<s>
A	O
medium	B-Protocol
dependent	I-Protocol
interface	I-Protocol
(	O
MDI	O
)	O
describes	O
the	O
interface	O
(	O
both	O
physical	O
and	O
electrical/optical	O
)	O
in	O
a	O
computer	B-Architecture
network	I-Architecture
from	O
a	O
physical	O
layer	O
implementation	O
to	O
the	O
physical	O
medium	O
used	O
to	O
carry	O
the	O
transmission	O
.	O
</s>
<s>
Ethernet	B-Protocol
over	I-Protocol
twisted	I-Protocol
pair	I-Protocol
also	O
defines	O
a	O
medium	B-Protocol
dependent	I-Protocol
interface	I-Protocol
crossover	I-Protocol
(	O
MDI-X	B-Protocol
)	O
interface	O
.	O
</s>
<s>
Auto	O
MDI-X	B-Protocol
ports	I-Protocol
on	O
newer	O
network	O
interfaces	O
detect	O
if	O
the	O
connection	O
would	O
require	O
a	O
crossover	O
,	O
and	O
automatically	O
chooses	O
the	O
MDI	O
or	O
MDI-X	B-Protocol
configuration	O
to	O
properly	O
match	O
the	O
other	O
end	O
of	O
the	O
link	O
.	O
</s>
<s>
The	O
popular	O
Ethernet	O
family	O
defines	O
common	O
medium-dependent	B-Protocol
interfaces	I-Protocol
.	O
</s>
<s>
For	O
10BASE5	O
,	O
connection	O
to	O
the	O
coaxial	O
cable	O
was	O
made	O
with	O
either	O
a	O
vampire	B-Protocol
tap	I-Protocol
or	O
a	O
pair	O
of	O
N	B-Protocol
connectors	I-Protocol
.	O
</s>
<s>
For	O
10BASE2	O
,	O
the	O
connection	O
to	O
the	O
coaxial	O
cable	O
was	O
typically	O
made	O
with	O
a	O
single	O
BNC	B-Protocol
connector	I-Protocol
to	O
which	O
a	O
T-piece	O
was	O
attached	O
.	O
</s>
<s>
For	O
fiber	O
,	O
a	O
variety	B-Device
of	I-Device
connectors	I-Device
are	O
used	O
depending	O
on	O
manufacturer	O
and	O
physical	O
space	O
availability	O
.	O
</s>
<s>
With	O
10BASE-T	B-Protocol
and	O
100BASE-TX	O
,	O
separate	O
twisted	O
pairs	O
are	O
used	O
for	O
the	O
two	O
directions	O
of	O
communication	O
.	O
</s>
<s>
Since	O
twisted	O
pair	O
cables	O
are	O
conventionally	O
wired	O
pin	O
to	O
pin	O
(	O
straight-through	O
)	O
there	O
are	O
two	O
different	O
pinouts	O
used	O
for	O
the	O
medium-dependent	B-Protocol
interface	I-Protocol
.	O
</s>
<s>
These	O
are	O
referred	O
to	O
as	O
MDI	O
and	O
MDI-X	B-Protocol
(	O
medium-dependent	B-Protocol
interface	I-Protocol
crossover	O
)	O
.	O
</s>
<s>
When	O
connecting	O
an	O
MDI	B-Protocol
port	I-Protocol
to	O
an	O
MDI-X	B-Protocol
port	I-Protocol
,	O
a	O
straight-through	O
cable	O
is	O
used	O
,	O
while	O
to	O
connect	O
two	O
MDI	B-Protocol
ports	I-Protocol
or	O
two	O
MDI-X	B-Protocol
ports	I-Protocol
,	O
a	O
crossover	B-Protocol
cable	I-Protocol
must	O
be	O
used	O
.	O
</s>
<s>
Conventionally	O
,	O
MDI	O
is	O
used	O
on	O
end	O
devices	O
and	O
routers	B-Protocol
while	O
MDI-X	B-Protocol
is	O
used	O
on	O
hubs	B-Protocol
and	O
switches	B-Protocol
.	O
</s>
<s>
Some	O
hubs	B-Protocol
and	O
switches	B-Protocol
have	O
an	O
MDI	O
uplink	O
port	O
(	O
often	O
switchable	O
)	O
to	O
connect	O
to	O
other	O
hubs	B-Protocol
or	O
switches	B-Protocol
without	O
a	O
crossover	B-Protocol
cable	I-Protocol
.	O
