<s>
X.25	B-Protocol
is	O
an	O
ITU-T	O
standard	O
protocol	O
suite	O
for	O
packet-switched	B-Protocol
data	O
communication	O
in	O
wide	O
area	O
networks	O
(	O
WAN	O
)	O
.	O
</s>
<s>
This	O
makes	O
it	O
one	O
of	O
the	O
oldest	O
packet-switching	B-Protocol
communication	O
protocols	O
available	O
;	O
it	O
was	O
developed	O
several	O
years	O
before	O
IPv4	B-Protocol
(	O
1981	O
)	O
and	O
the	O
OSI	O
Reference	O
Model	O
(	O
1984	O
)	O
.	O
</s>
<s>
It	O
also	O
supports	O
functionality	O
not	O
found	O
in	O
the	O
OSI	O
network	B-Application
layer	I-Application
.	O
</s>
<s>
Networks	O
using	O
X.25	B-Protocol
were	O
popular	O
during	O
the	O
late	O
1970s	O
and	O
1980s	O
with	O
telecommunications	O
companies	O
and	O
in	O
financial	O
transaction	O
systems	O
such	O
as	O
automated	O
teller	O
machines	O
.	O
</s>
<s>
An	O
X.25	B-Protocol
WAN	O
consists	O
of	O
packet-switching	B-Protocol
exchange	I-Protocol
(	O
PSE	O
)	O
nodes	O
as	O
the	O
networking	O
hardware	O
,	O
and	O
leased	B-Architecture
lines	I-Architecture
,	O
plain	O
old	O
telephone	O
service	O
connections	O
,	O
or	O
ISDN	B-Protocol
connections	O
as	O
physical	B-Application
links	I-Application
.	O
</s>
<s>
However	O
,	O
most	O
users	O
have	O
moved	O
to	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
systems	O
instead	O
.	O
</s>
<s>
X.25	B-Protocol
was	O
used	O
up	O
to	O
2015	O
(	O
e.g.	O
</s>
<s>
X.25	B-Protocol
was	O
also	O
available	O
in	O
niche	O
applications	O
such	O
as	O
Retronet	O
that	O
allow	O
vintage	O
computers	O
to	O
use	O
the	O
Internet	O
.	O
</s>
<s>
The	O
CCITT	O
(	O
later	O
ITU-T	O
)	O
Study	O
Group	O
VII	O
began	O
developing	O
a	O
standard	O
for	O
packet-switched	B-Protocol
data	O
communication	O
in	O
the	O
mid-1970s	O
based	O
upon	O
a	O
number	O
of	O
emerging	O
data	O
network	O
projects	O
.	O
</s>
<s>
Participants	O
in	O
the	O
design	O
of	O
X.25	B-Protocol
included	O
engineers	O
from	O
Canada	O
,	O
France	O
,	O
Japan	O
,	O
the	O
UK	O
,	O
and	O
the	O
USA	O
representing	O
a	O
mix	O
of	O
national	O
PTTs	O
(	O
France	O
,	O
Japan	O
,	O
UK	O
)	O
and	O
private	O
operators	O
(	O
Canada	O
,	O
USA	O
)	O
.	O
</s>
<s>
The	O
X.25	B-Protocol
specification	O
is	O
part	O
of	O
the	O
larger	O
set	O
of	O
X-Series	O
.	O
</s>
<s>
Publicly	O
accessible	O
X.25	B-Protocol
networks	O
,	O
commonly	O
called	O
public	O
data	O
networks	O
,	O
were	O
set	O
up	O
in	O
many	O
countries	O
during	O
the	O
late	O
1970s	O
and	O
1980s	O
to	O
lower	O
the	O
cost	O
of	O
accessing	O
various	O
online	O
services	O
.	O
</s>
<s>
Examples	O
include	O
Iberpac	B-Protocol
,	O
TRANSPAC	O
,	O
Compuserve	O
,	O
Tymnet	B-Application
,	O
Telenet	O
,	O
Euronet	O
,	O
PSS	B-Protocol
