Remote PHY

Remote PHY (also known as R-PHY, R PHY, and RPHY) is popular among service providers because it does an effective job of alleviating the rack space, power, and cooling constraints in the hub of a network. It does this by separating out the PHY layer and redistributing it out to the fiber node. CableLabs created the Remote PHY specifications that have become the standard for the industry. But this begs the question as to why there are constraints in the hub in the first place.

It’s due to the insatiable appetite of subscribers for bandwidth to support services such as 4k and IP video and other bandwidth intensive offerings. Add to this the fact that most households and businesses now have multiple devices accessing a network at any given time, and it becomes obvious why providers are rushing to provide gigabit speed services to the home and an increasing number of businesses.

For this to happen, an exponential number of node splits are required to reduce the size of the downstream service groups. In some cases, increasing the number of nodes by a multiple of 5-10 over the next five years. While this can increase speeds, it also creates new problems in the head-end or hub. More space/power/cooling capacity are required in the hub to accommodate the exponential number of nodes splits. Remote PHY relieves these issues.

Distributed Access Architectures

Remote PHY is part of a larger family of technologies called distributed access architectures (DAA) that alleviate congestion in the hub. In general, DAA technologies such as Remote PHY, R-MACPHY, and R-CCAP virtualize and move certain aspects of a network out of the hub and closer to subscribers.

Hubs are evolving from housing row after row of specialized equipment and RF splitting/combining networks, into potentially nothing more than a small collection of optical switches and routers (akin to a mini data center). R-MACPHY and R-CCAP have been adopted to some extent by the cable industry but Remote PHY is the most popular to-date due to its early adoption by established network equipment vendors and early CableLabs standardization.

Advantages of Remote PHY

Besides reducing space/power/cooling requirements in the hub, Remote PHY also eliminates the analog optical link and replaces it with a commodity digital 10G Ethernet link. This provides distinct advantages for a network moving forward. A digital link is easier to set up, taking less time to deploy. The link is more reliable, requiring less maintenance and manpower in the future. Significant signal to noise ratio (SNR) gains are also achieved using digital optical links versus the old amplitude modulated links, potentially enabling higher modulation orders for DOCSIS 3.1 downstreams.

The 10G Ethernet link creates an economical path for providers to add new services in the future. If an operator needs to spawn an FTTH connection to a high-usage customer or small business, this can be accomplished very economically with 10G Ethernet already nearby. The digital link also allows for longer fiber runs in a network enabling greater flexibility in collapsing hub sites. Finally pushing fiber deeper into the plant via DAA is directionally aligned with N+0 plans to support future Full-Duplex DOCSIS (FDX) rollouts.

R-PHY Challenges for Testing

Deploying Remote PHY creates some unique challenges for testing. Specifically, R-PHY removes the RF feed from the hub, which also removes the possibility for the use of dedicated test and monitoring gear in these locations. This change has a direct impact on actions such as return path monitoring, sweep and transmitting leakage tagger signals.

Remote PHY also has the potential of introducing timing issues between the MAC and PHY layers as R-PHY separates the previously co-located MAC and PHY layers. The separation of the creates distance between the two making it more difficult to keep the timing in sync. If precision timing protocol (PTP) messages are delayed for any reason, timing sync can be thrown off, and as a result, upstream packets from different modems collide and create upstream BER. Troubleshooting is difficult for technicians that don’t have the right knowledge and tools to recognize this type of issue.

Fiber has long been a part of the HFC, but distributed access architectures like R-PHY are rapidly accelerating the deployment of advanced fiber technologies like dense wavelength division multiplexing (DWDM) as fibers are migrated from analogue optical links to digital Ethernet links.

Turning up R-PHY nodes will require the certification or validation of the fiber physical layer links feeding the new R-PHY nodes to ensure that newly activated DWDM wavelengths route correctly from hub to node through any optic MUX/deMUX (optic combiner/splitter) and that the DWDM channels supporting the various R-PHY nodes are present and with the correct optic power levels. This applies equally to newly laid fiber and existing analogue fibers, after all those existing fibers may have been carrying light previously but not at these new DWDM wavelengths.

DWDM networks are more complicated to install, troubleshoot, and maintain versus previous technologies and require new equipment to support. The other fiber-related challenge created by R-PHY is the need for many more technicians to be equipped and skilled to work with fiber due to the massive increase in deployed fiber. The migration of 10G Ethernet from hubs/headends out into the field creates similar challenges. No longer is this a niche technology understood by just a handful of hub/headend engineers, maintenance tech’s must now be trained and equipped to properly maintain and troubleshoot 10G Ethernet.

Remote PHY has also created a proliferation of NEMs in the market giving providers more choices, but also creating confusion and complexity around testing. Many NEMs offer point-solution testing tools that only partially support other vendors' gear. This means technicians must be trained and equipped with a wider mix of testing combinations to accurately maintain and troubleshoot the situation they may face on any given day.

Leveraging the Remote PHY Unit

Virtualization of certain aspects of the hub for Remote PHY obviously has its challenges but also creates advantages moving forward. The R-PHY unit (RPU) can now assume several roles including monitoring the upstream RF, supporting the field find and fix for technicians, and enabling return sweep including real-time field meter interaction. It also provides the downstream RF tagging needed for leakage monitoring and troubleshooting systems.

For Remote PHY, Ethernet test gear designed for CATV is used to validate new R-PHY device turn-ups and troubleshoot timing issues for Remote PHY nodes that split the MAC and PHY layers. Virtualizing the upstream spectral analysis capabilities previously handled by hub-mounted hardware into the RPU enables continuity of these critical capabilities in a Remote PHY environment.

The RPU can further be leveraged to transmit and receive sweep telemetry signaling with existing field sweep meters, enabling a common sweep process between legacy and Remote PHY nodes. In this case, the technician uses the same process and meter regardless of the node type or service provision gear in use, effectively insulating the technician from the underlying complexity. Leakage tagger functionality is also virtualized into Remote PHY nodes to enable this critical plant maintenance capability in this new virtualized environment.

Testing and Monitoring Solutions for Remote PHY

VIAVI has developed the OneExpert CATV field instrument and the XPERTrak monitoring solution for networks that are deploying R-PHY and other DAA technologies. The OneExpert CATV tester has the ability to test both legacy and virtualized environments while also automating tests and displaying the pass/fail results on a dashboard. XPERTrak simplifies the transition to R-PHY by enabling continuity of critical test capabilities including interoperation with deployed VIAVI field meters for return sweep and ingress remediation.

For Ethernet testing in R-PHY environments, the T-BERD/MTS 5800 and MAP-2100 provide unmatched field test and monitoring capabilities. 

For the fiber element in R-PHY the 4100 series DWDM OTDR module with Smart Link Mapper (supported on the T-BERD/MTS 2000, 4000V2 & 5800), OCC-56C DWDM Optical Channel Checker and not forgetting fiber inspection tools such as the P5000i (also supported by the T-BERD/MTS platforms) or FiberChek Probe provide everything needed to deploy, certify and troubleshoot new or existing fiber links for DWDM services.

To gain more in-depth knowledge about Remote PHY read, “Remote PHY Architectures: Operational Challenges and Opportunities”. More detailed information regarding sweep testing for Remote PHY and DOCSIS 3.1 can be found in the application note, “Sweeping in an Evolving Network”.

Short on time? There are two great webinars that can help. “Remote PHY: Problems Solved, and Problems Created by DAA” and, “Exploring Distributed Access Architectures” will get you up to speed in no time.