1-The Theory of SDH

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OpxX 2500+ ESTM

The Theory of SDH

Issue 2.0 January 2001 1-1

Objectives:

To master the concept of SDH.

To master the characteristic of the SDH, the rate levels, the topologies of theSDH network, the basic NE type.

To understand the frame structure and the procedures of multiplexing of SDHsignals.

To understand the function and working principle of the section overheadbytes, the path overhead bytes, the pointer bytes.


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The Theory of SDH OptiX 2500+ ESTM

1-2 Issue 2.0 January 2001

1.1 Overview

1.1.1 What is SDH?

SDH is the abbreviation of Synchronous Digital Hierarchy. Like PDH ----plesiochronous digital hierarchy, SDH is a transmission system (protocol)which defines the characteristic of digital signals, including frame structure,multiplexing method, digital rates hierarchy, and interface code pattern, andso on.

SDH transmission network is an entirely brand new transmission network,with DXC, ADM, TM, REG constituting its nodes and high capacity optical

fiber constituting its transmission link. It also boasts of flexible networking andnetwork self-healing function.

SDH is a general technique system both for optical fiber transmission and formicrowave and satellite transmission.

1.1.2 The Characteristic of SDH over PDH

SDH uses universal interfaces to achieve the compatibility with differentequipment from different vendors. It also boasts of highly efficient andcoordinated management and operation through the whole network and

transmission process, flexible networking and traffic dispatching, and networkself-healing function. It greatly enhances the utilization ratio of networkresources and reduces the OAM costs due to the enhanced maintenancefunction.

1. Interface

Standardization of interfaces is the key to determine the possibility of realizingthe interconnection among different equipment from different vendors. SDHsystem provides universal standards for Network Node Interfaces (NNI).

1) Electrical interface

The electrical interface of the SDH equipment can be interconnected with theelectrical interface of the PDH equipment, the exchange equipment trunkinterface, the mobile equipment's trunk interface, the Code/Decodeequipment's trunk interface if these equipment's trunk interface is standard.

2) Optical interface

The optical interface of SDH adopts universal standards, so the SDHequipment from different vendors can be interconnected through the opticalinterface, while the PDH is not able to do this, as shown in Figure 1-1.


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A B

C

OptiX

equipment

OptiX

equipment equipment

OtherSDH

equipment

OtherSDH

Transmission network 1 Transmission network 2

Figure 1-1 Different SDH equipment interconnected through the optical interfaces

As shown in figure 1-1, the OptiX transmission equipment of HuaweiTechnologies Co., Ltd make up of network 1, while the other SDH equipmentfrom other vendor make up of network 2. Now there need 16 2Mbit/s circuitsbetween station A and B.

If these two transmission networks are not made up by SDH equipment butPDH equipment from different vendors, first we must drop 16 2M signals from

network 1 and 2 to DDF at the connection point "C", then add them to PDHequipment in network 2 from DDF. It is very complex and need a lot ofmultiplexing/de-multiplexing equipment. Even if 34Mbit/s electrical interfaceconnection is employed (multiplexing 16 2Mbit/s), while the equipment of twovendors are far away from each other, the direct connection of electricalinterface is hardly achieved. If SDH equipment is used, there only needs apair of optical fibers to achieve interconnection between these twotransmission networks. Its advantages are self-evident.

2. Multiplexing method

As low-rate PDH signals (e.g., the most common 2Mbit/s signal), with fixedpositions, are multiplexed into the frame structure of high-rate SDH signals(e.g., 622Mbit/s signal of STM-4) via byte interleaved multiplexing method,their locations of low-rate PDH signals in the frame of high-rate SDH signalare fixed and regular. Therefore, low-rate PDH signals, e.g. 155Mbit/s,(STM-1 ), can be directly added to or dropped from high-rate signals, e.g.,2.5Gbit/s (STM-16 ). This simplifies the multiplexing and de-multiplexingprocesses of signals, and makes SDH hierarchy especially suitable for highrate and large capacity optical fiber transmission systems, as shown in Figure1-2.



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2Mbit/s

622Mbit/s 622Mbit/sDe-m

ultiplexing

Multiplexing

Figure 1-2 the directly adding/dropping procedure

In PDH, only the signals of 1.544Mbit/s and 2.048Mbit/s are synchronous,while the signals of other rates are asynchronous. Since PDH adoptsasynchronous multiplexing method, low-rate signals can not be directlyadded/dropped from PDH high-rate signals. For example, 2.048Mbit/s signalscan not be directly added/dropped from 140Mbit/s signals. Here arises twoproblems:

1) Adding/dropping low-rate signals from high-rate signals must be conductedlevel by level. For example, to add/drop 2.048Mbit/s low-rate signals from140Mbit/s signals, the following procedures must be conducted, as shown in(Figure 1-3):

140Mbit/s

34Mbit/s 34Mbit/s8Mbit/s 8Mbit/s

2Mbit/s

140Mbit/s

de-multiplexer

de-multiplexer

de-multiplexer multiplexer

multiplexermultipilexer

Figure 1-3 Add/drop 2Mb/s signals from 140Mbit/s signals

2) Since adding/dropping low-rate signals to high-rate ones must go through

many stages of multiplexing and de-multiplexing, the impairment ton thesignals during multiplexing/de-multiplexing processes will be aggravated andtransmission performance will deteriorate. This is unbearable in large capacitytransmission. That's the reason why the transmission rate of PDH system hasnot been improved further.

3. Characteristics of operation, administration and maintenance

Abundant overhead bits for operation, administration and maintenance (OAM)function are arranged in the frame structures of SDH signals. This greatly



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strengthens the automation of network monitoring function and maintenance.,i.e. automatic maintenance. Some redundancy bits must be added during linecoding for line performance monitoring because few overhead bytes are

arranged in PDH signals.

The abundant overheads in SDH signals account for 1/20 of the total bytes ina frame. It greatly enhances the OAM function and reduces the cost ofsystem maintenance which occupies most large proportion of the overall costof telecommunication equipment. The overall cost of SDH system is less thanthat of PDH system, and estimated to be only 65.8% of that of the latter.

4. Compatibility

SDH has high compatibility, which means that PDH service can betransmitted via the SDH transmission network while SDH transmission

network is established. The two networks shall co-exist. And the existing PDHtransmission network can work together while establishing SDH transmissionnetwork.

Besides, SDH network can be used for transmitting signals of otherhierarchies, such as asynchronous transfer mode (ATM) signals and FDDIsignals. The basic transfer module (STM-1) of SDH signals in SDH networkcan accommodate three PDH digital signal hierarchies and other hierarchiessuch as ATM, FDDI and DQDB. This reflects the forward and backwardcompatibility of SDH and guarantees smooth transitions from PDH to SDHnetwork and from SDH to ATM. How does SDH accommodate signals ofthese hierarchies? It simply multiplexes the low-rate signals of differenthierarchies into the frame structure of the STM-1 signals at the boundary of

the network (e.g. SDH/PDH start point) and then de-multiplexes them at theboundary of the network (end point). In this way, digital signals of differenthierarchies can be transmitted in the SDH transmission network.

1.1.3 SDH Rate Level

SDH system provides a set of standard information structure levels, i.e. a setof standard rate levels. The basic module signal in SDH system is asynchronous transfer module ---- STM-1 at a rate of 155Mbit/s. STM-Nsignals of higher levels are formed by basic module signal--STM-1 via byteinterleaved multiplexing. Among it, in particular, N is a positive whole number,

N =1, 4, 16, 64. SDH signals of even higher levels include: STM-4 (622Mbit/s), STM-16 (2.5G bit/s) and STM-64 (10G bit/s).

1.1.4 Forms of Basic SDH Network Elements

An optical synchronous digital transmission network consists of two parts:SDH network element (NE) equipment and optical cable system. NEequipment provides functions of synchronous transmission, multiplexing andcross connection of messages. According to the functions provided, NEequipment is classified into regenerators (REG), terminal multiplexer (TM),


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add and drop multiplexer (ADM) and digital cross connection equipment(DXC).

1. Terminal Multiplexers (TM)

The following will describe the functions and features of terminal multiplexers(TMs) by taking STM-1 signal hierarchy as an example. Figure 1-4 illustratesthe functions of a terminal multiplexer.

155Mbit/s155Mbit/s

1.5 2 6 34 45 140 155

STM-1 TM

PowerOrder Wire Alarm TMN Interface

Figure 1-4 Functions of an STM-1 terminal multiplexer

As shown in the above figure, the main task of terminal multiplexer isresponsible for incorporating PDH tributary signals with low rates (e.g. twotypes of low rate PDH signals: 2Mbit/s, 34Mbit/s, 140Mbit/s and 1.5Mbit/s,

6Mbit/s, 45Mbit/s) and SDH electrical signals at a rate of 155Mbit/s intoSTM-1 frame structure, and converting these electrical signals into STM-1optical ones. Meanwhile, the terminal multiplexer implements the inverseprocess of the above procedures.

Theoretically, the terminal multiplexers of higher hierarchy are the same asSTM-1 ones except that the former can incorporate tributary signals withhigher rates and greater capacity.

Terminal multiplexers are principally used at the ends of point-to-point NEequipment or a chain network, as shown in Figure 1-5 and 1-6. Of course,terminal muptiplexers are often used in star, tree and train networks, as

terminals of SDH transmission network.

