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Recommendation G. 782

Recommendation G. 782

TYPES  AND  GENERAL  CHARACTERISTICS  OF  SYNCHRONOUS
DIGITAL  HIERARCHY  (SDH)  MULTIPLEXING  EQUIPMENT

    The CCITT,

considering

    (a)    that Recommendations G.707, G.708 and G.709 form a coherent
set of specifications for the synchronous digital hierarchy (SDH) and the
network node interface (NNI);

    (b)    that Recommendation G.781 gives the structure of Recommendations
on multiplexing equipment for the SDH;

    (c)    that Recommendation G.783 specifies the characteristics of SDH mul-
tiplexing equipment functional blocks;

    (d)    that Recommendation G.784 addresses management aspects of the
SDH;

    (e)    that Recommendation G.957 specifies characteristics of optical inter-
faces for use within the SDH;

    (f)    that Recommendation G.958 specifies digital line systems based on
the SDH for use on optical fibre cables;

    (g)    that Recommendation G.703 describes electrical interfaces for use
within the SDH,

recommends

    that SDH multiplexing equipment should have general characteristics
as described in this Recommendation.

1    Introduction

1.1    Scope

    Recommendation G.781 gives the structure of Recommendations on
SDH multiplexers. This Recommendation gives an overview of the func-
tions of SDH multiplexing equipment, examples of various multiplexing
equipment types and general performance requirements.

    The possibilities of add/drop features, mixed payloads and flexible
tributary/channel associations in SDH multiplexers make it difficult to pro-
vide a Recommendation which is unambiguous while remaining generic
enough not to constrain implementation. To overcome these difficulties, the
"functional reference model" approach has been adopted. Therefore this
series of Recommendations describes the equipment in terms of various
functional blocks. This logical partitioning is used to simplify and general-
ize the description. It does not imply any physical partitioning or implemen-
tation.

    Only external interface requirements will be specified. For payloads
these will conform to either STM-N (according to Recommendations
G.707, G.708 and G.709) or Recommendation G.703. The interface to the
transmission management network (TMN) will conform to Recommenda-
tion G.773. The points between function blocks exist only as logical refer-
ence points and not as internal interfaces;  there is therefore no interface
description or interface specification associated with these points.

1.2    Abbreviations

AIS        Alarm indication signal

AU        Administrative unit

AUG    Administrative unit group

DCC    Data communications channel

FEBE    Far end block error

FERF    Far end receive failure

HPA    Higher order path adaptation

HPC        Higher order path connection

HPT        Higher order path termination

LPA        Lower order path adaptation

LPC     Lower order path connection

LPT        Lower order path termination

MCF    Message communications function

MSOH    Multiplex section overhead

MSP    Multiplex section protection

MST     Multiplex section termination

MTPI    Multiplexer timing physical interface

MTS     Multiplexer timing source

NNI        Network node interface

NOMC    Network operators maintenance channel

PDH    Plesiochronous digital hierarchy

PI        Physical interface

POH    Path overhead

RSOH    Regenerator section overhead

RST        Regenerator section termination

SA        Section adaptation

SDH    Synchronous digital hierarchy

SEMF    Synchronous equipment management function

SOH    Section overhead

SPI        SDH physical interface

STM    Synchronous transport module

TMN    Telecommunications management network

TU        Tributary unit

TUG    Tributary unit group

VC        Virtual container

1.3    Definitions

    Note ù The following definitions are relevant in the context of SDH-
related Recommendations.

1.3.1    Administrative unit (AU)

    See Recommendation G.708.

1.3.2    Administrative unit group (AUG)

    See Recommendation G.708.

1.3.3    Data communications channel (DCC)

    See Recommendation G.784.

1.3.4    higher order path

    In an SDH network, the higher order (HO) path layers provide a
server network for the lower order (LO) path layers. The comparative terms
lower and higher refer only to the two participants in such a client/server
relationship. VC-1/2 paths may be described as lower order in relation to
VC-3 and VC-4 while the VC-3 path may be described as lower order in
relation to VC-4.

