SDH Networking and Protection

OTA000006

SDH Networking and Protection

ISSUE 1.01

OTA000006 SDH Networking and Protection ISSUE 1.01 Table of ContentsError! Style not defined.

Confidential Information of Huawei. No Spreading without Permission

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Table of Contents

Course Description ...........................................................................................................................1 Course Introduction......................................................................................................................1 Targets of the course ...................................................................................................................1 References...................................................................................................................................1

Chapter 1 SDH Network Topology...................................................................................................2 1.1 Chain (linear) Architecture .....................................................................................................2 1.2 Star Architecture ....................................................................................................................2 1.3 Tree Architecture ...................................................................................................................3 1.4 Ring Architecture ...................................................................................................................3 1.5 Mesh Architecture..................................................................................................................4

Chapter 2 SDH Network Protection .................................................................................................5 2.1 Basic Concepts ......................................................................................................................5 2.2 Categories of Survivable Networks........................................................................................8

2.2.1 Linear Multiplex Section Protection.............................................................................8 2.2.2 Protection Rings........................................................................................................11 2.2.3 Sub-network Connection Protection .........................................................................18

2.3 Comparison of the Network Protections ..............................................................................20

OTA000006 SDH Networking and Protection ISSUE 1.01 List of Tables


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List of Tables

Table 2-1 the switching criteria for OptiX equipment ............................................................... 19

OTA000006 SDH Networking and Protection ISSUE 1.01 Error! Style not defined.


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Course Description

Course Introduction

The product version of this teaching material is OTA051004.

Here explained the SDH transmission system five basic networks --- chain (line), star, tree, ring and mesh. As well we will cover the different protection mechanism used in the SDH survivable networks.

Targets of the course

Through this course, trainees should be able to:

� List the SDH different topologies structures, features and applications. � Have idea about the basic concept of the SDH network protection. � Understand the network objectives, application architecture, switching

initialization and restoration criteria, characteristics, network capacity of different types of network protection.

References

ITU-T recommendation G.841



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Chapter 1 SDH Network Topology

The network topology, the geometrical layout of SDH network nodes and transmission lines, reflects the physical connection of the network. The network topology is important in the sense that it determines the performance, reliability and cost-effectiveness of an SDH network.

1.1 Chain (linear) Architecture

Chain network, as shown in Figure 1-1 is such a topology in which all nodes are connected in sequence line, while the two nodes at both ends are connected at only one side. If the two end nodes are directly connected with each other with no nodes between them, this is called a point-to-point structure. Point-to-point structure can be considered as a special case of the chain network.

For a chain network, if there is a service between any two non-adjacent nodes, then we must configure the add/drop traffic at the two end nodes, and pass-through traffic in between these two nodes. For example, as shown in Figure 1-1, there is an ADM between two terminal multiplexers. This is a typical chain topology structure.

The chain network is simple and economical at the initial application stage of SDH equipment. For a chain network, it’s more difficult and more expensive to protect the traffic, compared with a ring network. The chain network is used in cases where the traffic is unimportant or where the traffic load is small so that we don’t have to care about the traffic protection. In terms of network protection for a chain, we can use 1+1 linear Multiplex Section protection and 1:N linear Multiplex Section protection. The simplest network configuration involves two multiplexers or multipoint of add/drop circuit a long the way linked by fiber with or without regenerator in the whole link.

A B C D E

Figure 1-1 Chain topology

1.2 Star Architecture

In an SDH transmission network, if a special node (central node or hub node) exists which has connections with all the other nodes, while between all the other nodes there are no direct connections, this network will be called a start or hub network. For a star network, the traffic between any other nodes other than the hub node must pass through (dispatched at this node) the hub node. The hub node selects routes and passes through the traffic signals for all the other nodes. As a result, the hub node is able to manage the bandwidth resources thoroughly and flexibly. On the other hand,



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there is the possibility of a potential bottleneck of bandwidth resources. Besides, the equipment failure of the hub node may result in the breakdown of the entire network.

The basic physical structure of a star network is shown in Figure 1-3. For star networks, the possible network protection is the same as a chain. 1+1 linear Multiplex Section protection and 1:N linear Multiplex Section protection can be used for a star network.

A

BC

D E

Figure 1-2 Star topology

1.3 Tree Architecture

In a point-to-point structure, if any end node is connected with several other nodes, a tree structure is formed. A tree structure can be considered as the combination of chain and star structures. It is suitable for broadcast service. However, due to the bottleneck problem and the optical power budget limit, it is not suitable for bidirectional traffic. The basic physical structure of a tree network is shown in Figure 1-3.

A

B C

D E

Figure 1-3 Tree topology

1.4 Ring Architecture

The ring network, as shown in Figure 1-4, is the most widely used network for SDH transmission networks. If the two end nodes in a chain network are connected together, the chain network will be converted into a ring. In such a structure, any traffic between two adjacent nodes can be directly add/drop between them. For traffic between two non-adjacent nodes, we have to configure the add/drop traffic at the source node and



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the sink node. And the pass-through traffic in between those two nodes must be created as well.

