is identical)
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Layered Networks
SDH is designed to support the exchange of information between access points in a synchronous transport network.
A synchronous transport network is a multi-layered transmission network, each layer of which may be accessed via a specific reference point known as an access point. An access point does not equate to an input or output port.
Information exchange between two or more access points in the same layer must be of the same type. A connection between any two access points is referred to as a ‘trail’.
Synchronous Digital Hierarchy (SDH)
Layer 2
Layer 1
To transport media Lower
layer (server)
Higher layer (client)
applications applications
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Connections Between Transport Layers
A single physical link connection in a lower layer of a multi-layered network may be used to support a number of logical link connections in the layer above. The logical connections in the higher layer may, in turn, be used to support a number of further logical connections resident in the layer above, and so on.
Note that the higher layers are closer to the end user, while the lower layers are closer to the network transmission medium. A lower layer directly supporting a higher layer is called the server layer, while the higher layer is referred to as the client layer.
The format of the transport signal within a specific layer can be referred to as the ‘characteristic information’ of the layer.
ITU-T
Jump MUX (or Skip MUX) (2–34 Mbit/s) – May include OLTE
Aggregate
First Order Layer 2.048 Mbit/s ( ±50 parts/million)
Circuit Layer
Second Order Layer 8.448 Mbit/s
Third Order Layer 34.368 Mbit/s
Physical Media Layer G.703 Physical Interface
Physical Interface
Optical Line Terminating Equipment Primary
Fourth Order Layer 139.264 Mbit/s 4th Order
Multiplexer
Input(s)
Output
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PDH Layered Network
A PDH network is a multi-layered transport network. Circuit layer signals are multiplexed via each PDH layer into a 140 Mbit/s signal for transport over an optical fibre system. In this instance, circuit layer information refers to voice or data circuits adapted for transport in a 64 kbit/s basic rate channel.
The PDH network elements provide primary and higher order multiplexing, whilst the OLTE (Optical Line Terminating Equipment) is an interface onto the (optical fibre) physical medium.
The boundaries between the various layers of the PDH network are defined by the physical, input and output ports of the PDH network elements.
In the European PDH system, higher order multiplexers (8, 34 and 140 Mbit/s) support four input tributaries. Primary layer multiplexers support 30 input channels. In either case, individual tributary ports are not shown in the diagram as separate entities. However, a single physical interface represents the total number of real physical input ports that may be connected at the appropriate layer, or a single output port.
Jump multiplexers have become popular in PDH systems. To the user, a number of lower-rate signals are, apparently, directly multiplexed onto a higher-rate aggregate one or two layers above the lower layer. For example, a sixteen by two (16 x 2) Jump multiplexer combines 16 x 2 Mbit/s input signals onto a 34 Mbit/s aggregate. In this case, there is no access point to second-order signals. However, the process of multiplexing from 2 to 8 Mbit/s still takes place as an internal function.
Synchronous Digital Hierarchy (SDH)
Layer
Note: All processes can be combined in one
Network Element, e.g. ADD and DROP MUX VC
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SDH Network Layers
In the PDH system, the characteristic information in each layer is generated by a multiplexer of the required order.
In SDH, the characteristic information in each layer is generated by a FP (Functional Process). In SDH, characteristic information is a VC of the required order, or an STM.
The physical media layer of an SDH network consists of a range of standardized optical or electrical NNIs. The TCPs (Termination Connection Points) of SDH networks may be considered to have the same functionality as PDH inter-layer physical interfaces.
PDH
NNI Standard Interfaces
Optical 1300 or 1550 nm – STM 1 / 4 / 16 (G.957)
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NNIs (Network Node Interfaces)
An SDH NE (Network Element) may be used entirely within an SDH network or as an access node supporting PDH interfaces. In the first case the network element supports all SDH physical interfaces, where each interface is a standardized NNI (SM3, for example). In the second case the SDH NE supports a mixture of SDH and PDH interfaces (SM2, for example). In all cases an SDH NE must support at least one SPI (SDH Physical Interface) at either STM-1, STM-4 or STM-16.
