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OptiX 155/622H(Metro1000) UMSDH Theory&ATM/IP Technology

Issue 2.22 September 2001 5-1

SDH Network Structure and

Network Protection Mechanism

Objectives:

Master the features and application range of SDH common topology.

Master network self-healing principle.

Master the feature, capacity and application range of different types of self-healingrings.

Understand the features of various kinds of common complex networks.

Understand the overall layer structure of SDH network.

Understand the strategy of transition from PDH to SDH.



SDH Network Structure and Network Protection MechanismOptiX 155/622H(Metro1000) UM

SDH Theory&ATM/IP Technology


(a) Link Shaped

(b) Star Shaped

(c) Tree Shaped

(d) Ring Shaped

(e) Mesh Shaped

TM

TM

TM

TM

TM TM TM

TM

TM

TM

ADM

ADM

ADM

ADM

ADM

ADM

ADM

ADM

DXC/ADM

DXC/ADM

DXC/ADM

DXC/ADM

DXC/ADM

DXC/ADM

Figure 5-1 Diagram of basic network topology

Tree shaped network

This kind of network topology can be seen as the combination of link shapedtopology and star shaped topology. The problems of security protection of thespecial node and the potential bottleneck of its processing capability also exist.

Ring shaped network

Ring shaped topology is in fact formed by connecting together the head and tail ofthe link shaped topology. In this form, none of the network element nodes are opento the outside. Currently, this is the most often used form of network topology. Themain reason for this is that it has very strong survivability. That is to say itsself-healing function is comparatively strong. Ring shaped networks are often usedin local networks (access networks and subscriber networks) and interoffice junctionnetworks.

Mesh shaped network





5-4Issue 2.22 September 2001

By connecting each network element node with every other network element node,mesh shaped network topology is formed. This kind of network topology provides a

number of transmission routes between two network element nodes, making thenetwork reliable and robust. The problems of bottleneck and invalidity will not exist.However, due to the high degree of redundancy of the system, system efficiency isdefinitely reduced, its cost is high and its structure complicated. Mesh shapednetwork is used mainly in long distance network in order to provide high reliability inthe network.

Currently, the most often-used network topologies are link shaped and ring shapednetwork topologies. Through flexible combination of these two topologies, morecomplex networks can be formed. This section mainly describes the constitution andfeatures of the link network, also the operating mechanism and features of variousmajor self-healing forms (self-healing rings) of the ring network.






5.2 Link Network and Self-healingRing

Based on direction of flow, services in the transmission network can be divided intounidirectional service and bi-directional service. Ring network is taken as anexample to show the difference between unidirectional service and bi-directionalservice, as shown in Figure 5-2.

A

B

C

D

Figure 5-2 Ring shaped network

For services inter-working between A and C, service route from A to C is assumed tobe A-B-C. If at the same time, service route from C to A is C-B-A, then, service routefrom A to C is the same as from C to A; they are called identical routes.

If service route from C to A is C-D-A, then service route from A to C is not the sameas from C to A; they are called separate routes.

We call services in identical routes as bi-directional services, while services inseparate routes as unidirectional services.

Service direction and route for common networking types are shown in Table 5-1.






Table 5-1 Table of service direction and route for common networking types

Networking Type Route Service directionLink shaped network Identical route Bi-directional

RingNetwork

Bi-directional path ring Identical route Bi-directional

Bi-directional Multiplex sectionring

Identical route Bi-directional

Unidirectional path ring Separate route Unidirectional

Unidirectional multiplexsection ring

Separate route Unidirectional

5.2.1 Link Shaped Network

Typical link shaped network is shown in Figure 5-3.

TM ADM ADM TM

A B C D

Tnbutary Service Tnbutary Service Tnbutary Service Tnbutary Service

Y X X X X X X Y

Time

Slot

Time

Slot

Time

Slot

Time

Slot

Time

Slot

Time

Slot

Time

Slot

Time

Slot

Figure 5-3 Link shaped network diagram

The feature of link shaped network is that it possesses time slot multiplexing function.That is to say in the signal of line STM-N, VC with a given sequence number can berepeatedly used in different transmission optical cable sections. As shown in Figure5-3, there are services between A-B, B-C, C-D and A-D. At this time, servicebetween A-B can occupy time slot X in A-B optical cable section (VC with sequencenumber X, e.g. the 48th VC-12 of 3VC-4). Service of B-C occupies time slot X in B-Coptical cable section (48th VC-12 of 3VC-4). Service of C-D occupies time slot X inC-D optical cable section (48th VC-12 of 3VC-4). This situation is the repeated use






of time slot. At this moment, since time slot X in the optical cable has already beenoccupied, service of A-D can only occupy another time slot, time slot Y in the optical

path, e.g. the 49th VC-12 of 3VC-4 or the 48th VC-12 of 7VC-4.

This kind of time slot repeated usage function of the link network enables thenetwork to have a comparatively large service capacity. Network service capacitydenotes the total amount of service that can be transferred through the network.Network service capacity is related to network topology, self-healing mode of thenetwork and the distribution of services between network element nodes.

The service capacity of a link network is at its minimum when the end station of thelink network has become the service host station. Service host station is so called todenote that every network element has service inter-working relation with the hoststation. There are no service inter-working relations between other network elements.

Take Figure 5-3 as an example, if A is the service host station, then, there are noservice inter-working relations between B, C and D. At this time, C, B, Dcommunicate with network element A respectively. Since at this time the maximumcapacity of the A-B optical cable section is STM-N (because data rate class of thesystem is STM-N), hence the network service capacity is STM-N.

