RMD Solution Introduction

Figure 1 DCA architecture protocol diagram

1. Evolution history of DAA
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‍Due to the impact of the COVID-19, many subscribers work and study at home. Meanwhile, with the demands for gigabit broadband, 4K/8K UHD video and other high-bandwidth services continue to grow. Operators urgently need to increase access network capacity to fulfill the growing network demands while controlling costs. This article discusses one solution to the above challenges based on cable network access.

As one of the current mainstream access network solutions, the cable broadband access solution has a history of more than 20 years since first launch in the 1990s. Among them, the changes that have a far-reaching impact on operator networks come from the proposal of distributed architecture: In 2015, Comcast first proposed the idea of DAA (Distributed Access Architecture), CableLabs subsequently established a transitional DCA (Distributed CCAP Architecture) working group, and officially opened the DOCSIS distributed architecture research. At the same time, CableLabs established the R-PHY and R-MACPHY working groups, and formally proposed two architectures, R-PHY and R-MACPHY. From 2015 to 2017, CableLabs completed the implementation of R-PHY related specifications. From 2017 to 2020, CableLabs shifted its focus to the R-MACPHY working group, and continued to discuss and implement FMA (Flexible MAC Architecture) related specifications. The evolution of related protocols is shown in Figure 1.

Although each manufacturer has its own technical preference, the main working group members of CableLabs (DAA-related) are the same. They are all from the world's top cable operators and DOCSIS-related equipment manufacturers. So the actual work of exchange discussions have promote the mutual learning between the R-PHY and R-MACPHY working groups.

2. RMD Deployment Solution

‍The R-MACPHY solution is one of the distributed CCAP access solutions (DCA) of the DOCSIS system. The core feature of the R-MACPHY solution is that the remote RMD (Remote MAC-PHY Device) device not only has the PHY layer function of CCAP, but also has the MAC layer function. It is precisely because of this feature that RMD can be used as an independently deployed "Layer 2 CCAP" device, or it can form a larger RMD cluster system with RMD Controller.

RMD can be installed at remote outdoor nodes. The remote node can be the position of the bidirectional CATV RX/TX or the amplifier in HFC network. The head-end gateway device and the RMD device use the "Layer 2" for networking communication, and 10GE Ethernet technology or 10G EPON/XGPON technology can be used for connection between them. The convergence layer is used to connect the RMD device to the user gateway device and converge network flow. The gateway device can be used as a three-layer switch or an OLT as shown in Figure 2.

Figure 2 RMD service networking diagram

RMD device can integrate CATV optical receiver, ONU/Ethernet uplink, DOCSIS and other functional modules. Among them, the DOCSIS module can realize the same functions as the traditional CMTS Core, and the downstream cable is connected to the user terminal equipment CM/STB.

2.1 How to deploy the RMD
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‍How to deploy RMD quickly and conveniently is very important to operators. Based on the RMD quantity, choosing a suitable deployment way would save costs effectively.

‍2.1.1 Batch deployment

‍When an operator deploys RMD in a centralized way, shown in Fig.3, a batch deployment plan is required. In such situation, RMD and RPD are the same, and both support zero-configuration activation, which is very convenient. RMD Controller is deployed on the head-end server, covering a wide range, while there’s no geographical constraints for RMD deployment. After RMD is powered on, it will request the management IP from the DHCP Server firstly, and then request configuration from the RMDC. The RMDC manages the RMD, CM and CPE centrally. There is no need for clock synchronization between RMD and RMDC, and there’s no special requirements for the connecting devices between RMD and RMDC. RMD uses PON or Switch for uplink, which is not restricted by brand and device model. Operators can choose the uplink device freely according to their actual conditions, while currently RPD (Remote PHY Device) mainly uses Switch for uplink.

Figure 3 RMD batch deployment diagram

2.1.2 Scattered deployment

‍When scattered deployment or a small amount of deployment is required, RMD has obvious advantages over RPD. It does not require the deployment of expensive head-end CCAP Core equipment. Meanwhile, RMD itself can also be used as a Controller, only one RMD needs to be deployed at the optical node. For example, the RMD can be deployed in small networks such as corporate campuses, besides, it’s suitable for MSO deployment in remote suburbs or cities with low density users as well, for those scenarios, the RMD solution has obvious cost-effective advantages. Because RMD has good support for IPTV, it is also very suitable for small networks such as hospitality industry.

‍2.2 How to upgrade from traditional CMTS to RMD

‍The typical network diagram of traditional CMTS is shown in Figure 4. With the rapid growth of broadband demand, traditional CMTS shows many disadvantages, such as serious user bandwidth sharing and low average bandwidth for subscribers, it is almost impossible for achieve Gigabit home access. In addition, there are also some other big problems, such as the high investment cost, the complex structure, the serious noise and high maintenance cost.

