Ring Architecture and Switching

SDH

1
 Protection Switch Architecture
(Basics)
• Path protection switch (PPS) / Line protection switch (LPS)

• 1+1 protection switch / 1:N (1:1) protection switch

• Stand-by Line Access (SLA)

• Unidirectional switch / Bidirectional switch

• Unidirectional ring / Biderectional ring

• Revertive switch / Non-revertive switch

• Manual switch operation

• Wait to restore (WTR) time

2
Path protection switch Line protection switch
P a th (V C -n ) L in e (S T M -n )

Working P a th S W

P a th S W P a th S W P a th S W

Protection

Path Protection Switch (PPS)

Working

Protection

equivalent

L in e S W (e q uiva le n t)

Line Protection Sw itch (LP S)

3
1. In above drawing, a thin line shows a path (VC-n or channel in STM-N) and a pipe
means a line (STM-N signal or multiplex section)

2. Path Protection Switch

2.1 Protection switches are set up at the ends of each path. Against a line
failure, protection switching take place at the ends of suffered paths. Against a
failure in a single path, switching is applied only to the path. Usually path
performance monitoring initiates the switching. A line failure results in a path
failure and the path failure detection causes switching, not the line failure
detection.
3. Line Protection Switch
3.1 Switches are set individually to all channels of the STM-N at both
ends of a section. Against a line failure, all channels are switched to
corresponding channels of the protection line. Although the switching is at a
channel (path) level, it is equivalent to that the line (fiber) is switched. The
switching is triggered by the section failure detection. Failure detection in a
path does not cause the line switching.

4
1+1, 1:1 and 1:N Protection Switch
Working
1+1 TX RX
Protection Protection
Switch TX RX

Working
1:1 TX RX
Protection Protection
Switch TX RX

Working 1
TX RX
Working 2
TX RX
1:N
Protection
Switch
Working N
TX RX
Protection
TX RX
Note : only one direction is shown.
5
1. 1+1 Protection Switch
1.1 At the transmitting side the normal signal (traffic) is permanently branched
onto both working and protection lines. Against failure on the working
switching takes place at the receiving side only. This switching scheme does
not require switching protocol.
2. 1:1 Protection Switch
2.1 Switches are installed at both transmitting and receiving side. This structure
requires switching protocol. Under normal condition, the protection line carries
an idle signal or a low priority extra traffic which will be removed when the
working fails.
3. N:1 Protection Switch
3.1 In this scheme one protection line is provided for N working lines. Switching
protocol is necessary and the protection line can carry an extra traffic while it
is not used by one of normal traffics.
4. Usually 1+1 uses nonrevertive switch and 1:1 and N:1 uses revertive switch, but of
course other way round is possible.

6
 Stand-by Line Access (SLA)

Normal Traffic Working

TX RX

Protection
TX RX

Extra Traffic

Normal Traffic Working

TX RX

Protection
TX RX
Extra Traffic service
Note : only one direction is shown.
is stopped.
Extra Traffic
7
1. In 1:1 or N:1 protection, when the protection line is not occupied by a normal traffic, it
can carry a low priority extra traffic. When failure occurs in working line the extra traffic
is removed form the protection line and the line is used by the affected normal traffic.
2. This arrangement is called Stand-by Line Access (SLA).
3. Bidirectional switch is required.
4. Usually a revertive switch is applied for SLA, but for 1:1 non-revertive type SLA is also
possible by using slightly complicated configuration.

1. Unidirectional Protection Switch

When the failure is only on one direction, only the affected direction traffic is transferred to the
protection line. For 1+1 system, this is a simple method because no protocol is necessary and
switching can be determined only at a receiving side. Disadvantage is that under protection
status different directions use different hardware and line and they might have different
propagation time.
2. Bidirectional Protection Switch
Even if a failure is only on one direction, both affected and unaffected directions are switched to
the protection line. This switching scheme requires a protocol, because the unaffected direction
receiver has no way to detect the failure until it is informed by the remote side. Both directions
of transmission maintain equal delays and this may be important where there is a significant
length imbalance for the working and protection.