T M T M

Figure 1-5 Point-to-point application



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TM TMA D M

Figure 1-6 A simple chain application

Additionally in practice, TMs can also be used in a ring-chain network asshown in Figure 1-7.

TMADMADM

ADM

ADM

Figure 1-7 Application in a ring-chain network

2. Add and Drop Multiplexers (ADM)

ADMs are most popular NEs in networks because they incorporate thefunctions of synchronous multiplexing and digital cross connection, andfeature the ability to add and drop any tributary signal flexibly. The followingwill describe the functions and features of add and drop multiplexers (ADMs)by taking STM-1 hierarchy as an example. Figure 1-8 illustrates the functionsof an ADM.

155Mbit/s155Mbit/s

1.5 2 6 34 45 140 155 Mbit/s

STM-1 ADM

PowerOrder Wire Alarm TMN Interface

Figure 1-8 Functions of an STM-1 ADM



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In addition to implementing the same signal multiplexing and demultiplexingfunction as a TM, the most important function of ADM is to implement thecross connection between line signals at both sides and between line signals

and tributary ones. For example, the accessed tributary signal of 2Mbit/sseries and 1.5Mbit/s series can be multiplexed not only into the STM-1 signalon the eastward line, but also into the STM-1 signal on the westward line. TheSTM-1 signals both on the eastward and westward lines can also beinterconnected with each other .

ADMs are widely used in chain, ring and hub networks. Please refer to Figure1-9, 1-10 and 1-11.

TM TMADM ADM

Figure 1-9 Application in a chain network

ADM

ADM

ADM ADM

Figure 1-10 Application in a ring network

TM

TM

TM

TM

ADM


Figure 1-11 Application in a hub network


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3. Regenerator (REG)

Regenerator is responsible for signal regeneration, amplification and relaying.Compared with TMs and ADMs, REGs do not provide functions of adding anddropping services at any site. Please refer to Figure 1-12.

STM-NRE G

Power Order Wire Alarm TMN Interface

STM-N

Figure 1-12 Functions of a regenerator

Regenerators are used to regenerate and relay services for long-distancetransmission in various networks.

4. Digital Cross Connection Equipment (DXC)

Digital cross connection equipment (DXC) is one of the important NEs in anSDH network. It provides functions of multiplexing, distribution,

protection/recovery, monitoring, and network management. The core functionof DXCs is cross connection.

The above text describes the types of NE equipment. In practice, theselection of NE equipment is based on the location of the NE on the network,characteristics of services added and dropped, and for the convenience ofnetwork management, etc.

In practical application, Huawei Technologies Co., Ltd. developed OptiXseries products can be configured to any of the above types. The mostcommon types are ADM and TM. With the development and improvement ofoptical amplification technique, REG is less to be used. OptiX series support

a simple application of DXC.

1.1.5 Basic Topological Structure of the SDH

Network

The topological structures of SDH networks basically include five types,namely chain, star, tree, ring, and mesh networks. SDH networks can makeup an even more complex network based on these basic topologicalstructures.



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1. Chain Networks

Chain network is such a topological structure in which all nodes areconnected in a chain, and the two nodes at both ends are directly connected.In such a structure, in order to establish a service connection between twonon-adjacent nodes, all of the nodes shall be connected in between. Thesetwo nodes should cooperate to enable the connection of the same service.For example, the connection of several Add/Drop Multiplexers ((ADMB, C andD)) between two Terminal Multiplexers (TM) is a typical application of chaintopological structure and an economical network topological structure at theinitial application stage of SDH equipment.

The basic physical topological structure of a chain network is shown in Figure1-13.

A B C D E

Figure 1-13 Topological structure of a chain network

2. Star (Hub) Networks

In a communication network, if there is a special node connected via direct

routes with all other nodes, between which there is no direct route, thisnetwork is considered to have a star or hub topological structure. In such astructure, the service connection between any nodes other than the specialhub node can only be established through route selection and transmitting viathe hub node. The hub node selects route for the passing service andestablishes a connection. The network topological structure connects thetransmission terminals of multiple optical fibers and has the capability tomanage bandwidth resources comprehensively and flexibly. Thus theinvestments and the operation costs can be significantly reduced. However, atthe hub node, potential bottle neck of bandwidth resources and equipmentfailure may result in the breakdown of the entire network. The basic physicaltopological structure of a star network is shown in Figure 1-14.

A

B

C

D E

Figure 1-14 Topological structure of a star network



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3. Tree Networks

In a point-to-point topological structure, if any end node is connected withseveral other special nodes, a tree topological structure is formed. A treetopological structure can be considered as the combination of chain and starstructures. It is suitable for broadcasting service. However, due to the bottleneck problem and the limitation of optical power budget, it is not suitable forproviding bi-directional communication services.

The basic physical topological structure of a tree network is shown in Figure1-15.

A

B C

D E

Figure 1-15 Topological structure of a tree network

4. Ring Networks

Ring networks are such communication networks in which all nodes areconnected in a chain and the two end nodes at both nodes are connectedtogether to form a ring, and no nodes are open. If the two end nodes in achain network are connected together, the chain network becomes a ring. Insuch a structure, in order to establish a service connection between twonon-adjacent nodes, all of the nodes in between should cooperate to enablethe connection of the same service. The most obvious advantage of a ringnetwork is its high survivability (i.e., self healing) which is essential to modernoptical networks with a large capacity. Thus, the ring network enjoys a verybroad application in the networking of SDH equipment.

The basic physical topological structure of a ring network is shown in Figure1-16.



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A

B

C D

E

Figure 1-16 Topological structure of a ring network

5. Mesh Networks

Mesh networks are such communication networks in which many nodes areinterconnected with each other via direct routes. In such topological structure,if direct routes are used in the interconnection of all nodes, this structure isconsidered as an ideal mesh topology. In a non-ideal mesh topologicalstructure, the service connection between nodes which are not connecteddirectly is established through route selection and transiting via other nodes.In a mesh network, no bottle neck problem or equipment failure exists. Sincemore than one routes can be selected between two nodes, when anyequipment fails, services can still be transmitted smoothly through otherroutes. Thus, the reliability of service transmission is increased. However,

such networks are more complicated, costly and difficult to manage. Meshnetworks are very suitable for the regions with large traffic.

The basic physical topological structure of a mesh network is shown in Figure1-17.

A

B

C

DE

Figure 1-17 Topological structure of a mesh network



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As stated above, each of these topological structures has its individualfeatures and different applications. In selecting a topological structure, manyfactors should be considered. For example, the network should be highly

survivable, easy to configure, suitable to add new services, and simple tomanage. In a practical communication network, different sections adoptdifferent topological structure. For example, local networks (i.e. accessnetworks or subscriber networks) normally use ring and star structures andsometimes chain structure are adopted. It may be helpful to use ring andchain topology in urban inter-exchange trunk networks. Meanwhile, tollbackbone networks may require mesh topological structure. The requiredstructure in practice should be determined according to specific situation.

The OptiX series products of Huawei Technologies Co., Ltd. support all theabove forms of network topological structures and their combinations.

1.2 The Frame Structure and

Multiplexing of SDH Signals

1.2.1 SDH Signal STM N Frame Structure

What kind of frame structure do SDH signals need?

The arrangement of the frame structure shall ensure that the low-rate tributary

signals are allocated as evenly and regularly in the frame as possible. Thismakes it easier to implement synchronous multiplexing, cross-connection(DXC), add/drop, and switching of tributaries. In a word, this arrangementfacilitates direct adding/dropping of low-rate tributary signals to/from high-ratesignals. Therefore, ITU-T defines the frame of STM-N as rectangle blockframes structures in unit of byte (8bit), as illustrated in Figure 1-18.


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9 270 N bytes

RSOH

AUPTR

MSOH

9 N

134

9

payload

261 N

5

Figure 1-18 STM-N frame structure

Tips:

What is a block frame?

For the convenience of signal analysis, the frame structures of the signals are

often illustrated as block frame structures. This is not the unique structure ofSDH signals. The frame structures of PDH signals, ATM signals and datapackets of packet switching are also block frames. For example, the frame of

E1 (2.048M bit/s) signals is a block frame of 1 row32 columns consisting of32 bytes. ATM signals have a block frame structure of 53 bytes. To illustrateframes structures of signals as block frames is merely for the convenience ofanalysis.

As shown in the above figure, the frame structure of STM-N signals is 9 rows

270N columns. The N here is equal to the N in STM-N, ranging: from 1,4, 16, 64,. The N indicates that this signal is multiplexed by N STM-1signals via byte interleaved multiplexing. This explains that the framestructure of STM-1 signals is a block structure of 9 rows270 columns. WhenN STM-1 signals are multiplexed into STM-N signal via byte interleavedmultiplexing, only the columns of STM-1 signals are multiplexed via byteinterleaved multiplexing. While the number of rows remains constantly 9.

It is known that signals are transmitted bit-by-bit in lines. But how is the blockframe transmitted in the line? Is the entire block transmitted simultaneously?



