What's inside the headend of cable television

    On a habr there is a post about IPTV headend. It was told about the methods of receiving and further transmitting a signal from satellites over IP networks. I’ll write about what is included in the headend of cable television and how it all works. Caution, a lot of pictures and text.



    General scheme


    As I wrote at the beginning, unlike IPTV, the KTV headend should be in every place where an abundance of subscribers is planned. The reason is simple - in KTV the signal arrives to the subscriber in a completely different environment - a coaxial cable, it will not be possible to transmit through the IP network. At the same time, broadcasting received from satellites can be easily transmitted from the Main Head Station (MGS) to the Regional (CSG) in the form of Multicast via an IP network. Below is an example from a circuit diagram.

    Of course, large operators may have several MGSs for the purpose of reservation and reception of channels from different geographically remote satellites.

    Elsewhere, you need to take local broadcast channels. You need to show local weather, ads, news. There are two options for this - either pick them up with the usual on-air antenna, digitize, convert and transfer to the subscriber, or pick them up directly from the copyright holders of the content (RTOC). The second option is usually more costly - you need to connect with a third-party organization, place your equipment with it . Therefore, mostly local channels are taken from the air.

    Bit of theory


    People often get confused about what they watch on their TV. In general, 3 types of broadcasting are now common
    1. Analog - you catch it with a conventional antenna in the attic or balcony, although the operator may sell it via cable
    2. Digital - DVB, now digital
    3. IPTV - a classic multicast that comes to your home via the Internet
    4. “Television over the Internet” - this usually means youtube, Smartv, in general, what you watch through regular queries

    Each type has its own advantage.

    Analog TV is good because it will work in any old TV without the use of any converters. For each channel, a band of 8 MHz is allocated here. If you look at the measurements by the spectrum instrument, you will clearly see the carrier of sound and image.

    Digital television (DVB) uses the same frequencies as analog. The key difference is that many channels can be shoved into the 8MHz band. You will no longer see a separate carrier in this band, the signal will be evenly distributed over it. In addition, due to the fact that the signal is now digital, it became possible to encrypt it. With this approach, it became possible to compose channel packages to subscribers. There is nothing tricky in them - all channels (in fact, not all) come to you in encrypted form, and the card inserted in the console contains the key to decrypt them.

    The DVB format itself defines the logic for compressing multiple channels into a single frequency band. There are various types of DVB, for example DVB-C (cable), DVB-T (terrestrial), DVB-S (satellite). The disadvantages of DVB include the fact that the subscriber now must install additional equipment and mess with the card.

    IPTV is great for subscribers, but it’s harder for your ISP to work with it. It gives additional load to the existing subscriber IP-network. Multicast rules here. As in DVB, you can receive absolutely all channels, but part will be encrypted. Unlike DVB, an IP network implies not only a channel from a provider to a subscriber, but also a reverse one. This allows you to use for decryption already, for example, a pair of login-password. In general, not bad about IPTV is written here .

    There is nothing much to talk about "Internet TV"; it seems to be understandable to the majority. Normal video content is transmitted in most cases via HTTP. The provider assumes no responsibility for its quality.

    The image below shows an example of joint broadcasting of analog and digital (DVB-C) television in the same frequency grid.



    In my example, the analogue broadcasts one channel (in my opinion, "Carousel"), and in the figure there are 8 different channels. It is clearly seen that in digital form the information is evenly distributed across the width, and in the analogue, two carrier images and sounds are clearly distinguished.

    The key distinguishing feature of cable television from IPTV / “over the Internet” is that information is transmitted onlyto the subscriber (we will not consider DOCSIS?), from him no answers will be sent to your equipment about a poor-quality signal, errors or something else. In any case, first of all you will have to go to him with your time-tested TV and measuring instruments.

    Also, unlike IP networks, if it’s gone away from you, it’s not a fact that this will come to the subscriber. There are no checksum checks (digitally actually exists, but if they are incorrect, the picture just disappears), information is verified ... I

    anticipate IPTV vs KTV.

