<s>
In	O
computer	B-Architecture
networking	I-Architecture
and	O
telecommunications	O
,	O
TDM	O
over	O
IP	B-Protocol
(	O
TDMoIP	B-Protocol
)	O
is	O
the	O
emulation	O
of	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
over	O
a	O
packet-switched	B-Protocol
network	I-Protocol
(	O
PSN	O
)	O
.	O
</s>
<s>
TDM	O
refers	O
to	O
a	O
T1	B-Protocol
,	O
E1	O
,	O
T3	O
or	O
E3	O
signal	O
,	O
while	O
the	O
PSN	O
is	O
based	O
either	O
on	O
IP	B-Protocol
or	O
MPLS	B-Protocol
or	O
on	O
raw	O
Ethernet	O
.	O
</s>
<s>
TDMoIP	B-Protocol
is	O
a	O
type	O
of	O
pseudowire	B-Protocol
(	O
PW	O
)	O
.	O
</s>
<s>
However	O
,	O
unlike	O
other	O
traffic	O
types	O
that	O
can	O
be	O
carried	O
over	O
pseudowires	B-Protocol
(	O
e.g.	O
</s>
<s>
ATM	O
,	O
Frame	B-Protocol
Relay	I-Protocol
and	O
Ethernet	O
)	O
,	O
TDM	O
is	O
a	O
real-time	O
bit	O
stream	O
,	O
leading	O
to	O
TDMoIP	B-Protocol
having	O
unique	O
characteristics	O
.	O
</s>
<s>
Another	O
issue	O
that	O
must	O
be	O
addressed	O
is	O
TDMoIP	B-Protocol
packet	O
loss	O
concealment	O
(	O
PLC	O
)	O
.	O
</s>
<s>
In	O
December	O
2007	O
TDMoIP	B-Protocol
was	O
approved	O
as	O
an	O
IETF	O
RFC	O
5087	O
authored	O
by	O
Dr.	O
Yaakov	O
Stein	O
,	O
Ronen	O
Shashua	O
,	O
Ron	O
Insler	O
,	O
and	O
Motti	O
Anavi	O
of	O
RAD	O
Data	O
Communications	O
.	O
</s>
<s>
Communications	O
service	O
providers	O
and	O
enterprise	O
customers	O
are	O
interested	O
in	O
deployment	O
of	O
voice	O
and	O
leased	O
line	O
services	O
over	O
efficient	O
Ethernet	O
,	O
IP	B-Protocol
and	O
MPLS	B-Protocol
infrastructures	O
.	O
</s>
<s>
While	O
Voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
is	O
maturing	O
,	O
its	O
deployment	O
requires	O
an	O
investment	O
in	O
new	O
network	B-Architecture
infrastructure	I-Architecture
and	O
customer	O
premises	O
equipment	O
(	O
CPE	O
)	O
.	O
</s>
<s>
TDMoIP	B-Protocol
presents	O
a	O
migration	O
path	O
,	O
whereby	O
modern	O
packet	B-Protocol
switched	I-Protocol
networks	I-Protocol
can	O
be	O
used	O
for	O
transport	O
,	O
while	O
the	O
end-user	O
equipment	O
need	O
not	O
be	O
immediately	O
replaced	O
.	O
</s>
<s>
TDMoIP	B-Protocol
was	O
first	O
developed	O
in	O
1998	O
by	O
RAD	O
Data	O
Communications	O
(	O
see	O
U.S.	O
patent	O
number	O
6,731,649	O
)	O
and	O
first	O
deployed	O
in	O
Sweden	O
in	O
1999	O
by	O
Utfors	O
(	O
later	O
acquired	O
by	O
Telenor	O
)	O
.	O
</s>
<s>
Utfors	O
employed	O
the	O
first	O
generation	O
TDMoIP	B-Protocol
product	O
(	O
known	O
as	O
IPmux-4	O
)	O
to	O
provide	O
bundled	O
services	O
including	O
TDM	O
private	O
lines	O
,	O
TDM	O
leased	O
lines	O
and	O
a	O
variety	O
of	O
IP	B-Protocol
and	O
Ethernet	O
services	O
.	O
</s>
<s>
In	O
2001	O
,	O
the	O
IETF	O
set	O
up	O
the	O
PWE3	O
working	O
group	O
,	O
which	O
was	O
chartered	O
