SDH Terms and Acronyms
SDH Terms
*** from ITU Recommendation G.707/Y1322
Synchronous Digital Hierarchy (SDH): The SDH is a hierarchical set of digital transport structures, standardized for the transport of suitably adapted payloads over physical transmission networks.
Synchronous Transport Module (STM): An STM is the information structure used to support section layer connections in the SDH. It consists of information payload and Section Overhead (SOH) information fields organized in a block frame structure which repeats every 125 ms. The information is suitably conditioned for serial transmission on the selected media at a rate which is synchronized to the network. A basic STM is defined at 155 520 kbit/s. This is termed STM-1. Higher capacity STMs are formed at rates equivalent to N times this basic rate. STM capacities for N=4, N=16, N=64 and N=256 are defined; higher values are under consideration.
*** STM-0 comprises a single administrative unit of level 3. The STM-N, N ³ 1, comprises a single administrative unit group of level N (AUG-N) together with the SOH. The STM-N hierarchical bit rates are given in 6.3.
Virtual Container-n (VC-n): A virtual container is the information structure used to support path layer connections in the SDH. It consists of information payload and Path Overhead (POH) information fields organized in a block frame structure which repeats every 125 or 500 ms. Alignment information to identify VC-n frame start is provided by the server network layer.
Two types of virtual containers have been identified.
– Lower order virtual container-n: VC-n (n=1, 2, 3)
This element comprises a single container-n (n=1, 2, 3) plus the lower order virtual container POH appropriate to that level.
– Higher order virtual container-n: VC-n (n=3, 4)
This element comprises either a single container-n (n=3, 4) or an assembly of Tributary Unit Groups (TUG-2s or TUG-3s), together with virtual container POH appropriate to that level.
Administrative Unit-n (AU-n): An administrative unit is the information structure which provides adaptation between the higher order path layer and the multiplex section layer. It consists of an information payload (the higher order virtual container) and an administrative unit pointer which indicates the offset of the payload frame start relative to the multiplex section frame start.
Two administrative units are defined. The AU-4 consists of a VC-4 plus an administrative unit pointer which indicates the phase alignment of the VC-4 with respect to the STM-N frame. The AU‑3 consists of a VC-3 plus an administrative unit pointer which indicates the phase alignment of the VC-3 with respect to the STM-N frame. In each case, the administrative unit pointer location is fixed with respect to the STM-N frame.
One or more administrative units occupying fixed, defined positions in an STM payload are termed an Administrative Unit Group (AUG).
An AUG-1 consists of a homogeneous assembly of AU-3s or an AU-4.
Tributary Unit-n (TU-n): A tributary unit is an information structure which provides adaptation between the lower order path layer and the higher order path layer. It consists of an information payload (the lower order virtual container) and a tributary unit pointer which indicates the offset of the payload frame start relative to the higher order virtual container frame start.
The TU-n (n=1, 2, 3) consists of a VC-n together with a tributary unit pointer.
One or more tributary units, occupying fixed, defined positions in a higher order VC-n payload is termed a Tributary Unit Group (TUG). TUGs are defined in such a way that mixed capacity payloads made up of different size tributary units can be constructed to increase flexibility of the transport network.
A TUG-2 consists of a homogeneous assembly of identical TU-1s or a TU-2.
A TUG-3 consists of a homogeneous assembly of TUG-2s or a TU-3.
Container-n (n=1-4): A container is the information structure which forms the network synchronous information payload for a virtual container. For each of the defined virtual containers there is a corresponding container. Adaptation functions have been defined for many common network rates into a limited number of standard containers. These include those rates already defined in ITU-T Rec. G.702. Further adaptation functions will be defined in the future for new broadband rates.
Network Node Interface (NNI): The interface at a network node which is used to interconnect with another network node.
Figure 3-1 gives a possible network configuration to illustrate the location of NNI specified in this Recommendation.
Figure 3-1/G.707/Y.1322 – Location of the NNI
Pointer: An indicator whose value defines the frame offset of a virtual container with respect to the frame reference of the transport entity on which it is supported.
Concatenation: A procedure whereby a multiplicity of virtual containers is associated one with another with the result that their combined capacity can be used as a single container across which bit sequence integrity is maintained.
SDH mapping: A procedure by which tributaries are adapted into virtual containers at the boundary of an SDH network.
SDH multiplexing: A procedure by which multiple lower order path layer signals are adapted into a higher order path or the multiple higher order path layer signals are adapted into a multiplex section.
SDH aligning: A procedure by which the frame offset information is incorporated into the tributary unit or the administrative unit when adapting to the frame reference of the supporting layer.
Bit Interleaved Parity-X (BIP-X): BIP-X code is defined as a method of error monitoring. With even parity an X-bit code is generated by the transmitting equipment over a specified portion of the signal in such a manner that the first bit of the code provides even parity over the first bit of all X-bit sequences in the covered portion of the signal, the second bit provides even parity over the second bit of all X-bit sequences within the specified portion, etc. Even parity is generated by setting the BIP-X bits so that there is an even number of 1s in each monitored partition of the signal. A monitored partition comprises all bits which are in the same bit position within the X-bit sequences in the covered portion of the signal. The covered portion includes the BIP-X.
