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SDH FUNDAMENTALS (0039.p65) • v1.0 • 11/94 1
SDH FUNDAMENTALS
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2 SDH FUNDAMENTALS (0039.p65) • v1.0 • 11/94
NOTICE
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Table of Contents
Page
Objectives ............................................................................................................................................................... 4
Advantages and Benefits of SDH ........................................................................................................................... 5
Network Overview ................................................................................................................................................. 6
Framing Structure Overview ................................................................................................................................ 14
Differences Between SDH and SONET Technologies ........................................................................................ 49
SDH Abbreviations .............................................................................................................................................. 51
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4 SDH FUNDAMENTALS (0039.p65) • v1.0 • 11/94
Objectives
The objectives of the SDH FUNDAMENTALS Training Manual are to enable a trainee to:
1. Describe the structure of SDH signals, including SDH overhead.
2. Explain the advantages of SDH systems over PDH systems.
3. Describe the three major SDH networks elements.
4. Mention three important differences between SDH and SONET technologies.
NOTE TO THE READER: To achieve these objectives, it is assumed the reader is familiar with PDH
technology.
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Advantages and Benefits of SDH
Advantages and benefits of SDH over PDH are:
• Reduced network installation, operation, and maintenance costs.
• A flexible transmission structure able to handle existing as well as new signals both simultaneously or non-
simultaneously without equipment modification or replacement. Examples of new signals include ATM
and IEEE 802.6 for MAN.
• High- and low-level compatibility among transmission line equipment manufacturers.
• Increased integrated maintenance and network management capabilities.
• Direct access to lower speed tributaries within a signal without need to multiplex/demultiplex the entire
high-speed signal.
• A unified worldwide standard, which eliminates signal conversion at international borders, dramatically
reducing the cost of international connections.
• SDH specification does not impose limits on the transmission capacity, so SDH will be able to satisfy both
current and future needs.
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6 SDH FUNDAMENTALS (0039.p65) • v1.0 • 11/94
Network Overview
Line/RadioSystems
DXC/EA Line/RadioSystems
SM
SM
SM
TR
TR
TR
SM
SM
SM
TR
TR
TR
NNINNINNINNI
TR: TributariesSM: Synchronous Multiplexer
NNI: Network-Node InterfaceDXC: Digital Cross-Connect System
EA: External access equipment
Location of the Network Node Interface
SDH, which stands for Synchronous Digital Hierarchy, is a group of ITU-T specifications describing the
Network Node Interface (NNI). The NNI is the interface between the transmission facility and the network
node which performs signal termination, switching, cross-connection or multiplexing/demultiplexing.
The transport networks built to comply with these specifications conform a system for the transmission of high-
speed signals based upon PDH, ATM, and other signals.
Previously to SDH, the specifications used are what is understood as Plesiochronous Digital Hierarchy (PDH).
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Line/RadioSystems
DXC/EA Line/RadioSystems
SM
SM
SM
TR
TR
TR
SM
SM
SM
TR
TR
TR
NNINNINNINNI
TR: TributariesSM: Synchronous Multiplexer
NNI: Network-Node InterfaceDXC: Digital Cross-Connect System
EA: External access equipment
Network Overview - Operations, Administration, Maintenance, and Provisioning (OAM&P)
Location of the Network Node Interface
One of the important benefits of SDH in respect to PDH is the enhanced operations, administration, maintenance,
and provisioning (OAM&P) capabilities made available. These capabilities allow network operators to provide
centralized control of the network, to better measure the quality of the information transferred, to implement
emergency connections and to allocate extra capacity for future, yet unknown services.
This is achieved in SDH by increasing the amount of overhead within the SDH frame structure (respect to PDH),
by specifying interfaces to access the Telecommunications Management Network (TMN), and by specifying basic
functionality of the elements in a network.
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Network Overview - Network Elements
Synchronous Digital Cross-Connect System
The principal component in an SDH network is the Digital Cross-Connect System (DCS or DXC), called
Synchronous Digital Cross-Connect System. The reasons for this are essentially the same as for PDH networks:
Often, not all the traffic carried to a network node by a high-capacity transmission link may be switched there;
some of the capacity may be required for Public Switched Telephone Network (PSTN) traffic routed through to
other nodes and for private circuits.
Also, the introduction of broadband services will necessitate the provision of channels carrying traffic at much
higher rates than 64 kbit/s.
SDHsignals
SDHsignals
M/N
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Network Overview - Network Elements
Synchronous Multiplexers
As in PDH, SDH uses multiplexers to combine lower-speed signals into higher-speed signals. These
multiplexers are typically referred to as synchronous multiplexers. There are two types of synchronous
multiplexers: Access and SDH multiplexers.
Access multiplexers have tributary (input) interfaces compatible with PDH and other signals (such as ATM,
FDDI, and IEEE 802.6 for MAN), and aggregate (output) parts compatible with SDH signals.
SDH multiplexers have both tributary interfaces and aggregate ports compatible with SDH signals.
Tributaries(PDH, ATM,IEEE 802.6)
SDHlines
Terminal Multiplexer
Higher orderSDH
signals
Terminal Multiplexer
Lower orderSDH
signals
symbol means "fiber optic line"
SDHlines
SDHlines
Tributaries (PDH, ATM, IEEE 802.6)
Higher orderSDH
signals
Lower order SDH signals
Higher orderSDH
signals
ACCESS MULTIPLEXERS
SDH MULTIPLEXERS
Add/Drop Multiplexer
Add/Drop Multiplexer
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Network Overview - Network Elements
Standard synchronousRegenerator
Drop & Insert synchronousRegenerator
Synchronous Regenerators
As in PDH systems, an SDH network may include signal regenerators, called synchronous regenerators.
However, fewer regenerators are needed in an SDH system than in a comparable PDH system. The reason for
this is that the use of optical transport medium makes the distances between regenerators (in excess of 70 km,
depending on the type of fiber used) much larger than in comparable PDH systems.