The interface on the radio path is proprietary.
Where PDH interfaces are required, the PDH signal becomes the circuit layer information.
Standardization of physical interfaces and transport structure ensures a high degree of cross vendor compatibility (the use of some overhead bytes are not fully standardized).
SDH NEs may be classified as SMs (Synchronous Multiplexers), digital cross connects, regenerators or SDH radios.
All NEs except SDH radios can be implemented using a standard set of functional processes.
Synchronous Digital Hierarchy (SDH)
FP
VC-12 VC-3 VC-4
Client
Server
TCP
FP
– Connection Point
– Termination Connection Point – Functional Processes
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SDH Functional Processes
NEs within the SDH must support functional processes in each network layer. Such functions replace those carried out in the PDH by multiplexers and termination equipment.
Examples of primary SDH functional processes that act directly upon circuit layer payloads or SDH transport structures include adaptation, termination and SNC (Sub-Network Connection).
Before addressing these functional processes it is necessary to review the terms ‘client layer’ and ‘server layer’.
Client Layers and Server Layers
In general terms, the functional processes of a server layer are bidirectional in that they operate on the characteristic information of the client layer to generate the characteristic information of the server layer.
The relationship between the characteristic information of server and client layers is best described in the following example.
In this example, the LOP (Low Order Path) functional processes of an SDH NE operate on a 2 Mbit/s PDH circuit layer payload in order to generate the VC (Virtual Container)-12 LOP transport structure. This process is bidirectional and in the receive direction recovers a 2 Mbit/s PDH circuit layer payload from a VC-12.
The diagram also shows the LOP functional process of an SDH NE operating upon the 34 Mbit/s PDH payload, which generates SDH transport structure VC-3.
In this example, the LOP layer is shown as a server layer for the client (circuit) layer. This server layer will be the client layer for the next functional process, namely the HOP (High Order Path) layer.
The diagram also shows the HOP functional process of an SDH NE operating upon the 140 Mbit/s client layer PDH payload ,which generates the HOP SDH transport structure VC-4. The HOP layer is acting as the server layer for the client layer signal.
Low Order Layer High Order Layer
Adaptation
Termination
Sub-Network Connection
(SNC) m
n Low Order Layer
High Order Layer Section Layers Low Order Layer High Order Layer Section Layer
Bidirectional Circuit Layer Information
Client
TCP
Access Point
Server
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SDH Functional Processes (continued) Adaptation Function
An adaptation function provides the conversion process between a server and a client layer, with one or more of the following processes being present: bit rate adaptation; encoding/decoding; scrambling/
descrambling; frame alignment; pointer generation and interpretation; multiplexing/demultiplexing;
payload identification/composition (signal label); frequency justification, and timing.
Trail Termination Function
The trail termination function provides for the signal integrity supervision of the layer. To this end in the source direction it adds some or all of the error detection code, i.e. BIP (Bit Interleaved Parity), CRC (Cyclic Redundancy Check) and trail trace identifier, i.e. source address. It conveys back the REI (Remote Error Indication) and RDI (Remote Defect Indicator).
In the sink direction it monitors for some or all of the following: bit errors, FEND (Far-end) and NEND (Near-end) performance, misconnection, SSF (Server Signal Fall) and signal loss.
In summary, a trail termination function processes either path layer or section layer overheads.
SNC (Sub-Network Connection)
SNC processes are implemented by specific NEs in either the LOP or HOP layers of an SDH network.
An SNC functional process carried out in the LOP layer of a network is referred to as a LPC (LOP Connection) and an SNC carried out in the HOP layer is referred to as a HPC (HOP Connection). LPC and HPC functional processes can both be modelled as a switching matrix supporting m inputs and n outputs.
Synchronous Digital Hierarchy (SDH)
Low Order Path ‘Trail’
LPA
LPT
LPA
LPT C-12
VC-12
LPOH LPOH Low Order Path (Virtual) Connection
LPT LPT
Low Order Path Layer
LPOH
– Access (Trail Termination Point) – Termination Connection Point – Low Order Path Overhead
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SDH Functional Processes (continued) LOP Functional Processes
The primary functional processes can be explained in greater detail by taking the case of a 2 Mbit/s PDH circuit and seeing how it is affected by the functional processes at the LOP, HOP and section layers of an SDH network.