Concatenation: The process of summing the bandwidth of a number of smaller containers into a larger bandwidth container. Two versions exist:
– Contiguous concatenation;
– Virtual concatenation.
Shortened Binary-BCH: A shortened version of the class of the block linear cyclic codes. These shortened binary BCH codes have the following common properties, i.e.:
n = 2m – 1 – s
k = n – t ´ m
d = 2 ´ t + 1
where:
n the size of the whole code word;
k the number of the information bits;
m the parameter of the BCH code;
t the number of the corrected errors within the block of the BCH code;
d the minimum code distance;
s the amount of information eliminated as part of the code shorting.
Generator Polynomial: The polynomial that is used for encoding of any cyclic codes. The remainder after division of the information polynomial by generating polynomial is the redundancy part of the encoded code word.
Systematic Code: The original data bits for binary codes are unchanged by the encoding procedure. Redundant bits or symbols (parity) are added separately to each code block.
SDH Acronyms
*** from ITU Recommendation G.707/Y1322
AIS Alarm Indication Signal
API Access Point Identifier
APS Automatic Protection Switching
ATM Asynchronous Transfer Mode
AU-n Administrative Unit-n
AUG-N Administrative Unit Group-N
BCH Bose-Chaudhuri-Hocquenghem
BCH-3 Triple error correcting BCH code
BER Bit Error Ratio
BIP-X Bit Interleaved Parity-X
C-n Container-n
CAS Channel Associated Signalling
CRC-N Cyclic Redundancy Check, width N
CTRL Control word sent from source to sink
DCC Data Communication Channel
DQDB Distributed Queue Dual Bus
DNU Do Not Use
EOS End of Sequence
FDDI Fibre Distributed Data Interface
FEBE Far End Block Error (renamed as REI)
FEC Forward Error Correction
FERF Far End Receive Failure (renamed as RDI)
FSI FEC Status Indication
GID Group Identification
HDLC High-level Data Link Control
HEC Header Error Control
HOVC Higher Order Virtual Container
IEC Incoming Error Count
IP Internet Protocol
ISF Incoming Signal Failure
ISDN Integrated Services Digital Network
ISID Idle Signal Identification
LAPD Link Access Protocol for D-channel
LAPS Link Access Procedure – SDH
LCAS Link Capacity Adjustment Scheme
LCD Loss of Cell Delineation
LOP Loss of Pointer
LOVC Low Order Virtual Container
LSB Least Significant Bit
MAN Metropolitan Area Network
MFI MultiFrame Indicator
MSB Most Significant Bit
MS-AIS Multiplex Section Alarm Indication Signal
MSF-AIS Multiplex Section FEC Alarm Indication Signal
MSOH Multiplex Section Overhead
MS-RDI Multiplex Section Remote Defect Indication
MS-REI Multiplex Section Remote Error Indication
MST Member Status
MSTE Multiplex Section Terminating Element
NDF New Data Flag
NNI Network Node Interface
NORM Normal Operating Mode
ODI Outgoing Defect Indication
ODUk Optical channel Data Unit-k
OEI Outgoing Error Indication
OH Overhead
OPUk Optical Payload Unit-k
OTN Optical Transport Network
OTUk Optical channel Transport Unit-k
PDH Plesiochronous Digital Hierarchy
PLM Payload Mismatch
POH Path Overhead
PPP Point-to-Point Protocol
PTE Path Terminating Element
PTR Pointer
RDI Remote Defect Indication (former FERF)
REI Remote Error Indication (former FEBE)
RFI Remote Failure Indication
RS-Ack Re-sequence Acknowledge
RSOH Regenerator Section Overhead
SDH Synchronous Digital Hierarchy
SLM Signal Label Mismatch
SOH Section Overhead
SQ Sequence Indicator
SSU Synchronization Supply Unit
STM(-N) Synchronous Transport Module (-N)
TCM Tandem Connection Monitoring
TC-RDI Tandem Connection Remote Defect Indication
TC-REI Tandem Connection Remote Error Indication
TCOH Tandem Connection Overhead
TCT Tandem Connection Trace
TCTE Tandem Connection Terminating Element
TIM Trace Identifier Mismatch
TSID Test Signal Identification
TTI Trail Trace Identifier
TU-n Tributary Unit-n
TUG(-n) Tributary Unit Group (-n)
UNEQ UNEQuipped
VC-n Virtual Container-n
VC-n-X X concatenated Virtual Container-ns
VC-n-Xc X Contiguously concatenated VC-ns
VC-n-Xv X Virtually concatenated VC-ns
VCG Virtual Concatenation Group
WAN Wide Area Network