to	O
develop	O
an	O
architecture	O
for	O
edge-to-edge	O
pseudowires	B-Protocol
,	O
and	O
to	O
produce	O
specifications	O
for	O
various	O
services	O
,	O
including	O
TDM	O
.	O
</s>
<s>
Other	O
standardization	O
forums	O
,	O
including	O
the	O
ITU	O
and	O
the	O
MPLS	B-Protocol
-	O
Frame	B-Protocol
Relay	I-Protocol
Alliance	O
,	O
are	O
also	O
active	O
in	O
producing	O
standards	O
and	O
implementation	O
agreements	O
for	O
pseudowires	B-Protocol
.	O
</s>
<s>
Although	O
TDM	O
can	O
be	O
used	O
to	O
carry	O
arbitrary	O
bit	O
streams	O
at	O
the	O
rates	O
defined	O
in	O
G.702	O
,	O
there	O
are	O
standardized	O
methods	O
of	O
carrying	O
bit	O
streams	O
in	O
larger	O
units	O
each	O
containing	O
the	O
same	O
number	O
of	O
bits	O
,	O
called	O
frames	B-Protocol
.	O
</s>
<s>
TDM	O
framing	O
locks	O
the	O
frame	O
rate	O
to	O
the	O
sampling	O
frequency	O
of	O
voice	O
traffic	O
,	O
so	O
that	O
there	O
are	O
always	O
8000	O
frames	B-Protocol
per	O
second	O
;	O
a	O
T1	B-Protocol
frame	O
consists	O
of	O
193	O
bits	O
and	O
an	O
E1	O
frame	O
of	O
256	O
bits	O
.	O
</s>
<s>
1	O
bit	O
per	O
T1	B-Protocol
frame	O
,	O
8	O
bits	O
per	O
E1	O
frame	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
for	O
E1	O
trunks	O
the	O
CAS	O
signaling	O
bits	O
are	O
updated	O
once	O
per	O
multiframe	O
of	O
16	O
frames	B-Protocol
(	O
every	O
2	O
milliseconds	O
)	O
while	O
for	O
T1	B-Protocol
ESF	O
trunks	O
the	O
superframe	O
is	O
24	O
frames	B-Protocol
(	O
3	O
milliseconds	O
)	O
.	O
</s>
<s>
TDMoIP	B-Protocol
transport	O
is	O
denoted	O
"	O
structured-agnostic	O
"	O
when	O
the	O
TDM	O
is	O
unframed	O
,	O
or	O
when	O
it	O
is	O
framed	O
or	O
even	O
channelized	O
,	O
but	O
the	O
framing	O
and	O
channelization	O
structure	O
are	O
completely	O
disregarded	O
by	O
the	O
transport	O
mechanisms	O
.	O
</s>
<s>
TDMoIP	B-Protocol
operates	O
by	O
segmenting	O
,	O
adapting	O
and	O
encapsulating	O
the	O
TDM	O
traffic	O
at	O
PSN	O
ingress	O
and	O
performing	O
the	O
inverse	O
operations	O
at	O
PSN	O
egress	O
.	O
</s>
<s>
For	O
the	O
MPLS	B-Protocol
case	O
,	O
ITU-T	O
Recommendation	O
Y.1413	O
contains	O
a	O
complete	O
description	O
of	O
the	O
packet	O
format	O
.	O
</s>
<s>
In	O
all	O
cases	O
a	O
TDMoIP	B-Protocol
packet	O
commences	O
with	O
PSN	O
headers	O
.	O
</s>
<s>
the	O
20-byte	O
header	O
of	O
UDP/IP	O
,	O
or	O
the	O
label-stack	O
of	O
MPLS	B-Protocol
.	O
</s>
<s>
After	O
these	O
headers	O
come	O
the	O
"	O
PW	O
label	O
"	O
,	O
a	O
four-byte	O
MPLS-like	O
label	O
that	O
serves	O
as	O
to	O
demultiplex	O
different	O
TDM	O
PWs	O
.	O
</s>
<s>
After	O
the	O
PSN	O
header	O
comes	O
the	O
four-byte	O
TDMoIP	B-Protocol
"	O
control	O
word	O
"	O
.	O
</s>
<s>
After	O
the	O
control	O
word	O
comes	O
the	O
TDMoIP	B-Protocol
payload	O
.	O
</s>
<s>