Unlike PDH regenerators, SDH regenerators must synchronize to the incoming frame structure. When a fault
occurs, for example, “Loss of Signal” or “Loss of Frame Synchronization,” synchronous regenerators produce a
proper SDH frame to keep the system “alive.” Also, SDH regenerators may provide access to the incoming
traffic; these are called Drop and Insert regenerators.
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Network Overview - Transmission Media
SDH Ring
To achieve cost reductions and increased reliability, SDH specifies the use of single-mode optical fiber across
the network. However, initial deployment of SDH systems have had to fit into a digital-distribution frame
regime dominated by coaxial plesiochronous interconnections at 34 Mbit/s and 140 Mbit/s. For this reason,
coaxial cable is currently being used for SDH connections within stations.
Many PTTs have invested heavily in coaxial cable for the transmission network. For this reason, copper cable
may be expected to continue in use for at least five more years. In addition, ITU-T has not specified optical
monitoring points, so optical/electrical signal conversion will still be needed.
In addition to coaxial cable and optic fiber, SDH radio systems have also been developed using new frequency
bands and using new modulation schemes in order to contain the bandwidth for compatibility with existing
frequency plans.
TR
TR
TR
TR
: Optic Fiber lines
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Network Overview - Timing Issues
timing signal
2 Mbit/stributaries
8 Mbit/sbuffer
timing signal
8 Mbit/s≈ 8 Mbit/s
PDH Network Node PDH Multiplexer
PDH Timing Issues
In PDH systems, differences in timing at a given hierarchical level (8 Mbit/s, for example), are handled by
using buffers. Multiplexing lower-level signals into higher-order signals uses the technique of bit justification
which allows frequency equalization of plesiochronous tributaries.
Unlike PDH, SDH systems make provision for differences in timing only at the first SDH level. There are no
provisions for handling differences in frequency at higher-rate SDH signals. The reason is that higher-order
SDH signals are synchronized by a highly stable, accurate network clock.
At the first SDH level, incoming PDH and other signals are synchronized with the framing structure by using
bit justification. Once this has occurred, subsequent frequency variations are handled by using a new technique
that does not use bit-justification.
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Network Overview - A Typical SDH Network
PDHDXS
FlexibleMux
FlexibleAccessSystem
64kDXS
(2-34)M links
(PSTN and Private Circuits)
PDHand
B-ISDN
2Mlinks
(voiceand data)
DigitalSwitch
(LE)
2M
STM-1 STM-1
STM-1 STM-1
STM-1 STM-43 140 Mbit/s links3 34 Mbit/s links
140 Mbit/s
34 Mbit/s
64klinks
(2-34)Mlinks
34 M
140 M4 STM-1s
"Regenerator Section"
STM-1
"RegeneratorSection"
"Multiplex Section"
"Path"
SkipMux
STM-1140 Mbit/s
Example of a Regional Implementation of SDH
The figure above shows a typical SDH network configuration. Three terms are especially important when
describing an SDH network: “Path”, “Multiplex”, and “Regenerator”.
“Path” is used to name any section of the SDH network between the points were an SDH signal is originated
and terminated.
“Multiplex Section” is used to name any section of the SDH network between two points where multiplexing or
demultiplexing of an SDH signal occurs, but no origination and termination occurs. If STM-4/STM-16 and
above multiplexers are used, then the network sections they delimit are also Multiplex sections.
“Regenerator Section” is used to name any section of the SDH network between two points with regeneration
capabilities (synchronous multiplexers and synchronous regenerators).
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Framing Structure Overview - Comparison of PDH and SDH
PDHDXS
FlexibleMux
FlexibleAccessSystem
64kDXS
(2-34)M links
(PSTN and Private Circuits)
PDHand
B-ISDN
2Mlinks
(voiceand data)
3 140 Mbit/s links3 34 Mbit/s links
140 Mbit/s
64klinks
(2-34)Mlinks
34 M
140 M
SkipMux
PDH
PDH
PDH
PDH
140 Mbit/s
How PDH and SDH Coexist in a Typical Network
As previously discussed, SDH technology is used in the interface between the transmission facility and the
network node which performs signal termination, switching, cross-connection or multiplexing/demultiplexing.
Before SDH, the technology used was PDH.
Although from the technology point of view PDH and SDH are different, the functionality and
applications of both PDH and SDH are essentially similar. For example, as in the case of PDH, SDH
framing is based upon a Frame Alignment signal, which is a binary word ranging from 7 to 12 bits in
PDH and a multiple of 6 octets in SDH.
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Framing Structure Overview - Introduction to STM-1 Frame
STM-1 Frame Structure
The basic SDH frame, called synchronous transport module at level 1 (STM-1) frame, is represented above.
The STM-1 frame consists of nine equal segments of serial bits. Each of these segments is composed of 270
octets. For each segment, the first 9 octets are used for “overhead” transport, and the remaining 261 octets are
used for “payload” transport. The length of an STM-1 frame is 270 octets/segment x 9 segments = 2430 octets.
The aggregate bit rate is therefore:
Aggregate bit rate = 2430 octets x 8 bits/octet ——> = 155.52 Mbit/s125 µ seconds
PDHDXS
STM-1 STM-1
STM-1 STM-1
STM-1
4 STM-1s
STM-1
STM-1
STM-1
STM-1 Signals Within a Typical Regional Network
1 2 43 5 6 7 8 9
Frame Period = 125 µ seconds
OVERHEAD PAYLOAD
9 bytes 261 bytes
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
C-4
VC-4
AU-4
AUG
STM-1 Frame
VC-4 Path Overhead
VC-4 Pointer
Section Overhead
Container for139264 kbit/s signal
Steps in Multiplexing a 140 Mbit/s signal into the STM-1 Frame
The diagram above shows the steps used to multiplex a 140 Mbit/s PDH signal into the STM-1 frame. Each of
those steps will be discussed separately next.
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
PDHDXS
STM-1 STM-44 STM-1s
C-4
VC-4
AU-4
AUG
STM-1 Frame
140M
VC-4 Path Overhead
VC-4 Pointer
Section Overhead
Multiplexing a 140Mbit/s Signal
In a PDH-SDH link containing a 140 Mbit/s signal, the STM-1 overhead and pointers are processed in the link
portion indicated in the figure above. Path overhead is processed at the nodes where virtual containers are
originated and terminated.* The VC-4 pointer is processed at the synchronous multiplexers and DCSs, and the
section overhead is processed at both synchronous multiplexers and line repeaters.