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Of course not. STM-N signals are also transmitted bit-by-bit. Then what is thesequence of transmission? Which bits are transmitted first and which later?The principle for SDH signal frame transmission is: the bytes (8-bit ) within the

frame structure is transmitted byte-by-byte (bit-by-bit) from left to right andfrom top to bottom. After one row is transmitted, the next row will follow. Afterone frame is completed, the next frame will start.

What is the frame frequency (i.e. the number of frames transmitted persecond) for STM-N signals? ITU-T defines the frequency to be 8000 framesper second for all levels in STM hierarchy. That means the frame length orframe period is a constant value of 125us. Maybe 8000 frames per second isfamiliar to you, because the frequency of E1 (2.048M bit/s) signals of PDH isalso 8000 frames per second.

Note that all frame frequencies of any STM hierarchical levels are 8000

frames per second. Constant frame period is a major characteristic of SDHsignals. Are the frame periods of signals of different PDH hierarchical levelsalso the same? The constant frame period makes the rates of STM-N signalsregular. For example, the data rate of STM-4 transmission is constantly 4times that of STM-1, and STM16 is 4 times of STM4 and 16 times ofSTM1. But the rate of E2 (8.448M bit/s) signals of PDH is not 4 times asthat of E1 (2.048M bit/s) signals. Such regularity of SDH signals makes itpossible to directly add/drop low-rate SDH signals to/from high-rate SDHsignals, especially for high-capacity transmission.

As illustrated in Figure 1-18, the frame of STM-N consists of three parts:Section Overhead (including Regenerator Section Overhead ---- RSOH andMultiplex Section Overhead ---- MSOH) ), Administrative Unit Pointer----AU-PTR and Information Payload. Next, we will describe the functions ofthese three parts.

1. Information Payload

The Information Payload is a place in the STM-N frame structure to storevarious information code blocks to be transmitted by STM-N. It functions asthe "wagon box" of the truck ---STM-N. Within the box are packed low-ratesignals ---- cargoes to be shipped. To monitor the possible impairment to thecargoes (the packed low-rate signals) on a real-time basis duringtransmission, supervisory overhead bytes ---- Path Overhead (POH) bytesare added into the signals while the low-rate signals are packed. As one part

of payload, the POH, together with the information code blocks, is loadedonto STM-N and transmitted on the SDH network. The POH is in charge ofmonitoring, administrating and controlling (somewhat similar to a sensor) thepath performances for the packed cargoes (the low-rate signals).

2. Section Overhead (SOH)

The Section Overhead (SOH) refers to the auxiliary bytes which is necessaryfor network operation, administration and maintenance (OAM) to guarantee anormal and flexible transmission of Information Payload. For example, theSection Overhead can monitor the impairment condition of all "cargoes" in


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STM-N during transmission. The function of POH is to locate the certainimpaired cargo in case any impairments occurs. SOH implements the overallmonitoring of cargo while the POH monitors a specific cargo. SOH and POH

also have some administration functions.

The Section Overhead is further classified into Regenerator SectionOverhead (RSOH) and Multiplex Section Overhead (MSOH). Theyrespectively monitor their corresponding sections and layers. As mentionedabove, section can be regarded as a large transmission path. The function ofRSOH and MSOH is to monitor this transmission path.

Then, what's the difference between RSOH and MSOH? In fact, they havedifferent monitoring domains. For example, if 2.5G bit/s signals aretransmitted in the fiber, the RSOH monitors the overall transmissionperformance of STM-16 while the MSOH monitors the performances of each

STM-1 of the STM-16 signals.

Technical details:

RSOH, MSOH and POH provide SDH signals with monitoring functions fordifferent layers. For example, for a 2.5G bit/s system, the RSOH monitors theoverall transmission performance of the STM-16 signal; the MSOH monitorsthe transmission performances of each STM-1 signal; and the POH monitorsthe transmission performances of each packaged low-rate tributary signal (e.g.2M bit/s) in STM-1. Through these complete monitoring and managementfunctions for all levels, you can conveniently conduct macro (overall) and

micro (individual) supervision over the transmission status of the signal, andeasily locate and analyze faults.

The Regenerator Section Overhead bytes in an STM-N frame are locatedwithin row 1-3 of column 1 to 9 N, 39N bytes in total . The MultiplexSection Overhead bytes in an STM-N frame are located within row 5-9 ofcolumn 1 to 9N, 59N bytes in total. Compared with the frame structureof PDH signals, the abundant section overhead is a significant characteristicof the frame structures of SDH signals.

3. Administrative Unit Pointer ---- AU-PTR

The Administrative Unit Pointer is located within column 9N of row 4 of theSTM-N frame and 9N bytes in total. What's the function of AU-PTR? Wehave mentioned before that low-rate tributary signals (e.g. 2Mbit/s) could beadded/ dropped directly from high-rate SDH signals. Why is it so? Becausethe locations of low-rate signals within a high-rate SDH frame structure arepredictable, i.e. regular. The predictability can be achieved via the pointeroverhead bytes function in the SDH frame structure. The AU-PTR indicatesthe exact location of the first byte of the information payload within the STM-N


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frame, so that the information payload can be properly extracted at thereceiving end according to the value of this location indicator (the value of thepointer) . Let's make it more easier. Suppose that there are many goods

stored in a warehouse in the unit of pile. Goods (low-rate signals) of each pileare regularly arranged (via byte interleaved multiplexing). We can locate apiece of goods within the house by only locating the pile this piece of goodsbelongs to. That is to say, as long as the location of the first piece of any pileis known, the precise location of any piece within the pile can be directlylocated according to the regularity of their arrangement. In this way you candirectly carry (directly add/drop) a given piece of goods (low-rate tributary)from the warehouse. The function of AU-PTR is to indicate the location of thefirst piece of goods within a given pile.

In fact, the pointer is further classified into high order pointer and low orderpointer. The high order pointer is AU-PTR while the lower order pointer isTU-PTR (Tributary Unit Pointer). The function of TU-PTR is similar to that ofAU-PTR except that the former indicates smaller "piles of goods".

1.2.2 Multiplexing Structure and Procedures of

SDH

SDH multiplexing includes two types: multiplexing of lower-order SDH signalsinto higher-order SDH signals; and multiplexing of low-rate tributary signals(e.g. 2Mbit/s, 34Mbit/s and 140Mbit/s) into SDH signals ----STM-N.

The first type of multiplexing has been mentioned above. Multiplexing is

conducted mainly via byte interleaved multiplexing, and by multiplexing fourinto one, i.e. 4STM1STM4 and 4STM4STM16. During themultiplexing, the frame frequency remains unchanged (8000-frame persecond), which means that the rate of the higher-level STM-N signals is 4times that of the next lower-level STM-N signals. During the byte interleavedmultiplexing, the information payload and pointer bytes of each frame aremultiplexed via interleaved multiplexing based on their original values, whilesome SOH will be accepted or rejected. In a multiplexed STM-N frame, theSOH is not formed via byte interleaved multiplexing with the SOHs of all thelower-order SDH frames. Some SOHs of the lower-order frame are rejected.The detailed multiplexing method will be described in the next section.

The second type of multiplexing is mostly used for multiplexing of PDH

signals into the STM-N signals.

There are two traditional ways to multiplex lower-rate signals into higher-ratesignals:

Bit inserting method (also known as the byte speed adjustment method)

This method uses the bit insertion indication in a fixed location to indicatewhether the inserted bit carries any signal data. It allows the multiplexedpayload has a big frequency difference (asynchronous multiplexing), as there


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exists the process of bit insertion and de-insertion (byte speed adjustment)and the tributary signals can not be directly connected into the high-speedmultiplexing signals nor lower-rate tributary signals be divided from

higher-rate signals. That is to say, lower-rate tributary signals can not goon/down higher-rate signals directly, but layer by layer. This is also themultiplexing method for PDH.

Fixed Position Projection Method

This method utilizes the special position of a lower-rate signal in a higher-ratesignal to carry lower-rate synchronous signals. It requires the lower-ratesignals to be synchronous to higher-rate signals, namely, their framefrequencies shall be the same, therefore, lower-rate tributary signals canconveniently go up/down higher-rate signals directly. But when there existsfrequency differences and phase differences (that is, asynchronous) between

higher-rate signals and lower-rate signals, 125s (8000 frame/second) buffershall be used to rectify the frequencies and locate the phases, thus causingsignal delay and slip losses.

From the above text, it can be seen that both multiplexing methods have theirshortcomings. The Bit Insertion Method can not enable lower-rate tributarysignals to go up/down higher-rate signals. And the Fixed Position ProjectionMethod causes quite a long time delay of the signals accessed.

The compatibility of SDH network requires the multiplexing mode of SDHshall both satisfy asynchronous multiplexing (for example, multiplexing PDHsignals into STM-N) and synchronous multiplexing (such as STM-1STM-4).Meanwhile, it shall enable the high-speed STM-N signals to easily add/droplower-speed signals without causing serious time delay of signals and sliplosses. Therefore, SDH shall adopt its own multiplexing procedures andmultiplexing structure. Within this multiplexing structure, the pointeradjustment and location technique is implemented to replace 125s buffer,which will rectify the frequency differences of tributary signals and adjust thephases. Each kind of service signals that are multiplexed into STM-N frameshall pass through three phases: the projection phase (equivalent to signalpackaging), the location phase (equivalent to pointer adjustment) and themultiplexing phase (equivalent to byte multiplexing).

