<s>
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
is	O
IBM	O
's	O
proprietary	O
networking	B-Architecture
architecture	O
,	O
created	O
in	O
1974	O
.	O
</s>
<s>
It	O
is	O
a	O
complete	O
protocol	B-Protocol
stack	I-Protocol
for	O
interconnecting	O
computers	O
and	O
their	O
resources	O
.	O
</s>
<s>
The	O
implementation	O
of	O
SNA	O
takes	O
the	O
form	O
of	O
various	O
communications	O
packages	O
,	O
most	O
notably	O
Virtual	B-Application
Telecommunications	I-Application
Access	I-Application
Method	I-Application
(	O
VTAM	B-Application
)	O
,	O
the	O
mainframe	B-Architecture
software	O
package	O
for	O
SNA	O
communications	O
.	O
</s>
<s>
SNA	O
was	O
made	O
public	O
as	O
part	O
of	O
IBM	O
's	O
"	O
Advanced	O
Function	O
for	O
Communications	O
"	O
announcement	O
in	O
September	O
,	O
1974	O
,	O
which	O
included	O
the	O
implementation	O
of	O
the	O
SNA/SDLC	O
(	O
Synchronous	B-Protocol
Data	I-Protocol
Link	I-Protocol
Control	I-Protocol
)	O
protocols	O
on	O
new	O
communications	O
products	O
:	O
</s>
<s>
They	O
were	O
supported	O
by	O
IBM	B-Device
3704/3705	I-Device
communication	O
controllers	O
and	O
their	O
Network	B-Protocol
Control	I-Protocol
Program	I-Protocol
(	O
NCP	B-Protocol
)	O
,	O
and	O
by	O
System/370	B-Device
and	O
their	O
VTAM	B-Application
and	O
other	O
software	O
such	O
as	O
CICS	B-Application
and	O
IMS	B-Application
.	O
</s>
<s>
This	O
announcement	O
was	O
followed	O
by	O
another	O
announcement	O
in	O
July	O
,	O
1975	O
,	O
which	O
introduced	O
the	O
IBM	O
3760	O
data	O
entry	O
station	O
,	O
the	O
IBM	B-Architecture
3790	I-Architecture
communication	O
system	O
,	O
and	O
the	O
new	O
models	O
of	O
the	O
IBM	B-Operating_System
3270	I-Operating_System
display	O
system	O
.	O
</s>
<s>
In	O
1999	O
there	O
were	O
an	O
estimated	O
3,500	O
companies	O
"	O
with	O
11,000	O
SNA	O
mainframes.	O
"	O
</s>
<s>
One	O
of	O
the	O
primary	O
pieces	O
of	O
hardware	O
,	O
the	O
3745/3746	O
communications	O
controller	O
,	O
has	O
been	O
withdrawn	O
from	O
the	O
market	O
by	O
IBM	O
.	O
</s>
<s>
A	O
robust	O
market	O
of	O
smaller	O
companies	O
continues	O
to	O
provide	O
the	O
3745/3746	O
,	O
features	O
,	O
parts	O
and	O
service	O
.	O
</s>
<s>
VTAM	B-Application
is	O
also	O
supported	O
by	O
IBM	O
,	O
as	O
is	O
the	O
NCP	B-Protocol
required	O
by	O
the	O
3745/3746	O
controllers	O
.	O
</s>
<s>
with	O
the	O
popularity	O
and	O
growth	O
of	O
TCP/IP	B-Protocol
,	O
SNA	O
is	O
changing	O
from	O
being	O
a	O
true	O
network	O
architecture	O
to	O
being	O
what	O
could	O
be	O
termed	O
an	O
"	O
application	O
and	O
application	O
access	O
architecture.	O
"	O
</s>
<s>
SNA	O
's	O
objective	O
was	O
to	O
reduce	O
the	O
costs	O
of	O
operating	O
