Synchronous Optical Network (SONET)

Synchronous Optical Network (SONET)
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MarthaKelly,Mexico,Researcher
Published Date:12-07-2017
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Synchronous Optical Network (SONET) Definition Synchronous optical network (SONET) is a standard for optical telecommunications transport formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), which sets industry standards in the U.S. for telecommunications and other industries. The comprehensive SONET standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades. Overview This tutorial provides an introduction to the SONET standard. Standards in the telecommunications field are always evolving. Information in this SONET primer is based on the latest information available from the Bellcore and International Telecommunications Union–Telecommunications Standardization Sector (ITU– T) standards organizations. Use this primer as an introduction to the technology of SONET. If more detailed information is required, consult the latest material from Bellcore and ITU–T, paying particular attention to the latest date. For help in understanding the language of SONET telecommunications, a comprehensive Glossary is provided. Topics 1. Introduction to SONET 2. Why Synchronize? 3. Frame Format Structure 4. Overheads 5. Pointers 6. SONET Multiplexing 7. SONET Network Elements Web ProForum Tutorials Copyright © 1/58 http://www.iec.org The International Engineering Consortium 8. SONET Network Configurations 9. What Are the Benefits of SONET? 10. SDH Reference 11. SONET Reference Materials Self-Test Correct Answers Glossary 1. Introduction to SONET Synchronous optical network (SONET) is a standard for optical telecommunications transport. It was formulated by the ECSA for ANSI, which sets industry standards in the United States for telecommunications and other industries. The comprehensive SONET/synchronous digital hierarchy (SDH) standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades. The increased configuration flexibility and bandwidth availability of SONET provides significant advantages over the older telecommunications system. These advantages include the following: • reduction in equipment requirements and an increase in network reliability • provision of overhead and payload bytes—the overhead bytes permit management of the payload bytes on an individual basis and facilitate centralized fault sectionalization • definition of a synchronous multiplexing format for carrying lower level digital signals (such as DS–1, DS–3) and a synchronous structure that greatly simplifies the interface to digital switches, digital cross- connect switches, and add-drop multiplexers • availability of a set of generic standards that enable products from different vendors to be connected • definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic–based transmission hierarchy. Web ProForum Tutorials Copyright © 2/58 http://www.iec.org The International Engineering Consortium Background Before SONET, the first generations of fiber-optic systems in the public telephone network used proprietary architectures, equipment, line codes, multiplexing formats, and maintenance procedures. The users of this equipment—regional Bell operating companies and interexchange carriers (IXCs) in the United States, Canada, Korea, Taiwan, and Hong Kong—wanted standards so that they could mix and match equipment from different suppliers. The task of creating such a standard was taken up in 1984 by the ECSA to establish a standard for connecting one fiber system to another. This standard is called SONET. Synchronization of Digital Signals To understand the concepts and details of SONET correctly, it is important to be clear about the meaning of synchronous, asynchronous, and plesiochronous. In a set of synchronous signals, the digital transitions in the signals occur at exactly the same rate. There may, however, be a phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may be due to propagation time delays or jitter introduced into the transmission network. In a synchronous network, all the clocks are traceable to one primary reference clock (PRC). The accuracy of the PRC is better than ±1 in 1011 and is derived from a cesium atomic standard. If two digital signals are plesiochronous, their transitions occur at almost the same rate, with any variation being constrained within tight limits. For example, if two networks must interwork, their clocks may be derived from two different PRCs. Although these clocks are extremely accurate, there is a difference between one clock and