Tektronix SONET User Manual

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 Nation al 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 i n 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 tec hnology of SONET. If more de tailed information is required, consult the latest mate rial 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

1/58

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 expe cted to provide the tra nsport 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 s ectionalization

• definition of a synchr onous multiplexing format for carrying lower

level digital signals (such as DS–1, DS–3) and a synchronous structure that greatly simplifies the interface to d igital switches, digital crossconnect 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 accom modating 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

2/58

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 sy nchronous, asynchronous, a nd 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 li e within specified limits. These phase differences may be due to propaga tion 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 e xample, 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 asy nchronous signals, the transitions of the signals do not necessari l y 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 differen t capacities through a synchron ous, flexible, optical hierarchy. This is ac complished 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

3/58

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. Thi s 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

4/58

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 multip lex up to DS–3. DS–3s are multiplexe d up to higher rates in the same manner. At the higher asynchronous rate , they cannot be accesse d 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 S TS– 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 ph ase wander and preven t 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, thi s terminal will serve as a mas ter for other SONET nodes, providing timing on its outgoing OC–N signal. Other SONET nodes will operate

Web ProForum Tutoria ls http://www.iec.org

The International Engineering Consortium

5/58

in a slave mode ca lled 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 tha t 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 S TS–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 transp orted and routed over the SONET n etwork. Once the payload is multiplexed into the synchronous payload envelope, it can be transported and switched through SONET without having to b e examined and possibly demultiplexed at intermediate nodes. Thus, SONET is said to be serviceindependent 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 payloa d has the capacity to transport up to the following:

• 28 DS–1s

Web ProForum Tutoria ls http://www.iec.org

The International Engineering Consortium

6/58

• 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. Wi th 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 bi t 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 (pay load 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 electri cal rate used primari ly 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 tran sport overhead, and the last 87 columns make up the SPE. SPEs can have any alignment within the frame, and this alignme nt is indicated by the H1 and H2 pointer bytes in the line overh ead.

Web ProForum Tutoria ls http://www.iec.org

The International Engineering Consortium

7/58

Figure 2. STS–1 Frame Elements

STS–1 Envelope Capacity and Synchronous Payload Envel ope (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 9row 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 contain ed in the transport overhead designates the loc ation 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

8/58

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 byteinterleaving 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 ST S–N multiplexers. Section overhead is used for communications between adjacent network elements such as regenerators.

Web ProForum Tutoria ls http://www.iec.org

The International Engineering Consortium

9/58

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 refle ct 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

10/58

• 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 and

framing bytes—These two bytes indicate the beginning of an STS–1

frame.

A2 J0 section trace (J0 )/ s ection 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 fi rst 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

11/58

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, D2, and D3

section data communications channel (DCC) bytes—Together,

these 3 bytes form a 192–kbps message channel providing a messagebased channel for OAM&P between pieces of section-terminating equipment. The channel is used from a cen tral location for alarms, control, monitoring, administration, and other communication needs. It is available for internally generated, externally generated, or manufacturerspecific messages.

Line Overhead

Line overhead contains 18 bytes of overhead accessed, generated, and processed by line-terminating equipment. This overhead supports functions such as th e following:

• locating the SP E 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

12/58

Table 4 shows line overhead byte by byte.

Table 4. Line Overhead

Byte Description

H1 and H2

STS payload pointer (H1 and H2)—Two bytes are allocated to a

pointer that ind icates the offset in bytes between the pointer and the first 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 thi s byte when it is not used to carry the SPE byte is undefined.

B2 line bit-interleaved parity code (BIP–8) byte—Thi s 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 and K2

automatic protection switching (APS channel) bytes—These 2

bytes are used for p rotection signaling between line-terminating entities for bidirectional autom atic protection switching and for detecting alarm indication signal (AIS–L) and remote defect indication (RDI) signals.

D4 to D12

line data communications channel (DCC) bytes—These 9 bytes

form a 576–kbps message c ha nnel from a central location for OAM& P 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 protoc ol analyzer is required to access the line–DCC informatio n.

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 S T S–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 Tutoria ls http://www.iec.org

The International Engineering Consortium

13/58

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 Tutoria ls http://www.iec.org

The International Engineering Consortium

14/58

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 repe titively

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-terminati ng

status and performance back to the originating path-ter minating 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 i s used for user communication

between path elements.

Web ProForum Tutoria ls http://www.iec.org

T multiframe indicator byte—This byte provides a generalized

The International Engineering Consortium

15/58

multiframe indicator for p ayload 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 a t the first byte of the VT SPE. VT POH provides for communi cation between the poin t 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 c onvey the VT path terminating performance back to an originating VT PTE. Bit 4 of th e 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

16/58

• error performance monitoring

• pointer adjustment information

• path status

• path trace

• section trace

• remote defect, error, and failure indications

• signal labels

• new data flag ind ications

• 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 actua l 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 performanc e 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 Tutoria ls http://www.iec.org

The International Engineering Consortium

17/58

Table 6. Anomalies, Defects, and Failures

Description Criteria

loss of signal (LOS)

out of frame (OOF) alignment

loss of frame (LOF) alignment

loss of pointer (LOP)

LOS is raised when the synchronous signal (STS–N) level drops below the threshold at which a BER of 1 in 103 is predicted. It could be due to a cut cable, excessive atten uation of the signal, or equipment fault. LOS state clears when two consecutive framing patterns are received and no new LOS condition is detected.

OOF state occurs when four or five consecutive SONET frames are received with invalid (errored) 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.

LOF state occurs when the OOF state exists for a specified time in milliseconds. LOF state clears when an in-frame condition exists continuously for a specified time in milliseconds.

LOP state occurs when N consecutive invalid pointers 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 poi nters or three consecutive AIS indications are received.

alarm indication signal (AIS)

Web ProForum Tutoria ls http://www.iec.org

LOP can also be ide ntified as follows:

• STS path loss of pointer (SP–LOP)

• VT path loss of pointer (VP–LOP)

The AIS is an all-ones characteristic or adapted 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:

The International Engineering Consortium

18/58

+ 40 hidden pages