1.3.5    higher order path adaptation (HPA)

    The HPA function adapts a lower order VC (VC-1/2/3) to a higher
order VC (VC-3/4) by processing the TU pointer which indicates the phase
of the VC-1/2/3 POH relative to the VC-3/4 POH and assembling/disassem-
bling the complete VC-3/4.

1.3.6    higher order path connection (HPC)

    The HPC function provides for flexible assignment of higher order
VCs (VC-3/4) within an STM-N signal.

1.3.7    higher order path termination (HPT)

    The HPT function terminates a higher order path by generating and
adding the appropriate VC POH to the relevant container at the path source
and removing the VC POH and reading it at the path sink.

1.3.8    Lower order path

    See higher order path above.

1.3.9    lower order path adaptation (LPA)

    The LPA function adapts a PDH signal to an SDH network by map-
ping/de-mapping the signal into/out of a synchronous container. If the signal
is asynchronous, the mapping process will include bit level justification.

1.3.10    lower order path connection (LPC)

    The LPC function provides for flexible assignment of lower order
VCs in a higher order VC.

1.3.11    lower order path termination (LPT)

    The LPT function terminates a lower order path by generating and
adding the appropriate VC POH to the relevant container at the path source,
removing the VC POH and reading it at the path sink.

1.3.12    Message communications function (MCF)

    See Recommendation G.784.

1.3.13    multiplex section overhead (MSOH)

    The MSOH comprises rows 5 to 9 of the SOH of the STM-N signal.

1.3.14    multiplex section protection (MSP)

    The MSP function provides capability for switching a signal between
and including two MST functions, from a working to a protection section.

1.3.15    multiplex section termination (MST)

    The MST function generates the MSOH in the process of forming an
SDH frame signal and terminates the MSOH in the reverse direction.

1.3.16    multiplexer timing physical interface (MTPI)

    The MTPI function provides the interface between an external syn-
chronization signal and the multiplexer timing source.

1.3.17    multiplexer timing source (MTS)

    The MTS function provides timing reference to the relevant compo-
nent parts of a multiplexing equipment and represents the SDH network ele-
ment clock.

1.3.18    Path overhead (POH)

    See Recommendation G.708.

1.3.19    regenerator section overhead (RSOH)

    The RSOH comprises rows 1 to 3 of the SOH of the STM-N signal.

1.3.20    regenerator section termination (RST)

    The RST function generates the RSOH in the process of forming an
SDH frame signal and terminates the RSOH in the reverse direction.

1.3.21    section adaptation (SA)

    The SA function processes the AU-3/4 pointer to indicate the phase of
the VC-3/4 POH relative to the STM-N SOH and assembles/disassembles
the complete STM-N frame.

1.3.22    Synchronous digital hierarchy (SDH)

    See Recommendation G.707.

1.3.23    synchronous equipment management function (SEMF)

    The SEMF converts performance data and implementation specific
hardware alarms into object-oriented messages for transmission over the
DCC(s) and/or a Q interface. It also converts object-oriented messages
related to other management functions for passing across the Sn reference
points.

1.3.24    Section overhead (SOH)

    See Recommendation G.708.

1.3.25    SDH physical interface (SPI)

    The SPI function converts an internal logic level STM-N signal into
an STM-N line interface signal.

1.3.26    Synchronous transport module (STM)

    See Recommendation G.708.

1.3.27    Telecommunications management network (TMN)

    See Recommendation M.30.

1.3.28    Tributary unit (TU)

    See Recommendation G.708.

1.3.29    Tributary unit group (TUG)

    See Recommendation G.708.

1.3.30    Virtual container (VC)

    See Recommendation G.708.

2    Overview of equipment functions

2.1    Multiplexing method

2.1.1    Generalized logical blocks

    Figure 2-1/G.782 is a generalized Multiplexer Logical Block Dia-
gram. It illustrates the steps that are required to assemble various payloads
and multiplex them into an STM-N output. It does not represent a useful or
practical network function. Examples of some configurations that may be
deployed are given in º3.

    The only function blocks that are payload specific are the physical
interface/path adaptation blocks used at the G.703 interfaces; all other func-
tions are non-payload specific. Therefore all operations functions, except
those associated with G.703 interfaces, are payload independent. New pay-
load types can be added by providing a new interface function; all other
parts of the system will be unaffected.

