After all of the taunts and name calling about the big cable providers in the US, it comes down to technology regarding the upload and download speed. When cable came about many years ago, did anyone think it would be as big as it is today?

Certainly not those folks hooking up huge antennas on the nearby hills to receive over the air broadcasts, and then reselling it to the local town. Since it became regulated and frequencies allotted, there has been little change in the frequency range, other than companies adding more on shared frequencies. This has led providers to find ways to pack as much as possible into the allowed space via digital compression and the like.

However, we are going to address the internet portion of these restrictions in this article and why there is such a difference between the upload and download speeds on cable internet in the US.

Cable Map

Back in the late 1920’s (yes that long ago), the then Federal Radio Commission (FRC), which would become the FCC, allocated VHF channels to use in the new era that was to come about. Television had been researched at that time and some short distance broadcasts had been seen. TV broadcasters were popping up left and right, from Boston to New York to Los Angeles.

This was an exciting time to be getting into Television, and this marked early antenna (OTA) broadcasts of sporting events, presidential addresses, and the like.

The FRC got to assigning channels to specific broadcasters, which were specific frequencies these broadcasters were allowed to transmit on in their area. For example, in 1948, in Boston, Massachusetts, WBZ-TV was given rights to broadcast on channel 4, which continues to operate on that channel to this day as a CBS affiliate.

Once these channels were beginning to get gobbled up in each area, the channel map was looked at and the number of available channels increased. Next, they allocated channels 7-13. And as the story goes, we ended up pushing past VHF and into UHF (Ultra High Frequency). Channels 14-83 were added, and this became what cable companies in the future called “expanded cable.”

Moving To Digital Cable

Once broadcasters and providers moved to digital processes, digital cable was introduced. What this does is uses video compression to take the normal 6MHz wide channel, and stuff anywhere from 10-18 digital streams into it. If you look at a cable map, you can see even if you only put 10 channels into one, that’s 810 channels of television (and still nothing on). This is done using QAM, which inserts a data stream on a carrier in an X/Y quadrant. If you’d like to know what that looks like, you should look here.

During all of this, high speed internet is getting life. Using DOCSIS, cable modems are able to connect via these data streams to the cable providers central office, and provide internet service. The trick with internet, is that its a two way operation, and cable TV is not.

This Is The Way Back

cable frequency

This image gives you an idea of how the frequency allocation works for a cable provider. The grey sections are broadcast television, and the brown is FM radio frequencies. The two in question here are the lime green and orange. These are the blocks of frequencies that determine how fast up and down you go.

First we’ll start with the orange. This is the downstream portion, which is the data going to you, the customer, from the internet. As you can see, the orange block is quite a bit larger than the lime colored one. Now the lime colored block is the upstream, which is you sending data to the internet. Useful nowadays for uploading YouTube videos and large images to websites and the like. However, I’m going to add a few more puzzles into the mix.

What if I told you, that since most cable providers are allowed to go all digital now (i.e. no cable direct or OTA broadcasts without a box or capable digital TV), that the orange block could be the ENTIRE right side of the red line. That would all be for downloading to you, and with DOCSIS 3.1, the modem can connect to 24 downstream data streams simultaneously.

This correlates to a theoretical maximum of 1 GBps! But, look at the lime green side. It’s stuck at a whole 37MHz wide, enough for 6 upstream channels. And at that low frequency, there is so much electrical noise, from TV’s and any other electric appliances, only 3-4 channels are usable. And because of the noise, the same strength QAM tuning can’t be used on the way back.

All of this, because TV started in the early 1900’s. Little did they know then, it would be powering so many things in our lives, all through a little modem on your desk.

About the author

Jeff Trocchio

Apple IIe green screen is whence I came. Where I go, only technology knows. If its Automotive, Mobile, Gaming or Computer tech I'll try my best to give my thoughts on it.

1,127 Comments

  • Great job +Jeff Trocchio , probably the best explanation from someone not in the industry I have heard. There are a couple of points I think should be mentioned.
    1. The FCC just granted permission to drop the analog lineup last year. As excited as we all are, nothing moves as fast as we want them to, which brings me to the next point

    2. There are lots of old people out there, LOTS. They are retired and don’t work any longer. This affords them a great amount of time to spend on the phone. They are paying customers and will cry bloody murder when (notice I said when and not if) we drop the analog line up. Preparations have to be made to accommodate these folks.

    3. You can place anywhere from 10 to 25 (this depends on things that I love talking about but I don’t think anyone but a headend tech cares about, if you want to know I would be more than happy to share) standard definition channels in each 6 MHz QAM. But who watches stupid standard definition? Ach!! SD streams are 3.75 mbps, HD streams are 12 mbps. An NTSC 6 MHz QAM is 38.8 mbps. So only 3 HD streams (more if your cable company uses a narrow cast switch digital system).

    4. Your chart shows the down stream data QAMs at 680-750. This is not accurate. Its not untrue, just inaccurate. The down stream data is placed on a 6 MHz digital channel called a QAM, just like the TV channels. These can be, and in fact are placed anywhere from 54 MHz (Ch 2) to 997 Mhz (Ch 158). Many people place them in the A tier (chs 95-99, the FM band) because placing analog channels there is just messy do to the FM band. So yes, the down stream can be located from 680-750, but is not exclusive to that range. In my system we start at ch 127, 810 MHz and go to ch 139.

    5. Hold on. Its coming. DOCSIS 3 and IP v6 are game changers. Prior to DOCSIS 3, the cable modems were not capable of channelizing the upstream (the upstream doesn’t use 6 MHz QAMs for reasons I wont go into). The last thing that had to fall into place was the FCC allowing us to drop the analog line up AND encrypt ALL digital channels. Now that this has happened most companies are preparing to drop analog and pull support of DOCSIS 2.0 modems (again this will cause a stink because many customers will have to get new modems)

    So, OK. That’s more than a couple of small points. But this is what I do and I love my job, so I was actually restraining myself. I could write a freaking novel on this!

    But, you DID do a pretty good of getting the point across as to why uploads are so slow. It really is a good article.