The ring network is highly survivable. The most obvious advantage of a ring network is its high survivability that is essential to modern optical networks with large capacity. Thus, the ring network enjoys very broad applications in SDH networks.

A

B

C D

E

Figure 1-4 Ring topology

1.5 Mesh Architecture

Mesh networks are such communications networks in which many nodes are interconnected with each other via direct routes. In such topological structure, if direct routes are used in the interconnection of all the nodes, this structure is considered as an ideal mesh topology. In a non-ideal mesh topological structure, the service connection between nodes that are not connected directly is established through route selection and transiting via other nodes. In a mesh network, no bottle neck problem exists. Since more than one route can be selected between any nodes, when any equipment fails, services can still be transmitted smoothly through other routes. Thus, the reliability of service transmission is increased. However, such networks are more complicated, costly and difficult to manage. Mesh networks are very suitable for those regions with large traffic.

A

B

C

DE

Figure 1-5 Mesh topology



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Chapter 2 SDH Network Protection

2.1 Basic Concepts

Modern society is getting more and more dependent on communications with the development of science and technologies, and so higher requirements to network security are being brought forward. Thus the concept of survivable network comes into being. The following will deal with the concepts of survivable network.

1. Unidirectional Traffic and Bidirectional Traffic

Unidirectional traffic and bidirectional traffic are named regarding the traffic flow directions in the ring. A unidirectional ring means that traffic travel in just one direction, e.g. clockwise or counter-clockwise, following a diverse route. While in a bidirectional ring, traffic signals go in two directions, one opposite to another, following a uniform route. As shown in Figure 2-2, a unidirectional ring is diversely routed; Figure 2-1, while a bidirectional ring is uniformly routed.

T he traffic shares the sam eequ ip m en t and link

B

A

Figure 2-1 Uniformly routed



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T1516670-94

A

B

b) Diversely routed

The traffic is ondifferent equipment

and links

Figure 2-2 Diversely routed

2. Unidirectional Traffic and Bidirectional Traffic

A network that is capable of restoring traffic in the event of a failure. The degree of survivability is determined by the network's ability to survive single line system failures, multiple line system failures, and equipment failures.

3. Bidirectional Protection Swtiching

A protection switching architecture in which, for a unidirectional failure (i.e. a failure affecting only one direction of transmission), both directions (of the "trail", "subnetwork connection", etc.) including the affected direction and the unaffected direction, are switched to protection.

4. Unidirectional Protection Swtiching

A protection switching architecture in which, for a unidirectional failure (i.e. a failure affecting only one direction of transmission), only the affected direction (of the "trail", "subnetwork connection", etc.) is switched to protection.

5. Bridge and Switch

Bridge is the action of transmitting identical traffic on both the working and protection channels. While the action of selecting normal traffic from the protection channels rather than the working channels is called switch.

6. Protection Channels

The channels allocated to transport the normal traffic during a switch event. Protection channels may be used to carry extra traffic in the absence of a switch event. When



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there is a switch event, normal traffic on the affected working channels is bridged onto the protection channels.

7. Working Channels

The channels over which normal traffic is transported when there are no switch events.

8. Subnetwork Connection Protection (SNC Protection)

A working subnetwork connection is replaced by a protection subnetwork connection if the working subnetwork connection fails, or if its performance falls below a required level.

9. Idle

A node that is not generating, detecting or passing-through bridge requests or bridge request status information.

10. Pass-through

The action of transmitting the same information that is being received for any given direction of transmission.

11. Switching Node

The node that performs the bridge or switch functions for a protection event.

12. Ring Switching

Protection mechanism, that applies to both two-fiber and four-fiber rings. During a ring switch, the traffic from the affected span is carried over the protection channels on the long path.

13. Revertive/non-revertive modes

In revertive mode of operation, when the protection is no longer requested, i.e. the failed working section is no longer in SD or SF condition (and assuming no other requesting sections), a local wait-to-restore state shall be activated. Since this state becomes the highest in priority, it is indicated on the sent K1 byte, and maintains the normal traffic signal from the previously failed working section on the protection section. This state shall normally time out and become a no request null signal (0) (or no request extra traffic signal (15), if applicable). The wait-to-restore timer deactivates earlier if the sent K1 byte no longer indicates wait-to-restore, i.e. when any request of higher priority pre-empts this state.

In non-revertive mode of operation, applicable only to 1 + 1 architecture, when the failed working section is no longer in SD or SF condition, the selection of the normal traffic signal from protection is maintained by activating a do-not-revert state rather than a no-request state.

Both wait-to-restore and do-not-revert requests in the sent K1 byte are normally acknowledged by a reverse request in the received K1 byte. However, no request is acknowledged by another No Request received.

