Cable network is a system receiving and distributing RF signals mainly inside apartment buildings. The borderline between a community antenna system and a cable TV network can be defined in many different ways, e.g. in Poland a cable network is defined as RF installation located in more than one building and having more than 250 outlets.
Cable TV was primarily planned to be signal installation allowing distribution of large (above 60) number of programs to large or very large group of subscribers. Currently, thanks to common use of HFC (Hybrid Fiber Coaxial) networks, it is possible to create networks serving tens of thousands of subscribers. In the beginning there were used the same channels as for terrestrial television only. To keep up with customers' demands for new channels, there have been utilized frequencies between the bands of terrestrial TV - so called cable channels - marked S.
Transmission capabilities of cable TV
In Poland there is used 8 MHz raster in D/K standard, with PAL color system. SECAM system is also allowed for original SECAM broadcasts (e.g. some satellite French or Russian channels). For stereo programs there is used digital Nicam Stereo system, but many cable networks also distribute some programs in A2 analog stereo.
The total capacity of such system is 99 channels, but the channels 1-5 are currently not used for distribution of TV programs. The rest - 94 channels - take continuous frequency range from 110 MHz to 862 MHz.
Older networks usually used only a subset of the available channels, which illustrates the table below:
Table 1. Frequency bands typically used in antenna community systems and cable TV networks in Poland
Frequency band [MHz]
87.5 - 108.0
Lower special band
110 - 174
174 - 230
Upper special band
230 - 302
S09 - S17
302 - 470
UHF IV (lower UHF)
470 - 606
K21 - K37
UHF V (upper UHF)
606 - 862
K38 - K69
Detailed TV channel frequency list can be found here.
Data transmission to the subscriber.
There are usually used only 60 channels for television transmission. On the assumption that, for the reason of possible distortions, there are left free the channels of terrestrial transmitters (usually 8) and 4 channels for modulators (of e.g. VCRs), it is still 22 channels available for data transmission. In practice, the number is further decreased because some channels have to be skipped due to distortions caused by other transmitters etc., so the real number is around 10.
These channels can be used for digital data broadcasting to subscribers (forward direction). Due to high quality of cable transmission (high S/N ratio, even including some specific distortions in cable networks), especially in the direction to subscribers, it is possible to use complex multi-level modulations. Such modulations ensure fast transmission within low bandwidth, i.e. high efficiency.
Typical examples are the 16QAM and 64QAM modulations. In practice, they are only used for broadcast channels, because they require relatively high signal to noise ratio. The advantage of this kind of modulation is high channel capacity, equal 4 b/Hz/s for 16QAM, and 6 b/Hz/s for 64QAM.
Reverse transmission - the return channel.
Obviously the users must have possibility of reverse transmission to the head station. Because of use of distribution amplifiers, the only possibility is frequency division, i.e. the forward transmission is performed in the range of television channels and the reverse transmission - in 5-65 MHz range. On account of the specificity of this kind of transmission, it is required to use interference-resistant modulations.
For this purpose there are usually used BPSK and QPSK modulations. Their basic advantages are high resistance to distortions and simplicity of the modulators and demodulators. These are the simplest phase modulations, with binary phase-shift keying and quadrature phase-shift keying adequately. The channel capacity is equal 1 b/Hz/s for BPSK and 2 b/Hz/s for QPSK.
Band selection for the return channel.
We have mentioned before that for the reverse transmission in cable networks there have been chosen frequencies lying below forward band, i.e. the 5-65 MHz range. It's worth trying to understand why.
There were two possible variants, either using the band lying below the lowest TV channel, or above the highest one. The frequencies above 862 MHz are less vulnerable to external interferences, as the range is a subject to regulations and the transmitters have limited output power.
However, distribution of so high frequency signals in cable TV networks encounters various problems, related to increase of cable attenuation and decrease of shielding effectiveness. In addition, the higher frequency, the bigger trouble with making filters with steep edges of frequency characteristics.
By contrast, the band below 65 MHz is the most commonly used frequency band, thus the environment is full of interferences. It is interfered by CB transceivers, household devices, car ignition systems, lighting controllers, computers etc. However, the basic advantage of this band for cable applications is low attenuation of the cables and possibility of making efficient filters. Besides that, it is easier to build active devices working in lower frequency bands.
In the very beginning, the upper frequency of the return channel was 30 MHz, to avoid any possibility of interference with the lowest TV channel beginning at 47 MHz. Later, since the lower channels were not used any more, the band has been widened.
The bands of forward and return channels
47 MHz - the lowest frequency of CH2 (raster B)
65.5 MHz - the lowest frequency of FM-OIRT band
87.5 MHz - the lowest frequency of FM band (CCIR)
Return channel bandwidth varies from 25 to 60 MHz, however we should remember that a part of it is useless due to too high interference levels.