The first process is LPA (Low Order Path Adaptation), a bidirectional process that operates on each 2 Mbit/s circuit layer payload to form an inter-layer transport structure known as a Container. Here, this container is referred to as a C-12. The LPA function is one of rate adaptation: the synchronizing and desynchronizing of circuit layer payloads. This process usually, but not necessarily, implies the use of bit justification techniques.
The next functional process in the LOP layer is LPT (Low Order Path Termination). The termination process is responsible for attaching path or section overhead data to the intermediate transport structures, or C-12. Section and Path overheads are used to provide local and far end error detection and far end receive failure indications in addition to a number of other OAM facilities. Termination (sink) functions extract the overhead data from the transport structures and validate the integrity of the appropriate (trail) connections. In this example of LPT, the path overhead is added to the Container (C-12) to form a Virtual Container (VC-(C-12). In this case, the LOP overhead is a single byte (V5 byte) and is transmitted in each VC-12 structure. The V5 byte contains sufficient information to validate the LOP connection between any two NEs acting as access nodes in an SDH network.
High Order Path ‘Trail’
HPA
HPT
HPA
HPT
VC-4
HPOH HPOH High Order Path (Virtual) Connection
HPOH
– Access (Trail Termination Point) – Termination Connection Point – High Order Path Overhead
VC-12(s)
Multiplexing and Pointer Processing
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SDH Functional Processes (continued) HOP Functional Processes
Like Low Order Path Adaptation, HPA (High Order Path Adaptation) is a bidirectional functional process that operates on LVC (Lower Order Virtual Containers) and VC-4 (HVC (High Order Virtual Containers)) transport structures. The HPA process can be seen in the diagram, as a multiplexing and demultiplexing function.
The bidirectional process operates on the LVC structure (VC-12) by allocating logical bandwidth in the VC-4 for each LVC. The bandwidth allocated to each order of the LVC is called a TU (Tributary Unit) or TU-n, where a TU-n is simply an integer number of byte columns in the VC-4, or a subdivision of the HVC. Therefore, in this example the tributary unit formed from the VC-12 is referred to as TU-12. The TU-12 undergoes two multiplexing functions before insertion in the VC-4. The HPA function also includes TU-n Pointer Processing.
The next high order process is HPT (High Order Path Termination). The only significant difference between the LPT process and HPT process is the number of bytes allocated for the path overhead which is attached to the product of HPA, the 4. Here, nine bytes of path overhead are allocated to each VC-4 structure within an STM-N signal. The fundamental reason for the path overhead added in HPT is to validate high order path connections within an SDH network and, like LPT, to support various OAM functions within the HOP layer.
Synchronous Digital Hierarchy (SDH)
SA
RST MST Multiplexer
Section Layer
Regenerator Section
Layer
RSOH
MSOH MSOH
RSOH
STM-N
STM-N Pointers AUG
Pointers AUG Multiplex
and Pointer Processing
MSOH
VC-4(s)
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SDH Functional Processes (continued) Section Functional Processes
SA (Section Adaptation) is a bidirectional process that operates on VC-4 (product of HPT) and STM-N transport structures.
The diagram shows the SA process operating on a single VC-4 by mapping the HVC into the AUG (Administrative Unit Group) of an STM-N frame structured signal. The SA process can combine mapping and multiplexing functions to support the loading of VC-4 high order path transport structures into an STM-N frame-structured signal. For example, four VC-4 structures will be loaded into an STM-4. The mapping process generates an AU pointer for each VC-4 mapped into the STM-N. The combination of the AU pointer and the VC-4 is now designated as an AU-4.