,	O
Datapac	B-Protocol
,	O
Datanet	O
1	O
and	O
AUSTPAC	B-Protocol
as	O
well	O
as	O
the	O
International	B-Protocol
Packet	I-Protocol
Switched	I-Protocol
Service	I-Protocol
.	O
</s>
<s>
Beginning	O
in	O
the	O
early	O
1990s	O
,	O
in	O
North	O
America	O
,	O
use	O
of	O
X.25	B-Protocol
networks	O
(	O
predominated	O
by	O
Telenet	O
and	O
Tymnet	B-Application
)	O
started	O
to	O
be	O
replaced	O
by	O
Frame	B-Protocol
Relay	I-Protocol
services	I-Protocol
offered	O
by	O
national	O
telephone	O
companies	O
.	O
</s>
<s>
Most	O
systems	O
that	O
required	O
X.25	B-Protocol
now	O
use	O
TCP/IP	B-Protocol
,	O
however	O
it	O
is	O
possible	O
to	O
transport	O
X.25	B-Protocol
over	O
TCP/IP	B-Protocol
when	O
necessary	O
.	O
</s>
<s>
X.25	B-Protocol
networks	O
are	O
still	O
in	O
use	O
throughout	O
the	O
world	O
.	O
</s>
<s>
A	O
variant	O
called	O
AX.25	B-Protocol
is	O
used	O
widely	O
by	O
amateur	O
packet	B-Application
radio	I-Application
.	O
</s>
<s>
Racal	O
Paknet	O
,	O
now	O
known	O
as	O
Widanet	O
,	O
remains	O
in	O
operation	O
in	O
many	O
regions	O
of	O
the	O
world	O
,	O
running	O
on	O
an	O
X.25	B-Protocol
protocol	O
base	O
.	O
</s>
<s>
In	O
some	O
countries	O
,	O
like	O
the	O
Netherlands	O
or	O
Germany	O
,	O
it	O
is	O
possible	O
to	O
use	O
a	O
stripped	O
version	O
of	O
X.25	B-Protocol
via	O
the	O
D-channel	O
of	O
an	O
ISDN-2	O
(	O
or	O
ISDN	B-Protocol
BRI	I-Protocol
)	O
connection	O
for	O
low-volume	O
applications	O
such	O
as	O
point-of-sale	O
terminals	O
;	O
but	O
,	O
the	O
future	O
of	O
this	O
service	O
in	O
the	O
Netherlands	O
is	O
uncertain	O
.	O
</s>
<s>
X.25	B-Protocol
is	O
still	O
used	O
in	O
the	O
aeronautical	O
business	O
(	O
especially	O
in	O
Asia	O
)	O
even	O
though	O
a	O
transition	O
to	O
modern	O
protocols	O
like	O
X.400	B-Protocol
is	O
without	O
option	O
as	O
X.25	B-Protocol
hardware	O
becomes	O
increasingly	O
rare	O
and	O
costly	O
.	O
</s>
<s>
As	O
recently	O
as	O
March	O
2006	O
,	O
the	O
United	O
States	O
National	O
Airspace	O
Data	O
Interchange	O
Network	O
has	O
used	O
X.25	B-Protocol
to	O
interconnect	O
remote	O
airfields	O
with	O
air	O
route	O
traffic	O
control	O
centers	O
.	O
</s>
<s>
France	O
was	O
one	O
of	O
the	O
last	O
remaining	O
countries	O
where	O
commercial	O
end-user	O
service	O
based	O
on	O
X.25	B-Protocol
operated	O
.	O
</s>
<s>
Known	O
as	O
Minitel	B-Application
it	O
was	O
based	O
on	O
Videotex	B-Operating_System
,	O
itself	O
running	O
on	O
X.25	B-Protocol
.	O
</s>
<s>
In	O
2002	O
,	O
Minitel	B-Application
had	O
about	O
9	O
million	O
users	O
,	O
and	O
in	O
2011	O
it	O
accounted	O
for	O
about	O
2	O
million	O
users	O
in	O
France	O
when	O
France	O
Télécom	O
announced	O
it	O
would	O