Throughput of the return channel.
Now we will try to estimate the throughput of the return channel. Transmission speed depends on the available bandwidth and spectral efficiency of the modulation used.
Rb - transmission speed in bps (bits per second)
n - spectral efficiency in bps/Hz, showing the number of bits that can be coded by one change of the carrier; n describes capacity of the modulation used (it is limited by the ratio of total signal power over the bandwidth and total noise power over the bandwidth).
Transmission speed is proportional to available bandwidth and channel capacity.
The more complex modulation, the higher n, reaching even 10 for 1024QAM modulation.
|modulation||n (real value)||η (theoretical)|
BPSK=2PSK, QPSK=4PSK, 4QAM=4PSK, 4QAM=4PSK
It seems that it is better to use modulations with larger
Influence of C/N parameter on choice of modulation.
It is not a problem to achieve signal to noise ratio better than 40dB in forward channel, so multi-level modulations are usually chosen e.g. 16QAM or 64QAM, that have better transmission characteristics than PSK. It's worth to remember that M-QAM (quadrature AM) modulations are identical to M-APK (amplitude phase keying).
The situation in the return channel is much worse - the signal to noise ratio is always smaller, and changeable within the band. In this case, there have to be chosen modulations featuring high resistance to distortions - usually QPSK, sometimes BPSK.
At this stage we can already determine the throughput of the return channel. We assume that the bandwidth of a single channel is 4 MHz (typical value) and we use QPSK modulation.
For the return channel we can use several such channels, e.g. five, so the total available throughput may reach 40Mbps.
Similarly, we can determine the throughput of the forward channel. Let's assume that single channel bandwidth is 6 MHz (allowance for some margin) and we use 64QAM modulation.
Also using several channels, we get maximum throughput of 210Mbps, which means a fast data transmission network. Additionally, thanks to using a few channels, we get increased reliability of the system. Different modulation types in the return and forward channels cause differences in transmission speed, however they match well with the traffic flow - the subscribers normally receive much more information than they send - the transmission asymmetry is not meaningful for a typical user.
Bandwidth of the return channel.
The total bandwidth of the return channel depends on the required channel capacity and the kind of modulation (which is connected with adequate equipment for the head station and that used by the subscribers). As we mentioned before, more complicated modulations are out of question in this channel, so the tendency for utilization of the whole available band.
Problems are caused by numerous interferences, both external and generated within the network.
There can be defined parameter describing availability of reversible channel. It says what percent of reverse channel width can be used for data transmission. We should be conscious that it can vary in different parts of network. In practice, it turns out that availability of reverse channel is only a fraction of its width. It is necessary to measure availability of the channel, or at least to estimate the ranges that are not available due to disturbing signals. This measurement should be performed with a spectrum analyzer capable of operation in return channel range.
Example spectrum of the return channel
Knowing distribution of disrupting spectral lines and their level, also available signal to noise ratio, we can zone these areas (channels) of reversible channel where transmission with required error level BER is possible.
Practically the strongest interferences occur on bands used by short-wave radio communication, on 27 MHz band and about 50 MHz, on the lowest frequencies, and on intermediate frequencies of radio and television receivers. Their arrangement is disordered which favors use of considerably narrow "elementary" channels. The narrow channels can be fitted between the interfering spectral lines.
Unfortunately the price we pay for it is complication and increase of the number of cable modems in head station. If we want to achieve the same bandwidth, we have to compensate narrow elementary bands by increased number of the the transmission channels. It requires additional modems in head station and increases the total cost of equipment.
Presently, on reversible channel different bandwidth are used.
Bandwidth of broadcast channel
Bandwidth of the broadcast channel cannot be wider than width of television channel. In D/K system it is equal to 8 MHz. The most common bandwidth in cable modems is equal to 6 MHz. One of reasons is compatibility with NTSC standard used in the US and other countries, with 6 MHz channel bandwidth.
Ability to use broadcast channel with significant width, results from better transmission parameters in comparison with reversible channel. Broadcast channel is usually located in UHF band, where the number of outer distortions is smaller, in addition there is significantly less disruptions generated in network itself.
Large spacing C/N, equal at least 43 dB allows to use multi-level modulations with high transmission speed. As the result we achieve speed 10-40 Mb/s. These speeds allow, depending on assumed average falling on single user, to service even 2000-4000 subscribers.
Distortions related to high output level of cable modems.
Cable modems, both those working in head stations and those installed at subscribers' places have high output levels. They typically are 120 dBuV. It is usually not a problem for broadcast channels (other signals distributed in the network have comparable levels), but the locally injected return signal may interfere some TV channels in local televisions.
