C:\WINWORD\CCITTREC.DOT_______________ Recommendation G. 782 Recommendation G. 782 TYPES AND GENERAL CHARACTERISTICS OF SYNCHRONOUS DIGITAL HIERARCHY (SDH) MULTIPLEXING EQUIPMENT The CCITT, considering (a) that Recommendations G.707, G.708 and G.709 form a coherent set of specifications for the synchronous digital hierarchy (SDH) and the network node interface (NNI); (b) that Recommendation G.781 gives the structure of Recommendations on multiplexing equipment for the SDH; (c) that Recommendation G.783 specifies the characteristics of SDH mul- tiplexing equipment functional blocks; (d) that Recommendation G.784 addresses management aspects of the SDH; (e) that Recommendation G.957 specifies characteristics of optical inter- faces for use within the SDH; (f) that Recommendation G.958 specifies digital line systems based on the SDH for use on optical fibre cables; (g) that Recommendation G.703 describes electrical interfaces for use within the SDH, recommends that SDH multiplexing equipment should have general characteristics as described in this Recommendation. 1 Introduction 1.1 Scope Recommendation G.781 gives the structure of Recommendations on SDH multiplexers. This Recommendation gives an overview of the func- tions of SDH multiplexing equipment, examples of various multiplexing equipment types and general performance requirements. The possibilities of add/drop features, mixed payloads and flexible tributary/channel associations in SDH multiplexers make it difficult to pro- vide a Recommendation which is unambiguous while remaining generic enough not to constrain implementation. To overcome these difficulties, the "functional reference model" approach has been adopted. Therefore this series of Recommendations describes the equipment in terms of various functional blocks. This logical partitioning is used to simplify and general- ize the description. It does not imply any physical partitioning or implemen- tation. Only external interface requirements will be specified. For payloads these will conform to either STM-N (according to Recommendations G.707, G.708 and G.709) or Recommendation G.703. The interface to the transmission management network (TMN) will conform to Recommenda- tion G.773. The points between function blocks exist only as logical refer- ence points and not as internal interfaces; there is therefore no interface description or interface specification associated with these points. 1.2 Abbreviations AIS Alarm indication signal AU Administrative unit AUG Administrative unit group DCC Data communications channel FEBE Far end block error FERF Far end receive failure HPA Higher order path adaptation HPC Higher order path connection HPT Higher order path termination LPA Lower order path adaptation LPC Lower order path connection LPT Lower order path termination MCF Message communications function MSOH Multiplex section overhead MSP Multiplex section protection MST Multiplex section termination MTPI Multiplexer timing physical interface MTS Multiplexer timing source NNI Network node interface NOMC Network operators maintenance channel PDH Plesiochronous digital hierarchy PI Physical interface POH Path overhead RSOH Regenerator section overhead RST Regenerator section termination SA Section adaptation SDH Synchronous digital hierarchy SEMF Synchronous equipment management function SOH Section overhead SPI SDH physical interface STM Synchronous transport module TMN Telecommunications management network TU Tributary unit TUG Tributary unit group VC Virtual container 1.3 Definitions Note — The following definitions are relevant in the context of SDH- related Recommendations. 1.3.1 Administrative unit (AU) See Recommendation G.708. 1.3.2 Administrative unit group (AUG) See Recommendation G.708. 1.3.3 Data communications channel (DCC) See Recommendation G.784. 1.3.4 higher order path In an SDH network, the higher order (HO) path layers provide a server network for the lower order (LO) path layers. The comparative terms lower and higher refer only to the two participants in such a client/server relationship. VC-1/2 paths may be described as lower order in relation to VC-3 and VC-4 while the VC-3 path may be described as lower order in relation to VC-4. 1.3.5 higher order path adaptation (HPA) The HPA function adapts a lower order VC (VC-1/2/3) to a higher order VC (VC-3/4) by processing the TU pointer which indicates the phase of the VC-1/2/3 POH relative to the VC-3/4 POH and assembling/disassem- bling the complete VC-3/4. 1.3.6 higher order path connection (HPC) The HPC function provides for flexible assignment of higher order VCs (VC-3/4) within an STM-N signal. 