the other. This is known as a plesiochronous difference. In the case of asynchronous signals, the transitions of the signals do not necessarily occur at the same nominal rate. Asynchronous, in this case, means that the difference between two clocks is much greater than a plesiochronous difference. For example, if two clocks are derived from free-running quartz oscillators, they could be described as asynchronous. Basic SONET Signal SONET defines a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy. This is accomplished by means of a byte-interleaved multiplexing scheme. Byte-interleaving simplifies multiplexing and offers end-to-end network management. The first step in the SONET multiplexing process involves the generation of the lowest level or base signal. In SONET, this base signal is referred to as Web ProForum Tutorials Copyright © 3/58 http://www.iec.org The International Engineering Consortium synchronous transport signal–level 1, or simply STS–1, which operates at 51.84 Mbps. Higher-level signals are integer multiples of STS–1, creating the family of STS–N signals in Table 1. An STS–N signal is composed of N byte-interleaved STS–1 signals. This table also includes the optical counterpart for each STS–N signal, designated optical carrier level N (OC–N). Synchronous and nonsynchronous line rates and the relationships between each are shown in Tables 1 and 2. Table 1. SONET Hierarchy Signal Bit Rate (Mbps) Capacity STS–1, OC–1 51.840 28 DS–1s or 1 DS–3 STS–3, OC–3 155.520 84 DS–1s or 3 DS–3s STS–12, OC–12 622.080 336 DS–1s or 12 DS–3s STS–48, OC–48 2,488.320 1,344 DS–1s or 48 DS–3s STS–192, OC–192 9,953.280 5,376 DS–1s or 192 DS–3s Note: STS = synchronous transport signal OC = optical carrier Table 2. Nonsynchronous Hierarchy Signal Bit Rate (Mbps) Channels DS–0 0.640 1 DS–0 DS–1 1.544 24 DS–0s DS–2 6.312 96 DS–0s DS–3 44.736 28 DS–1s 2. Why Synchronize? Synchronous versus Asynchronous Traditionally, transmission systems have been asynchronous, with each terminal in the network running on its own clock. In digital transmission, clocking is one of the most important considerations. Clocking means using a series of repetitive Web ProForum Tutorials Copyright © 4/58 http://www.iec.org The International Engineering Consortium pulses to keep the bit rate of data constant and to indicate where the ones and zeroes are located in a data stream. Because these clocks are totally free-running and not synchronized, large variations occur in the clock rate and thus the signal bit rate. For example, a DS– 3 signal specified at 44.736 Mbps + 20 parts per million (ppm) can produce a variation of up to 1,789 bps between one incoming DS–3 and another. Asynchronous multiplexing uses multiple stages. Signals such as asynchronous DS–1s are multiplexed, and extra bits are added (bit-stuffing) to account for the variations of each individual stream and combined with other bits (framing bits) to form a DS–2 stream. Bit-stuffing is used again to multiplex up to DS–3. DS–3s are multiplexed up to higher rates in the same manner. At the higher asynchronous rate, they cannot be accessed without demultiplexing. In a synchronous system such as SONET, the average frequency of all clocks in the system will be the same (synchronous) or nearly the same (plesiochronous). Every clock can be traced back to a highly stable reference supply. Thus, the STS– 1 rate remains at a nominal 51.84 Mbps, allowing many synchronous STS–1 signals to be stacked together when multiplexed without any bit-stuffing. Thus, the STS–1s are easily accessed at a higher STS–N rate. Low-speed synchronous virtual tributary (VT) signals are also simple to interleave and transport at higher rates. At low speeds, DS–1s are transported by synchronous VT–1.5 signals at a constant rate of 1.728 Mbps. Single-step multiplexing up to STS–1 requires no bit stuffing, and VTs are easily accessed. Pointers accommodate differences in the reference source frequencies and phase wander and prevent frequency differences during synchronization failures. Synchronization Hierarchy Digital switches and digital cross-connect systems are commonly employed in the digital network synchronization hierarchy. The network is organized with a master-slave relationship with clocks of the higher-level nodes feeding timing signals to clocks of the lower-level nodes. All nodes can be traced up to a primary reference source, a Stratum 1 atomic clock with extremely high stability and accuracy. Less stable clocks are adequate to support the lower nodes. Synchronizing SONET The internal clock of a SONET terminal may derive its timing signal from a building integrated timing supply (BITS) used by switching systems and other equipment. Thus, this terminal will serve as a master for other SONET nodes, providing timing on its outgoing OC–N signal. Other SONET nodes will operate Web ProForum Tutorials Copyright © 5/58 http://www.iec.org The International Engineering Consortium in a slave mode called loop timing with their internal clocks timed by the incoming OC–N signal. Current standards specify that a SONET network must be able to derive its timing from a Stratum 3 or higher clock. 