Drawing showing formation of disruptions caused by cable modem on television receiver
Because of limited separation between outputs of typical splitter (usually 25dB), at the input of TV set or stereo there is strong signal from cable modem. Assuming that modem's output level is 120dBuV, on the input of the receiver there is signal from 5-65 MHz band at 95 dBuV. Such high signal may cause strong intermodulation effects. This problem can be solved by the use of:
- outlets or filters blocking reversible channel band,
- branch joints (distributors) and multimedia outlets with raised separation between inputs R, TV and data transmission output - D, destined for cable modem connection.
Distortions forming on passive elements.
Another problem is non-linearity forming on passive elements. Primarily it concerns branch joints and building branches, which work in normal unidirectional network work with levels not exceeding 100 dBuV. However in bidirectional networks levels on reversible channel significantly exceed 100 dBuV.
Some passive elements are built with the use of ferrite cores that may become nonlinear when the levels are too high. Nonlinear characteristics are the reason of nonlinear distortions and generation of products that may interfere useful band. The countermeasure is to reduce these effects by using specially designed taps not susceptible to high signal levels.
Other source of non-linearity can be unsafe couplings and humid cables, where on oxidized surfaces parasite diode forms. As a result of it, coupling becomes the source of non-linearity, what effects another deterioration of signal to non-linearity ratio. Also here the fight with distortions depends on the use of couplings of renowned companies, security against the water contact, accurate making terminal and also routine controls of couplings.
The cables used should be gel-filled to avoid penetration of water/moisture in case of damaged outer sheath. Other components employed in the network should be designed for operation in systems with return channel (high signal levels).
Filters, crossovers and sockets.
The use of filters, crossovers and multimedia sockets allows to avoid detune of television receivers with strong signal of cable modem on reversible channel.
High-pass filters that strongly attenuate the band of reverse channel, should be used at receiver's input, which is connected to cable network together with the cable modem. Additionally, it is needed to group subscribers - those who have modem should be connected to the branch that is directly connected to building network, and the rest - should be connected to building network through high-pass filter.
The rejection band is usually 5-65 MHz (the upper value depends on chosen reverse channel band), and the pass band is 87-862 MHz. Minimum attenuation at cut-off frequency should be > 40dB, pass-band ripple less than 1-2 dB.
Return loss in pass band (input and output) should be 16.5 +/- 1.5 dB/octave. It's worth to notice that there are other, more expensive filters available, ensuring also matching in rejection band.
For subscribers that don't use cable modem, instead of high-pass filters connected in front of the subscriber's outlet, there can be used outlets with built-in filters. It both simplifies the system and protects the network against interferences (coming from the televisions).
Outlets with high pass filter and band pass filter in radio line.
Outlets can occur in two versions: with branch-joint and branch, sometimes can be found also without band pass filter in radio line. Then, at both outputs full signal of broadcast channel band 87-862 MHz is available.
For subscribers that have modem, so called multimedia outlets, also known as data transmission outlets are used. They have three outputs: two for unidirectional transmission, or for radio R and television TV and one for bidirectional transmission D, to which we connect modem. Basic parameters are: separation between inputs R+TV and input/output D and blocking distortions generated by radio and TV set on the reversible channel band. Also desired is for loss, or attenuation in data transmission line, to be small and the same in both directions.
It should be noticed that the use of branch-joint in RTV line causes that attenuation of TV and R outputs is the same and at TV output we have almost full band 87-862 MHz. However the use of band pass filter in radio line causes that radio receiver receives only signals from 87-108 MHz range. Sometimes radio outlet is connected not through branch-joint but through branch, usually with 8-10 dB attenuation.
Schemes of multimedia outlets with branch and branch-joint.
Modem connection is realized through branch with connection attenuation 10 dB, thanks to it we achieve good separation between input/output D and radio-television outputs.
Telkom-Telmor company manufactures interesting multimedia outlet with improved attenuation parameters. Advanced multimedia outlet GMF-351 has additional filters in data transmission line, thanks to which the insertion loss (D) has been reduced to 1 dB and the isolation between D and R outputs, or between D and TV outputs, is by 10 dB higher than in the typical case with one high-pass filter.
It is very good solution for subscribers using reversible channel, because it practically removes a problem with penetration of distortions from modem to receivers and distortions generated by receivers to reversible channel.
In the case when modem is installed in other place than TV set, division of the signal should not be made byTV set, but in other available place. Standard splitters are not suitable for this purpose - the TV set or stereo would be overdriven by strong return signal from the modem. The path leading to the television has to contain high-pass filter (it may be located in the outlet). The signal can be divided by typical splitterss only after such a filter. In the case of splitting the signal at the input (see the diagram below), the splitter has to be prepared for levels about 120 dB, and the R/TV outlet has to be equipped with high-pass filter.
Scheme of television receiver and modem connection that are placed in different rooms
Typical solution is the use of multimedia crossovers, also called multimedia distributors. The simplest of them have only branch to which we connect modem and high pass filter on television output. More advanced versions have a few filters, what improves the separation and hence increase their cost.