ITU-T defined a complete multiplexing structure (i.e. multiplexing routes).Through these routes, digital signals of three PDH hierarchies can be

multiplexed into STM-N signals via a variety of methods. The routes definedby ITU-T are illustrated in Figure 1-19.


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STM-N AUG AU-4 VC-4

AU-3 VC-3

TUG-3

TUG-2

TU-3

TU-2

TU-12

TU-11

VC-3

VC-2

VC-12

VC-11

C-4

C-3

C-2

C-12

C-11

N 1

1

1

3

3

3

4

7

7

139264kbit/s

44736kbit/s34368kbit/s

6312kbit/s

2048kbit/s

1544kbit/s

pointer processing

MultiplexingAligning adjustmentMapping

Figure 1-19 The multiplexing mapping structure defined in Rec. G.709

As illustrated in Figure 1-19, this multiplexing structure includes some basicmultiplexing units: C - Container, VC - Virtual Container, TU - Tributary Unit,TUG - Tributary Unit Group, AU - Administrative Unit, and AUG -Administrative Unit Group. The suffixes of these multiplexing units denotetheir corresponding signal levels. As illustrated in the figure, there are severalroutes (several multiplexing methods) through which a valid payload can bemultiplexed into STM-N signals. For example, there are two multiplexingroutes for 2Mbit/s signals, i.e. two methods for multiplexing 2Mbit/s signalsinto STM-N signals. You may have noted that 8Mbit/s PDH signals can't be

multiplexed into STM-N signals.

Although there are several routes for a kind of signals to be multiplexed intoSDH STM-N signals, the multiplexing route used in a country or an area mustbe the only one. In China, the SDH optical transmission network technologicalsystem stipulates that PDH signals based on 2Mbit/s signals shall beregarded as the valid payload of SDH and the multiplexing route of AU-4 shallbe employed. This multiplexing route structure is illustrated in Figure 1-20.

STM-N AUG AU-4 VC-4

TUG-3

TUG-2

TU-3

TU-12

VC-3

VC-12

C-4

C-3

C-12

N 1

1

3

3

7

139264kbit/s

34368kbit/s

2048kbit/s

Pointer processing

MultiplexingAligning justificationMapping

Figure 1-20 Basic multiplexing mapping structure employed in China



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1.3 Overhead

As mentioned before, the functions of overhead are to implement layeredmonitoring management for SDH signals. The monitoring is classified intosection layer monitoring and path layer monitoring. The section layermonitoring is further classified into regenerator section layer monitoring andmultiplex section layer monitoring, while the path layer monitoring is furtherclassified into higher order path layer monitoring and lower order path layermonitoring. Thus the layered monitoring for STM-N is implemented. Forexample, in a 2.5G system, the regenerator section overhead monitors theoverall STM-16 signal while the multiplex section overhead further monitorseach of the 16 STM-1. Furthermore, the higher order path overhead monitorsthe VC4 of each STM-1 and the lower order path overhead can monitor eachof the 63 VC12 in the VC4. Hence the multistage monitoring functions from2.5Gbit/s to 2Mbit/s are implemented.

Then, how are these monitoring functions implemented? They areimplemented via different overhead bytes.

1.3.1 Section Overhead

The section overhead of the STM-N frame is located in rows 1-9 of columns1-9N of with the frame structure. Notes: with the exception of that row 4 isAU-PTR. We are to describe the function of each section overhead byte withthe example of an STM-1 signal. For an STM-1 signal, the Section Overheadincludes RSOH which is located at rows 1-3 of columns 1-9, and ----RMSOH and which is located at rows 5-9 of columns 1-9 and MSOH withinits frame, as illustrated in Figure 1-21.


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A1 A1A1 A2 A2 A2 J0 *

*

Bytes reserved for use domestic

B1 E1 F1

D1 D2 D3

K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M1E2

RSOH

MSOH

9 bytes

9 rows

Media dependent bytes (temporary usage)

B2 B2 B2

Administrative Unit Pointer(s)

* Unscrambled bytesAll unmarked bytes are reserved for future international standardization

Figure 1-21 The diagram of section overhead bytes within the STM-N frame

Figure 1-21 illustrates the location of regenerator section overhead andmultiplex section overhead within the STM-1 frame. What is the differencebetween them? Their difference is in the monitoring scope i.e. the RSOH iscorresponding to a large scope ---- STM-N, while the MSOH is correspondingto a smaller scope ---- STM-1 within the large scope.

Framing bytes A1 and A2

Like a pointer, the function of the frame bytes, which is similar to that of apointer alignment, is for localization. As we know that SDH can add/droplower-rate tributary signals from higher-rate signals. Why? Because thereceiver can align the location of the lower-rate signals within the high-ratesignal via the pointers---- AU-PTR and TU-PTR. The first step of thisprocedure is to properly extract each STM-N frame from the received signalstream at the receiver, i.e. to align the start location of each STM-N frame,then to align the location of the corresponding lower-rate signals within each

frame. This procedure is similar to locating a person in a long queue. Youmust first align the specific square, then align the person via the row andcolumn within the square he is in. The function of the bytes A1 and A2 is toalign the square. So the receiver can align and extract the STM-N frame fromthe information stream via these two bytes and further align a specificlower-rate signal within the frame via the pointers.

How does the receiver align the frames via the A1 and A2 bytes? The A1 andA2 have fixed value, i.e. fixed bit patterns. A1: 11110110 (f6H) and A2:00101000 (28H). The receiver monitors each byte in the stream. Afterdetecting 3N successive f6H bytes followed by 3N 26H bytes (there are three



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A1 and three A2 bytes within an STM-1 frame), the receiver determines thatan STM-N frame starts to be received. By aligning the start of each STM-Nframe, the receiver can identify different STM-N frames and disassemble

them. In the case of N=1, what the frames identifies are STM-1 frames.

If the receiver doesn't receive correct A1 and A2 bytes within five or moresuccessive frames (625s), i.e. it can't identify the start of five successiveframes (identify different frames), it will enter out-of-frame status andgenerate out-of-frame alarm ---- OOF. If the OOF keeps for 3ms, the receiverwill enter loss-of-frame status ---- the equipment will generate loss-of-framealarm ---- LOF. Meanwhile, an AIS signal will be sent downward and the entireservices will be interrupted. Under LOF status, if the receiver stays in normalframe alignment status again for successive 1ms or more, the equipment willrestore the normal status.

Technical details:

STM-N signals shall be scrambled before being transmitted via the line sothat the receiver can extract timing signals from the line. But the A1 and A2framing bytes shall not be scrambled for the receiver to properly align them.To take both requirements into consideration, the STM-N signals don't

scramble the bytes in the first row (1 row9N columns, including A1 and A2bytes ) of the section overhead but transmit them transparently while theother bytes within the STM-N frame are scrambled before transmitting via theline. Thus it is convenient to extract the timing from the STM-N signals anddisassemble the STM-N signals at the receiver.

Regenerator Section Trace byte: J0

This byte is used to repeatedly transmit a Section Access Point Identifier sothat a section receiver can verify its continued connection to the intendedtransmitter. Within the domain of a single operator, this byte may use anycharacter. But at the boundaries between the networks of different operators,the format of J0 byte shall be the same (i.e. matched) between the receiverand transmitter of the equipment. Via J0 byte, the operator can detect and

clear faults early and shorten the network restoration time.

Another usage of the J0 byte is that J0 byte in each STM-N frame is definedas an STM identifier C1 and used to indicate the location of each STM-1within the STM-N ---- indicating which STM-1 within the STM-N this STM-1 is(the value of interleave depth coordinate) and which column this C1 byte islocated within the STM-1 frame (the value of the multi-column). It may beused to assist the A1 and A2 bytes in frame alignment.

Data Communication Channel (DCC) byte: D1-D12


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One of the features of SDH is its highly automation of OAM function, whichcan send conduct commands to issue and query data in performance autopoll to the networks element via network management terminals. SDH has

some functions which are not possessed by PDH systems, such as real-timeservice allocation, alarm fault location and on-line performance testing. Whereare these OAM data arranged to transmit? The data used for OAM functions,such as sent commands and checked alarm performance data, aretransmitted via D1-D12 bytes within the STM-N frame, i.e. the related data forOAM functions are arranged in the locations of the D1-D12 bytes andtransmitted by the STM-N signals via the SDH network. Thus the D1-D12bytes provide a common data communication channel accessible to all SDHnetwork elements. As the physical layer of the embedded control channel(ECC), the D1-D12 bytes transmit operation, administration and maintenance(OAM) information among the network elements and form a transmissionchannel of the SDH management network (SMN).

D1, D2 and D3 are Data Channel Character of Regenerator section DCCbytes (DCCR), with a rate of 364kbit/s192kbit/s and are used to transmitOAM information among regenerator section terminals. D4-D12 are DataChannel Character of Multiplex section DCC bytes (DCCM), with a sum rateof 964kbit/s=576kbit/s, and are used to transmit OAM information amongmultiplex section terminals.

The DCC has a total rate of 768kbit/s which provides a powerfulcommunication base for SDH network management.