Condition for the link network to reach its maximum service capacity is when serviceonly exists between adjacent network elements in the link network. As shown inFigure 5-3, in the network at this time, only services of A-B, B-C and C-D exist. No A-D service exists. Then, time slots can be repeatedly used. Services can occupy allthe time slots of the entire STM-N in each and every optical cable section. If the linknetwork has M network elements, then, the maximum service capacity in the

network will be (M-1)%STM-N, where M-1 is the number of optical cable sections.

Common link network uses two-fiber link it does not provide service protectionfunction (does not provide self-healing function); four-fiber link generally provides1+1 or 1:1 service protection. In a four-fiber link, two optical fibers are used for theactive receive/send channels, while the other two optical fibers are used for thestandby receive/send channels. In the last section, description of the MSP functionalblock, the 1+1, 1:1, 1: n self-healing functions of the link network have already beendescribed. It is pointed out here that in the 1: n protection mode, the maximumnumber of n can only be 14. Why? This is because it is limited by b5-b8 of the K1byte. In K1, the bits b5-b8 are 0001-1110 [1-14], indicating the code number of themaster channel that requires switching.

5.2.2 Ring Network– Self-healing Ring

1. Concept of self-healing

In the present society, all trades and professions are more and more dependent oninformation, requiring communication network to transfer information accurately andin time. Along with more and more information transferred through the network, thespeed of signal transmission becomes faster and faster. Once fault occurs in the






network, (this is unavoidable, such as optical cable damaged by construction work),extremely great damage will be done to the whole society. Therefore, survivability of

the network, or in other words, security of the network is a problem that need to befirst considered now.

The so called self-healing implies that when fault develops in the network (e.g.optical fiber broken), human intervention is not needed. The network automaticallyrestores transmission of service during the fault in an extremely short time (within50ms, as specified by ITU-T). The subscriber is practically unaware of the fact that afault has been occurred in the network. The basic principle is that the network hasthe capability of discovering a substituting transmission route and re-establishingcommunication. The substituting route may make use of standby equipment orredundancy capability of existing equipment, to satisfy the restoration of all orappointed priority services. From above, it is known that the prerequisite for a

network to have self-healing capability is to have redundant route, powerful crossconnect capability of the network element, and that the network element shouldhave a definite degree of intelligence.

Through using standby channels, self-healing only restores service that has beendisrupted. This does not involve the repair or replacement of specific faultycomponent parts or lines. Hence, repair at the point of fault can only beaccomplished by human intervention, such as broken optical cable still requiremanual re-connection.

Technical Details:

When self-healing occurs in a network, service is changed over to the standbychannel for transmission. There are two modes of switching, namely: restore modeand non-restore mode.

By restore mode, it is meant that when the master channel is in fault, service ischanged over to the standby channel. When the master channel has been repaired,service is again changed back to the master channel. Generally, after the repair ofthe master channel, it is necessary to wait for a period of time, usually a few minutesto not more than twenty minutes, so that the transmission properties of the masterchannel can become stabilized. Change back the service from the standby channelonly at this time.

By non-restore mode, it is meant that when the master channel is in fault, service ischanged over to the standby channel. When the master channel has been restored,service does not change back to the master channel. At this time, the original masterchannel becomes the standby channel. The original standby channel is now used asthe master channel. Only when the original standby channel becomes faulty, servicewill then change back to the original master channel.






2. Classification of self-healing rings

At present, ring shaped network topology is mostly used, because ring shapednetwork possesses strong self-healing function. Self-healing rings can be classifiedaccording to class of service protection, service direction on the ring, and thenumber of optical fibers between network element nodes.

Based on service direction on the ring, self-healing rings can be classified into twobig categories, namely: unidirectional ring and bi-directional ring. Based on thenumber of optical fibers between network element nodes, self-healing rings can beclassified as two- fiber ring (a pair of receive/send fibers) and four-fiber ring (twopairs of receive/send fiber). Based on class of service protection, self-healing ringscan be classified into two big categories, namely: path protection ring and multiplexsection protection ring.

In the following, the difference between path protection ring and multiplex sectionprotection ring will be described. For path protection ring, the protection of service isbased on the path. That is to say, a given VC (a given path of PDH signal) in theSTM-N signal is to be protected. Whether switching or not is determined by thetransmission quality of the signal in the given specific path. Usually, the fact ofwhether the receiving end has received the simple TU-AIS signal is used todetermine whether that path should proceed to switching. For example on a STM-16ring, if the receiving end has received TU-AIS in the 48th TU-12 of 4VC-4, then, onlythat path need to be changed over to the standby channel.

Multiplex-section switching ring is based on the multiplex section. Whether switchingor not is determined by the quality of multiplex section signal transferred on the ring.

Switching is initiated by the APS protocol carried along by K1, K2 (b1-b5) bytes.When problem occurs in the multiplex section, the entire STM-N or 1/2×STM-Nservice signals will switching to the standby channel. The conditions for multiplexsection protection switching are namely: LOF, LOS, MS-AIS and MS-EXC alarmsignals.

Technical Details:

Since there are only one K1 and one K2 in a STM-N frame, multiplex sectionswitching is to switching all master services in STM-N (four-fiber ring) or 1/2 STM-N(two-fiber ring) to the standby channel; not just switching a given path in it.