* (originating and terminating synchronous multiplexers)
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
C-4
VC-4VC-4 POH
Container for139264 kbit/s signal
C-4 and VC-4 Origination
In order to multiplex a 140 Mbit/s PDH signal into an STM-1 SDH signal, the first step is to synchronize the
PDH signal with the STM-1 structure. This is done by using the technique of “bit justification”. The created
signal is referred to as a “container”. For a 140 Mbit/s signal, this container is called a “C-4”.
The next step in the multiplexing process is to add overhead to the container just created. This overhead, which
is a group of 9 octets, is called “higher-order Path Overhead (POH)”. The C-4, along with its POH is referred to
as Virtual Container, order 4 (VC-4).
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
VC-4 Origination
Each of the nine VC-4 POH octets receives a unique name: J1, B3, C2, G1, F2, H4, Z3, Z4, and Z5. Any two
of these 9 octets are separated by 269 octets in the STM-1 frame. A brief description of each follows:
J1 (Path Validation and Trace): The octet is used to repetitively transmit a known pattern so the receiver can
verify its continued connection with the intended transmitter.
B3 (Path Error Monitoring): A bit interleaved parity of depth eight (BIP-8) is used to monitor for errors. This
works as follows: The number of ones in the position n in each octet of the previous VC-4 before scrambling is
counted. If the result is odd, then the corresponding bit in B3 is set to one. If the result is even, then the
corresponding bit is set to 0. At the receiver, even parity is verified.
C2 (Signal Label): This octet is reserved to indicate the composition of the VC-4, which could contain a 140
Mbit/s signal, ATM cells, MAN signals (DQDB protocol) or even FDDI signals. It also indicates if no valid
signal is being carried.
VC-4VC-4 POH
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
VC-4 Origination
G1 (Path Status): This octet conveys back to a VC-4 path originator the path terminating status and
performance. The first four bits return the count of BIP-8 errors Far End Block Errors (FEBE), and therefore
can be thought of as advanced 2 Mbit/s PDH REBEs. Bit 5 generates a Far End Receiver Failure (FERF) alarm
similar to PDH remote alarms. A FERF is sent in the opposite direction if signal failure occurs, or if Path Trace
Mismatch (checked with J1 octet) occurs. Bits 6, 7 and 8 are not currently used.
VC-4VC-4 POH
SDH PathOriginator/TerminatorVC-4 Path FEBE
BIP-8 errors SDH PathOriginator/Terminator
VC-4 Path LOPor
VC-4 Path AISor
Path Trace Mismatch
VC-4 Path FERF
FEBE1 2 3 4
FERF UNUSED5 6 7 8
F2, Z3 (Path User Channels): These two octets are reserved for user communication purposes between path
elements and are payload dependant.
H4 (Position Indicator): This octet provides a generalized position indicator for payloads and can be payload
specific.
Z4 (Spare octet): This octet is reserved for future uses.
Z5 (Network Operator octet): This octet is reserved for specific management purposes, such as tandem
connection maintenance.
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
AU-4 Origination
The next step in the multiplexing process of a 140 Mbit/s PDH signal is to make provision for differences in
frequency and phase between VC-4s. This is achieved by adding a “VC-4 pointer” to the existing VC-4. The
VC-4, along with its pointer, is called “Administrative Unit, order 4,” or AU-4.
The VC-4 pointer is a 10-bit string that indicates the position of the first octet of the VC-4 (J1 octet). Given that
the transmission frequency of an STM-1 is fixed, the VC-4 pointer allows VC-4 transmission rate and phase to
be controlled within certain limits. (Allows the VC-4 to “float” inside the STM-1).
There are three ways to change the value of a VC-4 pointer; those three will be reviewed in the next page.
AU-4VC-4 Pointer
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
VC-4 Pointer Value Definition
The first method to change a pointer value consists in incrementing or decrementing it by one unit at a time with
indication; the second, by forcing an arbitrary, non-unit change; and the third, by detecting a new pointer value
for three consecutive frames.
The first method of pointer value change is indicated by the bits in the pointer itself. When the value of
the pointer needs to be increased by one, 5 bits invert their value. To increase the value of the pointer by
one unit, bits 1,3,5,7, and 9 of the pointer are inverted. To decrease it, bits 2,4,6,8, and 10 of the pointer
invert their value.
The second method of pointer value change is achieved by the use of the New Data Flag (NDF) word. This is a
4-bit word indicated in the figure above by NNNN. When using this method, the normal NDF value of 0110 is
replaced by the value 1001. The new present pointer value takes place immediately.
The third method of pointer value change calls for accepting a new pointer value if it is received by three
consecutive frames.
N N N N S S 1 0 1 0 1 0 1 0 1 0
unspecifiedbits
VC-4 pointerNew DataFlag
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
How a VC-4 Pointer indicates the location of a VC-4
Contrary to PDH, which uses positive justification only to modify information speed, pointers allow
information speed and phase modification by using both positive and negative justification. Positive
justification allows transmission of “less” information within the frequency-constant STM-1 signal, whereas
negative justification allows transmission of “more” information within the frequency-constant STM-1 signal.
Typically, phase adjustments are implemented by using the New Data Flag (NDF) word. Also, in order to
compensate for frequency variations within the synchronous network, frequent pointer value adjustments are
used. Because of this, timing problems in a synchronous network can be pinpointed by observing the values of
pointers over time. A frequent variation would indicate potential timing problems.
N N N N S S 0 0
2 bytes
0 0 0 0 0 0 0 0
N N N N S S 0 0 0 0 0 0 0 0 0 1
2 bytes
NDF
NDF
VC-4 pointer
VC-4 pointer
3 bytes
3 bytes
VC-4 starts here
VC-4 starts hereinfo. from previous VC-4
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
AUG Origination
There is an SDH alarm dependent upon pointer activity. This alarm is called AU-4 Path AIS. Whenever an
SDH Path originator/terminator receives an AU-4 Path AIS, or an AU-4 Path LOP, it will transmit AU-4 Path
AIS, which is “all ones” in the entire AU-4, including the VC-4 pointer.