Order wire bytes: E1 and E2

Each of these two bytes provides a 64kbit/s order wire channel for voicecommunication, i.e. voice information is transmitted via these two bytes.

E1 is part of the RSOH and is used for regenerator section order wirecommunication. E2 is part of the MSOH and is used for direct order wirecommunication between terminals.

For example, the network is as follows:

A C DB

Regenerator RegeneratorMultiplexerTerminal

MultiplexerTerminal

Figure 1-22 Network diagram

If only E1 byte is used as order wire byte, A, B, C and D network elementscan communicate order wire. Why? Because the function of multiplexerterminals is to add/drop lower-rate tributary signals from SDH signal, RSOH



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and MSOH are required to be processed. So both E1 and E2 can be used tocommunicate order wire. The function of regenerators is signal regenerationand only RSOH is required to be processed. So E1 byte can also

communicate order wire.

If only E2 byte is used as order wire byte, then order wire voicecommunication is provided only between A and D. This is because B and Cnetwork elements don't process MSOH and E2 byte.

User channel byte: F1

This byte can be used to provide 64kbit/s data/voice channel. It is reservedfor user (often referring to network provider) to provide temporary order wireconnections for special maintenance purposes.

Bit Interleaved Parity 8 (BIP-8) byte: B1

This byte is allocated for regenerator section error monitoring (Byte B1 islocated in the regenerator section overhead).

What is the mechanism for monitoring? First, we'll discuss the BIP-8 parity.

Suppose that a signal frame is composed of 4 bytes: A1=00110011, A2=11001100, A3=10101010 and A4=00001111. The method of providing BIP-8parity to this frame is to divide it into 4 blocks with 8 bits (one byte) in a parityunit (each byte as a block because one byte has 8 bits, the same as a parityunit). These blocks are arranged as illustrated in Figure 1-23.

A1 00110011A2 11001100A3 10101010A4 00001111

B 01011010

BIP-8

Figure 1-23 BIP-8 parity

Compute the number of "1" over each column. Then fill a 1 in thecorresponding bit of the result (B) if the number is odd, otherwise fill a 0. Thatis, the value of the corresponding bit of B makes the number of "1" in thecorresponding column of A1A2A3A4 blocks even. This parity method is calledBIP-8 parity. In fact this is an even parity since it guarantees that the numberof "1" is even. B is the result of BIP-8 parity for the A1A2A3A4 block.

The mechanism for B1 byte is: the transmitting equipment processes BIP-8even parity over all bytes of the previous frame (1#STM-N) after scrambling,



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and places the result in byte B1 of the current frame (2#STM-N) beforescrambling. The receiver processes BIP-8 parity over all bits of the currentframe (1#STM-N) before de-scrambling, and conducts exclusive-OR

operation between the parity result and the value of B1 in the next frame(2#STM-N) after de-scrambling. If these two values are different, the result ofexclusive-OR will include 1. According to the number of "1", we can monitorthe number of error blocks occurred in 1#STM-N frame during transmission.

Technical details:

Since error performance of higher rate signals is reflected via error blocks, theerror status of STM-N signals is actually the status of error blocks. It can beseen from the BIP-8 parity method, that each bit of the parity result iscorresponding to a bit block, e.g. a column of bits in Figure 1-22. So a B1 byte

can at most monitor 8 error blocks from an STM-N frame that occur duringtransmission (The result of BIP-8 is 8 bits with each bit corresponding to acolumn of bits ---- a block).

Bit Interleaved Parity N24 code (BIP-N24) byte: B2

B2 is similar to B1 in operation mechanism except that it monitors the errorstatus of the multiplex section layer. The B1 byte monitors the transmissionerror of the complete STM-N frame signal. There is only one B1 byte in an

STM-N frame (Why? You'll get the answer when we discuss the interleavedmultiplexing of the section overhead during multiplexing of STM-1 into theSTM-N). The B2 bytes monitor the error performance status for each STM-1frame within the STM-N frame. There are N3 B2 bytes in an STM-N frame,with every three B2 bytes corresponding to an STM-1 frame. The mechanismfor monitoring is that the transmitting equipment computes BIP-24 (threebytes) over all bits of the previous STM-1 frame except for the RSOH (TheRSOH is included in the B1 parity for the complete STM-N frame), and placesthe result in bytes B2 of the current frame before scrambling. The receiverprocesses BIP-24 parity over all bits of the current frame STM-1 afterde-scrambling except for the RSOH, and conducts exclusive-OR operationbetween the parity result and B2 bytes in the next frame after de-scrambling.According to the number of "1" in the result of the exclusive-OR operation, we

can monitor the number of error blocks occurred in this STM-1 within theSTM-N frame during transmission. This method can at most monitor 24 errorblocks. Note: after the transmitting equipment writes B2 bytes, thecorresponding N STM-1 frames are multiplexed into an STM-N signal (thereare 3N B2 bytes). At the receiver the STM-N signal is de-interleaved into N STM-1 signals, then parity is conducted for the N groups of B2 bytes.

Automatic Protection Switching (APS) channel byte: K1, K2 (b1-b5)

These two bytes are allocated for transmitting Automatic Protection Switching


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(APS) signaling, which is used to guarantee that the equipment canautomatically switch on occurrence of a fault and restore the network traffic---- self-healing. These two bytes are used for the APS self-healing of the

multiplex section.

Multiplex Section Remote Defect Indication (MS-RDI): K2 (b6-b8)

This is an alarm message, returned to the transmitting end (source) by thereceiving end (sink), which means that the receiving end has detected anincoming section defect or is receiving the Multiplex Section Alarm IndicationSignal (MS-AIS). That is, when the receiving end detects receivingdeterioration, it returns an MS-RDI alarm signal to the transmitting end so thatthe later gets to know the status of the former. If the received b6-b8 bits of theK2 is 110, it means that this signal is an MS-RDI alarm signal returned by theopposite end. If the received b6-b8 bits of the K2 is 111, it means that this

signal is an MS-AIS alarm signal received by current end. Meanwhile, thecurrent end is required to send out MS-RDI signal to the opposite end, i.e.insert 110 bit pattern into the b6-b8 of the K2 within the STM-N signal frameto be sent to the opposite end.

Synchronization status byte: S1b5-b8

Different bit patterns, indicating different quality levels of clocks defined byITU-T, enable the equipment to determine the quality of the received clocktiming signal. This helps to determine whether or not to switch the clocksource, i.e. switch to higher quality clock source.

The smaller the value of S1 (b5-b8) is, the higher the level of clock quality is.

Multiplex Section Remote Error IndicationMSREIbyte: M1

This is a message returned to its transmitting end by the receiving end.The M1 byte is used to transmit the number of error blocks detected bythe receiving end via BIP-N24 (B2) so that the transmitting end can getthe receiving error status of the receiving end.

Media dependent bytes:

bytes are used to implement special functions of the specific transmission

media. For example, these bytes can be used to identify the direction of thesignal when bi-directional transmission is adopted in a single fiber.

All unmarked bytes are reserved for future international standardization.


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Tips:

SDH vendors usually use the reserved bytes within the section overhead ofthe STM frame to implement some special functions of their own equipment.

So far, the usage of bytes in the Section Overhead (RSOH, MSOH) of theSTM-N frame has been discussed. Via these bytes, OAM functions of theSTM-N section layer are implemented.

N STM-1 frames can be multiplexed into the STM-N frame viabyte-interleaved multiplexing. How is the Section Overhead multiplexed?During the byte-interleaved multiplexing, all bytes of the AU-PTR and payloadwithin the STM-1 frames are intact and are byte-interleaved. But themultiplexing method for the Section Overhead is different. Its multiplexingmethod is that when N STM-1 frames are multiplexed into the STM-N framevia byte-interleaved multiplexing, the Section Overhead of the first STM-1frame is kept while only the framing bytes and B2 bytes of the other N-1STM-1 Section Overheads are kept and the overhead bytes left are ignored.Figure 1-24 illustrates the Section Overhead structure of an STM-4 frame.

Notes: Bytes reserved for use domestic

RSOH

MSOH

36 bytes

9rows

A1 A1 A1A1A1 A1A1A1 A1A1 A1A1 A2 A2 A2A2 A2 A2A2 A2 A2A2 A2 A2 J0

B1 E1 F1

D1 D2 D3

K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M1 E2

B2 B2 B2B2 B2B2B2 B2B2B2 B2B2


* Unscrambled bytesAll unmarked bytes are reserved for future international standardization

CL Z0Z0Z0 * ** ** ** ***

Z0 For future international standardization

*

ally

Figure 1-24 Assignment of STM-4 SOH bytes

There is only one B1 in an STM-N while there are N 3 B2 bytes ( Since B2



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bytes are the result of the BIP-24 parity, each STM-1 has 3 B2 bytes, 38=24 bits). There is one D1-D12 byte, one E1 one E2 byte, one M1 byte, oneK1 byte and one K2 byte in an STM-N frame. Why?

Figure 1-25 is the structure of the STM16 Section Overhead.

