DOCSIS (Cable Internet) - What is it and how does it work?

By Michael Spalter
June 2021

About the author

Michael Spalter

Michael Spalter

Michael Spalter has been a networking technician for over 30 years and has been the CEO DrayTek in the UK since the company’s formation in 1997. He has written and lectured extensively on networking topics. If you’ve an idea for a blog or a topic you’d like explored, please get in touch with us.

All About Cable Modems & DOCSIS

DOCSIS is the modern standard used for 'cable modem' Internet (and triple-play) service in the UK and in most other countries. Compared to DSL (carried on twisted copper pair phone lines) and fibre (using light over glass-fibre cables), DOCSIS uses coaxial cables ("coax") to carry RF (radio frequency) signals to each home or office. Coax uses a single central conducting wire and an outer shield to prevent the signal from leaking along the path as shown in the picture at the top of this page.

A Brief History of cable service

In the UK, DSL service (ADSL and VDSL) have been and are still the most common medium for Internet connectivity. The reason for the widespread use of DSL is because premises could be connected using cabling which was already installed - DSL is carried over your existing copper telephone line and nearly every home and office has a phone line so no streets or front gardens need to be dug up, which would have otherwise been very costly.

Conversely, in North America, the most common broadband connection method is 'cable' which is used to refer to Internet services delivered over a coax cable. In the USA 'cable TV' is the most common medium for premium TV delivery (subscription channels) and also delivers 'over the air' free channels (ABC, NBC etc.) when broadcast signals are weak so most homes already had a coax cable for TV, so adding Internet to that existing coax cable was economical and, because of the topology (see later) it's also more economical to cable a street with coax/cable than DSL.

Cable TV in North America is now mostly all digital, but originally TV channels were delivered down the cables as multiple simultaneous analogue signals, much like over-the-air TV through an aerial. The set-top box would then tune into whichever programme you wanted. North America is (obviously) much bigger than European countries so the earliest cable TV systems would have a large local antenna which then fed the cable service to the local homes who were otherwise outside range of regular over-the-air broadcast channels. This was known as CATV (Community Antenna TV) and CATV is still commonly used as shorthand for 'Cable TV' (most people assume CATV is short for Cable TV!).

In the days of analogue cable TV, all channels were transmitted at once on the coax and there was no encryption. A common method of blocking channels which you didn't subscribe was the use of a selection of band filters locked into your cable TV receiver. As you can imagine, it wasn't difficult to abuse and get service without paying but even basic service was easy to steal simply by hooking up to a nearby utility pole and plugging it into your own TV or receiver. There was not only a whole industry of people helping pirate analogue cable TV, but would-be pirates being conned with fake devices - and you're not exactly going to complain to Trading Standard (or the Better Business Bureau).

There were cable TV services in the UK since the 1930's in limited areas from now defunct companies such as Rediffusion, Cablevision and Viewpoint but it was in the 1990's when media companies started large scale investment in programming and infrastructure. After various mergers and acquisitions, there were two main companies operating in the UK - Telewest and NTL, each serving different geographic regions. They them themselves merged in 2006, changing their name to Virgin Media a year later after acquiring Virgin Mobile (a cellular phone service provider).

Both NTL and Telewest (Virgin Media) provided almost exclusively digital TV service (with a few exceptions where they did for a time provide analogue TV). As well as using legacy cable installation, Telewest and NTL invested hugely into installing cable TV and Internet/phone service into cities around the UK.

Virgin Media has around 6 million customers in the UK today. They, as well as most cable TV subscribers in Europe and North America receive their service over coax cable using a technology called DOCSIS. Most DOCSIS services are version 3.0 but many are, or have been upgraded to, the newer DOCSIS 3.1. This digital system allows for 'triple play' services - phone, TV and Internet access down a single coax cable. In the home, that cable will go to a modem/router (for Internet access) and to set-top boxes at each of your TVs for TV viewing. If you have also subscribed to telephone services, that is normally now provided digitally by the modem (VoIP) but some earlier installations used a separate regular analogue telephone line combined with the coax.

Cablemodem and coax
A cable modem with the coax cable. The modem show also provides VoIP phone service via the socket on the left. The yellow RJ-45 Ethernet socket connects to your computer or, more commonly, a Wi-Fi router.

The modem and router may also be combined into one box but if your cable company has provided you with a combined router and modem but you wish to use your own router for more features, you can normally switch their device into 'modem only' mode which then acts as a simple bridge between the coax cable feed and your router, via Ethernet.


The signal on the coax which enters your home originates from a device called a CMTS (Cable Modem Termination System) which will typically be located in a street nearby. It may be in a manhole in the ground, in a street-side cabinet or up a utility pole. In less densely populated areas, a CMTS can actually be some distance from the premises, using powered amplifiers along the way to maintain signal strength where necessary. The CMTS is normally connected back to the cable company's backbone using fibre-optic connectivity. This is therefore called a HFC (Hybrid Fibre Network) and the CMTS is sometimes referred to as a Fibre Optic Node. For that reason, ISPs (cable and DSL) will often market services as "fibre" even though the connection to your premises is over copper wire (cheeky!).

