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Recommendation G.958

Recommendation G.958

DIGITAL  LINE  SYSTEMS  BASED ON  THE  SYNCHRONOUS  DIG-
ITAL  HIERARCHY
FOR  USE  ON  OPTICAL  FIBRE  CABLES

1    Introduction

1.1    General

    The CCITT,

considering

    (a)    that Recommendations G.707, G.708 and G.709 specify the bit
rates, the multiplexing structure and the detailed mappings associated with
the synchronous digital hierarchy (SDH);

    (b)    that Recommendations G.781, G.782, G.783 specify the general char-
acteristics and functions of synchronous multiplexing equipment and
RecommendationG.784 the management of SDH equipment and networks;

    (c)    that Recommendations G.703 and G.957 specify the physical parame-
ters of the electrical and optical interfaces of SDH equipment;

    (d)    that Recommendations of Serie G.780 specify SDH multiplexing
equipment in terms of functional blocks without any constraint on physical
implementation;

    (e)    that among the first generation of SDH equipment there will be syn-
chronous line systems;

    (f)    that there is a need to ensure that these new systems provide longitudi-
nal compatibility with plesiochronous line systems complying with
RecommendationsG.955 andG.956,

recommends

    that digital synchronous line systems comply with the requirements
described in the following.

1.2    Abbreviations

ADM    Add-drop multiplexer

AIS        Alarm indication signal

ALS        Automatic laser shutdown

BER    Bit error ratio

BIP-8    Bit Interleaved parity order 8

CID        Consecutive identical digit

DCC    Data communications channel

DXC    Digital cross-connect

E/O        Electrical/optical

ECC    Embedded control channel

I/F        Interface

LOF        Loss of frame

MCF    Message communications function

MS-AIS    Multiplex section alarm indication signal

MUX    Multiplexer

NE        Network element

O/E        Optical/electrical

OHA    Overhead access

OOF    Out of frame

OS/MD    Operations system/mediation device

PLL        Phase locked loop

PRBS    Pseudo random binary sequence

RSOH    Regenerator section overhead

RST        Regenerator section termination

RTG    Regenerator timing generator

SDH    Synchronous digital hierarchy

SEMF    Synchronous equipment management function

SMS    SDH management subnetwork

SOH    Section overhead

SPI        SDH physical interface

STM-N    Synchronous transport module

TMN    Telecommunications management network

1.3    Definitions

1.3.1    Agent

    See Recommendation G.784.

1.3.2    automatic laser shutdown (ALS)

    The ALS function of an optical line system automatically switches off
the transmitter of a regenerator section in case of cable break in this section.

1.3.3    Bit interleaved parity order 8 (BIP-8)

    See Recommendation G.708.

1.3.4    consecutive identical digit (CID) immunity

    The ability of a digital system component to sustain the occurrence of
a digital signal containing a continuous stream of binary ZEROs or ONEs.

1.3.5    Data communications channel (DCC)

    See Recommendation G.784.

1.3.6    Embedded control channel (ECC)

    See Recommendation G.784.

1.3.7    Loss of frame (LOF)

    See Recommendation G.783.

1.3.8    Message communications function (MCF)

    See Recommendation G.784.

1.3.9    Multiplex section alarm indication signal (MS-AIS)

    See RecommendationG.783.

1.3.10    Network element (NE)

    See Recommendation G.784.

1.3.11    Operations system/mediation device (OS/MD)

    See Recommendation G.784.

1.3.12    Out of frame (OFF)

    See Recommendation G.783.

1.3.13    Overhead access (OHA)

    See Recommendation G.783.

1.3.14    Regenerator section overhead (RSOH)

    See Recommendation G.782.

1.3.15    Regenerator section termination (RST)

    See Recommendation G.782.

1.3.16    regenerator timing generator (RTG)

    The RTG function provides a timing reference to the outgoing STM-
N signal of a regenerator. This timing reference is derived from the incom-
ing STM-N signal recovered by the SPI function in normal operation, or
from an internal oscillator included in the RTG in case of fault.

1.3.17    S/R reference points

    See RecommendationsG.955, G.956 and G.957.

1.3.18    SDH management subnetwork (SMS)

    See Recommendation G.784.

1.3.19    SDH physical interface (SPI)

    See Recommendation G.782.

1.3.20    Section overhead (SOH)

    See Recommendation G.708.

1.3.21    Synchronous digital hierarchy (SDH)

    See Recommendation G.707.

1.3.22    Synchronous equipment management function (SEMF)

    See RecommendationG.782.

1.3.23    Synchronous transport module (STM-N)

    See Recommendation G.708.

1.3.24    Telecommunications management network (TMN)

    See Recommendation M.30.

2    Applications

2.1    System types

    Figures 2-1/G.958 and 2-2/G.958 define the reference configurations
of the optical line systems specified in this Recommendation and the rela-
tionship between RecommendationsG.957, G.958 and serieG.780 for their
specification. The two following types of applications are identified:

1)    inter-office link, (see Figure 2-1/G.958 and º 4.2),

2)    intra-office link, (see Figure 2-2/G.958 and º 4.2).

