ETSI ETS 300 746 ed.1 (1997-02)

SLOVENSKI STANDARD
01-december-2003
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Transmission and Multiplexing (TM); Synchronous Digital Hierarchy (SDH); Network
protection schemes; Automatic Protection Switch (APS) protocols and operation
Ta slovenski standard je istoveten z: ETS 300 746 Edition 1
ICS:
33.040.20 Prenosni sistem Transmission systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN ETS 300 746
TELECOMMUNICATION February 1997
STANDARD
Source: ETSI TC-TM Reference: DE/TM-03042
ICS: 33.020
Key words: SDH, network, protection, interworking, protocol, transmission
Transmission and Multiplexing (TM);
Synchronous Digital Hierarchy (SDH);
Network protection schemes;
Automatic Protection Switch (APS) protocols and operation
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: secretariat@etsi.fr
Tel.: +33 4 92 94 42 00 - Fax: +33 4 93 65 47 16
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1997. All rights reserved.

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ETS 300 746: February 1997
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.

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ETS 300 746: February 1997
Contents
Foreword .7
1 Scope .9
2 Normative references.9
3 Abbreviations.9
4 Definitions and classifications .11
4.1 General definitions.11
4.2 Ring definitions .12
4.3 Protection classifications .13
5 Multiplex section trail protection protocols.13
5.1 Multiplex section trail linear protection.13
5.1.1 APS requirements .13
5.1.2 Switch initiation criteria.13
5.1.2.1 Externally initiated commands.13
5.1.2.1.1 Commands not signalled on the APS
channel .13
5.1.2.1.2 Commands using the APS bytes .14
5.1.2.2 Automatic initiated requests .14
5.1.3 Protection switch protocol .15
5.1.3.1 K1 Byte .15
5.1.3.2 Bit 1-5 of K2 byte .17
5.1.4 Protection switch operation .17
5.1.4.1 Node APS state .17
5.1.4.1.1 Idle state .17
5.1.4.1.2 Switching state.18
5.1.4.2 Node APS state transition rules.18
5.2 2-fibre multiplex section trail shared protection ring .18
5.2.1 APS requirements .18
5.2.2 Switch initiation criteria.19
5.2.2.1 Externally initiated commands.19
5.2.2.1.1 Commands not signalled on the APS
channel .19
5.2.2.1.2 Commands using the APS bytes .19
5.2.2.2 Automatically initiated commands .19
5.2.3 Protection switch protocol .20
5.2.3.1 K1 byte.20
5.2.3.2 K2 byte.21
5.2.4 Protection algorithm operation .21
5.2.4.1 Ring node APS state .22
5.2.4.1.1 Idle state .22
5.2.4.1.2 Switching state.22
5.2.4.1.3 Pass-through state.23
5.2.4.2 Ring node APS state transition rules.24
5.2.4.2.1 Transitions between the idle and full
pass-through state .24
5.2.4.2.2 Transitions between the idle and
switching states.25
5.2.4.2.3 Transitions between switching states .26
5.2.4.2.4 Transitions between switching and full
pass-through state .27
5.3 4-fibre multiplex section trail shared protection ring .28
5.4 2-fibre multiplex section trail dedicated protection ring.28
5.4.1 APS requirements .29

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ETS 300 746: February 1997
5.4.1.1 Requirements for the protocol. 29
5.4.1.2 Use of linear MS trail protocol. 29
5.4.2 Switch initiation criteria . 29
5.4.2.1 Externally initiated commands . 29
5.4.2.1.1 Commands not signalled on the APS
channel. 29
5.4.2.1.2 Commands using the APS bytes . 29
5.4.2.2 Automatically initiated commands. 30
5.4.3 Protection switch protocol. 30
5.4.3.1 K1 byte generation rules . 30
5.4.3.2 K2 byte generation rules . 31
