Some people say that the all-optical network is a point-to-multipoint structure. When a split ratio of 1:16 or higher is used, there will be problems such as "split bandwidth" and "insufficient bandwidth at a single point". It is not as good as the high-bandwidth network experience of the Ethernet all-optical network with 1:1 symmetry in the upstream and downstream. Is this true?
Of course not. The Ethernet all-optical network is 1:1 symmetric in the upstream and downstream on the surface, but in fact there is also an invisible split ratio. Taking the weak current room of a building as an example, the all-optical aggregation switch is 40GE uplink and there are 288 10GE ports on the user side. There is a convergence of 40GE/2880GE, that is, a convergence ratio of 1/72, and the congestion point is the uplink port. At the same time, because the all-optical aggregation switch does not have a dynamic bandwidth mechanism to guarantee bandwidth and maximum bandwidth, when the 40GE uplink port is congested, the 2880GE uplink data is indiscriminately dropped, which has a very bad impact on the service experience. It may cause a poor experience for small-bandwidth office services because the small-bandwidth traffic is squeezed by the large-bandwidth traffic of other users. The network congestion of the all-optical network is controllable, and it has the function of fine bandwidth management DBA, which can not only guarantee user services, but also improve the bandwidth utilization of ports. The 1:16 splitting ratio is far better than the invisible 1:72 splitting ratio of the all-optical aggregation switch.
（1）What is a DBA?
DBA (Dynamic Bandwidth Alocation) is a technology for implementing upstream bandwidth control in PON systems. OLT monitors the congestion of the PON port in real time and dynamically adjusts the bandwidth based on the congestion, current bandwidth utilization, and configuration, thereby improving bandwidth utilization.
（2）Working principle of DBA
1. The DBA module inside the OLT continuously collects DBA report information and performs calculations through the DBA algorithm.
2. The OLT sends the calculation results to the backup ONU in the form of BW Map.
3. Each ONU sends upstream burst data within the allowed time slot according to the BW Map information, occupying the upstream bandwidth.
（3）DBA Implementation Mechanism
In the GPON system, when the upstream data of multiple ONUs are combined on the trunk fiber and transmitted to the OLT, in order to avoid collision of data packets during the upstream process, the system needs an effective time division multiplexing mechanism to control the distribution of upstream traffic. This mechanism needs to solve the following problems:
1. Which T-OCNT data goes first?
2. How to allocate the data transmission time slot on each T-CONT?
At this time, DBA dynamic bandwidth allocation technology was born. OLT efficiently adjusts the time slots occupied by the backup ONU based on the report information uploaded by the ONU.
（4）Types of DBAs
1.The fixed bandwidth DBA template is completely reserved for a specific ONU or a specific service of the ONU. Even if the ONU has no upstream service flow, this part of the bandwidth cannot be used by other ONUs. Fixed bandwidth is mainly used for services that are very sensitive to service quality, such as: TDM, VoIP, etc.
2.The guaranteed bandwidth DBA template guarantees the bandwidth available when the ONU needs to use the bandwidth. When the actual service traffic of the ONU does not reach the guaranteed bandwidth, the DBA mechanism of the device should be able to allocate its remaining bandwidth to the services of other ONUs.
3.The guaranteed bandwidth + maximum bandwidth combination type guarantees users a certain amount of bandwidth while also allowing users to occupy a certain amount of bandwidth, but the total will not exceed the maximum bandwidth configured by the user. This bandwidth type is mainly used in VoIP and IPTV services.
4.The maximum bandwidth DBA template is the upper limit of the bandwidth that can be obtained when the ONU uses the bandwidth, which satisfies the contention resources used by the ONU to the greatest extent. The maximum bandwidth type is commonly used for common Internet access services.
5.The combination of fixed bandwidth + guaranteed bandwidth + maximum bandwidth not only reserves fixed bandwidth resources for users that cannot be preempted by other users, but also ensures that guaranteed bandwidth is available when bandwidth is needed. At the same time, it allows users to preempt certain bandwidth, but the total will not exceed the maximum bandwidth configured by the user.
（5）Benefits of a DBA
1. ONU actively reports the data to be sent in the upstream direction to OLT. OLT dynamically adjusts the upstream bandwidth of each ONU according to congestion and current bandwidth utilization, as well as DBA type configuration, thereby improving bandwidth utilization.
2. With improved bandwidth utilization, one GPON port can access more users.
3. With the dynamic adjustment of bandwidth by DBA, users can obtain the maximum bandwidth exceeding the fixed bandwidth allocation and enjoy better services.
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