Why Is an Advantange the DBA in the Upstream for GPON

Hi,

In response to your question, I would like to add a few words on this basis:

In the GPON downstream transmission, the backbone optical fiber from the OLT GPON port is divided into N branch fibers through an 1:N optical splitter, and the data is transmitted to branch fibers in broadcast mode. Each branch fiber bears the same data as the backbone fiber. After broadcast data reaches an ONU, the ONU only accepts data intended for it, and discards data not intended for it.

In the GPON upstream transmission, data from different ONUs is transmitted upstream through the respective branch optical fibers. The data is converged after passing through optical splitters. In the upstream direction, time division multiplexing (TDM) is used to transmit data. The upstream link is divided into different timeslots. The timeslot for each ONU to transmit data is centrally scheduled and authorized by the OLT.

An ONU can transmit data upstream only in the timeslot allocated to it by the OLT. Even if there is no data to be transmitted, the ONU needs to send a blank frame. If an ONU does not comply with the rule and sends data in a timeslot allocated to another ONU, data collision occurs, causing packet loss. A timeslot is a microscopic concept. Generally, the traffic or bandwidth is used to indicate the uplink transmission authorization obtained by an ONU. How to allocate upstream timeslots (namely bandwidth) is an important technology of the OLT. 

Dynamic bandwidth assignment (DBA) serves this purpose.

Thanks!

Hello,

What Is DBA?
In a GPON system, the upstream data of multiple ONUs is converged on the backbone optical fiber and then transmitted to the OLT. In order to avoid collision between data packets in the upstream direction, the system needs an effective time division multiplexing mechanism to control the allocation of the uplink traffic. This mechanism needs to solve the following problems:

                  

• Which T-CONT sends data first?                                                              

• How to allocate the data transmission timeslots for each T-CONT?                                                      
  

             

The Dynamically Bandwidth Assignment (DBA) monitors the PON congestion in real time. The OLT dynamically adjusts the bandwidth according to the congestion status, current bandwidth utilization, and configurations.
Bandwidth can also be allocated in static mode, or fixed mode. In this mode, an OLT periodically allocates a fixed bandwidth to each ONU based on the ONU's service level agreement (SLA), bandwidth, and delay indicators.
In fixed mode, an OLT uses a polling mechanism. The bandwidths allocated to ONUs may va****ut the bandwidth allocated to each ONU is the same in each polling period. The bandwidth guarantee depends on an ONU's SLA but not on its upstream service traffic. An ONU is allocated a fixed bandwidth even when it carries no upstream services.
This allocation mode is simple and applies to services (such as TDM) that have fixed traffic, but does not apply to IP services that have burst requirements on bandwidth. If this mode is applied to the IP services, the upstream bandwidth usage is low because the upstream bandwidth cannot be adjusted dynamically based on the upstream service traffic.

Why is DBA required?
                                                       

• The ONU proactively reports the data to be sent in the upstream direction to the OLT. The OLT dynamically adjusts the upstream bandwidth of each ONU according to congestion status, current bandwidth utilization, and DBA type configuration, thereby improving bandwidth utilization.                                                              

• With the increase of bandwidth utilization, one GPON port can connect more subscribers.                                                              

• The DBA mechanism dynamically adjusts the bandwidth so that subscribers can obtain a bandwidth higher than the maximum fixed bandwidth, thereby enjoying better services.                                                      
  

                                     
Implementation Principle of DBA

First, you need to know two DBA implementation modes: status reporting (SR) mode and non status reporting (NSR) mode. The difference between the two modes is that an ONU proactively sends a status report for data to be sent in SR mode. In NSR mode, the OLT monitors the upstream data of an ONU in real time, and the ONU does not need to report the data status. In NSR mode, OLT resources are occupied. Therefore, the SR mode is recommended.
                  