5.4.4 Protection algorithm operation. 31
5.4.4.1 Ring without failure. 32
5.4.4.2 Bi-directional failure. 32
5.4.4.3 Unidirectional failure. 33
5.4.4.4 The failure is repaired . 34
6 Path protection protocols. 35
6.1 LO/HO trail protection. 35
6.1.1 APS requirements. 35
6.1.2 Switch initiation criteria . 35
6.1.2.1 1+1 single-ended protection. 35
6.1.2.1.1 Externally initiated commands . 36
6.1.2.1.2 Automatically initiated commands. 36
6.1.2.1.2.1 Higher order automatically initiated
commands. 37
6.1.2.1.2.2 Lower order automatically initiated
commands. 37
6.1.2.2 1+1 dual-ended protection . 37
6.1.2.3 1:1 protection . 37
6.1.3 Protection switching protocol . 37
6.1.3.1 1+1 single-ended protection. 37
6.1.3.2 1+1 dual-ended protection . 38
6.1.3.3 1:1 protection . 38
6.1.4 Protection algorithm operation. 38
6.1.4.1 1+1 single-ended protection. 38
6.1.4.1.1 Control of the bridge. 38
6.1.4.1.2 Control of the selector . 38
6.1.4.1.2.1 Revertive mode . 38
6.1.4.1.2.2 Non-revertive mode. 38
6.1.4.2 1+1 dual-ended protection . 38
6.1.4.3 1:1 protection . 38
6.2 LO/HO SNC protection. 39
6.2.1 APS requirements. 39
6.2.2 Switch initiation criteria . 39
6.2.2.1 1+1 single-ended protection. 39
6.2.2.1.1 Externally initiated commands . 39
6.2.2.1.2 Automatically initiated commands. 40
6.2.2.1.2.1 Higher order automatically initiated
commands. 40
6.2.2.1.2.2 Lower order automatically initiated
commands. 40
6.2.2.2 Other architectures . 41
6.2.3 Protection switching protocol . 41
6.2.3.1 1+1 single-ended protection. 41
6.2.3.2 Other architectures . 41
6.2.4 Protection algorithm operation. 41
6.2.4.1 1+1 single-ended protection algorithm. 41
6.2.4.1.1 Control of the bridge. 41
6.2.4.1.2 Control of the selector . 41
6.2.4.1.2.1 Revertive mode . 41
6.2.4.1.2.2 Non-revertive mode. 42
6.2.4.2 Other architectures . 42

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Annex A (normative): Squelching mechanism in MS-Shared Protection Rings.43
A.1 Squelching of HO traffic .43
A.1.1 Case of single ring .43
A.1.2 Case of dual node ring interworking .44
Annex B (informative): Examples of protection switching in an MS shared protection ring.48
B.1 Unidirectional signal fail (ring) .48
B.1.1 Derivation of switching delay.48
B.2 Bi-directional signal fail (ring) .53
B.3 Unidirectional signal degrade (ring).57
B.4 Detection and clearing of a unidirectional SF-R in presence of another unidirectional SF-R on a
non-adjacent span.59
B.5 Unidirectional SF-R pre-empting a unidirectional SD-R on a non-adjacent span .61
B.6 Unidirectional SD-R pre-empting a unidirectional MS-R on an adjacent span.63
Annex C (informative): Bibliography.65
History.66

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Foreword
This European Telecommunication Standard (ETS) has been produced by Transmission and Multiplexing
(TM) Technical Committee of the European Telecommunications Standards Institute (ETSI).
This ETS is one of a family of related Technical Specifications (TSs) and ETSs covering the various
aspects of SDH protection:
Draft TS 101 009: "Transmission and Multiplexing (TM); Synchronous Digital Hierarchy (SDH)
network protection schemes; Types and characteristics";
Draft TS 101 010: "Transmission and Multiplexing (TM); Synchronous Digital Hierarchy (SDH)
protection interworking; Rings and other schemes";
ETS 300 746: "Transmission and Multiplexing (TM); Synchronous Digital Hierarchy
(SDH); Network protection schemes; Automatic Protection Switch (APS)
protocols and operation".
