How To – Cable Tv Systems – How do they work

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    With new products being offered by our cable tv providers we wanted to take a look at how all of these technologies are provided over one connection to your home.

    For the longest time Cable Tv meant that you were provided with a single selection of television broadcasts. The only variation to the basic cable choices was paid movie channels. Providing this type of service to customers was a relatively easy task for cable companies.

    The first thing that was needed was a local cable TV office site. This office would capture signals from satellite links and then send that signal out across their Coax Cable system to the neighborhoods they served.

    Because signals over coax cable have a limited distance the cable company would install signal amplifyers throughout the network to boost signal to the homes.

    The signal that was broadcast on the coax line was Analog meaning the signal was very similar to a antenna on your roof being connected to the back of your TV set with a coax line. Although the signal range was a little different the basic idea and the tuners needed to receive the signal were very similar. This allowed TV manufacturers to quickly add cable ready tuners to their new tv sets.

     Analog signals broadcast over Coax can be thought of in a similar way to your AM Radio. 

    Every station in your area can broadcast their AM Radio signal at the same time and to listen to that Radio Station you use your tuner to select the station that you want to listen to. In your city there is room for about 100 AM Radio Stations on your dial but across the country there are thousands of AM Radio Stations.

    With Analog over Cable Coax you have room for about 100 channels. To watch the channel you tune to the signal you want to watch.

    To scramble paid services the Cable company would capture the signal off of their satellite dish and then put it through a scrambling filter at their offices. This Scrambled signal would then be sent out on to their main Coax Cable Network. For the customer to subscribe to that channel a line filter would be installed between the main line that served your whole neighborhood and the single line that feeds just your house.  This would descramble the signal for just your home.

     At this point any channel signal that the cable company wanted to offer to any customer anywhere on their system was also available to every customer on their system and only restricted by line filters.

    These line filters were often stolen or bought on the black market and then inserted inside customers homes just before the cable box.

    To try to stop this problem cable companies would use a series of filtering techniques but as fast as they would change customers found a way around them.

    Two Way Signaling Begins 

    In the late 1980’s early 1990’s  cable companies started to introduce a smart signal box in customers homes. This box had internal filters that could be set when you purchased the box or temporarily set for pay-per-view broadcasts by sending a message out to the subscribers box. This meant that in-line filters were no longer necessary.

    Cable companies would still continue to broadcast the full spectrum of signals through analog technologies so that basic cable was still available to any customer that had a tv set that was cable ready or a box they purchased at a retail store. All paid and Pay-Per-View signals would be sent out scrambled and to descramble that signal meant you needed a 2way box that was able to communicate with the Cable Companies Office.

    When Two Way signals were introduced cable companies needed to expand their signal range from 800mhz to 1 gigahertz this  extra signal range was as if your AM Radio with 100 stations just got 120 stations.

    This allowed the cable companies to add additional television signals above the standard 99 channel range and to begin the offering of Internet across their systems.

    In the beginning Cable Internet was not all that is is today. The first instances of Cable Internet access was seen on a small family owned Cable Tv System in Upstate Eastern Pennsylvania.

    This Cable Tv Provider found that if they hooked their Novel Network to the Cable Tv line they could provide local residents access to the Internet. Unfortunately their system was still based on Coax wires and not Fiber so the access was limited to small areas of their system and at a slower speed then offered today. However the service that they provided was still better then local dialup or even ISDN from the telephone company.

    After researching this business for some time and talking with technical managers I personally took it on myself to contact Comcast Cable who was then headquartered just outside of Philadelphia, Pennsylvania.  It took about a day to find the right numbers but i weaseled my in to the back end of their offices and spoke to their CTO.  I took about 15 minutes of his time and gave him an overview of how internet over cable works. He seemed very interested in the concept. I then provided him the name and number of that small system in upstate PA so he could get a look at how they were doing it.

    This new technology had such potential impact on how people would access the internet that I felt that the efforts of this one small company that was just playing around with their own little system could not go unnoticed. Eventually Comcast purchased my local cable tv provider and then I evangelized the concept even more. Self Serving maybe because I wasn’t willing to pay for a T1 to be installed in my house but hopefully we all got something out of it.

    How 200mhz of untapped spectrum fostered the Digital Age

    As I was saying in its infantsy two way communications between the cable headend at the cable companies plant and the customers tv box was used as a filtering mechanism. If you wanted that special movie that was being broadcast at 9pm then you had to order it and a filter was sent  to your set top box to unlock that channel for a certain amount of time.

    We then saw that Ethernet communications could also take advantage of the coax already installed but there were a lot of headaches due to signal problems, incompatible amplifiers and other infrastructure limitations. Retrofitting systems would need to be done to unlock its full potential.

    At the same time that companies joined together with CableLabs to find a solution for delivering internet access there were huge advances in display technologies and video compression.

    Most people were just getting to see laptop LCD monitors transition to the desktop and within about a year some company decided to include a TV Tuner in the LCD Monitor.

    Movies that were once released on VHS only were now being introduced on DVD format because the companies that distributed Music CD and DVDs found that new compression and dedicated video processors could be combined to provide a better way to distribute films on disk.

    Mpeg1 and Mpeg2 gave away to new codecs like DivX and even before YouTube reached us all many Internet users were watching Video clips, Live Webcams and we all knew it wouldn’t be long until viewing video on our computers would be a higher quality then plain old tv.

    Digital Video, HybridFiberCoax and Docsis

    What happens next to unleash quantum leap in cable provisioning comes all at once.

