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- 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)
13
Synchronous Digital Hierarchy
(SDH) and Synchronous
Optical Network (SONET)
Thesynchronous hierarchy
digital (SDH) is
emergingthe
as universal
technology for
transmission telecommunications
in networks. the publication
Since first of international
standards by ITU-T in 1989, SDH equipment has been rapidly developedand deployed across the
world,andisrapidlytakingoverfromitspredecessor,thePlesiochronousDigitalHierarchy
(PDH).ThischapterdescribesSDHandtheNorthAmericanequivalentofSDH,SONET
(SynchronousOpticalNetwork),fromwhichitgrew.Inparticular,thechapterdescribesthe
features of SDH which characterize its advantages over PDH.
13.1 HISTORY OF THESYNCHRONOUS DIGITAL
HIERARCHY (SDH)
The synchronousdigitalhierarchy ( S D H ) was developedfromits 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 THEPROBLEMS OF PDHTRANSMISSION
Historically, digital telephone networks, modern data networks and the transmission
infrastructures serving themhave been based on atechnology 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
- 268 SYNCHRONOUS HIERARCHY
DIGITAL AND SYNCHRONOUS NETWORK
OPTICAL
(a) Europe
992 564 264 139 368 34 a448
2048 64
kbit/s
(El1 (E31 (E21
= X32 - X4 - X4 = X4 X4
- MUX - MUX - MUX - MUX - MUX
b
L
hierarchy level 0 1 2 3 4 5
(b) North America
176 274 736 44 6312 64
1544 kbiffs
(DSO)
. (DS1
.I 4 x 7
3 x 2 4 13 x 4
or T1)
(DS2 or T2)
(DS3
1
or T3)
H
-I
X 6 ,
MUX MUX MUX MUX
U U U U
hierarchy level
0 1 2 3 4
(c) Japan
728 97 064 32 2 631 64
1544 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/schannels,synchronized to someextent at thefirstmultiplexing level
(1.5 Mbit/s or 2 Mbit/s).
Theyrequiremultiplemultiplexingstages to reachthehigherbitrates, andare
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 is not locked synchronously in step (it free-
but is
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 three bits
or
more or less than another. If the system running slightly fasteris 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).
Extraframing bits areaddedat eachstageof thePDH multiplexingprocess.
Unfortunatelythismeansthatthe efficiency of thehigherorder line systems (e.g.
139264kbit/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 amultiplexingtechnique 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
Note
3xE3 1 =2 a= 3x E3
3x E2 3x E2
Note 2
-
4xEl 4xEl
IX B-C
El
t - - - -
- - - - )
ElEl
t W
6 3 ~ A-to-C
El
Note 1: 3 X E3 = 12 X E2 or 48 X E l after demultiplexing
Note 2: 3 X E2 = 12 X E l after demultiplexing
Figure 13.2 Breaking-out2Mbit/s (El) from140Mbit/s
a line system at intermediate
an
exchange
- 270 SYNCHRONOUS HIERARCHY
DIGITAL AND SYNCHRONOUS NETWORK
OPTICAL
A B C
STM-l STM-1
drop and
Insert
- -
multiplexor
El El
62 X El
Figure 13.3 Drop and insertmultiplexorused to break-out 2 Mbit/s (El) from a 155 Mbit/s
(STM-1) line at an intermediate exchange
Figure 13.4 Optical fibre back-up using
m standby
PDH system components
Before SDH, networks had to built up from separatemultiplex and line terminating
be
equipment (LTE),as the optical equipment interfaces in particular were manufacturer-
+
specific (i.e. proprietary). Back-uptended 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 THEMULTIPLEXING 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
- STRUCTURE
THE MULTIPLEXING OF SDH 271
XN X 1
pointer processing
c- multiplexing
Figure 13.5 Synchronous digital hierarchy (SDH) multiplexing structure (ITU-T/G.709)
so abbreviated), tributary units ( T U ) , tributary groups
unit ( T U G ) , administrative
units ( A U ) and finally administrativeunitgroups ( A U G ) into synchronoustransport
modules ( S T M ) .