Transposition dates
Date of adoption 7 February 1997
Date of latest announcement of this ETS (doa): 31 May 1997
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 30 November 1997
Date of withdrawal of any conflicting National Standard (dow): 30 November 1997

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1 Scope
This European Telecommunication Standard (ETS) specifies the Automatic Protection Switching (APS)
requirements, switching initiation criteria, and the APS protocols of Synchronous Digital Hierarchy (SDH)
multiplex section shared protection ring, multiplex section dedicated protection ring, multiplex section
linear protection, and path trail and Sub-Network Connection (SNC) protection schemes. The APS
protocols are specified in terms of their multiplex section or path overhead requirement, the signalling
messages and their operations under various failure conditions.
NOTE: The network objectives, architectures, functional modelling and operations of the
various SDH protection schemes are under study (draft TS 101 009, see annex C).
The protection interworking and interconnection scenarios for SDH network protection
schemes are also under study (draft TS 101 010, see annex C).
2 Normative references
This ETS incorporates by dated or undated references, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references subsequent amendments to, or revisions of, any of these publications
apply to this ETS only when incorporated in it by amendment or revision. For undated references the latest
edition of the publication referred to applies.
[1] ETS 300 417: "Transmission and Multiplexing (TM); Generic functional
requirements for Synchronous Digital Hierarchy (SDH) equipment".
[2] ITU-T Recommendation G.803: "Architectures of transport networks based on
the synchronous digital hierarchy (SDH)".
[3] ITU-T Recommendation G.708: "Network node interface for the synchronous
digital hierarchy".
[4] ITU-T Recommendation G.709: "Synchronous multiplexing structure".
[5] ITU-T Recommendation G.783: "Characteristics of synchronous digital hierarchy
(SDH) equipment functional blocks".
[6] ITU-T Recommendation G.841: "Types and characteristics of SDH network
protection architectures".
3 Abbreviations
For the purposes of this ETS, the following abbreviations apply:
ADM Add Drop Multiplex
AIS Alarm Indication Signal
APS Automatic Protection Switching
AU Administrative Unit
AU-AIS Administrative Unit - Alarm Indication Signal
AUG Administrative Unit Group
AU-n Administrative Unit (level) n
BER Bit Error Ratio
BIP-n Bit Interleaved Parity (of order) n
Br Bridged
CPE Customer Premises Equipment
DCC Data Communication Channel
DXC Digital Cross-Connect
EXER EXERcise
EXER-R EXERcise - Ring
FEBE Far End Block Error
FERF Far End Receive Failure
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FS Forced Switch
FS-P Forced Switch of the Protection channel to working
FS-R Forced Switch of working to protection - Ring
FS-W Forced Switch of a Working channel to protection
HO VC Higher Order Virtual Container
HO Higher Order
HOPA Higher Order Path Adaptation
HOPT Higher Order Path Termination
HPC Higher order Path Connection
HP-DEG Higher order Path-DEGraded
HP-SSF Higher order Path-Server Signal Fail
HP-TIM Higher order Path-Trace Identifier Mismatch
HP-UNEQ Higher order Path-UNEQuipped
ID IDentification
LO VC Lower Order Virtual Container
LO Lower Order
LOF Loss Of Frame
LOPA Lower Order Path Adaptation
LOPT Lower Order Path Termination
LOS Loss Of Signal
LP Lockout of Protection
LPC Lower order Path Connection
LP-DEG Lower order Path-DEGraded
LP-SSF Lower order Path-Server Signal Fail
LP-TIM Lower order Path-Trace Identifier Mismatch
LP-UNEQ Lower order Path UNEQuipped
MS Multiplex Section
MS-DPRing Multiplex Section - Dedicated Protection Ring
MS-P Manual Switch of the Protection channel to working
MS-R Manual Switch of working to protection - Ring
MS-SPRing Multiplex Section - Shared Protection Ring
MS-W Manual Switch of a Working channel to protection
NE Network Element
NNI Network Node Interface
NR No Request
OAM&P Operations, Administration Maintenance & Provisioning
OS Operations System
PC Private Circuit
POH Path OverHead
RC Remote Concentrator
RDI Remote Defect Indicator
RR Reverse Request
RR-R Reverse Request - Ring
RS Regenerator Section
SA Section Adaptation
SD Signal Degrade
SDH Synchronous Digital Hierarchy
SD-R Signal Degrade - Ring
SF Signal Fail
SF-R Signal Fail - Ring
SNC Sub-Network Connection
SNC/I Sub-Network Connection protection with Inherent monitoring
SNC/N Sub-Network Connection protection with Non-intrusive monitoring
SOH Section OverHead
SSF Server Signal Fail
ST Section Termination
STM Synchronous Transport Module
STM-N Synchronous Transport Module (level) N
Sw Switched
TMN Telecommunications Management Network
TSI Time Slot Interchange
TU Tributary Unit
TUG Tributary Unit Group
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TU-n Tributary Unit (level) n
VC Virtual Container
VC-n Virtual Container (level) n
WTR Wait To Restore
4 Definitions and classifications
For the purposes of this ETS, the following definitions and classifications apply:
4.1 General definitions
Administrative Unit (AU): See ITU-T Recommendation G.708 [3].
