<s>
A	O
packet	B-Protocol
assembler/disassembler	I-Protocol
,	O
abbreviated	O
PAD	O
is	O
a	O
communications	O
device	O
which	O
provides	O
multiple	O
asynchronous	O
terminal	O
connectivity	O
to	O
an	O
X.25	B-Protocol
(	O
packet-switching	O
)	O
network	O
or	O
host	O
computer	O
.	O
</s>
<s>
It	O
collects	O
data	O
from	O
a	O
group	O
of	O
terminals	O
and	O
places	O
the	O
data	O
into	O
X.25	B-Protocol
packets	O
(	O
assembly	O
)	O
.	O
</s>
<s>
A	O
Frame	B-Protocol
Relay	I-Protocol
assembler/disassembler	I-Protocol
(	O
FRAD	O
)	O
is	O
a	O
similar	O
device	O
for	O
accessing	O
Frame	B-Protocol
Relay	I-Protocol
networks	I-Protocol
.	O
</s>
<s>
The	O
structure	O
of	O
a	O
PAD	O
is	O
defined	O
by	O
the	O
ITU-T	O
in	O
recommendations	O
X.3	B-Protocol
,	O
X.28	O
,	O
and	O
X.29	O
.	O
</s>
<s>
X.3	B-Protocol
specifies	O
the	O
parameters	O
for	O
terminal-handling	O
functions	O
such	O
as	O
line	O
speed	O
,	O
flow	O
control	O
,	O
character	O
echo	O
,	O
et	O
al	O
.	O
</s>
<s>
for	O
a	O
connection	O
to	O
an	O
X.25	B-Protocol
host	O
.	O
</s>
<s>
The	O
X.3	B-Protocol
parameters	O
are	O
similar	O
in	O
function	O
to	O
present-day	O
Telnet	B-Protocol
options	O
.	O
</s>
<s>
X.28	O
defines	O
the	O
DTE-C	O
(	O
asynchronous	O
character	O
mode	O
)	O
interface	O
to	O
a	O
PAD	O
,	O
including	O
the	O
commands	O
for	O
making	O
and	O
clearing	O
down	O
connections	O
,	O
and	O
manipulating	O
the	O
X.3	B-Protocol
parameters	O
.	O
</s>
<s>
The	O
commands	O
were	O
very	O
crude	O
,	O
a	O
bit	O
like	O
(	O
but	O
not	O
at	O
all	O
compatible	O
with	O
)	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
.	O
</s>
<s>
X.29	O
defines	O
the	O
DTE-P	O
(	O
packet	O
mode	O
)	O
interface	O
to	O
a	O
PAD	O
,	O
i.e.	O
,	O
how	O
the	O
PAD	O
encapsulates	O
characters	O
and	O
control	O
information	O
in	O
X.25	B-Protocol
packets	O
.	O
</s>
<s>
Connections	O
are	O
established	O
using	O
X.121	B-Protocol
14-digit	O
X.25	B-Protocol
addresses	O
.	O
</s>
<s>
One	O
of	O
the	O
UK	O
Coloured	B-Protocol
Book	I-Protocol
protocols	I-Protocol
,	O
Green	O
Book	O
,	O
also	O
defines	O
two	O
PAD	O
protocols	O
.	O
</s>
<s>
Although	O
not	O
identical	O
to	O
Triple-X	O
,	O
Green	O
Book	O
is	O
sufficiently	O
similar	O
to	O
X.3	B-Protocol
and	O
X.29	O
that	O
generally	O
the	O
two	O
will	O
interwork	O
.	O
</s>
<s>
Green	O
Book	O
also	O
specifies	O
TS29	O
,	O
a	O
very	O
similar	O
protocol	O
which	O
ran	O
over	O
the	O
Yellow	O
Book	O
Transport	O
Service	O
,	O
which	O
is	O
another	O
of	O
the	O
Coloured	B-Protocol
Book	I-Protocol
protocols	I-Protocol
.	O
</s>
<s>
ITP	O
(	O
Interactive	O
Terminal	O
Protocol	O
)	O
was	O
an	O
early	O
PAD	O
protocol	O
for	O
use	O
over	O
X.25	B-Protocol
developed	O
in	O
the	O
1970s	O
for	O
use	O
with	O
UK	O
GPO	O
's	O
EPSS	O
(	O
Experimental	O
Packet	O
Switching	O
System	O
,	O
the	O
predecessor	O
of	O
PSS	B-Protocol
)	O
.	O
</s>