SOH data is provided to validate the integrity of virtual network connections in the multiplexer and regenerator layers of an SDH network as well as supporting various OAM functions of network elements operating within the two layers. The diagram shows the two functional processes that support the generation and processing of SOH data, namely MST (Multiplexer Section Termination) and RST (Regenerator Section Termination). The product of these termination processes, RSOH (Regenerator Section Overhead) and MSOH (Multiplexer Section Overhead), are contained within a three-row, 9 x N column area and five-row, 9 x N column area respectively of the STM-N signal.
LPA
VC-3
RST High Order
Path Layer
Section Layer Low Order Path Layer
LPT
VC-4
SA
MST HPA
HPT
(within STM-1) 34 Mbit/s
AUG MSOH
AUG C-3
AUG
MSOH RSOH
Synchronous Physical Interface SPI
TY2702/v3.1 © Wray Castle Limited 4.15
Further Examples of Functional Processes
Starting points to the primary functional processes include the 1.5 Mbit/s, 34 Mbit/s, 45 Mbit/s and 140 Mbit/s in addition to ETSI (European Telecommunications Standards Institute)-designated non-hierarchical signals.
The diagram takes the 34 Mbit/s PDH signal through the various functional processes.
Synchronous Digital Hierarchy (SDH)
LOP Layer
STM-1
STM-1 SPI SPI
63 LPC 63
LPA/T
63 VC-12
West East
HPA/T
SA/T
LPA/T HPA/T SA/T SPI
Lower Order Path Adaption and Termination Higher Order Path Adaption and Termination Section Adaption and Termination Synchronous Physical Interface
TY2702/v3.1
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LOP Connection
SNC functional processes can be broken down into LPC and HPC. The LPC process may operate on VC-12, VC-2 or VC-3 SDH transport structures, whilst the HPC function operates on connections at the VC-4 level only.
The LPC process can be seen in the diagram, which represents an STM-1 ADM (Add and Drop Multiplexer). Here, any individual VC-12 can be extracted from either of the two STM-1 signals, terminated, adapted and delivered to one of 63 2-Mbit/s ports.
Alternatively, the VC-12 can be extracted from one STM-1 signal and connected to the opposite STM-1 signal.
HOP Layer
STM-4
STM-4 SPI SPI
4 HPC 4
HPA/T
4(8) VC-4
West East
SA/T
HPA/T SA/T SPI
Higher Order Path Adaption and Termination Section Adaption and Termination Synchronous Physical Interface
LPC VC-12
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HOP Connection
The diagram illustrates how an STM-4 ADM can operate on the four VC-4 SDH transport structures which can be extracted from each of the two STM-4 ports.
The level of connectivity provided by the HPC functional process is, in this example, almost identical to that provided by the LPC functional process. However, if the HPC switching matrix is configured as a non-blocking time-space-time switch supporting the equivalent of eight VC-4 structured signals, then the ADM could access any low order VC (VC-12, VC-2 or VC-3) from either of the two STM-4 ports.
Synchronous Digital Hierarchy (SDH)
VC-4 C-4 140 Mbit/s PDH Structures
TUG-3 TU-3 VC-3 C-3 34/45 Mbit/s
TU-12 VC-12 C-12 2.0 Mbit/s
VC-11 C-11 1.5 Mbit/s
Designated by ETSI for
‘Non-Hierarchical’ Signals
The structure is bidirectional A multiplexer would be configured to support only one input option AUG Administrative Unit Group C Container
STM Synchronous Transport Module
TU Tributary Unit TUG Tributary Unit Group VC Virtual Container Note:
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Multiplexing Structures The ETSI Multiplexing Scheme
The structure of the ETSI multiplexing scheme is shown in the diagram. POH (Path Overhead) payloads are shown on the right hand side of the diagram and may be considered to represent single circuit layer traffic connections. Each 2 Mbit/s traffic stream connected to an SDH multiplexer will generate a C-12, VC-12 and TU-12.
Where a transport structure in the ETSI scheme is shown with more than one input, for example the VC-4, the multiplexer will be configured to support only one of the designated input options. For example, the VC-4 may be loaded with either a single C-4 structure (140 Mbit/s payload) or three TUG (Tributary Unit Group)-3 payloads, but not both simultaneously.
