<s>
Virtual	B-Application
Telecommunications	I-Application
Access	I-Application
Method	I-Application
(	O
VTAM	B-Application
)	O
is	O
the	O
IBM	O
subsystem	O
that	O
implements	O
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
for	O
mainframe	B-Architecture
environments	O
.	O
</s>
<s>
VTAM	B-Application
provides	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
for	O
communication	O
applications	O
,	O
and	O
controls	O
communication	O
equipment	O
such	O
as	O
adapters	O
and	O
controllers	O
.	O
</s>
<s>
In	O
modern	O
terminology	O
,	O
VTAM	B-Application
provides	O
a	O
communication	O
stack	O
and	O
device	O
drivers	O
.	O
</s>
<s>
VTAM	B-Application
was	O
introduced	O
in	O
1974	O
after	O
a	O
series	O
of	O
delays	O
as	O
a	O
major	O
component	O
of	O
SNA	O
along	O
with	O
the	O
370x	B-Device
Network	B-Protocol
Control	I-Protocol
Program	I-Protocol
(	O
NCP	B-Protocol
)	O
and	O
Synchronous	B-Protocol
Data	I-Protocol
Link	I-Protocol
Control	I-Protocol
(	O
SDLC	B-Protocol
)	O
.	O
</s>
<s>
In	O
IBM	O
terminology	O
,	O
VTAM	B-Application
is	O
an	O
access	B-Application
method	I-Application
software	O
allowing	O
application	O
programs	O
to	O
read	O
and	O
write	O
data	O
to	O
and	O
from	O
external	O
devices	O
.	O
</s>
<s>
It	O
is	O
called	O
'	O
virtual	O
 '	O
because	O
it	O
was	O
introduced	O
at	O
the	O
time	O
when	O
IBM	O
was	O
introducing	O
virtual	B-Architecture
storage	I-Architecture
by	O
upgrading	O
the	O
operating	O
systems	O
of	O
the	O
System/360	O
series	O
to	O
virtual	B-Architecture
storage	I-Architecture
versions	O
.	O
</s>
<s>
VTAM	B-Application
was	O
supposed	O
to	O
be	O
the	O
successor	O
to	O
the	O
older	O
telecommunications	B-Application
access	I-Application
methods	I-Application
,	O
such	O
as	O
Basic	B-Application
Telecommunications	I-Application
Access	I-Application
Method	I-Application
(	O
BTAM	B-Application
)	O
and	O
Telecommunications	B-Application
Access	I-Application
Method	I-Application
(	O
TCAM	O
)	O
,	O
which	O
were	O
maintained	O
for	O
compatibility	O
reasons	O
.	O
</s>
<s>
As	O
such	O
,	O
VTAM	B-Application
is	O
comparable	O
with	O
the	O
Virtual	B-Application
Storage	I-Application
Access	I-Application
Method	I-Application
(	O
VSAM	O
)	O
,	O
which	O
was	O
at	O
that	O
time	O
a	O
new	O
and	O
supposedly	O
converged	O
access	B-Application
method	I-Application
for	O
disk	O
storage	O
.	O
</s>
<s>
Originally	O
,	O
VTAM	B-Application
was	O
provided	O
free	O
of	O
charge	O
like	O
most	O
systems	B-Application
software	I-Application
of	O
that	O
time	O
.	O
</s>
<s>
However	O
,	O
VTAM	B-Application
2	O
was	O
the	O
last	O
version	O
to	O
be	O
freely	O
available	O
.	O
</s>
<s>
ACF/VTAM	O
(	O
Advanced	O
Communication	O
Function/Virtual	O
Telecommunications	O
Access	O
Method	O
)	O
was	O
introduced	O
in	O
1976	O
and	O
was	O
provided	O
for	O
a	O
licence	O
fee	O
.	O
</s>
<s>
The	O
major	O
new	O
feature	O
of	O
ACF/VTAM	O
was	O
the	O
Multisystem	O
Networking	O
Facility	O
,	O
which	O
introduced	O
"	O
implementation	O
of	O
intersystem	O
communication	O
among	O
multiple	O
S/370s	O
.	O
</s>
<s>
A	O
limited	O
version	O
of	O