8
Unidirectional Switch and Bidirectional Switch
Unidirectional Switch
TX RX
Working
RX TX

TX RX
Protection
RX TX

Bidirectional Switch
TX RX
Working
RX TX

TX RX
Protection
RX TX

9
Unidirectional Ring and Bidirectional Ring

Unidirectional Ring Bidirectional Ring

The drawing shows the traffic routing under normal (no failure) condition.

10
1. Unidirectional Ring
1.1 Normal routing of the traffic is that one direction of two-way connection
uses the right (or left) half of the CW (or CCW) line and the other direction uses the
left (or right) half of the same CW (or CCW) line. Thus both directions travel
around the ring in the same direction and uses capacity (a channel in STM-N) along
the entire circumference of the ring.
1.2 Total number of connections in the ring cannot exceed the capacity of each
section.
1.3 Unused line (above case CCW) is for protection capacity.
2. Bidirectional Ring
2.1 Normal routing of the traffic is that both directions of two-way connection
routed using the same section(s) and node(s). Corresponding channel of the STM-N
in unused half (above case left half) can be assigned for other connections which
has no overlap section each other.
2.2 Total number of connections in the ring can exceed the capacity of each
section.
2.3 Protection capacity is provided by dividing the section capacity half (for 2
fiber ring) or using additional fiber pair (4 fiber ring).

11
Revertive Switch and Non-reverteve
Switch
Revertive Switch and Non-reverteve Switch

Reverteve Switch
① Working Line normal failure normal

② Protection Line normal failure

Non-reverteve Switch
① Working Line normal failure normal

② Protection Line normal failure

Traffic flow

12
1. Rivertive Switch
1.1 When the failure in the working system is repaired, the traffic that has
been carried by the protection system is switched back to the working system.
When the SLA is employed, usually this switching scheme is used and also in N:1
system to vacate the protection system for the next failure.
2. Non-revertive Switch
2.1 After the recovery of the failed working system, the traffic is not
switched back to the working system. When the protection system fails later,
traffic is transferred to the working. Therefore it is not appropriate to name
“working” and “protection” and often they are called “0” and “1” system.
2.2 Whenever switching is applied and if a complicated hitless switching is
not employed it is inevitable to cause data hit, short period error. To avoid the
unnecessary errors this scheme is important, especially in 1+1 protection system.
3. Non-revertive 1:1
3.1 SLA is also possible by a different design method.

13
Manual Switch Operation
1. MSW Manual Switch

2. (SF) Signal Failure

3. FSW Forced Switch

4. LKOW Lock Out of Working

4. LKOP Lock Out of Protection

Note: Larger number has higher priority.

14
1. For the protection switching system, manual operations, which are listed above, are possible. They
are used for maintenance purposes.
2. The traffic on a selected system is transferred to the protection system by the MSW. When a
failure is detected on another system (SF, signal failure), the MSW is released and the failed
system takes over the protection system.
3. The FSW does the same switching as the MSW, but against other system failure it does not
release the protection system.
4. When the LKOW is applied to a working system, it is not switched to the protection system even
if it fails. Under this status neither MSW nor FSW can be applied.
5. The LKOP is practiced to the protection system, any switching to it is refused, either automatic
(SF) or manual (MSW and FSW.)
6. Priority order among operations is above order. The larger the number is, the higher the priority.
No priority difference between LKOP and LKOW.

1. In the revertive switching mode, to prevent frequent back-and-forth switching between the
working and protection because of intermittent fault, the failed working must be error-free. To
ensure this, the wait-to-restore (WTR) time is set. It is on the order of 5-12 minutes with 1-second
increment.
2. To prevent chattering of the protection switch due to an intermittent failure, the working channel
must be fault free for a fixed of time before the switch taken to transfer to protection.
3. During WTR period if a fault is detected on the waiting working system, the WTR time is reset.