large	O
numbers	O
of	O
terminals	O
and	O
thus	O
induce	O
customers	O
to	O
develop	O
or	O
expand	O
interactive	B-General_Concept
terminal-based	O
systems	O
as	O
opposed	O
to	O
batch	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
An	O
expansion	O
of	O
interactive	B-General_Concept
terminal-based	O
systems	O
would	O
increase	O
sales	O
of	O
terminals	O
and	O
more	O
importantly	O
of	O
mainframe	B-Architecture
computers	I-Architecture
and	O
peripherals	O
-	O
partly	O
because	O
of	O
the	O
simple	O
increase	O
in	O
the	O
volume	O
of	O
work	O
done	O
by	O
the	O
systems	O
and	O
partly	O
because	O
interactive	B-General_Concept
processing	O
requires	O
more	O
computing	O
power	O
per	O
transaction	O
than	O
batch	B-General_Concept
processing	I-General_Concept
.	O
</s>
<s>
under	O
CICS	B-Application
or	O
TSO	B-Application
)	O
from	O
the	O
same	O
location	O
.	O
</s>
<s>
SNA	O
also	O
aimed	O
to	O
overcome	O
a	O
limitation	O
of	O
the	O
architecture	O
which	O
IBM	O
's	O
System/370	B-Device
mainframes	B-Architecture
inherited	O
from	O
System/360	B-Application
.	O
</s>
<s>
Each	O
CPU	O
could	O
connect	O
to	O
at	O
most	O
16	O
I/O	B-Device
channels	I-Device
and	O
each	O
channel	O
could	O
handle	O
up	O
to	O
256	O
peripherals	O
-	O
i.e.	O
</s>
<s>
Thus	O
the	O
number	O
of	O
terminals	O
with	O
which	O
powerful	O
mainframes	B-Architecture
could	O
otherwise	O
communicate	O
was	O
limited	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
several	O
multi-layer	B-Protocol
communications	I-Protocol
protocols	I-Protocol
were	O
proposed	O
in	O
the	O
1970s	O
,	O
of	O
which	O
IBM	O
's	O
SNA	O
and	O
ITU-T	O
'	O
s	O
X.25	B-Protocol
became	O
dominant	O
later	O
.	O
</s>
<s>
IBM	B-Protocol
Network	I-Protocol
Control	I-Protocol
Program	I-Protocol
(	O
NCP	B-Protocol
)	O
is	O
a	O
communications	O
program	O
running	O
on	O
the	O
3705	B-Device
and	O
subsequent	O
37xx	B-Device
communications	O
processors	O
that	O
,	O
among	O
other	O
things	O
,	O
implements	O
the	O
packet	O
switching	O
protocol	O
defined	O
by	O
SNA	O
.	O
</s>
<s>
It	O
is	O
a	O
packet	O
forwarding	O
protocol	O
,	O
acting	O
like	O
modern	O
switch	O
-	O
forwarding	O
data	O
packages	O
to	O
the	O
next	O
node	O
,	O
which	O
might	O
be	O
a	O
mainframe	B-Architecture
,	O
a	O
terminal	O
or	O
another	O
3705	B-Device
.	O
</s>
<s>
The	O
communications	O
processors	O
supported	O
only	O
hierarchical	O
networks	O
with	O
a	O
mainframe	B-Architecture
at	O
the	O
center	O
,	O
unlike	O
modern	O
routers	B-Protocol
which	O
support	O
peer-to-peer	O
networks	O
in	O
which	O
a	O
machine	O
at	O
the	O
end	O
of	O
the	O
line	O
can	O
be	O
both	O
a	O
client	B-Protocol
and	O
a	O
server	B-Application