144 bytes

RSOH

MSOH

9 rows

M1 -

A 2 A 2 A 2 A 2A 2 A 2 J0

E1 F1

A 1 A 1A 1A 1 A 1 A 1

B 1

D1 D2 D3

K 1 K 2

D4 D5 D6

D7 D8 D9

D1 0 D1 1 D1 2

S 1 E2

B 2 B 2 B 2 B 2 B 2 B 2


CL Z 0C LC L

* ****

Notes: Bytes reserved for use in China* Unscrambled bytes

All unmarked bytes are reserved for future international standardization

Z0 For future international standardization

Figure 1-25 Assignment of the STM-16 SOH bytes

1.3.2 Path Overhead

The Section Overhead is responsible for section layer OAM functions whilethe Path Overhead for path layer OAM functions. Like transporting thecargoes loaded in the container: not only the overall impairment status of the

cargoes (SOH) but also the impairment status of each cargo (POH) shall bemonitored.

According to the "width" of the monitored path ( the size of the monitoredcargo) , the Path Overhead is further classified into Higher Order PathOverhead and Lower Order Path Overhead. In this curriculum the HigherOrder Path Overhead refers to the monitoring of VC4 level paths which canmonitor the transmission status of 140Mbit/s signal within the STM-N frame.The Lower Order Path Overhead implements the OAM functions for VC12path level, i.e. monitoring the transmission performance of 2Mbit/s signals



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within the STM-N frame.

Technical details:

According to the multiplexing route of the 34Mbit/s signal, the POH of the VC3can be classified into higher order or lower order path overhead. Its bytesstructure and function are the same as that of the VC4 Path Overhead. Sincethe multiplexing of 34Mbit/s signals into STM-N method is seldom used, thedetailed description of the VC3 POH is omitted here.

1. Higher Order Path Overhead: HO POH

The Higher Order Path Overhead, consisting of 9 bytes, is located in the firstcolumn of the VC4 frame, as illustrated in Figure 1-26.

J1B3C2G1F2

H4F3K3N1

VC4

1

1

261

9

Figure 1-26 The structure of Higher Order Path Overhead

J1Path trace byte

The AU-PTR pointer indicates the specific location of the start of the VC4within the AU-4, i.e. the location of the first byte of the VC4, so that thereceiving end can properly extract VC4 from the AU-4 according to the valueof this AU-PTR. The J1 is the start of the VC4, so the AU-PTR indicates thelocation of the J1 byte.

The function of J1 is similar to that of J0. The J1 byte is used to transmitrepetitively a Higher Order Path Access Point Identifier so that a pathreceiving terminal can verify its continued connection to the intendedtransmitter (this path is under continued connection). This requires that the J1bytes of the received and transmitting ends match. The default



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transmit/receive J1 byte values of the equipment provided by HuaweiTechnologies Co., Ltd. are: SBS Huawei 155 and SBS Huawei 622corresponding to Huawei's 155 and 622 transmission equipment respectively;

Huawei SBS corresponding to Huawei 2500 transmission equipment. Ofcourse the J1 byte can be configured and modified according to therequirement.

B3Path BIP8 Code

The B3 byte of path BIP-8 code is allocated for monitoring the transmissionerror performance of VC4 within the STM-N frame, e.g. monitoring thetransmission error performance of 140Mbit/s signal within the STM-N frame.Its monitoring mechanism is similar to that of the B1 and B2 except that it isused to process BIP-8 parity for the VC4 frame.

Once the receiving end detects error blocks, the number of error blocks willbe displayed in the performance monitoring event ---- HP-BBE (Higher OrderPath Background Block Error) of the equipment end. Meanwhile in thecorresponding VC4 path performance monitoring event ---- HP-REI (HigherOrder Path Remote Error Indication) of the transmitting end, the number ofreceived error blocks will be displayed. Like the B1 and B2 bytes, this methodcan implement real-time monitoring over the transmission performance of theSTM-N signal.

Technical details:

If the B1 of the receiving end has detected error blocks, the number of errorblocks detected by the B1 will be displayed in this end performance eventRS-BBE (Regenerator Section Background Block Error). Notes: that doesn'treturn to transmitting end.

If the B2 of the receiving end has detected error blocks, the number of errorblocks detected by the B2 will be displayed in this end performance eventMS-BBE (Multiplex Section Background Block Error). Meanwhile thecorresponding number of error blocks will be displayed in the transmitting endperformance event MS-REI (Multiplex Section Remote Error Indication) (TheMS-REI is sent by the M1 byte).

Notes:

When the error detected by the receiving end exceeds a given limitation, theequipment will report an error overflow alarm signal.



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C2: Signal label byte

The C2 is allocated to indicate the composition of multiplexing structure andinformation payload of the VC frame, e.g. such as equipped or unequippedstatus of the path, the type of loaded services and their mapping method. Forexample, C2=00H indicates that this VC4 path is unequipped. Then thepayload TUG3 of the VC4 is required to be inserted all "1" code---- TU-AIS,and the higher order path unequipped alarm ---- HP-UNEQ appears in theequipment. C2=02H indicates that the payload of the VC4 is multiplexed via aTUG structure multiplexing route. In China, the multiplexing of 2Mbit/s signalsinto VC4 adopts the TUG structure, as illustrated in the attached figure.C2=15H means that the payload of the VC4 is FDDI (Fiber Distributed DataInterface) signal. To configure the multiplexing of 2Mbit/s signals for

Huawei-developed equipment, the C2 is required to be configured as TUGstructure in 2M signal multiplexing.

Technical details:

The configuration of J1 and C2 bytes is required to ensure the consistencebetween the transmitting end and the receiving end ---- transmitting andreceiving ends match. Otherwise, the receiving equipment will generateHP-TIM (Higher Order Path Trace Identifier Mismatch) and HP-SLM (HigherOrder Signal Label Mismatch). These two alarms will make the equipmentinsert all "1" code---- TU-AIS alarm indication signal, into the TUG3 structure

of the VC4.

G1Path status byte

The G1 is allocated to convey the path terminal status and performance backto a VC4 path termination source. This feature permits the status andperformance of the complete duplex path to be monitored at either end, or atany point along that path. How to comprehend it? Actually the G1 byteconveys reply messages, i.e. the messages sent from the receiving end to thetransmitting end, by which the transmitting end can acquire the status of thecorresponding VC4 path signal received by the receiving end.

Bits 1 through 4 convey the count of error blocks in VC4 to transmit endtransmitting end, i.e. HP-REI, that have been detected by the receiving endusing the B3 (the path BIP-8 code). If the AIS, error overflow or J1 and C2mismatch is detected by the receiving end, an HP-RDI (Higher Order PathRemote Defect Indication) is sent back to the transmitting end via the fifth bitof the G1 byte so that the source can know the status of the correspondingVC4 signal received by the sink, and detect and locate the fault in time. Bits 6and 8 are reserved for future use.


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F2, F3: Path user channels bytes

These bytes are allocated for user order wire communication purposebetween path elements (and are related to payload).

H4: TU position indicator byte

H4 indicates the multi-frame type of effective load and position of payloads.For example, it can be used as a multi-frame indicator for the TU-12 or as acell boundary indicator for an ATM payload when it enters a VC4.

The H4 byte is effective only when the 2Mbit/s PDH signals are multiplexedinto the VC4. As mentioned above, a 2Mbit/s signal is multiplexed into a C12as a multi-frame consisting of 4 basic frames. To properly align and extractthe E1 signal, the receiver is required to know the sequence number (1, 2, 3,

4) of current basic frame within the multi-frame. The H4 byte, indicating thenumber of current TU-12 (VC12 or C12) within current multi-frame, has animportant function as a position indicator. It ranges from 01H to 04H. If the H4received by the receiving end is out of this range, the receiving end willgenerate a TU-LOM (Tributary Unit Loss of Multi-frame alarm).

K3: Idle byte

It is allocated for future use. The receiver is required to ignore its value.

N1: Network operator byte

This byte is allocated for specific management purposes.

2. Lower Order Path Overhead: LO POH

The LO-POH here refers to the path overhead of the VC12 which monitorsthe transmission performance of the VC12 path level, i.e. monitors thetransmission status of 2Mbit/s PDH signals within the STM-N frame.

Where is the LO-POH located within the VC12? Figure 1-27 displays a VC12multi-frame structure consisting of four VC12 basic frames. The lower orderPOH are located in the first byte of each VC12 basic frame. An LP-POHconsists of four bytes: denoted V5, J2, N2 and K4.


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1

1

9

500us VC12 multiframe

V5 J2 N2 K4

VC12 VC12VC12 VC12

4

Figure 1-27 The structure of a Lower Order Path Overhead

V5: Path status and signal label byte

The V5 byte is the first byte of the multi-frame. The TU-PTR locates the startof the VC12 multi-frame within the TU12 multi-frame, i.e. the TU-PTRindicates the specific location of the V5 byte within the TU12 multi-frame.

The V5 provides the functions of error check, signal label and path status ofthe VC12 paths. So this byte has the functions of the G1 and C2 bytes withinthe higher order path overhead. Figure 1-28 illustrates the structure of the V5byte.

Error Monitoring RemoteErrorIndication

RemoteFailureIndication

Signal Label

( BIP-2)

( REI) ( RFI)

Remote DefectIndication

( RDI )

1 2 3 4 5 6 7 8

Error monitoring:

Convey the BitInterleaved Parity codeBIP-2:

Bit 1 is set as such thatparity of all odd numberbits in all bytes in theprevious VC-12 is even.