Administrative Unit Group (AUG): See ITU-T Recommendation G.708 [3].
Automatic Protection Switching (APS): See ITU-T Recommendation G.783 [5].
Bit Interleaved Parity (BIP): See ITU-T Recommendation G.708 [3].
bridge: The action of transmitting identical traffic on both the working and protection trails.
dedicated protection: See ITU-T Recommendation G.803 [2].
dual ended operation: See ITU-T Recommendation G.803 [2].
head-end: The node that executes a bridge.
Loss Of Frame (LOF): See ITU-T Recommendation G.783 [5].
Loss Of Signal (LOS): See ITU-T Recommendation G.783 [5].
Lower Order Virtual Container (LO VC) access: The termination of a higher order VC for the purpose of
adding, dropping, or cross-connecting any individual LO VC or VC group.
misconnection: A condition in which traffic destined for a given node is incorrectly routed to another node
and no corrective action has been taken.
Multiplex Section (MS): See ITU-T Recommendation G.803 [2].
Multiplex Section AIS (MS-AIS): See ITU-T Recommendation G.783 [5].
Multiplex Section FERF (MS-FERF): See ITU-T Recommendation G.709 [4].
Network Node Interface (NNI): See ITU-T Recommendation G.708 [3].
pass-through: The action of transmitting the information that is being received from one multiplex section
terminating port of a node which is connected to the ring to the other multiplex section terminating port of
the same node.
path: See ITU-T Recommendation G.803 [2].
path AIS: See ITU-T Recommendation G.783 [5].
Path Overhead (POH): See ITU-T Recommendation G.708 [3].
protection trail: The trail allocated to transport the working traffic during a switch event. When there is a
switch event, traffic on the affected working trail is bridged onto the protection trail.
Regenerator Section (RS): See ITU-T Recommendation G.803 [2].
restoration: See ITU-T Recommendation G.803 [2].

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secondary traffic: Traffic that is carried over the protection trail when it is not used for the protection of
working traffic. This is sometimes called extra traffic. Secondary traffic is not protected and is pre-empted
when the protection trail is required to protect the working traffic.
Section Overhead (SOH): See ITU-T Recommendation G.708 [3].
shared protection: See ITU-T Recommendation G.803 [2].
single ended operation: See ITU-T Recommendation G.803 [2].
single point failure: Failure located at a single physical point in a sub-network. The failure may affect one
or more fibres. A single point failure may be detected by any number of Network Elements (NEs).
Sub-Network Connection (SNC): See ITU-T Recommendation G.803 [2].
sub-network connection protection: See ITU-T Recommendation G.803 [2].
switch: The action of selecting traffic from the protection trail rather than the working trail.
tail-end: The node that requests the bridge.
Time Slot Interchange (TSI): The capability of changing the timeslot position of through-connected traffic
(i.e. traffic that is not added or dropped from the node).
trail: See ITU-T Recommendation G.803 [2].
trail protection: See ITU-T Recommendation G.803 [2].
Tributary Unit (TU): See ITU-T Recommendation G.708 [3].
Tributary Unit Group (TUG): See ITU-T Recommendation G.708 [3].
Virtual Container (VC): See ITU-T Recommendation G.708 [3].