at	O
the	O
same	O
time	O
.	O
</s>
<s>
A	O
3705	B-Device
could	O
support	O
a	O
larger	O
number	O
of	O
lines	O
(	O
352	O
initially	O
)	O
but	O
only	O
counted	O
as	O
one	O
peripheral	O
by	O
the	O
CPUs	O
and	O
channels	O
.	O
</s>
<s>
Since	O
the	O
launch	O
of	O
SNA	O
IBM	O
has	O
introduced	O
improved	O
communications	O
processors	O
,	O
of	O
which	O
the	O
latest	O
is	O
the	O
3745	B-Device
.	O
</s>
<s>
Synchronous	B-Protocol
Data	I-Protocol
Link	I-Protocol
Control	I-Protocol
(	O
SDLC	O
)	O
,	O
a	O
protocol	O
which	O
greatly	O
improved	O
the	O
efficiency	O
of	O
data	O
transfer	O
over	O
a	O
single	O
link	O
:	O
</s>
<s>
It	O
is	O
a	O
Sliding	O
window	O
protocol	O
,	O
which	O
enables	O
terminals	O
and	O
3705	B-Device
communications	O
processors	O
to	O
send	O
frames	O
of	O
data	O
one	O
after	O
the	O
other	O
without	O
waiting	O
for	O
an	O
acknowledgement	O
of	O
the	O
previous	O
frame	O
-	O
the	O
communications	O
cards	O
had	O
sufficient	O
memory	O
and	O
processing	O
capacity	O
to	O
remember	O
the	O
last	O
7	O
frames	O
sent	O
or	O
received	O
,	O
request	O
re-transmission	O
of	O
only	O
those	O
frames	O
which	O
contained	O
errors	O
,	O
and	O
slot	O
the	O
re-transmitted	O
frames	O
into	O
the	O
right	O
place	O
in	O
the	O
sequence	O
before	O
forwarding	O
them	O
to	O
the	O
next	O
stage	O
.	O
</s>
<s>
These	O
frames	O
all	O
had	O
the	O
same	O
type	O
of	O
envelope	O
(	O
frame	O
header	O
and	O
trailer	O
)	O
which	O
contained	O
enough	O
information	O
for	O
data	O
packages	O
from	O
different	O
types	O
of	O
terminal	O
to	O
be	O
sent	O
along	O
the	O
same	O
communications	O
line	O
,	O
leaving	O
the	O
mainframe	B-Architecture
to	O
deal	O
with	O
any	O
differences	O
in	O
the	O
formatting	O
of	O
the	O
content	O
or	O
in	O
the	O
rules	O
governing	O
dialogs	O
with	O
different	O
types	O
of	O
terminal	O
.	O
</s>
<s>
Remote	O
terminals	O
(	O
e.g.	O
,	O
those	O
connected	O
to	O
the	O
mainframe	B-Architecture
by	O
telephone	O
lines	O
)	O
and	O
3705	B-Device
communications	O
processors	O
would	O
have	O
SDLC-capable	O
communications	O
cards	O
.	O
</s>
<s>
This	O
is	O
the	O
precursor	O
of	O
the	O
packet	O
communication	O
that	O
eventually	O
evolved	O
into	O
today	O
's	O
TCP/IP	B-Protocol
technology	O
.	O
</s>
<s>
VTAM	B-Application
,	O
a	O
software	O
package	O
to	O
provide	O
log-in	O
,	O
session	O
keeping	O
and	O
routing	O
services	O
within	O
the	O
mainframe	B-Architecture
.	O
</s>
<s>
A	O
terminal	O
user	O
would	O
log-in	O
via	O
VTAM	B-Application
to	O
a	O
specific	O
application	O
or	O
application	O
environment	O
(	O
e.g.	O
</s>
<s>
CICS	B-Application
,	O
IMS	B-Application
,	O
DB2	B-Application
or	O
TSO/ISPF	O
)	O
.	O
</s>
<s>
A	O