    A brief description of the signal flow between a Recommendation
G.703 interface and the STM-N output is provided in ºº2.1.2 and 2.1.3.
Description of functions performed by each of the logical blocks in
Figure2-1/G.782 is provided in Recommendations G.783 and G.784. Fur-
ther descriptions of the synchronous equipment management function
(SEMF) and message communications function (MCF) are given in º2.2
and descriptions of the multiplexer timing source (MTS) and multiplexer
timing physical interface (MTPI) are given in º4.

FIGURE 2-1/G.782 = 18,5 cm



2.1.2    Signal flow  G.703 input to STM-N output: multiplexing



    Physical interface/
lower order path
adaptation

Provides the appropriate G.703 interface and maps
the payload into the container as specified in
RecommendationG.709.

    Lower order path
termination

Adds the VC path overhead (VC-POH).

    Lower order path
connection

Allows flexible assignment of the VC-1/2 within the
VC-3/4.

    Higher order path
adaptation

Processes the TU pointer to indicate the phase of the
VC-1/2 POH relative to the VC-3/4 POH and assem-
bles the complete VC-3/4.

    Higher order path
termination

Adds the VC-3/4 path overhead.

        Higher order
path connec-
tion

Allows flexible assignment of the VC-3/4 within the
STM-N.





    Section adaptation

Processes the AU-3/4 pointer to indicate the phase of
the VC-3/4 POH relative to the STM-N SOH. Byte-
multiplexes the AU Groups (AUGs) to construct the
complete STM-N frame.

    Multiplex section
protection

Provides capability for branching the signal onto
another line system for protection purposes.

    Multiplex section
termination

Generates and adds rows 5 to 9 of the SOH.

    Regenerator section
termination

Generates and adds rows 1 to 3 of the SOH; the STM-
N signal is then scrambled except for row 1 of the
SOH.

    SDH physical inter-
face

Converts the internal logic level STM-N signal into
an STM-N interface signal. This may be an in-station
electrical signal, an in-station optical signal or an
inter-station optical signal.

2.1.3    Signal flow STM-N input to G.703 output: demultiplexing



    SDH physical inter-
face

Converts the interface signal into an internal logic
level and recovers timing from the line signal.

    Regenerator section
termination

Identifies the STM-N frame word, descrambles the
signal, and processes rows 1 to 3 of the SOH.

     The remaining operations are the inverse of those performed when multi-
plexing except that the C-1/2 interface function must provide a buffer store
and smoothing circuit to attenuate the clock jitter caused by the multiplex
process, pointer moves and bit stuffing (if applicable).

2.2    Operations, administration, maintenance and provisioning (OAM&P)

2.2.1    Overhead applications

    Recommendation G.708 specifies bandwidth allocated within the
SDH frame structure for various control and maintenance functions. Two
types of overhead are identified: Virtual Container Path Overhead (VC-
POH) and Section Overhead (SOH).

2.2.1.1    POH application

    Details of the functions provided by the POH are contained in Recom-
mendations G.708 and G.709.

    The VC-POH is generated and terminated at the point where the pay-
load is assembled or disassembled. Itisused for end to end monitoring of
the payload and may transit several multiplex and line systems. Some of the
VC-POH is completely payload independent, while other parts of the VC-
POH are used in specific ways according to the type of payload. In all cases,
the VC-POH is independent of user information. Thus it may be monitored
at any point within an SDH network to confirm network operation.

2.2.1.2    SOH application

    The section overhead (SOH) is subdivided into regenerator SOH
(RSOH) comprising rows 1 to 3 and multiplex SOH (MSOH) comprising
rows 5-9. The MSOH is accessible only at terminal equipments, whereas the
RSOH is accessible at both terminal equipments and regenerators.

    Details of the functions provided by RSOH and MSOH are given in
Recommendation G.708. These functions include performance monitoring
and section maintenance and operations functions.

    In order to permit regenerators to read from and write to the RSOH
without disrupting the primary performance monitoring, the RSOH is
excluded from the B2 (BIP-24) calculation. Since B1 is recomputed at each
regenerator, fault sectionalization is simplified.

