11 Chapter 6 SDH Fundamentals(Book)

Chapter 6: SDH Fundamentals [FUNDAMENTALS OF SDH AND OPTICAL FIBER ENGINEERING]

ReviewedJune2012 Page

Introduction The Synchronous Digital Hierarchy (SDH) forms the backbone of many service provider and carrier networks. SDH is used as time-division multiplexed (TDM) transport mechanism for voice and data. Frame- and cell-switched data networks, such as Frame Relay and ATM also depend on SDH TDM circuits. Why SDH? Early copper-based electrical transmission systems used time-division multiplexing to resolve capacity issues. However, with the massive increase in voice traffic, and commercialization of optical technology in the early 1980s, fiber-optic cable technology turned out to be the medium of choice. Information that previously required hundreds of copper pairs could be transmitted down a glass fiber only slightly thicker than a human hair. Carriers adopted fiber-optic technology and started testing and deploying fiber networks. These first-generation fiber-optic systems in the Public Switched Telephone Network (PSTN) used its private architectures, equipment, line codes, multiplexing formats, and maintenance procedures. Interconnection and interoperability between two carriers (each using different hardware vendors) was almost impossible to achieve. So the need of standardization was required to solve the issues of interconnection and interoperability between the carriers. The Consultative Committee for International Telegraph and Telephone (CCITT) had established a standard, Synchronous Digital Hierarchy (SDH), in Europe results from the SONET concepts proposed in the USA. The CCITT organization reorganized and formed the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) in 1993. The ITU-T SDH standards and equipment are implemented in Europe, Middle East, and Africa (EMEA), Latin America, and in the Asia Pacific region. Advantages of SDH over PDH

1. SDH provides universal standard for both electrical and optical interfaces including digital signal rate levels, frame structure, multiplexing method, monitoring, and management. So SDH equipment from different vendors can be easily interconnected.

2. Lower-rate signals are multiplexed into higher-rate signals via byte interleaved method. So their locations in SDH frame are known, therefore it can be added or dropped directly.

3. Abundant overhead bytes for operation, administration and maintenance (OAM) functions are arranged in the frame structures of SDH signals. This greatly enforces the network monitoring function, i.e. automatic maintenance.

4. SDH networks can be used for transmitting PDH signals as well as other Hierarchies like ATM, FDDI, SAN, and ESCON.

In this chapter, we will discuss SDH Architecture, SDH Frames, and SDH Multiplexing types. 6.1 SDH Architecture To illustrate The SDH Architecture we should explain its components: (signals, devices, and connections) and its Layers.



6.1.1 SDH Components Signals The SDH uses the term Synchronous Transport Module-N (STM-N) to indicate the optical as well as electrical signal level. The basic unit of transmission in SDH is (STM-1) =155.52 Mb/s, and Higher SDH rates are defined as shown in the table below.

Each STM-N interface rate contains SDH Overhead (OH) information to support Operation, Administration and Management (OAM) functions and a Payload capacity for the traffic. Devices The figure below shows a simple link using SDH devices. SDH transmission depends on three basic devices: Terminal Multiplexer (TM), Regenerator (R), and Add/Drop Multiplexer (ADM).

Regenerator or Repeater (R) The regenerator is a device that regenerates attenuated signals. A regenerator is needed when we have a long distance between multiplexers which make the signal level in the fiber gets attenuated and becomes too low to drive a receiver. Terminal Multiplexer/De-multiplexer (TM) A TM has two sides, a customer or tributary side that supports one or more tributary signals, and a network or line side that supports a line signal. A TM multiplexes tributary signals (PDH signals) into a line signal an (SDH STM-N) Add/Drop Multiplexer (ADM) An ADM has three sides, one tributary side and two line sides also referred to as east side and west side. An ADM is capable of multiplexing tributary signals for insertion (Add) into one of the two line interfaces, or de-multiplexing a tributary signals (Drop) from one of the two line interfaces, and also passing through tributary signals from one line interface to the other. In the simple configuration shown in the Figure above, a number of incoming tributary signals (from terminals) are fed into a TM multiplexer, where they are combined into a single

STM-N Rate(Mb/s) Abbreviated as STM-1 155.520 155 Mb/s STM-4 622.080 622 Mb/s

STM-16 2488.320 2.5 Gb/s STM-64 9953.280 10 Gb/s

STM-256 39813.12 40 Gb/s

Tributary Signals 2Mb/s

34Mb/s

140Mb/s

Line Signals STM-N

Tributary Signals 2Mb/s, 34Mb/s, 140Mb/s

Tributary Signals 2Mb/s

34Mb/s

140Mb/s



optical signal. The optical signal is transmitted to a regenerator, where it is recreated without the noise. The regenerated signals are then fed into an Add/Drop multiplexer. The Add/Drop multiplexer reorganizes these signals, if necessary, and sends them out as directed by information in the data frames. These re-multiplexed signals are sent to another regenerator and from there to the receiving TM de-multiplexer, where they are returned to its original form. Connections The devices defined in the previous section are connected together by using Regenerator sections, Multiplex sections, and path as shown in the figure below.