</s>
<s>
The	O
terminology	O
generally	O
refers	O
to	O
variants	O
of	O
the	O
Ethernet	B-Protocol
over	I-Protocol
twisted	I-Protocol
pair	I-Protocol
technology	O
that	O
use	O
a	O
female	O
8P8C	O
port	O
connection	O
on	O
a	O
computer	O
,	O
or	O
other	O
network	O
device	O
.	O
</s>
<s>
The	O
X	O
refers	O
to	O
the	O
fact	O
that	O
transmit	O
wires	O
on	O
an	O
MDI	O
device	O
must	O
be	O
connected	O
to	O
receive	O
wires	O
on	O
an	O
MDI-X	B-Protocol
device	O
.	O
</s>
<s>
Straight	O
through	O
cables	O
connect	O
pins	O
1	O
and	O
2	O
(	O
transmit	O
)	O
on	O
an	O
MDI	O
device	O
to	O
pins	O
1	O
and	O
2	O
(	O
receive	O
)	O
on	O
an	O
MDI-X	B-Protocol
device	O
.	O
</s>
<s>
Similarly	O
,	O
pins	O
3	O
and	O
6	O
are	O
receive	O
pins	O
on	O
an	O
MDI	O
device	O
and	O
transmit	O
pins	O
on	O
an	O
MDI-X	B-Protocol
device	O
.	O
</s>
<s>
The	O
general	O
convention	O
is	O
for	O
network	O
hubs	B-Protocol
,	O
bridges	O
and	O
switches	B-Protocol
to	O
use	O
the	O
MDI-X	B-Protocol
configuration	O
,	O
while	O
all	O
other	O
nodes	O
such	O
as	O
personal	B-Device
computers	I-Device
,	O
workstations	B-Device
,	O
servers	O
and	O
routers	B-Protocol
use	O
an	O
MDI	O
interface	O
.	O
</s>
<s>
Some	O
routers	B-Protocol
and	O
other	O
devices	O
had	O
an	O
uplink/normal	O
switch	O
to	O
go	O
back	O
and	O
forth	O
between	O
MDI	O
and	O
MDI-X	B-Protocol
on	O
a	O
specific	O
port	O
.	O
</s>
<s>
The	O
requirement	O
of	O
connecting	O
the	O
transmitter	O
of	O
one	O
side	O
to	O
the	O
receiver	O
on	O
the	O
other	O
side	O
and	O
vice	O
versa	O
makes	O
it	O
necessary	O
to	O
always	O
have	O
an	O
odd	O
number	O
of	O
crossovers	O
between	O
two	O
devices	O
,	O
with	O
an	O
MDI-X	B-Protocol
port	I-Protocol
containing	O
an	O
internal	O
crossover	O
.	O
</s>
<s>
Thus	O
,	O
connecting	O
MDI	O
to	O
MDI-X	B-Protocol
requires	O
a	O
straight-through	O
cable	O
(	O
one	O
crossover	O
in	O
total	O
)	O
.	O
</s>
<s>
Connecting	O
MDI	O
to	O
MDI	O
(	O
no	O
crossover	O
)	O
or	O
MDI-X	B-Protocol
to	O
MDI-X	B-Protocol
(	O
two	O
crossovers	O
)	O
requires	O
a(nother )	O
crossover	O
in	O
the	O
cable	O
to	O
get	O
an	O
odd	O
number	O
.	O
</s>
<s>
When	O
using	O
more	O
complicated	O
setups	O
through	O
multiple	O
patch	B-Protocol
panels	I-Protocol
in	O
structured	O
cabling	O
,	O
the	O
connection	O
can	O
use	O
multiple	O
patch	O
and	O
building	O
cable	O
segments	O
.	O
</s>
<s>
either	O
on	O
the	O
central	O
hub/switch	O
or	O
on	O
each	O
secondary	O
hub/switch	O
.	O
</s>
<s>
To	O
connect	O
two	O
ports	O
of	O
the	O
same	O
configuration	O
(	O
MDI	O
to	O
MDI	O
or	O
MDI-X	B-Protocol
to	O
MDI-X	B-Protocol
)	O
with	O
a	O
10	O
or	O
100	O
Mbit/s	O
connection	O
(	O
10BASE-T	B-Protocol
or	O
100BASE-TX	O
)	O
,	O
an	O
Ethernet	B-Protocol
crossover	I-Protocol
cable	I-Protocol
is	O
needed	O
to	O
cross	O
over	O
the	O
transmit	O
and	O
receive	O
signals	O
in	O
the	O
cable	O
,	O
so	O
that	O
they	O
are	O
matched	O
at	O