1.3.7 higher order path termination (HPT) The HPT function terminates a higher order path by generating and adding the appropriate VC POH to the relevant container at the path source and removing the VC POH and reading it at the path sink. 1.3.8 Lower order path See higher order path above. 1.3.9 lower order path adaptation (LPA) The LPA function adapts a PDH signal to an SDH network by map- ping/de-mapping the signal into/out of a synchronous container. If the signal is asynchronous, the mapping process will include bit level justification. 1.3.10 lower order path connection (LPC) The LPC function provides for flexible assignment of lower order VCs in a higher order VC. 1.3.11 lower order path termination (LPT) The LPT function terminates a lower order path by generating and adding the appropriate VC POH to the relevant container at the path source, removing the VC POH and reading it at the path sink. 1.3.12 Message communications function (MCF) See Recommendation G.784. 1.3.13 multiplex section overhead (MSOH) The MSOH comprises rows 5 to 9 of the SOH of the STM-N signal. 1.3.14 multiplex section protection (MSP) The MSP function provides capability for switching a signal between and including two MST functions, from a working to a protection section. 1.3.15 multiplex section termination (MST) The MST function generates the MSOH in the process of forming an SDH frame signal and terminates the MSOH in the reverse direction. 1.3.16 multiplexer timing physical interface (MTPI) The MTPI function provides the interface between an external syn- chronization signal and the multiplexer timing source. 1.3.17 multiplexer timing source (MTS) The MTS function provides timing reference to the relevant compo- nent parts of a multiplexing equipment and represents the SDH network ele- ment clock. 1.3.18 Path overhead (POH) See Recommendation G.708. 1.3.19 regenerator section overhead (RSOH) The RSOH comprises rows 1 to 3 of the SOH of the STM-N signal. 1.3.20 regenerator section termination (RST) The RST function generates the RSOH in the process of forming an SDH frame signal and terminates the RSOH in the reverse direction. 1.3.21 section adaptation (SA) The SA function processes the AU-3/4 pointer to indicate the phase of the VC-3/4 POH relative to the STM-N SOH and assembles/disassembles the complete STM-N frame. 1.3.22 Synchronous digital hierarchy (SDH) See Recommendation G.707. 1.3.23 synchronous equipment management function (SEMF) The SEMF converts performance data and implementation specific hardware alarms into object-oriented messages for transmission over the DCC(s) and/or a Q interface. It also converts object-oriented messages related to other management functions for passing across the Sn reference points. 1.3.24 Section overhead (SOH) See Recommendation G.708. 1.3.25 SDH physical interface (SPI) The SPI function converts an internal logic level STM-N signal into an STM-N line interface signal. 1.3.26 Synchronous transport module (STM) See Recommendation G.708. 1.3.27 Telecommunications management network (TMN) See Recommendation M.30. 1.3.28 Tributary unit (TU) See Recommendation G.708. 1.3.29 Tributary unit group (TUG) See Recommendation G.708. 1.3.30 Virtual container (VC) See Recommendation G.708. 2 Overview of equipment functions 2.1 Multiplexing method 2.1.1 Generalized logical blocks Figure 2-1/G.782 is a generalized Multiplexer Logical Block Dia- gram. It illustrates the steps that are required to assemble various payloads and multiplex them into an STM-N output. It does not represent a useful or practical network function. Examples of some configurations that may be deployed are given in §3. The only function blocks that are payload specific are the physical interface/path adaptation blocks used at the G.703 interfaces; all other func- tions are non-payload specific. Therefore all operations functions, except those associated with G.703 interfaces, are payload independent. New pay- load types can be added by providing a new interface function; all other parts of the system will be unaffected. A brief description of the signal flow between a Recommendation G.703 interface and the STM-N output is provided in §§2.1.2 and 2.1.3. Description of functions performed by each of the logical blocks in Figure2-1/G.782 is provided in Recommendations G.783 and G.784. Fur- ther descriptions of the synchronous equipment management function (SEMF) and message communications function (MCF) are given in §2.2 and descriptions of the multiplexer timing source (MTS) and multiplexer timing physical interface (MTPI) are given in §4. FIGURE 2-1/G.782 = 18,5 cm 2.1.2 Signal flow G.703 input to STM-N output: multiplexing Physical interface/ lower order path adaptation Provides the appropriate G.703 interface and maps the payload into the container as specified in RecommendationG.709. Lower order path termination Adds the VC path overhead (VC-POH). Lower order path connection Allows flexible assignment of the VC-1/2 within the VC-3/4. Higher order path adaptation Processes the TU pointer to indicate the phase of the VC-1/2 POH relative to the VC-3/4 POH and assem- bles the complete VC-3/4. Higher order path termination Adds the VC-3/4 path overhead. Higher order path connec- tion Allows flexible assignment of the VC-3/4 within the STM-N. Section adaptation Processes the AU-3/4 pointer to indicate the phase of the VC-3/4 POH relative to the STM-N SOH. Byte- multiplexes the AU Groups (AUGs) to construct the complete STM-N frame. Multiplex section protection Provides capability for branching the signal onto another line system for protection purposes. Multiplex section termination Generates and adds rows 5 to 9 of the SOH. Regenerator section termination Generates and adds rows 1 to 3 of the SOH; the STM- N signal is then scrambled except for row 1 of the SOH. SDH physical inter- face Converts the internal logic level STM-N signal into an STM-N interface signal. This may be an in-station electrical signal, an in-station optical signal or an inter-station optical signal. 2.1.3 Signal flow STM-N input to G.703 output: demultiplexing SDH physical inter- face Converts the interface signal into an internal logic level and recovers timing from the line signal. Regenerator section termination Identifies the STM-N frame word, descrambles the signal, and processes rows 1 to 3 of the SOH. The remaining operations are the inverse of those performed when multi- plexing except that the C-1/2 interface function must provide a buffer store and smoothing circuit to attenuate the clock jitter caused by the multiplex process, pointer moves and bit stuffing (if applicable). 2.2 Operations, administration, maintenance and provisioning (OAM&P) 2.2.1 Overhead applications Recommendation G.708 specifies bandwidth allocated within the SDH frame structure for various control and maintenance functions. Two types of overhead are identified: Virtual Container Path Overhead (VC- POH) and Section Overhead (SOH). 2.2.1.1 POH application Details of the functions provided by the POH are contained in Recom- mendations G.708 and G.709. The VC-POH is generated and terminated at the point where the pay- load is assembled or disassembled. Itisused for end to end monitoring of the payload and may transit several multiplex and line systems. Some of the VC-POH is completely payload independent, while other parts of the VC- POH are used in specific ways according to the type of payload. In all cases, the VC-POH is independent of user information. Thus it may be monitored at any point within an SDH network to confirm network operation. 2.2.1.2 SOH application The section overhead (SOH) is subdivided into regenerator SOH (RSOH) comprising rows 1 to 3 and multiplex SOH (MSOH) comprising rows 5-9. The MSOH is accessible only at terminal equipments, whereas the RSOH is accessible at both terminal equipments and regenerators. Details of the functions provided by RSOH and MSOH are given in Recommendation G.708. These functions include performance monitoring and section maintenance and operations functions. In order to permit regenerators to read from and write to the RSOH without disrupting the primary performance monitoring, the RSOH is excluded from the B2 (BIP-24) calculation. Since B1 is recomputed at each regenerator, fault sectionalization is simplified. The set of bytes E1, E2, F1 and D1 to D12 is referred to as the net- work operators maintenance channel (NOMC). 2.2.1.3 Protection of the Network operators maintenance channel (NOMC) In a 1+1 protection system, the NOMC will be on both channels. In a 1:n protection system, the NOMC will be on only one channel, normally channel 1. If channel 1 fails, the NOMC will be switched to the protection channel, along with traffic. It should be noted that failure of channel 1 will result in the loss of the NOMC under the following conditions: i) the protection channel is carrying extra traffic and a FORCED switch is in operation; ii) the protection channel is LOCKED OUT. Loss of the NOMC under conditions i) and ii) above, and in the case of diversely routed protection spans, requires further study. Bytes K1 and K2 shall be transmitted on the protection channel. In addition, they may also be transmitted on working channels. The receiver must be able to ignore bytes K1 and K2 on any of the working channels. 