     The set of bytes E1, E2, F1 and D1 to D12 is referred to as the net-
work operators maintenance channel (NOMC).

2.2.1.3    Protection of the Network operators maintenance channel (NOMC)

    In a 1+1 protection system, the NOMC will be on both channels. In a
1:n protection system, the NOMC will be on only one channel, normally
channel 1. If channel 1 fails, the NOMC will be switched to the protection
channel, along with traffic.

     It should be noted that failure of channel 1 will result in the loss of the
NOMC under the following conditions:

i)    the protection channel is carrying extra traffic and a FORCED
switch is in operation;

ii)    the protection channel is LOCKED OUT.

    Loss of the NOMC under conditions i) and ii) above, and in the case
of diversely routed protection spans, requires further study.

    Bytes K1 and K2 shall be transmitted on the protection channel. In
addition, they may also be transmitted on working channels. The receiver
must be able to ignore bytes K1 and K2 on any of the working channels.

2.2.1.4    Maintenance signals

    The maintenance signals defined in Recommendation G.709 º2.3.1
at the section layer are section AIS and far end receive failure (FERF). At
the path layer, Recommendation G.709 º2.3.2 defines path AIS and path
status information in the form of path FERF and far end block error (FEBE).
These path maintenance signals apply at both higher order and lower order
path level. Figure 2-2/G.782 illustrates the layer-to-layer and peer-to-peer
maintenance interaction provided in the SDH overhead.

2.2.1.5    Loss of signal at regenerators

    If a regenerator loses its input signal, a standby clock is activated and
a signal containing valid RSOH and MS-AIS is transmitted downstream.
This enables the NOMC functions carried by the RSOH to be activated if
required.

2.2.2    TMN access

    SDH multiplexers should provide interfaces for messages to or from
the TMN via either the DCC or a Qinterface or both. Messages arriving at
the interface not addressed to the local multiplex should be relayed to the
appropriate Q or DCC interface. The TMN can thus be provided with a
direct logical link to any SDH equipment via a single Q interface and the
interconnecting DCCs.

Figure 2-2/G.782 = 22 cm



2.2.2.1    Q -interface

    When access to the TMN is provided by a Q-interface, the interface
will conform to RecommendationG.773. A choice has to be made between
the B1, B2 and B3 protocol suites specified in that Recommendation.

2.2.2.2    Data communications channel (DCC)

    The use of the DCC is dependent on the network operator's mainte-
nance strategy and the specific situation. It may not always be required as it
is possible to carry out the required functions by other means.

    There are two ways of using the DCC:

i)    use of the D1 to D3 bytes located in the RSOH (DCCR) and accessi-
ble at regenerators and other NEs;

ii)    use of the D4 to D12 bytes located in the MSOH (DCCM) and not
accessible at regenerators. The specific use of the D4 to D12 bytes
is for further study.

    These channels are message based and provide communications
between network elements. They can be used to support communications
between sites and the TMN. Two examples are given in Figures 2-3/G.782
and2-4/G.782.

Figure 2-3/G.782= 10 cm



Figure 2-4/G.782 = 13 cm



2.2.2.3    Functionalities

2.2.2.3.1    Synchronous equipment management function (SEMF)

    This converts performance data and implementation specific hard-
ware alarms into object-oriented messages for transmission on the DCC(s)
and/or a Q-interface. It also converts object-oriented messages related to
other management functions for passing across the Sn reference points.

2.2.2.3.2    Message communications function (MCF)

    This function receives and buffers messages from the DCC(s), Q-and
F-interfaces and SEMF. Messages not addressed to the local site are relayed
to one or more outgoing DCC(s) in accordance with local routing proce-
dures and/or Q-interface(s). The function provides layer 1 (and layer 2 in
some cases)  translation between a DCC and a Q-interface or another DCC
interface.

2.2.3    Order-wire

    Use of the E1 and/or E2 bytes for providing an order-wire is optional.
Byte E1 can be accessed at all regenerators and terminals to provide a local
order-wire. Byte E2 can only be accessed at terminals and may be used to
provide an order-wire between terminal sites.

2.2.4    User channel

    Use of the F1 byte for providing a special user channel is optional.
Byte F1 can be accessed at all regenerators and terminals.

