6. Recommendation G.708 NETWORK NODE INTERFACE FOR THE SYNCHRONOUS DIGITAL HIERARCHY The CCITT, considering (a) that Network Node Interface (NNI) specifications are neces- sary to enable interconnection of synchronous digital network elements for transport of payloads, including digital signals of the asynchronous hierarchy defined in Recommendation G.702; (b) that Recommendation G.707 describes the advantages offered by a synchronous digital hierarchy and multiplexing method and spec- ifies a set of synchronous digital hierarchy bit rates; (c) that Recommendation G.709 specifies the multiplexing struc- tures; (d) that Recommendations G.707, G.708 and G.709 form a coherent set of specifications for the synchronous digital hierarchy and NNI; (e) that Recommendation G.802 specifies the interworking between networks based on different asynchronous digital hierarchies and speech encoding laws, recommends that the frame structure for multiplexed digital signals at the network node interface of a synchronous digital network including ISDN should be as described in this Recommendation. 1. Location of NNI Figure 1.1/G.708 gives a possible network configuration to illus- trate the location of network node interface specified in this Recommendation. 2. Basic multiplexing principle and multiplexing elements 2.1 General Frame structures and overheads in this document are mainly in the context of circuit mode connection types rather than Asynchronous Transfer Mode (ATM). ATM based multiplexing principles are under study. Figure 2.1/G.708 shows the relationship between various multi- plexing elements that are defined below, and illustrates possible multiplexing structures. Figures 2.2, 2.3 and 2.4/G.708 are examples of how various signals are multiplexed using these multiplexing elements. Details of the multiplexing method and mappings are given in Rec- ommendation G.709. Note - When signals at bit rates of the various multiplexing ele- ments of the synchronous digital hierarchy (Recommendations G.707, G.708, G.709) are different from existing hierarchy levels in Recommendation G.702, the signals are not required to be trans- ported via digital networks which are in line with Recommendation G.702. 2.2 Definitions 1) Container: C-n (n=1-4) This element is a defined unit of payload capacity which is sized to carry any of the levels currently defined in Recom- mendation G.702 and may also provide capacity for carriage of broadband signals which are not yet defined. 2) Virtual Container: VC-n Two types of virtual containers have been identified: - Basic Virtual Container: VC-n (n=1,2) This element comprises a single C-n (n=1,2) plus the basic virtual container path overhead (POH) appro- priate to that level. - Higher Order Virtual Container: VC-n (n=3,4) This element comprises a single C-n (n=3,4), an assembly of tributary unit groups (TUG-2s) or an assembly of TU-3s, together with virtual container POH appropriate to that level. 3) Tributary Unit: TU-n (n=1-3) This element consists of a virtual container plus a tributary unit pointer. A tributary unit pointer indicates the phase alignment of the virtual container (VC-n) with respect to the POH of the next higher level virtual containers in which it resides. The tributary unit pointer location is fixed with respect to this higher level POH. In certain applications (for example, synchronous mapping providing direct observability of 64 kbit/s channels) the basic virtual container has a fixed phase-alignment with respect to the higher level virtual container. In this case, the basic virtual container (VC-1) POH and TU-1 pointer are null. 4) Tributary Unit Group: TUG-2 This element consists of a homogeneous assembly of identical TU-n (n=1,2). 5) Administrative Unit: AU-n (n=3,4) This element consists of a VC-n (n=3,4) plus an administra- tive unit pointer. An administrative unit pointer indicates the phase alignment of the VC-n (n=3,4) with respect to the STM-1 frame. The administrative unit pointer location is fixed with respect to the STM-1 frame. 6) Synchronous Transport Module level 1: STM-1 This element is the basic building block of the synchronous digital hierarchy and it comprises either one AU-4 or multi- ple AU-3s, together with section overhead (SOH). 7) Synchronous Transport Module level N: STM-N This element defines the N-th level of the synchronous-digi- tal hierarchy and contains N synchronously multiplexed STM-1 sig- nals. The STM-N-signal can be obtained via single or multiple stage multiplexing. Values of N correspond to the synchronous digital hierarchy levels given in Recommendation G.707. 3. Frame structure 3.1 Level 1 - 155 520 kbit/s (STM-1) 3.1.1 Basic frame structure Frame structure is shown in Figure 3.1/G.708. The three main areas of the STM-1 frame are indicated: - section overhead; - AU pointers; - STM-1 payload. 3.1.2 Section overhead Rows 1-3 and 5-9 of columns 1-9 of the STM-1 in Figure 3.1/G.708 are dedicated to the section overhead. The allocation of section overhead capacity and functions is given in Figure 3.4a/G.708. An explanation of the overhead functions is given in section 5. 3.1.3 Administrative unit (AU) pointers Row 4 of columns 1-9 and row 1-3 of columns 11-14 in Figure 3.1/ G.708 are available for AU pointers. The positions of the pointers of the AUs for different organizations of the STM-1 payload are shown in Table 3.1/G.708. The application of pointers and their detailed specifications are given in Recommendation G.709. 