The next step in the multiplexing process of a 140 Mbit/s PDH signal is to generate an Administrative Unit
Group (AUG). No additional information is added to an AU-4 to create an AUG. This structure is part of SDH
primarily to remain compatible with the North American SONET structure.
AU-4
AUG
VC-4 Pointer
SDH PathOriginator/Terminator
AU-4 Path AISor
AU-4 Path LOPAU-4 Path AIS
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
STM-1 FrameSection Overhead
STM-1 Frame Origination
An AUG is multiplexed into an STM-1 signal by adding additional overhead, called Section overhead. Section
overhead includes framing information and information for maintenance, performance monitoring, and other
operational functions. The SOH includes both Regenerator Section Overhead (RSOH) , which is terminated at
regenerator functions, and Multiplex Section Overhead (MSOH), which passes transparently through
regenerators and is terminated where the AUGs are assembled and disassembled (synchronous multiplexers).
The section overhead is organized in octets. An explanation of the Section overhead follows.
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
STM-1 Frame
A1, A2 octets (Framing octets): Three A1 octets (11110110) and three A2 octets (00101000) are used to create
a Frame Alignment Signal (FAS) for frame alignment purposes. The used sequence is A1A1A1A2A2A2.
C1 octet (STM-N identifier): Indicates the level of the frame (1, 4, 16, etc.). Particularly useful when higher-
level signals are transported.
B1 octet (Regenerator Section Error Monitoring): Implements a bit interleaved parity eight (BIP-8) code using
even parity. The BIP-8 is computed over all bits of the previous STM-1 frame after scrambling and is placed in
B1 before scrambling.
E1, E2 octets (Orderwire channels): Use these two 64 kbit/s channels for voice communications. E1 is part of
the RSOH, and E2 is part of the MSOH.
F1 octet (User Channel): This 64 kbit/s channel is reserved for user purposes, for example, to provide
temporary data/voice channel connections for special maintenance purposes.
D1-D12 octets (Data Communication Channels): These channels can be used for alarms, maintenance, control,
monitoring, administration, and other purposes. These channels are divided into one 192 kbit/s channel at the
Regenerator section, and one 576 kbit/s channel at the Multiplex section.
B2 octets (Multiplex Section Error Monitoring): Three octets are used to implement a BIP-24 code using even
parity. The whole previous STM-1 frame, except the RSOH are included in the calculation.
FAS
A1 A1 A1 A2 A2 A2
2430 bytes
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
STM-1 Frame
K1, K2 octets (Automatic Protection Switching, APS): Those two octets are allocated to the function of
coordinating protection switching across a set of multiplex sections organized as a protection group. Bits 6, 7,
and 8 of K2 octet are used to signal MS-FERF (110) and MS-AIS (111).
Z1 octet (Synchronization status octet): Bits 1-4 of these three octets are unused. Bits 5-8 of those three octets
are used for synchronization status messages.
Z2 octet (Additional octet): This byte is reserved for functions not yet defined.
There are 39 additional octets as part of the section overhead whose use is still not defined.
FAS
A1 A1 A1 A2 A2 A2
2430 bytes
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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal
SDH MSElement
LOS, LOF,MS-AIS or
Excessive Errors
MS-FERF
MS-AIS
At the Multiplex Section level of an STM-1 SDH signal, there are two important alarms:
a) Multiplex Section AIS: Referred to as MS-AIS, this alarm is signaled by setting bits 6, 7 and 8 of the K2
octet equal to “1”. MS-AIS should be generated when signal is lost, when frame alignment is lost, when MS-
AIS is received, or when an excessive BER, as calculated using B2 octets, is detected.
b) Multiplex Section FERF: Referred to as MS-FERF, this alarm is signaled towards the opposite direction of
the data flow by setting bits 6, 7 and 8 of the K2 octet to 110. MS-FERF should be generated when signal is
lost, when frame alignment is lost, when MS-AIS is received, or when an excessive BER, as calculated using
B2 octets, is detected.
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Framing Structure Overview - Multiplexing a 34 Mbit/s Signal
C-3
VC-3
TU-3
TUG-3
VC-4
VC-3 Path Overhead
VC-3 Pointer
Container for34368 kbit/s signal
AU-4
AUG
STM-1 Frame
VC-4 Path Overhead
VC-4 Pointer
SectionOverhead
3 x
Steps in multiplexing a 34 Mbit/s signal into the STM-1 frame
The diagram above shows the steps used to multiplex a 34 Mbit/s PDH signal into the STM-1 frame. Following
is a separate discussion of these steps.
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Framing Structure Overview - Multiplexing a 34 Mbit/s Signal
C-3
VC-3
TU-3
TUG-3
VC-4
VC-3 Path Overhead
VC-3 Pointer
Container for34368 kbit/s signal
AU-4
AUG
STM-1 Frame
VC-4 Path Overhead
VC-4 Pointer
SectionOverhead
3 x
PDHDXS
(2-34)M links
(PSTN andPrivate Circuits)
STM-1
STM-1
STM-43 140 Mbit/s links3 34 Mbit/s links
140 Mbit/s
34 Mbit/s
(2-34)Mlinks
STM-1
1
2
1 , 2
STM-1
1 , 2
Multiplexing a 34 Mbit/s signal
The figure above shows an example of where in a PDH-SDH link containing a 34 Mbit/s signal the STM-1
overhead and pointers are processed. Path overhead is processed at the nodes where the respective virtual
containers are originated and terminated (originating and terminating synchronous multiplexers). The pointers
are processed at the synchronous multiplexers and DCCs, and the section overhead is processed at both
synchronous multiplexers and line repeaters.
