Remote PHY (also known as R-PHY, R PHY, and RPHY) is a type of distributed access architecture (DAA) that moves the physical layer from the headend or hub to the edge of the access network. Remote PHY architecture splits the components of the headend between the MAC layer and PHY layer. This innovative technology allows the MAC functionality, including bandwidth scheduling and signaling, to remain in the headend, while the PHY functionality is pushed to the network’s fiber nodes.
A Remote PHY device (RPD) at the network edge converts the downstream data transmitted from the core from digital to analog and transforms upstream data from end users from analog to digital format. The RPD includes QAM modulators for downstream translation and QAM demodulators for upstream traffic. The converged cable access platform core (CCAP core) at the headend or in a data center can include a CMTS core element to service DOCSIS transmission as well as an EQAM core for video signals.
The Importance of Remote PHY
Remote PHY has gained popularity among service providers by alleviating the rack space, power consumption, and cooling constraints in the hub of the network. CableLabs introduced the Remote PHY specifications in 2015 to take full advantage of DOCSIS and Ethernet transport technologies, allowing cable operators to efficiently transmit more data over the same amount of spectrum and meet customer demands for faster transport speeds.
The insatiable appetite for bandwidth to support services such as 4k and IP video has now validated the projections that Remote PHY explained. An increase in remote work and schooling has further transformed network usage patterns, with more demand for videoconferencing, educational platforms, and other more symmetric (equal upstream and downstream consumption) services. Upstream consumption has increased significantly singe the pandemic and shows no signs of slowing. Most households and businesses now have multiple devices accessing the network at any given time, further driving demand for higher service tiers.
For this to happen, an exponential number of node splits are required to reduce service group sizes. In some cases, the number of nodes will increase by a multiple of 5-10x over the next five years. While this can increase speeds and per-subscriber capacity, 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 known as distributed access architectures (DAA) that alleviate congestion in the hub by spreading the load across the access network. In general, DAA technologies such as R-PHY, R-MACPHY, and R-CCAP provide options to virtualize and move specific elements of a network out of the hub and closer to subscribers.
As more functionality moves away from the core, hubs are gradually evolving from housings with rows of specialized equipment and RF splitting/combining networks into collections of optical switches and routers, similar to “mini” data centers. R-MACPHY and R-CCAP have been adopted to some extent by the cable industry, although Remote PHY is the most popular DAA option to-date, based on enthusiastic early acceptance by network equipment manufacturers (NEMs) and a fully completed and released CableLabs standard.
Flexible MAC Architecture (FMA) represents the continued advancement of DAA technology beyond R-PHY, with an architecture conceived around the flexible location of MAC functions in the headend or distributed in the access network. The FMA library of specifications published by CableLabs in September of 2020 defines the disaggregation of management, control, and data planes into discrete elements. FMA also emphasizes equipment and interface interoperability, giving cable operators more opportunities to mix and match the best combination of technologies and suppliers.
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Advantages of Remote PHY
In addition to reducing space/power/cooling requirements in the hub, R-PHY also replaces the analog optical link with a commodity digital fiber 10G or higher Ethernet link. This provides distinct advantages for the network moving forward. A digital fiber link is easier to set up and more reliable, thereby requiring less maintenance and technician time. Significant signal to noise ratio (SNR) improvement is also achieved by using digital optical links instead of amplitude modulated links, potentially enabling more bits to pass through each Hz of spectrum via higher modulation orders in DOCSIS 3.1.
The 10G Ethernet link also creates an economical path for providers to add new services in the future. A new FTTH connection to a high-usage customer or small business can be accomplished very economically with a 10G Ethernet digital fiber connection already accessible. The digital link to the optical node also allows for longer fiber runs in the network, enabling greater flexibility in collapsing hub sites.
Pushing fiber deeper into the access network via DAA is also directionally aligned with fiber deep and N+0 plans, Full-Duplex DOCSIS (FDX) rollouts, and selective FTTH/PON connectivity for high bandwidth businesses and users. The speed and efficiency of R-PHY coupled with 10G EPON or XGS-PON is a dynamic combination for high symmetrical bandwidth with reduced power consumption.