14. Span

The set of multiplex sections between two adjacent nodes on a ring.



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15. Span Switching

Protection mechanism similar to 1:1 linear APS that applies only to four-fiber rings where working and protection channels are contained in separate fibers and the failure only affects the working channels. During a span switch, the normal traffic is carried over the protection channels on the same span as the failure.

16. Extra Traffic

Traffic that is carried over the protection channels when that capacity is not used for the protection of normal traffic. Extra traffic is not protected. Whenever the protection channels are required to protect the normal traffic, the extra traffic is pre-empted.

2.2 Categories of Survivable Networks

Ring and line are the two most commonly used networks, so the common protection network will be about ring and line. For the line network, we have 1:N linear multiplex section protection and 1+1 linear multiplex section protection. For ring protection, there might be several classification criteria. Based on the traffic protection level, it can be grouped into Path Protection ring, Multiplex Section Protection ring and Subnetwork Connection Protection ring. The basic protection entity for a path protection ring is Path (VC12 for E1, VC3 for E3/T3 and VC4 for E4). Multiplex section protection protects a MS, one STM-1 for example, while SNC protection protects one subnetwork connection.

2.2.1 Linear Multiplex Section Protection

Linear Multiplex Section (MS) protection is one of multiplex section protections. Linear multiplex section protection switching can be a dedicated or shared protection mechanism. It protects the multiplex section layer, and applies to point-to-point physical networks. One protection multiplex section can be used to protect the normal traffic from a number (N) of working multiplex sections. It cannot protect against node failures. It can operate in a unidirectional or bidirectional manner, and it can carry extra traffic on the protection multiplex section in bidirectional operation.

Protection modes can be divided into two kinds: 1+1 and 1:N. In 1+1 protection mode, every working system is protected by a dedicated protection system. But in 1:N protection mode, N systems share one protection system; and when the system is in normal operation, the protection system can also transmit extra traffic. Thus a higher efficiency can be obtained than that of 1+1 system, but a more complicated APS protocol is needed. This protection mode mainly protects the normal traffic in case optical cable of the working multiplex section is cut off or multiplex section performance degrades.

1. 1+1 linear multiplex section protection

Working mode of 1+1 linear multiplex section protection is shown in Figure 2-3.



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A B

Working section

Working section

Protection section

Protection section

Figure 2-3 1+1 Linear multiplex section protection working mode

From the viewpoint of switching mode, 1+1 linear multiplex section protection can be divided into unidirectional switching in non-revertive mode, unidirectinoal switching in revertive mode, bidirectional switching in non-revertive mode, and bidirectional switching in revertive mode.

Out of 1+1 linear multiplex section protections, some modes require APS protocol during the switching process, some don’t require. For 1 + 1 unidirectional switching, the signal selection is based on the local conditions and requests. Therefore each end operates independently of the other end, and bytes K1 and K2 are not needed to coordinate switch action.

Technical details:

Unidirectional switching mode means when switching occurs, it occurs only to one end, while the other end remains unchanged. Take 1+1 linear multiplex section protection as an example, as shown in Figure 2-3. If the transmitting optical fiber of the working section from Node A to Node B is cut off accidentally, Node B detects signals being invalid and switching will occur. The traffic signals that are sent by A will be received by the protection section optical fiber instead, while the status of Node A remains unchanged.

Bidirectional switching mode means switching will occur to both ends at the same time. As shown in Figure 3-2, if the transmitting optical fiber of working section from Node A to Node B is cut off accidentally and Node B has detected signals being invalid, then switching will occur. The protection section optical fiber will receive traffic signals sent by Node A instead and it will inform Node A with K1K2 bytes. Node A, being aware that Node B is under switching status, will also enter switching status. And then the protection section will receive traffic signals sent by Node B instead.

Revertive mode means when nodes are under switching status, and after working section is recovered, the switching status will be cleared so that the nodes recover their original normal status. While non-revertive mode means when nodes are under switching status, even if working section is recovered, nodes will no longer restore to its previous normal status; that is to say, the switching status will remain unchanged. As shown in Figure 3-2, if the transmitting optical fiber of the working section from Node A to Node B is cut off accidentally, Node B will detect signal being invalid, and execute switching to enter the switching status, and then receives traffic signals sent from Node A through protection section optical fiber. If the transmitting optical fiber of the working section from Node A to Node B recovers, the signal failure detected by Node B is



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cleared. At this moment, if Node B is in revertive mode, it will release the switching status and return to normal status. Then working section optical fiber will be used again to receive traffic signals sent by A. If Node B is in non-revertive mode, the switching status will be maintained, while the traffic signals sent by Node A will be received from the protection section optical fiber.