VTAM	B-Application
device	O
would	O
then	O
route	O
data	O
from	O
that	O
terminal	O
to	O
the	O
appropriate	O
application	O
or	O
application	O
environment	O
until	O
the	O
user	O
logged	O
out	O
and	O
possibly	O
logged	O
into	O
another	O
application	O
.	O
</s>
<s>
SNA	O
removed	O
link	O
control	O
from	O
the	O
application	O
program	O
and	O
placed	O
it	O
in	O
the	O
NCP	B-Protocol
.	O
</s>
<s>
Adding	O
communication	O
capability	O
to	O
an	O
application	O
program	O
was	O
much	O
easier	O
because	O
the	O
formidable	O
area	O
of	O
link	O
control	O
software	O
that	O
typically	O
requires	O
interrupt	O
processors	O
and	O
software	O
timers	O
was	O
relegated	O
to	O
system	O
software	O
and	O
NCP	B-Protocol
.	O
</s>
<s>
With	O
the	O
advent	O
of	O
Advanced	B-Architecture
Peer-to-Peer	I-Architecture
Networking	I-Architecture
(	O
APPN	B-Architecture
)	O
,	O
routing	O
functionality	O
was	O
the	O
responsibility	O
of	O
the	O
computer	O
as	O
opposed	O
to	O
the	O
router	B-Protocol
(	O
as	O
with	O
TCP/IP	B-Protocol
networks	I-Protocol
)	O
.	O
</s>
<s>
APPN	B-Architecture
stopped	O
the	O
need	O
to	O
maintain	O
Advanced	B-Protocol
Program-to-Program	I-Protocol
Communication	I-Protocol
(	O
APPC	O
)	O
routing	O
tables	O
that	O
explicitly	O
defined	O
endpoint	O
to	O
endpoint	O
connectivity	O
.	O
</s>
<s>
APPN	B-Architecture
sessions	O
would	O
route	O
to	O
endpoints	O
through	O
other	O
allowed	O
node	O
types	O
until	O
it	O
found	O
the	O
destination	O
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
the	O
way	O
that	O
routers	B-Protocol
for	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
and	O
the	O
Netware	O
Internetwork	B-Protocol
Packet	I-Protocol
Exchange	I-Protocol
protocol	O
function	O
.	O
</s>
<s>
(	O
APPN	B-Architecture
is	O
also	O
sometimes	O
referred	O
to	O
PU2.1	O
or	O
Physical	O
Unit	O
2.1	O
.	O
</s>
<s>
APPC	O
,	O
also	O
sometime	O
referred	O
to	O
LU6.2	B-Protocol
or	O
Logical	O
Unit	O
6.2	O
,	O
was	O
the	O
only	O
protocol	O
defined	O
to	O
APPN	B-Architecture
networks	O
,	O
but	O
was	O
originally	O
one	O
of	O
many	O
protocols	O
supported	O
by	O
VTAM/NCP	O
,	O
along	O
with	O
LU0	O
,	O
LU1	O
,	O
LU2	O
(	O
3270	B-Operating_System
Terminal	I-Operating_System
)	O
,	O
and	O
LU3	O
.	O
</s>
<s>
APPC	O
was	O
primarily	O
used	O
between	O
CICS	B-Application
environments	O
,	O
as	O
well	O
as	O
database	O
services	O
,	O
because	O
it	O
contact	O
protocols	O
for	O
2-phase	O
commit	O
processing	O
)	O
.	O
</s>
<s>
Physical	O
Units	O
were	O
PU5	O
(	O
VTAM	B-Application
)	O
,	O
PU4	O
(	O
37xx	B-Device
)	O
,	O
PU2	O
(	O
Cluster	O
Controller	O
)	O
.	O
</s>
<s>
Before	O
IBM	O
included	O
X.25	B-Protocol
support	O
(	O
NPSI	O
)	O
in	O
SNA	O