15
 Hold-Off & Wait to Restore (WTR) times

① Working Line normal failure normal

② Protection Line normal

HOLD OFF time WTR time

Traffic flow

16
 Ring Architecture

East West

Node C

West East

STM-N
Node B Node A

East West

17
1. Ring Configuration provides reliability to the network’s traffic.
2. Traffic is protected because it can flow between any two points in either of two directions in the ring: Clockwise or
Counterclockwise.
3. Rings can be Two Fiber (2F) or Four Fiber (4F). The ring shown above is a 2F Ring (one pair of fibers between adjacent nodes of
the ring). Additional reliability can be obtained by the deployment of 4F Rings.
4. In a 4F-Ring, nodes are interconnected by two pairs of fibers. The main pair is known as the Working Fibers (W), and the second
pair is known as the Protection Fibers (P).
5. There are two basic protection schemes for the Ring Architecture, which will be explained throughout this lesson:

1. Ring systems are classified into five types based on employed switching method combination (unidirectional or bidirectional
switch and path or line switch), routing of two-way traffic (unidirectional or bidirectional ring) and fiber number (2 or 4 fibers).
2. Theoretically any combination between switching and routing direction is possible. But in the ring system, above uni-uni and bi-
bi combination is used.
3. SNCP-ring (Unidirectional)
3.1 Also called 2-fiber Unidirectional Path protection Switch Ring (2F-UPSR)
3.2 No switching protocol is required. This is a simple and fast switching scheme.
4. MS Dedicated Protection Ring
4.1 2-fiber Unidirectional Line protection Switch Ring (2F-ULSR)
4.2 Protocol by K1 and K2 in MSOH is required and it is not standardized yet by ITU-T.
5. SNCP-ring (Bidirectional)
5.1 2-fiber Bidirectional Path protection Switch Ring (2F-BPSR)
5.2 Ring must be controlled at a path level and protocol by K3 or K4 in POH is required. It is not standardized yet by ITU-T.
6. 4F MS-SP ring
6.1 4-fiber Bidirectional Line protection Switch Ring (4F-BLSR)
6.2 Protocol by K1 and K2 in MSOH is required.
7. 2F MS-SP ring
7.1 2-fiber Bidirectional Line protection Switch Ring (2F-BLSR)
7.2 Protocol by K1 and K2 in MSOH is required.
8. There are two different algorithm in 4/2F MS-SP ring, the terrestrial application and the transoceanic application.
18
 Types of Ring System

PPS 2-fiber Subnetwork Connection Protection Ring (SNCP-ring)

(2F-UPSR)
Unidirectional
Ring / Switch
LPS 2-fiber MS Dedicated Protection Ring
(2F-ULSR) (for further study, G.841)
Ring System
PPS 2-fiber Subnetwork Connection Protection Ring (SNCP-ring)
(2F-BPSR) (for further study, G.841)
Bidirectional
Ring / Switch 4-fiber 4F MS Shared Protection Ring (MS-SP ring)
(4F-BLSR)
LPS
2-fiber 2F MS Shared Protection Ring (MS-SP ring)
(2F-BLSR)

LPS:Line Protection Switch (MS protection)

