<s>
A	O
time-slot	B-Device
interchange	I-Device
(	O
TSI	O
)	O
switch	O
is	O
a	O
network	B-Protocol
switch	I-Protocol
that	O
stores	O
data	O
in	O
RAM	B-Architecture
in	O
one	O
sequence	O
,	O
and	O
reads	O
it	O
out	O
in	O
a	O
different	O
sequence	O
.	O
</s>
<s>
It	O
uses	O
RAM	B-Architecture
,	O
a	O
small	O
routing	O
memory	O
and	O
a	O
counter	O
.	O
</s>
<s>
The	O
RAM	B-Architecture
stores	O
the	O
packets	B-Protocol
or	O
other	O
data	O
that	O
arrive	O
via	O
its	O
input	O
terminal	O
.	O
</s>
<s>
In	O
a	O
pure	O
time-slot	B-Device
interchange	I-Device
switch	O
,	O
there	O
is	O
only	O
one	O
physical	O
input	O
,	O
and	O
one	O
physical	O
output	O
.	O
</s>
<s>
When	O
a	O
packet	B-Protocol
(	O
or	O
byte	O
,	O
on	O
telephone	O
switches	O
)	O
comes	O
to	O
the	O
input	O
,	O
the	O
switch	O
stores	O
the	O
data	O
in	O
RAM	B-Architecture
in	O
one	O
sequence	O
,	O
and	O
reads	O
it	O
out	O
in	O
a	O
different	O
sequence	O
.	O
</s>
<s>
The	O
RAM	B-Architecture
location	O
chosen	O
for	O
the	O
incoming	O
data	O
is	O
taken	O
from	O
a	O
small	O
memory	O
indexed	O
by	O
the	O
counter	O
.	O
</s>
<s>
The	O
current	O
value	O
of	O
the	O
counter	O
also	O
selects	O
the	O
RAM	B-Architecture
data	O
to	O
forward	O
in	O
the	O
current	O
output	O
time	O
slot	O
.	O
</s>
<s>
In	O
a	O
time-slot	B-Device
interchange	I-Device
(	O
TSI	O
)	O
switch	O
,	O
two	O
memory	O
accesses	O
are	O
required	O
for	O
each	O
connection	O
(	O
one	O
to	O
read	O
and	O
one	O
to	O
store	O
)	O
.	O
</s>
<s>
In	O
higher-speed	O
switches	O
,	O
the	O
limit	O
from	O
T	O
can	O
be	O
halved	O
by	O
using	O
a	O
more	O
expensive	O
,	O
less	O
reliable	O
two-port	O
RAM	B-Architecture
.	O
</s>
<s>
The	O
switch	O
must	O
still	O
arbitrate	O
when	O
there	O
is	O
an	O
attempt	O
to	O
read	O
and	O
write	O
a	O
RAM	B-Architecture
slot	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
In	O
packet-switching	O
networks	O
,	O
a	O
time-slot	B-Device
interchange	I-Device
switch	O
is	O
often	O
combined	O
with	O
two	O
space-division	O
switches	O
to	O
implement	O
small	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
In	O
telephone	O
switches	O
,	O
time-slot	B-Device
interchange	I-Device
switches	O
usually	O
form	O
the	O
outer	O
layer	O
of	O
the	O
switching	O
fabric	O
at	O
a	O
central	O
office	O
's	O
switch	O
.	O
</s>