shut	O
down	O
the	O
service	O
by	O
30	O
June	O
2012	O
.	O
</s>
<s>
The	O
general	O
concept	O
of	O
the	O
X.25	B-Protocol
was	O
to	O
create	O
a	O
universal	O
and	O
global	O
packet-switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
Much	O
of	O
the	O
X.25	B-Protocol
system	O
is	O
a	O
description	O
of	O
the	O
rigorous	O
error	B-Error_Name
correction	I-Error_Name
needed	O
to	O
achieve	O
this	O
,	O
as	O
well	O
as	O
more	O
efficient	O
sharing	O
of	O
capital-intensive	O
physical	O
resources	O
.	O
</s>
<s>
The	O
X.25	B-Protocol
specification	O
defines	O
only	O
the	O
interface	O
between	O
a	O
subscriber	O
(	O
DTE	O
)	O
and	O
an	O
X.25	B-Protocol
network	O
(	O
DCE	O
)	O
.	O
</s>
<s>
X.75	B-Protocol
,	O
a	O
protocol	O
very	O
similar	O
to	O
X.25	B-Protocol
,	O
defines	O
the	O
interface	O
between	O
two	O
X.25	B-Protocol
networks	O
to	O
allow	O
connections	O
to	O
traverse	O
two	O
or	O
more	O
networks	O
.	O
</s>
<s>
X.25	B-Protocol
does	O
not	O
specify	O
how	O
the	O
network	O
operates	O
internally	O
many	O
X.25	B-Protocol
network	O
implementations	O
used	O
something	O
very	O
similar	O
to	O
X.25	B-Protocol
or	O
X.75	B-Protocol
internally	O
,	O
but	O
others	O
used	O
quite	O
different	O
protocols	O
internally	O
.	O
</s>
<s>
The	O
ISO	O
protocol	O
equivalent	O
to	O
X.25	B-Protocol
,	O
ISO	O
8208	O
,	O
is	O
compatible	O
with	O
X.25	B-Protocol
,	O
but	O
additionally	O
includes	O
provision	O
for	O
two	O
X.25	B-Protocol
DTEs	O
to	O
be	O
directly	O
connected	O
to	O
each	O
other	O
with	O
no	O
network	O
in	O
between	O
.	O
</s>
<s>
By	O
separating	O
the	O
Packet-Layer	B-Protocol
Protocol	I-Protocol
,	O
ISO	O
8208	O
permits	O
operation	O
over	O
additional	O
networks	O
such	O
as	O
ISO	O
8802	O
LLC2	O
(	O
ISO	O
LAN	B-General_Concept
)	O
and	O
the	O
OSI	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
.	O
</s>
<s>
X.25	B-Protocol
originally	O
defined	O
three	O
basic	O
protocol	O
levels	O
or	O
architectural	O
layers	O
.	O
</s>
<s>
In	O
the	O
original	O
specifications	O
these	O
were	O
referred	O
to	O
as	O
levels	O
and	O
also	O
had	O
a	O
level	O
number	O
,	O
whereas	O
all	O
ITU-T	O
X.25	B-Protocol
recommendations	O
and	O
ISO	O
8208	O
standards	O
released	O
after	O
1984	O
refer	O
to	O
them	O
as	O
layers	O
.	O
</s>
<s>
Physical	B-Application
layer	I-Application
:	O
This	O
layer	O
specifies	O
the	O
physical	O
,	O
electrical	O
,	O
functional	O
and	O
procedural	O
characteristics	O
to	O
control	O
the	O
physical	B-Application
link	I-Application
between	O
a	O
DTE	O
and	O
a	O
DCE	O
.	O
</s>
<s>
Common	O
implementations	O
use	O
X.21	B-Protocol
,	O
EIA-232	O
,	O
EIA-449	O
or	O
other	O
serial	O
protocols	O
.	O
</s>
<s>