PPS:Path Protection Switch (VC trail protection)
MS:Multiplex Section

19
Subnetwork Connection Protection (SNCP)
Ring
(2 Fiber Uni-directional Path Switch Ring - UPSR)

20
SNCP(Unidirectional Ring) / 2F-UPSR (1)

21
1.SNCP-unidirectional ring (2F-UPSR) uses
2 fiber unidirectional ring architecture and
Unidirectional 1+1path switch scheme.
2.The drawing shows a 2-way connection (A-D, as an example) which uses a channel (time slot) in the
STM-N signal. The solid line indicates the channel in the clockwise (CW) STM-N (fiber) and the
broken line the corresponding channel in the counter clockwise (CCW) STM-N. Other connections
between any other nodes are made by using different channels of the STM-N.
3.At the transmitting node (A) the traffic is branched onto both CW and CCW STM-N and at the
receiving node (D) one of them carried by CW and CCW is selected by a path protection switch (PPS).
Opposite direction (D to A) is arranged in the same way using the same channel on the unused
semicircle.
4.Under the normal status, PPSs at A and D select CW signals, making a unidirectional ring. The signals
on CCW are stand-by signals.
5. It is possible to set the one of the PPS to CCW. In this case both direction traffic travel on the same
route. This makes transmission delay of both directions equal. This is a bidirectional-like setting but the
system is still SNCP-unidirectional ring .
6.An important point of the system is that one path occupies one channel of the STM-N (both CW and
CCW) along the entire circumference of the ring. As a result, total number of paths in a ring cannot
exceeds STM-N’s capacity.
7.There is no limitation to the node number in an SNCP-ring from the point of view of the ring control, b
ut the STM-N capacity determines the limit.
8.Operators can selectively provision paths to be protected or unprotected. Unprotected path will only use
capacity in the selected route (semicircle). The other route will provide additional unprotected path(s).
22
SNCP(Unidirectional Ring) / 2F-UPSR (2)

23
1. Under the (line and path) failure status
1.1 When failure or degradation is detected on the currently selected signal, the
switch position of the PPS is changed (above case, CW to CCW at D node.)
Unaffected direction of the path does not activate the PPS (above case, at A.)
1.2 The activation of the PPS does not require protocol between nodes. Failure
detection and PPS activation are done by the receiving node alone. This makes
system control simple and as a result, the switching time short.
1.3 When the failure is at the line (STM-N) level, all (or many) paths in the ring are
suffered and PPS switching occur independently at every concerned nodes.
2. For the bidirectional-like setting, when the failure is at the line level, both directions are
switched at the same time. But they are considered as independent unidirectional
switching.
2.1 This cause data hits to both directions. But for normal unidirectional ring
arrangement, the data hits are only on one direction.

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Subnetwork Connection Protection (SNCP)

Aggregate 1

Aggregate 2
TCP Any Physical Structure TCP Aggregate - Aggregate
PPS PPS PPS
(Mesh,Ring or Mixed)
Case 1 Tributary

Path

Aggregate 1

Aggregate 2
Aggregate - Tributary
PPS

Tributary
TCP Any Physical Structure CP TCP
PPS PPS
(Mesh,Ring or Mixed)

Aggregate
Case 2
SNC-P SNC
Trubutary - Tributary
Path PPS

Tributary 1 Tributary 2

TCP CP Any Physical Structure CP TCP TCP : Termination Connection Point

PPS PPS CP : Connection Point
(Mesh,Ring or Mixed)
Case 3
SNC SNC-P SNC
Path

25
1. General SNCP has wider definition than previously explained SNCP-ring. The SNCP-
ring is a one type of the SNCP.
2. The SNCP can have any type of network physical structure (i.e. meshed, ring or
mixed) between PPSs. The SNCP-ring (2F-UPSR) is limited to the ring configuration.
3. A path can be subdivided to subnetwork connections (SNC) as described above. The
CP is a connection point of SNCs. The TCP is a path termination point.
4. The SNCP can be used to protect a portion of a path (e.i. a SNC), by setting PPSs at
two CPs (Case 3) or at a CP and a TCP (Case 2) or the full end-to-end path putting
PPSs at two TCPs (Case 1).
5. To make the SNCP flexible, three types of PPS should be available. They are
Aggregate-Aggregate PPS, Aggregate-Tributary PPS and Tributary-Tributary PPS.
Implementation of all or some of them to equipment depends on its design.
6. The SNCP-ring (2F-UPSR) uses only Aggregate-Aggregate PPS.

26
 4 Fiber
Multiplex Section Shared Protection (MS-SP) Ring
(4 Fiber Bi-directional Line Switch Ring - BLSR)