Path protection ring usually is dedicated protection. Under normal conditions,protection channel is also used for the transfer of master services (1+1 protection ofservices). Utilization ratio of the channel is not high. Multiplex section protection ringuses common protection. Normally master channel transfers master services.Standby channel transfers additional services (1:1 protection of services). Utilization






ratio of the channel is high.

3. Two-fiber unidirectional path protection ring

Two-fiber path protection ring uses two optical fibers to constitute two rings. One ofthem is the master ring - S1; the other is the standby ring - P1. Direction of serviceflow in the two rings must be opposite to each other. Protection function of the pathprotection ring is implemented by the "collective send, selective receive" function ofthe network element tributary board. That is to say the tributary board takes theservices added to the ring from the tributary and "collectively sends" them to mainring S1 and standby ring P1. Services on the two rings are exactly the same, but thedirections of flow are just opposite. Normally, the network element tributary board"selectively receives" the services dropped from the master ring to the tributary, asshown in Figure 5-4(a).

In case in the ring network, network element A has service inter-working relation withC, both network element A and C take the tributary services that are to be added tothe ring and "collectively send" them to rings S1 and P1. The services transferred onS1 and P1 are the same but the directions of flow are opposite S1 anti-clockwise,P1 clockwise. When the network is normal, both network elements A and C"selectively receive" services on master ring S1. Then, the service inter-workingmode of A and C is that services from A to C pass through network element D andtransferred to C (master ring service) via S1 fiber. Services also pass throughnetwork element B and transferred to C (standby ring service) via P1 fiber. Networkelement C tributary board "selectively receives" A->C services on the master ring S1.This completes the service transfer from network element A to network element C.Service transfer from network element C to network element A is similar to this.

CA

CA

AC

AC

S1

P1

A

C

D B

P1

S1

STM-N

Figure 5-4(a) Two-fiber unidirectional path protection ring

When the optical fibers in the BC optical cable section have been cut off at the same






time, notice that at this time the collective send function of the network elementtributary board has not changed. That is to say the services on S1 ring and P1 ring

remain the same, as shown in Figure 5-4(b).

Figure 5-4(b) Two-fiber unidirectional path protection ring

Now, we are going to see how service between network element A and networkelement C is protected. Tributary board of network element A collectively sends

service of network element A to network element C on S1 and P1 optical fibers. Theservice on S1 passes through optical fiber, passes through network element D andpasses along to network element C. Service on P1 passes through network elementB. Because the optical cable between B - C is broken, service on optical fiber P1cannot be transferred to network element C. Nevertheless, network element Cselectively receives the service on master ring S1 by default. Thus service ofnetwork element A to network element C has not been interrupted. Tributary board ofnetwork element C does not carry out protection switching.

Tributary board of network element C collectively sends service to network element A on S1 and P1 rings. The C to A service on P1 ring passes through networkelement D and passes along to network element A. For C to A service on S1 ring,because the optical fiber between B - C is broken, service cannot be transferred to

network element A. Network element A selectively receives the service on masterring S1 by default. At this time, since the C -> A service on S1 ring cannot betransferred through to A, R-LOS alarm will be generated at network element A line wside. So, all "1"- i.e. AIS will be drop inserted. Then, tributary board of networkelement A will receive alarm signal TU-AIS on the S1 ring. After the tributary board ofnetwork element A has received TU-AIS alarm on the S1 optical fiber, it immediatelyswitching to selective reception of C to A service on P1 optical fiber of the standbyring. So, C -> A service will be restored, completing the path protection for service onthe ring. At this time, tributary board of network element A is in the path protection






switching-state---switched to selective-receive standby ring mode.

After path protection switching has occurred for the network element, the tributaryboard monitors the service on master ring S1 at the same time. When TU-AIS hasnot been observed for a contiguous period of time (for Huawei equipment, it is about10 minutes), the tributary board of the network element where switching hasoccurred will change back to the selective reception of the master ring service, thusrestoring to the normal default status.

For the two-fiber unidirectional path protection switching, since service added to thering is collectively sent, selectively received, therefore protection for path service isactually 1+1 protection. The switching speed is fast (switching speed for Huaweiequipment is <=15ms). Service flow is simple, direct and clear, and it is easy toconfigure maintenance. The shortcoming is that the service capacity of the network

is not large. Network capacity of the two-fiber unidirectional protection ring is STM-N,which is constant and is not related to the number of nodes on the ring, and thedistribution of services among network elements. Why? Take an example. Considerthe case when there is a service occupying time slot X between network element Aand network element D. Because the service is a unidirectional service, then the A->D service occupies time slot X in the A-D optical cable section of the main ring(occupies time slot X in the A-B, B-C and C-D optical cable sections of the standbyring). D->A service occupies time slot X in D-C, C-B and B-A optical cable sectionsof the main ring (time slot X in the D-A optical cable section of the standby ring). Thatis to say, the service that occupies time slot X between A-D will occupy time slot X inall the optical cables on the ring (master ring, standby ring). Other services will notbe able to use that time slot (no time slot repeated usage function). Thus, whenservice between A-D is STM-N, other network elements will not be able to carry out

service inter-working hence there is no way to further increase service on the ring.This is because time slot resources of the entire STM-N have been occupied.Therefore, the maximum service capacity of a unidirectional path protection ring isSTM-N.

Two-fiber unidirectional path ring is used mostly on the ring where one of the stationpoints is a service master station - station of service concentration. For Huaweiequipment currently used for networking, two-fiber unidirectional path ring is mostly

used in 155, 622 systems.