Regenerator section Its the section between two Regenerators (R & R) or between a Regenerator and a Transport Node (R & TM) or (R & ADM). Multiplex section Its the section between two Transport Nodes (TM & ADM). Path Its the end-to-end portion of the network between two Transport nodes (TM & TM). 6.1.2 SDH Layers The SDH standard includes four functional layers: the Photonic layer, the Regenerator section layer, the Multiplex section layer, and the Path layer. They correspond to both the Physical and the Data link layers of OSI Model, the headers added to the frame at the various layers were discussed later in this chapter. See the figure below.

Reg. Section Reg. Section Reg. Section Reg. Section Reg. Section

Multiplex Section Multiplex Section

Path



Photonic layer The photonic layer corresponds to the physical layer of the OSI model. It includes physical specifications for the optical fiber channel, the sensitivity of the receiver, and so on. SDH uses NRZ encoding with the presence of light representing 1 and the absence of light representing 0. Regenerator Section Layer The Regenerator section in SDH is the portion of a transmission facility, including terminating points, between a transport node and a Regenerator, or two Regenerators. Multiplex Section Layer The multiplex section provides the means of transporting information between two consecutive transport nodes. One of the transport nodes originates the signal, and the other terminates the signal in either direction. Path Layer The path layer is responsible for the movement of a signal from its optical source to its optical destination. At the optical source, the signal is changed from an electronic form into an optical form, multiplexed with other signals, and encapsulated in a frame. At the optical destination, the received frame de-multiplexed, and the individual optical signals are changed back into their electronic forms. The figure below shows the relationship between the devices used in SDH transmission and the four layers of the standard. As you can see TM multiplexer is a four-layer device, Add/Drop multiplexer is a three-layer device and Regenerator is a two-layer device.

6.2 SDH Frames It is called a Synchronous Transport Module STM-N frame which is a two-dimensional matrix of bytes with 9 rows x 270 columns x N. For example, STM-1 frame is 9 rows x 270 columns x 1= (2430 bytes), and an STM-4 is 9 rows x 270 columns x 4 = (9720 bytes). The figure below shows the general format of STM-1 and STM-N.



6.2.1 Frame, Byte, and Bit Transmission One of the interesting points about SDH is that each STM-N signal is transmitted at a fixed rate of 8000 frames per second. This is the rate at which voice is digitized as discussed before. For each frame the bytes are transmitted from the left to the right, top to the bottom. For each byte, the bits are transmitted from (left to right). The figure below shows the order of frame and byte transmission.

6.2.2 STM-1 Frame Format The basic format of an STM-1 frame is shown in the figure below. As we said before, a STM-1 frame is a matrix of 9 rows of 270 bytes with total of 2430 bytes.

The STM-1 Frame consists of 1. The Section OverHead (SOH): which is further classified to Regenerator Section

OverHead (RSOH) and Multiplex Section OverHead (MSOH): these bytes implement OAM functions.

2. Administrative Unit pointer (AU-PTR): these bytes used to indicate the start of the payload.

3. Virtual Container (VC-4) or Payload: It contains Container (C-4) to carry user data and Path Overhead (POH) to monitor the data in real time.

The SDH rate is synchronous and is flexible enough to support many different signals. The SDH STM-1 rate is 155.520 Mb/s can be computed as follows: STM-1 rate = 270 columns x 9 rows x 8 bits/byte x 8000 frames/sec = 155.520 Mb/s.

Columns 10 to 270 constitute the VC with the POH that provides a payload data rate of 150.336 Mb/s. The actual data rate of the payload can be computed as follows:

Data rate = 261 columns x 9 rows x 8 bits/byte x 8000 frames/sec = 150.336 Mb/s.

Payload



6.3 SDH Multiplexing It is very important to understand the SDH multiplexing hierarchy to truly understand and appreciate SDH. There are Two types.

1. PDH to SDH signals (2Mb/s, 34Mb/s, and 140Mbit/s to STM-1). 2. Low rate to high rate SDH signals (STM-1 to STM-N).

6.3.1 Multiplexing PDH Signals to SDH signals E4 (140Mb/s) to STM-1

1. First the 140 Mb/s PDH signal adapted via bit stuffing into contair called C-4. The C-4 Container has 9 rows x 260 columns (2340 bytes). The frame rare of C-4 Container is 8000 frame/s. So the E4 signal after adaptation is = 9 rows x 260 columns x 8 bit/byte x 8000 frame/s =149.760 Mb/s.

2. A column of Path Overhead (POH) is added to every C-4 block to implement real-time monitoring over the path of 140Mb/s Signals. The resulting block is called VC-4. The rate of VC-4 = 9 rows x 261 columns x 8 bit/byte x 8000 frame/s =150.336 Mb/s.

3. A row of Administrative Unit Pointer (AU-PTR) is added to the VC-4 to indicate the start of the VC-4 in the payload. The resulting is called AU-4and it has the basic structure of STM-1 frame but without SOH.

4. The Section Overhead (RSOH + MSOH) is added to the AU-4 to complete the STM-1 Frame.



E3 (34Mb/s) to STM-1 1. First the 34Mb/s PDH signal adapted via bit stuffing into contair called C-3.

The C-3 Container has 9 rows x 84 columns (756 bytes).

2. A column of Path Overhead (POH) is added to every C-3 block to implement real-time monitoring over the path of 34Mb/s Signals. The resulting block is called VC-3.

3. A three bytes Tributary Uint pointer (TU-PTR) is added to every VC-3 block to indcate the start of the VC-3 in the payload. The resulting is called Tributary Unit 3(TU-3). i.e the TU-PTR indicates the start of VC-3 in TU-3.