    These two figures illustrate the fact that Recommendation G.957
specifies the performance of the components of the optical link, from the
electro-optic conversion to the opto-electronic one, whereas
RecommendationG.958 specifies the performance of the optical link con-
necting two SDH equipments (multiplexer, digital cross-connect, add-drop
multiplexer), including repeaters if any.

2.2    System components

    A digital line system on optical fibre cables at a given bit rate is usu-
ally defined as the means used to implement a digital line section at this bit
rate. It comprises line section terminating equipment at both ends, optical
fibre sections and regenerators (if needed).

    The basic frame structure of SDH provides the overhead necessary for
the operation and maintenance of line systems, and therefore terminal
equipment of synchronous line systems will include multiplexing functions.
For example, an STM-4 or STM-16 line system will include a synchronous
multiplexer to multiplex the synchronous tributaries. If it is used to imple-
ment digital line sections at a givenG.702 hierarchy level
(e.g.139264kbit/s), it will also include the functions needed to map the
plesiochronous tributary into the STM-1. The corresponding functions are
described in RecommendationG.783.

    In order not to constrain equipment design and to ensure compatibility
between the various options which may be useful to implement, synchro-
nous line systems are defined for the purpose of this Recommendation as
the means used to transport an STM-N stream between the two reference
pointsC of two consecutive synchronous multiplexers (according to the
functional description of RecommendationG.783 and assuming that it will
also be consistent with future Recommendations on digital cross-connect
systems û DXCs).

     Synchronous line systems therefore comprise the terminating parts of
synchronous multiplexers, i.e. from reference pointC to reference pointA
(see Figure1-1/G.783), the optical fibre sections and the regenerators if
needed.

    These components are described in º3 (transmission medium), ºº4,
5 and 6 (overall design features, line termination and regenerators).

    Wherever possible, references to the relevant parts of
RecommendationsG.782, G.783 and G.784 are made, in particular in terms
of functional organization and information flows.

    This is also illustrated in Figures 2-1/G.958 and 2-2/G.958.

FIGURE 2-1/G.958 = 7,5 cm = 293



FIGURE 2-2/G.958 = 9 cm = 352



    Note 1ûThe figures report functional partitioning, not necessarily physical
ones.

    Note 2ûThe functional blocks of Recommendation G.783 are not used
because the figures are only intended to describe the scope of the var-
ious Recommendations related to RecommendationG.958.

    Note 3ûA and C are reference points as described in Figure1-1/G.783.
Reference point A is equivalent to S/R reference points of
RecommendationsG.955 andG.956 (see also
RecommendationG.957).

3    Type of transmission medium

    Single mode optical fibre cables conforming to several Recommenda-
tions are allowed in these systems. RecommendationG.652 refers to disper-
sion-unshifted fibres, RecommendationG.653 to dispersion-shifted fibres,
and RecommendationG.654 to loss-minimized fibres. Both the attenuation
and dispersion aspects are of particular concern to RecommendationG.957;
only the latter are currently well defined in these Recommendations.

    The wavelength regions are around 1310 nm for G.652 fibre, and
around 1550nm forG.652, G.653, and G.654fibres. Within these regions,
the wavelength ranges are defined first by cut-off wavelength and by system
attenuation and length requirements. Wavelength ranges are specified in
RecommendationG.957 for each application.

    These fibres may be used with several transmitters; single-longitudi-
nal mode lasers, multi-longitudinal mode lasers, and light-emitting diodes.
Spectral characteristics such as chirp, mode-partition noise, and spectral
width induce a power penalty, depending upon the fibre dispersion. This
will then further limit the operating wavelength ranges.

4    Overall design features

4.1    System design and optical parameters

    Prior to the advent of SDH the scope of the design of optical line sys-
tems included a large number of parameters such as transmit power range,
receiver overload and sensitivity, line code, operating wavelength, operation
and maintenance features,etc. This led to a great variety of designs, each
having a specific combined optimization of the parameters, the only com-
mon parameters being the attenuation and dispersion of the optical path
between pointsS andR. Those systems specified in
RecommendationsG.955 andG.956 provided only longitudinal compatibil-
ity, i.e.the possibility of parallel installation on the same cable route. Inter-
connection between two operators could be achieved either through joint
engineering of the optical link, or interconnection at a hierarchical level.
(Joint engineering is defined in º4.3 below.)

    Synchronous line systems described in this Recommendation are
intended to provide transverse compatibility, i.e.the possibility of mixing
various manufacturers' equipments within a single optical section. This is
based on compliance with bit rates, frame structure and detailed mappings
as defined in RecommendationsG.707, G.708 andG.709, with general
characteristics and functionalities as given in RecommendationsG.782 and
G.783, and with operation and maintenance as specified in
RecommendationG.784.

     For the optical parameters of the interfaces used in synchronous line
systems, transverse compatibility is based on compliance with
RecommendationG.957. RecommendationG.957 is summarized in º4.2.