Technical Details:

When forming path rings, special attention should be paid to the fact that thedirection of service flow on active ring S1 and standby ring P1 must be opposite toeach other, otherwise, that ring will have no protection function.






? Think it Over:

When the optical fiber is not broken, communication can actually be carried out in aunidirectional S1 ring formed by one optical fiber. Why then another optical fiber hasto be used to form a P1 ring? This is because self-healing requires redundantchannel. P1 ring is the standby for master channel.

In case only P1 optical fiber of the B-C optical cable section in Figure 5-3 is broken,What will the outcome be? All of the services between A and C on the ring networkwill not have protection switching. Why? Think about it.

4. Two-fiber bi-directional path protection ring

Service on a two-fiber bi-directional path protection ring is bi-directional (identicalroute). Protection mechanism is also "collective send, selective receive" by thetributary. Service protection is 1+1. Service capacity on the network is the same asunidirectional path protection two-fiber ring. However, the structure is more complex.Comparing with two-fiber unidirectional path ring, it has no obvious advantage. So,this type of self-healing mode is not generally used. It is shown in Figure 5-5:

Node A

Node B

Node C

Node D OptiX 2500+

OptiX 2500+

OptiX 2500+

OptiX 2500+

Figure 5-5 OptiX 2500+ system two-fiber bi-directional path protection ring






5. Two-fiber unidirectional multiplex-section ring

As mentioned before, the service unit of multiplex-section ring protection is amultiplex section level service. The control of switching should be implementedthrough APS protocol which is carried by K1, K2 bytes in the STM-N signal. Becauseswitching has to be carried out through the operation of APS protocol, the switchingspeed is not as fast as path protection ring. The multiplex section switching speed ofHuawei equipment is <= 25ms.

Next, we are going to describe the self-healing mechanism of unidirectionalmultiplex-section protection switching ring, as shown in Figure 5-6.

CA

CA

AC

AC

S1

P1

A

C

D B

P1

S1

(b)Switching

CA

CA

AC

AC

S1P1

A

CD B

P1

S1

(a)

STM-N

Figure 5-6 Two-fiber unidirectional multiplex-section protection ring

In the case that network element A on the ring has service inter-working relation withnetwork element C, S1 and P1, the two optical fibers that constituted the ring, are






called master fiber and standby fiber respectively. The services transferred on themare not 1+1 services, but 1: 1 services. Master service is transferred on the master

ring S1, while standby service is transferred on the standby ring P1. Therefore, theservice protection mode on the multiplex-section protection ring is 1: 1 protection.This is different from that on the path protection ring.

When the ring is normal, network element A sends master service to networkelement C on the master fiber S1, and sends standby service to network element Con the standby fiber. Network element C selectively receives master service sent bynetwork element A on the master fiber S1, and receives standby service (additionalservice) sent by network element A on the standby fiber P1. In Figure 5-6, only thereception of master service has been illustrated. Service inter-working relation fromnetwork element C to network element A is similar to this, as shown in Figure 5-4(a).

When all optical fibers in the C-B optical cable section have been cut off, the twonetwork elements C and B at the end points of the fault will produce loop backfunction. See Figure 5-4(b). The master service from network element A to networkelement C is first sent by network element A onto optical fiber S1. Then it is loopedback to optical fiber P1 at the fault end point station B. At this time, the additionalservice on optical fiber P1 will be cleared. This is to give way for the transfer ofmaster service from network element A to network element C. This master servicepasses through network elements A and D, and transfers to network element C onoptical fiber P1. Because network element C extracts master service only from themaster fiber S1, the master service from network element A to network element C onoptical fiber P1 will be looped back to optical fiber S1 at point C (fault end pointstation). From optical fiber S1, network element C will download the master servicefrom network element A to network element C. For the master service from C to A,

since the master service route C->D->A has not been interrupted, the transfer ofmaster service C to A is normal and without difference. Only the standby service hasbeen cleared at this time.

By using this method, service in the faulty section is restored, accomplishing theservice self-healing function.

The method of calculating the maximum service capacity of a two-fiber unidirectionalmultiplex section ring is similar to that of a two-fiber unidirectional path ring. Only thatservice on the ring has 1: 1 protection, and that normally the standby-ring P1 cantransfer additional services. Therefore, the maximum service capacity of a two-fiberunidirectional multiplex section protection ring is normally 2×STM-N (includingadditional services). When protection switching occurs, it is 1×STM-N.

Because service capacity of a two-fiber unidirectional multiplex-section protectionring differs not much from a two-fiber unidirectional path ring, and its switching speedis slower than the two-fiber unidirectional path ring, it has no significant advantage.Hence, it is not much used in networking.






(b)

CA

CA

AC

AC

S1P1

A

C

D B

P1S1

S2P2

P2S2

STM-N

(a)

CA

CA

AC

AC

S1P1

A

C

D B

P1S1

S2P2

P2S2

STM-N

Switching

Switching

s

Figure 5-7 Four-fiber bi-directional multiplex-section protection ring

Four-fiber ring is definitely formed by four optical fibers. These four optical fibers areS1, P1, S2 and P2 respectively. Among them, S1 and S2 are master fibers, used fortransferring master services. P1 and P2 are standby fibers, used for transferringstandby services. That is to say P1 and P2 fibers are used to protect master

services on S1 and S2 respectively, when the master fibers are faulty. Please notethe direction of service flow in optical fibers S1, P1, S2 and P2. Direction of serviceflow in optical fibers S1 and S2 are opposite to each other (identical route,bi-directional ring). Direction of service flow in the two pairs of optical fibers S1, P1and S2, P2 are also opposite to each other. It can be found out from Figure 5-7(a)that service flow directions are same in optical fibers S1 and P2, and are also samein S2 and P1. (This is the basis for lectures on two-fiber bi-directionalmultiplex-section ring, to be given later. In a two-fiber bi-directional multiplex- sectionprotection-ring, just because the directions of service flow in optical fibers S1 and P2,






and also in S2 and P1 are the same, it is possible to transform a four-fiber ring into atwo-fiber ring). Furthermore, it should be noted that on a four-fiber ring, the

configuration of each network element node is required to be a double ADM system.Why? Because an ADM has only east/west two line ports (a pair of receive/sendoptical fibers is called a line port), while network element node on a four-fiber ringhas two line ports each on the east and on the west directions. So, it must beconfigured with double ADM system.