Figure 4 The network diagram of the traditional CMTS

With the development and progress of DOCSIS technology, a new generation of RMD solution have solved these problems. The typical network diagram of RMD solution is shown in Figure 5.

Figure 5 The network diagram of the RMD solution

Compared with the traditional CMTS solution, the main advantages of the new generation RMD solution include:

Saving more than 60% of the head-end space due to the fact that there are no needs for the optical receivers and mixers anymore;

Reducing the cost of the power;Saving the cost of the backbone fiber when using PON for uplink access;

Simplifying the debugging of the coaxial signal in optical node;Increasing the bandwidth for subscribers (10 times more than traditional CMTS);

Compatible with the DOCSIS2.0, DOCSIS3.0, and DOCSIS3.1 CMs;

Supporting smooth upgrading with license;Protecting existing investments when upgrading to the DOCSIS 4.0 due to the fact that DOCSIS4.0 RMD is compatible with existing DOCSIS2.0/3.0/3.1;

Supporting direct conversion of MPTS streams into RF signals (For CATV service, no need to change DVB STB).

2.3 RMD and FTTH hybrid networking

‍Considering the actual deployment and planning of the network, RMD device supports the hybrid deployment of DAA structure and FTTH, that is, the OLT is connected to the ONT and RMD device to achieve unified management and control.

For example, an operator had deployed FTTH solution for some of subscribers, but some areas cannot deploy fibers or the cost of deploying fibers is too high. At the same time, the cable network has been deployed. In this case, the RMD together with OLT solution is a good choice. Because there’s no need to add new equipment in the head-end. One thing to note is that when using the same OLT, RMD and FTTH subscribers need to be distributed on different OLT service boards. If this is not possible, at least the PON port needs to be separated, as shown in Figure 6.

Figure 6 The hybrid network diagram of FTTH and RMD solution

2.4 RMD can be used as a supplement to RPD

‍If the operator had deployed RPD device, but they need to expand their business to some cities with a small population. In that case, a small population is not enough to share the cost of CMTS Core. So the RMD solution is the best choice.

‍3. Evolution of DOCSIS 4.0

‍Now the DOCSIS 3.0/3.1 standard fulfill the subscribers needs for high bandwidth in most cases, but in order to better meet future challenges, CableLabs released the next-generation standard, which is the DOCSIS 4.0 standard in March 2020.

The main improvement of the DOCSIS4.0 standard is to provide higher uplink and downlink bandwidth. At the same time, it is also the first DOCSIS standard that combines full duplex DOCSIS (FDX, Full Duplex) and extended spectrum DOCSIS (ESD, Extended Spectrum DOCSIS). FDX will use existing spectrum for upstream and downstream bandwidth without a specific strategy, while ESD is committed to expanding the available spectrum and dedicate some of it to upstream or downstream traffic. In other words, FDX prioritizes the downlink while providing a strategy to expand the uplink spectrum, while ESD borrows spectrum from the downlink to allocate more spectrum to the uplink capacity.

As a part of supporting the 10G platform technologies,, DOCSIS 4.0 technology achieves up to 10 Gbps downstream speed (double times faster than DOCSIS 3.1 technology) and 6 Gbps upstream speed (4 times faster than DOCSIS 3.1 technology), which provide subscribers with higher bandwidth that can compete with FTTH.

The key of DOCSIS4.0 is the gigabit upstream capacity, which greatly improves the uploading speed for subscribers. Traditionally, companies need faster upload speeds to migrate large files. As cable operators respond to the huge connection needs of subscribers in the public health crisis, remote work and medical services will benefit from the enhanced uplink broadband after the deployment of DOCSIS 4.0 technology. Also, the industry has already begun to prepare for the arrival of DOCSIS 4.0 actively:

Considering the chipset development, CableLabs reported that it has more than 10 chipset manufactures plan to produce DOCSIS 4.0 already. Among them, the two main suppliers of DOCSIS chipsets, which are Broadcom and MaxLinear, already stated clearly that they will commit to developing DOCSIS 4.0 chipsets as soon as possible.

While considering the network applications, many operators have already planned to upgrade their cable networks from DOCSIS 3.0/3.1 to DOCSIS 4.0. For example, the US operator COMCAST confirmed that they are "fully committed" to the FDX-based DOCSIS 4.0 network upgrade plan. Besides, the Canadian operator Cogeco has already begun to evaluate and use ESD to upgrade the existing network to 1.8GHz DOCSIS 4.0 solution. Furthermore, in order to provide Gigabit bandwidth for 21 million households, Vodafone (Germany) made it clear that it will start field testing immediately after the first DOCSIS 4.0 prototype is released.