What is Remote MAC-PHY?

Remote PHY and Remote MAC-PHY are two approaches to distributed access architecture (DAA). Both of them move components away from the CMTS headend. The difference is in what parts of the network architecture are moved. As the names suggest, remote PHY moves the physical layer to the network edge. Meanwhile remote MAC-PHY moves the Media Access Control (MAC) layer, also.

Customers are demanding higher data rates. Remote PHY allows telecom companies to achieve the higher data rates that customers are demanding. Learn more in our blog post Challenges in Deploying Remote PHY.

Both are viable options, designed to work with the DOCSIS specifications. Though remote PHY is a more mature technology, remote MAC-PHY is more ambitious and might be preferable in the long run. The Remote MAC-PHY Technical Report describes its current status. There hasn’t been an actual specification since that 2015 report. CableLabs’ MAC-PHY working group is still active. Its work has accelerated in 2018. MAC-PHY is currently available from several vendors, but the lack of a full spec makes interoperability an issue.

Moving out the Lower Network Layers

In OSI terms, PHY is layer 1, while MAC is part of layer 2. Remote MAC-PHY may include a portion of layer 3 as well, making the remote node almost a satellite CMTS in its own right. Either way, there’s a shorter signal path to the customer’s equipment. This allows higher data rates with a better signal-noise ratio and less power consumption.

Moving the MAC portion out gives the remote node a considerable amount of control over the connection to the cable modem. With remote PHY alone, the CCAP core remains with the headend and handles the MAC layer. The PHY node converts the signal from digital to RF. It maps IP addresses to modems and handles QoS. Request-grant handshakes in DOCSIS take place between the modem and the MAC layer. Decreasing the distance between them improves throughput.

The developing standards will include support for full duplex DOCSIS, with multi-gigabit speed in both directions. This will be a huge benefit to businesses with high data delivery needs. To take advantage of those speeds, cable companies will need at least Remote PHY.

At the same time, remote nodes will change the way cable companies do equipment maintenance. With the distribution of equipment in many locations, they will have to use remote upgrades as much as possible. The hardware has to be hardy enough to minimize the trips to fix it. It has to be secure enough to be deployed in a less protected environment than the central office. Vendors are well aware of these concerns, and the equipment they deliver addresses these requirements.

Virtualizing the CMTS

With the lower levels of the data link layer removed, what remains for the CMTS is data. It can be virtualized, eliminating hardware dependencies. Keeping up with DOCSIS upgrades becomes easier. The service is more scalable. Virtualization is possible once the PHY layer is moved out, but moving the MAC layer out simplifies the task as well.

The possibilities for locating the Remote MAC-PHY unit are very flexible. One could be located in an office or apartment building, or it could be in a small space in a nearby building. It can even be mounted on a utility pole. It’s only necessary to be able to run high-speed data, such as Ethernet over fiber, to the unit.

For the present, remote PHY is the safer, more conservative approach to DAA. Certified units conform to the specification and are interchangeable. There’s no vendor lock-in. Remote MAC-PHY means going with a particular vendor’s equipment. Once a full specification is available, current equipment will at least need software upgrades. On the positive side, making both MAC and PHY remote gives the CMTS more flexibility and could help to eliminate data bottlenecks.

Making the Upgrade

It’s becoming clear that moving the cable connection close to the end user makes sense. Squeezing the maximum performance out of DOCSIS 3.1 requires shortening the distance data travels over RF. A centralized access architecture is too limiting. The question is whether to use the well-established Remote PHY or jump ahead to the less settled world of Remote MAC-PHY. Each approach has its advantages.

Customers are demanding higher data rates. Remote PHY allows telecom companies to achieve the higher data rates that customers are demanding. Learn more in our blog post Challenges in Deploying Remote PHY.

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