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13
Synchronous Digital Hierarchy
(SDH) and Synchronous
Optical Network (SONET)
The synchronous digital hierarchy (SDH) is emerging as the universal technology for
transmission in telecommunications networks. Since the first publication
of
international
standards by ITU-T in
1989,
SDH equipment has been rapidly developed and deployed across the
world, and is rapidly taking over from its predecessor, the Plesiochronous Digital Hierarchy
(PDH). This chapter describes SDH and the North American equivalent of SDH, SONET
(Synchronous Optical Network), from which it grew. In particular, the chapter describes the
features of SDH which characterize its advantages over PDH.
13.1
HISTORY
OF
THE SYNCHRONOUS DIGITAL
HIERARCHY (SDH)
The
synchronous digital hierarchy
(SDH)
was developed from its North American
forerunner
SONET
(synchronous optical network).
SDH is the most modern type of
transmission technology, and as its name suggests it is based on a synchronous multi-
plexing technology. The fact that SDH is synchronous adds greatly to the efficiency
of

the transmission network, and makes the network much easier to manage.
13.2 THE PROBLEMS
OF
PDH TRANSMISSION
Historically, digital telephone networks, modern data networks and the transmission
infrastructures serving them have been based
on
a technology called
PDH
(Plesio-
chronous Digital Hierarchy)
as we discussed in Chapter 5. As we also discussed, three
distinct PDH hierarchies evolved, as we summarize in Figure
13.1.
They share three
common attributes.
267
Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
268 SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK
(a)
Europe
64 2048 a448 34 368 139 264 564 992
kbit/s
(El
1
(E21 (E31
=

X4
X4
=
X4
-
X4
-
X32
-
MUX
-
MUX
-
MUX
-
MUX
-
MUX
b
L
hierarchy level
0
1
2
3
4
5
(b)
North
America

64 1544 6312 44 736 274 176
kbiffs
(DSO) (DS1
or
T1) (DS2
or
T2) (DS3
or
T3)
.
.
1
-
3x241 3x4
I
4x7 HX6
I
,
MUX MUX MUX MUX
U
U
U U
hierarchy level
0
1
2
3
4
(c)
Japan

64 1544 631 2 32 064 97 728
kbiffs
X 24
X4
X5
X3
MUX
MUX
MUX
MUX
b
hierarchy level
0
1
2
3
4
Figure
13.1
The various plesiochronous multiplexing hierarchies
(ITU-T/G.571)
They are all based on the needs of telephone networks, i.e. offering integral multiples
of
64
kbit/s channels, synchronized to some extent at the first multiplexing level
(1.5
Mbit/s or
2
Mbit/s).
They require multiple multiplexing stages to reach the higher bitrates, and are

therefore difficult to manage and to measure and monitor performance, and relatively
expensive to operate.
They are basically incompatible with one another.
Each individual transmission line within a
PDH
network runs
plesiochronously.
This
means that it runs on a clock speed which is nominally identical to all the other line
systems in the same operator’s network but is not locked
synchronously
in step (it is
free-
running
as we discussed in Chapter
5).
This results in certain practical problems. Over a
relatively long period of time (say one day) one line system may deliver two or three bits
more or less than another. If the system running slightly faster is delivering bits for the
second (slightly slower) system then a problem arises with the accumulating extra bits.
Eventually, the number of accumulated bits becomes too great for the storage avail-
able for them, and some must be thrown away. The occurrence is termed
slip.
To keep
this problem in hand,
framing
and
stufJing
(or
justlJication)

bits are added within the
THE PROBLEMS
OF
PDH TRANSMISSION
269
normal multiplexing process, and are used to compensate. These bits help the two end
systems to communicate with one another, speeding up or slowing down as necessary
to keep better in step with one another. The extra framing bits account for the differ-
ence, for example, between 4
X
2048(E1 bitrate)
=
8192 kbit/s and the actual E2 bitrate
(8448 kbit/s, see Figure 13.1).
Extra framing bits are added at each stage of the PDH multiplexing process.
Unfortunately this means that the efficiency of the higher order line systems (e.g.
139264 kbit/s, usually termed 140Mbit/s systems) are relatively low (91%). More
critically still, the framing bits added at each stage make it very difficult to break out a
single 2 Mbit/s
tributary
from a 140 Mbit/s line system without complete demultiplexing
(Figure 13.2). This makes PDH networks expensive, rather inflexible and difficult to
manage.
SDH, in contrast with PDH, requires the synchronization of all the links within
a network. It uses a multiplexing technique which has been specifically designed to
allow for the
drop and insert
of
the individual
tributaries