4.2    Transverse compatibility (Recommendation G.957)

    Recommendation G.957 (Optical interfaces for equipments and sys-
tems relating to the synchronous digital hierarchy), provides specifications
for the optical interfaces of SDH equipment, described in
RecommendationsG.782 andG.783, and line systems described in the
present Recommendation, to achieve the possibility of transverse compati-
bility on elementary cable sections, i.e.mixing various manufacturers'
equipments within a single optical section.  These specifications also pro-
vide longitudinal compatibility with line systems of comparable hierarchical
level and application which are in accordance with
RecommendationsG.955 andG.956.

    The applications belong to these three categories with regard to the
achievable repeater span:

û    intra-office for distances less than approximately 2 km;

û    short-haul inter-office for distances of approximately 15 km;

û    long-haul inter-office for distances of approximately 40 km or more.

    Within each category further subdivisions are made depending on the
fibre type and wavelength region as described in º3 above. This leads to
one intra-office specification, two inter-office short-haul specifications and
three inter-office long-haul specifications for each bit rate.

    For each application separate specifications are given for the transmit-
ter at point S, the receiver at pointR and the optical path betweenS andR.

    Recommendation G.957 also contains the definition of each parame-
ter used. Corresponding measurement methods are under study and should
eventually be included in RecommendationG.957.

    The relationship between the parameters of the constituents of the
optical link is also described in RecommendationG.957 º5 in order to
establish a a common system design approach for engineering SDH optional
links. This can be used to select a suitable interface for a given regenerator
section, depending on the characteristics of the optical path of this section.

    It should be noted that the specifications given in
RecommendationG.957 are based on worst-case parameter values to pro-
vide simple design guidelines for network planners and explicit component
specifications for manufacturers. It is recognized that, in some cases, this
may lead to more conservative design than could be obtained through joint
engineering of the optical link, the use of statistical equipment design
approaches (statistical and semi-statistical design approaches are described
in RecommendationG.957), or in applications and environments more con-
strained than those permitted under the standard operating conditions.

     It is believed that this approach is sufficient to meet the requirements
of the great majority of cases. º4.3 below discusses those cases where there
is a need for an improved level of performance.

4.3    Joint engineering

    For a limited number of cases joint engineering may be envisaged to
meet the requirements of optical sections where the interface specifications
of RecommendationG.957 prove inadequate. This will probably occur
where the required section loss is greater (e.g.2dB) than that specified in
RecommendationG.957 but may also be considered for other parameters.

    For those cases it is up to the Administrations/operators concerned to
specify more closely the aspects of the system where the specifications of
RecommendationG.957 are not satisfactory. It is important to stress that
every situation requiring ôjoint engineeringö is likely to be different û hence
it is meaningless to try to standardize any of the parameter values for these
systems. Instead, it is for the Administrations/operators concerned to come
to an agreement as to what is required and then negotiate with manufactur-
ers as to what is actually feasible. This process is very likely to lead to both
ends of a transmission link being supplied by the same manufacturer, who
meets the required performance by jointly optimizing the transmitters and
receivers.

    It should be pointed out that, in spite of the futility of specifying any
parameter values for ôjointly engineeredö systems, it would be advisable for
Administrations/operators or manufacturers involved to follow the general
guidelines and system engineering approach used in
RecommendationG.957. In particular it would be helpful to use the same
parameter definitions as RecommendationG.957 (e.g.receiver sensitivity at
Rreference point including all temperature and aging effects).

4.4    Pattern dependence testing

    STM-N signals contain regions within the data stream where the pos-
sibility of bit errors being introduced is greater due to the structure of the
data within these regions.

    Three cases in particular may be identified:

1)    errors resulting from eye-closure due to the tendency for the mean
level of the signal within the equipment to vary with pattern-den-
sity due to alternative current couplings (ôDC wanderö);

2)    errors due to failure of the timing recovery circuit to bridge regions
of data containing very little timing information in the form of data
transitions;

3)    errors due to failure of the timing recovery circuit as in2) above but
compounded by the occurrence of the first row of the STM-N sec-
tion overhead bytes preceding a period of low timing content
(these bytes have low data content, particularly for largeN).

    In order to verify the ability of STM-N equipment to operate error-free
under the above conditions, a possible method to assess the consecutive
identical digit (CID) immunity of a circuit block is presented in AppendixI.

    This method may be employed during the design phase of the equipment
and appropriate points in the production assembly process.

5    Transmission overheads

    This section describes the regenerator section overhead (RSOH) pro-
cessing functionality in a synchronous line system. The definition of a
regenerator section (see Figure5-1/G.958 below) and the functional
description of a regenerator (see Figure5-2/G.958 below) are based on the
functional block description of RecommendationG.783. The functional
blocks and the signals are bi-directional where necessary. These descrip-
tions are logical descriptions, not suggested implementations.

5.1    Regenerator section model

    The regenerator section model is illustrated in Figure 5-1/G.958. The
definitions of the functions and signals at reference points are given in
RecommendationG.783.