When the ring network is normal, master services from network element A tonetwork element C go through optical fiber S1 and pass network element B to reachnetwork element C. Services from network element C to network element A passthrough optical fiber S2 and network element B to reach network element A(bi-directional service). Additional services of network element A and networkelement C are transferred through optical fibers P1 and P2 respectively. Through

receiving services in the master fiber by the network element A and network elementC, inter-working of master services between the two network elements can berealized. Through receiving services on the standby fiber, inter-working of standbyservices between the two network elements can be realized. See Figure 5-7(a).

When optical fibers in the optical cable section between B - C have been cut off,optical fibers S1, P1, S2 and P2 of the network elements B and C at the two ends ofthe fault have loop back functions. See Figure 5-7(b) (network element loop back atthe fault end point). At this time, master service from network element A to networkelement C is transferred to network element B along optical fiber S1. Loop backfunction is executed at this network element B. Master service from network element A to network element C on optical fiber S1 is looped back for transmission throughoptical fiber P1. Additional services on optical fiber P1 will be interrupted. Meanwhile

master service on P1 passes through network element A and network element D(the other network elements execute pass-through function). Then it is transferred tonetwork element C. At network element C, service on optical fiber P1 is looped backto optical fiber S1 (network element at the fault end point executes loop backfunction). Network element C by receiving service on optical fiber S1 receives themaster service from network element A to network element C.

For service from network element C to network element A, network element C firstloops its master service onto optical fiber P2. Additional service on optical fiber P2will be interrupted. Then, along optical fiber P2, the master service from networkelement C passes through network element D and network element A to reachnetwork element B. Network element B executes loop back function. Master servicefrom network element C to network element A on optical fiber P2 will be looped back

to optical fiber S2, which will then transfer the service back to network element Athrough optical fiber S2. Network element A drops the service on optical fiber S2.Through this kind of loop back and pass through method, multiplex sectionprotection of services is accomplished, enabling the network to self-heal.

The service capacity of four-fiber bi-directional multiplex-section protection ring hastwo extreme modes. For one of the two modes, there is a service concentrationstation on the ring. Each network element communicates services with this station.Direct services between network elements do not exist. In this situation, the






minimum service capacity on the ring is 2 % STM-N (master service) and 4% STM-N (including additional services). This is because both the east and west sides

of the service concentration station can only pass through STM-N (master) or 2% STM-N (including additional services). Why? This is because data rate level of theoptical cable section is only STM-N. Another situation is that services only existbetween adjacent network elements on the ring network. No cross network elementservices exist. Then, each optical cable section is dedicated to the inter-working ofservices between adjacent network elements. For example, A-D optical cablesection transfers only the bi-directional services between A and D. D-C optical cablesection transfers only the bi-directional services between D and C, etc. Servicesbetween adjacent network elements do no occupy time slot resources of otheroptical cable sections. In this way, each optical cable section can transfer a

maximum of STM-N (master) or 2% STM-N (including standby) services (time slotscan be repeatedly used). The number of optical cable sections on the ring is equal to

the number of network element nodes on the ring. Therefore, service capacity of thenetwork will reach its maximum in this case: N% STM-N or 2N% STM-N.

Protection switching speed of multiplex-section ring is slower than path ring.Switching is controlled by APS protocol in K1, K2 bytes. More single boards areinvolved in equipment switching, faults can easily occur. Despite all these, thebiggest advantage of bi-directional multiplex-section ring is that its service capacityon the ring is large. The more the distribution of service is scattered, the more thenumber of network element nodes in the ring, the more voluminous is its capacity.Channel utilization ratio is much higher than path ring. Hence, bi-directionalmultiplex-section ring is widely used.

Bi-directional multiplex-section ring is used mainly in networks where the distribution

of services is comparatively scattered. Since four-fiber ring requires that the systemhave a comparatively high degree of redundancy 4 fiber, double ADM --- the costis comparatively high. Hence it is not much used. How to solve this problem? Pleaselook at the two-fiber bi-directional multiplex-section protection ring --- sharedtwo-fiber multiplex-section protection ring.

Technical Details:

The number of network element nodes on the multiplex section protection ring (notincluding REG, because REG does not participate in the function of multiplex sectionprotection) is not unlimited. It is determined by K1, K2 bytes. The maximum number

of nodes on the ring is 16.

7. Two-fiber bi-directional mult iplex-section protection r ing - Shared two-fibermultiplex-section protection ring






Due to the fact that the cost of four-fiber bi-directional multiplex-section ring iscomparatively high, a new revised version of it has come into being: the two-fiber

bi-directional multiplex-section protection ring. Their protection mechanism is similar.The only difference is that it uses the two-fiber mode. Only a single ADM is used forthe network element node. Hence it is widely used.