- Terrestrial Application -

27
4 Fiber MS-SPRing / 4F-BLSR (1)
Terrestrial Application -

28
1. 4 fiber MS-SP ring (4F-BLSR) uses
1. 4 fiber bidirectional ring architecture and
2. Bidirectional 1:1 line switch scheme.
A pair of fiber is for the working line and the other for the protection line.
2. The solid and broken lines show the same channels (time slots) in the working and protection
STM-Ns respectively.
3. Different from the SNCP-ring, both directions of a 2-way path are directed to the same route and
they are not branched onto the protection channel. The corresponding channel on the second
semicircle (left, in the drawing) is vacant, so it is possible to arrange other paths, like another A-D
or D-E and E-A, etc., by using the same channel. Of course those paths must not have overlaps.
The overlapped paths must use different channel of the STM-N. As a result, total number of paths
in the ring can exceed the STM-N capacity.
4. The SLA (Stand-by Line Access) is possible.
5. It requires the switching protocol carried by K1 and K2 of MSOH.
6. The maximum number of nodes is limited to sixteen (16). This is determined by the control
protocol (standard). Four bits in K1 are assigned to indicate the message destinations, resulting in
maximum 16 nodes.

29
4 Fiber MS-SPRing / 4F-BLSR (2)
- Terrestrial Application -

30
1. Against a total section failure (both working and protection), the traffic on the working fiber is
looped to the corresponding channel of the opposite direction protection fiber at the both ends of
the failed section (B and C).
2. The looped traffic passes through both originating and destination nodes and looped back to the
working fiber at the other end of the failed section (C and B).
3. The same applies to all other channels which are used by other path, i.e. the loop at B and C and
the through at other nodes. The result is just as if the traffic is looped at the STM-N level or the
fiber. That is why this system is categorized to the line switch.
4. The line switch takes place at the failed section side of the node. That is, at C node it is not at
the side facing D node but at the side facing B node.
5. Now the long route (C-D-E-F-A-B) is used as a protection line for the short rout (B-C). In this
way, the protection ring is shared by all sections (MS) in the ring.
6. This switching against a total section failure is named “Ring Switch .”
7. Unaffected paths, e.g. paths between C-D,E-A etc., remain on the working fiber.
8. Against multiple section failure, all of paths cannot survive, some of them are killed.
9. When a node failure occurs, e.g. at C node, ring switches take place at B node and D node. They
are at the sides facing C node.

31
4 Fiber MS-SPRing / 4F-BLSR (3)
- Terrestrial Application -

32
1. Against a section failure on the working line only, all traffics carried by the failed
section are transferred to the protection fiber of the same section. This switching is
named “Span Switch”.
2. When multiple section failures occur and if they are all working-line-only type
failures, the span switches are applied to all of them. And all normal traffics are
protected. The 4F MS-SP ring can protect multiple failures of a certain type.
3. This means network operators can expect 4F MS-SP higher survivability than 2F MS-
SP in addition to higher capacity. (cf. 2F MS-SP explanations)
4. Extra traffics by the SLA that pass through the failed section will be removed.
5. The system automatically select the ring switch or the span switch depending on the
failure mode. Maintenance crews do not have to intervene.
6. Both in the ring and span switching mode, all traffics are switched to the protection.
Unlike SNCP ring, it is impossible to set protected paths selectively. Always all paths
in the MS-SP ring are protected.

33
 2 Fiber
Multiplex Section Shared Protection (MS-SP) Ring
(2 Fiber Bi-directional Line Switch Ring - BLSR)
- Terrestrial Application -

34
2 Fiber MS-SPRing / 2F-BLSR (1)
- Terrestrial Application -

35
1. 2 fiber MS-SP ring (2F-BLSR) uses
2 fiber bidirectional ring architecture and
Bidirectional 1:1 line switch scheme.
2. To provide protection channels, the STM-N capacity is divided into two parts. The
first half is assigned to the working channels and the second half is for the
protection.
3. The path arrangement is same as the 4 fiber MS-SP ring including reuse of channels.
4. The total maximum number of paths in the ring is half of 4 fiber MS-SP ring, under
the same conditions.
5. The SLA is possible.
6. Same protocol as 4 fiber ring is used and the maximum node number is also sixteen
(16).