3. Frame Format Structure SONET uses a basic transmission rate of STS–1 that is equivalent to 51.84 Mbps. Higher-level signals are integer multiples of the base rate. For example, STS–3 is three times the rate of STS–1 (3 x 51.84 = 155.52 Mbps). An STS–12 rate would be 12 x 51.84 = 622.08 Mbps. STS–1 Building Block The frame format of the STS–1 signal is shown in Figure 1. In general, the frame can be divided into two main areas: transport overhead and the synchronous payload envelope (SPE). Figure 1. STS–1 Frame Format The synchronous payload envelope can also be divided into two parts: the STS path overhead (POH) and the payload. The payload is the revenue-producing traffic being transported and routed over the SONET network. Once the payload is multiplexed into the synchronous payload envelope, it can be transported and switched through SONET without having to be examined and possibly demultiplexed at intermediate nodes. Thus, SONET is said to be service- independent or transparent. Transport overhead is composed of section overhead and line overhead. The STS–1 POH is part of the synchronous payload envelope. The STS–1 payload has the capacity to transport up to the following: • 28 DS–1s Web ProForum Tutorials Copyright © 6/58 http://www.iec.org The International Engineering Consortium • 1 DS–3 • 21 2.048 Mbps signals • combinations of each STS–1 Frame Structure STS–1 is a specific sequence of 810 bytes (6,480 bits), which includes various overhead bytes and an envelope capacity for transporting payloads. It can be depicted as a 90-column by 9-row structure. With a frame length of 125 µs (8,000 frames per second), STS–1 has a bit rate of 51.840 Mbps. The order of transmission of bytes is row-by-row from top to bottom and from left to right (most significant bit first). As shown in Figure 1, the first three columns of the STS–1 frame are for the transport overhead. The three columns contain 9 bytes. Of these, 9 bytes are overhead for the section layer (for example, each section overhead), and 18 bytes are overhead for the line layer (for example, line overhead). The remaining 87 columns constitute the STS–1 envelope capacity (payload and POH). As stated before, the basic signal of SONET is the STS–1. The STS frame format is composed of 9 rows of 90 columns of 8-bit bytes, or 810 bytes. The byte transmission order is row-by-row, left to right. At a rate of 8,000 frames per second, that works out to a rate of 51.840 Mbps, as the following equation demonstrates: (9) x (90 bytes/frame) x (8 bits/byte) x (8,000 frames/s) = 51,840,000 bps = 51.840 Mbps This is known as the STS–1 signal rate—the electrical rate used primarily for transport within a specific piece of hardware. The optical equivalent of STS–1 is known as OC–1, and it is used for transmission across the fiber. The STS–1 frame consists of overhead, plus an SPE (see Figure 2). The first three columns of each STS–1 frame make up the transport overhead, and the last 87 columns make up the SPE. SPEs can have any alignment within the frame, and this alignment is indicated by the H1 and H2 pointer bytes in the line overhead. Web ProForum Tutorials Copyright © 7/58 http://www.iec.org The International Engineering Consortium Figure 2. STS–1 Frame Elements STS–1 Envelope Capacity and Synchronous Payload Envelope (SPE) Figure 3 depicts the STS–1 SPE, which occupies the STS–1 envelope capacity. The STS–1 SPE consists of 783 bytes, and can be depicted as an 87-column by 9- row structure. Column 1 contains 9 bytes, designated as the STS POH. Two columns (columns 30 and 59) are not used for payload but are designated as the fixed-stuff columns. The 756 bytes in the remaining 84 columns are designated as the STS–1 payload capacity. Figure 3. STS–1 SPE Example STS–1 SPE in Interior of STS–1 Frames The STS–1 SPE may begin anywhere in the STS–1 envelope capacity (see Figure 4). Typically, it begins in one STS–1 frame and ends in the next. The STS payload pointer contained in the transport overhead designates the location of the byte where the STS–1 SPE begins. STS POH is associated with each payload and is