within a high speed bit rate.
Thus, for example, a single
drop and insert multiplexor
is required to break out a single
2 Mbit/s
tributary
from an
STM-I
(synchronous transport module)
of
155
520
kbit/s
(Figure 13.3).
Other major problems of the PDH are the lack of tools for network performance
management and measurement
now
expected by most public and corporate network
managers, the relatively poor availability and range of high speed bit rates and the
inflexibility
of
options for line system back-up (Figure 13.4).
A
B
C
3xE3
Note
1
=2
3x E2

Note
2
4xEl
IX
El
B-C
t )
-
El El
t
W
63~ El
A-to-C
Note
1
:
3
X
E3
=
12
X
E2
or
48
X
El
after demultiplexing
Note
2: 3

X
E2
=
12
X
El
after demultiplexing
a=
3x E3
3x E2
4xEl
Figure
13.2
Breaking-out 2Mbit/s
(El)
from a 140Mbit/s line system at an intermediate
exchange
270
SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK
A
B
C
STM-l
STM-1
drop and
Insert
multiplexor
-
-
El

El
62
X
El
Figure
13.3
Drop and insert multiplexor used to break-out
2
Mbit/s
(El)
from a
155
Mbit/s
(STM-1) line at an intermediate exchange
m
standby
Figure
13.4 Optical fibre back-up using PDH system components
Before
SDH,
networks had to be built up from separate multiplex and
line terminating
equipment
(LTE),
as the optical equipment interfaces in particular were manufacturer-
specific (i.e. proprietary). Back-up tended to be on a
I
main
+
I

standby
protection basis,
making back-up schemes costly (Figure
13.4)
and difficult to manage. These problems
have been eliminated in the design of
SDH
through in-built flexibility of the bitrate
hierarchy, integration of the optical units into the multiplexors, ring structure topologies
and in-built performance management and diagnostic functions.
13.3
THE MULTIPLEXING STRUCTURE
OF
SDH
As
is shown in Figure
13.5,
the
containers
(i.e. available bitrates)
of
the
synchronous
digital hierarchy
have been designed to correspond
to
the bit rates of the various
PDH
hierarchies. These
containers

are multiplexed together by means of
virtual containers
(abbreviated to VCs but are not to be confused with
virtual channels
which are also
THE MULTIPLEXING STRUCTURE
OF
SDH
271
XN
X1
pointer processing
c-
multiplexing
Figure
13.5
Synchronous digital hierarchy
(SDH)
multiplexing structure
(ITU-T/G.709)
so
abbreviated),
tributary units (TU), tributary unit groups (TUG), administrative
units (AU)
and finally
administrative unit groups (AUG)
into
synchronous transport
modules (STM).
The basic building block of the SDH hierarchy is the

administrative unit group
(AUG).
An
AUG comprises one AU-4 or three AU-3s. The AU-4 is the simplest form
of AUG, and for this reason we use it to explain the various terminology of SDH
(con-
tainers, virtual containers, mapping, aligning, tributary units, multiplexing, tributary unit
groups).
The
container
comprises sufficient bits to carry a full
frame
(i.e. one cycle) of user
information of
a
given bitrate. In the case of
container
4
(C-4
)
this is a field of 260
X
9
bytes
(i.e. 18 720 bits). In common with PDH, the
frame repetition rate
(i.e. number of cycles per
second) is 8000Hz. Thus a
C4-container
can carry a maximum user throughput rate

(information payload)
of 149.76 Mbit/s (18 720
X
8000).
This can either be used as a raw
bandwidth or, say, could be used to transport a PDH link of 139.264 Mbit/s.
To
the container is added a
path overhead
(POH)
of 9 bytes (72 bits). This makes
a
virtual container
(VC).
The process
of
adding the POH is called
mapping.
The
POH
information is communicated between the point of
assembly
(i.e. entry to the
SDH
network) and the point of disassembly.
It
enables the management of the SDH system
and the monitoring
of
its performance.