3.1.4 Administrative units in the STM-1 The STM-1 payload can suppport the following types and numbers of administrative units: - one AU-4; - or three AU-32s; - or four AU-31s. The VC-n associated with each AU-n does not have a fixed phase with respect to the STM-1 frame. The location of the first byte of the VC-n is indicated by the AU-n pointer. The AU-n pointer is in a fixed location in the STM-1 frame as illustrated in Figures 2.2, 2.3, 2.4, 3.1, 3.2 and 3.3/G.708. The AU-4 may be used to carry, via the VC-4, three TU-32s or four TU-31s. This nested arrangement is illustrated in Figures 2.2 and 3.3/G.708. The VC-3 associated with each TU-3 does not have a fixed phase relationship with respect to the start of the VC-4. The TU-3 pointer is in a fixed location in the VC-4 and the loca- tion of the first byte of the VC-3 is indicated by the TU-3 pointer (illustrated in Figures 2.2 and 3.3/G.708). 3.1.5 VC-4 and VC-3 path overheads The allocation of the VC-4 and VC-3 path overhead capacity and functions is given in Figure 3.4b/G.708. An explanation of the overhead functions is given in section 5. The position of the VC-4 and VC-3 path overhead is specified in Recommendation G.709. 3.2 Level 4 - 622 080 kbit/s (STM-4) This level is obtained by one byte interleaving four STM-1s as illustrated in Figure 3.5/G.708. The SOH of the STM-1s shall be 125 ｵs phase aligned prior to mul- tiplexing such that the SOH of the STM-4 is contained in the first 36 columns. The AU pointer value(s) of each STM-1 is/are adjusted to indicate the start of the VC(s) with respect to this new posi- tion of the AU pointer(s) which is fixed relative to the STM-4 SOH. 4. Interconnection of STM-1s The synchronous digital hierarchy, specified in Recommendations G.707, G.708 and G.709 is designed to be universal allowing trans- port of a large variety of signals including those specified in Recommendation G.702. However, there are a numebr of options for structuring an STM-1. This section provides guidelines for the interconnection of STM- 1s. Two general cases are considered: Case 1 - STM-1s having the same structure (detailed in section 4.1) Case 2 - STM-1s having different structures (detailed in section 4.2). 4.1 Interconnection of STM-1s having the same structure The interconnection unit used between STM-1s is the VC associated with the AU. This arrangement is shown in Table 4.1i)/G.708. 4.2 Interconnection of STM-1s having different structures In the case of STM-1s having different structures, the following guidelines should be used to facilitate interconnection by a bilateral agreement or to resolve the contention. The method of interconnection between STM-1s having different structures depends on whether the type of AU is different or whether the type of TUG is different. The cases are considered in three categories: - different types of AU-3 carrying a C-3 payload; - different types of AU carrying the same type of TUG-2; - different types of TUG-2s. 4.2.1 Different types of AU-3s carrying a C-3 payload For the interconnection of different types of AU-3s carrying a C- 3 payload, the C-3 payload is transferred from the AU-3 to a cor- responding TU-3. This TU-3 is then assembled into a VC-4 using the nested approach illustrated in Figure 3.3/G.708. This arrangement is shown in Table 4.1ii)/G.708, and is intended to facilitate the transit of C-3 in a VC-3 across a network which cannot support the associated AU-3. 4.2.2 Different types of AU carrying the same type of TUG For the interconnection of a different type of AU carrying the same type of TUG-2, the TUG-2s are transferred between the dissim- ilar AUs. In the absence of bilateral agreement on an AU-3 type, the AU-4 shall be used. This arrangement is shown in Table 4.1iii)/G.708. 4.2.3 Different types of TUG-2s For the interconnection of different types of TUG-2s, the TU-1s are transferred from the TUG-22 to the TUG-21. The TUG-21 is used as the interconnection unit. In the absence of bilateral agreement on an AU-3 type, the TUG-21s are directly assembled into a VC-4. This arrangement is shown in Table 4.1iv)/G.708. The method of interconnection between an AU-31 containing TUG-21s and an AU-31 containing TUG-22s is for further study. 5. Overhead functions 5.1 Types of overhead Several types of overhead have been identified for application in the synchronous digital hierarchy. 1) Section overhead: SOH Section overhead capacity is added to either an AU-4 or an assembly of AU-3s to create an STM-1. The content always includes STM-1 framing. Content representing section performance monitoring and other maintenance and operational functions can be added or modified without disassembly of the STM-1 as appropriate for various configurations of elements (for example, interme- diate regenerator monitoring, protection switching control). 