used to communicate various information from the point where a payload is mapped into the STS–1 SPE to where it is delivered. Web ProForum Tutorials Copyright © 8/58 http://www.iec.org The International Engineering Consortium Figure 4. STS–1 SPE Position in the STS–1 Frame STS–N Frame Structure An STS–N is a specific sequence of Nx810 bytes. The STS–N is formed by byte- interleaving STS–1 modules (see Figure 5). The transport overhead of the individual STS–1 modules are frame aligned before interleaving, but the associated STS SPEs are not required to be aligned because each STS–1 has a payload pointer to indicate the location of the SPE (or to indicate concatenation). Figure 5. STS–N ` 4. Overheads SONET provides substantial overhead information, allowing simpler multiplexing and greatly expanded operations, administration, maintenance, and provisioning (OAM&P) capabilities. The overhead information has several layers, which are shown in Figure 6. Path-level overhead is carried from end-to-end; it is added to DS–1 signals when they are mapped into VTs and for STS–1 payloads that travel end-to-end. Line overhead is for the STS–N signal between STS–N multiplexers. Section overhead is used for communications between adjacent network elements such as regenerators. Web ProForum Tutorials Copyright © 9/58 http://www.iec.org The International Engineering Consortium Enough information is contained in the overhead to allow the network to operate and allow OAM&P communications between an intelligent network controller and the individual nodes. Figure 6. Overhead Layers The following sections detail the different SONET overhead information: • section overhead • line overhead • STS POH • VT POH This information has been updated to reflect changes in Bellcore GR–253, Issue 2, December 1995. Section Overhead Section overhead contains 9 bytes of the transport overhead accessed, generated, and processed by section-terminating equipment. This overhead supports functions such as the following: • performance monitoring (STS–N signal) • local orderwire • data communication channels to carry information for OAM&P Web ProForum Tutorials Copyright © 10/58 http://www.iec.org The International Engineering Consortium • framing This might be two regenerators, line-terminating equipment and a regenerator, or two sets of line-terminating equipment. The section overhead is found in the first three rows of columns 1 to 9 (See Figure 7). Figure 7. Section Overhead–Rows 1 to 3 of Transport Overhead Table 3 shows section overhead byte by byte. Table 3. Section Overhead Byte Description A1 framing bytes—These two bytes indicate the beginning of an STS–1 and frame. A2 J0 section trace (J0)/section growth (Z0)—The byte in each of the N STS–1s in an STS–N that was formally defined as the STS–1 ID (C1) byte has been refined either as the section trace byte (in the first STS–1 of the STS–N), or as a section growth byte (in the second through Nth STS–1s). B1 section bit-interleaved parity code (BIP–8) byte—This is a parity code (even parity), used to check for transmission errors over a regenerator section. Its value is calculated over all bits of the previous STS–N frame after scrambling then placed in the B1 byte of STS–1 before scrambling. Therefore, this byte is defined only for STS–1 number 1 of an STS–N signal. E1 section orderwire byte—This byte is allocated to be used as a local orderwire channel for voice communication between regenerators, hubs, and remote terminal locations. Web ProForum Tutorials Copyright © 11/58 http://www.iec.org The International Engineering Consortium F1 section user channel byte—This byte is set aside for the users' purposes. It terminates at all section-terminating equipment within a line. It can be read and written to at each section-terminating equipment in that line. D1, section data communications channel (DCC) bytes—Together, D2, these 3 bytes form a 192–kbps message channel providing a message- and based channel for OAM&P between pieces of section-terminating D3 equipment. The channel is used from a central location for alarms, control, monitoring, administration, and other communication needs. It is available for internally generated, externally generated, or manufacturer- specific messages. Line Overhead Line overhead contains 18 bytes of overhead accessed, generated, and processed by line-terminating equipment. This overhead supports functions such as the following: • locating the SPE in the frame • multiplexing or concatenating signals • performance monitoring • automatic protection switching • line maintenance Line overhead is found in rows 4 to 9 of columns 