<s>
Link	B-Protocol
Access	I-Protocol
Procedure	I-Protocol
for	I-Protocol
Modems	I-Protocol
(	O
LAPM	B-Protocol
)	O
is	O
part	O
of	O
the	O
V.42	O
error	B-Error_Name
correction	I-Error_Name
protocol	O
for	O
modems	B-Application
.	O
</s>
<s>
LAPM	B-Protocol
is	O
an	O
error	B-Error_Name
control	I-Error_Name
protocol	O
defined	O
in	O
ITU-T	O
recommendations	O
V.42	O
.	O
</s>
<s>
Like	O
many	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocols	O
,	O
it	O
is	O
a	O
variant	O
of	O
HDLC	O
.	O
</s>
<s>
Like	O
the	O
Microcom	B-Device
Networking	I-Device
Protocols	I-Device
that	O
preceded	O
it	O
,	O
LAPM	B-Protocol
uses	O
cyclic	O
redundancy	O
checking	O
(	O
CRC	O
)	O
and	O
retransmission	O
of	O
corrupted	O
data	O
(	O
ARQ	B-Error_Name
)	O
to	O
ensure	O
data	O
reliability	O
.	O
</s>
<s>
Error	B-Error_Name
control	I-Error_Name
protocols	O
such	O
as	O
LAPM	B-Protocol
use	O
frames	O
of	O
variable	O
lengths	O
,	O
each	O
beginning	O
with	O
a	O
header	O
and	O
ending	O
with	O
a	O
frame	B-Protocol
check	I-Protocol
sequence	I-Protocol
trailer	O
(	O
a	O
cyclic	O
redundancy	O
check	O
)	O
.	O
</s>
<s>
Correctly	O
received	O
packets	O
are	O
acknowledged	O
,	O
and	O
unacknowledged	O
packets	O
are	O
automatically	O
retransmitted	O
(	O
ARQ	B-Error_Name
)	O
.	O
</s>
<s>
Usually	O
LAPM	B-Protocol
adds	O
only	O
about	O
5	O
percent	O
framing	O
overhead	O
.	O
</s>
<s>
LAPM	B-Protocol
has	O
an	O
optional	O
Selective	O
reject	O
(	O
SREJ	O
)	O
functionality	O
which	O
allows	O
it	O
to	O
resend	O
only	O
the	O
corrupted	O
frames	O
,	O
providing	O
faster	O
recovery	O
from	O
an	O
error	O
.	O
</s>
<s>
The	O
ITU-T	O
V.42	O
LAPM	B-Protocol
procedure	O
is	O
considered	O
more	O
robust	O
than	O
the	O
Microcom	B-Device
Networking	I-Device
Protocols	I-Device
(	O
MNP	O
)	O
that	O
preceded	O
it	O
,	O
and	O
has	O
a	O
more	O
sophisticated	O
and	O
powerful	O
data	O
compression	O
option	O
in	O
V.42bis	O
,	O
allowing	O
much	O
greater	O
data	O
throughput	O
.	O
</s>
<s>
V.42bis	O
added	O
a	O
string	O
compression	O
algorithm	O
called	O
BTLZ	O
(	O
British	O
Telecom	O
Lempel	B-Algorithm
Ziv	I-Algorithm
)	O
,	O
in	O
which	O
frequently	O
occurring	O
strings	O
of	O
characters	O
are	O
replaced	O
by	O
code	O
words	O
.	O
</s>
<s>
As	O
the	O
data	O
may	O
not	O
always	O
be	O
compressible	O
(	O
for	O
example	O
encrypted	O
or	O
pre-compressed	O
data	O
)	O
the	O
modem	B-Application
has	O
the	O
ability	O
to	O
switch	O
between	O
compressed	O
and	O
uncompressed	O
modes	O
of	O
operation	O
.	O
</s>
<s>
Compression	O
performance	O
is	O
continually	O
monitored	O
and	O
,	O
if	O
no	O
compression	O
is	O
obtained	O
,	O
the	O
modem	B-Application
transmits	O
the	O
data	O
in	O
uncompressed	O
form	O
.	O
</s>
<s>
Even	O
while	O
in	O
uncompressed	O
mode	O
,	O
the	O
modem	B-Application
monitors	O
the	O
characteristics	O
of	O
the	O
data	O
and	O
switches	O
compression	O
back	O
on	O
as	O
soon	O
as	O
some	O
benefit	O
can	O
be	O
achieved	O
.	O
</s>