Data	B-Protocol
link	I-Protocol
layer	I-Protocol
:	O
The	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
consists	O
of	O
the	O
link	O
access	O
procedure	O
for	O
data	O
interchange	O
on	O
the	O
link	O
between	O
a	O
DTE	O
and	O
a	O
DCE	O
.	O
</s>
<s>
In	O
its	O
implementation	O
,	O
the	O
Link	B-Protocol
Access	I-Protocol
Procedure	I-Protocol
,	I-Protocol
Balanced	I-Protocol
(	O
LAPB	B-Protocol
)	O
is	O
a	O
data	O
link	O
protocol	O
that	O
manages	O
a	O
communication	O
session	O
and	O
controls	O
the	O
packet	O
framing	O
.	O
</s>
<s>
It	O
is	O
a	O
bit-oriented	O
protocol	O
that	O
provides	O
error	B-Error_Name
correction	I-Error_Name
and	O
orderly	O
delivery	O
.	O
</s>
<s>
Packet	O
layer	O
:	O
This	O
layer	O
defined	O
a	O
packet-layer	B-Protocol
protocol	I-Protocol
for	O
exchanging	O
control	O
and	O
user	O
data	O
packets	O
to	O
form	O
a	O
packet-switching	B-Protocol
network	O
based	O
on	O
virtual	O
calls	O
,	O
according	O
to	O
the	O
Packet	B-Protocol
Layer	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
The	O
X.25	B-Protocol
model	O
was	O
based	O
on	O
the	O
traditional	O
telephony	O
concept	O
of	O
establishing	O
reliable	O
circuits	O
through	O
a	O
shared	O
network	O
,	O
but	O
using	O
software	O
to	O
create	O
"	O
virtual	O
calls	O
"	O
through	O
the	O
network	O
.	O
</s>
<s>
These	O
calls	O
interconnect	O
"	O
data	B-General_Concept
terminal	I-General_Concept
equipment	O
"	O
(	O
DTE	O
)	O
providing	O
endpoints	O
to	O
users	O
,	O
which	O
looked	O
like	O
point-to-point	B-Architecture
connections	I-Architecture
.	O
</s>
<s>
Closely	O
related	O
to	O
the	O
X.25	B-Protocol
protocol	O
are	O
the	O
protocols	O
to	O
connect	O
asynchronous	O
devices	O
(	O
such	O
as	O
dumb	O
terminals	O
and	O
printers	O
)	O
to	O
an	O
X.25	B-Protocol
network	O
:	O
X.3	B-Protocol
,	O
X.28	O
and	O
X.29	O
.	O
</s>
<s>
This	O
functionality	O
was	O
performed	O
using	O
a	O
packet	B-Protocol
assembler/disassembler	I-Protocol
or	O
PAD	O
(	O
also	O
known	O
as	O
a	O
triple-X	O
device	O
,	O
referring	O
to	O
the	O
three	O
protocols	O
used	O
)	O
.	O
</s>
<s>
Although	O
X.25	B-Protocol
predates	O
the	O
OSI	O
Reference	O
Model	O
(	O
OSIRM	O
)	O
,	O
the	O
physical	B-Application
layer	I-Application
of	O
the	O
OSI	O
model	O
corresponds	O
to	O
the	O
X.25	B-Protocol
physical	B-Application
layer	I-Application
,	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
to	O
the	O
X.25	B-Protocol
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
and	O
the	O
network	B-Application
layer	I-Application
to	O
the	O
X.25	B-Protocol
packet	O
layer	O
.	O
</s>
<s>
The	O
X.25	B-Protocol
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
LAPB	B-Protocol
