Introduction What is DOCSIS? What is downstream vs upstream? What are cable modem channels? What is channel bonding? What is QAM?


If you’ve tried to purchase a cable modem recently, you’ll be met with a barrage of terminology that you may have never heard before. I’ll try and explain each of these new technologies to make you a better informed consumer. A quick search on Amazon and you’ll find cable modem specifications that look like this: Downstream vs Upstream? DOCSIS? QAM? What do these mean to me as a consumer? Most of us subscribe to the general rule of thumb that bigger is better, but if you’re like me, you’ll want to know exactly why it is better.

What is DOCSIS?

This stands for Data Over Cable Service Interface Specification. It is a standard that describes several technologies that are used to communicate data over the cable television network. There have been several iterations of the standard to include 1.0, 1.1, 2.0, 3.0 and 3.1. With each new iteration, performance and throughput have been increased.  This increase in bandwidth comes from DOCSIS 3.0 and greaters ability to use multiple downstream/upstream channels and an increase in QAM performance (both described below).  Bottom Line: To achieve the fastest internet speeds, you’ll need a cable modem to conforms to the latest DOCSIS standard. 

What is Downstream vs Upstream?

You can think of downstream as the information that comes from the internet to your home. Upstream is the information you send from your home to the internet. As an example, when you request the webpage, your initial request out to google uses your upstream channels and when google delivers you the contents of their webpage, that uses the downstream channels. In terms of speed, your cable ISP will typically offer something like 50Mbps down and 5Mbps up. This means that you can download information quicker than you can upload it. This makes sense as residential internet users are more likely to download large amounts of data than they are to upload large amounts of data. Bottom line: Download and Upload streams are used to identify the direction of data and is usually used by your ISP to tell you how much data can be sent in either direction. 99% of consumers need only concern themselves with the downstream rate. a 5Mbps downstream connection is good for the casual downloader, checking email and internet browsing, but you may notice lag when trying to stream HD video via Netflix or Hulu. 50Mbps is good for the more avid downloader and is perfect for streaming HD video to several devices at the same time. As of this writing the highest downstream rate offered by my local cable provider is 300Mbps, which is great for aggressive downloader that doesn’t want to wait a day to download the 50GB Ashley Madison database that was leaked.

What are Cable Modem Channels?

Cable modems will advertise that they support 8 downstream and 4 upstream channels.  A channel in this context is almost synonymous with channels you use to watch television shows, but instead of being used to broadcast TV, they are used to transmit data. Each channel uses a different frequency (just like normal television channels) and therefore, more channels means more data can be sent or received. Bottom Line: More channels means, more data can be sent or received, but only if your ISP supports the latest DOCSIS standard and also supports that many channels (read below on “Channel Bonding” to know why). For example, if you have a cable modem that supports 8 downstream channels, but your ISP only uses 4 channels for downstream, then you’ll only get the 4. You’ll notice that this technology is backwards compatible. So how do you find out how many channels your ISP supports? I dont know the answer to this yet, but it’s not easily found on their website, but I imagine you’d have to talk to level 2 or 3 technician. If somone knows a way to identify the number of channels being used by the cable modem, let me know.

Into the weeds…

Traditionally analog television transmits each channel in 6Mhz frequency intervals between the 85MHz and 850Mhz range, as illustrated below.

What’s left over is unused bandwidth in the 5-85Mhz and 850-1000Mhz range. In DOCSIS 3.0 and earlier, this unused bandwidth is reclaimed to create additional 6Mhz channels and they are used as downstream/upstream channels for transmitting internet data. Understand that this analogy has been greatly oversimplified in order help someone who is unfamiliar with these concepts. In reality, the upstream frequencies are different than the downstream frequencies and as of DOCSIS 3.1, the 6Mhz frequency band is no longer used and is replaced by 20kHz to 50,kHz wide channels that are bonded (channel bonding is described below).  Having that said, the concept is the same. 

What is Channel Bonding?

Channel bonding allows multiple channels to be aggregated together to send more data. This is significant because it was a major improvement from DOCSIS 2.0 to DOCSIS 3.0.

DOCSIS 2.0 In DOCSIS 2.0, every customer is assigned a single channel. this limits the customer to the maximum bandwidth for the channel.


DOCSIS 3.0 In DOCSIS 3.0, customers can use cable modems that support channel bonding to aggregate several channels and get higher throughout. If a channel goes down or is experiencing high utilization, your cable modem can bond with another channel, giving you better reliability and throughput.


Bottom line: To achieve the fastest internet speeds available as of the time of this writing (for Time Warner this is 300Mbps), you need to use a cable modem that is supported by your ISP, which usually means it supports DOCSIS 3.1 and uses 16×4 channel bonding, 16 downstream and 4 upstream channels. If you your cable modem only supports 8×4 channel bonding, you won’t be able to achieve 300Mbps speeds.

What is QAM?

Quadrature Amplitude Modulation is a technique (actually a combination of techniques) used to encode data. The number that follows the acronym, such as QAM64 and QAM256, represents the numbers of bit patterns that can be represented. The greater the number of bit patterns, the more information that can be transmitted at one time. QAM is implemented on a per channel basis. Bottom line: The higher the QAM that is supported the higher the theoretical throughput.

Into the weeds…

In order to understand QAM, it is important to  understand how an analog signal can be used to represent binary data. The illustration below shows a technique known as phase shift keying PSK. Here, the phase of an analog signal is shifted 90 degrees. As you can see, each 90 degree shift creates a new distinct wave pattern and each wave pattern can be used to present a bit pattern. Each of these patterns is called a symbol.


To take this concept a step further, QAM not only adjusts the phase but also the amplitude (as its name implies). For example, QAM16 uses quarter increments of the amplitude, 25%, 50%, 75% and 100%, in addition to 90 degree phase shifts. This gives a total of 16 4-bit symbols. Through a combination of these techniques QAM256 is able to represent 256 distinct 8-bit symbols. Ultimately, this means that more data can be transmitted.