2. 1:N linear multiplex section protection

Working mode of 1:N linear multiplex section protection is shown in Figure 2-4

A B

Bridging Selector

Protection section (Transmit)

Working section 1(Transmit)

Protection section (Receive)

Working section 1(Receive)

Working section 2(Transmit)

Working section 2(Receive)

Working section N(Transmit)

Working section N(Receive)

Selector Bridging

Figure 2-4 1:N Linear multiplex section protection working mode

Out of all the possible switching modes, 1:N linear multiplex section protection supports only one working mode—bidirectional switching in revertive working mode, in consideration of the extra traffic.

Bidirectional switching in revertive working mode of 1:N linear protection follows multiplex section protection protocol as well. In the course of switching, K1K2 bytes between nodes are transmitted through protection section optical fiber.

Technical details:

The MSP functions, at the ends of a multiplex section, make requests for and give acknowledgements of switch action by using the APS bytes (K1 and K2 bytes in the MSOH of the protection section). The bit assignments for these bytes and the bit-oriented protocol are defined as follows.

K1 byte: The K1 byte indicates a request of a traffic signal for switch action. A request can be:

1- A condition (SF and SD) associated with a section. A condition has high or low priority. The priority is set for each corresponding section;

2- A state (wait-to-restore, do not revert, no request, reverse request) of the MSP function; or

3- An external request (lockout of protection, forced or manual switch, and exercise).

K1 byte Bits 1-4 indicate the type of request. Bits 5-8 indicate the number of the traffic signal or section for which the request is issued.



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K2 byte Bits 1-4 indicate a signal number. Bit 5 indicates the type of the MSP architecture: set 1 indicates 1:n architecture and set 0 indicates 1 + 1 architecture. Bits 6-8 are used for MS-AIS and MS-RDI indication.

2.2.2 Protection Rings

For path protection ring, traffic protection is based on paths. Switching or not is determined by signal qualities of each path on the ring. For multiplex section protection ring, traffic protection is based on multiplex section. Switching or not is determined by signal qualities of the multiplex section between each span of nodes. An important difference between a path protection ring and a multiplex section protection ring is that the former usually adopts dedicated protection. That is to say, in normal conditions protection section also sends traffic and protection channel is dedicated to the whole ring; while the later usually adopts shared protection, i.e., in normal condition protection section is idle and protection channel is shared by each span of the ring. Thus protection rings can be divided into dedicated protection ring and shared protection ring. It is certain that multiplex section protection ring can also adopt the dedicated protection, yet it has no distinctive advantages over a path protection ring.

According to the traffic flow direction, we can have unidirectional ring and bidirectional ring. From the number of optical fibers between two adjacent nodes, the rings can be further divided into two-fiber rings and four-fiber rings.

From the above-mentioned criteria, we might have 2-fiber unidirectional path protection ring, 2-fiber bidirectional path protection ring, 2-fiber unidirectional multiplex section protection ring, 2-fiber bidirectional multiplex section protection ring, 2-fiber unidirectional SNC protection ring, 2-fiber bidirectional SNC protection and 4-fiber bidirectional multiplex section protection ring.

1. Two-fiber unidirectional path protection ring

Two-fiber unidirectional path protection rings use the 1+1 protection mode and the structure of "head-end bridging, while the tail-end switching". One optical fiber is the working fiber; call the S fiber, while the other is the protection fiber, called the P fiber as shown in Figure 2-5.

A

B

C

D

S1

P1

Figure 2-5 Illustration of a two-fiber unidirectional path protection ring in normal condition



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A bridge is used to simultaneously transmit signals onto the working and protection fibers, so the same traffic signals are sent on the two fibers, but in opposite directions. At the receiving end, either the working or the protection fiber is chosen to receive the traffic signals according to the signal quality. The receiver uses a switch to select the working trail under normal operating conditions.

Switching is effected by judging the quality of the path signals according to the path alarm signals (e.g. TU-AIS, TU-LOP, etc.) as well as error bits status of path signals. The switch completion time of the OptiX equipment is superior to the 50ms switching time as stipulated in the ITU-T recommendations. The short switching time is attributable to the efforts to optimize the path protection in system hardware and software and is of great significance to the traffic which is sensitive to error bits, such as signaling, data, video, etc.

A

B

C

D

switch

S1

P1

Figure 2-6 Illustration of a two-fiber unidirectional path protection ring in switched condition

After path protection switch takes place in the NE, the tributary board monitors the status of traffic on the working fiber S1 at the same time. When no TU-AIS is found for a while (10 minutes for Huawei equipment), the tributary board of the switched NE will restore to receive traffic from the working fiber, back to the default status in normal conditions.

Because path protection is a dedicated protection mechanism, this means that the timeslots of each fiber cannot be reused. In the two-fiber unidirectional path protection ring, because the traffic added to the ring is sent concurrently and received selectively, the path protection is actually in 1+1 protection mode. This mode features fast switch (Huawei equipment switches at the speed �15ms) and simple traffic flow, making configuration and maintenance easy. The disadvantage is its limited network capacity. The network capacity is the maximum traffic load that a network can carry. The network capacity of the two-fiber unidirectional protection ring is constantly STM-N, which is not related to the number of nodes on the ring and the traffic distribution between NEs. Why?