the	O
connector	O
level	O
.	O
</s>
<s>
The	O
confusion	O
of	O
needing	O
two	O
different	O
kinds	O
of	O
cables	O
for	O
anything	O
but	O
hierarchical	O
star	B-Architecture
network	I-Architecture
topologies	I-Architecture
prompted	O
a	O
more	O
automatic	O
solution	O
.	O
</s>
<s>
Auto	O
MDI-X	B-Protocol
(	O
aka	O
"	O
auto	O
crossover	O
"	O
)	O
automatically	O
detects	O
the	O
required	O
cable	O
connection	O
type	O
and	O
configures	O
the	O
connection	O
appropriately	O
,	O
removing	O
the	O
need	O
for	O
crossover	B-Protocol
cables	I-Protocol
to	O
interconnect	O
switches	B-Protocol
or	O
connect	O
PCs	B-Device
peer-to-peer	O
.	O
</s>
<s>
For	O
auto	O
MDI-X	B-Protocol
to	O
operate	O
correctly	O
,	O
the	O
data	O
rate	O
on	O
the	O
interface	O
and	O
duplex	O
setting	O
must	O
be	O
set	O
to	O
"	O
auto	O
"	O
.	O
</s>
<s>
Auto	O
MDI-X	B-Protocol
was	O
developed	O
by	O
Hewlett-Packard	O
engineers	O
Daniel	O
Joseph	O
Dove	O
and	O
Bruce	O
W	O
.	O
Melvin	O
.	O
</s>
<s>
When	O
two	O
auto	O
MDI-X	B-Protocol
ports	I-Protocol
are	O
connected	O
together	O
,	O
which	O
is	O
normal	O
for	O
modern	O
products	O
,	O
the	O
algorithm	O
resolution	O
time	O
is	O
typically	O
<	O
500	O
ms.	O
</s>
<s>
Subsequently	O
,	O
Dove	O
promoted	O
auto	O
MDI-X	B-Protocol
within	O
the	O
1000BASE-T	O
standard	O
and	O
also	O
developed	O
patented	O
algorithms	O
for	O
"	O
forced	O
mode	O
auto	O
MDI-X	B-Protocol
"	O
which	O
allow	O
a	O
link	O
to	O
be	O
automatically	O
established	O
even	O
if	O
the	O
port	O
does	O
not	O
auto-negotiate	O
.	O
</s>
<s>
This	O
may	O
or	O
may	O
not	O
be	O
implemented	O
on	O
a	O
given	O
device	O
,	O
so	O
occasionally	O
a	O
crossover	B-Protocol
cable	I-Protocol
may	O
still	O
be	O
necessary	O
when	O
connecting	O
auto	O
MDI-X	B-Protocol
to	O
MDI-X	B-Protocol
(	O
hub	B-Protocol
or	O
switch	O
)	O
,	O
especially	O
when	O
autonegotiation	B-Protocol
is	O
deactivated	O
.	O
</s>
<s>
Newer	O
routers	B-Protocol
,	O
hubs	B-Protocol
and	O
switches	B-Protocol
(	O
including	O
some	O
10/100	B-Protocol
,	O
and	O
all	O
1-gigabit	O
or	O
10-gigabit	O
devices	O
in	O
practice	O
)	O
use	O
auto	O
MDI-X	B-Protocol
for	O
10/100Mbit	O
connections	O
to	O
automatically	O
switch	O
to	O
the	O
proper	O
configuration	O
once	O
a	O
cable	O
is	O
connected	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
there	O
are	O
no	O
dedicated	O
transmit	O
and	O
receive	O
pairs	O
,	O
and	O
consequently	O
,	O
crossover	B-Protocol
cables	I-Protocol
are	O
never	O
required	O
for	O
1000BASE-T	O
communication	O
.	O
</s>
<s>
The	O
physical	O
medium	O
attachment	O
sublayer	O
(	O
PMA	O
)	O
provides	O
identification	O
of	O
each	O
pair	O
and	O
usually	O
continues	O
to	O
work	O
over	O
crossover	B-Protocol
cables	I-Protocol
as	O
well	O
,	O
even	O
if	O
the	O
pairs	O
are	O
unusually	O
swapped	O
,	O
crossed	O
,	O
or	O
if	O
the	O
polarity	O
of	O
a	O
pair	O
is	O
unexpectedly	O
inverted	O
.	O
</s>
