What is cable broadband?

Cable broadband is a method of delivering high-speed Internet to residences and businesses using cable television infrastructure.

Similar to DSL, fiber, and fixed wireless connections, cable bridges the “last mile” between the mainstream Internet “backbone” and customer residences.

Cable bridges the “last mile” between the mainstream Internet “backbone” and customer residences.

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Cable is similar to DSL in that both use copper cables to deliver data as electrical signal, as opposed to fiber cables which use optical signal (essentially, light). While DSL uses old-fashioned copper telephone wires, cable broadband uses the same coaxial copper cables that deliver sound and video to cable television subscribers.

Cable provides speeds and bandwidth similar to DSL offerings, with most residential plans offering between 20–100Mbps for downloads and 1–10Mbps for uploads.

In theory, cable wires are more advanced than DSL wiring, allowing for greater bandwidth and faster speeds. In practice, cable connections are “shared” within neighborhoods using a technology called Statistical Multiplexing. This brings down prices for consumers and providers at the cost of slightly reduced speeds, particularly during peak use times.

Components of a cable broadband network

Most “cable” broadband connections are technically Hybrid Fiber Coaxial (HFC) connections.

In an HFC connection, a “trunk” fiber optical cable covers the distance between the provider and a local service area. Once there, it terminates at an optical node, which translates the optical signal to electrical signal. The electrical signal is routed via “branches” of coaxial cable to individual homes, where cable modems split Internet and TV signal and deliver it to a home network.

In some cases repeaters or amplifiers are used to boost signal every 600 meters or so.

Components of a cable broadband network

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How coaxial cables work

Digital data is, in essence, strings of binary “on/off” signals, generally represented as zeros and ones. Everything you experience when you use the Internet is stored as digital data.

How coaxial cables work

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When you tune in to a TV channel, you’re selecting a particular “slice” of the signal being transmitted over that cable, just like selecting a channel on wireless radio. In simple terms, broadband cable Internet takes up one of those “channels” on a coaxial cable.

The biggest difference between broadband Internet data and traditional TV data is that Internet data goes both ways as users upload and download content. To prevent signals from interfering with one another, coaxial cables are “split,” using 52–750 MHz for downloads and 5–42 MHz for uploads.

Upstream and Downstream Frequencies

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Anatomy of a coaxial cable

The traditional “cable networks” that cable broadband uses to bridge the “last mile” are composed of copper coaxial cables.

Coaxial cables have a variety of parts, each designed to protect and transmit electrical signals as precisely and efficiently as possible.

Anatomy of coaxial cable

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• Central copper conductor: Carries electromagnetic signal. This will often be copper-coated steel rather than pure copper, which increases the strength of the cable. High frequencies tend to travel on the outside of a conductive wire, a phenomena called “skidding,” allowing the steel central strength member to avoid interfering with the cable’s carrying capacity.
• Plastic dielectric insulator: Maintains uniform distance between center wire and outer foil.
• Aluminum foil: Foil essentially bounces high frequency signal back to the center wire as it tries to escape the cable. Foil also blocks high frequency electromagnetic interference from outside the cable.
• Braided copper: This layer has a similar function to the foil, but the braided copper construction makes it particularly effective at canceling out low-frequency electromagnetic interference from outside.
• Outer jacket: Usually made from PVC, the outer jacket protects the interior of the cable, repelling water, animals, and other contaminants once installed.

The term “coaxial” refers to the shared axis between the central copper conductor and outer foil/mesh layer. Maintaining this uniform axis or distance between the center conductor wire and outer mesh/foil creates a stable environment for signal to move, defining the “capacity” of the cable.

Anatomy of coaxial cable

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The mesh also serves to protect the signal inside from electromagnetic interference outside the cable, allowing the signal to travel farther before it deteriorates.

To envision this, think about mass transportation. A traditional DSL cable is like a bus: it’ll get from point A to point B, but it’s constantly slowed down by outside traffic. Coaxial cable is like a subway line, with the mesh wrapper acting as the tunnel wall: no interference from other vehicles translates to faster, more reliable transportation.

Translating data: how DOCSIS works

In an ideal situation, broadband data would be delivered to subscribers via direct bi-directional cables. Cable has two big roadblocks to this sort of ideal setup. First, coaxial cables are not designed to be bidirectional, since they were originally designed for one-way broadcast media, specifically television. Second, they are not direct, since residence connections are shared starting at the local node.

Cable broadband providers work around these issues by using DOCSIS, an acronym for Data Over Cable Service Interface Specifications. DOCSIS is a suite of specifications developed by CableLabs, a nonprofit research organization.

How Docsis works

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DOCSIS essentially oversees the transmission and “translation” between optical and digital signal through coaxial cable. It specifies a standard protocol for cable modems for modulating and demodulating electromagnetic signal on either end of a coaxial cable.

Without a universal standard like DOCSIS for how to send, receive, and translate electromagnetic signal, it would be impossible for equipment from different manufacturers to operate in complex real-world setups.

DOCSIS and cable modems

The DOCSIS standard is constantly evolving to increase service quality and boost upload capacity. As such, cable modems will include information about which DOCSIS version they use. Older DOCSIS specifications will still work, but an older DOCSIS modem will act like a bottleneck, slowing down broadband connections with faster throughput than that older DOCSIS can handle.

The latest version, DOCSIS 3.1, is designed to compete with 1Gb up/down synchronous fiber connections.

Approximate capacity of DOCSIS versions

Approximate capacity of DOCSIS versions

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• DOCSIS 1: 38Mbps Down, 9Mbps Up
• DOCSIS 2.0: 38Mbps Down, 27Mbps Up
• DOCSIS 3.0: 152Mbps Down, 108Mbps Up (CableLabs says it can now go even higher)
• DOCSIS 3.1: 10,000Mbps Down, 1,000Mbps Up

Pros and Cons of cable broadband