The
virtual container
is
aligned
within an
administrative unit (AU)
(this is the key to
synchronization). Any spare bits within the AU are filled with a defined filler pattern
called
fixed stuf.
In addition, a
pointer
field of
9
bytes (72 bits) is added. The
pointers
(3 bytes for each VC, up to three VCs in total (9 bytes maximum)) indicate the exact
position of the virtual container(s) within the AU frame. Thus in our example case, the
AU-4 contains one 3 byte pointer indicating the position of the VC-4. The remaining
6 bytes of pointers are filled with an idle pattern. One AU-4 (or three AU-3s containing
three pointers for the three VC-3s) are
multiplexed
to form an AUG.
To a single AUG is added 9
X
8
bytes (576 bits)
of
section overhead(S0H).
This makes

a single
STM-1
frame (of 19 440 bits). The SOH is added to provide for
block framing
and
for the maintenance and performance information carried on a transmission line
section
272
SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK
basis. (A
section
is an administratively defined point-to-point connection in the network,
typically an SDH-system between two major exchange sites, between two intermediate
multiplexors or simply between two regenerators). The
SOH
is split into
3
bytes of RSOH
(regenerator section overhead)
and
5
bytes of
MSOH
(multiplex section overhead).
The
RSOH
is carried between, and interpreted by,
SDH
line system
regenerators

(devices
appearing in the line to
regenerate
laser light or other signal, thereby avoiding signal
degeneration). The
MSOH
is carried between, and interpreted by the devices assembling
and disassembling the AUGs. The
MOH ensures integrity of the
AUG.
As the
frame repetition rate
of
an
STM-1
frame is
8000Hz,
the total line speed is
155.52 Mbit/s
(19
440
X
8000).
Alternatively, power of four
(1,
4,
16,
etc.) multiples of
AUGs may be multiplexed together with a proportionately increased section overhead,
AUG frame (in this case one AU-4)

1
bvte 260 bytes
.
1-
Pot-
C-4
container
AUG frame (in this case one AU-4)
9
bytes 261 bytes
4
b
row
41
X
(X)
(X;
VC-4 virtual container
pointers (up to
~
3;
here only
one is used)
9
bytes
STM-1 frame
261 bytes
RSOH
MSOH
row

4
AUG (administrative unit group)
Figure
13.6
Basic
structure
of
an
STM-I
frame
THE
TRIBUTARIES
OF
SDH
273
to make larger STM frames. Thus an STM-4 frame (4AUGs) has a frame size of
77760 bits, and a line rate of 622.08 Mbit/s. An STM-16 frame (16AUGs) has a frame
size of 31
l
040 bits, and a line rate of 2488.32 Mbit/s.
Tributary unit
groups
(TUGS)
and
tributary units (TUs)
provide for further break-
down of the VC-4 or VC-3 payload into lower speed tributaries, suitable for carriage of
today’s
T1,
T3, El or E3 line rates (1.544Mbit/s, 44.736 Mbit/s, 2.048 Mbit/s or

34.368 Mbit/s).
Figure 13.6 shows the gradual build up of a C-4 container into an STM-1 frame. The
diagram conforms with the conventional diagrammatic representation of the STM-1
frame as a matrix of 270 columns by 9 rows of bytes. The transmission of bytes, as
defined by ITU-T standards
is
starting at the top left hand corner, working along each
row from left to right in turn, from top to bottom row. The structure is defined in
ITU-T recommendations G.707. G.708 and G.709.
13.4
THE
TRIBUTARIES
OF
SDH
The structure
of
an AUG comprising
3
AU-3s is similar to that for an AUG of one
AU-4, except that the area used in Figure 13.6 for VC-4 is instead broken into 3 separate
areas of 87 columns, each area carrying one VC-3 (Figure 13.7). In this case all three
pointers are required to indicate the start positions within the frame of the three separate
VCs. The various other TU and VC formats follow similar patterns to the AUs and VCs
presented (TUs also include pointers like AUs). Table 13.1 presents the various
-c
row
41 1 2 3 AU-3 (1
)
AU-3 (2)
AU-3 (3)

