Cable’s 10G Platform to Provide Synchronization for 5G
Cable service providers operate an extensive hybrid fiber coax (HFC) infrastructure to serve residential and business fixed broadband. In recent weeks, the world witnessed how cable networks around the globe have withstood the test of a dramatic surge in capacity demand due to the work-from-home (WFH) and other xFH practices induced by COVID-19 pandemic and are holding up extremely well.
As the economy opens again and 5G deployments resume, a large part of the time lost due to the COVID-19 pandemic can be regained by leveraging the extensive wireline networks to transport the mobile 5G traffic, be it fronthaul, midhaul or backhaul (collectively termed “xhaul”) between the radio units (RUs) or Base Stations (BSs) and the RAN Infrastructure. A critical impediment that stood in the way of leveraging the ubiquitous HFC infrastructure was the inability to provide timing and synchronization to the radio units which is crucial to their operation.
For nearly two years, the CableLabs Mobile Xhaul vendor and operator team has been working on equipping the DOCSIS® technology to provide better xhaul for mobile traffic.
Today, we are happy to announce the publication of the first release of the Synchronization Techniques for DOCSIS Technology Specification. When coupled with the Low Latency Xhaul Specification (LLX) standardized last year, which specifies requirements to reduce the latency on the DOCSIS network for mobile traffic, the two together provide the performance needed for DOCSIS network to xhaul mobile traffic. The ubiquity of the HFC plant will greatly assist the economic and timely deployment of these new 5G radios.
Synchronization Over DOCSIS Network
The mobile network is synchronous by design and requires the sharing of a common clock. This is achieved in practice by means of the radios and “their controllers” connecting to the Global Navigation Satellite System (GNSS). This works well for outdoor macro deployments. For small cell deployments, especially indoors, more often than not GPS signals are either not available or not economical. Instead, an equivalent global clock signal is transported over the IP network using precision time protocol (PTP), specified in the IEEE 1588-2008 family of specifications.
Transporting PTP over the DOCSIS network is particularly challenging due to the asymmetrical nature of the DOCSIS network. Leveraging the DOCSIS Time Protocol (DTP) to address the asymmetry issue offers a practical solution. A high-level architecture of the solution framework is illustrated in the figure below (technical details can be obtained in this SCTE white paper). DTP was invented back in 2011 and incorporated into the DOCSIS 3.1 specifications in 2013. In the newly issued SYNC specification, the Mobile Xhaul team updated the DTP profiles, defined timing system architectures and specified requirements on the DOCSIS network equipment to make PTP work end-to-end. As a result, the DOCSIS specification when bolstered with the newly issued SYNC spec and the LLX spec, is capable to support the LTE and 5G timing requirements.
The Mobile Xhaul team invites cable and mobile operators as well as vendors to provide input to these latest set of specifications. Several HFC equipment vendors have already demonstrated the feasibility of DTP in various proof of concept (PoC) implementations. In the upcoming months, our team will complete additional requirements and timing architectures.
Soon, cable MSOs will be upgrading their HFC plants to the distributed access architecture (DAA). DAA nodes are already PTP-compatible, as PTP is needed for the R-PHY device and the CMTS core to be on the same timing island. The MSOs and cable equipment vendors are better off designing their new network architectures with mobile requirements in mind and ensure that the DAA nodes can support the 1.5µs of end-to-end timing requirement needed for LTE and majority of the 5G deployments as specified in the SYNC spec.
We are excited to offer the ability of the DOCSIS technology to provide reliable and precision timing services. This will aid the ubiquitous HFC wireline network to become an obvious choice for the mobile operators as a low CAPEX and fast-to-deployment xhaul solution. We are working hard to converge the 10G and the 5G technologies, and SYNC is one of the areas that has come to fruition.
We acknowledge the tremendous efforts of the Mobile Xhaul team in driving these specifications to a timely publication, specifically those who did heavy lifting in the SYNC spec: John Chapman (Cisco), Elias Chavarria Reyes (Cisco), Peter Meyer (Microchip) and Yair Neugeboren (CommScope).