Wait To Restore (WTR): The condition in which a working trail meets the restoral threshold after an SD
or SF condition. The transport of working traffic is ready to be reverted to the working trail from the
protection trail.
working traffic: Traffic that is normally carried in a working trail, except in the event of a protection
switch.
working trail: The trail over which working traffic is transported when there is no switch events.
4.2 Ring definitions
add traffic: Traffic that is inserted into a working trail at a ring node.
drop traffic: Traffic that is extracted from a working trail at a ring node.
long path: The path segment away from the span for which a ring request is initiated. Typically, there are
other intermediate nodes along this path segment.
ring: A ring is constructed within a layer consisting of a set of nodes, each of which is connected to its
immediate neighbour (adjacent) nodes by a trail/link connection, forming a closed loop. The capacity
offered by the ring between any pair of adjacent nodes is the same.
ring request: The request sent over the long path away from the span for which the request is initiated,
i.e. a long path request.
ring switching: Protection mechanism in a ring, which in the event of a switch the working traffic is
carried over the protection trail away from the failure.

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short path: The path segment over the span for which a span request is initiated. This span is always the
one to which both the head-end and tail end are connected.
span: The set of multiplex sections between two adjacent nodes on a ring.
squelching: The process of inserting path AIS in order to prevent misconnection.
4.3 Protection classifications
Classification of SDH protection schemes is based on the ITU-T Recommendation G.803 [2] layering
concept of a transport network model.
NOTE: See also draft TS 101 009 (annex C).
5 Multiplex section trail protection protocols
5.1 Multiplex section trail linear protection
5.1.1 APS requirements
An APS protocol is required to co-ordinate the bridge and switch operations between the nodes. The
requirements for the protocol are listed below:
Switch time. For MS trail linear protection the switching time shall be less than 50 ms.
Secondary traffic. For 1:n MS trail linear protection, access to the protection trails may be provided as an
option to accommodate secondary, low priority traffic.
Switching types. MS trail linear protection shall support both single ended and dual ended switching.
Operation Modes. The mode of operation shall be both revertive and non-revertive.
Manual control. External commands shall be provided for manual control of protection switching by the
operations systems or the craftpersons.
5.1.2 Switch initiation criteria
The requests to perform protection switching can be initiated either externally or automatically.
Externally initiated commands are entered by way of the Operations System (OS) or the craftperson
interface. Subclause 5.1.2.1 describes these externally initiated commands available at the OS,
craftsperson, or both interfaces.
APS requests can also be initiated based on multiplex section and equipment performance criteria.
Subclause 5.1.2.2 provides the automatically initiated command criteria.
5.1.2.1 Externally initiated commands
External requests are initiated at an NE by either the OS or the craftsperson. The external request may be
transmitted to the appropriate NE via the APS bytes, the TMN, or over the local craft interface. The
requests are evaluated by the priority algorithm in the protection switching controller.
5.1.2.1.1 Commands not signalled on the APS channel
The descriptions of the externally initiated commands are provided below:
clear: This command clears the externally initiated command at the node to which the command was
addressed. For 1:n and 1+1 revertive architecture, the NE-to-NE signalling following removal of the
externally initiated commands is performed using the NR code.

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The following command is useful if one span has excessive switching to protection. Another use for this
command includes blocking protection access for some channels that have only traffic that does not need
protection or before it become active during installation. The command is not time critical (i.e. not needed
to be completed in tens of milliseconds). Thus, it can be transmitted over the Data Communication
Channel (DCC).
lockout of a working channel: This command prevents the working channel from switching to the
protection channel by disabling the node's capability to request a protection switch of any kind. If any
working traffic is already on protection, it is switched back to the regular working channel regardless of its
condition. For 1:n and 1+1 revertive architecture, if no other requests are active, the NR code is
transmitted. The lockout of a working channel should be done at both ends of the line termination to avoid
unnecessary alarms. If the channel is locked out only at one end and the NE at the other end of the line
initiates a switch request, after waiting a pre-set time the NE that initiated the request will assume that the
response is not coming because of a failure situation and will send an alarm. This should be avoided by
locking out both sides.
