PON System as a 1588 Distributed Boundary Clock

The massive roll-out of LTE base stations to satisfy the ever increasing demand for traffic has forced operators to consider a variety of transport networks to carry backhaul traffic. GPON as a fiber based access network, among other wireline options is well suited for this function. As with any other backhaul network for LTE services, GPON must also deliver stringent phase/time to the subtending radio access network comprising of eNodeBs.

 

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Technologies such as PON and DSL have defined inherent mechanisms to transport accurate frequency synchronization across the links. GPON systems can carry an accurate and stable frequency reference in the physical layer. DSL can transport a frequency reference using the NTR (Network Timing Reference) attribute. In addition, GPON can transport a precise phase reference that is not necessarily based on IEEE 1588TM PTP. However when such networks are required to carry LTE backhaul traffic from eNodeBs that have a native IEEE 1588 PTP slave, then there is a need for interworking between the inherent mechanisms and IEEE 1588 nodes. This is where the concept of “distributed boundary clock (BC)” comes into play and some of the BC functions are split into separate network elements. For example: the Optical Line Terminal (OLT) and Optical Network Unit (ONU) in PON systems. The functionally is akin to a separate PTP slave function in OLT and a separate PTP master function in the ONU.

In this approach, the PTP slave implemented in OLT will act as a slave to a central GM in the CO and obtain its traceable Time of the Day (ToD) from it. This ToD can be used to set the ToD for the PON system. A method such as the one defined in G.984.3 Amendment 2 could be used to distribute the ToD from the OLT to the ONU. Once the ONU has a traceable ToD, it could then act as a boundary clock as long as it has the other attributes of a T-BC (Telecom Boundary Clock). For example, the ONU (as a T-BC) may have an alternate time reference, say via GNSS, then a combination of physical layer and PTP as assist mechanisms to GNSS will go a long way in providing good holdover time with a relatively inexpensive oscillator during GNSS outages.

Typically, there are multiple ONUs connected to a single upstream OLT. Each OLT provides the slave half of the PTP-BC function and the ONU (or ONT) acts as PTP the master to the endpoint application (eg: LTE pico cell). The OLT’s slave may also support hybrid SyncE function in some customer requirements.

Qulsar’s managed clock engine M88 is the optimal choice for both the OLT and ONU applications. For more details on the M88, please check out the M88 product page.

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