    A regenerator section is defined as the part of an SDH link between
two adjacent reference pointsC, i.e.where the RSOH is generated and
included in the STM-N frame and where the RSOH is extracted from the
STM-N frame and terminated. Regenerator section end equipments may be
multiplexers (or cross-connect systems) and/or regenerators. In case of
intra-office links both ends are multiplexers (or DXCs).

FIGURE 5-/1G.958 = 12 cm = 469



 5.2    Regenerator model and functionality

    The regenerator model is shown in Figure 5-2/G.958. The functional
blocks and the signals at the reference points are the same as those described
in RecommendationG.783, except where noted below.

    In the following description, signal flows from left to right of
Figure5-2/G.958.

    The signal at reference point A(1) is the STM-N line signal. Refer-
ence point A(1) physically corresponds to reference pointR in
RecommendationsG.955 andG.956. The characteristics of the optical sig-
nal at this reference point are given in RecommendationG.957.

    STM-N signal entering at reference point A(1) is electrically regener-
ated by the SDH physical interface function SPI(1) at reference pointB(1).

FIGURE 5-2/G.958 = 11,5 cm = 449



    SPI(1) converts the signal at reference point A(1) into the sequence of logi-
cal levels forming the signal at reference pointB(1), for which SPI(1) must
grant the characteristics necessary to meet the required transmission and
network performance. Transmission performance requirements for synchro-
nous optical systems are given in ºº6 and7 and the network performance
requirements are given in RecommendationG.782. Timing is extracted
from the incoming signal and is made available at reference pointT1 to the
regenerator timing generator (RTG), and at reference point B(1) to the
RST(1). RTG requirements are contained in º6.

    The status of the received signal is monitored to detect input signal failures.
Input signal fail conditions and related parameters are defined in º7.

     The signal fail condition is reported to the synchronous equipment manage-
ment function (SEMF) through reference point S1 and to RST(1) through
reference pointB(1). The SEMF monitors all the regenerator functions for
management and control as described in º5.2.5.

    The RST(1) function recovers the frame alignment from the fully formatted
and regenerated STM-N data and associated timing at B(1). Criteria for
frame alignment algorithm, for out-of-frame condition (OOF), loss of frame
state (LOF) and the associated reporting to SEMF through the S2 reference
point are described in RecommendationG.783.

    Then the RST(1) function descrambles the signal at B(1), using the recov-
ered frame alignment and extracts the RSOH bytes. Scrambling in the
regenerator is described in º5.2.1.

    In an STM-N frame, the use of only a subset of the RSOH bytes is defined.
The definitions of these bytes and their positions in the STM-N frame are
given in RecommendationG.708 and described in detail for SDH equip-
ment in RecommendationG.783. In this section of RecommendationG.958
features specific to regenerators of synchronous line systems are described.

    The B1 byte is used to locate faulty regenerator sections. The B1 byte is
monitored and the result is reported to SEMF through reference point S2.

    Byte E1 provides an orderwire voice channel between section terminations.
Byte E1 is passed to the overhead access (OHA) function at reference point
U1. The OHA function in the regenerator provides the means for accessing
specific overhead capacities in RSOH. In the case of a 1:N line protection
system, it is not necessary for all regenerators within the same repeater sta-
tion to access the order-wire signal.

    The F1 byte is the user channel and is also passed to the OHA function. The
access of the user channel in the regenerator is optional. An example of use
of the F1 byte to identify a failed section in a chain of regenerator sections is
reported in RecommendationG.783.

    Data communication channels (DCC) bytes D1-D3 are routed to the mes-
sage communication function(MCF) through reference pointN. The use of
DCC is described in RecommendationsG.783 andG.784.

    Regenerators should be capable of ignoring the national use bytes and bytes
reserved for future international standardization.

    The RST function might need to access other bytes for medium dependent
use (see º5.2.3 below).

    The signal at reference point C is an STM-N frame with the associated tim-
ing signal. The RST(2) function inserts RSOH bytes to the data at reference
pointC, performs the scrambling and presents the fully formatted STM-N
data to SPI(2) at reference pointB(2).

    The RSOH bytes to be inserted are generated at RST(2), taken either from
the OHA through reference pointU1 or from the MCF through reference
pointN, or relayed from RST(1).

     Under normal operation (i.e.in-frame condition at RST(1)):

û    A1, A2 and C1 bytes are either generated or relayed. Relaying the
received framing bytes reduces the delay in the detection of OOF
and recovery from failure in a chain of regenerators. Fault section-
alization capability is not affected because B1 is recalculated for
each regenerator section. From a management viewpoint, it is pref-
erable that all the regenerators in a line system conform to either
one or other approach;

û     B1 is generated as described in Recommendation G.783;

û    E1 and F1 are taken from the OHA; optionally they may be relayed;

û    D1-D3 are taken from the MCF;

û    national use bytes and bytes reserved for future international stan-
dardization in the RSOH are either relayed or generated as
described in RecommendationG.783.