NOTE: This external command is not currently present in the MS linear protection protocol
described in the ITU-T Recommendation G.783 [5].
5.1.2.1.2 Commands using the APS bytes
The following commands are carried over the APS bytes:
Lockout of Protection (LP): This command prevents any working channel (and the extra traffic, if
possible) from access the protection channel by issuing a Lockout of Protection Request. If any working
traffic is already on protection, it is switched back to the regular working channel regardless of its
condition. Only channel number 0 is allowed with a Lockout of Protection request.
Forced Switch of a Working channel to protection (FS-W): This command performs the switch from
the addressed working channel to the protection channel, unless a higher or equal priority switch
command is active or the protection channel is in a SF condition.
Forced Switch of the Protection channel to working (FS-P): because 1+1 system can be
non-revertive, forced switch of a working channel to protection command is not adequate if the traffic is
already on the protection channel. Thus a command to switch the traffic back to the working channel is
added: forced switch of the protection channel to working command performs the switch from the
protection channel to the (regular) working channel, unless a higher or equal priority switch command is
active. Since forced switch has higher priority than SF, this command will be carried out regardless of the
condition of the working channel. This command is used only in 1+1 architecture.
Manual Switch of a Working channel to protection (MS-W): This command performs the switch from
the addressed working channel to the protection channel, unless a higher or equal priority switch
command is active or the protection channel is in a SF condition. MS-W has a lower priority than Forced
Switch (FS) as shown in table 1.
Manual Switch of the Protection channel to working (MS-P): because 1+1 system can be non-
revertive, manual switch of a working channel to protection command is not adequate if the traffic is
already on the protection channel. Thus a command to switch the traffic back to the working channel is
added: manual switch of the protection channel to working command performs the switch from the
protection channel to the (regular) working channel, unless a higher or equal priority switch command is
active. Since manual switch has lower priority than SF, this command will be carried out only if the working
channel is not in SD or SF condition. This command is used only in 1+1 architecture.
EXERcise (EXER): This command exercises protection switching for the addressed channel without
completing the actual bridge and switch, unless the protection channel is in use. The command is issued
and the responses are checked, but no working traffic is affected. The aim of exercise is to test the APS
channel and K1, K2 bytes processing in the APS controller.
5.1.2.2 Automatic initiated requests
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

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ETS 300 746: February 1997
conditions (i.e., after a switch has been completed, all appropriate performance monitoring is continued).
The NE initiates the following requests automatically: Signal Failure (SF), Signal Degrade (SD), Reverse
Request (RR) and Wait to Restore (WTR). The requests are transmitted from NE to NE (not from OS to
NE).
The SF request is used to protect working traffic affected by a hard-failure, while the SD request is used to
protect against a soft failure. The node that receives the request, performs the activity according to the
priority level, and sends the bridged indication.
The WTR request is used to prevent frequent oscillation between the protection channels and the working
channels. The intent is to minimize oscillations, since hits are incurred during switching. The WTR request
is issued after the clearing of the defect condition on the working channels. The WTR is issued only after
an SF or an SD condition and, thus, does not apply for externally initiated requests.
DNR and NR requests are used when there is no need for protection, after the clearing of an external
command or of WTR condition. In particular, the DNR command is issued in 1+1 non-revertive system
when it is used together with 1:n system.
The definitions of the automatically initiated requests and their trigger conditions are provided below:
Signal Fail (SF): The SF condition is defined in ETS 300 417 [1]; this command is used to request
protection switching for signal failures.
Signal Degrade (SD): The SD condition is defined in ETS 300 417 [1]; this command is used to request
protection switching for signal degradation.
Reverse Request (RR): This command is transmitted to the tail-end NE as an acknowledgement for
receiving the request. It assumes the priority of the request to which it is responding.
Wait To Restore (WTR): This command is issued when working channels meet the restoral threshold
after an SD or SF condition. It is used to maintain the state during the WTR period unless it is pre-empted
by a higher priority request. It is used for revertive system only.
