<s>
LAPDm	B-Protocol
in	O
telecommunications	O
is	O
a	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocol	O
used	O
in	O
GSM	O
cellular	O
networks	O
.	O
</s>
<s>
LAPDm	B-Protocol
forms	O
Layer	B-Protocol
2	I-Protocol
of	O
the	O
Um	O
interface	O
between	O
the	O
Base	B-General_Concept
Transceiver	I-General_Concept
Station	I-General_Concept
and	O
Mobile	O
station	O
,	O
which	O
is	O
to	O
say	O
that	O
it	O
is	O
used	O
in	O
the	O
radio	O
link	O
between	O
the	O
cellular	O
network	O
and	O
the	O
subscriber	O
handset	O
.	O
</s>
<s>
LAPDm	B-Protocol
is	O
derived	O
from	O
a	O
much	O
older	O
link	O
layer	O
protocol	O
called	O
HDLC	O
and	O
specified	O
in	O
3GPP	O
specifications	O
TS	O
04.05	O
and	O
TS	O
04.06	O
.	O
</s>
<s>
LAPDm	B-Protocol
is	O
similar	O
to	O
the	O
ISDN	B-Protocol
Layer	B-Protocol
2	I-Protocol
,	O
LAPD	O
,	O
but	O
with	O
these	O
simplifications	O
:	O
</s>
<s>
LAPDm	B-Protocol
frames	O
are	O
always	O
184	O
bits	O
,	O
with	O
segmentation	O
for	O
larger	O
messages	O
.	O
</s>
<s>
LAPDm	B-Protocol
allows	O
no	O
more	O
than	O
one	O
outstanding	O
unacknowledged	O
I-frame	O
(	O
GSM	O
04.06	O
Sections	O
5.8.4	O
and	O
6	O
)	O
.	O
</s>
<s>
LAPDm	B-Protocol
does	O
not	O
support	O
extended	O
header	O
formats	O
(	O
GSM	O
04.06	O
Section	O
3	O
)	O
.	O
</s>
<s>
LAPDm	B-Protocol
supports	O
only	O
the	O
SABM	O
,	O
DISC	O
,	O
DM	O
,	O
UI	O
and	O
UA	O
U-Frames	O
(	O
GSM	O
04.06	O
Sections	O
3.4	O
,	O
3.8.1	O
)	O
.	O
</s>
<s>
LAPDm	B-Protocol
supports	O
the	O
RR	O
and	O
REJ	O
S-Frames	O
(	O
GSM	O
04.06	O
3.4	O
,	O
3.8.1	O
)	O
,	O
but	O
not	O
RNR	O
(	O
GSM	O
04.06	O
Sections	O
3.8.7	O
and	O
6	O
)	O
.	O
</s>
<s>
LAPDm	B-Protocol
has	O
just	O
one	O
internal	O
timer	O
,	O
T200	O
(	O
GSM	O
04.06	O
5.8	O
)	O
.	O
</s>
<s>
LAPDm	B-Protocol
supports	O
only	O
one	O
terminal	O
endpoint	O
,	O
whose	O
TEI	O
is	O
implied	O
.	O
</s>
<s>
LAPDm	B-Protocol
can	O
never	O
be	O
in	O
a	O
receiver-not-ready	O
condition	O
(	O
GSM	O
04.06	O
Section	O
3.8.7	O
)	O
.	O
</s>
<s>
LAPDm	B-Protocol
supports	O
only	O
two	O
SAPs	B-Protocol
:	O
SAP3	O
for	O
SMS	B-Device
and	O
SAP0	O
for	O
everything	O
else	O
(	O
GSM	O
04.06	O
Sections	O
3.3.3	O
and	O
6	O
)	O
.	O
</s>
<s>
Another	O
important	O
difference	O
between	O
LAPDm	B-Protocol
and	O
LAPD	O
is	O
the	O
establishment	O
contention	O
resolution	O
procedure	O
of	O
GSM	O
04.06	O
Section	O
5.4.1.4	O
,	O
wherein	O
the	O
MS	O
sends	O
an	O
L3	O
message	O
in	O
the	O
information	O
field	O
of	O
the	O
SABM	O
frame	O
which	O
is	O
then	O
echoed	O
back	O
by	O
the	O
BTS	O
in	O
the	O
corresponding	O
UA	O
frame	O
.	O
</s>
<s>
This	O
procedure	O
is	O
required	O
in	O
LAPDm	B-Protocol
because	O
of	O
the	O
possibility	O
of	O
a	O
handset	O
accidentally	O
attempting	O
to	O
use	O
the	O
wrong	O
channel	O
.	O
</s>
<s>
LAPD	O
does	O
not	O
require	O
contention	O
resolution	O
,	O
since	O
hard-wired	O
ISDN	B-Protocol
devices	O
cannot	O
accidentally	O
use	O
a	O
wrong	O
channel	O
.	O
</s>
<s>
The	O
Associated	O
Control	O
Procedure	O
used	O
in	O
Layer	B-Protocol
2	I-Protocol
of	O
the	O
iDEN	O
RF	O
interface	O
is	O
very	O
similar	O
to	O
LAPDm	B-Protocol
.	O
</s>