Bit 2 is set as such thatparity of all evennumber bits in all bytesis even.

RemoteErrorIndication

(formerFEBE):

If one ormore errorblocks were

detected bythe BIP-2,one1 is sentbacktowards theVC12 pathoriginator,andotherwise0zero issent.

RemoteFailureIndication:

If a failure isdeclared, thisbit sendsone1,otherwise itsends zero0.

Signal Label:

The signal label indicates thestatus and mapping method ofthe payload. Eight binary valuesare possible in these three bits:

000 VC path Unequipped VCpath

001 VC path equipped, nonspecific payload

010 Asynchronous floatingmapping

011 Bit synchronous floatingmapping

100 Byte synchronous floatingmapping

Remote DefectIndication(former FERF):

It sends 1 if adefect isdeclared,otherwise itsends 0.



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101 Reserved for future use

110 Test signal, O.181 specificmapping

111 VC-AIS

Figure 1-28 The structure of the VC12 POH (V5).

If the error blocks were detected by the receiver via the BIP-2, the number oferror blocks detected by the BIP-2 is displayed in this end performance eventLP-BBE (Lower Order Path Background Block Error), and meanwhile anLP-REI (Lower Order Path Remote Error Indication) is sent back to thetransmitter via the b3 of the V5 byte. Thus the corresponding number of error

blocks errors can be displayed in the transmitter performance event LP-REI.Bit 8 of the V5 is allocated for the VC12 Path Remote Defect Indication. AnLP-RDI (Lower Order Path Remote Defect Indication) is sent back to thesource if either a TU12 AIS signal or signal failure condition is being detectedby the sink. Note: In this curriculum, RDI is called remote deteriorationindication or remote defect indication.

If the defect condition lasts beyond the maximum time allocated to thetransmission protection mechanisms, the defect becomes a failure. Then anLP-RFI (Lower Order Path Remote Failure Indication) is sent back to thesource via the b4 of the V5 by the sink to inform the source that a receivingfailure arises on the corresponding VC12 path at the sink.

b5-b7 provide a signal label. If only its value is not 0, the VC12 path isequipped, i.e. the VC12 package is not empty. If the value of b5-b7 is 000, theVC12 is unequipped and an LP-UNEQ (Lower Order Path Unequipped) alarmis aroused at the termination sink. Then all 0 code is inserted (not all 1 code---- AIS). If the b5-b7 of V5 at the transmitter and the receiver mismatch, anLP-SLM (Lower Order Path Signal Label Mismatch) alarm is generated at thetermination sink.

J2VC12 path trace byte

The function of the J2 is similar to that of the J0 and J1. It is used to transmitrepetitively a Lower Order Path Access Point Identifier agreed mutually by the

transmitter and the receiver so that the path receiving terminal can verify itscontinued connection to the intended transmitter.

N2Network operator byte

This byte is allocated for specific management purposes.

K4Reserved byte


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It is reserved for future use.

Questions:

What did you learn from this section?

This section described the layered methods of implementing the STM-N OAMfunctions, such as the Regenerator Section Overhead, Multiplex SectionOverhead, Higher Order Path Overhead and Lower Order Path Overhead.Via those overhead bytes, you can completely monitor the whole STM-Nsignal and lower rate signals equipped in the STM-N frame.


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1.4 Pointers

The function of the pointers is aligning via which the receiver can properlyextract the corresponding VC from the STM-N, and then disassemble the VCand C packages to extract the lower rate PDH signals, i.e. directly drop lowerrate tributary signals from the STM-N signal.

What is aligning? Aligning is a procedure by which the frame offsetinformation is incorporated into the Tributary Unit or the Administrative Unit,i.e. via the Tributary Unit Pointer (or Administrative Unit Pointer) attached tothe VC to indicate and determine the start of the lower order VC frame withinthe TU payload (or the start of the higher order VC frame within the AUpayload). When relative differences occur in the phases of the frames and

make the VC frames "float", the pointer value will be justified to ensure that itconstantly and properly designates the start of the VC frame. For a VC4, itsAU-PTR indicates the location of the J1 byte while for a VC12, its TU-PTRindicates the location of the V5 byte.

The TU pointer or AU pointer provides a method of allowing flexible anddynamic alignment of the VC within the TU or AU frame because these twopointers are able to accommodate differences, not only in the phases of theVC and the SDH, but also in the frame rates.

Two pointers are provided: AU-PTR and TU-PTR which are used forrespectively aligning of the Higher Order VC (here referring to VC4) and the

Lower Order VC (here VC12) within the AU-4 and TU12 respectively. Theiroperation mechanisms are described below.

1.4.1 Administrative Unit Pointer AU-PTR

The AU-PTR, located in row 4 of columns 1 to 9 within the STM-1 frame, isused to indicate the specific location of the fist byte J1 of the VC4 within theAU-4 payload so that the receiver can properly extract the VC4, as illustratedin Figure 1-29.


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RSOH

H1YYH2FF H3H3H3

H1YYH2FFH3H3H3

MSOH

RSOH

MSOH

Negative justification opportunity

0 1 - - 86

696 697 - - 782

0 1 - - 86

1 9 270

1

4

91

4

9

125us

250us

522 435 436 ------ 521

523 ------ 608

Positive justification opportunity

Figure 1-29 The location of the AU-4 pointer in the STM frame

As can be seen from the figure, the AU-PTR consists of 9 bytes:H1YYH2FFH3H3H3 with Y=1001SS11 (S bits are unspecified) andF=11111111. The pointer value is contained in the last ten bits of H1 and H2

bytes. In the frame, three bytes form a justification opportunity ---- a cargounit.

What is the function of the justification opportunity? Let's take the example oftransporting cargoes with a truck. The cargoes ---- VC4 are continuouslyloaded onto the cargo box ---- information payload area three byte (one unit)-by-three byte. The stop time of the truck is 125us.


1) If the frame rate of the VC4 is higher than that of the AU-4, i.e. the packagerate of AU-4 is lower than the loading rate of the VC4, then the time forloading a VC4 (cargo) is less than 125us (the stopping time of the truck). TheVC4 will be continuously loaded before the truck leaves. However, the cargobox of the truck (the information payload area of AU-4) is already full andunable to accommodate more cargoes. At that time, the three H3 bytes (onejustification unit) are used to accommodate the cargoes. These three H3bytes are like a backup space temporarily added to the truck. Then thelocation of the all cargoes will be moved forward by one unit (three bytes), sothat more cargoes [one VC4 +plus 3 bytes] can be added into the AU-4. Thusthe location of each cargo unit (one unit includes 3 bytes) will be changed.This justification method is called negative justification. The three H3 bytesappear immediately after the two FF bytes are called negative justificationopportunity. At that time, the three H3 bytes are filled with VC4 payload. Viathis justification method, the first three bytes of the VC4 of the next truck areloaded on current truck.


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2) If the frame rate of the VC4 is slower than that of the AU-4, i.e. a VC4 can'tbe completely loaded during the stopping time of the AU-4 "truck", then thelast three bytes ---- cargo unit of the VC4 shall be transported by the next

truck. Since the AU-4 hasn't been filled with VC4 (lack of a 3-byte unit), thecargo box has an spare space of 3 bytes. To prevent the cargoes fromstraggling during transmission due to the spare space within the cargo box,three additional H3 bytes are required to be inserted immediately after thethree H3 bytes of the AU-PTR. And the H3 bytes are filled withpseudo-random information (like the stuff inserted into the space of the cargobox). Then all the 3-byte units within the VC4 are required to move backwardby one unit (3 bytes). Thus the position of the cargo units will be changedaccordingly. This justification method is called positive justification. Thecorresponding position of the three inserted H3 bytes is called positivejustification opportunity. If the rate of the VC4 is much lower than that of theAU-4, more than one positive justification unit (3 H3 bytes) will be required tobe inserted into the AU-4 payload area. Note that there is only one negativejustification opportunity (3 H3 bytes). And the negative justification opportunityis located within the AU-PTR while the positive justification opportunity islocated within the payload area of the AU-4.

3) Either positive justification or negative justification will change the locationof the VC4 within the AU-4 payload, i.e. the location of the first byte of theVC4 within the AU-4 payload will be changed. Then the AU-PTR will makecorresponding positive or negative justification. For the convenience ofaligning each byte of the VC4 (each cargo unit, actually) within the AU-4payload, each cargo unit is allocated with a location value, as illustrated inFigure 1-30. The location value of the 3-byte unit immediately after the H3bytes is set as 0, and so on. Thus an AU-4 payload area has 2619/3783

locations, and the AU-PTR designates the location value of the J1 byte withinthe AU-4 payload. Admittedly, the AU-PTR shall be in the range of 0 to 782,otherwise it is an invalid pointer value. If invalid pointer values were receivedconsecutively in 8 frames, the equipment will generate an AU-LOP (AU Lossof Pointer) alarm and insert an AIS alarm signal- TU-AIS.

Either positive or negative justification is processed once a unit, then thepointer value will be modified incremented +1 (pointer positive justification) ordecreased -1 (pointer negative justification) by one.

4) If there is no difference in the rates and the phases between the VC4 andthe AU-4, i.e. the stopping time of the truck and the rate for loading the VC4match, the AU-PTR value is 522, as indicated by the frame arrow in Figure

1-29.