/
pointers
1 87 8% 174 175 261
AU-3
=
87
columns
X
9
rows
of
bytes
Figure
13.7
AUG frame arranged as 3
X
AU-3
Table
13.1
Payload rates
of
SDH
containers
Container Container Frame Capable of carrying
tY
Pe frame size repetition rate
PDH
line type
c-l
1

193 bits
8000 T1 (1 544 kbit/s)
c-l2 256 bits
8000
El (2048 kbit/s)
c-2
1
789 bits
8000
T2 (63 12 kbit/s)
C-22 1056 bits
8000
E2 (8448 kbit/s)
C-3
1
4296 bits
8000
E3 (34 368 kbit/s)
C-32 5592 bits
8000 T3 (44 736 kbit/s)
c-4 260
X
9 bytes
8000
139 264 kbit/s
274
SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK
POH

1

2 3.45
67s
il'.:: ;
,.,.
~~~


,:'261







,






r.1
1:.
86
.,,.
.',:;
,!
B
86

~. ">'

1
'
:


''Cl
TUG-3 TUG-3
86
columns
X
TUG-3
9
rows
of
bytes
86'
Figure
13.8
VC-4
submultiplexing scheme as 3
X
TUG-3 using byte interleaving
container rates available within
SDH.
Note that the terminology C-l2 is intended to
signify the hierachical structure and should not therefore be called C-twelve, but instead
C-one-two. The relevant VC is VC-one-two, etc.
Figure

13.8
shows an alternative demultiplexing scheme, based upon the sub-
multiplexing of a VC-4 container into three
tributary unit group3
(TUG-3s).
In this
case, the first three columns are used as path overhead, and each
TUG
occupies a total
TU-l 1
123
TU-l
2
1234
1



N.,

TU-2
l23456789101112
1
2
3
4
5
6 7 8 9

16

23

30

37

44

51

58

65

72

79

86

Figure
13.9
TUG-3
submultiplexing into 7
X
TUG-2; TUG-2 submultiplexing
THE
TRIBUTARIES
OF
SDH
275
of
86
columns, but the individual TUGS are
byte interleaved.
This sub-multiplexing
scheme lends itself better to the carriage of
PDH
signals.
The sub-multiplexing of the TUG-3s themselves may be continued as shown in
Figure 13.9, where each TUG-3 is sub-multiplexed into
7
X
TUG-2, also using
byte
interleaving.
Finally, as Figure 13.9 also shows, the TUG-2s may be subdivided into
byte interleaved
TU-l
l tributaries (for T1 rate of 1.544Mbit/s) or TU-l2 tributaries

(for El rate of 2.048 Mbit/s).
The individual
containers
(C-l
1
or
C-12) may be packed into the TU-l 1 (synonymous
with VC-l 1) or TU-l2 (synonymous with VC-12) in one of three manners, using either
a
no framing (i.e.
asynchronously)
a
bit synchronous framing
a
byte synchronous framing
VC-3
/
VC-4
POH
(9
rows
X
1
byte!8
bitsn
Figure
13.10
Path overhead
(POH)
formats

for
VC-l, VC-2, VC-3
and
VC-4
276 SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK
The
asynchronous
and
bit synchronous
framing methods allow a certain number of bits
for
justzjication.
This enables 1.5 Mbit/s or 2 Mbit/s tributaries
of
an SDH transmission
network to operate in conjunction with PDH or other networks running on separate
clocks (i.e. not running synchronously with the SDH network; we covered the subject of
justiJication
in Chapter
5).
Byte synchronous framing,
in contrast, demands common
clocking. The advantage is the ability to directly access 64 kbit/s subchannels within the
1.5
Mbit/s or 2Mbit/s tributary using
drop and insert
methods (Figure 13.3). In addi-
tion,
byte synchronous
streams are simpler for the equipment to process.

13.5
PATH OVERHEAD
Figure
13.10
illustrates the
path
overhead
(POH)
formats used for creating VC-l, VC-2,
VC-3 and VC-4 containers. This information is added to the corresponding
container.
The meanings and functions
of
the various bits and fields are given in Table 13.2.
13.6
SECTION
OVERHEAD (SOH)
The diagram and table of Figure 13.1 1 illustrate the constitution
of
the section
overhead.
Table
13.2 Meaning and function of the fields in the