Do Not Revert (DNR): This command is issued when there are no commands initiated from the OS ,
craftperson, or either of the NEs. It is used only in 1+1 non-revertive system compatible with 1:n system. If
the line-terminating NE receives the RR code in response to the DNR code, it shall keep transmitting the
DNR code until pre-empted.
No Request (NR): The NR code is transmitted when there is no need to use the protection channel. Extra
traffic can be put on the protection channel when there is no request from either of the line-terminating
NEs.
5.1.3 Protection switch protocol
Byte K1 and bit 1-5 of byte K2, shall be used for protection switching. See subclause 5.1.4 for details on
the operational usage of these bytes.
Byte K1 and bit 1-5 of byte K2 shall be transmitted within the multiplex section overhead of the STM-N that
is carrying the protection sections. Although they can also be transmitted identically on working sections,
receivers should not assume so, and should have the capability to ignore this information on the working
sections. Note that bits 6-8 of byte K2 are used on all STM-N line signals to signal MS-RDI and MS-AIS.
A detected failure of the received K1 or K2 is considered as equivalent to a SF condition on the protection
section.
5.1.3.1 K1 Byte
These bits shall be assigned as shown in table 1 K1 bits 1-4 carry request codes, listed in descending
order of priority in table 1 K1 bits 5-8 carry the requesting channel ID for the request code indicated in K1
bits 1-4.
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Table 1: Byte K1 functions
Request code (Bits 1-4) Requesting channel identification
(Bits 5-8)
bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 bit 8
1111 Lockout of Protection (note 1) Requesting channel ID is set to the channel's
ID for which the request is issued:
1110 Forced Switch
1101 Signal fail - High Priority Channel 0 Null Channel (note 6)
(note 2)
1100 Signal fail - Low Priority Channel 1-14 Working Channels (note 7)
(note 2)
1011 Signal Degrade - High Priority 15 Extra Traffic Channel (note 8)
Channel (note 2)
1010 Signal Degrade - Low Priority Channel
(note 2)
1001 Not used (note 3)
1000 Manual Switch
0111 Not used (note 3)
0110 Wait-To-Restore
0101 Not used (note 3)
0100 Exercise
0011 Not used (note 3)
0010 Reverse Request (note 4)
0001 Do Not Revert (note 5)
0000 No Request
NOTE 1: Only channel number 0 is allowed with a Lockout of Protection request.
NOTE 2: For Signal Fail and Signal Degrade only, bit 4 indicates the priority assigned to the
channel requesting switch action: 1 indicates high priority channel and 0 indicates
low priority channel. For protection channel only code 1 is allowed. For 1+1 system
only code 1 is allowed.
NOTE 3: Some network operators may use these codes for network specific purposes. The
receiver shall be able of ignoring these codes.
NOTE 4: Reverse Request assumes the priority of request to which it is responding.
NOTE 5: Do Not Revert is used in 1+1 non-revertive system compatible with 1:n system
only.
NOTE 6: Allowed associated priority channel: 1.
NOTE 7: Allowed associated priority channel: 0 or 1. For 1+1 system only channel number 1
is allowed with priority 1.
NOTE 8: Allowed only in 1:n system.

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5.1.3.2 Bit 1-5 of K2 byte
Bit 1-5 of byte K2 shall be assigned as shown in table 2.
Table 2: Bit 1-5 of byte K2 functions
Channel identification (Bits 1-4) Protection
architecture (bit 5)
bit 1 bit 2 bit 3 bit 4 bit 5
null channel (0) if the received K1 byte indicates 0 = 1+1 architecture
either null channel or the number of a locked out
working channel
number of channel which is bridged (1-15), in all 1 = 1:n architecture
other cases
5.1.4 Protection switch operation
This subclause is structured as follows:
First, a number of general APS algorithm rules are given. Detailed rules then follow. The first subclause
covers the two classes of node APS states, and the steady-state behaviour of the node in these states.
The second subclause describes the transition rules among the node APS states.
These rules apply conceptually to a single MS linear protection APS controller operating at a node. It is
choosing switching and signalling actions based on all incoming K-byte signalling, detected failures, local
equipment failures, and externally initiated commands. In general, this conceptual controller looks at all
incoming information, chooses the highest priority input, and takes action based on that choice.