Note:

The AU-PTR indicates the location of the J1 byte within the next VC4 frame.In the case that the network is synchronous, the pointer justification seldomappears. So mostly the H3 bytes are filled with pseudo information.



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As mentioned before, the pointer value is located in the last ten bits within theH1H2 bytes. Thus the value of the ten bits ranges from 0 to 1023 (210). If theAU-PTR value is not within 0-782, it is an invalid pointer value. How do the 16

bits of the H1H2 implement pointer justification control? Please see Figure1-30.

N N N N S S I D I D I D I D I D

New Data FlagNDF

means that thecapacity of thepayload has changed.

If no change occursto the payload, the

normal value ofNNNN is "0110".Within the frame withits payload changed,the NNNN inverted to"1001" ---- NDF. Thepointer value of theframe with an NDFshall be changed intothe new locationvalue of the VC,called new data. If thepayload no longer

changes, the NDF ofthe next frame shallreturn to the normalvalue "0110". Nosubsequentincrement ordecrement operationis allowed for at leastthree frames followingthis operation.

AU/TUtype

For AU-4and TU-3,SS=10

10 bit pointer value

The range for AU-4 pointer is 0782 with three bytes as an offsetunit.

The pointer value indicates the offset between the last H3 byte ofthe AU-4 pointer and the first byte of the VC4 frame.

Pointer justification regulation

(1) During normal work, the pointer locates the start of the VC4within the AU-4 frame. The NDF is set to "0110".

(2) If the frame rate of the VC4 is lower than that of the AU-4, theinversion of the five I-bits indicates that a positive frequencyjustification is required. Then the start point of the VC frame ismoved backward by one unit and in the subsequent frame, pointervalue shall be increased by plus one.

(3) If the frame rate of the VC4 is higher than that of the AU-4, theinversion of the five D-bits indicates that a negative frequencyjustification is required. Then the negative justification opportunity

H3 bytes are overwritten with the actual information data of the VC4and the start point of the VC frame is moved forward by one unit. Inthe subsequent frame, pointer value shall be decreased by minusone.

(4) If the NDF appears refreshed value 1001, the change of thepayload capacity is indicated. The pointer value shall be increasedor decreased correspondingly, then the NDF shall return to thenormal value 0110.

(5) Following a pointer justification operation, no subsequentincrement or decrement operation is allowed for at least threeframes.

(6) When the pointer is interpreted by the receiver, any variationfrom the current pointer value is ignored except a consistent newpointer value received three times (three frames) consecutively.

Figure 1-30 The 16-bit pointer code consisting of H1 and H2 within the AU-4

The pointer value is carried in bits 7-16 of H1 and H2. The odd number bits ofthe ten bits are denoted I-bits while the even number bits are denoted D-bits.The operation of the pointer value increment or decrement by one is indicated


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by the inversion of all or the majority of the five I-bits or five D-bits. So theI-bits are also called increment bits while the D-bits are called decrement bits.

No subsequent frame pointer justification is allowed for every other threeframes, i.e. if the frame in which the pointer value inversion is regarded as thefirst frame, the subsequent pointer inversion isn't allowed until the fifth frame(the subsequent pointer value will be increased or decreased by one).

The inversion of the NDF indicates the change of the AU-4 payload. Then thepointer value will leap, i.e. the step-length of the pointer value increment ordecrement is not 1one. If the receiver detects NDF inversion in eight framesconsecutively, the equipment will generate an AU-LOP alarm.

The receiver only interprets the received pointer which is consistent in threeor more consecutive times (frames), i.e. the system considers that the pointer

values of the three frames following the pointer justification are consistent. If asubsequent pointer justification occurs, a VC4 aligning error will appear at thereceiver and result in transmission performance defects.

In a word, if the 5 I-bits or 5 D-bits invert at the transmitter, the subsequentAU-PTR value shall be increased or decreased by one. The receiverdetermines whether to justify in the subsequent frame according to theinversion status of the majority of the I-bits or D-bits, i.e. to align the first byteof the VC4 and restore the timing of the signal before the pointer adaptationand alignment.

1.4.2 Tributary Unit Pointer TU-PTR

The TU pointer is used to indicate the specific location of the first byte V5 ofthe VC12 within the TU12 payload so that the receiver can properly extractthe VC12. The TU-12 pointer provides a method of allowing flexible anddynamic alignment of the VC12 within the TU-12 multi-frame. The TU-PTR islocated in the bytes denoted V1, V2, V3 and V4 within the TU12 multi-frame,as illustrated in Figure 1-31.

70 71 72 73 105 106 107 108 0 1 2 3 35 36 37 38

74 75 76 77 109 110 111 112 4 5 6 7 39 40 41 42

78 The firstBasic framestructure ofC-12 94-2 32W

2Y

81 113 Thesecondbasicframestructureof C-12 :94-2 32W

1Y 1G

116 8 The thirdbasic framestructure ofC-12: 94-232W

1Y 1G

11 43 The fourthbasic framestructure ofC-12: 94-1 31W 1Y

1M+1N

46

82 85 117 120 12 15 47 50


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86 89 121 124 16 19 51 54

90 93 125 128 20 23 55 58

94 97 129 132 24 27 59 62

98 101 133 136 28 31 63 66

102 103 104 V1 137 138 139 V2 32 33 34 V3 67 68 69 V4

Figure 1-31 Numbering of the TU-12 pointer location and offset

The TU12 PTR consists of four bytes denoted: V1, V2, V3 and V4.

From the byte immediately following the V2 within the TU-12 payload, each

byte is in sequence specified an offset number such as "0" and "1" accordingto the offset from the byte to the last V2, with one byte as a positivejustification unit. Total offset numbers are from 0 to 139. The first byte V5 ofthe VC12 frame is located in the location with an offset number correspondingto the binary value of the TU-12 pointer value.

The V3 byte of the TU-12 PTR is the negative justification opportunity. Apositive justification opportunity immediately follows it. V4 is a reserved byte.The pointer value is located in the last ten bits of the V1 and V2 bytes. Thefunction of the 16 bits of the V1 and V2 bytes is similar to that of the 16 bits ofthe H1 and H2 bytes within the AU-PTR.

Notes:

Positive/negative justification is implemented via the V3 byte.

The justification unit of the TU-PTR is one (byte). Thus the range of thepointer value is 0 to 139. If the invalid pointer or NDF is being received ineight frames consecutively, a TU-LOP (Tributary Unit Loss of Pointer) alarmwill be generated at the receiver and an AIS alarm signal shall be inserted.

If there is no difference in the phases and frequencies between the VC12 andTU12, the location value of the V5 byte is 70, i.e. the TU-PTR value is 70.

The pointer justification and pointer interpretation methods of the TU-PTR issimilar to that of the AU-PTR.



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1.5 SDH Networking Protection

Methods

One of the major advantages of SDH networking is that different basicnetwork structures can make up different combinations, thus enabling thetransmission network as a whole to have the capacity to deal with networkfailures and enhancing the reliability of the network in operation.

SDH networking mainly relies on Protection and Restoration--- these twodifferent mechanisms, to guarantee that the communication services can bemaintained in case of a failure. So-called Protection usually refers to arelatively quick transition process, whose execution is determined by theswitching part automatically. As the protection function occupies somepre-pointed space between various network points, the switched paths alsohave also their pre-determined routes. Hereafter, we would like to introducethe networking protection mothods with the transmission equipment made byHuawei Technologies Co., Ltd. as an example.

OptiX equipment supports the following four types of self-healing protectionmechanisms recommended by ITU-T:

1. Route protection type

Protection methods of this type include:

11 route protection (11 linear multiplexing section protection)

Two-fiber unidirectional multiplexing section special protection ring

Two-fiber bi-directional multiplexing section shared protection ring

Two-fiber unidirectional path protection ring

Two-fiber bi-directional path protection ring

2. Sub-netinterconnection protection type

3. Double-point interconnection (DNI) inter-connection serviceprotection

4. Virtual fiber shared path protection


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1.5.1 Route Protection Type

1. 1 1 Line Protection

The working principle of SDH line protection is that when the working systemtransmission is stopped or the performance deteriorates to a certain degree,the system switching equipment will automatically switch the key signals tothe spare fiber system for transmission. It is mainly adopted to protect thetransmission media and the line terminal interfaces of regenerators, TM andADM (such as the optical/electrical and electrical/optical switching part).However, it doesn't protect the failures occurring at end TM or ADM point.OptiX equipment supports 1+1 route protection at the tributary side. And ittakes less than 50ms to restore the services, even better than ITU-Trecommended standard.

The so-called 1+1 line protection refers to that each working system has aspecial designated standby system. These two systems back up each otherand adopt the non-restoration mode. The source end of the main system andthe backup system are interconnected and the receiving end decides whetherto accept signals from the main system or the backup system according to thequality of the signals received. 1+1 line protection doesn't require theparticipation of APS protocol. It can be automatically executed based onwhether there is any failure or shortcoming of the received signals. Or,enforced switching or locking can be performed according to the externalorders.

2. Two-fiber bi-directional MS shared protection rings

For two-fiber bi-directional MS shared protection rings, as their services usethe common route and are transmitted bi-directionally, the time slots on t

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