The ETSI map indicates that a TUG-3 can be assigned to carry seven TUG-2 structures (or a single TU-3) and each TUG-2 structure can in turn support three TU-12 intermediate payloads or a single TU-2.
STM-1 STM-1 ADM
PDH Circuit Layer Traffic
STM-1 Payload Options (examples):
63 x VC-12 (63 x 2 Mbit/s)
42 x VC-12 and 1 x VC-3 (42 x 2 Mbit/s and 1 x 34(45) Mbit/s) 21 x VC-12 and 2 x VC-3 (21 x 2 Mbit/s and 2 x 34(45) Mbit/s) 3 x VC-3 (3 x 34(45) Mbit/s)
1. The options shown are for the MAXIMUM payloads of the STM-1s.
2. Other PDH payload options are valid.
3. VC-2 may be used for ‘non-hierarchical payloads’ (ETSI).
The VC-2 structure is used in SONET to support 6 Mbit/s PDH traffic.
Notes:
West
Drop and Insert
East Through Traffic
Drop and Insert
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Payload Options
Certain NEs can support a mixture of PDH payloads subject to the rules of the ETSI multiplexing scheme. The payload areas of the STM-1 transport structures could be configured to carry various combinations of adapted PDH payloads derived from the circuit layer tributary ports of the ADM or passed-through between the two NNIs.
The maximum payload of either of the two STM-1 signals could be made up of one of the following combinations:
63 x VC-12
(42 x VC-12) + (1 x VC-3)
(21 x VC-12) + (2 x VC-3)
3 x VC-3
Further permutations of PDH payloads options, including VC-2 structured traffic, are equally valid.
However, ETSI has designated the VC-2 for the use of non-hierarchical payloads. The VC-2 structure is used in SONET systems to support North American DS2 (6 Mbit/s) PDH traffic.
Synchronous Digital Hierarchy (SDH)
261 Cols RSOH
AUG 3
4
5
9
AU-4 PNTR
J1 A B C D
Payload Area
C4 or 3 x TUG-3s
POH
260 Cols
VC-4 9
9 9
MSOH
A B C D
J1
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Mapping Transport Structures into the VC-4
The STM-1 signal is a byte-interleaved structure and comprises a number of functional areas, as shown in the diagram. The AUG comprises the AU-4 Pointer (Row 4 columns 1-9) and the Payload Area, which is a 261 x 9 structure.
As shown in the ETSI mapping structure, all payload types are mapped into the VC-4 via other transport structures. In turn, the VC-4 is mapped into the AUG. The first byte of the VC-4 (the J1 byte) may appear at any position within the payload area. The AU-4 pointer points to the position of the J1 byte enabling a receiving terminal to locate the start of the VC-4.
A VC-4 may be equipped with a single C4 carrying an adapted 140 Mbit/s PDH payload or three TUG3s.
9& 3
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TUs and TU Designations
A TU consists of a VC and a TU pointer (pointers are omitted from this diagram).
From the ETSI mapping structure it can be seen that:
TU-12 contains a VC-12 which contains 1 x 2 Mbit/s
TUG 2 contains 3 x TU-12s
TUG 3 contains 7 x TUG-2s
VC4 contains 3 x TUG-3s
The VC4 can therefore transport a total of 63 TU-12s, (3 x 7 x 3), hence 63 x 2 Mbit/s signals. When configuring network elements, for example an ADM, a user must be able to map any 2 Mbit/s input to any TU-12. To do this, each TU-12 must be uniquely identified. This may be achieved using the k (TUG-3); l (TUG-2) and m (TU-12) numbering scheme. The valid numbering range is 1.1.1 to 3.7.3.
The diagram also identifies the four columns allocated in the VC-4 to T-12 (1.1.1) and TU-12 (3.7.3) TU-3 AND TU-2 Designators
It should be easy to deduce that a TU-3 can be designated by a k.o.o code and the TU-2 by a k.l.o code.
For example, suppose that TUG3, k=2, in the diagram is configured to hold a TU-3, the k.l.m code for this
For example, suppose that TUG3, k=2, in the diagram is configured to hold a TU-3, the k.l.m code for this