It can be seen from Figure 5-7(a) that the directions of service flow in optical fibersS1 and P2 are the same as in S2 and P1. Thus, we can use time division techniqueto combine the two pairs of optical fibers into two optical fibers - S1/P2, S2/P1. Then,the first half time slot of the optical fiber (e.g. 1# - 8#STM-1 of STM-16 system) isused to transfer master services. The other half time slot (e.g. 9# - 16#STM-1 ofSTM-16 system) is used to transfer additional services. That is to say the protectiontime slot of one optical fiber is used for the protection of master services in the otheroptical fiber. For example, time slot P2 on S1/P2 optical fiber is used to protect the

S2 service on S2/P1 optical fiber. Why? Because on a four-fiber ring, S2 and P2themselves are a pair of master/standby optical fibers. Therefore, there are nodedicated master/standby optical fibers for the two-fiber bi-directionalmultiplex-section protection ring. The first half time slot of each optical fiber is themaster channel, while the latter half time slot is the standby channel. Service flow inthe two optical fibers is in opposite directions. The protection mechanism of atwo-fiber bi-directional multiplex-section protection ring is shown in Figure 5-8.

Under normal network conditions, master service from network element A to networkelement C is placed in time slot S1 of the S1/P2 optical fiber (for STM-16 system,master services can only be placed in the first 8 time slots of STM-N, in 1# -8#STM-1 [VC-4]). Standby service is placed in time slot P2 (for STM-16 system, itcan only be placed in 9# - 16#STM-1 [VC-4]). Along optical fiber S1/P2 the service

passes through network element B and passes to network element C. Networkelement C extracts the master and standby services from timeslots S1 and P2 onthe S1/P2 optical fiber respectively. Master service from network element C tonetwork element A is placed in time slot S2 of optical fiber S2/P1. Additional serviceis placed in time slot P1 of optical fiber S2/P1. The service passes through networkelement B and passes to network element A. Network element A extracts thecorresponding service from optical fiber S2/P1. See Figure 5-8(a).






CA

CA

AC

AC

S1/P2

S2/P1

A

C

D B

S2/P1

S1/P2

Figure 5-8(a) Two-fiber bi-directional multiplex-section protection ring

When the optical cable section between B - C in the ring network has been cut off,master service from network element A to network element C will pass along opticalfiber S1/P2 before being transferred to network element B. It is looped back atnetwork element B (loop back at fault end point). Loop back is to have all the servicein time slot S1 of optical fiber S1/P2 looped back to time slot P1 of optical fiberS2/P1 (e.g. for STM-16 system, 1# - 8#STM-1 [VC-4] in its entirety is looped back to9# - 16#STM-1 [VC-4]). At this time, additional services in time slot P1 of optical fiberS2/P1 will be interrupted. The service then passes along optical fiber S2/P1 andpasses through network element A and network element D to reach network element

C. Loop back function is executed at network element C (fault end point station).Thus, the master service from network element A to network element C, carried bytime slot P1 of optical fiber S2/P1 will be looped back to time slot S1 of optical cableS1/P2. Network element C then extracts service from that time slot, accomplishingthe reception of master service from network element A to network element C. SeeFigure 5-8(b).






CA AC

CA AC

S1/P2

S2/P1

A

C

D B

S2/P1

S1/P2

Switching

STM-N

Figure 5-8(b) Two-fiber bi-directional multiplex-section protection ring

As to service from network element C to network element A, master service S2 fromnetwork element C to network element A is looped back to time slot P2 of opticalfiber S1/P2 by network element C. At this time, additional services in time slot P2 willbe interrupted. The service then passes along optical fiber S1/P2, and passesthrough network element D and network element A. Then, it is transferred to networkelement B. Loop back function is executed at network element B. --- Take theservice in time slot P2 of optical fiber S1/P2, loop it back to time slot S2 of opticalfiber S2/P1. Via optical fiber S2/P1, the service is transferred to network element A

and dropped.

By using the above-mentioned method, self-healing of services is accomplishedwhen the ring network is faulty.

The service capacity of two-fiber bi-directional multiplex-section protection ring isone half of that of four-fiber bi-directional multiplex-section protection ring, i.e.M/2(STM-N) or M×STM-N (including additional services), where M is the number ofnodes.

Two-fiber bi-directional multiplex-section protection ring is often used for networking,mainly for 622 and 2500 systems. It is also suitable for use in networks where

services are scattered.






? Think it Over:

Think for a moment why two-fiber bi-directional multiplex-section protection ring isnot used in a 155 system.

This is because the basic service unit of multiplex-section protection is ofmultiplex-section level, while STM-1 is the smallest unit in a multiplex-section, whichcannot be divided further. Two-fiber bi-directional multiplex-section protection ringrequires that the optical fiber be divided into half by using time slot division technique.Thus, each time slot in the optical fiber should be able to transfer 1/2STM-1 signal,which is impossible to attain in a two-fiber bi-directional multiplex-section protectionring of a 155 system.

In current networking, there are only two kinds of self-healing rings in common use,namely: two-fiber unidirectional path protection ring and two-fiber bi-directionalmultiplex-section protection ring. Comparison of them follows.

8. Comparison of two kinds of self healing rings

Service capacity (only master service is considered)

The maximum service capacity of a unidirectional path protection ring is STM-N.

Service capacity of a two-fiber bi-directional multiplex-section protection ring is M/2% STM-N (M is the number of nodes on the ring).