A CMTS may have multiple coax outputs but each connection can serve many subscribers and that's why DOCSIS is economical. A coax cable comes out of the CMTS and passes a home, a splitter (a T-connection) is put in and a drop-wire run into that home. The cable then continues down the road and provides a drop-wire into the next home, and so on. In this way, only one cable needs to be run down the street. With DSL, you need a cable from every home all the way back to the DSLAM or MSAN (the DSL equivalent of a CMTS).

Cable Poles
A coax service running down the street, providing two drops into homes as it passes. Right: Entering the property

As everyone is sharing the same street coax 'backbone' they are all sharing the available bandwidth on that cable, but also the available bandwidth on the fibre part of the network back to the ISP. This contention is why your 300Mb/s service may not actually deliver that speed all of the time for Internet connectivity, however your cable company will apply QoS (Quality of Service) assurance which provides guaranteed bandwidth for TV services in order that programmes are not interrupted due to congestion.

Taps & Splitters

In order to split one coax feed into two or more feeds withing your home, you can use a splitter. This is exactly what has already happened in the street outside your home. Outside a building, splitters may split to separate apartments or to multiple locations within one home.  The splitters and cables outside a building are not always tidy and be easily damaged if they are in a vulnerable position as well as degrade if left exposed to weather:

Cables outside a building exposed
Caption: Cables outside a building with exposed splitter - it should be in a weatherproof box 

A splitter will have a reduced output compared to the input - the signal strength of each output is 1/n of the input, where 'n' is the number of outputs so, every time a signal is split, it becomes weaker. The coax enters your home and is typically then split to each of the locations within the home that you want a cable TV receiver (set top box) or your Internet cable modem. An alternative type of splitter is called a 'tap'. A tap works like a regular splitter but splits one coax into only two outputs, one of which will have minimal loss, and the other has greater loss - i.e. the loss of signal is not balanced across both outputs. A tap is therefore useful where one output has a short run to its destination whereas the other output, with the stronger signal has further to go, say continuing down the street. Therefore, in the street, a tap is used to drop a coax into each home. If you want to extend a coax cable within your home, use a connector, not a splitter, to avoid loss (unless you need more than one output).

Cable Splits
This was the mess I found in the basement of a house. 6 splitters in series. The DOCSIS signal to the modem was too low, causing disconnections - I wonder why!

Technical Background

To calculate the speed of a communications channel, you multiply the symbol rate by the modulation rate. The 'symbol rate' is how many symbols are transmitted per second. The modulation rate is how many bits each symbol can represent. As an example, a symbol rate of 5360 symbols/second is standard on DOCSIS 3.0. QAM256 gives us 8 bits per symbol. So 5360 * 8 = 42880bps (42.88Mb/s). After overheads and signalling, that's about 38Mb/s for data payload. You can then bond multiple channels (8, 16, 24 etc.) to get an even higher total speed. 

DOCSIS 3.1 makes use of OFDM so uses much narrower channels, spaced just 25Khz apart (optionally 50Khz) so it can support up to 7680 subcarriers, grouped into OFDM sub-channels of 192Mhz each. Compared to single frequency QAM, OFDM is more 'spectrally efficient' - it makes use of more, but narrower sub-channels within the range/band, turning off any which are unreliable or subject to interference. DOCSIS supports up to 4096QAM - that's 12 bits per symbol and 'variable bit loading' means that different levels of QAM can be applied to different subcarriers to make best use of the medium. Symbol times are longer than in DOCSIS 3.1 to help maintain fidelity and the frequency range of the allocated band (for all channels) was extended to 1.2Mhz (optionally to 1794Mhz depending on local regulations).

The DOCSIS 3.1 specification actually optionally allows for 16384QAM but there's no real-world support for that currently and the precision required would need a much more costly CPE (QAM is maths intensive so needs processing power). You'd also need very good quality lines with minimal interference. Currently, the speeds provided by DOCSIS 3.1 are sufficient for most likely use cases for the forseeable future, just as Bill Gates said in 1981 that "640K is more memory than anyone will ever need" (except that that is a myth - he never said that, though it's widely held lore!). Here's a long lecture if you really love this stuff:

The bottom line: DOCSIS 3.1 gives us up to 1.89 Gbps per downstream channel so potentially multi-Gigabit service with channel bonding.

Modem Connection & Registration

With a DOCSIS 3.0 connection, when you turn on your cable modem. The first thing it does after some self-diagnostics, is to scan the whole frequency range on the incoming coax cable for a primary downstream QAM signal to lock onto which has been generated by the CMTS. The primary capable channel is used for SYNC (not data/payloads) and will provide information about the line capability and, importantly, provide information on a primary upstream channel which the cable modem can use, to talk back to the CMTS and register itself. Once bi-directional telemetry is established on the primary channels, the CMTS and your cable modem can exchange further information about the line capabilities and frequency profiles and establish a working connection for data transmission. This happens by the cable modem downloading a config file from the CMTS using TFTP. This is where the cable provider can set parameters such as service speed and QoS.