Two-fiber unidirectional path ring is usually used in the case when a site on the ring is the main traffic station, i.e. centralized traffic station, between which and all the other nodes there are traffic signals, while between all the other nodes, there are no or few traffic signals. In the current networking, the two-fiber unidirectional path ring of Huawei equipment is usually used in STM-1 and STM-4 systems.



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2. Two-fiber Bidirectional path protection ring

The protection switching principle of two-fiber bidirectional path protection ring is basically the same as that of unidirectional path protection ring, except that in two-fiber bidirectional path protection ring, the route of receiving signals is consistent with that of sending signals, as shown in Figure 2-7.

A

B

C

D

Figure 2-7 Two-fiber bidirectional path protection ring without protection switching

3. Two-fiber bidirectional multiplex section protection ring — Two-fiber bidirectional MS shared protection ring

The two-fiber bidirectional multiplex section protection ring (two-fiber bidirectional MS shared protection ring in ITU-T recommendations. Note: the following paragraphs will use those two terms interchangeably) requires only two fibers for each span of the ring. Each fiber carries both working channels and protection channels. On each fiber, half the channels are defined as working channels and half are defined as protection channels. The normal traffic carried on working channels in one fiber are protected by the protection channels in another fiber traveling in the opposite direction around the ring (See Figure 2-8). This permits the bidirectional transport of normal traffic. Only one set of overhead channels is used on each fiber.

Two-fiber MS shared protection rings support ring switching only. When a ring switch is invoked, the normal traffic is switched from the working channels to the protection channels in the opposite direction.



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A

B

C

D

Protection channels

Working channels

Figure 2-8 Normal traffic flow for two-fiber bidirectional MS protection ring

The AU groups that traverse the span between any two adjacent nodes are divided into working channels and protection channels. In the case of the two-fiber ring, the STM-N can be viewed as a multiplex of N AU-4s, where the AU-4s are numbered from 1 to N according to the order that they appear in the multiplex. AU-4s numbered from 1 to N/2 shall be assigned as working channels, and AU-4s numbered from (N/2) + 1 to N shall be assigned as protection channels. The normal traffic carried on working channel m is protected by protection channel (N/2) + m

For example, a STM-16 system shall assign #1--- #8VC4 as the working channels, #9---#16 as the protection channels. One fiber of #9---#16 are to protect #1---#8 of another fiber. For another example, an STM-4 can be considered a multiplex of four AU-4s numbered one to four. AU-4s number one and two would be assigned as working channels, and AU-4s number three and four would be assigned as protection channels. This assignment applies to both directions of transmission and to all spans.

The ring APS protocol shall be carried on bytes K1 and K2 in the multiplex section overhead. Functions that are required in real time and required to make a protection switch are defined in the ring APS protocol using bytes K1 and K2. Each node on the ring shall be assigned an ID that is a number from 0 to 15, allowing a maximum of 16 nodes on the ring. This node ID is called MSP node information for OptiX equipment.

For two-fiber bidirectional multiplex section protection rings, as their traffic have uniform routes and are sent bidirectionally, time slots in the ring can be shared by all nodes, so the total capacity is closely related to the traffic distribution mode and quantity of nodes on the ring. The network capacity for two-fiber bidirectional multiplex section ring is ½*M*STM-N (M is the number of nodes on the ring, STM-N is the STM level). If we count the protection channels as well, the maximum traffic load that a two-fiber bidirectional MS shared protection ring can carry is M*STM-N. Nevertheless, half of the traffic would not be protected in case of fiber failures.

Technical details:

APS requests are also initiated based on multiplex section and equipment performance criteria detected by the NE. All the working and protection channels are monitored



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regardless of the failure or degradation conditions (i.e. after a switch has been completed, all appropriate performance monitoring is continued). The NE initiates the following bridge requests automatically: Signal Failure (SF), Signal Degrade (SD), Reverse Request (RR), and Wait to Restore (WTR). The bridge requests are transmitted from NE to NE (not from NMS to NE).

The SF (signal failure) bridge request is used to protect normal traffic affected by defects, while the SD (signal degrade) bridge request is used to protect against signal degradations due to bit errors. R-LOS, R-LOF, MS-AIS, AU-LOP etc. are all examples of SF. B2-SD (the error bit ratio of B2 is above 10E-6), B2-EXC (the error bit ratio of B2 is above 10E-3) are examples of SD. The bridge requests are transmitted on both the short and long paths. Each intermediate node verifies the destination node ID of the long-path bridge request and relays the bridge request. The destination node receives the bridge request, performs the activity according to the priority level, and sends the bridged indication.

When a node determines that a switch is required, it sources the appropriate bridge request in the K-bytes in both directions, i.e. the short path and long path.