    When RST(1) is in a failure state described in º5.2.2:

û    A1, A2 and C1 are generated;

û     B1 is generated as described in RecommendationG.783;

û    E1 and F1 are taken from the OHA;

û    D1-D3 are taken from the MCF;

û    national use bytes and bytes reserved for future international stan-
dardization in the RSOH are generated as specified in
RecommendationG.783.

    When RST(1) is in OOF condition (but not in a failure state as described in
º5.2.2) all RSOH bytes may be relayed.

    SPI(2) converts the logical levels of the signal at reference point B(2) into
optical pulses at reference pointA(2). The SPI function must provide the
characteristics of the signal necessary to meet the required transmission and
network performance. Reference point A(2) physically corresponds to refer-
ence pointS in RecommendationsG.955 andG.956. The characteristics of
the optical signal at this reference point is given in RecommendationG.957.
Parameters related to the status of the transmitter are sent to SEMF through
reference pointS1. Parameters to be monitored are defined in º7.

5.2.1    Scrambling in the regenerator

    In order to clearly define the scrambling and descrambling processes,
a functional diagram of the signal path in the RST is shown in Figure5-3/
G.958, according to the algorithm reported in RecommendationG.709.

    The upper part shows the transmission side of RST. Firstly the com-
plete STM-N frame, including B1 byte computed on the previous frame, is
built, the STM-N frame is then scrambled, except for the first SOH row,
i.e.the first N┤9bytes, and finally the BIP-8 is computed over the entire
scrambled frame. The BIP-8 value will be included in the next frame as B1
byte.

    Similarly the central part shows the receive side of RST. Before
descrambling, frame alignment is searched or verified on the received STM-
N signal and BIP-8 is computed. Then the STM-N frame is descrambled,
except for the first SOH row, i.e.the first N ┤9bytes, and the RSOH is sub-
sequently used, including B1 byte.

    The lower part of the Figure 5-3/G.958 shows the entire STM-N
frame.

    This description is only functional, and does not imply any particular
physical implementation. The regenerator has to access only the RSOH
bytes, and in principle needs to descramble and scramble only these bytes.
Therefore data at C to be passed from RST(1) to RST(2) may actually be
passed transparently from B(1) to B(2) rather than being descrambled at
RST(1) and scrambled at RST(2).

5.2.2    Alarm indication signal (AIS)

    Under the failure cases mentioned in Recommendation G.783 (i.e.
loss of signal or loss of frame), resulting in a logical all ONES signal at ref-
erence pointC, the signal atC and valid RSOH added at B(2) results in an
MS-AIS. RecommendationG.783 specifies the delay to activate and deacti-
vate MS-AIS.

5.2.3    Medium dependent use of overhead bytes

    It is possible that in the future some bytes may be dedicated to func-
tions specific to a particular transmission medium. These bytes could be
taken from those reserved for national use and future international standard-
ization. Otherwise bytes already defined could be modified in their use to
include medium specific requirements.

    As an example the need for and implementation of a function to iden-
tify the signal direction in bi-directional optical transmission over a single
fibre is for further study.

5.2.4    Intra-office link

    Reduced functionalities to be used in intra-office link regenerator sec-
tions are for further study.

5.2.5    Management

    The general SDH control and management principles and the inter-
working with TMN illustrated in RecommendationG.784 apply to the
regenerator. The SDH management architecture, the communication net-
work structure among different network elements (NE) and a model for the
regenerator are shown in RecommendationG.784.

FIGURE 5-3/G.958 = 15,5 cm = 606



    The regenerator includes a SEMF. It contains a number of filtering functions
that translate primitive information, coming from the functional blocks, to
forms usable by the network management and vice versa. Some information
is not processed by a filtering function. The filtering functions used in a
regenerator are described in detail in RecommendationG.783. The possible
use of internal storage for performance parameters history retrieval, the abil-
ity of generating autonomous alarm reports on threshold crossings and the
possibility of setting externally the threshold values are described in
RecommendationG.784. In the regenerator an Agent is present in the
SEMF, which controls the exchange of information with other SDH network
elements or with the TMN for management purposes. Characteristics of the
Agent are given in RecommendationG.784.

    Messages are sent over the embedded control channel (ECC) that utilizes
DCC, i.e. D1-D3 bytes as the physical layer. The protocol stack used and the
message generation and termination methods are described in
RecommendationG.784.

    Messages are transmitted and received by the message communication
function (MCF), which is connected to the SEMF through Vreference point
and to a Qinterface when provided.

     Incoming bytes D1-D3 are extracted by the RST function and routed to the
MCF through reference pointN. Relayed messages and locally generated
messages are sent through reference pointN as D1-D3 bytes to the
RSTfunction, which inserts them into the RSOH of the outgoing STM-N
frame.

    Two interfaces towards elements external to the SDH network may be used.
The Qinterface can connect the regenerator to an operation system/media-
tion device (OS/MD). The Finterface may be used to connect the regenera-
tor to a workstation for monitoring and maintenance purposes.