Complexity

So far as the complexity of control protocol or the complexity of operation isconcerned, two-fiber unidirectional path protection ring is the simplest among allkinds of switching rings. Due to the fact that it does not involve the processingprocedure of APS protocol, service switching time is also the shortest. The controllogic of two-fiber bi-directional multiplex-section protection ring is the most complexamong all kinds of switching rings.

Compatibility

Two-fiber unidirectional path protection ring uses the path AIS signal, which hasbeen completely specified, to determine whether switching is required. This isentirely compatible with current SDH standards. Therefore, it is easy to satisfyproduct compatibility requirements of many manufacturers.

Two-fiber bi-directional multiplex-section protection ring uses APS protocol to






determine switching. APS protocol has not yet been standardized. Thereforemultiplex-section switching rings cannot satisfy product compatibility requirements of

many manufacturers at present.






5.3 Topology and Feature ofComplex Networks

Through the combination of links and rings, some comparatively complex networkscan be formed. In the following, the commonly used network topology in networkingwill be described. The 2500 system is taken as example to enhance the focus.

1. T type network

T type network is actually a kind of tree shaped network, as shown in Figure 5-9.

TM

TM

TM ADM ADM

ADM

ADM

STM-16

STM-4

A

Figure 5-9 Diagram of T shaped network

We set up STM-16 system as the trunk line and STM-4 system as the tributary line.The purpose of the T shaped network is to take STM-4 services of the tributary,make them go up/down to/from STM-16 system trunk line by means of networkelement A. In this case, the tributary line is connected to the tributary of networkelement A. Tributary line services, as the low speed tributary signal of networkelement A, add/drop through network element A.

2. Ring-plus-link

Network structure is shown in Figure 5-10.

Ring-plus-link is formed by two basic topological forms, namely: ring network and

link network. The link is attached at network element A. STM-4 service of the link isthe low speed tributary service of network element A. It goes up/down the ring bymeans of the add/drop function of network element A. There is no protection forSTM-4 service on the link. Once it goes up the ring, it will enjoy the protectionfunction of the ring. For example: network element C inter-works service withnetwork element D. When optical cable section A- B happens to be broken, servicetransmission on the link will be interrupted. If it is the A- C optical cable section thathappens to be broken, by means of the protection function of the ring, servicebetween network element C and network element D will not be interrupted.






TM ADM

ADM

ADM ADM

ADM

STM-4

STM-16

C

A B D

Figure 5-10 Diagram of ring-plus-link topology

3. Tributary crossover of ring shaped sub-network.


The two STM-16 rings are partly connected together through the tributary of the twonetwork elements A and B. Any two network elements in the two rings can inter-worktheir services through the tributary between A and B. Furthermore, there are manyselectable routes, and system redundancy is high. All the services inter-workingbetween the two rings have to pass through the low-speed tributary transmissionpath of the two network elements A and B. Hence the problem of security guaranteefor the low-speed tributary exists.

ADM

ADM ADM

ADM

ADM

ADM

ADM

ADM

STM-1/4STM-16 STM-16

AB

Figure 5-11 Topology of tributary crossover network connecting ring shaped sub-network

4. Tangent rings


In the diagram, the three rings are tangent to each other at common node networkelement A. Network element A can be a DXC, or the equivalent ADM can be used(both Ring II and Ring III are low-speed tributary of network element A). This kind ofnetworking mode enables arbitrary service inter-working between rings. It has more






service relief capability than passing through a tributary cross over ring network. Ithas more selectable routes for services, and system redundancy is higher. However,

the problem of security protection for the important node (network element A) exists.

ADM

ADM ADM

ADM

ADM

ADM

ADM

ADM 2500

STM-1

STM-1

STM-1

ADM

STM-16

STM-16

STM-16

A

II

III

155

622

STM-4

STM-4

STM-4

DXC/ADM

I

Figure 5-12 Diagram of tangent ring topology

5. Intersecting rings

Tangent rings can be extended to form intersecting rings, as shown in Figure 5-13.This will provide standby important node and more selectable routes. Systemredundancy is increased.






ADM

ADM

ADM

ADM

ADM

ADM

STM-16

STM-16

STM-16

622

STM-4

STM-4

STM-4

DXC/ADM

DXC/ADM

Figure 5-13 Diagram of intersecting ring topology

6. Hub network


Network element A, as the hub point, can access each STM-1 or STM-4 link or ringon the tributary side. By means of the cross connect function of network element A,add/drop of tributary services to the backbone line and inter-working of servicesbetween tributaries can be provided. Inter-working of services between tributaries

passes through the add/drop of network element A. Laying of direct routes andequipment between tributaries can thus be avoided. Also, there is no need to occupyresources on the backbone network.






ADM

ADM ADM

ADM

ADM

ADM

ADM ADM

ADM

ADM

STM-16STM-16

STM-1

STM-1

STM-16

STM-4

STM-4

DXC/ADM

A

STM-4/1

aa

a

Figure 5-14 Diagram of hub network topology.






5.4 Overall Layer Structure of SDHNetwork

Compared with PDH, SDH has immense advantage. However this kind ofadvantage can only be brought into full play when SDH network is formed.

In the concept of conventional networking, the increase in utilization ratio oftransmission equipment has been given first priority. For the increase of line fillcoefficient, many direct paths have been established at each node, making thenetwork structure very complicated. As for the development of moderncommunication, the most important task is to simplify the network structure, and toprovide powerful operation, maintenance and management (OAM) functions. This

will lower the cost of transmission, and support the development of new services.