1 to 9 (see Figure 8). Figure 8. Line Overhead: Rows 4 to 9 of Transport Overhead Web ProForum Tutorials Copyright © 12/58 http://www.iec.org The International Engineering Consortium Table 4 shows line overhead byte by byte. Table 4. Line Overhead Byte Description H1 STS payload pointer (H1 and H2)—Two bytes are allocated to a and pointer that indicates the offset in bytes between the pointer and the first H2 byte of the STS SPE. The pointer bytes are used in all STS–1s within an STS–N to align the STS–1 transport overhead in the STS–N and to perform frequency justification. These bytes are also used to indicate concatenation and to detect STS path alarm indication signals (AIS–P). H3 pointer action byte (H3)—The pointer action byte is allocated for SPE frequency justification purposes. The H3 byte is used in all STS–1s within an STS–N to carry the extra SPE byte in the event of a negative pointer adjustment. The value contained in this byte when it is not used to carry the SPE byte is undefined. B2 line bit-interleaved parity code (BIP–8) byte—This parity code byte is used to determine if a transmission error has occurred over a line. It is even parity and is calculated over all bits of the line overhead and STS–1 SPE of the previous STS–1 frame before scrambling. The value is placed in the B2 byte of the line overhead before scrambling. This byte is provided in all STS–1 signals in an STS–N signal. K1 automatic protection switching (APS channel) bytes—These 2 and bytes are used for protection signaling between line-terminating entities K2 for bidirectional automatic protection switching and for detecting alarm indication signal (AIS–L) and remote defect indication (RDI) signals. D4 line data communications channel (DCC) bytes—These 9 bytes to form a 576–kbps message channel from a central location for OAM&P D12 information (alarms, control, maintenance, remote provisioning, monitoring, administration, and other communication needs) between line entities. They are available for internally generated, externally generated, and manufacturer-specific messages. A protocol analyzer is required to access the line–DCC information. S1 synchronization status (S1)—The S1 byte is located in the first STS–1 of an STS–N, and bits 5 through 8 of that byte are allocated to convey the synchronization status of the network element. Z1 growth (Z1)—The Z1 byte is located in the second through Nth STS–1s of an STS–N (3 = N = 48) and are allocated for future growth. Note that an OC–1 or STS–1 electrical signal does not contain a Z1 byte. Web ProForum Tutorials Copyright © 13/58 http://www.iec.org The International Engineering Consortium M0 STS–1 REI–L (M0)—The M0 byte is only defined for STS–1 in an OC–1 or STS–1 electrical signal. Bits 5 through 8 are allocated for a line remote error indication function (REI–L, formerly referred to as line FEBE), which conveys the error count detected by an LTE (using the line BIP–8 code) back to its peer LTE. M1 STS–N REI–L (M1)—The M1 byte is located in the third STS–1 (in order of appearance in the byte-interleaved STS–N electrical or OC–N signal) in an STS–N (N = 3) and is used for a REI–L function. Z2 growth (Z2)—The Z2 byte is located in the first and second STS–1s of an STS–3 and the first, second, and fourth through Nth STS–1s of an STS–N (12 = N = 48). These bytes are allocated for future growth. Note that an OC–1 or STS–1 electrical signal does not contain a Z2 byte. E2 orderwire byte—This orderwire byte provides a 64–kbps channel between line entities for an express orderwire. It is a voice channel for use by technicians and will be ignored as it passes through the regenerators. STS POH STS POH contains 9 evenly distributed POH bytes per 125 microseconds starting at the first byte of the STS SPE. STS POH provides for communication between the point of creation of an STS SPE and its point of disassembly. This overhead supports functions such as the following: • performance monitoring of the STS SPE • signal label (the content of the STS SPE, including status of mapped payloads) • path status • path trace The POH is found in rows 1 to 9 of the first column of the STS–1 SPE (see Figure 9). Web ProForum Tutorials Copyright © 14/58 http://www.iec.org The International Engineering Consortium Figure 9. POH in Rows 1 to 9 Table 5 describes POH byte by byte. Table 5. STS POH Byte Description J1 STS path trace byte—This user-programmable byte repetitively transmits a 64-byte, or 16-byte E.164 format string. This allows the receiving terminal in a path to verify its continued connection to the intended transmitting terminal. B3 STS path bit-interleaved