5.3    Regenerator interfaces

    The regenerator has the following interfaces:

û    S reference point on both transmitting fibres: the interface character-
istics at this reference point are specified in
RecommendationG.957;

û    R reference point on both receiving fibres: the interface characteris-
tics at this reference point are specified in
RecommendationG.957;

û    interface for orderwire channel: to be defined;

û    interface for user channel: to be defined;

û    use of a Q interface may be foreseen in some applications;

û    F interface to a workstation: its characteristics are under study.

6    General characteristics of synchronous optical line systems

6.1    Synchronization and timing signal

    The structure and details of synchronization and timing signals are
described in RecommendationG.782.

6.2    Regenerator timing

    Figure 6-1/G.958 illustrates the timing functions for regenerators. The
regenerator timing generator (RTG) includes an internal oscillator. In nor-
mal operation, the SPI function recovers the timing from the incoming
STM-Nsignal at reference pointA and passes the data and timing to RST at
reference pointB, and passes the timing signal also to the RTG function at
reference pointT1. The RTG function provides the timing signal to the out-
going STM-N signal at reference pointT0. The directionality of the timing
signals is maintained.

    When transmitting MS-AIS the RTG shall provide timing for the out-
going STM-N signal at reference pointT0 using the internal oscillator. The
long-term frequency stability of the internal oscillator in free-running mode
shall be equal to or better than ▒20ppm.

    The RTG and SPI functions must accommodate timing from an
incoming MS-AIS signal.

6.3    Jitter performance

    This section deals with jitter requirements for optical interfaces at the
STM-N levels as defined in RecommendationG.707. Specifications for
multiplex jitter and wander at STM-N and RecommendationG.703 inter-
faces are described in RecommendationsG.782 andG.783.



    The purpose of the jitter requirements in these sections is to control
the accumulation of jitter within SDHline systems. SDH line equipment jit-
ter specifications are organized into limits for the following: jitter genera-
tion, jitter transfer and jitter tolerance.

6.3.1    Jitter generation

    Jitter generation is defined as the amount of jitter at the STM-N out-
put of SDH equipment.

     An SDH regenerator shall not generate more than 0.01 UI RMS jitter,
with no jitter applied at the STM-Ninput. The measurement bandwidth and
technique are under study.

6.3.2    Jitter transfer

    Jitter transfer specification applies only to SDH regenerators.

    The jitter transfer function is defined as the ratio of jitter on the output
STM-N signal to the jitter applied on the input STM-N signal versus fre-
quency.

    The jitter transfer function of an SDH regenerator shall be under the
curve given in Figure 6-2/G.958, when input sinusoidal jitter up to the mask
level in Figure 6-3/G.958 is applied, with the parameters specified for Type
A in Table1/G.958 for each bit rate.

FIGURE 6-2/G.958 = 7 cm



    If an SDH regenerator meets the jitter transfer specification for TypeB, it is
classified as Type B regenerator.



6.3.3    Jitter tolerance

    Jitter tolerance is defined as the peak-to-peak amplitude of sinusoidal
jitter applied on the input STM-Nsignal that causes a 1dB optical power
penalty at the optical equipment. Note that this is a stress test to ensure that
no additional penalty is incurred under operating conditions. This technique
is described in Supplement No.3.8 of the O-Series Recommendations.

FIGURE 6-3/G.958 = 7,5 cm = 293



    SDH equipment shall tolerate, as a minimum, the input jitter applied accord-
ing to the mask in Figure6-3/G.958, with the parameters specified in
Table2/G.958 for each bit rate.

    In a line system using Type A regenerators, the SDH regenerators and termi-
nal shall meet TypeA jitter tolerance specification. In a line system using
TypeB regenerators, the SDH regenerators and terminals shall meet TypeB
jitter tolerance specifications. In this case a terminal or regenerators meeting
TypeA jitter tolerance specifications may also be used. In a line system
without regenerators, the SDH terminal shall meet either TypeA or TypeB
jitter tolerance specifications. The use of regenerators of TypeA and TypeB
within the same line system is for further study.



6.4    Error performance

    The synchronous line systems specified in this Recommendation
should meet the relevant performance objectives of RecommendationG.821
under the worst environmental conditions. In particular they are required to
provide at least error performance in accordance with ôsection quality
classification1ö, defined in RecommendationG.821.

6.5    Availability and reliability

    For further study.

6.6    Environmental conditions

    For further study.

6.7    Laser safety

    For safety considerations, according to Reference [1] or national reg-
ulations, it may be necessary to provide for an automatic laser shutdown
(ALS) facility of the laser in case of cable break.

    This function is considered as optional.

    Appendix II shows the required functionality of automatic laser shut-
down when implemented.

    In case automatic laser shutdown facility is implemented the follow-
ing Command, Configuration & Provisioning information will flow over the
S1 reference point(refer to Table5-12/G.783):



    If automatic laser shutdown is implemented it should not impair fault
sectionalization capability in case of loss of signal at the transmitter or the
receiver due to causes other than a cable break.