ACF/VTAM	O
known	O
as	O
ACF/VTAME	O
(	O
Advanced	O
Communications	O
Function	O
for	O
the	O
Virtual	B-Application
Telecommunications	I-Application
Access	I-Application
Method	I-Application
Entry	O
)	O
was	O
made	O
available	O
for	O
DOS/VSE	O
systems	O
running	O
on	O
the	O
IBM	B-Device
4300	I-Device
.	O
</s>
<s>
VTAM	B-Application
has	O
been	O
renamed	O
to	O
be	O
the	O
SNA	B-Application
Services	I-Application
feature	O
of	O
Communications	O
Server	O
for	O
OS/390	B-Application
.	O
</s>
<s>
This	O
software	O
package	O
also	O
provides	O
TCP/IP	B-Protocol
functions	O
.	O
</s>
<s>
VTAM	B-Application
is	O
used	O
in	O
multiple	O
enterprises	O
as	O
of	O
September	O
2019	O
.	O
</s>
<s>
VTAM	B-Application
supports	O
several	O
network	O
protocols	O
,	O
including	O
SDLC	B-Protocol
,	O
Token	B-Device
Ring	I-Device
,	O
start-stop	O
,	O
Bisync	B-Protocol
,	O
local	O
(	O
channel	O
attached	O
)	O
3270	O
devices	O
,	O
and	O
later	O
TCP/IP	B-Protocol
.	O
</s>
<s>
In	O
a	O
VTAM	B-Application
network	O
,	O
communication	O
took	O
place	O
through	O
an	O
integrated	O
communication	O
adapter	O
in	O
the	O
mainframe	B-Architecture
itself	O
,	O
or	O
by	O
a	O
separate	O
programmable	O
front-end	O
processor	O
,	O
the	O
IBM	O
3745/3746	O
Communications	O
Processor	O
,	O
with	O
its	O
own	O
operating	O
system	O
,	O
the	O
Network	B-Protocol
Control	I-Protocol
Program	I-Protocol
NCP	B-Protocol
.	O
</s>
<s>
IBM	O
provides	O
hardware	O
maintenance	O
and	O
microcode	O
updates	O
for	O
the	O
estimated	O
20,000	O
installed	O
3745/3746	O
controllers	O
.	O
</s>
<s>
A	O
robust	O
third	O
party	O
industry	O
of	O
smaller	O
3745/3746	O
specialty	O
companies	O
provide	O
such	O
controllers	O
,	O
upgrades	O
,	O
features	O
,	O
and	O
related	O
support	O
services	O
.	O
</s>
<s>
VTAM	B-Application
and	O
SNA	O
are	O
still	O
in	O
use	O
by	O
many	O
enterprises	O
.	O
</s>
<s>
Initially	O
,	O
VTAM	B-Application
only	O
allowed	O
communication	O
between	O
mainframes	B-Architecture
and	O
peripheral	O
equipment	O
such	O
as	O
terminals	O
,	O
distributed	O
processors	O
and	O
minicomputers	O
.	O
</s>
<s>
Later	O
,	O
'	O
cross-domain	O
'	O
services	O
were	O
introduced	O
(	O
not	O
to	O
be	O
confused	O
with	O
TCP/IP	B-Protocol
domains	O
)	O
allowing	O
SNA	O
networks	O
with	O
more	O
than	O
one	O
mainframe	B-Architecture
.	O
</s>
<s>
Finally	O
,	O
Advanced	O
Peer	O
to	O
Peer	O
Networking	O
functions	O
(	O
APPN	B-Architecture
)	O
were	O
added	O
to	O
VTAM	B-Application
.	O
</s>
<s>
In	O
APPN	B-Architecture
the	O
mainframe	B-Architecture
is	O
no	O
longer	O
the	O
heart	O
of	O
the	O
network	O
,	O
but	O
all	O
nodes	O
in	O
the	O
network	O
are	O
considered	O
as	O
peers	O
.	O
</s>
<s>
One	O
of	O
the	O
reasons	O
why	O
APPN	B-Architecture
was	O
introduced	O
was	O
to	O
act	O
as	O
a	O
DECnet-killer	O
,	O
but	O
by	O
the	O
time	O
APPN	B-Architecture
was	O
actually	O
shipped	O
the	O
popularity	O
of	O
DECnet	B-Protocol
had	O
already	O
declined	O
.	O
</s>
<s>
APPN	B-Architecture
was	O
considered	O
overly	O
complex	O
and	O
as	O
of	O
2012	O
has	O
been	O
largely	O
superseded	O
by	O
TCP/IP	B-Protocol
.	O
</s>
