TELECOM Digest OnLine - Sorted: Re: Phone Line on cat 5 10-Base-T Ethernet?

Re: Phone Line on cat 5 10-Base-T Ethernet?

Robert Bonomi (
Tue, 10 May 2005 00:56:32 -0000

In article <>,
<> wrote:

> I'm a little bit new to the actual specifications of hardware
> operations so this is something I wanted to ask because it seems to be
> the case according to what I've read. (I've been primarily a software
> person myself.)

> I understand that standard 4-pair wire (cat 5) running data at 10mbps
> does not use the blue/white and blue pair (wires 4 and 5), which is
> typically the standard color for running a phone line along an
> ethernet wire.


> If this is correct, is it possible to run a standard analog phone line
> over cat 5 ethernet sold in hardware or computer stores, simply by
> connecting to the blue & blue/white pair and using that? Since the
> wire is typically twisted pairs, I had the impression this was
> possible without crosstalk between either the ethernet and the phone
> line.

Yup. This is *expressly* part of the original design.

> Also, if the network cards being used are of the typical 10/100 type
> that sell these days for $20 or less, or are included on the
> motherboard of the user's PC, does that mean you can't do this because
> 100mbps will use all 4 pairs, or is it that you can run 100mbps
> service over the other pairs and it won't really use the inner
> blue-blue/white pair?

Nope. Standard 100mbit Ethernet uses the same 4 wires, and those four
wires only. There are some early 'non-standard' 100mbit
implementations that did use all 8 wires. Most common was one with
'VG' as the last part of the protocol name. Also, beware of
"100Base-TX". that trailing 'X' is signficant.

> The things I have read indicate there are two types of 100mbps
> service, 100Base-T, 100Base-T4 and 100Base-T8, where 100base-T8 uses
> all of the wires in a 4-pair ethernet cable, and 100-Base-T4 uses only
> two of the pairs. How would I know which is being used in ordinary
> connections?

Virtually everything 100mbit on the market (in recent history) uses
only the standard 4 wires that plain old 10mbit Ethernet uses

Note: _gigabit_ Ethernet, aka "1000mbit Ethernet" is a different story.
it _does_ use all 8 wires.

As far as I know anything that advertises itself as 10/100mbit
Ethernet will use only the 4 wires used by 10mbit.

> This also seems to imply that the other pair (wires 7 & 8) is also
> available for use as phone service, conceivably implying you can run
> 100mbps ethernet and two analog phone lines on the same 4-pair cable
> without problems or interference. I'd like to know if this is the
> case.


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