7    Operational overview

7.1    Overview

    The operation, administration and maintenance features of digital
synchronous line systems should be designed in accordance with
RecommendationsM.20 (Maintenance philosophy for telecommunication
networks), M.30 (Principles for a telecommunications management net-
work) andG.784 (SDH management).

     In particular management principles should be based on the concepts
defined in RecommendationM.30:

û    functional organization of management functions (configuration,
performance, faults),

û    functional description of network elements in managed objects.

    The synchronous line system may be considered from the point of
view of management as a SDHManagement Sub-network (SMS as defined
in RecommendationG.784). The architecture, embedded control channel
(ECC) functions, information model and ECC protocols should therefore
conform to the specifications given in RecommendationG.784.

    In particular the information model should follow the specifications
given in RecommendationG.784.

    It should also be noted that synchronous line systems defined in this
Recommendation should provide autonomous management functions (per-
formance monitoring, fault location, alarm generation) for early implemen-
tations where the connection to a TMN is not possible. The way in which
this could be done while retaining forward compatibility with the full
deployment of TMN features is for further study.

7.2    General management functions

    The synchronous line systems should provide the general manage-
ment functions described in RecommendationG.784.

7.3    Fault (maintenance) management

    The synchronous line systems should support the fault management
functions described in RecommendationG.784.

7.3.1    Alarm surveillance

    This section describes parameters which should be monitored in the
synchronous line systems. In general, these parameters are monitored to
assist with fault localization. They are not intended to act as the primary
indication of link failure.

7.3.1.1    Parameters to be monitored at the S1 reference point

7.3.1.1.1    Signal status (transmitter)

    This parameter should indicate whether the transmitter power level is
in the range specified in RecommendationG.957 for the defined application
code. It will therefore have two values: within range, and out of range. Some
form of hysteresis and integration time has to be provided (for further
study). It is recognized, that without the use of a coupler and additional
detector, the only parameter that can give an indication of the transmitter
output power is the current passing through the laser back-facet monitor
diode. Under certain fault conditions, the circuit controlling this current may
mask significant variations in the laser output power. The exact power level
at which this parameter takes on the value ôout of rangeö is not specified.
The purpose of monitoring this parameter is to indicate whether a serious
fault exists in the transmitter.

7.3.1.1.2    Loss of incoming signal

    This parameter should take on the value ôincoming signal absentö
when the incoming power level at the receiver has dropped to a level which
is lower than that required to cause a BER of1 in 10-3. The purpose of mon-
itoring this parameter is to indicate either:

i)    transmitter failure,

ii)    optical path break.

7.3.1.1.3    Laser bias

    This parameter should be used to monitor the bias current of the laser
of the transmitter. The purpose of monitoring this parameter is to indicate
laser degradation well in advance of catastrophic failure of the link. The
value at which this parameter takes on the value ôbias out of limitsö is not
specified.

7.3.1.1.4    Laser temperature

    This parameter can have the values ôtemperature within range/tem-
perature out of rangeö. The purpose of monitoring this parameter is to indi-
cate failure of the transmitter temperature control circuitry. The value at
which this parameter takes on the value ôtemperature out of rangeö is not
specified.

7.3.1.2    Parameters to be monitored at the S2 reference point

    The corresponding requirements are contained in Recommendation
G.783.

7.3.2    Testing

7.3.2.1    Loopbacks

    It is considered that the loss of signal indications at the receiver and at
the transmitter provide sufficient resolution for practical fault sectionaliza-
tion and that loopbacks, optical or otherwise, are not necessary.

    The need for test points or loopbacks for testing purposes is under
study.

7.3.3    External events

    This point concerns the case where there is a need to monitor through
the synchronous line system site related alarms (door opening or fire in an
unmanned station,etc.) or more generally a non-SDH network element. The
corresponding implementation and requirements are under study.

7.4    Performance management

    The synchronous line systems should support the performance man-
agement functions described in RecommendationG.784.

    These functions should be implemented using information flows at
reference points S1 and S2 and filtering functions described in
RecommendationG.783.

7.5    TMN interfaces

    Synchronous line systems should provide at least one interface at each
end conforming to RecommendationG.773.

7.6    Orderwire

    The E1 byte may be used for conferencing between line terminal sta-
tions and/or regenerator stations. The E2byte may be used for express
point-to-point communication between terminal stations.

    The definition of orderwire ports and associated signalling procedures
is not in the scope of this Recommendation.

APPENDIX I

(to Recommendation G.958)

Implementation of the cid immunity measurement

    Summary

    Alternating digital signal patterns may be used to verify the adequacy
of timing-recovery and low-frequency performance of STM-N equip-
ments.

    Appropriate pattern sequences are defined below and in FigureI-1/G.958.

    This test does not attempt to simulate conditions which may occur under
anomalous operating conditions to which the equipment may be subjected.

    Description

    The specific test patterns are made up of consecutive blocks of data of four
types:

a)    all 1s (zero timing content, high average signal amplitude);

b)    pseudo-random data with a mark-density ratio of 1/2;

c)    all 0s (zero timing content, low average signal amplitude);

d)    a data block consisting of the first row of section overhead bytes for
the STM-N system under test.