For	O
structure-agnostic	O
transport	O
(	O
SAToP	O
)	O
this	O
is	O
simply	O
a	O
predetermined	O
number	O
of	O
TDM	O
octets	O
,	O
while	O
for	O
the	O
structure-locked	O
format	O
the	O
payload	O
is	O
an	O
integer	O
number	O
of	O
TDM	O
frames	B-Protocol
.	O
</s>
<s>
For	O
structure-indication	O
and	O
structure-reassembly	O
TDMoIP	B-Protocol
draws	O
on	O
proven	O
adaptation	O
mechanisms	O
originally	O
developed	O
for	O
ATM	O
.	O
</s>
<s>
For	O
statically	O
allocated	O
,	O
constant	O
bit-rate	O
(	O
CBR	O
)	O
TDM	O
links	O
,	O
TDMoIP	B-Protocol
employs	O
ATM	O
adaptation	O
layer	O
1	O
(	O
AAL1	O
)	O
.	O
</s>
<s>
TDMoIP	B-Protocol
allows	O
concatenation	O
of	O
any	O
number	O
of	O
AAL1	O
cells	O
into	O
a	O
packet	O
(	O
note	O
that	O
these	O
are	O
AAL1	O
cells	O
and	O
not	O
ATM	O
cells	O
,	O
i.e.	O
</s>
<s>
By	O
allowing	O
multiple	O
cells	O
per	O
packet	O
,	O
TDMoIP	B-Protocol
facilitates	O
flexible	O
tradeoffs	O
of	O
buffering	O
delay	O
(	O
which	O
decreases	O
with	O
fewer	O
cells	O
per	O
packet	O
)	O
for	O
bandwidth	O
efficiency	O
(	O
which	O
increases	O
with	O
more	O
cells	O
per	O
packet	O
,	O
due	O
to	O
the	O
per	O
packet	O
overhead	O
)	O
.	O
</s>
<s>
For	O
dynamically	O
allocated	O
TDM	O
links	O
,	O
whether	O
the	O
information	O
rate	O
varies	O
due	O
to	O
activation	O
of	O
time	O
slots	O
or	O
due	O
to	O
voice	O
activity	O
detection	O
,	O
TDMoIP	B-Protocol
employs	O
ATM	O
adaptation	O
layer	O
2	O
(	O
AAL2	O
)	O
.	O
</s>
<s>
TDMoIP	B-Protocol
concatenates	O
the	O
minicells	O
from	O
all	O
active	O
time	O
slots	O
into	O
a	O
single	O
packet	O
.	O
</s>
<s>
For	O
time	O
slots	O
carrying	O
high-level	O
data	O
link	O
control	O
(	O
HDLC	O
)	O
data	O
,	O
such	O
as	O
data	O
for	O
common	O
channel	O
signaling	O
(	O
CCS	O
)	O
,	O
TDMoIP	B-Protocol
has	O
a	O
special	O
adaptation	O
that	O
encapsulates	O
stretches	O
of	O
non-idle	O
data	O
.	O
</s>
<s>
By	O
contrast	O
,	O
IP-based	O
systems	O
typically	O
add	O
various	O
forms	O
of	O
delay	O
,	O
one	O
of	O
which	O
is	O
based	O
on	O
the	O
time	O
it	O
takes	O
to	O
form	O
packets	O
(	O
packetization	O
delay	O
)	O
,	O
which	O
is	O
proportional	O
to	O
the	O
packet	O
size	O
divided	O
by	O
the	O
data	O
rate	O
.	O
</s>
<s>
The	O
other	O
form	O
of	O
delay	O
introduced	O
by	O
IP	B-Protocol
systems	O
is	O
the	O
playout	O
delay	O
,	O
which	O
needs	O
to	O
be	O
added	O
at	O
the	O
recipient	O
to	O
buffer	O
packet	O
delay	O
variation	O
and	O
ensure	O
a	O
smooth	O
playout	O
.	O
</s>
<s>
VoIP	B-Application
systems	O
that	O
try	O
to	O
be	O
very	O
bandwidth	O
efficient	O
may	O
also	O
add	O
tens	O
of	O
milliseconds	O
of	O
algorithmic	O
delay	O
in	O
the	O
voice	O
codec	O
.	O
</s>
<s>
In	O
contrast	O
,	O
TDMoIP	B-Protocol
maps	O
TDM	O
octets	O
directly	O
into	O
the	O
