Is there cross-over circuitry in a HUB?

[rnwalker]

I got curious the other day. I hooked my DSL modem to a 16 port hub at home
for a small network I am building. I was told to connect the modem to the
UPLINK port, but I didn't have an UPLINK port. They told me to use a
cross-over cable and connect to port 1 (which I did), and all worked fine.

The UPLINK port is used for cascading HUBS. Is the connection to the UPLINK
port crossed over (Tx-Rx) inside the box at the UPLINK port? Someone
suggested to me last week that the UPLINK port is straight through, but that
every port on a HUB is automatically crossed over inside the HUB.

I am curious how the basic circuitry is layed out inside a simple HUB.

Thank you

[Bill Sanderson]

This is a confusing area, but I think the person who told you all the
connections are crossed over in the hub is correct.

"rnwalker" <rnwalke...@sympatico.ca> wrote in message
news:Uwbf6.48610$Pm2.9...@news20.bellglobal.com...

[Terry Kennedy]

In comp.dcom.lans.ethernet rnwalker <rnwalke...@sympatico.ca> wrote:
> The UPLINK port is used for cascading HUBS. Is the connection to the UPLINK
> port crossed over (Tx-Rx) inside the box at the UPLINK port? Someone
> suggested to me last week that the UPLINK port is straight through, but that
> every port on a HUB is automatically crossed over inside the HUB.
>
> I am curious how the basic circuitry is layed out inside a simple HUB.

The physical interface for copper Ethernet is MDI (Media Dependent Inter-
face). End stations like LAN cards in workstations are configured as MDI,
and straight-through patch cables will preserve that pinout when connected
to the hub/switch/whatever.

Almost all hubs are MDI-X (MDI with crossover), so the straight-through
cable connects the user's TX to the hub's RX and vice versa. Some hubs will
have either a dedicated "Uplink" port that's wired as MDI, or a port with
a switch that flips it from MDI-X to MDI.

Switches can be wired either way, though the current trend is mostly toward
MDI-X. Different switches, even within the same vendor's product line, might
be wired differently, though.

For the connection to work, you need an *odd* number of crossovers. But you
need to count any MDI-X port as a crossover. So:

MDI-X hub -> straight cable -> MDI LAN card - 1 crossover (on MDI-X hub)
MDI-X hub -> crossover cable -> MDI-X switch - 3 crossovers

Terry Kennedy http://www.tmk.com
te...@tmk.com Jersey City, NJ USA

[news-server.rochester.rr.com]

The discussion and intend of the need for a crossover or not has always been
a hassle for folks. Hewlett-Packard has an created the AUTO-MDIX feature
on their latest hubs and switches. This let's the hub or switch figure
out if you need the MDI or MDI-X config, and alters its internal electronics
to make the needed connection. It will even fix reversed polarity wiring.
See www.hpprocurve.com for more details.

"rnwalker" <rnwalke...@sympatico.ca> wrote in message
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