From: Jay Hennigan (jay@xxxxxxxx)
Date: Tue Nov 06 2001 - 00:46:36 GMT-3
On Mon, 5 Nov 2001, Alex Hsieh wrote:
> hi group:
>
> Is there any difference between rollover and crossover cable?Or they are
> the same thing?Thanks.
They perform similar functions but are or may be wired differently depending
on the nature of the crossover cable. A rollover cable is essentially the
same as a "null modem" cable.
When connecting two devices, each of which transmits data on a wire or
pair of wires and receives on a different wire or pair of wires, it is
necessary to ensure that the transmit side of one device is connected to
the receive side of the other.
In the case of twisted-pair ethernet, switches and hubs are intentionally
wired opposite of end node devices such as computers and (most) routers.
Therefore, connecting a switch or hub to an end mode device uses a straight
through cable as the reversal of transmit and receive is inherent in the
devices being connected.
When directly connecting two end node devices or two hubs or switches,
one must use a crossover cable to connect the transmit side of one end
to the receive side of the other.
A similar situation relates to T-1 and 56K four-wire serial lines,
the transmit side of an end device connects straight through to the
receive side of a standard RJ jack going to a carrier's network.
Cisco console and aux ports are wired in such a manner that the
corresponding transmit and receive data and control lines are
symmetrical around the center of the RJ-45 jack, so that "rolling
over" the wires by connecting the extreme left pin on one end to the
extreme right pin on the other end and so on will cause the appropriate
lines to be connected together.
The pins used for ethernet crossover cables are different. Those used
for T-1 crossover cables are different from both ethernet and the
rollover cable. 56K DDS lines can use a rollover cable as a crossover.
For more details, CCO has diagrams of many cables scattered throughout
many of the installation documents. Also search the web for "null modem",
DCE vs DTE.
-- Jay Hennigan - CCIE #7880 - Network Administration - jay@west.net NetLojix Communications, Inc. - http://www.netlojix.com/ WestNet: Connecting you to the planet. 805 884-6323
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