SDH
path overhead
(POH)
Field
Name
Function
BIP-2

FEBE
L1,
L2,
L3
remote alarm
J1
B3
c2
G1
F2
H4
23,
24,
z5
bit inserted parity
far end block error
signal label
remote alarm
path trace
BIP-8
parity code
signal label
path status
path user channel
multiframe indicator
bytes reserved for national
network operator use
error check function
indication of received BIP error
indication of VC payload type

indication of receiving failure to transmitting
end
verification of VC-n connection
error check function
indication of VC payload type and composition
indication of received signal status to
transmitting end
provides communication channel for network
operating staff
multiframe indication
reserved
NETWORK
TOPOLOGY
OF
SDH
NETWORKS
277
*
Row
4
is
used
for
the
AUG
.frame
pointers
Field Function
AI, A2 framing
Bl,

B2
parity check for error detection
c1
identifies STM-1
in
STM-n
frame
Dl-D12 data communications channel (DCC
-
for network management use)
El, E2 orderwire channels (voice channels for technicians)
F1
user
channel
K1,
K2 automatic protection switchins (APS) channel
z1,z2 reserved
Figure
13.11
The
SDH
section overhead
(SOH)
13.7
NETWORK TOPOLOGY
OF
SDH
NETWORKS
SDH equipment is designed to be used in the construction of synchronous (in par-
ticular, optical fibre) transmission networks in redundant ring topologies.

A
number of
specific equipment types are foreseen by the standards as the building blocks
of
such
networks. These are illustrated in Figure 12.12.
SDH multiplexors allow
2
Mbit/s and other sub STM-1 rate tributaries to be
multiplexed for carriage by an SDH network.
Drop and insert multiplexors
(also called
278 SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL
NETWORK
ADM=add/drop multiplexor
(drop and inserl multiplexor)
STM-4
MUX=multiplexor
Ring
DXGdigital crossconnect
Figure
13.12 Ring
topology and
generic equipment types used in
SDH
networks
addldrop
multiplexors
(ADM))
allow tributaries to be removed from the line at an

intermediate station without complete demultiplexing (as we discussed in Figures 13.2
and 13.3).
Crossconnectors
(or
DXC,
digital crossconnectors)
allow for the flexible
interconnection and reconfiguration
of
tributaries between separate sub-networks or
rings. STM-4 and STM-16 multiplexors allow concentration of STM-1 signals onto
high speed 622 Mbit/s (STM-4) or
2.5
Gbit/s (STM-16) backbone networks.
Structuring of the network in interconnected rings allows for easy back-up
(restora-
tion)
of failed connections in the network. In a highly meshed network
n
:
1 (as opposed
to
1
:
1)
restoration is possible by choosing any alternative route to the destination.
In simpler networks and single rings a 1
:
1
restoration may be possible, but leaves at

least
50%
of
the capacity unused for most
of
the time.
N:
1 restoration is useful in
reducing the amount of normally unused plant (and thus costs) in cases where failures
are rare (see Chapter 37).
13.8 OPTICAL INTERFACES FOR
SDH
ITU-T recommendation G.957 covers the optical interfaces defined for use with SDH,
according to the light wavelength to be used and the application.
A
number of SDH
system types are defined (Table 13.3).
13.9 MANAGEMENT
OF
SDH
NETWORKS
Compared with PDH networks, SDH networks are more efficient and easier to
administrate (due to the availability
of
drop
and insert
(addldrop)
multiplexors). Using
SONET (SYNCHRONOUS OPTICAL NETWORKS) 279
Table

13.3
Classification
of
optical fibre interfaces
for
SDH
equipment
Application
Inter-office
Intra-
office Shorthaul Longhaul
