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Re: Can cable and DSL modems do data compressions?

Cross-posted to 5 groups.

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On 8/13/13 5:14 AM, Elmo P. Shagnasty wrote:
> In article <GLSdnfEcJenRHJTPnZ2dnUVZ_vidnZ2d@earthlink.com>,
>   ANTant@zimage.com (Ant) wrote:
>
>>> Cable modems aren't modems, so you can't compare dial-up modems to what
>>> the cableco puts in the middle between your coax and your ethernet.
>>
>> Interesting. Why did they call it modems then? What about DSL?
>
> You don't know much about human nature, do you.  People were used to
> getting online with modems--so when the cable company came by and said
> "we can do it too," they called their thingy a "modem" as well so as to
> give people a familiar point for conversation.
>
> DSL, same thing.
>
> In other words, they took the term that is short for
> "modulate/demodulate" and simply applied it to something else that does
> not modulate and demodulate, all for the sake of convenience.  Now
> "modem" is a general term that defines that thing that provides you paid
> broadband, and no longer means "modulate/demodulate".
>
> And since there's no modulation/demodulation, there's no compression to
> be offered.
>

Sorry, wrong.

Cable modems are very much mod/demod devices. Read this from

http://www.vicomsoft.com/learning-center/cable-modems-part-2/

14. What Is A QPSK And QAM?

Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude 
Modulation (QAM) are two different forms of modulation techniques used 
with cable modem technology.

Quadrature Phase Shift Keying (QPSK) is easy to implement and fairly 
resistant to noise. It is used primarily for sending data from the 
cable subscriber upstream to the Head-End, but sometimes it can also 
be used to send data downstream.

Quadrature Amplitude Modulation (QAM) is primarily used for sending 
data downstream. QAM is very efficient, but QAM's susceptibility to 
interfering signals makes it ill-suited to noisy upstream 
transmissions from the subscriber to the Head-End. There are three 
variations, 16QAM, 64QAM and 256QAM. A single downstream 6 MHz channel 
may support up to 27 Mbps of data throughput from the Head-End using 
64QAM while speeds can be boosted to 36 Mbps using 256QAM. A single 
6MHz upstream channel may support between 500Kbps to 10Mbps of data 
throughput from cable subscribers homes using 16QAM or QPSK depending 
on the amount of frequency spectrum allocated for the
service.



BTW, so are DSL modems. See

http://www.vicomsoft.com/learning-center/dsl-part-2/#3

In implementing ADSL, line coding techniques were an important issue. 
There are two flavours, Discrete Multitone Modulation (DMT) and 
Carrierless Amplitude Phase Modulation (CAP). The conclusion is that 
although CAP is widespread, more and more products are issued with DMT.

Both Cable and DSL modems had to be designed to operate over wire 
cable plants that were built to expect to carry analog signals. Cable 
TV originally carried RF analog TV channels on coax. Telco local loops 
carried analog voice on twisted pair.

Also, analog modulated signals carry farther over wire than raw 
digital data such as TTL, RS-232, Ethernet, USB, etc.

Also, modulation schemes and data compression schemes are not mutually 
exclusive. You can do one without the other if necessary for the 
application.

There are 2 main reasons compression is not done in Cable or DSL modems
1. A high percentage of TCP/IP data packets are already compressed via 
such as JPEG, MPEG and other schemes. So further attempts at 
compression yield no advantage.
2. The data rates are so high already, the added processing needed to 
perform compression algorithms in the modem would only serve to delay 
data throughput.

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Thread

Can cable and DSL modems do data compressions? ANTant@zimage.com (Ant) - 2013-08-09 18:26 -0500
  Re: Can cable and DSL modems do data compressions? Andrew Rossmann <andysnewsreply@no_junk.comcast.net> - 2013-08-10 07:28 -0500
  Re: Can cable and DSL modems do data compressions? "David H. Lipman" <DLipman~nospam~@Verizon.Net> - 2013-08-10 10:26 -0400
    Re: Can cable and DSL modems do data compressions? ANTant@zimage.com (Ant) - 2013-08-10 14:04 -0500
      Re: Can cable and DSL modems do data compressions? "David H. Lipman" <DLipman~nospam~@Verizon.Net> - 2013-08-11 07:35 -0400
        Re: Can cable and DSL modems do data compressions? ANTant@zimage.com (Ant) - 2013-08-11 18:16 -0500
          Re: Can cable and DSL modems do data compressions? "David H. Lipman" <DLipman~nospam~@Verizon.Net> - 2013-08-11 20:29 -0400
          Re: Can cable and DSL modems do data compressions? "Elmo P. Shagnasty" <elmop@nastydesigns.com> - 2013-08-11 21:45 -0400
            Re: Can cable and DSL modems do data compressions? ANTant@zimage.com (Ant) - 2013-08-12 19:43 -0500
              Re: Can cable and DSL modems do data compressions? "Elmo P. Shagnasty" <elmop@nastydesigns.com> - 2013-08-13 05:14 -0400
                Re: Can cable and DSL modems do data compressions? Retired <Retired@home.com> - 2013-08-13 13:35 -0400
                Re: Can cable and DSL modems do data compressions? "Elmo P. Shagnasty" <elmop@nastydesigns.com> - 2013-08-13 21:37 -0400
                Re: Can cable and DSL modems do data compressions? "ps56k" <pschuman_no5pam_m3@interserv.com> - 2013-08-29 22:54 -0500
                Re: Can cable and DSL modems do data compressions? Retired <Retired@home.com> - 2013-08-30 13:13 -0400
                Re: Can cable and DSL modems do data compressions? "ps56k" <pschuman_no5pam_m3@interserv.com> - 2013-09-10 16:13 -0500
  Re: Can cable and DSL modems do data compressions? jeffj@panix.com (Jeff Jonas) - 2013-08-27 19:33 -0400

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