4. TU-3 frame structure is incomplete. Six Random bytes (R) are stuffed to fill the gap of

the TU-3. The new block is called Tributary Unit Group 3 (TUG-3).



5. Three (TUG-3) are byte interleaved into container C-4.

6. C-4 container is not complete. We add two columns of stuffed bytes (R).

7. Multiplexing C-4 to STM-1 is similar to the previous process discussed in multiplexing 140Mb/s PDH signal to STM-1.

i.e three E3 (34Mb/s) PDH signals Can be multiplexed into one STM-1 E1 (2Mb/s) to STM-1

1. First the 2 Mb/s PDH signal adapted via bit stuffing into contair called C-12. The C-12 Container can mount (34 bytes).

2. One byte of Low-Order Path Overhead (LO-POH) is added to the notch in the top left corner of each C-12 block to implement real-time monitoring over the path of 2Mb/s Signals. The resulting block is called VC-12.



3. One byte of Tributary Uint pointer (TU-PTR) is added to every VC-12 block to indcate the start of the VC-3 in the payload. The resulting is called Tributary Unit 12(TU-12). i.e the TU-PTR indicates the start of VC-12 in TU-12.

4. Three Tributary Unit 12 (TU-12) are byte interleaved to form Tributary unit group 2 (TUG-2).

5. Seven TUG-2 are byte interleaved with the same method and add two columns of stuffed bytes to form TUG-3.

6. Multiplexing TUG-3 to STM-1 is similar to the previous process discussed in multiplexing 34Mb/s PDH signal to STM-1.

The process of multiplexing E1 to STM1 pass through three byte interleaved processes: 3 TU-12, 7 TUG-2, and 3 TUG-3. So the number of multiplexed E1s = 3 x 7 x 3= 63 E1.(i.e 63 E1 (2Mb/s) PDH signals Can be multiplexed into one STM-1).

SDH Multiplexing structure ITU-T defined a complete multiplexing structure, through which digital signals of three PDH hierarchies can be multiplexed into STM-N signals. This multiplexing structure includes some basic multiplexing units:


ReviewedJune2012 Page10

C (Container), VC (Virtual Container), TU (Tributary Unit), TUG (Tributary Unit Group), AU (Administrative Unit), AUG (Administrative Unit Group).

Low-rate tributaries are multiplexed into STM-N signals through three procedures: mapping, aligning, and multiplexing. SDH mapping: is a procedure by which tributaries are adapted into virtual containers at the boundary of an SDH network, e.g. E1 into VC-12. SDH aligning: is a procedure to add TU-PTR or AU-PTR into the VC-12, VC-3 or VC4. The pointer value constantly locates the start point of VC within the TU or AU so that the receiving end can correctly separate the corresponding VC. SDH multiplexing: is the procedure by which the TUs are organized into the higher order VC or the AUs are organized into STM-N via byte interleaving. 6.3.2 Low rate SDH Signals to high rate SDH signals STM-N Frame Structure



From STM-1 to STM-N

6.4 Overhead The functions of overhead are to implement layered monitoring management (OAM) for SDH signals. The monitoring is classified into Section layer monitoring and Path layer monitoring. The section layer monitoring is further classified into Regenerator section layer monitoring and Multiplex section layer monitoring while the path layer monitoring is further classified into Higher order path layer and Lower order path layer. Example The figure below shows that the incoming tributary signals are adapted into the suitable containers, after that the terminal multiplexer generates the POH and SOH (RSOH1, MSOH1), and then sends the STM-N signal to the regenerator. The regenerator evaluates the RSOH1 and generates new RSOH2 and sends the STM-N signal to the second regenerator. The second regenerator evaluates the RSOH2 and generates RSOH3 and sends the STM-N signal to the terminal multiplexer. The terminal multiplexer evaluates the POH, MSOH1, and RSOH3and then drop the tributary signals.

VC-4 #1 VC-4 #1

VC-4 #2

VC-4 #3

VC-4 #N VC-4 #N

VC-4 #3

VC-4 #2 RSOH

MSOH



We can now define (Path, Multiplex section, and Regenerator section) again as follows Regenerator section Its the section between two Regenerators (R & R) or between a Regenerator and a Transport Node (R & TM) where RSOH generated or evaluated or extracted. Multiplex section Its the section between two Transport Nodes (TM & ADM) where the MSOH generated or evaluated or extracted. Path Its the end-to-end portion of the network between two Transport nodes (TM & TM) where the POH generated or extracted. 6.4.1 Section Overhead (SOH) Section Overhead Functions

a) Frame alignment. b) STM-1 Identification. c) Error Control. d) Engineering orderwire. e) User channel. f) Data communication channel. g) Automatic protection switching. h) Synchronization quality level.

a) Frame alignment (A1, A2)

Indicate the beginning of the STM-N frame. Bytes are unscrambled. A1 = f6H (11110110), A2 = 28H (00101000). STM-N: (3N) A1 bytes, (3N) A2 bytes.