,	O
connecting	O
to	O
an	O
X.25	B-Protocol
network	O
would	O
have	O
been	O
awkward	O
.	O
</s>
<s>
Conversion	O
between	O
X.25	B-Protocol
and	O
SNA	O
protocols	O
could	O
have	O
been	O
provided	O
either	O
by	O
NCP	B-Protocol
software	O
modifications	O
or	O
by	O
an	O
external	O
protocol	O
converter	O
.	O
</s>
<s>
Attempts	O
to	O
reduce	O
SNA	O
network	O
complexity	O
by	O
adding	O
IBM	B-Architecture
Advanced	I-Architecture
Peer-to-Peer	I-Architecture
Networking	I-Architecture
functionality	O
were	O
not	O
really	O
successful	O
,	O
if	O
only	O
because	O
the	O
migration	O
from	O
traditional	O
SNA	O
to	O
SNA/APPN	O
was	O
very	O
complex	O
,	O
without	O
providing	O
much	O
additional	O
value	O
,	O
at	O
least	O
initially	O
.	O
</s>
<s>
SNA	O
software	O
licences	O
(	O
VTAM	B-Application
)	O
cost	O
as	O
much	O
as	O
$	O
10,000	O
a	O
month	O
for	O
high-end	O
systems	O
.	O
</s>
<s>
And	O
SNA	O
IBM	B-Device
3745	I-Device
Communications	O
Controllers	O
typically	O
cost	O
over	O
$100K	O
.	O
</s>
<s>
TCP/IP	B-Protocol
was	O
still	O
seen	O
as	O
unfit	O
for	O
commercial	O
applications	O
e.g.	O
</s>
<s>
in	O
the	O
finance	O
industry	O
until	O
the	O
late	O
1980s	O
,	O
but	O
rapidly	O
took	O
over	O
in	O
the	O
1990s	O
due	O
to	O
its	O
peer-to-peer	O
networking	B-Architecture
and	O
packet	O
communication	O
technology	O
.	O
</s>
<s>
APPN	B-Architecture
added	O
a	O
new	O
dimension	O
to	O
state	O
logic	O
with	O
its	O
concept	O
of	O
differing	O
node	O
types	O
.	O
</s>
<s>
APPN	B-Architecture
was	O
n't	O
without	O
issues	O
;	O
in	O
the	O
early	O
days	O
many	O
shops	O
abandoned	O
it	O
due	O
to	O
issues	O
found	O
in	O
APPN	B-Architecture
support	O
.	O
</s>
<s>
Over	O
time	O
,	O
however	O
,	O
many	O
of	O
the	O
issues	O
were	O
worked	O
out	O
but	O
not	O
before	O
TCP/IP	B-Protocol
became	O
increasingly	O
popular	O
in	O
the	O
early	O
1990s	O
,	O
which	O
marked	O
the	O
beginning	O
of	O
the	O
end	O
for	O
SNA	O
.	O
</s>
<s>
Within	O
SNA	O
there	O
are	O
three	O
types	O
of	O
data	O
stream	O
to	O
connect	O
local	O
display	O
terminals	O
and	O
printers	O
;	O
there	O
is	O
SNA	O
Character	O
String	O
(	O
SCS	O
)	O
,	O
used	O
for	O
LU1	O
terminals	O
and	O
for	O
logging	O
on	O
to	O
an	O
SNA	O
network	O
with	O
Unformatted	O
System	O
Services	O
(	O
USS	O
)	O
,	O
there	O
is	O
the	O
3270	B-Operating_System
data	I-Operating_System
stream	I-Operating_System
mainly	O
used	O
by	O
mainframes	B-Architecture
such	O
as	O
the	O
System/370	B-Device
and	O
successors	O
,	O
including	O
the	O
zSeries	O
family	O
,	O
and	O
the	O
5250	B-Device
data	O
stream	O
mainly	O
used	O
by	O
minicomputers/servers	O
such	O
as	O
the	O
System/34	B-Device
,	O
System/36	B-Device
,	O