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Framing Structure Overview - Multiplexing a 34 Mbit/s Signal
C-3
VC-3
TU-3
VC-3 POH
VC-3 Pointer
Container for34368 kbit/s signal
C-3, VC-3 and TU-3 Origination
The first three steps in multiplexing a 34 Mbit/s PDH signal into an STM-1 are similar to the first three steps in
the multiplexing of a 140 Mbit/s signal. The 34 Mbit/s signal is first synchronized with the STM-1 signal, then
Path Overhead identical to that used in a 140 Mbit/s signal is added, and then a VC-3 pointer is added to the
created Virtual Container, to create a Tributary Unit, order 3 (TU-3).
The alarms for a VC-3 Path are similar to those of a VC-4 Path.
The only major difference between a VC-4 pointer and a VC-3 pointer is that when adjusting the value of a VC-
4 pointer by one, the position of the payload is moved by three octets, but when adjusting the value of a VC-3
pointer by one, the position of the payload is moved by one octet. The alarms found in an AU-4 are similar to
those in a TU-3.
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Framing Structure Overview - Multiplexing a 34 Mbit/s Signal
TU-3
TUG-3
VC-4VC-4 Path Overhead
3 x
TUG-3 and VC-4 Origination
The next step in the multiplexing process is to generate a Tributary Unit Group of order 3 (TUG-3). To
generate a TUG-3, no pointer processing occurs or extra overhead is added to the existing TU-3s. The reason
for having this extra step in the multiplexing process is to allow an STM-1 signal to carry mixed capacity
payloads made up of different size TUs. This makes an STM-1 signal more flexible as a transport structure.
The next step in the multiplexing process is to create a VC-4. This is done by octet-interleaving three TUG-3s,
and adding a higher-order Path Overhead. In general, the contents of the involved 3 TUG-3s may or may not be
the same type of signals (2 Mbit/s or 34 Mbit/s signals).
From this point on, the remaining multiplexing steps are similar to the ones discussed for a 140 Mbit/s signal.
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
Steps in multiplexing a 2 Mbit/s signal into the STM-1 frame
The diagram above shows the steps used to multiplex a 2 Mbit/s PDH signal into the STM-1 frame. Each of
those steps will be discussed separately next.
C-12
VC-12
TU-12
TU-12
VC-4
VC-12 Path Overhead
VC-12 Pointer
Container for2048 kbit/s signal
AU-4
AUG
STM-1 Frame
VC-4Path Overhead
VC-4 Pointer
SectionOverhead
3 x
TUG-2
TUG-3
7 x
3 x
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
Multiplexing a 2Mbit/s signal
The figure above shows an example of where in a PDH-SDH link containing a 2 Mbit/s signal the STM-1
overhead and pointers are processed. Path overhead is processed at the nodes where the respective virtual
containers are originated and terminated (originating and terminating synchronous multiplexers). The pointers
are processed at the synchronous multiplexers and DCCs (not shown), and the section overhead is processed at
both synchronous multiplexers and line repeaters.
C-12
VC-12
TU-12
TU-12
VC-4
VC-12 Path Overhead
VC-12 Pointer
Container for2048 kbit/s signal
AU-4
AUG
STM-1 Frame
VC-4Path Overhead
VC-4 Pointer
SectionOverhead
3 x
TUG-2
TUG-3
7 x
3 x
PDHDXS
64 kDXS
STM-1
STM-1
34 Mbit/s
2
1 , 2
FlexibleAccessSystem
FlexibleMux
DigitalSwitch(LE)
2M
1 , 2
2M link
1
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
C-12 Origination
Unlike 140 Mbit/s and 34 Mbit/s PDH signals, which are asynchronous in nature, 2 Mbit/s signals in a PDH
network may be either synchronous or asynchronous. The reason for this is that PDH network synchronization
is carried with 2048 kHz reference signals.
The advantages of 2 Mbit/s asynchronous multiplexing are an unrestricted payload, timing transparency, and
minimum mapping delay, while its main disadvantage is the inability to access 64 kbit/s timeslots within the 2
Mbit/s signal.
There are two ways to multiplex a synchronous 2 Mbit/s signal: bit-synchronous and octet-synchronous
multiplexing. Bit-synchronous multiplexing is typically used with unframed signals that are synchronized with
the SDH transport signal. Framed signals, however, can also be multiplexed with bit-synchronous mapping.
Byte-synchronous multiplexing provides timeslot visibility and is therefore appropriate where 64 kbit/s signals
are to be added, dropped, or cross-connected directly from an STM-1 signal.
C-12Container for2048 kbit/s signals
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
C-12
VC-12VC-12 Path Overhead
Container for2048 kbit/s signal
VC-12 POH: • • • • • •V5 J2 Z6 Z7
VC-12 Origination
The next step in the multiplexing process is to add overhead to the C-12 container just created. This overhead
consists of 4 octets, called V5, J2, Z6 and Z7. It is referred to as “lower-order Path Overhead (POH)”. These
octets are added at the beginning of the C-12 container. The C-12 along with the added octets is referred to as
Virtual Container, order 12, VC-12.
The first VC-12 Path Overhead octet called V5 will be described next.
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
Description of V5 byte
BIP-2 (Path error monitoring): Bits 1 and 2 are used for error monitoring using Bit Interleaved Parity (BIP).
Bit 1 is set such that parity of all odd-numbered bits (1, 3, 5 and 7) in all octets in the previous VC-12 is even.
Bit 2 is set similarly for the even-numbered bits (2, 4, 6 and 8). At the receiver, even parity is verified.
FEBE (far end block error): Bit 3 is a VC-12 Path Far End Block Error (FEBE) indication. This bit is set to
one and sent back to the VC-12 originator if one or more errors are detected by the BIP-2.
Remote Failure Indication: Bit 4 is a VC-12 path Remote Failure Indication (RFI). This bit is set to one if a
failure occurs. A failure is a Defect* that persists beyond the maximum time allocated to the protection
mechanism.
Signal Label: This three-bit code is used to indicate to the receiver whether a valid 2 Mbit/s signal is being
transported in the VC-12, and what type of multiplexing is being employed (asynchronous, bit or octet
synchronous).
FERF (Far End Receive Failure): This bit is set to one if the associated receiver in the SDH Path element
receives a TU-12 Path AIS or TU-12 Path LOP.
*In this context, Defect is understood as defined in ITU-T Recommendation G.826.