The first step is to properly extract each STM-N frame from the received continuous signal stream at the receiver. The function of the bytes A1 and A2 is to locate the start of the STM-N



frame. So the receiver can align and extract the STM-N frame from the information stream via these two bytes. How does the receiver align the frames via the A1 and A2 bytes? The A1 and A2 have fixed value, i.e. fixed bit patterns: A1: 11110110 (f6H) and A2: 00101000(28H). The receiver monitors each byte in the stream. After detecting 3N successive f6H bytes followed by 3N 28H bytes (there are three A1 and three A2 bytes within an STM-1 frame), the receiver determines that an STM-N frame starts to be received. By aligning the start of each STM-N frame, the receiver can identify different STM-N frames and disassemble them. If the receiver doesn't receive A1 and A2 bytes within five or more successive frames (625us), i.e. it can't identify the start of five successive frames (identify different frames), it will enter out-of-frame status and generate out-of-frame alarm ---- OOF. If the OOF keeps for 3ms, the receiver will enter loss-of-frame status ---- the equipment will generate loss-of-frame alarm ---- LOF. Meanwhile, an AIS signal will be sent downward and the entire services will be interrupted. Under LOF status, if the receiver stays in normal frame alignment status again for successive 1ms or more, the equipment will restore the normal status. b) STM-1 Identification (J0) Its used for repeatedly transmission to give a path through an SDH network a (predefined Name between the receiver & intended transmitter), this (predefined Name) enables the receiver to check the continuity of its connection with the intended transmitter. Also its used as an STM-1 identifier, each STM-1 frame take an ID number before being interleaved into an STM-N signal. Note Before the STM-N frame transmitted by the physical link (optical fiber) it is always scrambled at the transmitter except RSOH first row and at the receiver side each frame is descrambled to restore the original data sequence. The RSOH first row (A1, A2, and J0) is not scrambled because it contains bytes locating frames beginning. c) Error monitoring (B1, B2) B1 is one byte uses the technique of Bit Interleaved Parity Code (BIP-8) at the regenerator section level (for detailed description about BIP-N Technique please see appendix 2). The B1 Byte is only defined in the first STM-1 frame in an STM-N signal. To understand the mechanism of B1 see the figure below and follow its related discussion.



1- The transmitting equipment processes BIP-8 even parity over all bytes of the previous frame (1#STM-N) after scrambling [parity process result = x] and places the result in byte B1 of the current frame (2#STM-N) before scrambling [B1= x]. 2- The receiver processes BIP-8 parity over all bits of the current frame (1#STM-N) before de-scrambling [result =y]and compares the parity result and the value of B1 in the next frame (2#STM-N) after de-scrambling[which is x]. 3-If xy , it means an error occurred and the different bit means error block. According to the number of different bits, we can monitor the number of error blocks occurred in 1#STM-N frame during transmission. 4- If the B1 of the receive end has detected error blocks, the number of error blocks detected by the B1 will be displayed in receive end performance event RS-BBE (Regenerator Section Background Block Error). 5-When the error bits detected by the receive end exceed a given threshold, the equipment will report corresponding alarms, i.e. when the error bit ratio (EBR) is greater than 10-6, the given alarm is B1-SD(signal degrade) & When the error bit ratio (EBR) is greater than 10-3, B1-EXC (another name B1-OVER) will be given. 6- It is necessary to differentiate between events & alarms; all alarms are events not vice versa, and the alarm is an indication for a problem need to be solved almost quickly. B2 is similar to B1 in operation mechanism except that it monitors the error status of the multiplex section layer. The B1 byte monitors the transmission error of the complete STM-N frame signal. There is only one B1 byte in an STM-N frame. There are N*3 B2 bytes in an STM-N frame with every three B2 bytes corresponding to an STM-1 frame. The B2 monitoring adopts BIP-24 (three bytes) method, which can at most monitor 24 error blocks one time. As can be seen from the BIP-24 parity method, each bit of the parity result is corresponding to a bit block. So three B2 bytes can at most monitor 24 error blocks from an STM-N frame that occur during transmission (The result of BIP-24 is 24 bits with each bit corresponding to a column of bits ---- a block). If the B2 of the receive end has detected error blocks, the number of error blocks detected by the B2 will be displayed in this end performance event MS-BBE (Multiplex Section Background Block Error). At the same time, M2 will be used to report to the transmit end that error blocks have been detected, and the transmit end will report MS-FEBBE (Multiplex Section Far End Background Block Error) performance event and MS-REI (Multiplex Section Remote Error Indication) alarm. When the error bits detected by the receive end exceed a given threshold, the equipment will report corresponding alarms. When the error bit ratio (EBR) is greater than 10-6, the given alarm is B2-SD. When the error bit ratio (EBR) is greater than 10-3, B2-EXC will be given.

Tx Rx

1# STM-N

2# STM-N

2# STM-N

1# STM-N



Multiplexing Section Remote Error Indication MS-REI(M1)

(1) When NE2 receive the traffic it will evaluate B2 bytes. (2) If NE2 find bit errors it will generate a performance event MS-BBE. (3) At the same time NE2 will return M1 to report NE1 that error blocks have been

detected. (4) NE1 will generate performance event MS-FEBBE. (5) And generate MS-REI alarm.

