Upstream Bandwidth Assignment-DBA [GPON Technical Posts 09]

Hello, dear friends!

This post is about Upstream Bandwidth Assignment - DBA, as part of the GPON Technical Posts section on the Community. Please see more details below.

BACKGROUND INFORMATION

In the introduction to GPON, we have mentioned the DBA multiple times. Then, what is DBA? What are its functions? You will get an understanding after reading this chapter. 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 do 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 vary but 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 PRINCIPLES

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 that 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.

TYPES

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.

Type 2 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.

Type 3 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 services.

Type 4 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.

Type 5 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.

Now, do not hesitate to read through the post and give us your correct answers. If you have any difficulty understanding the content or want to learn more about the GPON technology, please leave a message and reply to the post. We are looking forward to your comments. Answers for the previous session: BD Questions for this session: (Multiple choice with more than one correct answer)

Which of the following statements about the DBA is correct?

1. The adjustment of DBA for the upstream bandwidth is dynamic.

2. DBA can improve upstream bandwidth utilization.

3. The NSR mode does not occupy the OLT resources.

4. DBA has five types. [GPON Technical Posts 08] Resource Sharing: Multiplexing Principles

LINKS OF GPON TECHNICAL POSTS

[GPON Technical Posts 01] Introduction to the Evolution of the Access Network

[GPON Technical Posts 02] Development of Optical Access Network Technology

[GPON Technical Posts 03] PON Family History

[GPON Technical Posts 04] Evolution of the GPON Technology

[GPON Technical Posts 05] Composition of a GPON Network

[GPON Technical Posts 06] How Does GPON Implement Upstream and Downstream Transmission?