,	O
provides	O
a	O
reliable	O
data	O
path	O
across	O
a	O
data	O
link	O
(	O
or	O
multiple	O
parallel	O
data	O
links	O
,	O
multilink	O
)	O
which	O
may	O
not	O
be	O
reliable	O
itself	O
.	O
</s>
<s>
The	O
X.25	B-Protocol
packet	O
layer	O
provides	O
the	O
virtual	O
call	O
mechanisms	O
,	O
running	O
over	O
X.25	B-Protocol
LAPB	B-Protocol
.	O
</s>
<s>
The	O
packet	O
layer	O
includes	O
mechanisms	O
to	O
maintain	O
virtual	O
calls	O
and	O
to	O
signal	O
data	O
errors	O
in	O
the	O
event	O
that	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
cannot	O
recover	O
from	O
data	O
transmission	O
errors	O
.	O
</s>
<s>
All	O
but	O
the	O
earliest	O
versions	O
of	O
X.25	B-Protocol
include	O
facilities	O
which	O
provide	O
for	O
OSI	O
network	B-Application
layer	I-Application
Addressing	O
(	O
NSAP	B-Protocol
addressing	O
,	O
see	O
below	O
)	O
.	O
</s>
<s>
X.25	B-Protocol
was	O
developed	O
in	O
the	O
era	O
of	O
computer	B-General_Concept
terminals	I-General_Concept
connecting	O
to	O
host	O
computers	O
,	O
although	O
it	O
also	O
can	O
be	O
used	O
for	O
communications	O
between	O
computers	O
.	O
</s>
<s>
Instead	O
of	O
dialing	O
directly	O
“	O
into	O
”	O
the	O
host	O
computer	O
which	O
would	O
require	O
the	O
host	O
to	O
have	O
its	O
own	O
pool	O
of	O
modems	O
and	O
phone	O
lines	O
,	O
and	O
require	O
non-local	O
callers	O
to	O
make	O
long-distance	O
calls	O
the	O
host	O
could	O
have	O
an	O
X.25	B-Protocol
connection	O
to	O
a	O
network	O
service	O
provider	O
.	O
</s>
<s>
Now	O
dumb-terminal	O
users	O
could	O
dial	O
into	O
the	O
network	O
's	O
local	O
“	O
PAD	O
”	O
(	O
packet	B-Protocol
assembly/disassembly	I-Protocol
facility	O
)	O
,	O
a	O
gateway	O
device	O
connecting	O
modems	O
and	O
serial	O
lines	O
to	O
the	O
X.25	B-Protocol
link	O
as	O
defined	O
by	O
the	O
X.29	O
and	O
X.3	B-Protocol
standards	O
.	O
</s>
<s>
Having	O
connected	O
to	O
the	O
PAD	O
,	O
the	O
dumb-terminal	O
user	O
tells	O
the	O
PAD	O
which	O
host	O
to	O
connect	O
to	O
,	O
by	O
giving	O
a	O
phone-number-like	O
address	O
in	O
the	O
X.121	B-Protocol
address	I-Protocol
format	I-Protocol
(	O
or	O
by	O
giving	O
a	O
host	O
name	O
,	O
if	O
the	O
service	O
provider	O
allows	O
for	O
names	O
that	O
map	O
to	O
X.121	B-Protocol
addresses	O
)	O
.	O
</s>
<s>
The	O
PAD	O
then	O
places	O
an	O
X.25	B-Protocol
call	O
to	O
the	O
host	O
,	O
establishing	O
a	O
virtual	O
call	O
.	O
</s>
<s>
Note	O
that	O
X.25	B-Protocol
provides	O
for	O
virtual	O
calls	O
,	O
so	O
appears	O
to	O
be	O
a	O
circuit	B-Protocol
switched	I-Protocol
network	I-Protocol
,	O
even	O
though	O
in	O
fact	O
the	O
data	O
itself	O
is	O
packet	B-Protocol
switched	I-Protocol
internally	O
,	O