    Let's just count: one SD-quality channel (480p) can be transmitted with acceptable quality with a bitrate of 3-4 MBps. HD quality - 8-10 MBps. Total, having in the set of channels 180SD + 20HD, the operator needs to spend only about 1GBps (or even more) on the uplinks of their equipment. This is not considering video with recordings from different places, several audio tracks. To date, common mass operators of wired Internet have common uplinks between 1GBps home nodes. It’s difficult to put television in the current infrastructure.
    On the other hand, after construction for the Internet service, there were “dark” (unused) fibers in the fiber optic cables, because they are usually stored with a margin. They can be used for our purposes. In addition, there are a huge number of places where technologies such as G (E) PON are gaining momentum, with which cable television is easily integrated.

    In addition, the cable television system itself is less whimsical and simpler than IP networks. Here you do not need to take into account the lack of QoS, port coordination, channel dubbing, incorrect routing. And this means that less attention is required to the home nodes, you can try to “set and forget” (of course, when connecting a new client, you may have to tighten the frequency response and power on the home receiver).

    In addition, the average user has not yet gotten used to IPTV, and home TVs that support this technology out of the box without consoles also do not yet appear in a huge mass.
    My opinion is that a mass user is not yet ready for IPTV. Like the mass operator.

    What's inside


    The very concept of the headend is very vague. For example, a video capture device with an AUX OUT port can be called a “head station”. She will broadcast the whole channel for some reason. However, we have a very specific goal - to organize the broadcasting of many channels with real content for a bunch of subscribers. For this, the following is included in our station:

    1. Equipment for receiving local channels - consider that there are a lot of TV tuners, they just take the broadcast (or some other) signal and convert it to IP-multicast.
    2. Antenna post - several conventional antennas for receiving terrestrial television
    3. Decoders / modulators - convert IP multicast into a signal that can be received by custom TVs (digital or analog)
    4. Network equipment - usually an L3 switch (yes, you can also L2) to combine the mixing of multicast local and backbone
    5. Channel-forming equipment - optical transmitter and amplifier


    Now I’ll give an example diagram of a specific CWG:



    It is clear that the composition may vary depending on the situation. For example, if you have a convenient location and it’s easy to connect with local channels via an IP network, you may not need an antenna post in principle.

    If the image quality of local channels is unsatisfactory due to poor reception at the antenna post, it can be moved to another place, anyway it will leave a clean multicast that can be driven through an IP network. In several cities, we have succeeded.
    As I wrote in the theoretical part, the signal to the subscriber can go either analog or digital. Considering that the digital signal works in the same frequency grid, this does not constitute any problems.

    For transmission between optical amplifiers / receivers, a laser signal is used through an optical fiber at a wavelength of 1550 nm. For connections, oblique APC polishing is used. Something like DWDM.

    The complex itself is not very large - a couple of racks:



    In the picture, the leftmost rack does not count, there the equipment is intended for another. It is also useful to add monitoring equipment to the rack.

    Reception of local broadcasting


    As I wrote above, it is necessary to organize the reception of local federal channels (for example, “Russia 1”, “ORT”). In our case, they are taken as an analog signal and then converted to multicast. We use Anevia Flamingo 660 , these are analog encoders in multicast . By and large, this is a system unit with several TV tuners installed in it. Below is the image of the encoders at the back and front. Photos were not very good due to not the best lighting.



    On the left is also a bar for crossing outputs in antennas with inputs to encoders. A kind of patchpanel.

    Antenna Post


    The signal of the air channels should appear on the encoders described above from somewhere. To do this, an antenna post is placed nearby (on the roof) from the analog encoder. Since the broadcasting in our country is in the meter and decimeter bands, two antennas are being installed. They are connected to encoders. Below is a photo from the roof (maybe someone will recognize their city?)



    network hardware


    So, we figured out where to get the content from. Part we take away local channels, part from the main head. For something, you need to accept Multicast traffic, route it, filter out the excess. As I wrote earlier, this is a regular switch (better L3).