Bit Number
BIP-2 FEBE RFI SIGNAL LABEL
1 2 3 4
FERF
85 6 7
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
• • • • • •V5 J2 Z6 Z7
VC-12 Path Overhead
J2 octet is provisionally used to repetitively transmit an identifier signal so that a path-receiving terminal can
verify its continued connection to the intended transmitter. The E.164 numbering format is used. Similar to J1
octet in higher-order POH.
Z6 octet is used to provide a tandem-connection monitoring function. Similar to Z5 octet in the higher-order
POH.
Z7 octet is reserved for future use.
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
TU-12 Origination
The following alarms are used in association with 2 Mbit/s Virtual Containers:
The next step in the multiplexing process of a 2 Mbit/s PDH signal is to make provision for differences in
frequency and phase between VC-12s. This is achieved by adding a “VC-12 pointer” to the existing VC-12.
The VC-12, along with its pointer, is called “Tributary Unit, order 12,” or TU-12.
The VC-12 pointer is very similar to the VC-3 and VC-4 pointers in structure and function. As in the case of
VC-3 pointers, single-octet increments for VC-12 pointers are possible.
VC-12
TU-12VC-12 Pointer
SDH PathElement
VC-12 Path FEBE
BIP-2 errorsSDH Network
Element
TU-12 Path LOPor
TU-12 Path AIS
VC-12 Path FERF
TU-12 Path AIS
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
TU-12 Origination
There are two possible multiplexing modes of the TU-12 structure: Floating and Locked multiplexing modes.
Floating mode implies the use of the VC-12 pointer previously discussed to allow frequency and phase
alignment of VC-12s. The V5 octet is generated and used for the purposes previously described. Note that if
this terminology is used, then the VC-4 for a 140 Mbit/s and the VC-3 for a 34 Mbit/s PDH signals employ
Floating mode.
In contrast, Locked mode of a TU-12 implies that the VC-12 pointer is not used (in fact, the pointer is set to
zero), the VC-12 occupies a fixed location within the higher-order VC (VC-4), and no VC-12 POH is
generated. This mode of multiplexing is essentially equivalent to mapping N x 64 kbit/s signals directly into the
VC-4. This multiplexing mode was introduced as a potentially less expensive structure for implementing
subnetworks with 64 kbit/s flexibility as the need for pointer processing could be avoided.
VC-12
TU-12VC-12 Pointer
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Framing Structure Overview - Multiplexing a 2 Mbit/s Signal
TUG-2 and TUG-3 Origination
The next step in the multiplexing process is to generate a Tributary Unit Group of order 2 (TUG-2) by octet
interleaving three TU-12s. For this, no pointer processing occurs or extra overhead is added. One reason for
having this step in the multiplexing process is to allow an STM-1 signal to carry any of the first-level PDH
signals (European 2 Mbit/s or American 1.544 Mbit/s), and also to be compatible with second-level PDH
signals.
The following step in the multiplexing process is to generate a Tributary Unit Group of order 3 (TUG-3) by
octet interleaving seven TUG-2s. Again, here no pointer processing occurs or extra overhead is added. From
this point on, subsequent multiplexing of a 2 Mbit/s signal into an STM-1 signal is similar to the multiplexing of
a 34 Mbit/s PDH signal.
TU-123 x
TUG-2
TUG-3
7 x
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Framing Structure Overview - Multiplexing Other Signals
B-ISDN in a typical Regional SDH Implementation
Payloads that require more than one C-4 can also be transported at the STM-1 level. This is done by
concatenating AU-4s. Sets of X contiguous AU-4s may have their payloads locked together by setting the
pointer value in all but the leading AU-4 to a specific state known as the Concatenator Indicator (CI). Pointer
adjustments indicated for the leading AU-4 are then replicated in all the concatenated AU-4s in the set,
maintaining bit sequence integrity over the whole broadband payload. Such a set of concatenated AU-4s is
designated AU-40Xc.
An STM-1 signal can transport not only synchronous signals (such as 2 Mbit/s synchronous) and
plesiochronous signals (most PDH signals), but can also transport asynchronous signals. An example of such
asynchronous signals that is likely to become common in the future is B-ISDN based upon Asynchronous
Transfer Mode (ATM).
PDHDXS
PDHand
B-ISDN
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Framing Structure Overview - Higher Order SDH Signals
PDHDXS
STM-4
Use of an STM-4 link in a typical Regional SDH Implementation
Network growth and the demand for broadband services are leading to very high-rate optical transmission
systems at 622 Mbit/s, 2488 Mbit/s, and possibly beyond. These should ultimately replace 140 Mbit/s and 565
Mbit/s systems. Consequently, there is a requirement for synchronous transport modules operating at rates
higher than 155 Mbit/s.
These higher-order synchronous transport modules can be assembled by further multiplexing. At each stage,
four tributaries are combined by extracting the payload from each, recalculating their pointer values, then phase
aligning and octet interleaving them and finally adding a new section overhead.
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Framing Structure Overview - Higher-Order SDH Signals
Byte interleaving STM-1s to form STM-4 and STM-16
The resulting digit rates are 4n x 155.52 Mbit/s. STM-N is the generic term for these higher-rate modules. For
example, STM-4 is at 622.08 Mbit/s. STM-16 is at 2488.32 Mbit/s and can carry 16 times the payload of an
STM-1. The resulting hierarchy is extendible to arbitrarily-high bit rates.
The multiplexing explained before needs no addition of information, because the 155.52 Mbit/s signals are bit
and frame synchronized. This allows the use of the Frame Alignment Signal (FAS) from the lower-order level
to align at the higher level.
1234
4 3 2 1 . . .
5678
8 7 6 5 . . .
9101112
13141516
12 11 10 9 . . .
16 15 14 13 . . .
STM-4s
STM-1s . . . 11
7
3 14 10 6 2 13 9 5 1 . . .
STM-16
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Framing Structure Overview - Framing Synchronization Strategy
In-FrameState (IF)
OOF LOF
A B
. . .