similar	O
to	O
the	O
way	O
TCP	O
provides	O
connections	O
even	O
though	O
the	O
underlying	O
data	O
is	O
packet	B-Protocol
switched	I-Protocol
.	O
</s>
<s>
Two	O
X.25	B-Protocol
hosts	O
could	O
,	O
of	O
course	O
,	O
call	O
one	O
another	O
directly	O
;	O
no	O
PAD	O
is	O
involved	O
in	O
this	O
case	O
.	O
</s>
<s>
In	O
theory	O
,	O
it	O
does	O
n't	O
matter	O
whether	O
the	O
X.25	B-Protocol
caller	O
and	O
X.25	B-Protocol
destination	O
are	O
both	O
connected	O
to	O
the	O
same	O
carrier	O
,	O
but	O
in	O
practice	O
it	O
was	O
not	O
always	O
possible	O
to	O
make	O
calls	O
from	O
one	O
carrier	O
to	O
another	O
.	O
</s>
<s>
While	O
the	O
PAD	O
function	O
defined	O
by	O
X.28	O
and	O
X.29	O
specifically	O
supported	O
asynchronous	O
character	O
terminals	O
,	O
PAD	O
equivalents	O
were	O
developed	O
to	O
support	O
a	O
wide	O
range	O
of	O
proprietary	O
intelligent	O
communications	O
devices	O
,	O
such	O
as	O
those	O
for	O
IBM	O
System	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
.	O
</s>
<s>
Error	B-Error_Name
recovery	I-Error_Name
procedures	O
at	O
the	O
packet	O
layer	O
assume	O
that	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
is	O
responsible	O
for	O
retransmitting	O
data	O
received	O
in	O
error	O
.	O
</s>
<s>
Restart	O
packet	O
,	O
which	O
clears	O
down	O
all	O
virtual	O
calls	O
on	O
the	O
data	O
link	O
and	O
resets	O
all	O
permanent	O
virtual	B-Protocol
circuits	I-Protocol
on	O
the	O
data	O
link	O
.	O
</s>
<s>
X.25	B-Protocol
supports	O
two	O
types	O
of	O
virtual	B-Protocol
circuits	I-Protocol
;	O
virtual	O
calls	O
(	O
VC	O
)	O
and	O
permanent	O
virtual	B-Protocol
circuits	I-Protocol
(	O
PVC	O
)	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
permanent	O
virtual	B-Protocol
circuits	I-Protocol
are	O
preconfigured	O
into	O
the	O
network	O
.	O
</s>
<s>
VC	O
may	O
be	O
established	O
using	O
X.121	B-Protocol
addresses	O
.	O
</s>
<s>
The	O
X.121	B-Protocol
address	O
consists	O
of	O
a	O
three-digit	O
data	O
country	O
code	O
(	O
DCC	O
)	O
plus	O
a	O
network	O
digit	O
,	O
together	O
forming	O
the	O
four-digit	O
data	B-Protocol
network	I-Protocol
identification	I-Protocol
code	I-Protocol
(	O
DNIC	O
)	O
,	O
followed	O
by	O
the	O
national	B-Protocol
terminal	I-Protocol
number	I-Protocol
(	O
NTN	O
)	O
of	O
at	O
most	O
ten	O
digits	O
.	O
</s>
<s>
NSAP	B-Protocol
addressing	O
facility	O
was	O
added	O
in	O
the	O
X.25(1984 )	O
revision	O
of	O
the	O
specification	O
,	O
and	O
this	O
enabled	O
X.25	B-Protocol
to	O
better	O
meet	O
the	O
requirements	O
of	O
OSI	O
Connection	O
Oriented	O
Network	O
Service	O
(	O
CONS	B-Protocol
)	O
.	O
</s>
<s>
Public	O
X.25	B-Protocol
networks	O
were	O
not	O
required	O
