<s>
Synchronous	B-Protocol
Data	I-Protocol
Link	I-Protocol
Control	I-Protocol
(	O
SDLC	O
)	O
is	O
a	O
computer	O
communications	O
protocol	O
.	O
</s>
<s>
It	O
is	O
the	O
layer	O
2	O
protocol	O
for	O
IBM	O
's	O
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
.	O
</s>
<s>
SDLC	O
supports	O
multipoint	B-General_Concept
links	O
as	O
well	O
as	O
error	O
correction	O
.	O
</s>
<s>
This	O
de	O
facto	O
standard	O
has	O
been	O
adopted	O
by	O
ISO	O
as	O
High-Level	O
Data	O
Link	O
Control	O
(	O
HDLC	O
)	O
in	O
1979	O
and	O
by	O
ANSI	O
as	O
Advanced	B-Protocol
Data	I-Protocol
Communication	I-Protocol
Control	I-Protocol
Procedures	I-Protocol
(	O
ADCCP	B-Protocol
)	O
.	O
</s>
<s>
The	O
most	O
common	O
alternatives	O
for	O
SNA	O
with	O
SDLC	O
were	O
probably	O
DECnet	B-Protocol
with	O
Digital	B-Protocol
Data	I-Protocol
Communications	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
DDCMP	B-Protocol
)	O
,	O
Burroughs	O
Network	O
Architecture	O
(	O
BNA	O
)	O
with	O
Burroughs	O
Data	O
Link	O
Control	O
(	O
BDLC	O
)	O
,	O
and	O
ARPANET	O
with	O
IMPs	B-Protocol
.	O
</s>
<s>
the	O
Zilog	B-Device
SCC	I-Device
but	O
was	O
not	O
incorporated	O
into	O
HDLC	O
is	O
SDLC	O
loop	O
mode	O
.	O
</s>
<s>
In	O
this	O
mode	O
,	O
a	O
primary	O
and	O
a	O
number	O
of	O
secondaries	O
are	O
connected	O
in	O
a	O
unidirectional	O
ring	B-Architecture
network	I-Architecture
,	O
with	O
each	O
one	O
's	O
transmit	O
output	O
connected	O
to	O
the	O
next	O
's	O
receive	O
input	O
.	O
</s>
