JP Vossen on 6 Oct 2009 12:45:09 -0700

[Date Prev] [Date Next] [Thread Prev] [Thread Next] [Date Index] [Thread Index]

Re: [PLUG] Comcast "CDV" device & firewalls

> Date: Tue, 6 Oct 2009 09:55:23 -0400
> From: Carl Johnson <>
>>>> (cable "modems" aren't really "modems")<<<
> can you elaborate?
> from what i know of comcast's systems, they are modems in the sense that the
> downstream data gets modulated onto a 256QAM carrier (mostly by cisco
> uBR10k's) and sent to the device. the device then DEMODulates the RF carrier
> and pulls the data out of it and, spits it out the LAN port. on the return
> the device MODulates the data from the LAN port and puts in onto 16QAM/QPSK
> carrier and sends it back to the CMTS.

Huh, I think you've got me.  I was thinking more along the lines of the 
network layer, where you bridge dissimilar physical segments.  What I 
was *not* thinking of was the actual physical signaling.  Your 
description really does sound like a modem...

> it may not be a "modem" in the sense of dial-up types where the digital data
> is transformed back and forth into analog signal; but there's still some
> demodulation/modulation being done.

And yes, I was also thinking of that digital <--> analog conversion, or 
lack thereof.

I stand corrected, thanks for the info.

As an aside I was also amused by the inverse nature of what we are 
talking about.  Old modems convert digital (data) to analog (voice) to 
send over phone (voice) lines.  Now VoIP converts analog (voice) to 
digital (data) to send over digital (data) lines.  :-)

JP Vossen, CISSP            |:::======|
My Account, My Opinions     |=========|
"Microsoft Tax" = the additional hardware & yearly fees for the add-on
software required to protect Windows from its own poorly designed and
implemented self, while the overhead incidentally flattens Moore's Law.
Philadelphia Linux Users Group         --
Announcements -
General Discussion  --