    The test pattern is shown in Figure I-1/G.958 where the regions A, B,
C and D are identified.

FIGURE I-1/G.958 = 6,5 cm = 254



    The duration of the zero-timing-content periods A and C is made equal to
the longest like-element sequences expected in the STM-N signal. A value
of 9bytes (72bits) is provisionally proposed for this.

    The duration of the pseudo-random periods should allow recovery of both
the zero base line offset of the signal and of the timing recovery circuit fol-
lowing occurrence of theA and Cperiods. Therefore it should be longer
than the longest time constant in the regenerator. In the case of a PLL based
clock extraction this could give a value of the order of 10000bits. Taking
into account possible limitations of test equipment a minimum value of
2000bits is considered acceptable.

    The content of the pseudo-random section should be generated by a scram-
bler having the same polynomial as defined in CCITT
RecommendationG.709. Ideally, the scrambler should ôfree-runö i.e.the
beginning of the pattern should be uncorrelated with the frame alignment
section. This arrangement will ensure that the system experiences the worst
possible phasing of the PRBS at some point during the course of the test.
However it is recognized that test equipment limitations may preclude the
use of a free running scrambler. Hence it may be necessary to specify a
worst-case phasing of the PRBS. This is for further study.

    The D period is defined as the first row of the section overhead of the STM-
N signal, including valid C1bytes (consecutive binary numbers).

    It is recommended that this test be applied to SDH systems at any appropri-
ate point in time during the design or production phase. This would be done
to demonstrate the ability of both timing-recovery and decision circuits ade-
quately to handle worst-case SDH signals.

    It should be emphasized that the test pattern may be rejected by or cause
malfunction of certain equipments because, for example, the occurrence of
the frame alignment bytes within the pattern. The test should therefore only
be used for assemblies not so affected, such as timing recovery units,
receiver amplifier chains,etc.

     However, the test may be applicable in certain cases at the available user
ports. It is not proposed as a general acceptance test which might require
special defined access ports and connection arrangements within the equip-
ment.



APPENDIX II

(to Recommendation G.958)

Description of automatic laser shutdown (ALS) capability
in case of cable break

FIGURE II-1/G.958 = 5,5 cm =,293



    If a cable break happens at point A, the consecutive loss of signal at RX2 is
used to cut TX2 which is the adjacent transmitter in the opposite direction.
This in turn leads to a loss of signal in RX1 which switches off TX1.

    For test and monitoring purposes it is possible to override the shutdown
mechanism by switching on the laser manually.

    When the cable has been repaired either an automatic or a manual action
according to FigureII-2/G.958, at TX1 or TX2 is necessary to restore cor-
rect transmission.

    The response time of the transmitter/receiver combination, measured from
receiver input (pointR) to transmitter output (pointS) should be less than
0.85seconds. This response time of 0.85seconds refers to the time differ-
ence between the moment light enters the receiver at pointR and the
moment the transmitter starts light emitting at pointS in case the transmitter
is in the shut down situation.

    ôManual restartö or ôManual restart for testöcan only be activated when the
laser is shut down.

    In case 1+1 protection switching is implemented a working channel receiver
should shut down a working channel transmitter. Similarly, a protection
channel receiver should shut down a protection channel transmitter.

 FIGURE II-2/G.958 = 23,5 cm = 919



Reference

[1]    IEC 825 Standard Radiation safety of laser products equipment, classifi-
cation, requirements and user's guide.











INTERNATIONAL  TELECOMMUNICATION  UNION







CCITT    G.958

THE  INTERNATIONAL
TELEGRAPH  AND  TELEPHONE
CONSULTATIVE  COMMITTEE











DIGITAL  NETWORKS,  DIGITAL  SECTIONS

AND  DIGITAL  LINE  SYSTEMS





DIGITAL  LINE  SYSTEMS  BASED  ON  THE
SYNCHRONOUS  DIGITAL  HIERARCHY
FOR  USE  ON  OPTICAL  FIBRE  CABLES









Recommendation  G.958







Geneva, 1990







FOREWORD

    The CCITT (the International Telegraph and Telephone Consultative
Committee) is the permanent organ of the International Telecommu-
nication Union (ITU). CCITT is responsible for studying technical,
operating and tariff questions and issuing Recommendations on them
with a view to standardizing telecommunications on a worldwide
basis.

    The Plenary Assembly of CCITT which meets every four years,
establishes the topics for study and approves Recommendations pre-
pared by its Study Groups. The approval of Recommendations by the
members of CCITT between Plenary Assemblies is covered by the
procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988).

    Recommendation G.958 was prepared by Study Group XV and was
approved under the Resolution No. 2 procedure on the 14th of December
1990.





___________________





CCITT  NOTE

    In this Recommendation, the expression "Administration" is used for
conciseness to indicate both a telecommunication Administration and
a recognized private operating agency.



















πITU1990

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