A: 5 consecutive frames with FAS errors occurB: 3 mseconds without detection of two consecutive non-FAS errored frames
For STM-N; N=1,4,16 signals, the framing synchronization strategy for a network element is as follows:
a) Frame synchronization is declared when two consecutive frames without FAS errors are received. This
condition is called In-Frame (IF) state.
b) If In-Frame state has been achieved, when five consecutive frames with FAS errors are detected, the receiver
equipment declares Out-of-Frame (OOF) state.
c) When in Out-of-Frame state, if two consecutive frames without FAS errors are not received within 3
milliseconds, the network equipment declares Loss-of-Frame (LOF) state.
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Framing Structure Overview - Common STM-1 Representation
1 2 43 5 6 7 8 9
OVERHEAD
PAYLOAD
123456789
Common STM-N Frame Representation
The diagram above shows how an STM-1 frame is typically rearranged to display the information in a block-
like form. This representation is normally used because it is more compact than a serial bit stream
representation.
In this representation, the STM-N frame is represented as a block of 9 rows by 270 x N columns. The first 9 x
N octet columns of the STM-N contain overhead information, and the remaining columns contain path
overhead, virtual container pointers, and users information.
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J1
B3
C2
G1
F2
H4
Z3
Z4
Z5
Section Overhead
Section Overhead
PDH 140M orStructured SDH Signal
1
3
4
5
9
Regenerator
Multiplex
VC-4POH
9 x N 261 x N
270 x N columns (bytes)
9 rows
A1 A1 A1 A2 A2 A2 C1
B1 ∆ E1 F1
D1 D2 D3
B2 B2 B2 K1 K2
D4 D5 D6
D7 D8 D9
D10 D11 D12
Z1 Z1 Z1 Z2 Z2 Z2 E2
Bytes reserved for national use
Unscrambled bytes. Therefore care should be takenwith their content
Media dependent bytes
9 r
ow
s
RSOH
MSOH
9 bytes
AU-n pointers
∆
∆ ∆
∆
∆
∆
Framing Structure Overview - Common STM-1 Representation
Common STM-N Frame Representation
The figure above shows the block representation as used by the ITU-T. Note the location of the Section
Overhead in this representation. If the STM-1 signal carries a 140 Mbit/s signal, then the corresponding VC-4
POH is also part of the STM-1 signal as shown above.
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Framing Structure Overview - Common STM-1 Representation
If three 34 Mbit/s signals are carried within an STM-1 signal, then the ITU-T representation is as shown in the
following figure:
J1
B3
C2
G1
F2
H4
Z3
Z4
Z5
Container-3
85 columns
86 columns
TUG-3 = TU-3
VC-3
VC-3 POH
H1
H2
H3
Fixe
d st
uff
86 columns
VC-12
TUG-2 TUG-2
TUG-3(7 x TUG-2)
3 VC-12s
Fixe
d st
uff
NPI
POH
POH
TU-12PTRs
If 63 2 Mbit/s signals are carried within an STM-1 signal, then the corresponding representation is as shown in
the following figure.
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Differences Between SDH and SONET Technologies
Most of the important differences between SONET and SDH were influenced by the need of a transport system
able to support the main European PDH rates, mixed payloads, and the emerging broadband standards. Indeed,
SONET and SDH are compatible, but not identical, digital hierarchies.
Both hierarchies define similar sets of overheads and functions, however, there are differences in the usage of
the two overhead structures and pointer processing. Nevertheless, these differences are beyond the scope of this
document. For those differences, please see AT&T Communications Document “A Technical Report on A
Comparison of SDH and SONET.”
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Differences Between SDH and SONET Technologies (Continued)
The following are major differences between SONET and SDH:
1. The 51.84 Mbit/s Synchronous Transport Signal - Level 1 (STS-1) is the basic building block of SONET.
All lower-rate payloads are mapped into the STS-1, and all higher-rate signals are created by synchronously
multiplexing N STS-1s to form an STS-N. In contrast, the 155.52 Mbit/s Synchronous Transport Module -
Level 1 (STM-1) is the basic building block of SDH. All lower-rate payloads are mapped into the STM-1,
and all higher-rate signals are created by synchronously multiplexing N STM-1s to form an STM-N.
2. Eight different transmission rates have been defined for SONET: 51.84 Mbit/s, 155.52 Mbit/s, 466.56
Mbit/s, 622.08 Mbit/s, 933.12 Mbit/s, 1244.16 Mbit/s, 1866.24 Mbit/s and 2488.32 Mbit/s. In contrast,
only three different transmission rates have been defined for SDH: 155.52 Mbit/s, 622.08 Mbit/s and
2488.32 Mbit/s.
3. Unlike SONET, SDH allows different mappings for the same payload. All of the SDH payloads which can
be mapped into an AU-3 can also be mapped into an AU-4. SONET provides only one choice for the
defined payload mappings.
4. SONET and SDH differ in some of their payload mappings. See table below (compatible SONET/SDH
appear in brackets).
