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What is Remote PHY?

Here’s a general overview on Remote PHY, a CableLabs approach to distributed access architecture (DAA) in a digital hybrid fiber/coax (HFC).

By Lucía Quiles

TL;DR

Here’s a general overview on Remote PHY, a CableLabs approach to distributed access architecture (DAA) in a digital hybrid fiber/coax (HFC).

Key takeaways

  • Remote PHY is an approach to distributed access architecture (DAA) in a digital hybrid fiber/coax (HFC) network.
  • It moves the physical layer from the CMTS headend to the network edge.
  • By decreasing the distance to the customer’s equipment, it can achieve higher bandwidth and throughput than a centralized system.

Remote PHY is an approach to distributed access architecture (DAA) in a digital hybrid fiber/coax (HFC) network. It moves the physical layer from the CMTS headend to the network edge. By decreasing the distance to the customer’s equipment, it can achieve higher bandwidth and throughput than a centralized system. The customer sees no difference except in performance.

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The specification is a free download from CableLabs. It’s written for engineers working on its implementation. Here is a general overview for those looking to understand the concepts.

The architecture

The central component in a CMTS is the converged cable access platform (CCAP). It unifies delivery of voice, video, and data through the DOCSIS protocols. The older form is called an integrated CCAP or I-CCAP, and it implements OSI layers 1 through 4 to send data over HFC. This includes Layer 1, the physical or PHY layer.

That degree of centralization has its limits. Remote PHY moves Layer 1 from the CCAP to a remote PHY device (RPD) and connects to it by UDP and TCP/IP over Ethernet. The RPD includes QAM modulators and demodulators, letting it with the customer equipment over linear optics and coax cables. This doesn’t affect the DOCSIS functionality. The network connecting to the RPD is called the converged interconnect network, or CIN; it includes routers, switches, or both.

In addition to shortening the distance from the node to the customer, the remote PHY approach increases scalability. The CCAP is no longer limited by the number of lines that can connect to one box.

Strictly speaking, the RPD includes some Layer 2 and 3 functionality, including media access control (MAC) and pseudowires to the CCAP.

RPD functionality

RPDs may be located anywhere, possibly a long distance from the main equipment center and the CCAP. Any reasonable number can connect to a central router. Each one may have its own router port, or they may be daisy-chained. A single box may contain multiple RPDs.

An RPD needs to run reliably without much onsite intervention, and it has to deal with a possibly insecure physical connection to the CIN. It uses encrypted communication to ensure the authenticity and integrity of the data. Each RPD authenticates itself as part of its startup sequence, then it uses DHCP to get its IP address. IPv6 support is required in the specification.

Software updates are occasionally necessary to fix bugs and improve security. With devices located on the provider’s premises, this isn’t difficult, but with devices that might be a hundred miles away or more, a rigorous approach is needed. Remote PHY defines a secure software download protocol. An X.509 certificate validates the source of the update. The update process builds upon existing DOCSIS methods for updating cable modems.

If an RPD conforms to the specification, it will work with any CCAP that has Remote PHY support.

The devices

RPDs come in shelves and nodes. A shelf is a box located at the headend or hub, containing one or more RPDs. An RPD node is located at the optical node. The technical difference is that a node supports optical node RF technology and a shelf doesn’t.

With a “fiber deep” configuration, a node may be located very close to the customer, perhaps serving just a few blocks or a single office building. A node may be located almost anywhere, even in an outdoor environment. That’s why secure connections are so important.

Cable Labs has arranged with Kyrio to run a qualification program for RPDs, to make sure that they conform to the technical requirements and are interoperable. Qualified devices have to support DOCSIS 3.1.

Making the transition from an integrated CCAP to one with remote PHY takes a concerted effort, but many things remain the same, and the benefits are significant.

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