parity code (path BIP–8) byte—This is a parity code (even) used to determine if a transmission error has occurred over a path. Its value is calculated over all the bits of the previous SPE before scrambling. C2 STS path signal label byte—This byte is used to indicate the content of the STS SPE, including the status of the mapped payloads. G1 path status byte—This byte is used to convey the path-terminating status and performance back to the originating path-terminating equipment. Therefore, the duplex path in its entirety can be monitored from either end or from any point along the path. Bits 1 through 4 are allocated for an STS path REI function (REI–P, formerly referred to as STS path FEBE). Bits 5, 6, and 7 of the G1 byte are allocated for an STS path RDI (RDI–P) signal. Bit 8 of the G1 byte is currently undefined. F2 path user channel byte—This byte is used for user communication between path elements. H4 VT multiframe indicator byte—This byte provides a generalized Web ProForum Tutorials Copyright © 15/58 http://www.iec.org The International Engineering Consortium multiframe indicator for payload containers. At present, it is used only for tributary unit structured payloads. Note: The POH portion of the SPE remains with the payload until it is demultiplexed. VT POH VT POH contains four evenly distributed POH bytes per VT SPE starting at the first byte of the VT SPE. VT POH provides for communication between the point of creation of an VT SPE and its point of disassembly. Four bytes (V5, J2, Z6, and Z7) are allocated for VT POH. The first byte of a VT SPE (i.e., the byte in the location pointed to by the VT payload pointer) is the V5 byte, while the J2, Z6, and Z7 bytes occupy the corresponding locations in the subsequent 125-microsecond frames of the VT superframe. The V5 byte provides the same functions for VT paths that the B3, C2, and G1 bytes provide for STS paths—namely error checking, signal label, and path status. The bit assignments for the V5 byte are illustrated in Figure 10. Figure 10. VT POH—V5 Byte Bits 1 and 2 of the V5 byte are allocated for error performance monitoring. Bit 3 of the V5 byte is allocated for a VT path REI function (REI–V, formerly referred to as VT path FEBE) to convey the VT path terminating performance back to an originating VT PTE. Bit 4 of the V5 byte is allocated for a VT path remote failure indication (RFI–V) in the byte-synchronous DS–1 mapping. Bits 5 through 7 of the V5 byte are allocated for a VT path signal label to indicate the content of the VT SPE. Bit 8 of the VT byte is allocated for a VT path remote defect indication (RDI–V) signal. SONET Alarm Structure The SONET frame structure has been designed to contain a large amount of overhead information. The overhead information provides a variety of management and other functions such as the following: Web ProForum Tutorials Copyright © 16/58 http://www.iec.org The International Engineering Consortium • error performance monitoring • pointer adjustment information • path status • path trace • section trace • remote defect, error, and failure indications • signal labels • new data flag indications • data communications channels (DCC) • automatic protection switching (APS) control • orderwire • synchronization status message Much of this overhead information is involved with alarm and in-service monitoring of the particular SONET sections. SONET alarms are defined as follows: • anomaly—This is the smallest discrepancy that can be observed between the actual and desired characteristics of an item. The occurrence of a single anomaly does not constitute an interruption in the ability to perform a required function. • defect—The density of anomalies has reached a level where the ability to perform a required function has been interrupted. Defects are used as input for performance monitoring, the control of consequent actions, and the determination of fault cause. • failure—This is the inability of a function to perform a required action persisted beyond the maximum time allocated. Table 6 describes SONET alarm anomalies, defects, and failures. Web ProForum Tutorials Copyright © 17/58 http://www.iec.org The International Engineering Consortium Table 6. Anomalies, Defects, and Failures Description Criteria loss of signal LOS is raised when the synchronous signal (STS–N) (LOS) level drops below the threshold at which a BER of 1 in 103 is predicted. It could be due to a cut cable, excessive attenuation of the signal, or equipment fault. LOS state clears when two consecutive framing patterns are received and no new LOS condition is detected. out of frame OOF state occurs when