On Fri, 05 Oct 2012 09:12:14 -0700, Jeff Liebermann
wrote:

On Thu, 04 Oct 2012 21:25:59 -0700, josephkk
wrote:

On Thu, 04 Oct 2012 09:20:07 -0700, Jeff Liebermann
wrote:


Not bigger network, but different network. The definition of a router
is a device that connects two DIFFERENT networks at the IP layer (ISO
layer 3). One port is connected to the greater internet network. The
other port is connected to a local area network, that uses
non-routeable IP addresses.

I mostly agree with your distinction on the difference between modem and
router.

It's an oversimplified definition, which falls apart on devices like a
brouters (bridge-router) and using a DSL modem to simultaneously
connect to multiple services. Simple definitions never seem to stay
simple.

My definitions, which might be different than yours:

DSL Modem: DSL (actually ATM) to ethernet bridge. Everything done at
the MAC layer (ISO Layer 2) with no involvement with IP layer (ISO
layer 3) except for configuration management.

I do not think that what is appearing at the DSL end user terminals is ATM
but closer to VT45 or VT135 with highly compressed data. Moreover you are
looking at data likely being a shared service with a much more broadband
(TV) type service on the same pair.

?-)

Ok, you got me. What is V45 and V135? Google wasn't particularly
helpful by suggesting that VT meant vacuum tube. Are you thinking of
AT&T U-Verse IPTV service which uses VDSL? If so, I know nothing
because AT&T doesn't offer it in my area.

VT45 is the payload portion of an STS-1 (about equal to a DS-3), likewise
VT135 is the payload portion of an STS-3.

The DSL modem with diagnostics include "ATM Ping" which should be a
clue. Articles on how DSL works always mention ATM as the underlying
virtual circuit mechanism to connect to the DSLAM (ATM switch) which
does the IP packet reassembly from the tiny ATM pieces.
http://www.eetimes.com/electronics-news/4140135/ADSL-Technology-Explained-Part-2-Getting-to-the-Application-Layer
See section "ADSL and ATM".

Thanks for the link. It ties up a lot of loose ends for me. But the
signal coming down the wire to my modem-router does not seem to include
the ATM overhead.

?-)