The basicbuilding block of the SDH hierarchy is the administrativeunitgroup
( A U G ) . 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, theframe 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 transporta PDH link of 139.264 Mbit/s.
To the container is added a path overhead ( P O H ) of 9 bytes (72 bits). This makes a
virtual container ( V C ) . 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 ( A U ) (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 A U 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 (of19 440 bits). The SOHis added toprovide forblock framing and
for the maintenance and performance information carried on a transmission section line
- 272 SYNCHRONOUS HIERARCHY
DIGITAL AND SYNCHRONOUS NETWORK
OPTICAL
basis. (Asection is an administratively defined point-to-point connectionin the network,
typically an SDH-system between two major exchange sites, between two intermediate
multiplexors or simply betweentwo 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 interpretedby 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)
260 1 bvte bytes
. 1-
Pot- C-4 container
AUG frame (in this case one AU-4)
9 bytes bytes 261
4 b
row 4 X
1 (X) (X; VC-4 virtual container
pointers (up to ~
3; here only
one is used)
STM-1 frame
9 bytes 261 bytes
RSOH
row 4 AUG (administrative unit group)
MSOH
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 aframe size of
77760 bits, and a line rate of 622.08 Mbit/s. An STM-16 frame (16AUGs) has frame a
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 intolower speed tributaries, suitable for carriageof
today’s T1, T3, El or E3 line rates(1.544Mbit/s, 44.736 Mbit/s, 2.048 Mbit/sor
34.368 Mbit/s).
Figure 13.6 shows the gradual build up of a C-4 container intoan 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 topleft 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 indicate the start positions within the frameof the three separate
to
VCs. The various other U and VC formats follow similar patterns to the AUs and
T VCs
presented (TUs also includepointers like AUs). Table 13.1 presents various
the
-c
row 41
AU-32 1
(1
3 ) AU-3
(2) AU-3 (3)
/
pointers
1 87 8%
261 175 174
AU-3 = 87 columns X 9 rows of bytes
Figure 13.7 AUG frame arranged as 3 X AU-3
Table 13.1 Payloadrates of SDH containers
Container Container Frame Capable of carrying
tYPe frame size repetition rate PDH line
type
c-l 1 193 bits 8000 544 (1
T1 kbit/s)
c-l2 256 bits 8000 E l (2048 kbit/s)
c-21 789 bits 8000 T2 (63 12 kbit/s)
C-22 1056 bits 8000 E2 (8448 kbit/s)
C-3 1 4296 bits 8000 (34E3 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 HIERARCHY
DIGITAL AND SYNCHRONOUS NETWORK
OPTICAL
,!
POH
r ''Cl
.1
....
1 2 3.45 6 7 s il'.::..;..,.,. ..........
~~~ . . . . . . ,:'261 ....
.......
.... ... ....
....
..,....
. . . . .. . . .. . . .
..
... ..... ...
. . . . ... '.. :
1
.
: 86 .,,. .',:; B 86 ~. ">'.... 1 86'..
....... .... 86 columns X
TUG-3 9 rows ofTUG-3
bytes
TUG-3
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 called C-twelve, but instead
be
C-one-two. The relevant VC is VC-one-two, etc.
Figure 13.8 shows an alternative demultiplexing
scheme,based upon thesub-
multiplexing of a VC-4 container into three tributary unit g r o u p 3 (TUG-3s). In this
case, the first three columns are used as path overhead, and each TUG occupies a total
TU-l 1 TU-l 2 TU-2
123 1234 l23456789101112
1..N.,
..
.
...
.
.
.
.
.
. .
.
... ... ...
... ... .. .
.. .. .
.. .. .
1 2 3 4 5 6 7 8......16......23......30......37...... ......51...... ......65...... ......79.................86
9 44 58 72
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-multiplexingofthe TUG-3s themselves may be continuedasshown 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 E l rate of 2.048 Mbit/s).
