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Two Ways To Split Nodes

If you spend much time listening to cable tech people talk about their bandwidth-development options, you’ve probably heard the one about the “node split.” It’s one of the six or seven “tools in the toolbox” that operators are actively applying to make sure they have enough shelf space for everything that’s coming.

Lately, though, the “node-split” talk comes with a prefix. Some say “logical” — the “logical node split.” Others say “virtual node split.”

Just to make sure it’s confusing, the “virtual node split” has lots of aliases: “One-to-one combining,” “serving-area group reduction” and “laser fan-out reduction.”

Don’t be afraid. It’s just tech talk. If you haven’t yet stumbled upon this language, chances are high that you soon will. Let’s sort it out.


First things first. In this particular instance, the word “physical” implies the actual fiber-optic cables in a cable system and where they physically are. “Logical” refers to the laser that’s positioned at or near the center of the network, to transmit “downstream” into fibers.

Tactically, logical node splits tend to precede physical node splits. It’s a bandwidth two-step. That’s partly because logical node splits happen “higher up” in the network — again, at that spot where lasers launch light into the fibers heading out toward neighborhoods. This can be at the headend or at a regional “distribution hub.”

In the early days of hybrid fiber-coax (HFC) design, you see, laser transmitters were expensive. Digital services were just getting started, with very low subscription rates. For that reason, most operators configured their laser transmitters to split their load across multiple (usually four) 500-home nodes.

This means that right now, your cable modem is connected to a piece of coax, which connects to a 500-home node, which, along with three or so other 500-home nodes, links over fiber — to one transmit laser.

There’s a certain beauty to this, especially if you’re an efficiency buff. Say one side of town contains a college. Or a bunch of Cisco employees. Or both. Bandwidth usage is probably pretty high there. But on the other side of town, let’s assume that broadband service penetration and usage is comparatively low.

Say you have one node in the high-usage area and three in the low-usage area. The campus node starts to get hot — way more people using way more bandwidth. What to do?

Step one is the logical node split: Get that node off the shared laser. Instantly, the campus node gets more bandwidth, and the three nodes still on the first laser get less congestion. (This costs about $2,000 for the laser, and is considered a part of normal headend maintenance.)

This sequence goes on, in a usage-initiated way, until each node is service by a single laser. That’s why some people call this “one-to-one combining” — one laser to one node.

Now let’s say you’ve completed all of the logical node splits you can do. You’re at 1:1 combining. And again, that campus node is redlining in usage.


What to do? Step two: the physical node split.

Physical node-splitting means lighting up another strand of fiber between that node, and the headend. It involves putting a laser transmitter/receiver on each end of the glass, and it costs about $2,500, all-in.

How much unlit glass is there, running out to nodes? When cable’s hyrid fiber-coaxial networks were first being built, 20 or so years ago, engineers did have the presence of mind to leave some growing room in the form of dark fibers. Generally speaking, four to six individual fibers run out to each node. One to two of them are usually in use before the first split needs to happen.

On split No. 1, the 500-home node becomes a 250-home node. Split No. 2 halves the 250-home node to a 125-home node. And so on, down to around 31 homes per node (split No. 4). Kind of makes Verizon Communications’s 32-home serving areas seem a bit less daunting, no?

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