The destination node is the node that is adjacent to the source node across the failed span. When a node that is not the destination nodes receives a higher priority bridge request, it enters the appropriate pass-through state. In this way, the switching nodes can maintain direct K-byte communication on the long path. Note that in the case of a bidirectional failure such as a cable cut, the destination node would have detected the failure itself and sourced a bridge request in the opposite direction around the ring.

When the destination node receives the bridge request, it performs the bridge and bridges the channels that were entering the failed span onto the protection channels in the opposite direction. In addition, for signal fail-ring switches, the node also performs the switch to protection channels.

APS controller status and status transition

The APS controller is responsible for generating and terminating the APS information carried in the K1K2 bytes and implementing the APS algorithm. With the switching state of each NE, the APS controller status is also changed.

S SwitchingP Pass-throughI Idle WTR Wait to Restore

I

I I

I

S

S P

P WTR

WTR P

P

APS Controller Status

Figure 2-9 The status of APS Controller



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4. Two-fiber unidirectional multiplex section protection ring – two-fiber unidirectional Multiplex Section dedicated protection ring

Two-fiber unidirectional MS dedicated protection ring is composed of two fibers. Working channels and protection channels are carried over different optical fibers. The S1 is used to carry the working channels, while the P1 carries the protection channels. The low-rate tributary payload is added and dropped only in the S1 optical fiber, while the protection fiber P1 is left idle for protection purpose. Of course, fiber P1 can be used to carry extra traffic when not used for protection.

P 1

C A A C

C A A C

S 1

S 1

P 1

D

A

C

B

Xs w itc h in g

(b )

P 1

C A A C

C A A C

S 1

S 1

P 1

D

A

C

B

(a )

Figure 2-10 Two-fiber unidirectional MS dedicated switching

In case the two optical fibers between Node B and Node C are cut, the protection switch at Node B and Node C adjacent to the broken point will start the bridge function specified in the APS protocol, as shown in Figure 2-10. At Node B, the line signals (AC) previous carried over the S1 optical fiber is bridged to the P1 fiber and can still reach Node C counter clockwise via Node A and Node D. The other nodes (A and D) serve to pass through the working traffic carried over the P1 optical fiber and send them to the Node C smoothly. The bridge function guarantees the continuity of the ring even in time of failures so that the working traffic on the low-rate tributaries will not be interrupted. When the fault is finished, the switch will return to its original position.

The two-fiber unidirectional Multiplex Section dedicated protection ring is seldom used in actual applications since it has no advantages over either the two-fiber unidirectional path protection ring or two-fiber bidirectional multiplex section shared protection.

The OptiX SDH Optical Transmission System fully supports the two-fiber unidirectional MS dedicated protection ring, which is similar to the two-fiber bidirectional MS switching ring in switching condition, configuration and switching time.

5. Four-fiber bidirectional multiplex section protection ring – Four-fiber bidirectional Multiplex Section shared protection ring

Four-fiber MS shared protection rings require four fibers for each span of the ring. As illustrated in Figure 2-11, working and protection channels are carried over different fibers: two multiplex sections transmitting in opposite directions carry the working



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channels while two multiplex sections, also transmitting in opposite directions, carry the protection channels. This permits the bidirectional transport of normal traffic. The multiplex section overhead is dedicated to either working or protection channels since working and protection channels are not transported over the same fibers.

Node A Node B

Node D Node C

See exploded view

Fibre carrying working traffic (arrow indicates transmission direction)

Fibre carrying protection traffic (arrow indicates transmission direction)

Figure 2-11 Four-fiber MS shared protection ring – view of entire ring

Four-fiber MS shared protection rings support ring switching as a protection switch, as well as span switching, though not concurrently. Multiple span switches can coexist on the ring since only the protection channels along one span are used for each span switch. Certain multiple failures (those that affect only the working channels of a span such as electronic failures and cable cuts severing only the working channels) can be fully protected using span switching.

The AU groups that traverse the span between any two adjacent nodes are divided into working channels and protection channels. In the case of the four-fiber ring, each working and protection STM-N is carried on a separate fiber.

The ring APS protocol shall be carried on bytes K1 and K2 in the multiplex section overhead. In the case of the four-fiber ring, the APS protocol is only active on the fibers carrying protection channels. Functions that are required in real time and required to make a protection switch are defined in the ring APS protocol using bytes K1 and K2.

Each node on the ring shall be assigned an ID that is a number from 0 to 15, allowing a maximum of 16 nodes on the ring. This ID is called MSP node information for OptiX equipment.

For four-fiber bidirectional multiplex section protection rings, as their traffic have uniform routes and are sent bidirectionally, time slots in the ring can be shared by all nodes, so the total capacity is closely related to the traffic distribution mode and quantity of nodes on the ring. The network capacity for four-fiber bidirectional multiplex section ring is M*STM-N (M is the number of nodes on the ring; STM-N is the STM level). If we count the protection channels as well, the maximum traffic load that a four-fiber bidirectional MS shared protection ring can carry is 2*M*STM-N. Nevertheless, half of the traffic would not be protected in case of fiber failures.