four or five consecutive (OOF) SONET frames are received with invalid (errored) alignment framing patterns (A1 and A2 bytes). The maximum time to detect OOF is 625 microseconds. OOF state clears when two consecutive SONET frames are received with valid framing patterns. loss of frame LOF state occurs when the OOF state exists for a (LOF) alignment specified time in milliseconds. LOF state clears when an in-frame condition exists continuously for a specified time in milliseconds. loss of pointer LOP state occurs when N consecutive invalid pointers (LOP) are received or N consecutive new data flags (NDFs) are received (other than in a concatenation indicator), where N = 8, 9, or 10. LOP state clears when three equal valid pointers or three consecutive AIS indications are received. LOP can also be identified as follows: • STS path loss of pointer (SP–LOP) • VT path loss of pointer (VP–LOP) alarm indication The AIS is an all-ones characteristic or adapted signal (AIS) information signal. It is generated to replace the normal traffic signal when it contains a defect condition in order to prevent consequential downstream failures being declared or alarms being raised. AIS can also be identified as follows: Web ProForum Tutorials Copyright © 18/58 http://www.iec.org The International Engineering Consortium • line alarm indication signal (AIS–L) • STS path alarm indication signal (SP–AIS) • VT path alarm indication signal (VP–AIS) remote error This is an indication returned to a transmitting node indication (REI) (source) that an errored block has been detected at the receiving node (sink). This indication was formerly known as far end block error (FEBE). REI can also be identified as the following: • line remote error indication (REI–L) • STS path remote error indication (REI–P) • VT path remote error indication (REI–V) remote defect This is a signal returned to the transmitting indication (RDI) terminating equipment upon detecting a loss of signal, loss of frame, or AIS defect. RDI was previously known as FERF. RDI can also be identified as the following: • line remote defect indication (RDI–L) • STS path remote defect indication (RDI–P) • VT path remote defect indication (RDI–V) remote failure A failure is a defect that persists beyond the indication (RFI) maximum time allocated to the transmission system protection mechanisms. When this situation occurs, an RFI is sent to the far end and will initiate a protection switch if this function has been enabled. RFI can also be identified as the following: • line remote failure indication (RFI–L) • STS path remote failure indication (RFI–P) • VT path remote failure indication (RFI–V) Web ProForum Tutorials Copyright © 19/58 http://www.iec.org The International Engineering Consortium B1 error Parity errors evaluated by byte B1 (BIP–8) of an STS– N are monitored. If any of the eight parity checks fail, the corresponding block is assumed to be in error. B2 error Parity errors evaluated by byte B2 (BIP–24 x N) of an STS–N are monitored. If any of the N x 24 parity checks fail, the corresponding block is assumed to be in error. B3 error Parity errors evaluated by byte B3 (BIP–8) of a VT–N (N = 3, 4) are monitored. If any of the eight parity checks fail, the corresponding block is assumed to be in error. BIP–2 error Parity errors contained in bits 1 and 2 (BIP–2: bit interleaved parity–2) of byte V5 of an VT–M (M = 11, 12, 2) are monitored. If any of the two parity checks fail, the corresponding block is assumed to be in error. loss of sequence Bit error measurements using pseudo-random synchronization sequences can only be performed if the reference (LSS) sequence produced on the synchronization receiving side of the test set-up is correctly synchronized to the sequence coming from the object under test. To achieve compatible measurement results, it is necessary to specify that the sequence synchronization characteristics. Sequence synchronization is considered to be lost and resynchronization shall be started if the following occur: • Bit error ratio is greater than or equal to 0.20 during an integration interval of 1 second. • It can be unambiguously identified that the test sequence and the reference sequence are out of phase. Note: One method to recognize the out-of-phase condition is the evaluation of the error pattern resulting from the bit-by-bit comparison. If the error pattern has the same structure as the pseudo-random test sequence, the out-of-phase condition is reached. Web ProForum Tutorials Copyright © 20/58 http://www.iec.org The International Engineering Consortium

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