SONET/SDH Payload Mappings
Payload STS-1 STS-3c AU3 Based STM-1 AU4 Based STM-1
DS1E1DS1CDS2E3DS3E4ATMATMFDDIDQDB
1.5 Mbit/s2.048 Mbit/s3.152 Mbit/s6.312 Mbit/s
34.368 Mbit/s44.736 Mbit/s
139.264 Mbit/s149.760 Mbit/s599.040 Mbit/s125.000 Mbit/s149.760 Mbit/s
(VT1.5)(VT2)VT 3
(VT 6)None
(STS-1 SPE)NoneNoneNoneNoneNone
NoneNoneNoneNoneNoneNone
(STS-3c SPE)(STS-3c SPE)
VC4-4c(STS-3c SPE)(STS-3c SPE)
(VC11) or VC12*(VC12)None(VC2)VC3
(VC3)NoneNoneNoneNoneNone
VC11 or VC12*VC12NoneVC2VC3VC3
(VC4)(VC4)VC4
(VC4)(VC4)
( ) Compatible SONET/SDH mappings.* In SDH, a DS1 may be carried in a VC12 (2 Mbps)
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SDH Abbreviations
ACSE Association control service elementADM Add/Drop MultiplexerAITS Acknowledged information transfer serviceAIS Alarm indication signalALS Automatic laser shutdownANSI American National Standards InstituteAP Access PointAPDU Application protocol data unitASE Application service elementAPS Automatic protection switchingASN.1 Abstract syntax notation oneATM Asynchronous transfer modeAU Administrative unitAU-n Administrative unit of order nAUG Administrative unit groupBBER Background block error ratioBER Binary error rateBIP Bit interleaved parityBIP-8 Bit interleaved parity of order 8BIP-X Bit interleaved parity-XBITS Building integrated timing supplyB-ISDN Broadband integrated services digital networkC ContainerC-n Container of order nCAS Channel associated signalingCC Connect confirmCCITT The International Telegraph and Telephone Consultative CommitteeCCS Common channel signalingCEPT Committee European de Post et TelegraphCI Concatenation indicatorCLNP Connectionless network layer protocolCLNS Connectionless network layer serviceCM Connection matrixCMI Coded mark inversionCMIP Common management information protocolCMISE Common management information service elementCONP Connection oriented network-layer protocolCP Connection pointCR Connection requestCV Code violationDCC Data communication channelDCN Data communication networkDCS Digital crossconnect systemsDDF Digital distribution frameDIN Deutsche Industrie NormenausschussDPRing Dedicated protection ringDQDB Distributed queue dual bus protocolDS Degraded SecondDXC Digital crossconnectEA External access equipment
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EBU European Broadcasting UnionECC Embedded control channelEEC European Economic CommunityEOW Engineering order-wireES Errored secondESR Errored second ratioETSI European Telecommunications Standards InstituteEX Extinction ratioFAL Frame alignment lossFAS Flexible Access System, British Telecom’s fractional 2 Mbit/s service based upon ISDNFAW Frame alignment wordFBS Flexible Bandwidth System, another name for the Flexible Access SystemFDDI Fiber digital data interfaceFDM Frequency Division MultiplexingFEBE Far end block errorFERF Far end receiver failureFLS Frame loss secondFU Functional unitGNE Gateway network elementHDTV High Definition TelevisionHEC Header error controlHOP High order pathHPA Higher order path adaptationHPC Higher order path connectionHPT Higher order path terminationHVC Higher order virtual containerI/D Increment/decrementIDN Integrated Digital NetworkIEC International exchange carriersIEE British Institution of Electrical EngineersIEEE North American Institute of Electrical and Electronics EngineersIFU Interworking functional unitIP Interworking protocolIS Intermediate systemISDN Integrated services digital networkISO International Standards OrganizationITU-T The ITU Telecommunication Standardization SectorKILOSTREAM British Telecom’s 64 kbit/s service based upon X.25 circuitsKILOSTREAM+ British Telecom’s fractional 2 Mbit/s service based upon X.25 circuitsLAN Local Area NetworkLCN Local communications networkLEC Local Exchange CarriersLED Light emitting diodeLO Lower orderLOF Loss of frameLOM Loss of multiframeLOP Loss of PointerLOP Low order pathLOS Loss of signalLPA Lower order path adaptationLPC Lower order path connectionLPT Lower order path termination
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LVC Lower order virtual containersMAF Management applications functionMAN Metropolitan Area NetworkMCF Message communication functionMD Mediation deviceMEGASTREAM British Telecom’s 2 Mbit/s serviceMF Mediation functionMLM Multi-longitudinal modeMO Managed objectMOC Managed object classMRTIE Maximum relative time interval errorMS Multiplex sectionMS-AIS Multiplex section alarm indication signalMS-FERF Multiplex section far end receive failureMSOH Multiplex section overheadMSP Multiplex section protectionMSPG MS protection groupMST Multiplex section terminationMTG Multiplexer timing generatorMTIE Maximum time interval errorMTPI Multiplexer timing physical interfaceMTS Multiplexer timing sourceNA Not applicableNDF New data flagNE Network elementNEF Network element functionN-ISDN Narrowband integrated services digital networkNLR Network layer relayNNE Non-SDH network elementNNI Network node interfaceNOMC Network operators maintenance channelNPI Null Pointer indicationNPDU Network protocol data unitNRZ Nonreturn to zeroNSAP Network service access pointNU National useOAM&P Operations, administration, maintenance and provisioningOFS Out-of-frame secondOHA Overhead accessOOF Out of frameORL Optical return lossOS Operations systemOSF Operations system functionOSI Open systems interconnectionPDH Plesiochronous digital hierarchyPDU Protocol data unitPI Physical interfacePJE Pointer justification eventPJC Pointer justification countPOH Path overheadPPDU Presentation protocol data unitPS Protection Switch
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54 SDH FUNDAMENTALS (0039.p65) • v1.0 • 11/94
PSN Packet switched networkPSTN Public switched telephone networkPJE Pointer justification eventPOH Path overHeadPSTN Public Switched Telephone NetworkPTR PointerRACE Research into advanced communications for Europe programmeRMS Root-mean-squareROSE Remote operations service elementRS Regenerator sectionRSOH Regenerator section overheadRST Regenerator section terminationRZ return to zeroSA Section adaptationSAPI Service access point identifierSD Signal degradeSDCN SDH data communication networkSDH Synchronous digital hierarchySEMF Synchronous equipment management functionSES Severely errored secondSF Signal failSLM Single-longitudinal modeSM Synchronous multiplexerSMN SDH management networkSMS SDH management sub-networkSNDCF Sub-network dependent convergence functionSOH Section overheadSONET Synchronous optical networkSPDU Session protocol data unitSPI SDH physical interfaceSTM-N Synchronous transport module at level NSVC Switched virtual circuitTEI Terminal end-point identifierTMN Telecommunications management networkTPDU Transport protocol data unitTR TributaryTSAP Transport service access pointTU Tributary unitTU-n Tributary unit of order nTUG-n Tributary unit group of order nUAS Unavailable secondUAT UnAvailable timeUI Unit intervalUI Unnumbered informationUITS Unacknowledged information transfer serviceVC Virtual containerVC-n Virtual container of order nVC-n-Xc X time concatenated VC-n (n=2 or 4)VTG Virtual tributary groupWDM Wavelength-division multiplexing