Wavelength
nominal/nm
1310
1310
1550 1310 1550
Fibre
type
G.652 G.652 (3.652 G.652 G.652 G.653
(3.654
Distance
kilometres
<2
15
40
40
60
STM
level
STM-

1
I-
1
S-1.1
S-1.2
L-1.1
L-1.2 L-1.3
STM-4
1-4
S-4.1
S-4.2 L-4.1 L-4.2 L-4.3
STM-16 1-16 S-16.1
S-16.2
L-16.1
L-16.2 L-16.3
a C-4 container at its full capacity (i.e. 149.76Mbit/s) we achieve a system efficiency
using SDH of 96% (cf. 91% with PDH). However, apart from these benefits there is
one other significant advantage: SDH networks are much easier to manage in
operation. Partly this is due to the fact that SDH was conceived as a technology for a
whole network (rather than a set
of
individual links); partly this is due
to
the fact that
SDH is simply more modern, and therefore the available network management tools
are more advanced.
The SDH standards, in contrast to PDH standards, set out a set of functions for
monitoring and reconfiguration
of
remote equipment. This is achieved by the dedica-

tion of a defined management channel within the section overhead. This is referred to as
the
data communication channel
(DCC)
or sometimes the
embedded communication
channel
(ECC).
A
number of ITU-T recommendations are in course of preparation
to
define functions compatible with the
telecommunications management network
(TMN,
Chapter 27) which can be supported by the DCC. These will include facilities for
performance monitoring (by sending a continuous given bit pattern and measuring the
received signal), remote loopback and testing facilities, as well as remote configuration
capability.
13.10
SONET
(SYNCHRONOUS OPTICAL NETWORK)
SONET is the name of the North American variant of SDH. It is the forerunning
technology which led to the ITU’s development of SDH. The principles of SONET are
very similar to those of SDH, but the terminology differs. The SONET equivalent of an
SDH synchronous transfer module (STM) has one
of
two names, either
optical carrier
(OC)
or

synchronous transport system
(STS).
The SONET equivalent of an SDH
virtual container (VC) is called a
virtual tributary
(VT).
Some SDH STMs and VCs
correspond exactly with SONET STS and VT equivalents. Some do not. Table 13.4
presents a comparison of the two hierarchies.
280
SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK
Table
13.4
Comparison
of
SDH
and
SONET hierarchies
North American SONET Carried Bitrate/Mbit/s
SDH
VT 1.5
VT 2.0
VT 3.0
VT 6.0
STS- 1 (OC- 1)
STS-3 (OC-3)
STS-6 (OC-6)
STS-9 (OC-9)
STS- 12 (OC- 12)
STS- 18 (OC- 18)

STS-24 (OC-24)
STS-36 (OC-36)
STS-48 (OC-48)
STS-96 (OC-96)
STS-192 (OC-192)
-
1
S44
2.048
3.152
6.312
8.448
34.368
44.736
149.76
51.84
155.52
31 1.04
466.56
622.08
933.12
1244.16
1866.24
2488.32
4976.64
9953.28
VC-I 1
VC-l2
VC-21
VC-22

VC-3 1
VC-32
VC-4
-
STM- 1
-
STM-4
STM- 16
-
STM-64
13.11
SDH AND ATM (ASYNCHRONOUS TRANSFER MODE)
Finally, it is worth mentioning here the integration of SDH into the specifications for
broadband-ISDN (B-ZSDN)
and
ATM
(asynchronous transfer mode).
These techniques
will form the basis of future
broadband networks.
The C-4 container may be used
directly for carriage of ATM, and will be one of the standard speeds at which ATM will
be used. ATM cells
(of
53
bytes or
octets)
do not fit an integral number of times into the
C-4 frame
(2340

bytes), but this is not important. The SDH standards require only that
the ATM
octets
are aligned with the bytes of the SDH container. Individual ATM cells
can be split between container frames when necessary. We return to B-ISDN and ATM
in Chapters
25
and
26.