Crossovers (multimedia distributors) with branch and branch-joint
The main advantage of these distributors is integration, in one device, of the tap/splitter and set of filters blocking penetration of unwanted signals from the reverse channel and those generated in the TV tuners. Similarly to the outlet of Telkom-Telmor company, the attenuation in dataa line is asymmetric, 1 dB from the modem and 10 dB to the modem.
There are also available variants with branch-joint instead of branch, that characterize even attenuation on broadcast line between RTV and D inputs that is 4dB, but also lower separation between these inputs.
The sources of internal distortions.
Resistance of cable network to external distortions must be connected with elimination of internal distortions generated in the network. Due to structure of cable networks, penetration of interferences to the broadcast band (87.5-862 MHz) is significantly minimized because of directional characteristics of passive devices. The major source of distortions are cascades of amplifiers, between the head station and the destination point.
Diffusion of distortions on broadcast channel
There is a bigger problem with the return channel, where the distortions from different parts of the network "accumulate" in the signal. Interference and noise generated by amplifiers and other active devices sum up in the node, decreasing parameters of the signal combined from the signals of individual modems and received by devices in the head station.
Diffusion of distortions on reversible channel
Basic source of distortions are all kind radio-television devices and computers connected to cable network. Each of these devices generates during normal work, especially in intermediate frequency band, distortions that in dominant quantity occur on reversible channel. The level of distortions inputted to network by TV set can reach 50 dBuV. It should be noticed that, distortions can be generated also when device is currently in stand-by mode.
Additionally, there is a lot of interferences in the city environment - both generated in the home (household appliances), or coming from outside. Some of them can penetrate the network through the connected devices (i.e. TV sets, stereo, computers).
It is the reason why return channel should be provided only to the points where it is really needed. The rest of home network should be separated by high-pass filter/s. In the place of using cable modem all receivers have to be connected via diplexers or multimedia outlets.
The conclusion is that the network has to be as tight as possible, especially for the unwanted signals in the return channel band. The network has to be composed of suitable equipment (e.g. passive elements linear with high levels), and carefully tested/measured. There should not be used pass-through configurations.
Level of network traffic generated by subscribers.
The level of the traffic generated by subscribers enforces adequate means to ensure good quality of service. Let's try to estimate traffic capacity of data transmission network based on cable TV network. To simplify the issue, we assume one broadcast and one return channel and the following parameters:
- - broadcast channel with 6 MHz bandwidth, modulation 64 QAM, transmission speed 31 Mb/s,
- reversible channel with 1,66 MHz bandwidth, modulation QPSK, transmission speed 2,5 Mb/s.
As we mentioned before, for most subscribers the asymmetry of transmission speed in different directions isn't a problem - they usually download much larger amounts of data than upload to the network.
Now we have to determinate what transmission speed we want to reserve for each subscriber. On account of that no subscriber would accept lower transmission speed than that offered by telephone / ISDN modem, we have to assume that the minimal transmission speed can't drop below 64 kbps for a single subscriber.
Number of channels with given bandwidth can be count using following pattern:
- N - number of channels,
- Rb - transmission speed in b/s,
- P - transmission speed for single subscriber in b/s
- Nd - number of channels - broadcast direction Here we assumed 64 kb/s speed.
- Nz - number of channels - reversible direction
Here we assumed, on account of traffic asymmetry, that reversible traffic to broadcast traffic ratio is like 1:10, or equal 6,4 kb/s.
Presented values inform that maximal number of simultaneously working subscribers can't exceed 400. If the number of working subscribers began to increase, the only way to service the increased traffic is to decrease transmission speed for single user.
Obviously, not all subscribers work simultaneously, so the maximum number of subscribers can be greater. It also depends on the traffic generated by the active subscribers. Some approximation is given by the ratio of active subscribers to the total number of subscribers.
Measure of average generated traffic is Erlang. Traffic value 1E means that user generates continuous traffic and i.e. traffic value 0,1E that user generates traffic only in 10% of network using time.
In the case of telephony it is assumed that subscriber generates traffic with 0,1E intensity, however in the case of data transmission - 0,06 - 0,1E. Estimated subscribers number connected to one node equipped only with one reversible and one broadcast channel can be count with the use of following patterns:
nz=Nz/a=390/0,06=6500 n - subscribers number,
nd=Nd/a=484/0,06=8066 a - average traffic generated by single subscriber.
It can be assumed that in our case the number of connected subscribers cannot exceed 8100. Obviously, 64kbps isn't what subscribers really want; cable networks usually offer much higher speeds. Due to proportion, it is easy to estimate that 4050 subscribers can be serviced at 128 kbps, 2025 at 256 kbps, 1012 at 512 kbps, 506 at 1Mbps, and so on.