to	O
make	O
use	O
of	O
NSAP	B-Protocol
addressing	O
,	O
but	O
,	O
to	O
support	O
OSI	O
CONS	B-Protocol
,	O
were	O
required	O
to	O
carry	O
the	O
NSAP	B-Protocol
addresses	O
and	O
other	O
ITU-T	O
specified	O
DTE	O
facilities	O
transparently	O
from	O
DTE	O
to	O
DTE	O
.	O
</s>
<s>
Later	O
revisions	O
allowed	O
multiple	O
addresses	O
in	O
addition	O
to	O
X.121	B-Protocol
addresses	O
to	O
be	O
carried	O
on	O
the	O
same	O
DTE-DCE	O
interface	O
:	O
Telex	B-Application
addressing	O
(	O
F.69	B-Application
)	O
,	O
PSTN	O
addressing	O
(	O
E.163	O
)	O
,	O
ISDN	B-Protocol
addressing	O
(	O
E.164	O
)	O
,	O
Internet	B-Protocol
Protocol	I-Protocol
addresses	O
(	O
IANA	O
ICP	O
)	O
,	O
and	O
local	O
IEEE	O
802.2	O
MAC	B-Protocol
addresses	O
.	O
</s>
<s>
However	O
,	O
in	O
practice	O
not	O
many	O
of	O
the	O
national	O
X.25	B-Protocol
networks	O
supported	O
PVCs	O
.	O
</s>
<s>
One	O
DTE-DCE	O
interface	O
to	O
an	O
X.25	B-Protocol
network	O
has	O
a	O
maximum	O
of	O
4095	O
logical	O
channels	O
on	O
which	O
it	O
is	O
allowed	O
to	O
establish	O
virtual	O
calls	O
and	O
permanent	O
virtual	B-Protocol
circuits	I-Protocol
,	O
although	O
networks	O
are	O
not	O
expected	O
to	O
support	O
a	O
full	O
4095	O
virtual	B-Protocol
circuits	I-Protocol
.	O
</s>
<s>
Logical	O
channel	O
identifiers	O
remain	O
assigned	O
to	O
a	O
virtual	B-Protocol
circuit	I-Protocol
for	O
the	O
duration	O
of	O
the	O
connection	O
.	O
</s>
<s>
The	O
range	O
of	O
possible	O
logical	O
channels	O
is	O
split	O
into	O
4	O
groups	O
:	O
channels	O
assigned	O
to	O
permanent	O
virtual	B-Protocol
circuits	I-Protocol
,	O
assigned	O
to	O
incoming	O
virtual	O
calls	O
,	O
two-way	O
(	O
incoming	O
or	O
outgoing	O
)	O
virtual	O
calls	O
,	O
and	O
outgoing	O
virtual	O
calls	O
.	O
</s>
<s>
All	O
International	O
networks	O
are	O
required	O
to	O
implement	O
support	O
for	O
permanent	O
virtual	B-Protocol
circuits	I-Protocol
,	O
two-way	O
logical	O
channels	O
and	O
one-way	O
logical	O
channels	O
outgoing	O
;	O
one-way	O
logical	O
channels	O
incoming	O
is	O
an	O
additional	O
optional	O
facility	O
.	O
</s>
<s>
Logical	O
channel	O
identifier	O
zero	O
will	O
not	O
be	O
assigned	O
to	O
a	O
permanent	O
virtual	B-Protocol
circuit	I-Protocol
or	O
virtual	O
call	O
.	O
</s>
<s>
The	O
logical	O
channel	O
identifier	O
of	O
zero	O
is	O
used	O
for	O
packets	O
which	O
do	O
n't	O
relate	O
to	O
a	O
specific	O
virtual	B-Protocol
circuit	I-Protocol
(	O
e.g.	O
</s>
<s>
In	O
public	O
networks	O
,	O
X.25	B-Protocol
was	O
typically	O
billed	O
as	O
a	O
flat	O
monthly	O
service	O
fee	O
depending	O
on	O
link	O
speed	O
,	O
and	O
then	O
a	O
price-per-segment	O
on	O
top	O
of	O
this	O
.	O
</s>
<s>