The individual containers (C-l 1 or C-12) may be packed into the TU-l (synonymous
1
with VC-l 1) or TU-l2(synonymous with VC-12) in one of three manners, using either
a noframing (i.e. asynchronously)
a bitsynchronous framing
a bytesynchronousframing
VC-3 / VC-4 POH (9rows X 1 byte!8 bitsn
Figure 13.10 Path overhead (POH) formats for VC-l, VC-2, VC-3 and VC-4
- 276 SYNCHRONOUS HIERARCHY
DIGITAL AND SYNCHRONOUS NETWORK
OPTICAL
The asynchronous and bit synchronous framing methods allow a certain number of bits
of
for justzjication. This enables 1.5 Mbit/s or 2 Mbit/s tributaries 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 thepath overhead ( P O H ) 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)
Thediagramandtable of Figure 13.1 illustratetheconstitution
1 of thesection
overhead.
Table 13.2 Meaning and function of the fields in the SDH path overhead (POH)
Field Name Function
BIP-2 bit inserted parity error check function
FEBE far end block error indication of received BIP error
L1, L2, L3 signal label indication of VC payload type
remote alarm remote alarm indication of receiving failure to transmitting
end
J1 path trace verification of VC-n connection
B3 BIP-8 parity code error check function
c2 signal label indication of VC payload type and composition
G1 path status indication of received signal status to
transmitting end
F2 path user channel provides communication channel for network
operating staff
H4 multiframe indicator multiframe indication
2 3 , 24, z5 bytes reserved for national reserved
network operator use
- NETWORK TOPOLOGY OF SDH NETWORKS 277
* Row 4 i used for the AUG .frame pointers
s
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 channel
(APS)
z1,z2 reserved
Figure 13.11 The SDH sectionoverhead (SOH)
13.7 NETWORK TOPOLOGY OF SDH NETWORKS
SDH equipment is designed to be used in the construction of synchronous (in par-
A
ticular, optical fibre) transmission networks in redundant ring topologies. 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 multiplexorsallow 2 Mbit/sandothersubSTM-1ratetributariesto be
multiplexed for carriage by an SDH network. Drop and insert multiplexors (also called
- 278 SYNCHRONOUS HIERARCHY
DIGITAL AND SYNCHRONOUS
OPTICAL NETWORK
ADM=add/drop multiplexor
(drop and inserl multiplexor)
STM-4 MUX=multiplexor
Ring crossconnect
DXGdigital
Figure 13.12 Ring topology and generic equipment types used in SDH networks
addldrop multiplexors ( A D M ) ) allowtributariesto be removedfromthe line at an
intermediate station without complete demultiplexing (as we discussed in Figures 13.2
and 13.3). Crossconnectors (or DXC, digitalcrossconnectors) allow forthe 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 ahighly 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 more are efficient and easier to
administrate (due to the availability of drop and insert (addldrop) multiplexors). Using
- CHRONOUS SONET 279
Table 13.3 Classification of optical fibre interfaces for SDH equipment
Application
Inter-office
Intra-
office Shorthaul Longhaul
Wavelength nominal/nm 1310 1550 1310 1550
1310
Fibre type G.653G.652
G.652G.652
G.652
(3.652
(3.654
Distance kilometres
- 280 SYNCHRONOUS HIERARCHY
DIGITAL AND SYNCHRONOUS NETWORK
OPTICAL
Table 13.4 Comparison of SDH and SONET hierarchies
North American SONET Carried Bitrate/Mbit/s SDH
VT 1.5 1 S44 VC-I 1
VT 2.0 2.048 VC-l2
VT 3.0 3.152 -
VT 6.0 6.312 VC-21
8.448 VC-22
34.368 VC-3 1
44.736 VC-32
- 149.76 VC-4
STS- 1(OC- 1) 51.84
STS-3 (OC-3) 155.52 STM- 1
STS-6 (OC-6) 31 1.04 -
STS-9 (OC-9) 466.56
STS- 12 (OC- 12) 622.08 STM-4
STS- 18 (OC- 18) 933.12
STS-24 (OC-24) 1244.16
STS-36 (OC-36) 1866.24
STS-48 (OC-48) 2488.32 STM- 16
STS-96 (OC-96) 4976.64 -
STS-192 (OC-192) 9953.28 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-4containermaybe used
directly for carriage of ATM, and be one of the standard speeds at which ATM will
will
be used. ATM cells (of 53 bytes or octets) do notfit an integral numberof 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.
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