2.3    STM-N protection switching

    Protection switching of a signal provides a capability, using equip-
ment redundancy and switching action, such that in the event of the failure
of a ôworkingö channel, the signal is available via a protection channel.

     The use of protection switching is dependent on the network opera-
tor's maintenance strategy. It may not always be required. If required on
SDH line systems, redundancy is provided for functions and physical
medium between, and including, two MST functions, i.e. for the multiplex
section. Thus, the Multiplex Section Protection (MSP) function included in
multiplexing equipment provides protection for the STM-N signal against
failures within a multiplex section.

    The MSP function communicates with the corresponding far end
MSP function to coordinate the switch action, via a bit-oriented protocol
defined for the K bytes of the MSOH. It also communicates with the SEMF
for automatic and manual switch control. Automatic protection switching is
initiated based on the condition of the received signals. Manual protection
switching provides both local and remote switching from commands
received via the SEMF. The details of switch initiation, control and opera-
tion are described in Recommendation G.783.

2.3.1    MSP architectures

    Two MSP architectures are defined: 1+1 (one plus one) and 1:n
(one forn).

2.3.1.1    1+1 architecture

    In a 1+1 MSP architecture shown in Figure 2-5/G.782, the STM-N
signal is transmitted simultaneously on both multiplex sections, designated
working and protection sections; i.e. the STM-N signal is permanently con-
nected (bridged)  to the working and protection sections at the transmitting
end. The MSP function at the receiving end monitors the condition of the
STM-N signals received from both sections and connects (selects) the
appropriate signal. Due to permanent bridging of the working channel, the
1+1 architecture does not allow an unprotected extra traffic channel to be
provided.

Figure 2-5/G.782= 4,5 cm



2.3.1.2    1:n architecture

    In a 1:n MSP architecture shown in Figure 2-6/G.782, the protection sec-
tion is shared by a number of working channels; the permitted values for n
are 1 through 14. At both ends, any one of the n STM-N channels or an extra
traffic channel (or possibly a test signal) is bridged to the protection section.
The MSP functions monitor and evaluate the conditions of the received sig-
nals and perform bridging and selection of the appropriate STM-N signals
from the protection section.

    Note that 1:1 architecture is a subset of 1:n (n=1) and may have the
capability to operate as 1+1 for interworking with a 1+1 architecture at
the other end.

2.3.2    Operation modes

    The MSP may operate either bi-directionally or uni-directionally and
in either a revertive or non-revertive mode, depending on the network man-
agement.

    In bi-directional operation, the channel is switched to the protection
section in both directions, and switching of only one direction is not
allowed. In uni-directional operation, the switching is complete when the
channel in the failed direction is switched to protection.

    In revertive mode of operation, the working channel is switched back
to the working section, i.e. restored, when the working section has recov-
ered from failure. In non-revertive mode of operation, the switch is main-
tained even after recovery from failure. For 1:n architectures, only
revertive mode is allowed.

2.4    Integrated interfaces

    Section 3 describes multiplexer configurations for multiplexer func-
tions that may be integrated with the line terminating function. It is envis-
aged that such direct SDH interfaces will also be provided on other network
elements such as digital cross-connects or digital switches. These interfaces
may be either intra-station or inter-station.

Figure 2-6/G.782 = 10,5 cm



3    Multiplexing equipment types

    This section provides some examples of equipment configurations
and network applications for SDH equipment, based on the generalized
multiplexer logical block diagram (Figure 2-1/G.782). The description of
these examples is generic and no particular physical partitioning of func-
tions is implied. The examples are not a complete set; other configurations
may be useful in other network applications.

3.1    Type I (Figure 3-1/G.782)

    This provides a simple G.703 to STM-N multiplex function. For
example, 63┤2048 kbit/s signals could be multiplexed to form an STM-1
output or, 12┤44736 kbit/s signals could be multiplexed to form an STM-
4. The location of each of the tributary signals in the aggregate signal is
fixed and dependent on the multiplex structure chosen.

3.2    Type Ia (Figure 3-2/G.782)

    The ability to provide flexible assignment of an input to any position
in the STM-N frame can be provided by including a VC-1/2 and/or VC-3/4
path connection function.