This is a message returned to its transmit end by the receive end so that the transmit end can get the receiving error status of the receive end. For STM-N levels this byte conveys the count (in the range of [0, 255]) of interleaved bit blocks that have been detected in error by the BIP-24uN (B2). For rates of STM-16 and above, this value shall be truncated to 255. M1 byte is only defined is STM-1 frame in an STM-N signal. d ) Engineering orderwire (E1, E2) E1 is part of the RSOH and may be accessed at regenerators. E2 is part of the MSOH and can be accessed at multiplex section terminations. Each of these two bytes provides a 64kb/s orderwire channel for voice communication, i.e. voice information is transmitted via these two bytes. The orderwire provides a convenient communication function during maintenance and troubleshooting. E1and E2 bytes are only defined is STM-1 frame in an STM-N signal. e) User channel (F1) It is reserved for the network operators and can be used to provide a 64 kb/s data or voice channel i.e. this byte can be used for services and maintenance purposes. This byte is only defined in the first STM-1 frame in an STM-N frame. f) Data communication channel

RS-DCC D1 ~ D3 192 Kbit/s (3x64 Kbit/s). MS-DCC D4 ~ D12 576 Kbit/s (9x64 Kbit/s).



One of the features of SDH is its highly automatic OAM function which can conduct commands issues and performance auto poll to the networks element via NMT Network Management Terminals. SDH has some functions which do not exist in PDH systems, such as real-time service allocation, alarm fault location and on-line performance testing. Where are these OAM data arranged to transmit? The data used for OAM functions, such as sent commands and checked alarm performance data, are transmitted via D1-D12 bytes within the STM-N frame. Thus the D1-D12 bytes provide a common data communication channel accessible to all SDH network elements. As the physical layer of the embedded control channel (ECC), the D1-D12 bytes transmit OAM information among the network elements and form a transmission channel of the SDH management network (SMN). The DCC has a total rate of 768kb/s that provides a powerful communication base for SDH network management. D1, D2 and D3 are regenerator section DCC bytes (DCCR) with a rate of 364kb/s192kb/s and are used to transmit OAM information among regenerator section terminals. D4-D12 are multiplex section DCC bytes (DCCM) with a sum rate of 964kb/s=576kb/s and are used to transmit OAM information among multiplex section terminals. g) Automatic protection switching (K1, K2 (b1-b5)) Automatic Protection Switching (APS) is the capability of a transmission system to detect a failure on a working facility and to switch to a standby facility to recover the traffic. This capability has a positive effect on the overall system availability. The Multiplex Section in SDH is protected in this automatic fashion. The Multiplex Section protection (MSP) mechanisms are coordinated by the K1 and K2 bytes in the Multiplex Section Overhead. You should know that Protection switching is initiated as a result of one of the following situations:

1- Signal failure. 2- Signal degradation. 3- In response to commands from a local craft terminal or a remote network manager.

There are two modes of APS, 1+1 protection switching and 1: N protection switching. More about APS and K1, K2 bytes will be discussed later in chapter 8. Multiplex Section Remote Defect Indication (MS-RDI) K2 (b6-b8): The bits 6, 7 and 8 of K2 are used for error indication alarm function; you can understand the function of these bits by the following examples: As shown in figure below , If the signal from REG to TM2 was defected resulting in bit error rate exceed 10-3 then TM2 sets K2(b6-b8) to 110 and transmits a new STM-N frame to TM1, and TM1 receiver interprets that as an error message of MS-RDI return by the opposite side.



Another probability may occur; If the optical line between TM1and REG is disconnected ,then a status of loss of frame (LOF) will happen and K2(b6-b8) will be set to 111 and will be transmitted to TM2 ,then TM2 receiver interprets the message to an MS-AIS(multiplex section alarm indication signal )as an alarm received by the current side , meanwhile TM2 sets K2(b6-b8) to 110 and sends it to the opposite side , after that normally TM1 generates an MS-RDI alarm return by the opposite direction ,as shown in the figure below.

As a brief summary: we can say that if the SDH MUX receiver detects: x K2 =110, then the MUX will generate MS-RDI alarm. x K2=111, then the MUX will generate MS-AIS alarm, and return MS-RDI to the

opposite direction MUX. h) Synchronization quality level (S1) Bits 5 to 8 of S1 byte are allocated for Synchronization Status Messages (SSM) to indicate the synchronization quality level. The table below gives the assignment of bit patterns to the four synchronization levels agreed to ITU-T.

bits 5 ~ 8 Description 0000 Quality unknown (existing sync. Network) 0010 G.811 PRC 0100 SSU-A (G.812 transit) 1000 SSU-B (G.812 local) 1011 G.813 (Sync. Equipment Timing Clock) 1111 Do not use for sync (DNU).

Different bit patterns, indicating different quality levels of clocks defined by ITU-T, enable the equipment to determine the quality of the received clock timing signal. This helps to determine whether or not to switch the clock source, i.e. switch to higher quality clock source. The smaller the value of S1 (b5-b8), the higher the level of clock quality. Bits 1 to 4 of S1 byte indicate the different external clock sources ID for clock protection purposes. This byte is only defined in the first STM-1 frame in an STM-N signal. Z1 and Z2 Bytes are spare bytes for future use

BER > 10-3 TM 1 REG TM 2 MS-RDI

110 110

Line disconnection REG TM 2 TM 1

110 110

111 111

110

MS-AIS MS-RDI



6.4.2 Path Overhead (POH) The Section Overhead is responsible for section layer OAM functions while the Path Overhead for path layer OAM functions. In other words, by considering the STM-N frame is analogue to a truck while the SOH take care of the whole truck the POH take care of the goods inside. According to the "width" of the monitored path (the size of the monitored cargo), the Path Overhead is further classified into Higher Order Path Overhead and Lower Order Path Overhead. In this curriculum the Higher Order Path Overhead refers to the monitoring of VC-4 level paths within the STM-N frame. The Lower Order Path Overhead implements the OAM functions for VC-12 path level, i.e. monitoring the transmission performance of 2Mb/s signals within the STM-N frame. The Higher Order Path Overhead (HO-POH) It consists of 9 bytes, located in the first column of the VC-4 frame as shown in the figure below.