36
2 Fiber MS-SPRing / 2F-BLSR (2)
- Terrestrial Application -

37
1. Against a section failure, the loop switching takes place at both end of the failed section
(B and C). The traffic carried by a channel of the working half of STM-N is transferred
to the corresponding channel of the protection half of opposite direction STM-N. It
travels the long route passing originating, destination and other nodes (A, F, E and D).
At the remote end it is switched back to the working half of the opposite direction
(original fiber).
2. There is no Span Switch for 2 fiber ring. Unlike 4F MS-SP, no survivability against
multiple failures.
3. Other points are same as 4 fiber ring.

38
Node ID Map of 4F/2F MS-SPRing

39
1. Each node on the MS-SP ring must be assigned an ID that is a number between 0 and 15. The ID assignment
is not necessarily in order. The SNCP ring does not require them.
2. In the protocol exchange, the node ID is used to indicate a message destination.
3. The cross connection table of each node must be provided with information that shows the originating and
destination node of each path indicated by the node ID.
4. Each node must have knowledge of the IDs assigned to all other node in the ring and their sequence. The
node ID map, which is provisioned to each node, provides this information.
5. When nodes are added to the ring, the node ID map must be revised.

1. The 4F/2F MS-SP ring has possibility of making a misconnection when a node fails.
2. Misconnection
2.1 Two paths, A-C and C-F, are terminated at the failed node (C) and they use a same channel of the
STM-N. In this case, their stand-by channel in the protection capacity is the same one.
2.2 When C node fails ring switches take place at B and D nodes. B node does it to save A-C path and D
node to save C-F path. And they are switched to the same protection channel.
2.3 The result is that previous two independent paths are changed to single A-F connection.
2.4 This is inevitable and the system send out an AIS (Alarm Indication Signal) to the misconnection paths
in order to avoid inconvenience.
3. No misconnection
3.1 When two paths that use the same channel of the STM-N are not terminated at the failed node (through
connection), A-E and E-F, the misconnection does not occur. No AIS is sent out to those paths.
4. This control is named “Squelch Control.” Its procedure is show at the right of the drawing.
5. For the squelch control the squelch table (next slide) must be provisioned to each node.

40
 MS-SPRing Misconnection and Squelch Control
A-F Squelch Control
C-F E-F
with AIS
MS-SP switch

A-C E-F VC-4 no

A-E
F F path ?
yes
A-E
A-F
A E A E
with AIS no A/D
on the same channel on the same channel at C ?
A/D no
of the STM-N. of the STM-N. yes
at C ?

B D B D no AU AIS
yes
no TU AIS

C C for individual AU
at B and D
TU AIS
for individual TU
at A, E and F
AU AIS

The ring-switch is applied after

working channel AU AIS insertion at B and D.
protection channel For VC-12/3, AU AIS is inserted
A- C C-F
at B and D first, then it is
stopped after TU AIS
misconnection no misconnection completion, in order to avoid
short term misconnections.

41
 Squelch Table of MS-SPRing
MS-SP
Ring STM-N Node ID No. STM-N Node ID No. STM-N Node ID No.
West i East West j East West k East

VC-4 (VC organized AU) through

(at intermediate nodes)

AU-4 #m
VC-3/VC-12 VC-4 (VC organized AU)
AU-4 in AU-4 through

AU-4 #n

VC-3/VC-12 VC-3/VC-12
Add/Drop all through
AU-4 #m AU-4 #m AU-4 #m

AU-4 Path ()i (j) (k)

AU-4 #n AU-4 #n AU-4 #n AU-4 #n

West East West East West East

Squelch Table AU-4 #m ?1 k AU-4 #m -* -* AU-4 #m i ?3
AU-4 #n ?2 j AU-4 #n i k AU-4 #n j ?4
* VC-4 cross connect level requires
no squelch table.

AU-4 through node : The node where VC-4 cross connect level is applied to the AU-4.
AU-4 termination node :The node where VC-3/VC-12 cross connect level is applied to the AU-4 in question,
regardless cross connections, either all through or several add/drop connections.