Calls	O
invoking	O
the	O
Fast	O
Select	O
facility	O
(	O
allowing	O
128	O
bytes	O
of	O
data	O
in	O
call	O
request	O
,	O
call	O
confirmation	O
and	O
call	O
clearing	O
phases	O
)	O
would	O
generally	O
attract	O
an	O
extra	O
charge	O
,	O
as	O
might	O
use	O
of	O
some	O
of	O
the	O
other	O
X.25	B-Protocol
facilities	O
.	O
</s>
<s>
The	O
network	O
may	O
allow	O
the	O
selection	O
of	O
the	O
maximal	O
length	O
in	O
range	O
16	O
to	O
4096	O
octets	O
(	O
2n	O
values	O
only	O
)	O
per	O
virtual	B-Protocol
circuit	I-Protocol
by	O
negotiation	O
as	O
part	O
of	O
the	O
call	O
setup	O
procedure	O
.	O
</s>
<s>
The	O
maximal	O
length	O
may	O
be	O
different	O
at	O
the	O
two	O
ends	O
of	O
the	O
virtual	B-Protocol
circuit	I-Protocol
.	O
</s>
<s>
Data	B-General_Concept
terminal	I-General_Concept
equipment	O
constructs	O
control	O
packets	O
which	O
are	O
encapsulated	O
into	O
data	O
packets	O
.	O
</s>
<s>
The	O
packets	O
are	O
sent	O
to	O
the	O
data	O
circuit-terminating	O
equipment	O
,	O
using	O
LAPB	B-Protocol
Protocol	O
.	O
</s>
<s>
Data	O
circuit-terminating	O
equipment	O
strips	O
the	O
layer-2	B-Protocol
headers	O
in	O
order	O
to	O
encapsulate	O
packets	O
to	O
the	O
internal	O
network	O
protocol	O
.	O
</s>
<s>
X.25	B-Protocol
provides	O
a	O
set	O
of	O
user	O
facilities	O
defined	O
and	O
described	O
in	O
ITU-T	O
Recommendation	O
X.2	O
.	O
</s>
<s>
X.25	B-Protocol
also	O
provides	O
X.25	B-Protocol
and	O
ITU-T	O
specified	O
DTE	O
optional	O
user	O
facilities	O
defined	O
and	O
described	O
in	O
ITU-T	O
Recommendation	O
X.7	O
.	O
</s>
<s>
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
(	O
LAN	B-General_Concept
)	O
use	O
)	O
while	O
maintaining	O
compatibility	O
with	O
the	O
CCITT/ITU	O
-T	O
specifications	O
.	O
</s>
<s>
The	O
user	O
facilities	O
and	O
other	O
features	O
supported	O
by	O
each	O
version	O
of	O
X.25	B-Protocol
and	O
ISO/IEC	O
8208	O
have	O
varied	O
from	O
edition	O
to	O
edition	O
.	O
</s>
<s>
Several	O
major	O
protocol	O
versions	O
of	O
X.25	B-Protocol
exist	O
:	O
</s>
<s>
The	O
X.25	B-Protocol
Recommendation	O
allows	O
many	O
options	O
for	O
each	O
network	O
to	O
choose	O
when	O
deciding	O
which	O
features	O
to	O
support	O
and	O
how	O
certain	O
operations	O
are	O
performed	O
.	O
</s>
<s>
This	O
means	O
each	O
network	O
needs	O
to	O
publish	O
its	O
own	O
document	O
giving	O
the	O
specification	O
of	O
its	O
X.25	B-Protocol
implementation	O
,	O
and	O
most	O
networks	O
required	O
DTE	O
appliance	O
manufacturers	O
to	O
undertake	O
protocol	O
conformance	O
testing	O
,	O
which	O
included	O
testing	O
for	O
strict	O
adherence	O
and	O
enforcement	O
of	O
their	O
network	O
specific	O
options	O
.	O
</s>