System/38	B-Device
,	O
and	O
AS/400	B-Device
and	O
its	O
successors	O
,	O
including	O
System	O
i	O
and	O
IBM	B-Device
Power	I-Device
Systems	I-Device
running	O
IBM	B-Application
i	I-Application
.	O
</s>
<s>
This	O
is	O
also	O
used	O
for	O
non-SNA	O
3270	B-Operating_System
devices	O
supported	O
by	O
TCAM	B-Application
or	O
VTAM	B-Application
.	O
</s>
<s>
LU2	O
devices	O
are	O
IBM	B-Operating_System
3270	I-Operating_System
display	O
terminals	O
.	O
</s>
<s>
LU3	O
devices	O
are	O
printers	O
using	O
3270	B-Operating_System
protocols	O
.	O
</s>
<s>
LU4	O
devices	O
are	O
batch	B-General_Concept
terminals	O
.	O
</s>
<s>
LU7	O
provides	O
for	O
sessions	O
with	O
IBM	B-Device
5250	I-Device
terminals	O
.	O
</s>
<s>
The	O
primary	O
ones	O
in	O
use	O
are	O
LU1	O
,	O
LU2	O
,	O
and	O
LU6.2	B-Protocol
(	O
an	O
advanced	O
protocol	O
for	O
application	O
to	O
application	O
conversations	O
)	O
.	O
</s>
<s>
The	O
term	O
37xx	B-Device
refers	O
to	O
IBM	O
's	O
family	O
of	O
SNA	O
communications	O
controllers	O
.	O
</s>
<s>
The	O
3745	B-Device
supports	O
up	O
to	O
eight	O
high-speed	O
T1	B-Protocol
circuits	I-Protocol
,	O
the	O
3725	O
is	O
a	O
large-scale	O
node	O
and	O
front-end	B-General_Concept
processor	I-General_Concept
for	O
a	O
host	O
,	O
and	O
the	O
3720	B-Device
is	O
a	O
remote	O
node	O
that	O
functions	O
as	O
a	O
concentrator	O
and	O
router	B-Protocol
.	O
</s>
<s>
VTAM/NCP	O
PU4	O
nodes	O
attached	O
to	O
IBM	B-Device
Token	I-Device
Ring	I-Device
networks	O
can	O
share	O
the	O
same	O
Local	O
Area	O
Network	B-Architecture
infrastructure	I-Architecture
with	O
workstations	O
and	O
servers	O
.	O
</s>
<s>
NCP	B-Protocol
encapsulates	O
SNA	O
packets	O
into	O
Token-Ring	B-Device
frames	O
,	O
allowing	O
sessions	O
to	O
flow	O
over	O
a	O
Token-Ring	B-Device
network	I-Device
.	O
</s>
<s>
The	O
actual	O
encapsulation	O
and	O
decapsulation	O
takes	O
place	O
in	O
the	O
3745	B-Device
.	O
</s>
<s>
As	O
mainframe-based	O
entities	O
looked	O
for	O
alternatives	O
to	O
their	O
37XX-based	O
networks	O
,	O
IBM	O
partnered	O
with	O
Cisco	O
in	O
the	O
mid-1990s	O
and	O
together	O
they	O
developed	O
Data	B-Protocol
Link	I-Protocol
Switching	I-Protocol
,	O
or	O
DLSw	B-Protocol
.	O
</s>
<s>
DLSw	B-Protocol
encapsulates	O
SNA	O
packets	O
into	O
IP	O
datagrams	O
,	O
allowing	O
sessions	O
to	O
flow	O
over	O
an	O
IP	B-Protocol
network	I-Protocol
.	O
</s>
<s>
The	O
actual	O
encapsulation	O
and	O
decapsulation	O
takes	O
place	O
in	O
Cisco	O
routers	B-Protocol
at	O
each	O
end	O
of	O
a	O
DLSw	B-Protocol
peer	O
connection	O
.	O
</s>
<s>
At	O
the	O
local	O
,	O
or	O
mainframe	B-Architecture
site	O
,	O
the	O
router	B-Protocol
uses	O
Token	B-Device