2) Virtual container path overhead: POH Virtual container path overhead provides for communication between the point of assembly of a virtual container and its point of disassembly. Two categories of virtual container path overhead have been identified: - Basic virtual container path overhead (VC-1,2 POH) Basic virtual container POH is added to the container (C-1,2) when the VC-1,2 is created. Among the functions included in this overhead are virtual container path performance monitoring, signals for maintenance purposes and alarm status indications. - Higher order virtual container path overhead (VC-3,4 POH) VC-3 POH is added to either an assembly of TUG-2s or a C-3 to form a VC-3. VC-4 POH is added to either an assembly of TU-3s or a C-4 to form a VC-4. Among the functions included within this overhead are virtual container path performance monitoring, alarm status indications, sig- nals for maintenance purposes and multiplex structure indications (VC-3,4 composition). The types of overhead described above and their applications are shown in Figure 5.1/G.708. 5.2 Overhead descriptions The location of the various section and VC-3,4 path overhead bytes in the STM-1 frame is illustrated in Figure 3.4/G.708. 5.2.1 SOH byte descriptions 1) Framing: A1, A2 Six bytes are dedicated to each STM-1. The pattern shall be A1A1A1A2A2A2 (A1=11110110, A2=00101000). These bytes shall be provided in all STM-1 signals within an STM-N. 2) Data communication channels: D1 - D12 Twelve bytes are allocated for section data communication. These bytes are defined only for STM-1 #1 of an STM-N signal. 3) STM identifier: C1 This is a unique number assigned to an STM-1 prior to it being multiplexed to a higher STM-N level. Upon demultiplexing, this byte may be used to identify the position of any particular STM-1 within the incoming STM-N signal. 4) Orderwire: E1, E2 These two bytes provide orderwire channels for voice communication. These bytes are defined only for STM-1 #1 of an STM-N signal. 5) User channel: F1 This byte is reserved for user purposes (for example, network operations). This byte is defined only for STM-1 #1 of an STM-N signal. 6) BIP-8: B1 One byte is allocated in each STM-1 for an elementary regen- erator section bit error monitoring function. This function shall be a Bit Interleaved Parity 8 (BIP-8) code using even parity. The BIP-8 is computed over all bits of the previous STM-N frame after scrambling and is placed in byte B1 before scrambling. (For details of the scrambling process, see Recommendation G.709.) The B1 byte shall be monitored and recomputed at every regenerator. Note - Bit Interleaved Parity - N (BIP-N) code is defined as a method of error monitoring. With even parity, an N 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 pro- vides even parity over the first bit of all N-bit sequences in the covered portion of the signal, the second bit provides even parity over the second bits of all N-bit sequences within the specified portion, etc. Even parity is generated by setting the BIP-N bits so that there are an even number of 1s in each of all N-bit sequences including the BIP-N. 7) BIP-24: B2 x 3 Three bytes are allocated in each STM-1 for a section bit error monitoring function. This function shall be a Bit Interleaved Parity 24 code (BIP-24) using even parity. The BIP-24 is com- puted over all bits of the previous STM-1 frame except for the first three rows of section overhead (A1 through D3) and is placed in bytes B2 before scrambling. This parity code is not recomputed at regenerators. These bytes shall be provided in all STM-1 signals within an STM-N signal. 8) APS channel: K1, K2 Two bytes are allocated for Automatic Protection Switching (APS) signalling. These bytes are defined only for STM-1 #1 of an STM-N signal. 9) Spare: Z1, Z2 Six bytes are allocated for functions not yet defined. These bytes have no defined value. These bytes are reserved in all STM-1s of an STM-N. 5.2.2 AU pointer descriptions 1) Pointer value Two bytes are allocated for a pointer which indicates the offset in bytes between the pointer and the first byte of the asso- ciated virtual container POH. For a complete specification and location of these bytes, see Recommendation G.709. (2) Pointer action Three pointer action bytes are allocated in an AU-4 for fre- quency justification purposes. One pointer action byte is allocated for AU-3s and TU-ns. For complete specification of these bytes, refer to Recommendation G.709. In the event of a negative justification, they carry valid infor- mation. 5.2.3 VC-n (n=3,4) POH byte descriptions 1) Path BIP-8: B3 One byte is allocated in each virtual container for a path bit error monitoring function. This function shall be a BIP-8 code using even parity. The BIP-8 is computed over all bits of the previous container and is placed in the B3 byte. 2) Path status: G1 One byte is allocated to return the VC-n path terminating status performance information to the VC-n path originating point. 3) Signal label: C2 One byte is allocated to indicate the composition of the VC-n payload. 4) VC-n path-user channel: F2 One byte is allocated for user communication purposes. 5) VC-n path trace: J1 This byte is used at the VC-n termination point to verify the VC-n path connection. 6) Spare: Z3 - Z5 Three bytes are allocated for as yet undefined purposes. 7) Multiframe indicator: H4 This byte is allocated to provide a multiframe indication, when required. 8) Physical specification of the NNI Specification for physical, electrical or optical character- istics of the NNI will be contained in another Recommendation which is under study. TABLE 3.1/G.708 Position of AU pointers +末末末末末末末末末+末末末末末末末末末末末末+ _ AU _ Position of AU Pointer _ +末末末末末末末末末+末末末末末末末末末末末末+ _ 31 _ Area A and B _ +末末末末末末末末末+末末末末末末末末末末末末+ _ 32 _ Area A _ +末末末末末末末末末+末末末末末末末末末末末末+ _ 4 _ Area A _ +末末末末末末末末末+末末末末末末末末末末末末+