On DOCSIS 3.1, a similar procedure operates but when scanning, your modem is looking for a 'Physical Link Channel (PLC) preamble'. That PLC Preamble sends an 'OFDM Channel Descriptor' (OCD) and the 'Downstream Profile Descriptor'. This informs your cable modem of the line's capabilities - the frequency range in use, the OFDM type and details of the downstream profiles that the modem can use. The PLC should always be on a good reliable part of the frequency spectrum. If the modem can't detect the DOCSIS 3.1 PLC preamble, it will fall back to DOCSIS 3.0 (3.1 is backward compatible).

Your cable modem will use multiple bonded channels for downstream and upstream, for example 16x8 would be 16 downstream channels and 8 downstream. The number will depend on the type of service you receive. Your modem can report its channel usage like so (truncated in the image for clarity purposes):

Cable status

The physical (PHY) layer is now established and the modem itself can now register with the cable company in order to activate live service (Internet, phone or cable TV connectivity). It does this by checking your modem's MAC address from the database of authorised devices. If the MAC address is recognised as belonging to a registered modem, service is then activated and the IP layer is set up.

Using your own cable modem

Depending on where in the world you live, you may get your cable modem from your cable provider free of charge, rent it from them for a fee, or have the option to buy your own. In the UK, Virgin Media mandate the use of their modem/router only (The 'SuperHub') in order to keep a uniform network and make support easier.

In North America you can choose to purchase your own modem. Each ISP will normally have a list of 'approved' modems which will include plain modems and combined modem routers. They are all widely available but you should ensure that the modem you purchase is compatible with your cable service provider, compatible with the right version of DOCSIS and that it supports the speed you're paying for (and ideally allow for future service upgrades). DOCSIS 3.1 modems should be backward compatible with a DOCSIS 3.0 service. If your cable service is 600Mb/s on 16x8 channels, that means 16 downstream and 8 upstream channels, so you need a modem that supports at least 16x8, and is compatible with the cable provider.

Cable Company Diagnostics

In the UK, Virginmedia provide the cable modem and it's normally one-box also containing the Wi-Fi capable router. In other countries, the cable company (cableco) will either rent you a modem (typically $10/month) or allow you to buy your own, from a list of known supported models. That may be just a modem, or combined with Wi-Fi enabled router. If you have just a cable modem, you add your own router. Cable modems will provide operational statistics and telemetry back to the cable company using SNMP or TR-069 (when the connection is up) as well as physical layer diagnostics within the DOCSIS telemetry and the CMTS reporting. In this way, the cableco can tell if your connection is up and your modem online. Cable companies may also be able to reboot your device and upgrade its firmware.   For details on diagnosing faults and unreliability on your own cable line, see my separate article on Diagnosing Cable Modem Line Faults, coming soon.

Cable Modem Vulnerabilities

A cable modem itself provides a simple function from the user's point of view but like any other modem or router it's still subject to abuse by bad actors and may have vulnerabilities. In 2020, researchers warned of what they called the CableHaunt vulnerability which potentially left 200 million worldwide cable modem users 'vulnerable'. Patches were released by the chipset manufacturer (Broadcom) thought whether hardware vendors and then cable companies used or pushed the new code out is variable. Always keep your firmware up to date and always change default passwords for the GUI (and Wi-Fi).

DOCSIS Security

As users within a neighbourhood are sharing common cabling, your neighbours' signals (data) are also present on your own cabling. This would be a security risk so DOCSIS includes built-in encryption called Baseline Privacy Interface (BPI). BPI and the later BPI+ used only 56-bit DES encryption which is now obsolete, so were updated and renamed to 'SEC' in DOCSIS 3.0.  SEC runs between the CMTS and each cable modem to avoid users 'listening in' to each other. Your individual cable modem is identified by its MAC address and only that modem will be able to authenticate and validate the encryption certificate using X.509 methods and 128-bit AES encryption.

When you install your cable modem, initial setup requires that you register the modem to the line. The MAC address on the cable port will be unique and the cable company will permit only one modem to be registered at a time. If you change your modem, you must re-register the new one though this is normally easy to do through a captive portal which appears on your browser if you're connected through an unregistered modem. 

I hope you found this article useful and/or interesting.  In a later article I'll be covering fibre services which operate according to some of the same principles. As always, please let us have your comments below and any suggestions for new articles and please do share on social media, networks etc.

Note : EuroDOCSIS differ from standard DOCSIS, using 8Mhz wide channels (6.952Msym/s), giving 54Mb/s per channel.


Cable Modem
Virgin Media