Ring	I-Device
topology	O
to	O
connect	O
natively	O
to	O
VTAM	B-Application
.	O
</s>
<s>
At	O
the	O
remote	O
(	O
user	O
)	O
end	O
of	O
the	O
connection	O
,	O
a	O
PU	O
type	O
2	O
emulator	O
(	O
such	O
as	O
an	O
SNA	O
gateway	O
server	B-Application
)	O
connects	O
to	O
the	O
peer	O
router	B-Protocol
via	O
the	O
router	B-Protocol
's	O
LAN	O
interface	O
.	O
</s>
<s>
End	O
user	O
terminals	O
are	O
typically	O
PCs	O
with	O
3270	B-Operating_System
emulation	O
software	O
that	O
is	O
defined	O
to	O
the	O
SNA	O
gateway	O
.	O
</s>
<s>
The	O
VTAM/NCP	O
PU	O
type	O
2	O
definition	O
becomes	O
a	O
Switched	O
Major	O
Node	O
that	O
can	O
be	O
local	O
to	O
VTAM	B-Application
(	O
without	O
an	O
NCP	B-Protocol
)	O
,	O
and	O
a	O
"	O
Line	O
"	O
connection	O
can	O
be	O
defined	O
using	O
various	O
possible	O
solutions	O
(	O
such	O
as	O
a	O
Token	B-Device
Ring	I-Device
interface	O
on	O
the	O
3745	B-Device
,	O
a	O
3172	O
Lan	O
Channel	O
Station	O
,	O
or	O
a	O
Cisco	O
ESCON-compatible	O
Channel	O
Interface	O
Processor	O
)	O
.	O
</s>
<s>
The	O
proprietary	O
networking	B-Architecture
architecture	O
for	O
Honeywell	O
Bull	O
mainframes	B-Architecture
is	O
Distributed	O
Systems	O
Architecture	O
(	O
DSA	O
)	O
.	O
</s>
<s>
DSA	O
is	O
also	O
no	O
longer	O
supported	O
for	O
client	B-Protocol
access	O
.	O
</s>
<s>
Bull	O
mainframes	B-Architecture
are	O
fitted	O
with	O
Mainway	O
for	O
translating	O
DSA	O
to	O
TCP/IP	B-Protocol
and	O
VIP	O
devices	O
are	O
replaced	O
by	O
TNVIP	O
Terminal	B-Application
Emulations	I-Application
(	O
GLink	O
,	O
Winsurf	O
)	O
.	O
</s>
<s>
GCOS	B-Application
8	I-Application
supports	O
TNVIP	O
SE	O
over	O
TCP/IP	B-Protocol
.	O
</s>
<s>
The	O
networking	B-Architecture
architecture	O
for	O
Univac	O
mainframes	B-Architecture
was	O
the	O
Distributed	O
Computing	O
Architecture	O
(	O
DCA	O
)	O
,	O
and	O
the	O
networking	B-Architecture
architecture	O
for	O
Burroughs	O
mainframes	B-Architecture
was	O
the	O
Burroughs	O
Network	O
Architecture	O
(	O
BNA	O
)	O
;	O
after	O
they	O
merged	O
to	O
form	O
Unisys	O
,	O
both	O
were	O
provided	O
by	O
the	O
merged	O
company	O
.	O
</s>
<s>
DECnet	B-Protocol
is	O
a	O
suite	O
of	O
network	O
protocols	O
created	O
by	O
Digital	O
Equipment	O
Corporation	O
,	O
originally	O
released	O
in	O
1975	O
to	O
connect	O
two	O
PDP-11	B-Device
minicomputers	B-Architecture
.	O
</s>
<s>
It	O
evolved	O
into	O
one	O
of	O
the	O
first	O
peer-to-peer	O
network	O
architectures	O
,	O
thus	O
transforming	O
DEC	O
into	O
a	O
networking	B-Architecture
powerhouse	O
in	O
the	O
1980s	O
.	O
</s>
<s>
SNA	O
initially	O
aimed	O
at	O
competing	O
with	O
ISO	B-General_Concept
's	I-General_Concept
Open	O
Systems	O
Interconnection	O