3.3    Type II (Figure 3-3/G.782)

    This provides the ability to combine a number of STM-N signals into
a single STM-M signal. For example, four STM-1 signals (from multiplex-
ers or line systems) could be multiplexed to provide a single STM-4 signal.
The location of each of the VC-3/4s of the STM-N signals is fixed in the
aggregate STM-M signal.

3.4    Type IIa (Figure 3-4/G.782)

    The ability to assign flexibly a VC-3/4 on one STM-N to any position
in the STM-M frame can be provided by including a VC-3/4 path connec-
tion function.

3.5    Types IIIa and IIIb

    These provide the ability to access any of the constituent signals
within an STM-N signal without demultiplexing and terminating the com-
plete signal. The interface provided for the accessed signal could be either
according to G.703 or an STM-M (M<N). These are described in more
detail below.

3.5.1    Type IIIa (Figure 3-5/G.782)

    Figure 3-5/G.782 illustrates the case of a Type IIIa multiplexer where
access to the constituent signal is via a G.703 interface.

    The higher order path connection function allows the VC-3/4 signals
within the STM-N signal to be either terminated locally or re-multiplexed
for transmission. It also allows the VC-3/4 signals generated locally to be
assigned to any vacant position in the STM-N output. The lower order path
connection function allows the VC-1/2 signals (from the C-3/4 terminated
by the VC-3/4 POH function) to be terminated locally or directly re-multi-
plexed back into an outgoing C-3/4. The lower order path connection func-
tion also allows the locally generated VC-1/2 signals to be routed to any
(vacant) position on any outgoing C-3/4.

3.5.2    Type IIIb (Figure 3-6/G.782)

    Figure 3-6/G.782 illustrates the case of a Type IIIb multiplexer where
access to the constituent signal is via an STM-M interface.

    This type has some additional functions to those required for Type
IIIa, namely those for demultiplexing the STM-N signal into VC-1/2 sig-
nals.

3.6    Type IV (Figure 3-7/G.782)

    This provides the translation function to allow C-3 payloads in a VC-
3 to transit a network that uses SDH equipment which cannot support AU-3.
Information on interworking is given in Recommendation G.708.

Figure 3-1/G.782= 14 cm



Figure 3-2/G.782= 14 cm



Figure 3-3/G.782 = 14 cm



Figure 3-4/G.782 = 14 cm



Figure 3-5/G.782 = 14 cm



Figure 3-6/G.782 = 19,5 cm



Figure 3-7/G.782 = 14 cm



4    General performance requirements

4.1    Timing and synchronization overview

4.1.1    General

    The SDH has been designed to operate as a synchronized network,
accommodating G.811 plesiochronous operation and network wander by a
scheme of pointer adjustments. SDH network jitter/wander performance is
determined by SDH internal and external clock performance, network out-
put wander at synchronization interfaces, and SDH line system jitter/wan-
der. Pointer adjustment statistics, and related G.703 tributary output jitter/
wander, are determined by SDH network jitter/wander performance and the
design of the SDH demultiplexer at the boundary of an SDH network. This
section provides general principles and applications guidelines for synchro-
nization of SDH multiplexing equipment. Detailed timing and synchroniza-
tion specifications are given in Recommendation G.783.

    Figure 2-1/G.782 includes the following functional blocks related to
timing and synchronization:

    MTPI    ù    provides the appropriate interface for G.703 based synchroni-
zation inputs/outputs;

    MTS    ù    provides the internal timing signals to the multiplexer equipment
based on either an external input or internal oscillator.

4.1.2    Guidelines for synchronization

4.1.2.1    SDH network application

    An SDH network application is one in which at least one of the tribu-
tary signals is an SDH signal, thus requiring pointer processing in the TU
and/or AU paths. Two examples of SDH network applications are given
below:

ù    SDH network comprising externally synchronized SDH network
elements containing internal clocks. The specification of the qual-
ity of these clocks is in the province of SG XVIII;

ù    SDH network including network elements for which the transmit
clock for a particular signal is derived directly from the corre-
sponding receive clock (loop timing). Loop timing is typically
used in small terminal stations, particularly in star networks, where
an external synchronization reference interface is not available;
e.g. access networks and equipment in customer premises.