Remote PHY creates new opportunities to support 5G deployment, as the same 10G Ethernet links can be leveraged for 5G xHaul applications. R-PHY expands upon the 5G monetization opportunities for cable providers that include leasing of deployed dark fiber, offering Ethernet bandwidth as a paid service, and utilizing FDX links for 5G xHaul.
The Generic Access Platform (GAP) could potentially promote the service flexibility of R-PHY deployments further by creating a standardized node housing that accepts pluggable modules for 5G, WiFi, FTTH, or DOCSIS. Each slot in a GAP enclosure is allotted a budget for power consumption and thermal dissipation and is fully adaptable to new technology deployment within each service group.
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 negates 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 can also introduce timing issues between the MAC and PHY layers, since R-PHY separates these previously co-located elements of the CCAP core, making it more difficult to keep the timing in sync. If precision timing protocol (PTP) messages are delayed, timing sync can be thrown off, and upstream packets from different modems can collide and introduce errors that are difficult to troubleshoot without the right equipment and training.
Video Verification is another important test challenge for Remote PHY cable operators, since the combined video RF is created at the RPD and cannot be tested before the RPD is deployed and activated. Common video issues include missing channels, programs not running, missing packet identifiers (PIDs), and Out of Band (OOB) carriers that are not enabled. A lengthy and complex manual video verification test process requiring multiple instruments can be reduced to 5 minutes or less, using the OneCheck video analysis application of the handheld OneExpert CATV signal analysis meter.
Remote PHY has created a proliferation of new NEMs entering the market, which gives providers more choices but can also create confusion and complexity concerning test practices. 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 test tool combinations to accurately maintain and troubleshoot the situations they may encounter.
Remote PHY and Fiber
Fiber has been an important element of cable networks since the first hybrid fiber-coaxial (HFC) networks were deployed, but distributed access architectures like R-PHY are rapidly accelerating the deployment of digital fiber and complicating the fiber testing challenges. Advanced fiber transmission technologies like dense wavelength division multiplexing (DWDM) require more comprehensive testing as fibers are migrated from analog optical links to digital Ethernet links.
DWDM networks are more complicated to install, troubleshoot, and maintain versus previous technologies and require new equipment to support. Turning up R-PHY optical nodes requires the certification or validation of the fiber physical layer links feeding the new R-PHY optical nodes. Newly activated DWDM wavelengths must be verified to route correctly from hub to node through any MUX/deMUX (optical combiner/splitter), and the optical power of each channel must be tested at the appropriate wavelength.
This applies to newly laid fiber as well as any legacy analog fibers that are repurposed for Remote PHY. Dispersion of legacy fibers should also be evaluated, since chromatic dispersion (CD) and polarization mode dispersion (PMD) levels beyond IEEE 802.3ae-2002 limits can impair digital fiber performance at 10G transmission rates.
R-PHY is leading to a massive increase in deployed fiber, meaning more technicians must be trained and equipped appropriately. Complete fiber characterization for high- speed links includes connector inspection, bi-directional loss testing, distance and reflectance measurement, in addition to any DWDM, synchronization, and dispersion testing required by the configuration. R-PHY fiber test tools from VIAVI have empowered technicians by introducing advanced automation, portability, and cloud connectivity to the E2E Remote PHY fiber tool kit.
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 technology within each R-PHY device (RPD) can be useful for 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.
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 RPD enables continuity of these critical capabilities in a R-PHY environment.
The RPD can be leveraged further 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 optical 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
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 meter can be used to test both legacy and virtualized environments while automating tests and conveniently displaying 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. As the industry’s smallest 10G handheld tester, the T-BERD/MTS 5800 performs automated Ethernet service activation and integrated timing/synchronization testing. The MAP-2100 is a rack-mounted test solution for remote Ethernet bit-error-rate (BER) testing that can be utilized effectively for service activation and ongoing transmission quality verification.
For the essential fiber testing processes in R-PHY, the 4100 series DWDM OTDR module with Smart Link Mapper Smart Link Mapper (supported on the T-BERD/MTS 2000, 4000V2 & 5800), OCC-56C DWDM Optical Channel Checker along with fiber inspection tools such as the P5000i (also supported by the T-BERD/MTS platforms) or FiberChek Probe provide a complete fiber testing solution with everything required to deploy, certify, and troubleshoot new or existing fiber links, including those with 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.