,	O
which	O
was	O
an	O
attempt	O
to	O
create	O
a	O
vendor-neutral	O
network	O
architecture	O
that	O
failed	O
due	O
to	O
the	O
problems	O
of	O
"	O
design	O
by	O
committee	O
"	O
.	O
</s>
<s>
The	O
TCP/IP	B-Protocol
suite	I-Protocol
for	O
many	O
years	O
was	O
not	O
considered	O
a	O
serious	O
alternative	O
by	O
IBM	O
,	O
due	O
in	O
part	O
to	O
the	O
lack	O
of	O
control	O
over	O
the	O
intellectual	O
property	O
.	O
</s>
<s>
This	O
protocol	O
allows	O
existing	O
VTAM	B-Application
applications	O
(	O
CICS	B-Application
,	O
TSO	B-Application
)	O
to	O
run	O
with	O
little	O
or	O
no	O
change	O
from	O
traditional	O
SNA	O
by	O
supporting	O
traditional	O
3270	B-Operating_System
terminal	I-Operating_System
protocol	O
over	O
the	O
TCP/IP	B-Protocol
session	O
.	O
</s>
<s>
This	O
protocol	O
is	O
widely	O
used	O
to	O
replace	O
legacy	O
SNA	O
connectivity	O
more	O
than	O
DLSw	B-Protocol
and	O
other	O
SNA	O
replacement	O
technologies	O
.	O
</s>
<s>
A	O
similar	O
TN5250	O
(	O
Telnet	B-Protocol
5250	B-Device
)	O
variant	O
exists	O
for	O
the	O
5250	B-Device
.	O
</s>
<s>
Non-IBM	O
SNA	O
software	O
allowed	O
systems	O
other	O
than	O
IBM	O
's	O
to	O
communicate	O
with	O
IBM	O
's	O
mainframes	B-Architecture
and	O
AS/400	B-Device
midrange	O
computers	O
using	O
the	O
SNA	O
protocols	O
.	O
</s>
<s>
Some	O
Unix	O
system	O
vendors	O
,	O
such	O
as	O
Sun	O
Microsystems	O
with	O
its	O
SunLink	O
SNA	O
product	O
line	O
,	O
including	O
PU2.1	O
Server	B-Application
,	O
and	O
Hewlett-Packard/Hewlett	O
Packard	O
Enterprise	O
,	O
with	O
their	O
SNAplus2	O
product	O
,	O
provided	O
SNA	O
software	O
.	O
</s>
<s>
Microsoft	O
introduced	O
SNA	B-Application
Server	I-Application
for	O
Windows	O
in	O
1993	O
;	O
it	O
is	O
now	O
named	O
Microsoft	B-Application
Host	I-Application
Integration	I-Application
Server	I-Application
.	O
</s>
<s>
Digital	O
Equipment	O
Corporation	O
had	O
VMS/SNA	O
for	O
VMS	B-Operating_System
.	O
</s>
<s>
Third-party	O
SNA	O
software	O
packages	O
for	O
VMS	B-Operating_System
,	O
such	O
as	O
the	O
VAX	O
Link	O
products	O
from	O
Systems	O
Strategies	O
,	O
Inc.	O
,	O
were	O
also	O
available	O
.	O
</s>
<s>
Hewlett-Packard	O
offered	O
SNA	B-Application
Server	I-Application
and	O
SNA	O
Access	O
for	O
its	O
HP	B-Device
3000	I-Device
systems	O
.	O
</s>
<s>
IBM	O
supported	O
using	O
several	O
non-IBM	O
software	O
implementations	O
of	O
APPC/PU2.1/LU6.2	B-Protocol
to	O
communicate	O
with	O
z/OS	B-Application
,	O
including	O
SNAplus2	O
for	O
systems	O
from	O
HP	O
,	O
Brixton	O
4.1	O
SNA	O
for	O
Sun	B-Application
Solaris	I-Application
,	O
and	O
SunLink	O
SNA	O
9.1	O
Support	O
for	O
Sun	B-Application
Solaris	I-Application
.	O
</s>