42
1. There are two kind of AU-4 (VC-4). One type carries low order VCs (LOVC, VC-3, VC-12
etc.) and it is called “VC organized AU.” The other is a pure VC-4 that carries a 140Mb/s,
for example.
2. When the cross connect level of a VC organized AU-4 is set to LOVC level, i.e. other than
VC-4, the NE must have information (squelch table) where its cross connect level is set to
LOVC level again, to both east and west directions. It dose not matter whether the LOVC’s
cross connection is add/drop or through or mixed, both at own node and at the other side of
AU-4.
3. When the cross connect level is VC-4 level, regardless VC organized or not, the squelch table
is not required. Because its terminating node is clear from the cross connect map.

43
 4/2 Fiber
Multiplex Section Shared Protection (MS-SP) Ring

(4/2 Fiber Bi-directional Line Switch Ring - BLSR)

- Transoceanic Application -

44
4/2 Fiber MS-SPRing / 4/2F-BLSR
- Transoceanic Application -

45
1. If the MS-SP ring has sections with very long length, e.g. a transoceanic submarine cable, and the
ring switch is applied like a terrestrial system, the traffic must cross a ocean back and forth under
failure condition, resulting in very long delay. To solve the problem different algorithm called the
transoceanic application is standardized (also in G.841).
2. In this algorithm, instead of the ring switch, the affected path is switched to the corresponding
channel of the protection capacity of opposite direction at the path terminating node. The same
occurs at all nodes that have suffered paths. Actually, this is not a line switch but a path switch.
The result is same as the SNCP bidirectional ring but the protocol does not use K3 or K1 of POH
and algorithm is different.
3. When a section failure occurs, B and C exchange protocol via the protection fiber of the long
route (B-A-F-D-E-C) using K1 and K2. Nodes A, F, E and D can monitor the exchange, decide
which of their paths are in trouble and take the above action at the path level. For this protocol the
DCCr is also used to exchange the path mapping information.
4. Against the on-only-working type failure on 4F MS-SP, the span switch(s) are applied in the same
way as the terrestrial system.

46
 Inter Locked Ring (ILR)

Secondary Secondary
node node

No. 1 Ring No. 2 Ring

Primary Primary
node node
ring failure
link failure

47
1. When a path is laid over two rings, it can survive against a ring failure by the self-healing
function of the ring. But the failure is on the link that connects two rings, it will be killed.
2. By connecting two rings by two links and diverting the traffic to the second link against the
first link failure, the survivability of the path can be improved. This method is named Inter
Locked Ring (ILR).
3. The ILR is possible for any combination of two rings, MS-AP~MS-AP or SNCP~SNCP or
MS-SP~SNCP.
4. Two nodes where the links are connected are named the primary node and the secondary
node. The two nodes are not necessarily neighbor nodes. It is possible to put nodes between
the primary and the secondary.
5. The ILR setting is on the path bases. It is not necessary to set ILR to all of paths that pass the
link.

48
 Inter Locked MS-SPRing (1)
(P-S Connection: on working channel)
MS-SP Ring No.2 MS-SP Ring No.1 MS-SP Ring No.2 MS-SP Ring No.1

F F
B B
Secondary node Secondary node

working working

protection protection
G G
A A
SS SS
SS
Primary node SS
Primary node

C C

D normal D

ring failure

E E

49
1. There are two types of MS-SP ~ MS-SP ILR depending on whether the working capacity or the
protection capacity connects the primary and the secondary node. This drawing shows on-
working case.
2. MS-SP~ MS-SP ILR (P-S on-working)
2.1 At the transmitting side primary node the traffic is branched on to the first and second link
routes. At the receiving side primary node a service selector (SS) is installed and it choose a
normal signal carried by the first or the second link. Either failure on the first or the second
link can be protected. The path connections between four node (two primaries and two
secondary nodes) is same as the SNCP. The SS works as the PPS.
2.2 The right side drawing explains the protection against a section failure on the ring. Usual
ring switch or span switch (not shown) takes place. Thus a double failure on the link and the
ring can be protected.
3. The connection between the primary and the secondary nodes uses the working capacity. A
demerit of this configuration is that it consumes a part of the working capacity for protection
purpose and reduces efficiency of the ring.