2.2.1.4 Maintenance signals The maintenance signals defined in Recommendation G.709 §2.3.1 at the section layer are section AIS and far end receive failure (FERF). At the path layer, Recommendation G.709 §2.3.2 defines path AIS and path status information in the form of path FERF and far end block error (FEBE). These path maintenance signals apply at both higher order and lower order path level. Figure 2-2/G.782 illustrates the layer-to-layer and peer-to-peer maintenance interaction provided in the SDH overhead. 2.2.1.5 Loss of signal at regenerators If a regenerator loses its input signal, a standby clock is activated and a signal containing valid RSOH and MS-AIS is transmitted downstream. This enables the NOMC functions carried by the RSOH to be activated if required. 2.2.2 TMN access SDH multiplexers should provide interfaces for messages to or from the TMN via either the DCC or a Qinterface or both. Messages arriving at the interface not addressed to the local multiplex should be relayed to the appropriate Q or DCC interface. The TMN can thus be provided with a direct logical link to any SDH equipment via a single Q interface and the interconnecting DCCs. Figure 2-2/G.782 = 22 cm 2.2.2.1 Q -interface When access to the TMN is provided by a Q-interface, the interface will conform to RecommendationG.773. A choice has to be made between the B1, B2 and B3 protocol suites specified in that Recommendation. 2.2.2.2 Data communications channel (DCC) The use of the DCC is dependent on the network operator's mainte- nance strategy and the specific situation. It may not always be required as it is possible to carry out the required functions by other means. There are two ways of using the DCC: i) use of the D1 to D3 bytes located in the RSOH (DCCR) and accessi- ble at regenerators and other NEs; ii) use of the D4 to D12 bytes located in the MSOH (DCCM) and not accessible at regenerators. The specific use of the D4 to D12 bytes is for further study. These channels are message based and provide communications between network elements. They can be used to support communications between sites and the TMN. Two examples are given in Figures 2-3/G.782 and2-4/G.782. Figure 2-3/G.782= 10 cm Figure 2-4/G.782 = 13 cm 2.2.2.3 Functionalities 2.2.2.3.1 Synchronous equipment management function (SEMF) This converts performance data and implementation specific hard- ware alarms into object-oriented messages for transmission on the DCC(s) and/or a Q-interface. It also converts object-oriented messages related to other management functions for passing across the Sn reference points. 2.2.2.3.2 Message communications function (MCF) This function receives and buffers messages from the DCC(s), Q-and F-interfaces and SEMF. Messages not addressed to the local site are relayed to one or more outgoing DCC(s) in accordance with local routing proce- dures and/or Q-interface(s). The function provides layer 1 (and layer 2 in some cases) translation between a DCC and a Q-interface or another DCC interface. 2.2.3 Order-wire Use of the E1 and/or E2 bytes for providing an order-wire is optional. Byte E1 can be accessed at all regenerators and terminals to provide a local order-wire. Byte E2 can only be accessed at terminals and may be used to provide an order-wire between terminal sites. 2.2.4 User channel Use of the F1 byte for providing a special user channel is optional. Byte F1 can be accessed at all regenerators and terminals. 2.3 STM-N protection switching Protection switching of a signal provides a capability, using equip- ment redundancy and switching action, such that in the event of the failure of a “working” channel, the signal is available via a protection channel. The use of protection switching is dependent on the network opera- tor's maintenance strategy. It may not always be required. If required on SDH line systems, redundancy is provided for functions and physical medium between, and including, two MST functions, i.e. for the multiplex section. Thus, the Multiplex Section Protection (MSP) function included in multiplexing equipment provides protection for the STM-N signal against failures within a multiplex section. The MSP function communicates with the corresponding far end MSP function to coordinate the switch action, via a bit-oriented protocol defined for the K bytes of the MSOH. It also communicates with the SEMF for automatic and manual switch control. Automatic protection switching is initiated based on the condition of the received signals. Manual protection switching provides both local and remote switching from commands received via the SEMF. The details of switch initiation, control and opera- tion are described in Recommendation G.783. 