What are the common reasons that affect the quality of home broadband services?

Although about 60% of home broadband service quality problems occur in home networks (see the article “What are the main quality issues of home broadband?”, users thought all problems are caused by the operator’s network quality. So what is real?

From the perspective of end-to-end connection, the operator’s home broadband network is mainly composed of broadband fiber access network, xxNET, transmission network and content source, as shown in Figure 1. The transmission network mainly provides connections between various network elements of xxNET, and IDC and CDN belong to content sources. In recent years, as operators have increased the introduction of content sources, content sources are no longer the main aspect affecting service quality in home broadband networks. Quality problems of home broadband services mainly occur in broadband fiber access networks.

The impact of broadband fiber access network on the quality of home broadband services is often related to ODN fiber link interruption, ONU weak light, GPON access to gigabit users, OLT uplink bandwidth limitation and OLT overload.

ODN fiber link interruption

From the reasons for users’ complaints about the quality of home broadband services, ODN fiber link interruption accounts for more than 70%. The main reason is the cable interruption caused by municipal road reconstruction and residential community reconstruction, and link interruption by active fiber connection. Optical cable interruption caused by municipal construction is difficult to prevent, and we can only work hard to shorten the repair time limit. Link interruption caused by active fiber connection is often related to the quality of active connectors. Group purchase makes product prices cheaper, but the maintenance cost caused by product quality problems is getting higher and higher.

However, after the ODN fiber link interruption was repaired, users’ dissatisfaction with the quality of home broadband was eliminated. However, the main factor affecting user satisfaction is the lag and slowness when using the network.

ONU weak light

ONU weak light (ONU received optical power is less than -27dBm) will significantly increase the probability of ONU failure. For example, the probability of a weak light ONU flashing off more than 5 times in a week is as high as 41%, which is 2.4 times that of a non-weak light ONU, as shown in Figure 2 (the horizontal axis in the figure is the ONU received optical power).

The main reason for ONU weak light is that the fiber macro bending loss is too large (see “The impact of insufficient fiber bending radius on ODN link loss” and “What is the difference G657A2 vs G652D?”), and large loss of active connections. Too many active connections in the ODN link (more than 8), and end face contamination caused by unused active connectors not covered with dust caps (as shown in Figure 3) will increase the loss of active connections in the link.

Weak light mainly occurs on the ONU side. If it occurs on the OLT side, it may be because the OLT uses a Class B+ optical module (the downstream power budget of the Class B+ optical module is 1.5dB lower than that of the Class C+, but the upstream power budget is 4.0dB lower than that of the Class C+).

GPON access to gigabit users

The question of whether GPON can access gigabit users has always been controversial. It is generally believed that the number of gigabit users accessed by GPON ports should be strictly controlled. The specific number of users varies from place to place, usually 2 to 5. However, according to the speed test results for high-utilization PON port gigabit users during the 2022 Lantern Festival Gala, as shown in Table 1, GPON can fully meet the needs of gigabit development.

The actual bandwidth utilization of existing GPON ports is very low. For example, in a first-tier city J, the peak value of GPON port downstream bandwidth utilization (second level, within a week) higher than 20% accounts for less than 0.3%, as shown in Table 2. The average network speed of gigabit users is not significantly higher than that of ordinary users, so even if multiple gigabit users are connected to the GPON port, the speed test of a single gigabit user is very good.

However, user traffic is often bursty, and the millisecond peak speed of a single user is always close to the user bandwidth. For example, the burst rates of video services such as IPTV and 4K/8K at different time granularities are shown in Figure 4. When gigabit users use the network on a daily basis, although the average network speed is not high (usually a few megabits to tens of megabits per second), the millisecond peak speed will still reach the limit of the contracted bandwidth (the limit of the contracted bandwidth is generally set to 100% to 120% of the contracted bandwidth).

Therefore, even if there are only two Gigabit users connected to the GPON port, as long as one Gigabit user is testing the speed and the other Gigabit user is using IPTV, it may cause millisecond-level packet loss. Although the impact of millisecond-level packet loss on the network is limited and the probability of occurrence is not high, if there are more Gigabit users connected to the GPON port, the probability of packet loss will increase, thus affecting the network use of other users on the same PON port.

OLT uplink bandwidth is limited

Currently, the bandwidth of OLT uplink circuits is mainly 10GE (2 or 4, half for primary and half for backup). In the article “What is the bandwidth utilization rate of the OLT uplink before expansion is required?”, it is analyzed that the expansion threshold of 10GE circuits should be 70%.

However, the article only analyzes the bandwidth requirements of the second-level peak. The peak rate difference of services at different time granularities (1ms, 10ms, 100ms, 1s) is large. The greater the difference between user bandwidth and user’s average network speed, the more significant this difference is. For example, the millisecond peak of a single gigabit user may be more than 900Mbps higher than the second peak.

The cache of OLT is only at the millisecond level. When the millisecond level peak of the OLT’s upstream exceeds the bandwidth, packet loss may occur. Therefore, the bandwidth of the OLT’s upstream should be redundant based on the maximum bandwidth of a single user, while meeting the second level peak. If the expansion threshold of the 10GE circuit is 70%, sudden traffic at the millisecond, 10 millisecond, and 100 millisecond levels may be lost when the circuit utilization rate approaches the threshold. Therefore, it is recommended that the expansion threshold of the 10GE circuit should be lower than 60%.

There are still a small number of GE circuits in the uplink of the existing OLT, and when there are gigabit users under the OLT, GE circuits are obviously not suitable for use as OLT uplink. Moreover, when the GE circuit connected to the OLT is docked with the BRAS/SR, it is usually converged through a three-layer switch, as shown in Figure 5, which also increases the latency of the service.

OLT overload

When the number of users connected to an OLT exceeds 5,000, it is usually called OLT overload. OLT overload is also considered one of the important reasons affecting the quality of home broadband. Operators in some provinces and cities even require that the number of users connected to a single OLT does not exceed 1,000 households.

Based on 16 user boards per OLT, 16 PON ports per user board, and 64 users connected to each PON port, the maximum access capacity of the OLT is 16,384 households, and 5,000 households is only 30.5% of the device’s access capacity. If the OLT is overloaded when the number of users connected exceeds 5,000, then there must be a problem with the performance of the device!

According to HW’s analysis of the number of users connected to OLT and the proportion of IPTV freezes/color screens, as shown in Table 3, it can be seen that the number of users connected to OLT has a certain correlation with user experience. However, this correlation may also be related to the limited uplink bandwidth of OLT. When the uplink bandwidth of OLT remains unchanged, the more users connected to OLT, the worse the user experience will be.

The impact of OLT overload on the user’s Internet experience is not obvious. Based on the pressure of assessment, in order to prevent a single OLT failure from affecting too many areas, it may be the main reason for setting a limit on the number of users connected to a single OLT.

Conclusion

Due to the different network status, the above factors have different impacts on home broadband quality in different metropolitan area networks. The existing PON network management has limited ability to analyze service data packet loss. The above analysis on the impact of Gigabit user burst traffic on home broadband quality is only based on subjective judgment. We also hope to get feedback from front-line operation and maintenance personnel on the actual situation of the network.