Figure 1 illustrates the conceptual operation of an MS linear protection APS controller.
5.1.4.1 Node APS state
There are two classes of node APS states: the idle state and the switching state.
5.1.4.1.1 Idle state
This state is allowed only for 1:n architecture and for 1+1 revertive architecture.
A node is in the idle state when it is not generating or detecting requests.
Rule I #1 - IDLE STATE SOURCED K BYTES: Any node in the idle state shall source the K-bytes as
given in table 3.
Table 3: Byte K1 and K2 values sourced in the idle state
K1 [1-4] = 0000 (No Request code) or
0001 (Do Not Revert code - see note)
K1 [5-8] = 0000 protection channel ID
K2 [1-4] = 0000 protection channel ID
K2 [5] = 0/1 (1+1 or 1:n code)
NOTE: Only for 1+1 non-revertive systems compatible with 1:n systems.

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5.1.4.1.2 Switching state
A node is in a switching state when it is either sourcing a request (automatically or externally), or
terminating a request.
Rule S #1 - SWITCHING STATE SOURCED K-BYTES: Any node in the switching state shall source
K-bytes as shown in table 4:
Table 4: Byte K1 and K2 values sourced by a node in the switching state
K1 [1-4] = REQUEST code
K1 [5-8] = requesting channel ID
K2 [1-4] = null channel (0) if the received K1 byte
indicates either null channel or the
number of a locked out working
channel.
Number of channel which is bridged
(1-15), in all other cases.
K2 [5] = 0/1 (1+1 or 1:n code)
5.1.4.2 Node APS state transition rules
For further study.
Externally initiated com m ands
APS
Local equipm ent
Detected failures
failures
Controller
Incom ing K-byte signalling
Figure 1: Conceptual MS linear protection APS controller
5.2 2-fibre multiplex section trail shared protection ring
5.2.1 APS requirements
NOTE: For the APS related objectives in SDH network protection schemes, see
draft TS 101 009, subclause 6.2 (details in annex C).

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5.2.2 Switch initiation criteria
The requests to perform protection switching can be initiated either externally or automatically. Externally
initiated commands are entered by way of the Operations System (OS) or the craftsperson interface.
Subclause 5.2.2.1 describes these externally initiated commands available at the OS, craftsperson, or
both interfaces. APS requests can also be initiated based on multiplex section and equipment
performance criteria. Subclause 5.2.2.2 provides the automatically initiated command criteria.
5.2.2.1 Externally initiated commands
External bridge requests are initiated at an NE by either the OS or the craftsperson. The external bridge
request may be transmitted to the appropriate NE via the APS bytes, the TMN, or over the local craft
interface. The bridge requests are evaluated by the priority algorithm in the protection switching controller.
5.2.2.1.1 Commands not signalled on the APS channel
The descriptions of the externally initiated commands are provided below:
clear: This command clears the externally initiated command and WTR at the node to which the
command was addressed. The NE-to-NE signalling following removal of the externally initiated commands
is performed using the NR code.
The following command is useful if one span has excessive switching to protection. Another use for this
command includes blocking protection access for some spans that have only traffic that does not need
protection. The command is not time critical (i.e. not needed to be completed in tens of milliseconds).
Thus, it can be transmitted over the DCC.
lockout of working channels - ring switch: This command prevents the working channels over the
addressed span from accessing the protection channels for a ring switch by disabling the node's capability
to request a ring protection switch of any kind. If any working traffic is already on protection, the ring bridge
is dropped regardless of the condition of the working channels. If no other bridge requests are active on
the ring, the NR code is transmitted. This command has no impact on the use of protection channels for
any other span. For example, the node can go into full pass-through state.
5.2.2.1.2 Commands using the APS bytes
The following commands are carried over the APS bytes:
Forced Switch of working to protection - Ring (FS-R): This command performs the ring switch from
working channels to the protection channels for the span between the node at which the command is
initiated and the adjacent node to which the command is destined. As this request has the highest priority,
this command will be executed regardless of the
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