GPON’s Dynamic Bandwidth Assignment (DBA)

Dynamic Bandwidth Assignment (DBA) is used on Passive Optical Network’s to allow better use of upstream bandwidth, and it is especially good for dealing with bursty upstream traffic. GPON and many other PON technologies use Time Division Multiple Access (TDMA) for managing upstream access by ONUs, and at any one point in time, TDMA provides unshared timeslots (fraction of the upstream bandwidth over time) to each ONU for upstream transmissions.

DBA allows upstream timeslots to grow and shrink based on the distribution of upstream traffic loads and operates on a timescale of milliseconds. Of course, the total of all timeslots on a PON cannot be greater than the length of a single upstream frame.

DBA functions on Transmission Containers (T-CONTs), which are upstream timeslots, and each is identified by a particular AllocID. An ONU must have at least one T-CONT, but most have several T-CONTs, and each corresponds to a particular upstream time slot on the PON. Without DBA support in the OLT, upstream bandwidth is statically assigned to T-CONTs, cannot be shared, and can be changed only through a management system.

There are two methods to determine the bandwidth requirements of ONTs. The first, called Status Reporting (SR) DBA, involves explict T-CONT buffer status provided by the ONTs. With this method, the OLT solicits T-CONT buffer status, and the ONUs respond with a report for each assigned T-CONT. The other method is known as Traffic Monitoring (TM). With TM DBA. the OLT imputes how much bandwidth is required by monitoring the number of idle frames sent in a particular T-CONT. A GPON OLT must support both methods, but ONUs need not provide any support for DBA; the OLT will just use Traffic Monitoring DBA for ONUs that provide no support for DBA.

DBA provides a way for an OLT to oversubscribe its upstream bandwidth and provides an effective increase to average bandwidth available to each of its ONTs. But DBA brings with it the possibility of a carrier not being able to satisfy all the upstream bandwidth granted to ONTs. Static bandwidth assignment does not have this problem, but it does not allow for any more than the 1.244 Gbps of the upstream to be assigned to the ONTs. The bottom line is that DBA allows more ONUs on a single GPON, and it allows more bandwidth to be assigned to each ONU, with no changes to the optics in the OLT or the ONUs.

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