The Chinese SDH network structure is divided into four layers, as shown in Figure 5-15.

The top layer is the long distance class one trunk network. This includes majorprovincial capital cities and transit node cities of comparatively large service volume.In these cities, DXC4/4 are installed. Between these cities, high-speed optical linksSTM-4/STM-16 are formed, constituting a large capacity, highly reliable meshshaped national backbone network structure. This is supplemented by a lesseramount of linear networks.

Since DXC4/4 also have 140Mbit/s interface for PDH system, hence existing140Mbit/s and 565Mbit/s systems of PDH can also be brought into the long distanceclass one trunk network under the unified management of DXC4/4.

The second layer is the class two trunk network. DXC4/4 or DXC4/1 are installed inmajor transit nodes. In between these nodes are STM-1/STM/4, formingintra-provincial mesh shaped or ring shaped backbone network structure. This issupplemented by a lesser amount of linear network structures. Since DXC4/1 have2Mbit/s, 34Mbit/s or 140Mbit/s interfaces, hence existing PDH systems can also bebrought into the class two trunk network under unified management. It also has thecapability of flexible circuit dispatching.

The third layer is the junction network (i.e. the portion between long distance endoffice and local office and between local telephone offices). This network can be

divided into a number of rings according to districts. Self-healing rings can be formedusing ADM with speeds of STM-1/STM-4. It can also be two node rings using routebackup mode. These rings have very high survivability, and it possesses servicevolume relief function. In ring shaped networks, multiplex-section switching ringmode is mainly employed. Whether it's four-fiber or two-fiber is determined bycomparing the service capacity and economy. Rings are linked up by DXC4/1,accomplishing the relief of service volume and other management functions. At thesame time, it can also be used as the gateway or interface between long distancenetwork and junction network and between junction network and subscriber network.






Finally it can be used as the gateway between PDH and SDH.

The lowest layer is the subscriber access network. Because it is located at theboundary of the network, service capacity requirement is low. Furthermore, a largepart of the service capacity converges to one node (end office). Therefore pathswitching ring and star shaped network are all very suitable for this applicationenvironment. Besides ADM, the equipment required also includes optical subscriberloop carrier system (OLC). The speed of it is STM-1/STM-4. The interfaces can beSTM-1 optical/electrical interface, 2Mbit/s, 34Mbit/s or 140Mbit/s interface for PDHsystems, ordinary telephone subscriber interface, PBX interface, 2B+D or 30B+Dinterface and MAN interface etc.

Subscriber access network is the largest and most complex part of SDH network. Itoccupies more than 50% of the total communication network investment. The use of

optical fiber in the subscriber network is a gradual process. When we talk about fiberto the curb (FTTC), fiber to the building (FTTB), and fiber to the home (FTTH), theyare the different stages of this process. At present, when China is popularizingoptical subscriber access network, consideration must be given to the deployment ofa unified SDH/CATV network. Not only will it offer telecommunication services it willalso provide CATV services. This is more suitable for the situation in China.






OLC OLC

DXC4/4

DXC4/4

DXC4/4

DXC4/4

STM-4 or

STM-16

Class two trunk

network

STM-1or

STM-4

DXC DXC DXC

DXC

DXC DXC

4/1 4/1 4/1

4/1

4/4 4/4

ADM

ADM ADM

ADM ADMADM ADM

ADM

ADM

STM-1

Or STM-4

OLC OLC OLCOLC OLC OLC

TM

Ring

Shaped

Class one trunk

network

Junction

network

Subscriber

network

Ring

Shaped

Star

Shaped

Figure 5-15 SDH network structure.






5.5 The Strategy of Transition FromPDH to SDH

Transition from PDH to SDH is inevitable. Long term coexistence of PDH and SDHis also an objective reality. The strategy of introducing SDH to the telecommunicationnetwork in different countries can usually be classified as follows: "From top down".First introduce SDH into national or regional large capacity core networks. This willincrease the capacity of the long distance network rapidly. "From bottom up". Firstdeploy synchronization-island in a part of the core network or the access network.This can bring into full play early the merit of SDH synchronization, so as to increaseone's own competitiveness. "Overlay network". For the support of dedicated services,deploy end to end SDH overlay network in national and regional networks, such asin certain new development zones. This kind of strategy can provide high qualitycommercial telecommunication-services. Due to the fact that the long distancenetworks in China still need extension, and the public network infrastructure needsto be perfected, hence the above mentioned three kinds of strategies for introducingSDH can all be used in China.

? Think it Over:

Think for a moment, what you have learnt in this chapter?

1. Basic forms of network topology and their features.

2. Protection mechanism of self-healing rings and their range of application.

3. Various forms of the more complex network topology.

4. The four-layer structure of Chinese SDH network.

5. Approximate strategy for the transition from PDH to SDH.

No. 2 of above is very important, and must be completely mastered. Have youalready mastered it?






Summary

This chapter mainly describes the basic topology of SDH networks, self-healing ringmechanism, and features of the more complex networks. Master mainly theoperating mechanism, application range and service capacity of unidirectional pathprotection ring and bi-directional two-fiber multiplex-section protection ring.






Exercises

Triggering condition for unidirectional path protection ring is ________ alarm.

Triggering condition for two-fiber bi-directional multiplex-section protection ring is ________, ________, ________, ________ alarms.

The service capacity of a 4-network element, two-fiber bi-directionalmultiplex-section protection ring (2.5G system) is ________ 2M channels.

01 05 sdh network structure

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