2.3.1 MSP architectures Two MSP architectures are defined: 1+1 (one plus one) and 1:n (one forn). 2.3.1.1 1+1 architecture In a 1+1 MSP architecture shown in Figure 2-5/G.782, the STM-N signal is transmitted simultaneously on both multiplex sections, designated working and protection sections; i.e. the STM-N signal is permanently con- nected (bridged) to the working and protection sections at the transmitting end. The MSP function at the receiving end monitors the condition of the STM-N signals received from both sections and connects (selects) the appropriate signal. Due to permanent bridging of the working channel, the 1+1 architecture does not allow an unprotected extra traffic channel to be provided. Figure 2-5/G.782= 4,5 cm 2.3.1.2 1:n architecture In a 1:n MSP architecture shown in Figure 2-6/G.782, the protection sec- tion is shared by a number of working channels; the permitted values for n are 1 through 14. At both ends, any one of the n STM-N channels or an extra traffic channel (or possibly a test signal) is bridged to the protection section. The MSP functions monitor and evaluate the conditions of the received sig- nals and perform bridging and selection of the appropriate STM-N signals from the protection section. Note that 1:1 architecture is a subset of 1:n (n=1) and may have the capability to operate as 1+1 for interworking with a 1+1 architecture at the other end. 2.3.2 Operation modes The MSP may operate either bi-directionally or uni-directionally and in either a revertive or non-revertive mode, depending on the network man- agement. In bi-directional operation, the channel is switched to the protection section in both directions, and switching of only one direction is not allowed. In uni-directional operation, the switching is complete when the channel in the failed direction is switched to protection. In revertive mode of operation, the working channel is switched back to the working section, i.e. restored, when the working section has recov- ered from failure. In non-revertive mode of operation, the switch is main- tained even after recovery from failure. For 1:n architectures, only revertive mode is allowed. 2.4 Integrated interfaces Section 3 describes multiplexer configurations for multiplexer func- tions that may be integrated with the line terminating function. It is envis- aged that such direct SDH interfaces will also be provided on other network elements such as digital cross-connects or digital switches. These interfaces may be either intra-station or inter-station. Figure 2-6/G.782 = 10,5 cm 3 Multiplexing equipment types This section provides some examples of equipment configurations and network applications for SDH equipment, based on the generalized multiplexer logical block diagram (Figure 2-1/G.782). The description of these examples is generic and no particular physical partitioning of func- tions is implied. The examples are not a complete set; other configurations may be useful in other network applications. 3.1 Type I (Figure 3-1/G.782) This provides a simple G.703 to STM-N multiplex function. For example, 63´2048 kbit/s signals could be multiplexed to form an STM-1 output or, 12´44736 kbit/s signals could be multiplexed to form an STM- 4. The location of each of the tributary signals in the aggregate signal is fixed and dependent on the multiplex structure chosen. 3.2 Type Ia (Figure 3-2/G.782) The ability to provide flexible assignment of an input to any position in the STM-N frame can be provided by including a VC-1/2 and/or VC-3/4 path connection function. 3.3 Type II (Figure 3-3/G.782) This provides the ability to combine a number of STM-N signals into a single STM-M signal. For example, four STM-1 signals (from multiplex- ers or line systems) could be multiplexed to provide a single STM-4 signal. The location of each of the VC-3/4s of the STM-N signals is fixed in the aggregate STM-M signal. 3.4 Type IIa (Figure 3-4/G.782) The ability to assign flexibly a VC-3/4 on one STM-N to any position in the STM-M frame can be provided by including a VC-3/4 path connec- tion function. 3.5 Types IIIa and IIIb These provide the ability to access any of the constituent signals within an STM-N signal without demultiplexing and terminating the com- plete signal. The interface provided for the accessed signal could be either according to G.703 or an STM-M (M