5G! Is your network ready?

With Mobile World Congress having just wrapped up, thought I'd take a moment to talk with Hal Roberts of Calix who has been actively involved in the standards work that will enable NG-PON2 to be used for 5G fronthaul.

Teresa: Hal, I understand that NG-PON2 is being considered for 5G fronthaul for various features of the technology. Can you briefly touch upon these features?

Hal: 5G fronthaul requires high speeds and lower latency and jitter. NG-PON2 with its multiple wavelengths delivers speeds up to 40Gbps, and its low latency and jitter make it uniquely suitable amongst PON technologies to deliver 5G fronthaul. Segregating 5G traffic onto a separate wavelength from other traffic avoids the delay associated with the ranging window, where all upstream traffic must halt to allow a new ONU to range into the system. In addition, the standard has introduced finer granularity for upstream bandwidth allocations, further reducing latency.

Teresa: Is there anything being done in the ITU standards to enhance the performance of NG-PON2 for fronthaul?

Hal: Yes, work is being done in the ITU standards to define a methodology called Cooperative Dynamic Bandwidth Assignment (Cooperative DBA) which has the potential of reducing latency that is associated with the standard form of DBA.

Teresa: From my understanding of mobile fronthaul, specifically CPRI, the bandwidth needs are constant for each CPRI service. Why would DBA be needed? Couldn't a constant bit rate service be allocated for each CPRI link?

Hal: That is correct, CPRI will not benefit from DBA, cooperative or otherwise. However, CPRI is a very inefficient transport for fronthaul. The industry has been considering other more efficient protocols and ‘eCPRI’ was recently defined at the end of last summer as a more efficient replacement for CPRI. eCPRI has a bandwidth demand that varies with user traffic, unlike CPRI. Therefore, eCPRI can benefit from the ability of DBA to efficiently allocate bandwidth resources where they are needed as traffic on each eCPRI service ebbs and flows.

Teresa: How does C-DBA avoid the normal time lag associated with DBA?

Hal: To understand this we must look to the fronthaul network. A fronthaul transport connects the BBU (baseband unit) with the RRU (remote radio head). These two components comprise the ‘mobile base station’ from previous architectures. These interfaces have different terminology with eCPRI, but the concept remains the same. The BBU knows when bandwidth will be needed in the upstream from the RRU to the BBU as it is needed. There is no need to wait for packets to queue up at the ONU waiting for an upstream allocation. The concept of C-DBA is that that BBU informs the OLT when bandwidth is needed in advance of the packet's arrival at the ONU. The OLT grants the upstream timeslots as they are needed.

Teresa: What specifically, is the ITU planning to put into the NG-PON2 requirements to define C-DBA?

Hal: At the December 2017 ITU-Q2 Shanghai meeting it was agreed that a C-DBA interface, between the BBU and the OLT, should be defined. The group set an aggressive target date for the consent of a new addition to G.989.3 section 7 to define this interface by the next ITU Plenary meeting in October of 2018.

Teresa: Thanks, Hal, it’s really exciting to see the continued expansion of applications for NG-PON2.