    At the same time that larger companies are installing Hybrid Fiber Coax systems the video compression technology explodes this leaves cable providers in a bit of a sticky situation. The better they make their Internet Access the higher the probability for their network content providers to totally side step them in distribution. On the other hand Verizon begins to offer very limited trials of FIOS fiber directly to the home. The Verizon offering has no impact from the Tv Network distribution path as Fiber to the home will allow Verizon into the Television Distribution business on top of securing their dieing Dialup and ISDN Services. To this point DSL has not even started but within months it will.

    It is at this point that Microsoft gives 1 Billion Dollars to Comcast to foster their Hybrid Fiber Backbone because they know that the more people that connect to the internet means more content on the internet and more content means more servers using their operating system. Not only that Microsoft is now in the Broadcast Industry through MSNBC so they want a part of anything that might improve their market share including Windows Media Distribution.

    So faced with the decision to go Internet or Analog Video the Cable companies grasp at Digital Distribution of video over Ethernet. This technology harnesses the good aspects of all they have been working on.

    First they upgrade their lines from pure Coax to Hybrid Fiber to the Neighborhoods and then they make use of the already installed Coax from the Fiber box to make the last high speed hop into the house. This means lower overhead because most homes already have Coax into the house. Verizon on the other hand that is installing Fiber to every home will have an advantage once every home is installed. At this point only about 2 million of the 150 million homes have this service so don’t hold your breath. You are more likely to see electric cars being sold by Arab Oil Producers before you get Fiber to your house that can compete with your Cable Companies Hybrid Fiber Network.

    So, lets recap before we go on

    Analog systems were first introduced– They used satellite dishes at the cable plant to get video into the coax wires that were sent to your home. Everyone got all of the channels and Pay channels were filtered by inline filters. (this can be thought of as a dumb system) 

    Addressable Cable Boxs were introduced and with an extra 200mhz of signal cable companies could now apply filters in their set top box. Everyone still got the full signal to their home but 2 way communication and instant filtering allowed for temporary filters that would let you watch a movie that was being broadcast at a set time from the cable plant.

    Tests in Data Communications– A small company with a few crazy techs decided to hook up their company network to the Cable Tv line and it worked kinda. This was the start of Packet Communications over the Cable Network. Packets of Ethernet data were sent to peoples computers as they were requested. (By browsing the internet, You request a webpage and it is sent to you and only you on the network.)

    Cable Companies Upgrade to Hybrid Fiber Coax– This jump in technology was first needed to allow segmentation of the Cable Network. Until that time the Analog signal that was broadcast was sent out to all homes just the same but because of the need for Internet Data Communications a Cable plant that served 10,000 customers could not have data flying around its network. Local Loops of (maybe 100) Coax Connected homes were segmented and a fiber optic line was sent to that area and then when internet traffic was used in that area only 100 posible homes would be in the Ethernet cloud. (this can be seen on your cable modem when you are not using it there is often blinking lights…. this is data that is skipping across your modem and trying to find your neighbors)

    Video Compression Technology Quantum Leaps– This allows full screen video to be broadcast over Ethernet or the internet. It also has the effect of fostering HDTV High Definition Digital Television where more data is needed to fill a larger more fine grain picture.

    Back to Now

    Packet Transmission of Data Merges with Video Transmission– with so much data running around their networks for internet traffic Set Top Boxes begin to emerge. Now not only can Analog signals be broadcast on the Cable Network but with a little software and a few video processing chips and by harvesting part of their cable modem the cable companies can start to deliver Video On Demand.

    Video on demandwas the next progression of video distribution. With their Networks built up for Internet traffic and the ability to address each device on their cable tv network companies started to look at ways they could make better use of their technology.

    The technology was there
    Two Way Addressable Network Packet Devices was in the Cable Modem
    Video Codecs and Processors were available from DVD video players

    Now all they needed was simple client and server software to run on the set top box to allow customers to select video feeds that would be sent only to that individual networked device.

    This is the basic progression of video distribution from cable tv providers today they have also provisioned portions of traffic for VOIP telephone services. In the next year or so all analog signals both over the air antenna and cable tv will end and all video will be distributed digitaly over ethernet packets on cable systems or a similar broadcast over the air.

    At this point Cable Tv Providers will be in total control of exactly what video content is available to each individual home and each individual set top box.

    This means as subscribers we should be able to require our Cable Tv Providers to allow Ala Cart purchases of video broadcasts. No longer should we have to pay for network broadcasts  if we never watch them. Never watch Disney or Fox News or the Golf Channel then why should you have to pay for it when each and every chanel lineup can be taylored for each television.

    All in all HFC Hybrid Fiber Coax systems used in most cable tv company networks today are really not all that complex.

    You have the headend at the Cable Plant which sends and receives information from the network and you have basically an ethernet based network that has to comply with all of the other headaches found in basic networks. Signals need to be amplified and split and terminated. Routers Switches and enduser devices are used for control of information.

    Once you are done with the hardware then you have the information that needs to be sent. All information is sent in a packet stream and once it reaches its destination device like a cable modem or a Set Top Box  it is converted into RF Signal for tv watching or it contnues its way to your computer for Internet Services.

    SNMP Signals are used for managing network devices and anything you can do on any other network can be done on the Cable System Network. Mac addresses and IP addresses are both used in the network and ISP services are connected to at Network NAP backbone access sites.

    This overview is not the complete picture but it should give you some of the basic principles of how video and other services are sent to your home and next time you are driving down the street and you see a big box hanging from a wire you will know its just a basic network device like a signal amplifier and it is not some mystical magic thing that only the cable companies need to use on their networks.

     Check our other HowTos for information about setting up a home network.

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