    This reduces the entire accepted trunk (from the MGS) and the local (from the antenna post) multicast. In our case, this is the Catalyst 3750 with an additional power supply. Multicast groups are mixed here, part is given for monitoring, part of the local content can be transferred to other nearby cities. For Multicast routing, PIM SM is raised. On the switch, we still over-control all the glands from the RGS complex (and there are many addresses - each board has its own address and you can go to it).



    If your antenna post is removed from the modulators, then it will also require some kind of switch.

    Decoders / Modulators


    We have the right content, it's time to transfer it to the cable TV network. To do this, you need to turn it into a form that is understandable for TVs or set-top boxes. For this, the so-called signal delivery systems are used .

    This is the most important piece of iron. Just it converts everything into the form that can be swallowed by custom TVs or set-top boxes. Specifically, we have three AppearTV DC 1000 chassis with boards.



    In the above case, the two upper ones are encoded into an analog signal, and the lower one into digital. An analog signal is issued as follows - on each board there are 2 “nipples”, from each of which two channels can be given. Total 4 channels from the board. With a figure, the principle is the same, only in each “channel” (frequency range) there is now a bunch of channels. With one lower chassis with 2 boards, as many real channels go as with the two upper fully-stuffed chassis! A bunch of wires from each port goes to the adder and goes to the optical transmitter. Unfortunately, specifically with this CSG I do not have their photos, they will be from the station from another city.

    Our glands are modular - this is the chassis (basket) in which the boards are mounted for various needs. For example, for digital broadcasting, you can set up encryption cards to sell television packages to subscribers.

    A bunch of coaxial wires go from the decoders to the common adder, what comes next will be transmitted over the network. There is a fashionable adder here, but in general terms it can just be a bar with several electric dividers / taps.

    Channel forming equipment


    Well, we have all the signal we need! Now we need to deliver it directly to subscribers. In general, these are already elements of the cable network and here you need to think what you need. But in general terms, the RGS uses an optical transmitter and amplifier for this.
    A coaxial cable enters the optical transmitter and the optical fiber exits. A signal is transmitted along it at a wavelength of 1550 nm. The signal is mixed - information is available both in analog form and in digital.



    In the image above is an optical transmitter, below is an optical amplifier. Please note - polishing on APC patch cords.

    Well, then everything. Here the elements of a cable television network are already beginning - dividers, home optical receivers, electric amplifiers and subscribers with their idle televisions, hanging consoles, leads to wires ...

    Ancillary equipment


    We launched the headend and our subscribers can (if the local technicians did everything right) watch the TV! Now the fun begins - our station needs to be serviced. To do this, it is useful to install auxiliary equipment.

    Specifically, we have a regular computer with a TV tuner to remotely view the availability of channels from the RGS output, VLC to view the Multicast stream at the entrance to the RGS. So far, we haven’t come up with anything smarter than just connecting via RDP and watching what is shown through the TV tuner. Of course, it’s not very convenient, but typical problems (lack of image, lack of sound, incorrect PAL / SECAM color rendition, etc.) can be solved.

    Also connected to it is a measuring device (planar) for evaluating indicators of signal strength and signal-to-noise ratio. Well, at the same time, there is a constant broadcasting of the picture “the channel is temporarily unavailable”, which is automatically turned on in case of failure of any channel.



    Finally


    I showed a basic idea of ​​what the headend of cable television consists of and how it works. It is important to remember the general principle of the passage of content from the antenna to the subscriber:
    1. antenna
    2. encoders (TV tuners)
    3. switches
    4. modulators
    5. transmitters

    I’ll leave some useful links:
    1. Table of frequencies of television channels in our country (OIRT)
    2. Frequencies of broadcasting channels in Russian cities
    3. Good site with a detailed description of operating principles
    4. Frequencies of broadcasting and satellite channels

    PS Thanks to ProgerXP for gliffy.com in which the diagrams were compiled for this article.

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