Path trace byte (J1) The J1 is the start of the VC-4. The J1 byte is used as an Identifier so that a path receiving terminal can verify its continued connection to the intended transmitter. This requires that the J1 bytes of the received and transmit ends match. The default transmit/receive J1 byte values differ according to equipment vendors and it may be 16 or 64 byte character string ABCD and transmitted over 16 or 64 consecutive STM frames. The received J1 should match or be same with the expected J1. If not, HO-Path Trace Identifier Mismatch (HP-TIM) alarm will be generated, which perhaps interrupts the service within the VC4 sometimes.



Path BIP-8 byte (B3) The B3 byte is allocated for monitoring the transmission error performance of VC-4. Its monitoring mechanism is similar to that of the B1 and B2 except that it is used to process BIP-8 parity for the VC-4 frame. Once the receive end detects error blocks, the number of error blocks will be displayed in the performance monitoring event ---- HP-BBE (Higher Order Path Background Block Error) of the received end. At the same time, G1 (b1-b4) will be used to report to the transmit end that error blocks have been detected, and the transmit end will report HP-FEBBE (Higher-order Path Far End Background Block Error) performance event and HP-REI (Higher-order Path Remote Error Indication) alarm. When the error bits detected by the receive end exceed a given threshold, the equipment will report corresponding alarms. When the error bit ratio (EBR) is greater than 10-6, the given alarm is B3-SD. When the error bit ratio (EBR) is greater than 10-3, B3-EXC will be given. Signal label byte (C2) This byte indicates type and composition of the VC-4 tributary information. As shown in the figure below. The received C2 should match or be same with the expected C2. If not, HO-Path Payload Label Mismatch (HP-PLM) alarm will be generated, which interrupts the service within the VC4 sometimes. And if the received C2 is 00H a HO-Path Unequipped (HP-UNEQ) alarm will be generated. As shown in the figure above. Path Status Byte (G1) It is used to give to the VC3/VC4 path source some important information about the actual status and performance of the path sink or termination.

Unequipped C2 C2 C2 C2 C2

C-4(140Mb/s) 3 X TUG-3(34 or 2Mb/s)

ATM Cells FDDI signal

C2= 00000000 C2= 00010010 C2= 00000010 C2= 00010011 C2= 00010101

For 5 frames



As shown in figure above the G1 byte is divided into three portions:

The first 4 bits contain the High order Path Remote Error Indication (HP-REI), and its value range from 0-to-15.If the value is included in the range 0-8, it is equal to the number of errors and violations detected using the BIP-8 code (i.e. the received byte B3);All other values (between 9 and 15) mean: no error detected.

The fifth bit contains the High order Path Remote Defect Indication (HP-RDI), this flag is set to 1, and returned in the opposite direction when the receiving end has detected an incoming path failure (i.e. no valid signal was received in the VC-4, due to many causes such as an AIS, error overflow, J1&C2 mismatch.etc); otherwise, it is reset to 0.

The last three bits are actually not used yet. Path User Channel Bytes (F2 and F3) Two 64 kb/s channels available for communication between the path start and path end. Multiframe Indication Byte (H4) A multiframe synchronization is required if the VC-4 contains VC-2s, VC-12s, VC-11s. These multiframes are spread over 500s. It is therefore necessary to indicate in which VC-4 they start. The value of this byte ranges from 00H to 03H. When multiframe indicator value (H4) not recovered for X ms, where X = 1 to 5 ms, an alarm of Tributary Unit Loss of Multiframe (TU-LOM) will be generated. Other bytes

Byte K3 (Automatic Protection Switching): it is used to protect the higher order path level. Only the first 4 bits are actually used for this purpose while the last 4 are not used yet.

Byte N1 (Network Operator): it is used for management and maintenance purposes. The Lower Order Path Overhead (LO-POH) The LO-POH here refers to the path overhead of the VC-12 that monitors the transmission performance of the VC-12 path level, i.e. monitors the transmission status of 2Mb/s PDH signals within the STM-N frame. Where is the LO-POH located within the VC-12? The lower order POH is located in the first byte of each VC-12 basic frame. An LO-POH consists of four bytes denoted V5, J2, N2 and K4. Usually, V5 byte is the most important in LO-POH.

5 1 2 3 4 6 7 8

HP-REI HP-RDI NOT USED



V5 It is the first byte in VC-12. The TU-12 pointer points at this byte and represents thus the reference point of the VC-12. V5 is used for transmitting the following information. Bit 1, 2 Error Performance Monitoring (BIP-2) These two bits are used for error monitoring over the complete lower-order path. The result is calculated to obtain an even parity. Bit 3 Remote Error Indication (REI) It is set to 1 and returned to the opposite direction if one or more errors were detected via the BIP-2. Bit 4 Remote Failure Indication (RFI) It is set to 1 and returned to the opposite direction if a fault or failure were detected. Bit 5, 6, and 7 Signal Label These three bits correspond with the C2 byte of the higher-order POH. The use of the three special mapping indicators 010, 011 and 100 is optional. However, these values must not be used for other purposes.

