A New Cable Network Architecture

coaxial cableThere seems to be constant press about the big benefits that are going to come when cable coaxial networks upgrade to DOCSIS 3.1. Assuming a network can meet all of the requirements for a DOCSIS 3.1 upgrade the technology is promising to allow gigabit download speeds for cable networks and provide cable companies a way to fight back against fiber networks. But the DOCSIS 3.1 upgrade is not the only technological path that can increase bandwidth on cable networks.

All of the techniques that can increase speeds have one thing in common – the network operator needs to have first freed up channels on the cable system. This is the primary reason that cable systems have converted to digital – so that they could create empty channel slots on the network that can be used for broadband instead of TV.

The newest technology that offers an alternative to DOCSIS 3.1 is being called Distributed Access Architecture (DAA). This solution moves some or all of the broadband electronics from the core headend into the field. In a traditional DOCSIS cable network the broadband paths are generated to customers using a device called a CMTS (cable modem termination system) at the core. This is basically a router that puts broadband onto the cable network and communicates with the cable modems.

In the most extreme versions of DAA the large CMTS in the headend would be replaced by numerous small neighborhood CMTS units dispersed throughout the network. In the less extreme version of DAA there would be smaller number of CMTS units placed at existing neighborhood nodes. Both versions provide for improved broadband in the network. For example, in the traditional HFC network a large CMTS might be used to feed broadband to tens of thousands of customers. But dispersing smaller CMTS units throughout the network would result in a network where fewer customers are sharing bandwidth. In fact, if the field CMTS units can be made small enough and cheap enough a cable network could start to resemble a fiber PON network that typically shares bandwidth with up to 32 customers.

There are several major advantages to the DAA approach. First, moving the CMTS into the field carries the digital signal much deeper into the network before it gets converted to analog. This reduces interference which strengthens the signal and improves quality. And sending digital signals deeper into the network allows support for higher QAM, which is the signaling protocol used to squeeze more bits per hertz into the network. Finally, the upgrade to DAA is the first step towards migrating to an all-digital network – something that is the end game for every large cable company.

There is going to be an interesting battle between fans of DOCSIS 3.1 and those that prefer the DAA architecture. DOCSIS 3.1 was created by CableLabs, and the large cable companies who jointly fund CableLabs tend to follow their advice on an upgrade path. Today DOCSIS 3.1 is still in first generation deployment and is just starting to be field tested and there is already a backlog on ordering DOCSIS 3.1 core routers. This opens the door for the half dozen vendors that have developed a DAA solution as an alternative.

While CableLabs didn’t invent DAA, they have blessed three different variations of network design for the technology. The technology has already been trialed in Europe and the Far East and is now becoming available in the US. It’s been rumored that at least one large US cable company is running a trial of the equipment, but there doesn’t seem to be any press on this.

Cable networks are interesting in that you can devise a number of different migration paths to get to an all-digital network. But in this industry the path that is chosen by the largest cable companies tends to become the de facto standard for everybody else. As the large companies buy a given solution the hardware costs drop and the bugs are worked out. As attractive as DAA is, I suspect that as Comcast and others choose the DOCSIS 3.1 path that it will become the path of choice for most cable companies.

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