What Are The Common Reasons Affecting The Quality of Home Broadband Services in Broadband Fiber Access Networks?

The impact of broadband fiber access networks on the quality of home broadband services is often related to ODN fiber link interruptions, ONU weak light, GPON access to Gigabit subscribers, OLT uplink bandwidth limitations, and OLT reloading.

1 ODN fiber link interruption

From the viewpoint of the causes of user complaints about the quality of home broadband services, ODN fiber link interruptions account for more than 70%. Fiber optic cable interruption caused by municipal road reconstruction and residential community reconstruction is the main reason for fiber optic link interruption, followed by link interruption caused by fiber optic active connection. It is difficult to prevent fiber optic cable interruptions caused by municipal construction, and efforts can only be made to shorten the time frame for emergency repairs. Link interruptions caused by fiber optic active connections are often related to the quality of active connectors. Collective procurement has made the products cheaper and cheaper, but the maintenance costs due to product quality problems have become higher and higher.

However, complaints due to ODN fiber optic link interruptions are eliminated after the fault is repaired and the user's dissatisfaction with the quality of the home broadband is eliminated, and it is the card and slowdown felt by the user when using the network that is the main aspect that affects the user's satisfaction.

2 ONU weak light

ONU low light (ONU received optical power less than -27dBm) will make the probability of ONU failure significantly higher, for example: low-light ONU flash off more than five times a week probability as high as 41%, which is 2.4 times that of the non-low-light ONU.

ONU weak light is mainly due to excessive macro-bend loss of the fiber, followed by the attenuation of the active connection is larger. too many active connections in the ODN link (more than 8), unused active connectors are not covered with dust caps resulting in end-face contamination, will lead to an increase in the attenuation of the link active connection.

Weak light mainly occurs on the ONU side, and if it occurs on the OLT side, it may be due to the use of Class B+ optical modules in the OLT (the downlink optical power budget of Class B+ optical modules is 1.5 dB lower than that of Class C+, but the uplink optical power budget is 4.0 dB lower than that of Class C+).

3 GPON Access to Gigabit Users

The question of whether GPON can access gigabit users has been controversial. It is generally recognized that the number of gigabit users accessed by GPON ports should be strictly controlled, and the specific number of users required varies from place to place, usually from 2 to 5. However, according to the results of jsCM's speed measurement for high-utilization PON port gigabit users during the 2022 Lantern Festival party, GPON can fully meet the demand for gigabit development.

The actual bandwidth utilization of established GPON ports is very low, for example, in a first-tier city J, the percentage of GPON ports with peak downstream bandwidth utilization (second level, within a weekly interval) higher than 20% is less than 0.3%. The average Internet speed of gigabit users is not significantly higher than that of ordinary users, so even if multiple gigabit users are accessed under the GPON port, the speed measurement of individual gigabit users is very good.

However, user traffic is often bursty, and the millisecond network speed peak of a single user is always close to the user's bandwidth, e.g., the burst rate of IPTV, 4K/8K and other video-type services at different time granularity. When Gigabit users use the network on a daily basis, although the average network speed is not high (usually a few megabytes to tens of megabytes per second), the peak millisecond network speed will still reach the limit of the contracted bandwidth (the limit of the contracted bandwidth is usually set at 100% to 120% of the contracted bandwidth).

Therefore, even if there are only two Gigabit subscribers accessing under the GPON port, as long as one Gigabit subscriber is speed testing while the other Gigabit subscriber is using IPTV, it may cause millisecond packet loss. Although millisecond packet loss has a limited impact on the network and the probability of occurrence is not high; however, if the number of gigabit users accessed by the GPON port is higher, the probability of packet loss increases, which brings about an impact on the network usage of other users under the same PON port.

4 OLT uplink bandwidth constraints

Currently, the bandwidth of OLT uplink circuits is mainly 10GE (2 or 4 lines, half of the main and half of the backup), and the expansion threshold of 10GE circuits is suitable to be 70%.

The peak rate gap between services at different time granularities (1ms, 10ms, 100ms, 1s) is large, and the larger the gap between the user bandwidth and the user's average network speed, the more significant this gap is. For example, a millisecond peak for a single gigabit user may be more than 900Mbps higher than a second peak.

The cache of the OLT is only at the millisecond level, which may cause packet loss when the millisecond peak of the OLT uplink circuit exceeds the circuit bandwidth, so the bandwidth of the OLT uplink circuit should be redundant to a certain extent according to the maximum bandwidth of a single user based on meeting the second peak. 10GE circuits with a capacity expansion threshold of 70% may lose packets if the utilization rate of the circuits is close to the threshold, and the millisecond, 10-millisecond, 100-millisecond levels of bursty traffic may drop packets; therefore, it is recommended that the expansion threshold for 10GE circuits should be lower than 60%.

There are still a small number of GE circuits in the existing OLT uplinks, and when there are gigabit users under the OLT, the GE circuits are obviously not suitable for use as OLT uplinks. Moreover, when the GE circuits in the OLT uplink are interfaced with BRAS/SR, they are generally aggregated through Layer 3 switches, which also increases the service delay.

5 OLT reloading

When the number of subscribers accessed by one OLT exceeds 5,000, it is usually called OLT heavy load.OLT heavy load is also regarded as one of the important reasons affecting the quality of home broadband, and operators in some provinces and cities even require that the number of subscribers accessed by a single OLT should not exceed 1,000.

According to each OLT 16 user boards, each user board 16 PON ports, each PON port access to 64 users, OLT's maximum access capacity of 16,384 households, 5,000 households is only 30.5% of the access capacity of the equipment. If the OLT access to users over 5000 heavy load, then the performance of the equipment must have a problem.

According to HW's analysis of the number of OLT access users and the percentage of IPTV stuttering / splash screen, it can be seen that the number of OLT access users and user experience have a certain correlation. However, this correlation may also be related to the restricted OLT uplink bandwidth. When the OLT uplink bandwidth remains unchanged, the more the number of users accessed by OLT, the worse the user experience naturally becomes.

The impact of OLT reloading on user Internet experience is not obvious. Based on the pressure of assessment, in order to prevent a single OLT failure from affecting too large an area, may be the main reason for setting a limit on the number of users accessed by a single OLT.

6 Summary

Due to the different network status quo, the above factors have different degrees of impact on the quality of home broadband in different MANs. Existing PON network management of service data packet loss analysis capability is limited, the above analysis of the impact of gigabit user burst traffic on the quality of home broadband is only based on subjective judgments, the actual situation of the network requires the feedback of the front-line operation and maintenance personnel.