payload	O
with	O
no	O
voice	O
compression	O
algorithms	O
and	O
no	O
resultant	O
algorithmic	O
delay	O
.	O
</s>
<s>
The	O
packetization	O
latency	O
added	O
by	O
TDMoIP	B-Protocol
depends	O
on	O
the	O
number	O
of	O
cells	O
per	O
packet	O
but	O
is	O
typically	O
in	O
the	O
single	O
millisecond	O
range	O
due	O
to	O
the	O
higher	O
data	O
rate	O
of	O
a	O
complete	O
multiplex	O
as	O
compared	O
to	O
a	O
single	O
VoIP	B-Application
flow	O
.	O
</s>
<s>
Playout	O
delay	O
considerations	O
do	O
not	O
differ	O
materially	O
between	O
TDMoIP	B-Protocol
and	O
VoIP	B-Application
,	O
however	O
,	O
so	O
both	O
work	O
best	O
on	O
paths	O
with	O
controlled	O
packet	O
delay	O
variation	O
(	O
strong	O
overprovisioning	O
or	O
"	O
QoS	O
"	O
)	O
.	O
</s>
<s>
The	O
problem	O
arises	O
when	O
at	O
most	O
one	O
side	O
of	O
the	O
TDMoIP	B-Protocol
tunnel	O
has	O
a	O
highly	O
accurate	O
time	O
standard	O
.	O
</s>
<s>
For	O
ATM	O
networks	O
,	O
which	O
define	O
a	O
physical	O
layer	O
that	O
carries	O
timing	O
,	O
the	O
synchronous	O
residual	O
time	O
stamp	O
(	O
SRTS	O
)	O
method	O
may	O
be	O
used	O
;	O
IP/MPLS	B-Protocol
networks	O
,	O
however	O
,	O
do	O
not	O
define	O
the	O
physical	O
layer	O
and	O
thus	O
cannot	O
specify	O
the	O
accuracy	O
of	O
its	O
clock	O
.	O
</s>
<s>
Hence	O
,	O
in	O
many	O
cases	O
the	O
only	O
alternative	O
is	O
to	O
attempt	O
to	O
recover	O
the	O
clock	O
based	O
exclusively	O
on	O
the	O
TDMoIP	B-Protocol
traffic	O
,	O
a	O
technology	O
known	O
as	O
"	O
adaptive	O
clock	O
recovery	O
"	O
.	O
</s>
<s>
The	O
TDMoIP	B-Protocol
control	O
word	O
described	O
above	O
includes	O
a	O
16-bit	O
sequence	O
number	O
for	O
detecting	O
and	O
handling	O
lost	O
and	O
mis-ordered	O
packets	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
lost	O
packets	O
,	O
TDMoIP	B-Protocol
requires	O
insertion	O
of	O
interpolation	B-Algorithm
packets	O
to	O
maintain	O
TDM	O
timing	O
.	O
</s>
<s>
The	O
precise	O
effect	O
of	O
packet	O
loss	O
on	O
voice	O
quality	O
and	O
the	O
development	O
of	O
packet	O
loss	O
concealment	O
algorithms	O
have	O
been	O
the	O
subject	O
of	O
detailed	O
study	O
in	O
the	O
VoIP	B-Application
community	O
,	O
but	O
their	O
results	O
are	O
not	O
directly	O
applicable	O
to	O
the	O
TDMoIP	B-Protocol
case	O
.	O
</s>
<s>
This	O
is	O
because	O
VoIP	B-Application
packets	O
typically	O
contain	O
between	O
80	O
samples	O
(	O
10	O
ms	O
)	O
and	O
240	O
samples	O
(	O
30	O
ms	O
)	O
of	O
the	O
speech	O
signal	O
,	O
while	O
TDMoIP	B-Protocol
packets	O
may	O
contain	O
only	O
a	O
small	O
number	O
of	O
samples	O
.	O
</s>
<s>
Since	O
TDMoIP	B-Protocol
packets	O
are	O
so	O
small	O
,	O
it	O
is	O
acceptable	O
to	O
simply	O
insert	O
a	O
constant	O
value	O
in	O
place	O
of	O
any	O
lost	O
speech	O
samples	O
.	O
</s>