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Inter Locked MS-SPRing (2)
(P-S Connection: on working channel)
MS-SP Ring No.2 MS-SP Ring No.1

F
B
Secondary node
working

protection
G
A
SS
SS
Primary node

51
Inter Locked MS-SPRing (3)
(P-S Connection: on protection channel)
MS-SP Ring No.2 MS-SP Ring No.1 MS-SP Ring No.2 MS-SP Ring No.1

F F B
B
Secondary node
Secondary node SS
protection SS
protection
working transfer
working
G G
A A
SS SS
Primary node Primary node
SS

C C

D normal D

ring failure

E E

52
1. MS-SP ~ MS-SP ILR (P-S on-protection)
Connection between the primary and the secondary uses protection capacity. The
switching against the link failure is quite same as previous one. One against the ring
failure is completely different and complicated.
Against the ring failure, the same ring switches take place at the failed section. The
traffic is looped to the protection capacity but when it reaches to the primary node it has
no way to go further, because a part of the protection capacity is occupied by the ILR.
To vacate the protection capacity to the ring-switched traffic, automatic reconfiguration
of the ILR is carried out at the primary and the secondary. The SS and the branching for
the opposite direction at the primary is transferred to the secondary as shown in the
drawing. And the traffic on the first link is directly connected to the working capacity
of the ring. Now the ILR is ready for a failure on the links. This is named “SS transfer.”
The merit of this configuration with complicated ring control is it does not consume any
of the working capacity of the MS-SP ring. The same total ring capacity as an
independent MS-SP can be realized.

53
Inter Locked MS-SPRing (4)
(P-S Connection: on protection channel)
MS-SP Ring No.2 MS-SP Ring No.1

F B
Secondary node
SS

SS
protection
transfer
working
G
A
SS
Primary node

54
 Inter Locked MS-SPRing
(SNCP Ring - SNCP Ring)
SN-CP Ring No.1 SN-CP Ring No.2

1. SNCP ring ~ SNCP ring ILR

The ILR arrangement of SNCP
rings is shown above, the PPS
setting at the ILR node is
slightly different from that of
normal one.
For transmitting direction
( connection from the tributary
to aggregates) branching onto
the CW and CCW fibers is not
applied, only to one direction.
To the received traffics from
aggregate lines, the drop-and-
continue to the other ILR node
is applied.

55
 Double Failure on Link and Ring (SNCP/ILR)
unprotected bouble failure

Secondary Secondary
node node

SNCP ring
or SNCP ring PPS

MS-SP ring
Primary Primary
node node

protected bouble failure

1. On the SNCP ring, the input traffic from a link ( tributary ) line is transmitted to only
one direction, the first link to CCW and the second link to CW.
Therefore, depending on the mode of double failure on the ring and link, there is a
case where the path cannot be protected. Above drawing explain the protected and
unprotected situations.
2. The MS-SP ring does not have this inconvenience. 56
Inter Locked Ring
(SNCP - MS-SP)
SN-CP Ring MS-SP Ring

1. The ILR between the SNCP

ring and the MS-SP ring is also
possible. As shown above, each
ring sets its own ILR
connection.
SS
2. The drawing shows only on-
working type ILR for the MS-
SP, but on-protection type ILR
can be used without affecting
the setting of the SNCP ring
side.

57
 Review Questions
Fill up the space enclosed in parentheses in the following sentence with correct words.
1. For the 1+1 line system, only the failure line is transferred to the protection line, this
transfer is called ( a ). Even if failure is only on one direction, both lines are
transferred to the protection lines. This transfer is called ( b ).
2. For the 1+1 linear system, the protection line can be used to transport an ( a )
traffic. This system is called ( b ) ( c ) access system.
3. For the APS of the ring, the maximum number of nodes on a ring is ( a ). The
node number start from ( b ) to ( c ). The number assignment is not
necessarily in ( d ).
4. For linear APS, there are various switching priority, the top of the list of “switch
preemption priority” is ( a ).
5. Both line and path switched rings can be implemented by either as a ( a )-
directional ring, or as a ( b )-directional ring.
6. What stands for ILR( a ) ( b )( c )?

58