Protection switching description

APS requests are also initiated based on multiplex section and equipment performance criteria detected by the NE. All the working and protection channels are monitored regardless of the failure or degradation conditions (i.e. after a switch has been completed, all appropriate performance monitoring is continued). The NE initiates the following bridge requests automatically: Signal Failure (SF), Signal Degrade (SD),



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Reverse Request (RR), and Wait to Restore (WTR). The bridge requests are transmitted from NE to NE (not from NMS to NE).

The SF (signal failure) bridge request is used to protect normal traffic affected by defects, while the SD (signal degrade) bridge request is used to protect against signal degradations due to bit errors. R-LOS, R-LOF, MS-AIS, AU-LOP etc. are all examples of SF. B2-SD (the error bit ratio of B2 is above 10E-6), B2-EXC (the error bit ratio of B2 is above 10E-3) are examples of SD. The bridge requests are transmitted on both the short and long paths. Each intermediate node verifies the destination node ID of the long-path bridge request and relays the bridge request. The destination node receives the bridge request, performs the activity according to the priority level, and sends the bridged indication.

A two-fiber ring only uses ring switches to restore traffic. A four-fiber ring has the additional option of span switching. Specifically, from the perspective of a node in a four-fiber ring, two protection channels exist: a short path over the span used in the span switch, and a long path over the long way around the ring used in a ring switch. With span switching, each span in a four-fiber ring can behave similar to a 1:1 linear protection system. Therefore, failures that only affect the working channels and not the protection channels can be restored using a span switch. Four-fiber rings should use span switching when possible so that multiple span switches can coexist. Therefore, span switching has priority over ring switching for bridge requests of the same type (e.g. Signal Fail, Signal Degrade, and Forced Switch). Lower priority span switches shall not be maintained in the event of a higher priority ring bridge request.

When a node determines that a switch is required, it sources the appropriate bridge request in the K-bytes in both directions, i.e. the short path and long path.

In the case of unidirectional failures, signaling on the short path may permit faster switch completion. Since the node across the failed span will typically see the short-path bridge request much sooner than the long-path bridge request status (or bridge request), it can initiate its own bridge requests more quickly. In the case of span bridge requests on four-fiber rings, signaling on the long path informs other nodes on the ring that a span switch exists elsewhere on the ring. This mechanism denies lower priority ring switches.

The destination node is the node that is adjacent to the source node across the failed span. When a node that is not the destination nodes receives a higher priority bridge request, it enters the appropriate pass-through state. In this way, the switching nodes can maintain direct K-byte communication on the long path. Note that in the case of a bidirectional failure such as a cable cut, the destination node would have detected the failure itself and sourced a bridge request in the opposite direction around the ring.

When the destination node receives the bridge request, it performs the bridge. If the bridge request is of a ring type, the node bridges the channels that were entering the failed span onto the protection channels in the opposite direction. In addition, for signal fail-ring switches, the node also performs the switch to protection channels.

2.2.3 Sub-network Connection Protection

As network structures are becoming more and more complicated, the sub-network connection protection (SNCP) is the only traffic protection mode that can be adapted to various network topological structures with a fast switching time. LO/HO SNC protection is another path layer protection. It is a dedicated protection scheme which can be used in different network structures: meshed networks, rings, etc.

As shown in Figure 2-12, SNCP uses the 1+1 protection mode. Traffics are simultaneously sent on both the working and protection sub-network connection. When the working sub-network connection fails, or when its performance deteriorates to a



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certain level, at the receiving end of the sub-network connection, the signal from the protection sub-network connection is selected according to the preference selection rule. Switching usually takes the unidirectional switching mode, thus it needs no APS protocol.

SNC Starting

Node

SNC Termination Node

ProtectionSNC

WorkingSNC

Sub-network 1

Sub-network 2

NE A NE B

Selector

123

123

Figure 2-12 Sub-network connection protection

SNCP is dedicated 1 + 1 in which traffic at the transmit end of a subnetwork connection is transmitted two separate ways over working and protection paths. In the case of 1 + 1 dedicated protection, the transmit end is permanently bridged, where the traffic will be transmitted on both the working and protection subnetwork connections. At the receive end of the SNC, a protection switch is effected by selecting one of the signals based on purely local information. No APS protocol is required for this protection switching scheme if it is unidirectional.