Now, it is assumed that an ONU currently has 3 T-CONTs which need to send data upstream.
The ONU adds a status report for data to be sent in the current T-CONTs in the upstream data flow. That is, the ONU sends an upstream dynamic bandwidth report (DBRu) to the OLT to report the status of T-CONTs and apply for the bandwidth for the next upstream data transmission.
                                                       

After receiving a status report from an ONU, the OLT calculates the bandwidth map (BW map) according to the DBA algorithm logic, identifies the start and end time for each T-CONT to transmit data, and delivers the information to the ONU along with the downstream data.
                  
                  After receiving a BW map message from the OLT, the ONU sends data in the specified timeslot with a DBRu.

Five DBA types

                  When the OLT receives DBA report messages from an ONU, that is, after the ONU uploads the T-CONT messages, how does the OLT allocate timeslots to T-CONTs? Which T-CONT sends data first and which later? This is why DBA types are introduced. There are five DBA types: type1, type2, type3, type4, type5. T-CONTs are the physical resources of an ONT. A T-CONT can allocate a bandwidth only after it is bound to a DBA profile. After a T-CONT is successfully bound to a DBA profile, the T-CONT can provide flexible dynamic bandwidth allocation solutions based on different settings of the DBA profile.
      

• Type1 indicates a DBA profile of the fixed bandwidth type. The fixed bandwidth is reserved for a specified ONU or certain services of the ONU. It cannot be used for other ONUs even when the upstream service stream is not transmitted on the ONU. This type of bandwidth is mainly used for services, such as TDM and VoIP, that have a high QoS requirement.       

                                            

• Type2 indicates a DBA profile of the assured bandwidth type. The assured bandwidth is the available bandwidth of an ONU when the ONU requires the bandwidth. When the actual service stream does not reach the assured bandwidth, the DBA mechanism of the device is used to allocate the remaining bandwidth to services of other ONUs.                                                                     

• Type3 indicates a DBA profile of the assured bandwidth + maximum bandwidth type. This type of bandwidth is the bandwidth of the combined type. When it is used, a subscriber is allocated with a certain bandwidth and at the same time occupies certain bandwidths. The total bandwidth, however, cannot exceed the maximum bandwidth configured for the subscriber. This type of bandwidth is mainly used for VoIP and IPTV service.                                                      
  

                                                       

• Type4 indicates a DBA profile of the maximum bandwidth type. This type of bandwidth is the maximum bandwidth that can be used by an ONU to meet the ONU bandwidth requirement to the greatest extent. It is used for services such as Internet access service.                                                      
  

                                                       

• Type5 indicates a DBA profile of the fixed bandwidth + assured bandwidth + maximum bandwidth type. This type of bandwidth is the bandwidth of the combined type. When it is used, a subscriber is allocated with a fixed bandwidth that cannot be used by other subscribers. In addition, the subscriber can use the assured bandwidth when necessary and can occupy certain bandwidths. The total bandwidth, however, cannot exceed the maximum bandwidth configured for the subscriber.  

In the GPON upstream transmission, data from different ONUs is transmitted upstream through the respective branch optical fibers. The data is converged after passing through optical splitters. In the upstream direction, time division multiplexing (TDM) is used to transmit data. The upstream link is divided into different timeslots. The timeslot for each ONU to transmit data is centrally scheduled and authorized by the OLT. 

An ONU can transmit data upstream only in the timeslot allocated to it by the OLT. Even if there is no data to be transmitted, the ONU needs to send a blank frame. If an ONU does not comply with the rule and sends data in a timeslot allocated to another ONU, data collision occurs, causing packet loss. A timeslot is a microscopic concept. Generally, the traffic or bandwidth is used to indicate the uplink transmission authorization obtained by an ONU. How to allocate upstream timeslots (namely bandwidth) is an important technology of the OLT. Dynamic bandwidth assignment (DBA) serves this purpose.

Detail

https://support.huawei.com/huaweiconnect/carrier/en/thread-446015.html

https://support.huawei.com/huaweiconnect/carrier/en/thread-445969.html

Thanks