b5 b6 b7 Meaning 0 0 0 Unequipped 0 0 1 Equipped non specific payload 0 1 0 Asynchronous 0 1 1 Bit - Synchronous 1 0 0 Byte Synchronous 1 0 1

Equipped - unused 1 1 0 1 1 1

Bit 8 Remote Defect Indication (RDI) Its set to 1 and returned to the source if either a TU-12 AIS or Signal failure condition is being detected by the sink. J2 Path Trace byte Its used as an Identifier so that a path receiving terminal can verify its continued connection to the intended transmitter. This requires that the J2 bytes of the received and transmit ends match. K4 Automatic Protection Switching byte Bits 1 to 4 are provided for controlling automatic protection switching processes at the lower-order level. Bits 5 to 8 are reserved for future applications. N2 Network operator byte Its used for specific management purposes.

5 1 2 3 4 6 7 8

BIP-2 LP-REI LP-RFI Signal Label LP-RDI



6.5 Pointers SDH network is intended to be a synchronous network. However, there will always be slight timing difference because different clocks are being used or the same clock is being distributed over long distances. SDH pointers allow this limited asynchronous operation within the synchronous network. The VC has no fixed phase relation to the frame it can start at any position within the payload area. Most often it begins in one frame and ends in another. This ability of the VC to move relative to the STM frame is known as floating. Pointer types In the Synchronous Digital Hierarchy (SDH), there are two pointer types: the AU pointer and the TU pointer which are used for aligning of VC-4 and VC-12 within the AU-4 and TU-12 respectively. 6.5.1 AU-4 Pointer In the AU-4, only every third byte is provided with a counting no.. The three bytes with number 0 start to the right of the last pointer byte (H3). The bytes from 522 to 782 are located in front of the pointer in rows 1 to 3. Consequently, pointer values higher than 521 point to the next STM-1 frame. Structure of AU-4 Pointer H3 is the Pointer action byte, if the frequency of the STM-N frame is not exactly identical with the one of the VC-4; the pointer value is increased or decreased by 1 at regular intervals, while frame matching is simultaneously performed by a positive or negative justification process. After each pointer correction, at least three frames must be transmitted without pointer modification. The frequency matching process for AU-4 pointers Positive justification If the frame frequency of the VC-4 is lower than that of the STM-N frame, stuffing bytes must be inserted and the pointer value must be increased by 1 at regular intervals.



The stuffing bytes are inserted directly behind the last H3 byte, for an AU-4 three stuffing bytes are inserted. The new pointer (P+1) is then transmitted starting at the next frame. The next VC-4 starts at the position indicated by the new pointer. Negative justification If the frame frequency of the VC-4 is higher than that of the STM-N frame, additional information of the VC-4 must be transmitted in the H3 bytes and the pointer value must be decreased by 1 at regular intervals. The following three H3 bytes are filled with information. The new pointer (P-1) is transmitted starting at the next frame. The next VC starts at the position indicated by the new pointer.



H1 and H2 are read as a 16-bit data word including the New Data Flag NDF and the pointer value. In case of modifications of the payload, it may be necessary to set a new pointer value. In order to indicate this change, the so-called New Data Flag (NDF) is set. Then the new pointer value is transmitted. On the receive side, the NDF is evaluated. The new pointer value received indicates the new position of the Virtual Container VC-4. The NDF with the new pointer value is transmitted only once, i.e. in the first frame. There must not be any further pointer operations within the next three frames.

a) New Data Flag values i. NDF 0110 = disabled Maintain pointer value ii. NDF 1001 = enabled Set new pointer value

For AU-4 SS = 10.

b) Pointer value

Bits 7 to 16 represent the pointer value. The pointer value indicates the offset between the VC-4 start (J1 byte) and the reference point in 3-byte increments. The bits are by turns referred to as I bit and D bit (Increment and Decrement). If the pointer value is to be increased by a positive justification process, this is indicated by the inversion of all five I bits (bits 7, 9, 11, 13 and 15). On the decoder side, an inversion of I bits is followed by a majority decision, i.e. if at least three I bits have been inverted, the current pointer value is increased by 1 and the justification bytes contained in the payload of the current VC-4 are ignored. In negative justification processes, the five D bits (bit 8, 10, 12, 14 and 16) are inverted. On the decoder side, the D bits are evaluated in the same way and the information contained in H3 is inserted into the payload of the current VC4. 6.5.2 TU-12 pointer The bytes required for the pointer operations are referred to as V1, V2 and V3. These bytes are located in the first byte position of four consecutive TU-12s. The definition of the byte available in the current TU-12 is performed by means of the H4 multiframe indicator of the VC-3 POH or VC-4 POH.

V1 and V2 can be read as a 16-bit data word. Bits 1 to 4 represent the New Data Flag NDF. The following two values have been defined:



a) New Data Flag values i. NDF 0110 = disabled

ii. NDF 1001 = enabled

TU-12: SS = 10

a) Pointer value Bits 7 to 16 represent a 10-bit word, the so-called pointer value. As a binary value, the pointer value indicates the offset between the VC-12 start and the reference point expressed in bytes. The bits are by turns referred to as I and D bit (Increment and Decrement). If the pointer value is to be increased by positive justification, this is indicated by the inversion of all five I bits (bits 7, 9, 11, 13 and 15). On the decoder side, an inversion of the I bits is followed by a majority decision, i.e. if at least three I bits have been inverted, the current pointer value is increased by 1 and the justification bytes contained in the payload of the current VC-12 are ignored. In the negative justification process, the five D bits (bits 8, 10, 12, 14 and 16) are inverted. On the decoder side, the D bits are evaluated in the same way and the information contained in V3 is inserted into the payload of the current VC-12. 6.6 SDH Alarms Alarms can be generally divided into 2 main different groups:

Internal Alarms (Card Fail or Physical alarms) Line Alarms (Logical alarms).

