The DOCSIS standard
Peter Tirsek has written the following very detailed overview of the Docsis standard used to transmit internet via coaxial cables
"Peter Tirsek" <usenet-spamtrap@tirsek.com> wrote in a message news:Pine.LNX.4.33.0112221049120.20302-100000@wolfie.tirsek.com
> On Fri, 21 Dec 2001, Jens wrote:
>
> > [...] downstream data traffic on a cable TV network has a total
> > capacity of approx. 40 Mbit/s, provided the DOCSIS standard is used.
>
> Just a bit of extra information:
>
> Downstream can be operated using two modulation types, 64-QAM and 256-QAM. The
> latter allows for approximately 33 per cent higher speeds within almost
> the same layer-1 bandwidth (frequency allocation on the cable), but it
> consequently also places much greater demands on the quality of the antenna system,
> which means that most operators presumably use 64-QAM
> modulation instead of 256-QAM.
>
> This results in the following downstream speeds:
>
> With 64-QAM, 6 bits are transmitted per symbol at a symbol rate of
> 5.056941 Msps (millions of symbols per second). This gives a raw data
> bandwidth of 6 * 5.056941 = 30.34 Mbps. In addition, there is error
> correction and MPEG transport stream overhead (this is where MPEG comes
> into the picture, and not in connection with compression).
>
> The error correction overhead amounts to 16 bytes of FEC per 188 bytes (fixed length)
> MPEG transport stream payload, resulting in an utilisation rate of 188 / 204
> = approx. 92%. In addition, there is a 4-byte header within the 188-byte MPEG
> packet, which reduces the actual payload to only 184 / 204
> = approx. 90 per cent.
>
> With the 30.341646 Mbps pre-FEC bandwidth mentioned earlier, we therefore obtain a total
> downstream bandwidth of 6 * 5.056941 * 184 / 204 = 27.37 Mbps
> post-FEC.
>
>
> With 256 QAM, 8 bits are transmitted per symbol at a symbol rate of
> 5.360537 Msps, giving 8 * 5.360537 = 42.88 Mbps pre-FEC and
> 8 * 5.360537 * 184 / 204 = 38.68 Mbps post-FEC.
>
>
> If EuroDOCSIS is used, the figures are slightly different. Here,
> a wider Layer-1 bandwidth is used (8 MHz channel spacing compared
> to DOCSIS’s 6 MHz), which also allows for greater data
> bandwidth.
>
> With 64-QAM: 6 bits per symbol and 6.952 Msps = 41.71 Mbps pre-FEC and
> 37.62 Mbps post-FEC.
>
> With 256-QAM: 8 bits per symbol and 6.952 Msps = 55.62 Mbps pre-FEC and
> 50.16 Mbps post-FEC.
>
>
>
> Although these ever-increasing figures sound fantastic, the reality is
> probably that most people use the American DOCSIS standard (the equipment is cheaper), and
> with 64-QAM (which causes fewer problems due to lower requirements for signal
> quality), a DOCSIS downstream channel is therefore effectively around 27 Mbps.
>
>
> > The same applies to upstream data traffic, which has a total capacity
> > of around 10 Mbit/s.
>
> Again, this depends on which of the various options
> the operator chooses. The upstream can operate at five different symbol
> rates – 160, 320, 640, 1280 and 2560 ksps – and with two different
> modulation schemes, QPSK and 16-QAM. Once again, it is the case that higher
> symbol rates and better modulation schemes provide greater data
> bandwidth, but also place greater demands on the quality of the antenna system.
>
> QPSK provides 2 bits per symbol, and 16-QAM provides 4 bits per symbol. This
> results in the following options for upstream bandwidth, prior to error correction:
>
> ----------- ------------ ------------------------
> | Symbol | Layer-1 | Pre-FEC bandwidth with |
> | rate | bandwidth | QPSK 16-QAM |
> ----------- ------------ ----------- ------------
> | 160 | 0.2 MHz | 0.26 Mbps | 0.64 Mbps |
> | 320 | 0.4 MHz | 0.64 Mbps | 1.28 Mbps |
> | 640 | 0.8 MHz | 1.28 Mbps | 2.56 Mbps |
> | 1280 | 1.6 MHz | 2.56 Mbps | 5.12 Mbps |
> | 2560 | 3.2 MHz | 5.12 Mbps | 10.25 Mbps |
> ----------- ------------ ----------- ------------
>
> The default (at least for Cisco CMTS[1] units) is 1280 ksps with QPSK
> modulation, or 2.56 Mbps pre-FEC per upstream channel. Due to the
> more limited upstream capacity, it is not unusual for the CMTS to be
> equipped with several upstream ports per downstream port, typically in
> a ratio of 4:1, 6:1 or 8:1.
>
> Error correction and overhead on the upstream depend on the type of data
> being transmitted (user data or internal DOCSIS data), as well as how the CMTS
> is configured.
>
>
> > Various compression methods are used, which I am not familiar with, though I
> > believe I have heard that MPEG-2 is used.
>
> Data compression as such is not used. It simply requires
> too much processor capacity to compress the aforementioned up to
> 50 Mbps in real time.
>
> In DOCSIS version 1.1, however, a technique known as Payload
> Header Suppression can be applied on the upstream, whereby the cable modem omits
> certain data from the IP header, after which the CMTS reinstates the missing
> data before the packet is forwarded out into the network.
>
>
> > > So what are the speeds on the TV network and the radio network?
> > I don’t know, but I’m sure there are some experts in the field who
> > can answer that for you. I’ve heard that, with today’s digital technology, you can squeeze
> > around 3–4 channels into the space of a single analogue channel.
>
> It’s difficult to talk about ‘speed’ on an analogue TV channel, which
> Scorpio seems to be asking about. Here we’re getting down to a completely different level than
> we normally discuss in relation to data transmission, so they aren’t particularly easy to
> compare.
>
> However, one could say that a TV channel takes up approximately 7 MHz, and at that
> bandwidth (MHz bandwidth, not Mbps bandwidth) you can have one of
> the aforementioned 27, 38, 37 or 50 Mbps data channels. However, it is not a fair
> comparison to conclude that a TV channel therefore takes up, for example, 27 or
> 50 Mbps, because in this form it is analogue – not digital.
>
>
> With digital TV, things become a little more comparable (and probably
> correspondingly more confusing as well). An MPEG-2 compressed TV signal
> typically takes up between 4 and 8 Mbps, depending on the quality. DVB (the digital TV
> standard used in Europe) is, in terms of transmission, very close
> to EuroDOCSIS; so if we say we have 37 Mbps data capacity in
> the space of a single analogue TV channel, that means there’s room for 4–9 digital TV
> channels in the same space as a single analogue one.
>
>
>
> [1] For the uninitiated: CMTS = Cable Modem Termination System, which is
> the term for the central equipment (in the cable TV world most often
> referred to as headend equipment) that controls all
> cable modems.
>
> --
> Kind regards | On Usenet, I speak solely as
> Peter Tirsek | a private individual, and not on behalf of my
> | employer, Telia Stofa A/S, or any
> | other company in the Telia group.
>