    All SDH network elements whose synchronization is traceable to a
primary reference clock(s), shall be integrated into existing synchronization
hierarchies. Primary reference and slave clocks are specified in
RecommendationsG.811 andG.812, respectively.

    NoteùSpecification of network output wander requirements at syn-
chronization interfaces is in the province of SG XVIII.

4.1.2.2    SDH point-to-point application

    An SDH point-to-point application is one in which all tributary sig-
nals are asynchronous or plesiochronous according to Recommendation
G.703, with no pointer processing in either the TU or AU paths. Synchroni-
zation is not required in this application but must be provided as soon as net-
working is extended beyond simple point-to-point.

4.1.2.3    External synchronization interfaces

    Timing reference in a network element can be derived from three
types of inputs:

i)    G.703 external synchronization interface (for 2048 kHz, Recom-
mendation G.703 applies; the case of 1544 kHz is for further
study);

ii)    G.703 tributary interface (carrying reference synchronization);

iii)    STM-N interface.

    Depending on the type of network element, one or more timing refer-
ence inputs may be available. SDH equipment should have the ability to
switch automatically to another timing reference if the selected timing refer-
ence is lost. Timing reference is considered to be lost under the following
conditions:

ù    loss of signal on the selected timing reference interfaces;

ù    AIS on the selected timing reference interface.

    If the selected timing reference is an STM-N signal, switching to
another timing reference should only take place after it has been established
that any available protection switching of the STM-N and its terminating
circuitry has failed to recover the STM-N.

4.1.2.4    Loss of timing reference

    Loss of all incoming timing reference is a major fault calling for
immediate maintenance action. In cases where some traffic remains, a suffi-
cient timing accuracy can be maintained over a limited time period by using
a clock in holdover mode. The action taken by the synchronous multiplexer
under such conditions will depend on the network synchronization strategy.
The effect of this on national and international paths is in the province of SG
XVIII.

    In some cases, where loss of reference timing signal due to a loss of
the incoming signal results in loss of data from the network element, the
only requirement for signalling loss of timing reference is to transmit AIS,
for which entry into free-running mode is necessary. This is applicable, for
example, to regenerators.

4.1.3    Specification of jitter and wander

    SDH jitter and wander is specified at both STM-N and G.703 inter-
faces in order to control overall network jitter/wander accumulation. In
order to assure control of this accumulation, the jitter and wander character-
istics of all SDH based equipment are specified. The jitter and wander char-
acteristics of SDH based multiplex equipment are given in G.783 and those
of SDH based line systems are given in G.958.

4.2    Equipment error performance

    The general error performance design objective is that no errors shall
be introduced by the multiplexing equipment when operating within speci-
fied limits, under the most adverse environmental conditions given in º4.4
below.

    The specific requirement is that, when operating within specified lim-
its under the environmental conditions given in º4.4 below, the equipment
should be capable of providing a level of performance which is consistent
with the support of paths meeting the ôhigh gradeö performance classifica-
tion identified in Recommendation G.821.

4.3    Availability and reliability

    For further study.

4.4    Environmental conditions

    For further study.







INTERNATIONAL  TELECOMMUNICATION  UNION







CCITT    G.782

THE  INTERNATIONAL
TELEGRAPH  AND  TELEPHONE
CONSULTATIVE  COMMITTEE









GENERAL  ASPECTS  OF  DIGITAL

TRANSMISSION  SYSTEMS;

TERMINAL  EQUIPMENTS





TYPES  AND  GENERAL  CHARACTERISTICS
OF  SYNCHRONOUS  DIGITAL  HIERARCHY
(SDH)  MULTIPLEXING  EQUIPMENT










Recommendation  G.782







Geneva, 1990







FOREWORD

    The CCITT (the International Telegraph and Telephone Consultative
Committee) is a permanent organ of the International Telecommuni-
cation 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.782 was prepared by Study Group XV and was
approved under the Resolution No. 2 procedure on the 14 of December
1990.





___________________





CCITT  NOTE

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



















πITU1990

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