, but at that time Combo was a disc burner that could both read DVDs and burn CDs. Combo probably means two functions in one. The Combo we are talking about today is a component in the OLT equipment that has both GPON and XG-PON functions.
Since GPON and XG-PON use different uplink and downlink wavelengths, if a 2-in-1 multiplexer/demultiplexer is used on the OLT side to combine the GPON and XG-PON ports into one, the original ODN can be used for arbitrary access. GPON or XG-PON optical modem to achieve smooth upgrade from GPON to XG-PON.
The optical module including GPON optical devices, XG-PON optical devices and 2-in-1 multiplexer/demultiplexer (WDM1r in figure 2) is called Combo optical module. If the user interfaces on the OLT user board all use Combo optical modules, it is called a Combo user board. The Combo optical module on the user board is also commonly known as a Combo port.
The bandwidth access capability of a Combo port is the sum of the access capabilities of a GPON port (2.5G) and an XG-PON port (10G). The GPON optical modem attached to a Combo port shares the 2.5G bandwidth of GPON, and the XG-PON optical modem shares the 10G bandwidth of XG-PON.
Possible problems may occur in use Combo port
Since the unit price of a Combo port is several times higher than that of a GPON port, the unit price of an XG-PON optical modem is more than twice as high as that of a GPON optical modem. However, the current bandwidth utilization rate of GPON is generally low, and GPON still has certain Gigabit connectivity.
Therefore, when operators develop Gigabit services, they often use existing GPON ports to develop Gigabit users (see ① in the figure 3). When the number of Gigabit users under the GPON ports reaches a certain number, they will then migrate users to Combo port (unplug the patch cord connecting the with the GPON port in ODN link and then plug it into the Combo port, see ② in the figure 3).
However, this will result in too few users accessing through the XG-PON through the Combo port, causing the access cost for XG-PON users to be too high.
As of the end of 2020, the number of broadband users of the three major operators has reached 455 million. Obviously, the number of broadband users cannot increase significantly, and the development of broadband services must mainly focus on speeding up existing users.
After the GPON port is migrated to the Combo port, due to the additional cost of replacing the XG-PON optical modem, the original Gigabit users are not willing to change if the model itself support gigabit internet. Only in the following two situations, users need to configure XG-PON optical modem.
- New broadband is added and the bandwidth requirement is Gigabit. Although GPON optical modems can also be used in this situation, due to the large user bandwidth, they are not suitable for accessing GPON.
- The user’s bandwidth is upgraded to Gigabit, but the user’s existing optical modem does not support it.
Obviously, within the range covered by a Combo port, the probability of the above two situations occurring is low. Currently, the average number of XG-PON optical modems connected to Combo ports nationwide is less than 2pcs, which leads to a serious imbalance in the use of XG-PON and GPON resources in Combo boards. It not only causes a waste of XG-PON resources, but also causes It will affect the Internet experience of GPON users.
Solution to improve XG-PON resource utilization
By expanding the use scope of XG-PON optical modems, relatively concentrating XG-PON optical modems on Combo ports, and improving the ODN splitting ratio, the number of XG-PON users under Combo ports can be effectively increased and the occupation of Combo ports can be reduced, thus finally improving the utilization of XG-PON resources.
3.1 Expand the use scope of XG-PON optical modem
Limited by business needs, the development of Gigabit must be a step-by-step process. If XG-PON only accesses Gigabit users, the proportion of XG-PON optical modems under most Combo ports will be very low. Therefore, it is necessary High-bandwidth users (such as 500M and above) will be uniformly accessed through XG-PON.
For users who have access to the Combo port, if the user bandwidth is upgraded to 500M or above, the operator can replace the original GPON optical modem with an XP-PON optical modem. The replaced GPON optical modem can be used for new broadband user below 500M. New users with bandwidths of 500M and above use XG-PON optical modems, and users with bandwidths below 500M use GPON optical modes.
3.2 XG-PON optical modems are relatively concentrated at the Combo port
In areas where one-level optical splitting is used, add another optical splitter to the location where the splitters were originally concentrated and connected to the Combo port. Users with bandwidth upgraded to 500M and above and new broadband users can access the newly added optical splitter and try to ensure that the number of users 500M and above and users below 500M under the Combo port does not exceed half of the splitter capacity.
In areas where second-level optical splitting is used, another first level splitter can be added to the location where the first-level splitter was originally set up and connected to the Combo port. If a second-level optical splitter has a new or upgraded 500M and above business needs, the second-level splitter will be moved to the new first-level optical splitter.
3.3 Improve ODN splitting ratio
Currently, ODN mainly uses 1:64 splitting. Due to the uneven occupation of the splitter ports, the average number of users connected to each GPON port is only about 30 households. The bandwidth capability of the Combo port fully supports the mixed access of 64 GPON users (below 500M) and 64 XG-PON users (500M and above). Therefore, under the current high price of Combo port, 1:64 splitting will obviously cause a lot of waste.
But does the Combo port support the ODN splitting ratio to be increased to 1:128?
The receiving power of existing GPON optical links at the user side (fiber distribution box) is generally -20dBm ~ -24dBm, while the receiving sensitivity of GPON optical modems is -28dBm, and the link’s attenuation maintenance margin is at least 4.0dB. . If the ODN splitting ratio is doubled, the link will only increase loss by about 3.0dB. Therefore, from the perspective of ODN link loss, existing GPON generally has the ability to increase the splitting ratio to 1:128.
The Combo port contains two channels, GPON and XG-PON. The optical power parameters of the GPON channel are the same as those of the existing GPON. The minimum transmit optical power of the GPON is consistent; therefore, both the GPON and XG-PON channels of the Combo port support the ODN splitting ratio to be increased to 1:128.
When ODN adopts the first-level splitting mode, new splitters will be enabled only after the ports of the optical splitter are fully occupied (except for a small amount of reservation). Therefore, if the splitting ratio is increased to 128 under the first-level splitting, it will cause There are too many users connected to one Combo port, and it is difficult to meet the bandwidth needs of long-term users. Therefore, it is recommended to only increase the splitting ratio of the ODN link with secondary optical splitting. But it should be noted:
- Due to the increase in ODN link attenuation, the maintenance margin is reduced. Not all ODN links meet the attenuation requirements.
- XG-PON is very sensitive to the bending loss of optical fiber (see “How much difference of bending resistance between fiber G.657 and G.652?” ), when users use XG-PON optical cats for access, they should pay special attention to the bending loss of the drop cable, and avoid using G.652 fiber.
Conclusion
In response to the requirements of the “Dual Gigabit” Network Collaborative Development Action Plan (2021-2023) recently issued by the Ministry of Industry and Information Technology, operators have significantly increased their procurement needs for Combo. For example, some operators have made it clear that they will only allow purchase Combo user boards instead of GPON user boards. However, if follow the current Combo port usage strategy, it will inevitably lead to a very low utilization rate of XG-PON resources in the Combo user board, resulting in greater waste.