Each single alarm can be associated to one of these severities:

Urgent Not Urgent Unabled

6.6.1 Internal Alarms Card Missing This alarm indicates that in a specific slot a card has been declared but the slot is actually empty. The alarm is cancelled when a new card is inserted into the slot or when the cards declaration is removed from the rack configuration. Card Mismatch This alarm indicates that in a specific slot a card has been declared but the slot is actually filled in with a different kind of card. The alarm is cancelled when the right card is inserted into the slot or when the cards declaration is modified into the rack configuration.



Card Failure This alarm indicates that the card filled in a specific slot is out of service. The alarm is cancelled when the old card is substituted by a new one. LAP-D KO This alarm indicates that the data channels connecting the present NE and remote units are down. In the remote side a LAP-D FAIL Alarm is generated There are two different types of LAP-D KO Alarm: - MS LAP-D Fail (data channels of Multiplexer Section are down) - RS LAP-D Fail (data channels of Regenerator Section are down). It is a NOT URGENT Alarm. The problem can be solved checking the LAP-D settings in the Application Local Configuration, or in the LAP_D Configuration and Local Configuration Table. 6.6.2 Line Alarms Los Of Signal (LOS) It is an alarm indicating that the present (receiving) unit is waiting for an external signal but actually it is receiving nothing. It is declared when no light pulses persist for 100msec. If it persists for a period longer than 2.5sec, then an URGENT alarm LOS is generated and sent. It requires an immediate check about incoming line equipment (connectors, cables, fibres) and about transmitters (lasers). Loss Of Frame (LOF) it is an alarm indicating that the present (receiving) unit has detected a persistent error into incoming fixed pattern bytes (A1&A2) of RSOH. It is declared when an error frame is detected for a period longer than 3ms, while its really considered by receiver only if four consecutive frames carry incorrect patterns: in this case an URGENT alarm LOF is generated and sent. Loss Of Pointer (LOP) It is an alarm indicating a generic problem about pointers. It is declared when a pointer failure occurs, such as out_of_range_value, wrong_NDF, or any other failures related to the pointer processing mechanism used for TU-PTR or AU-PTR. In this case an alarm LOP is generated and sent. Loss of Sequence Synchronization (LSS) It is an alarm that reports a loss of a primary or secondary timing source reference. In this case an URGENT alarm LSS is generated and sent. Signal Degraded (SD) It is an URGENT alarm indicating that an incoming STM-N signal has a value of BER (Bit Error Rate) t 10-6 . It means that NE is actually receiving a degraded SDH signal. Excessive Bit Error Rate (EXBER) It is an URGENT alarm indicating that an incoming STM-N signal has a value of BER (Bit Error Rate) t 10-3 . It means that NE is actually receiving a very degraded SDH signal.



Alarm Indication Signal (AIS) There are two different kinds of AIS alarms related to hierarchical signal kinds: PDH-AIS and SDH-AIS. When a problem about a PDH signal occurs (along its transmission path) a PDH-AIS is generated and forwarded. In this way, all the equipment involved in the transmission path (starting from the point of failure) are correctly alerted. In presence of a PDH-AIS, the data channel is filled with a sequence of 1 that can be detected and handled only by PDH devices. When a problem about a SDH signal occurs (along its transmission path) a SDH-AIS is generated and forwarded. In this way, all the equipment involved in the transmission path (starting from the point of failure) are correctly alerted. In presence of a SDH-AIS, the data channel is filled with a sequence of 1 that can be detected and handled only by SDH devices. Administrative Unit Alarm Indication Signal (AU-AIS) If the high level SDH frame content is abnormal, an AU-AIS is generated: in this case both AU-PTR and Payload Area are filled with all 1 while Section Overhead Area (RSOH&MSOH) is unmodified. A detected AU4-AIS is really considered by receiver only if three consecutive frames contain all AU-PTR bit set to 1. Multiplex Section Alarm Indication Signal (MS-AIS) It is generated in consequence of a LOS or a LOF detected on the incoming side of a multiplexer. When a MS-AIS is generated, all bytes of the payload area, MSOH and AU-PTR are set to 1. A received MS-AIS is considered true and therefore considered only if the related pattern 111 (bit 6,7,8 of K2 byte into MSOH) for at least three consecutive frames. Multiplex Section Remote Defect Indication (MS-RDI) When a MTF (Multiplex Termination Function) receives a SDH very degraded signal (LOS, EXBER, LOF, LOP) or detects a MS-AIS, it sends backwards a MS-RDI alarm. For this purpose, bits 6,7,8 of byte K2 belonging to MSOH are set to 110. Multiplex Section Far End Receive Failure (MS-FERF) It is generated (as acknowledgement alarm) when MST function has read the configuration 110 into bits 6,7and 8 of received byte K2 for at least 3 consecutive frames.

11 Chapter 6 SDH Fundamentals(Book)

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