World’s first demonstration of modularization of dynamic bandwidth assignment; a crucial function to QoS in optical access system

November 20, 2018

- To achieve future optical access systems quickly to provide various services in 5G era -

Nippon Telegraph and Telephone Corporation (headquartered in Chiyoda-ku, Tokyo; CEO Jun Sawada; hereinafter “NTT”) has developed a prototype of an optical line terminal (OLT) with a softwarized and modularized dynamic bandwidth assignment (DBA*1) function, which represents a significant impact on quality of service (QoS) in optical access system (e. g. FTTH; Fiber to the Home). The prototype successfully supports, for the first time, the replacement of the DBA software module to meet changes in service requirements. This advance enables carriers to use a common access system for a diverse range of services including accommodation of base stations for the fifth generation (5G) and beyond 5G mobile systems.

NTT has already introduced the FASA*2 (Flexible Access System Architecture) concept, and tried to modularize the functions essential to access system operation. Although DBA is well known to be one of the most difficult functions to modularize because of its timing-critical feature, we have succeeded in modularization by dividing it into a software part that is replaced to suit the service requirements, and a hardware part independent from the requirements. Furthermore, an API*3 (Application Programming Interface) that facilitates the cooperation and control of the two parts is introduced. Combining this approach with low-latency optical access technology*4 offering coordination with 5G mobile systems will yield optical access systems that can be quickly altered to realize various new services in the 5G era. In addition to the DBA function, continued modularization of other various functions and the replacement of software components will avoid the significant investment of redeveloping hardware-level equipment from scratch. This will result in cost-effective access systems that can quickly meet various requirements.

In order to establish the API as open specifications for use by various partners, we are currently working to advance standardization activities in the Broadband Forum. NTT will push forward its R&D activities which are even now contributing to the expansion of the application area of optical access systems in cooperation with other carriers, system vendors, standardization organizations, and open source software development organizations.

The accomplishments will be presented at NTT R&D Forum 2018 (Autumn)*5 and an international exhibition on network virtualization technologies, ONF CONNECT*6.


The current optical access systems used by the FTTH service are based on equipment purpose-built for just the service intended. Therefore, in order to support a new service with new requirements, it is necessary to redesign and develop complete equipment sets (mainly OLT) from scratch. This hinders the agile adaptation of the system to other services, and cost-reductions are hard to achieve if the initial production scale is constrained. Meanwhile, optical access systems are expected to better support future mobile systems in order to establish the efficient construction of 5G (and beyond 5G) mobile networks, which require the installation of many more base stations than before.
In Feb. 2016, NTT launched the new concept of FASA with the target being the technology to realize the access network of the future. Since then, NTT has conducted R&D activities on primary technologies and specifications. FASA does not use service-specific hardware, unlike conventional access network systems, as it is based on the concept of "modularizing the various individual functions of access equipment as much as possible to enable the free combination of these individual components." The goal is to incorporate newly desired functions flexibly and economically to satisfy the latest service requirements while maintaining service quality.

What we have achieved [Fig. 1]

The application area of optical access systems can now be significantly expanded as NTT has achieved one of the key milestones in modularization by FASA, the technology for softwarizing the DBA function. NTT has also succeeded in developing and testing OLT prototypes with the API that enables the smooth replacement of DBA software modules. NTT has successfully demonstrated, for the first time, that the DBA software can be replaced to meet new service requirements. As this validation experiment examined several DBA software modules designed to support mobile, factory, and other usage scenarios, NTT has explicitly showed that the OLT can be quickly applied to support different services by simple software replacement.

Fig. 1 Replacement of softwarized DBA functions to meet each service requirements.

Fig. 1 Replacement of softwarized DBA functions to meet each service requirements.

The technical achievements are shown below.

1) “Softwarization technology of DBA function” to significantly expand the application area of optical access systems

Modularization (disaggregation) of optical access systems is under extensive discussion and development activities in open source software (OSS) and standardization organizations (SDO), such as Open Networking Foundation (ONF) and BBF. However, DBA function modularization was thought to be out of scope by such organizations because of the extremely high-speed processing requirements of the DBA function (of the order of sub-milliseconds). However, NTT remained resolute in its R&D activities because drastic progress in optical access system flexibility is possible once the DBA function is modularized. NTT’s current solution to DBA modularization is to divide it to a software part, which can be replaced to meet service requirements, a hardware part, which is independent of service requirements, and an API to facilitate the cooperation and control of the two parts. In order to establish the API as open specifications that can be used by various partners, we are currently active in standardization meetings in the Broadband Forum.

2) The world’s first demonstration of software replacement of DBA functions in response to service requirements.

NTT conducted the proof-of-concept demonstration in order to verify DBA software replacement in response to new service requirements by using OLT prototypes that implemented our API. The box-type OLT is assumed to be used in the central offices of carriers for FTTH services. It is also expected to be applied to 5G mobile systems with the use of a low-latency optical access technology. The module-type OLT is assumed to be used with a commodity server. Among the conventional OLT functions, only the hardware-oriented functions are left inside the OLT module while the other functions, all realized by software, are located in the server. This architecture well suits small-start services, for example, local area networks in factories, campuses, and office buildings. As this validation experiment examined several DBA software modules that were designed to create services suitable for mobile, factory, and other application scenarios, NTT has successfully shown that its OLT rapidly implement different services by simply replacing software. Beyond the DBA function, the modularization of other various functions, which will permit the replacement of more software components, will avoid the significant costs demanded by redeveloping OLT hardware from scratch. This will result in a cost-effective access system that can quickly meet various requirements.

Looking forward

NTT will pursue the international standardization of API specifications enabling the replacement of softwarized DBA functions. NTT will work to modularize the remaining access functions in cooperation with carriers, system vendors, SDOs, and OSS organizations. NTT will continue pursuing these approaches to achieve future optical access systems that can quickly and cost-effectively implement the widest possible range of services.


*1 DBA - Dynamic Bandwidth Assignment. This is a dynamic bandwidth control function for point-to-multi point optical access networks. In order to avoid the collision of upstream data from multiple ONUs, the DBA function in an OLT assigns times to each ONU at which to start upstream transmission and the transmission duration.

*2 FASA - Flexible Access System Architecture. “Introducing the New FASA Concept for Future Access Systems” - NTT news release Feb 8, 2016
FASA Home page:

*3 API - Application Programming Interface. This is an interface that specifies how components should interact.

*4 Low-latency optical access technology - “Development and trial of low-latency optical access technology that operates in coordination with a 5G mobile system” - NTT news release Feb 14, 2018

*5 NTT R&D Forum 2018 (Autumn) - To be held November 29-30, 2018 at the NTT Musashino R&D Center (Musashino, Tokyo).

*6 ONF CONNECT - To be held December 4-6, 2018 (Santa Clara, United States of America).

Attachments / References

Fig. 1: Replacement of softwarized DBA functions to meet each service requirement.

FASA is a registered trademark of NTT.

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