For OptiX series equipment, the alarms causing the SNC protection switching are listed in the Table 2-1:

Table 2-1 the switching criteria for OptiX equipment

��

��

1 R-LOS default 11 HP-LOM default

2 R-LOF, R-OOF default 12 HP-UNEQ default

3 MS-AIS default 13 B3-EXC default

4 B2-EXC default 14 B3-SD optional

5 B2-SD optional 15 Unplug line card default

6 AU-LOP default 16 LP-TIM optional

7 AU-AIS default 17 LP-SLM optional

8 HP-TIM optional 18 LP-UNEQ optional

9 TUAIS default 19 BIP-EXC optional

10 TULOP default 20 BIP-SD optional



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2.3 Comparison of the Network Protections

SDH is a complicated transmission network. It’s highly survivable and flexible in the application architecture. One might be easily confused about the network protections. The following paragraphs list the comparison between the network protections supported by OptiX equipment.

� Linear multiplex section protection is the simplest networking application in survivable networks. The traffic recovery of this protection is instant, which is very effective for the faults caused by optical or electrical components of nodes. But this protection mode does not work when optical cables are cut off (which is a serious fault that occurs frequently) because usually all optical fibers are in one cable (including working and protection) and would be cut off at the same time. Further improvement is to adopt a different geographic route for protection fibers. Thus when optical fibers of working path are cut off; optical fibers of protection route can still send signals safely to the opposite end. This route backup method is easy to configure, and network is simple to manage, while traffic can be recovered rapidly. But this method needs at least two sets of optical cables and equipment, and usually the backup route is long and costly. Furthermore, this method can protect only transmission link failure, it cannot protect node failure. (Survivable rings can protect against node failures). So this method is chiefly applied to point-to-point networking application.

� For two-fiber unidirectional path protection ring, all tributary signals entering into the ring will arrive at the receiving nodes in two directions. That is to say, the signals have to travel along the whole ring, so the timeslots can not be reused. As a result the network capacity of the ring is limited to STM-N. For two-fiber bidirectional path protection ring, the network is the same as a unidirectional path protection ring, but the traffic signals go along a uniform route. It’s a little bit more confusing than a two-fiber unidirectional path protection ring. In China, the customers tend to use unidirectional path protection ring, while outside China, the customers are happier to use bidirectional path protection ring.

� For multiplex section protection rings, most customers will choose bidirectional rings because of its higher network capacity than other forms. Two-fiber unidirectional MS dedicated protection ring is used in some special situations such as ring-line architecture where the transmission is only STM-1 level and it’s a must to protect the traffic signals between the ring and the line. Two-fiber unidirectional MS dedicated protection ring is seldom used for system at STM-4 level or above. Four-fiber bidirectional MS shared protection ring is very expensive to construct, it’s not so easy to maintain neither. Put it in one word, four-fiber bidirectional MS shared protection ring is used for system at STM-16, STM-64 or above.



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� The protection mechanism of SNC protection is almost the same as that of a path protection ring. They are both dedicated protection, so they have the same network capacity, which is a constant of STM-N, regardless of the number of nodes. SNC protection is more effective in complicated networks, such as tangent rings, ring inter-working where two rings are connected at two points and operate such that failure at either of these two nodes will not, because lost of any traffic, except for traffic inserted or dropped at the point of failure.

� When the traffic distribution is of a concentrated type where there is a central node and all other nodes have traffic to and from this central node, while between any other two nodes there is no or very little traffic, then a unidirectional ring is more cost-effective than a bidirectional ring. On the contrary, when the traffic distribution is of a scattered type where there isn’t a central node and between all other nodes there is much traffic to and from each other, then a bidirectional ring is more cost-effective than a unidirectional ring. For the latter case, it’s better to adopt the two-fiber or four-fiber bidirectional MS shared protection rings.

� When the transverse compatibility (the capability to interconnect different products together) in concerned, it’s better to use 1+1 linear MS protection, 1:N linear MS protection, two-fiber or four-fiber bidirectional MS shared protection ring, two-fiber unidirectional or bidirectional path protection ring, two-fiber unidirectional or bidirectional SNC protection (unidirectional protection switching, revertive or non-revertive mode). For two-fiber unidirectional MS dedicated protection and bidirectional SNC protection (in revertive or non-revertive mode), because ITU-T is yet to give the technical recommendations, so transverse compatibility can not be guaranteed until new recommendations are given. At the moment, it’s up to the discretion of the customers to use those types of protection mechanism at their own risks.

� If the traffic is time sensitive, it’s better to select two-fiber unidirectional or bidirectional path protection ring thanks to its shorter protection switching completion time (about 15ms). For multiplex section protection rings, the switching completion time is about 25ms. But if your rings are longer than 1200km, you’d better to consider the K1K2 bytes transmission delay which will be noticeable. For a ring with no extra traffic, all nodes in the idle state, and with less than 1200km of fiber, the ring switch or span switch completion time for a failure on a single span shall be less than 50ms. On rings under all other conditions, the switch completion time can exceed 50ms to allow time to remove extra traffic, or to negotiate and accommodate coexisting APS requests. The specific time interval is under study in ITU-T, so for those types of application architecture, there might be some discrepancies. These discrepancies are permissible according to ITU-T recommendations.



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Date post:	13-Apr-2015
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