Dynamic bandwidth allocation(DBA) technology refers to the technology that adapts to various changes in the user rate by changing the real-time bandwidth dynamics of each ONU in the GPON system to improve the bandwidth utilization of the system. The uplink access of the PON system is generally based on the combination of central control on-demand allocation and fixed allocation, that is, the static bandwidth allocation and dynamic bandwidth allocation specified in ITU-T G983.4, and is assigned according to the business level. For data communication in GPON, this variable rate service is not suitable for static bandwidth allocation. It is necessary to greatly increase the system bandwidth utilization through dynamic bandwidth allocation, that is, DBA technology according to the requirements of ONU burst services. The utilization efficiency of uplink bandwidth is improved by dynamically adjusting bandwidth between ONUs. According to the characteristics of GPON and G983 recommendations, It can be known that the specific requirements for dynamic bandwidth allocation are: transparent business, high bandwidth utilization, low jitter and delay characteristics, fair bandwidth allocation, strong signal, real-time, QoS that guarantees different services.
1.1 Types of bandwidth
The GPON standard follows the regulations for QoS support in BPON. ITU-T clearly identifies four priority levels of bandwidth in the BPON standard G. 983.4 and GPON standard G984.3. They are the four types of bandwidth, Fixed, Assured, Non-Resured, and Best-Ffort, which are fixed types, guaranteed types, non-guaranteed types, and the best possible type of bandwidth. At the same time, the transmission container(T-CONT) of the bandwidth allocation basic unit in the GPON uplink system is divided into five types according to the type of bandwidth it uses. The priority allocation scheme based on business level can provide good QoS protection for business while implementing efficient DBA. Therefore, it is important for DBA to clearly recognize the characteristics of various bandwidth resources and understand the scope of application of T-CONT.
1.2 Transmission vessel T-CONT
As pointed out earlier, GPON has four types of bandwidth, each type of bandwidth can support a certain QoS required business: T_CONT indirectly guarantees the QoS requirements of the service by providing specific types of bandwidth for the business. Whether a T-CONT can support a certain QOS requirement depends entirely on whether the T-CONT can provide the bandwidth required to meet the QoS requirements of the service. If the T-CONT provides the bandwidth required to meet the QoS requirements of a business, the business can be hosted. Of the five types of T-CONT, type L type T-CONT only provides Fixed type bandwidth; Type 2 type provides only the bandwidth of the Assembly type: Type 3 type provides the bandwidth of the Assembly type and the Non-aid type; The tyPe4 type provides only Best-effort type bandwidth: the type5 type is the first four types of fusion that can provide all types of bandwidth and support all types of business. These five T-CONTs are sometimes referred to in this article as T-CONTs for Types 1, 2, 3, 4, and 5, respectively.
1.3 GPON Implementation of DBA
According to the protocol, GPON's DBA process includes the following five steps: congestion status detection, congestion status reporting to OLI, OLT re-allocation according to the supplied parameters, OLT distribution information according to the updated bandwidth distribution, and T-CONT type dispatch authorization, Implement consultation for management information in DBA. The DBA process is shown in Figure 2.
On the Structure and Key Techniques of GPON System
Figure 2 GPONDBA Process Chart
According to the terms of the agreement, GPON's DBA is divided into two modes: one is that ONU reports its own state and required bandwidth to OLT, and OLT performs DBA on ONU based on reported data, the so-called state-based reporting. Dynamic bandwidth allocation(SR-DBA, Status Report DBA): The other is that ONU does not need to report to OLT. OLT implements automatic dynamic bandwidth allocation from its own traffic monitoring function, that is, dynamic bandwidth allocation based on non-state-based reports(NSR-DBA, On Status Report DBA).