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All drawings appearing in this Fascicle have been done in Autocad.
Recommendation Q.700
INTRODUCTION TO CCITT SIGNALLING SYSTEM No. 7
1 General
This Recommendation provides an overview of the Signalling System by
describing the various functional elements of CCITT No. 7 and the relationship
between these functional elements. This Recommendation provides a general
description of functions and capabilities of the Message Transfer Part (MTP),
Signalling Connection Control Part (SCCP), Telephone User Part, ISDN User Part
(ISDN-UP), Transaction Capabilities (TC), and the Operations, Maintenance and
Administration Part (OMAP) which are covered elsewhere in the Q.700 to Q.795
series of Recommendations. However, in the case of contradiction between the
specifications and Q.700, the Q.700 to Q.795 specification shall apply.
Supplementary Services in CCITT S.S. No.7 ISDN applications are described
in the Q.73x series of Recommendations.
In addition to these functions in the CCITT No. 7 signalling system, the
Q.700 to Q.795 series of Recommendations describes the CCITT No. 7 network
structure, and also specifies the Tests and Measurements applicable to CCITT No.
7.
This Recommendation is also a specification of those aspects such as CCITT
S.S. No. 7 Architecture, Flow Control and general compatibility rule which are
not specified in separate Recommendations, and are applicable to the overall
scope of S.S. No. 7.
The remainder of this Recommendation describes:
- S 2: Signalling network concepts components and modes;
- S 3: The functional blocks within CCITT Signalling System No. 7 and the
services provided by them;
- S 4: CCITT Signalling System No. 7 protocol layering and its
relationship to OSI modelling;
- S 5: Node, application entity and user part addressing;
- S 6: Operations, administration and maintenance aspects of CCITT S.S.
No. 7;
- S 7: Performance aspects of the functional blocks within CCITT S.S. No.
7;
- S 8: Flow control for both the signalling network and within nodes;
- S 9: Rules for evolving CCITT S.S. No. 7 protocols while preserving
compatibility with earlier versions;
- S 10: A cross-reference to a glossary of terms.
1.1 Objectives and fields of application
The overall objective of Signalling System No. 7 is to provide an
internationally standardised general purpose common channel signalling (CCS)
system:
- optimised for operation in digital telecommunications networks in
conjunction with stored program controlled exchanges;
- that can meet present and future requirements of information transfer
for inter-processor transactions within telecommunications networks for
call control, remote control, and management and maintenance
signalling;
- that provides a reliable means for transfer of information in correct
sequence and without loss or duplication.
Fascicle VI.7 - Rec. Q.700 PAGE1
The signalling system meets requirements of call control signalling for
telecommunication services such as the telephone, ISDN and circuit switched data
transmission services. It can also be used as a reliable transport system for
other types of information transfer between exchanges and specialised centres in
telecommunications networks (e.g. for management and maintenance purposes). The
system is thus applicable for multipurpose uses in networks that are dedicated
for particular services and in multiservices networks. The signalling system is
intended to be be applicable in international and national networks.
The scope of CCITT S.S. No. 7 encompasses both circuit related and
non-circuit related signalling.
Examples of applications supported by CCITT S.S. No. 7 are:
- PSTN,
- ISDN,
- Interaction with Network Databases, Service Control Points for service
control,
- Mobiles (Public Land Mobile Network),
- Operations Administration and Maintenance of Networks.
The signalling system is optimized for operation over 64-kbit/s digital
channels. It is also suitable for operation over analogue channels and at lower
speeds. The system is suitable for use on point-to-point terrestrial and
satellite links. It does not include the special features required for use in
point-to-multipoint operation but can, if required, be extended to cover such an
application.
1.2 General characteristics
Common channel signalling is a signalling method in which a single channel
conveys, by means of labelled messages, signalling information relating to, for
example, a multiplicity of circuits, or other information such as that used for
network management. Common channel signalling can be regarded as a form of data
communication that is specialised for various types of signalling and information
transfer between processors in telecommunications networks.
The signalling system uses signalling links for transfer of signalling
messages between exchanges or other nodes in the telecommunication network served
by the system. Arrangements are provided to ensure reliable transfer of
signalling information in the presence of transmission disturbances or network
failures. These include error detection and correction on each signalling link.
The system is normally applied with redundancy of signalling links and it
includes functions for automatic diversion of signalling traffic to alternative
paths in case of link failures. The capacity and reliability for signalling may
thus be dimensioned by provision of a multiplicity of signalling links according
to the requirements of each application.
1.3 Components of CCITT S.S. No. 7
CCITT S.S. No. 7 consists of a number of components or functions which are
defined as a series of Q.700 to Q.795 Recommendations.
CCITT S.S. No. 7 function Recommendations
Message Transfer Part (MTP) Q.701-Q.704, Q.706,
Q.707
Telephone User Part (TUP) (including Q.721-Q.725
supplementary services)
Supplementary services Q.730
Data User Part (DUP) Q.741 (note 1)
ISDN User Part (ISDN-UP) Q.761-Q.764, Q.766
Signalling Connection Control Part (SCCP) Q.711-Q.714, Q.716
Transaction Capabilities (TC) Q.771-Q.775
Operations Maintenance and Administration Part Q.795
(OMAP)
Note 1 - Functions of the DUP are fully specified in Recommendation X.61.
PAGE24 Fascicle VI.7 - Rec. Q.700
Other Q.700 to Q.795 series Recommendations which describe other aspects
of the signalling system but not part of the CCITT S.S. No. 7 signalling
interfaces are:
Title Recommendations
Signalling Network Structure Q.705
Numbering of International Signalling Point Codes Q.708
Hypothetical signalling reference connection Q.709
PABX application Q.710
CCITT S.S. No. 7 Test Specification (General) Q.780
MTP Level 2 Test Specification Q.781
MTP Level 3 Test Specification Q.782
TUP Test Specification Q.783
Monitoring and measurements for the CCITT S.S. No.7 Q.791
network
S 3 of Q.700 describes the relationship between these
components.
1.4 Description techniques in the Q.700 to Q.795 series of Recommendations
The CCITT S.S. No. 7 Recommendation series define the signalling system
using prose description which is complemented by SDL diagrams and state
transition diagrams. Should any conflict arise between the text and the SDL
definition, the textual description is taken as definitive.
Message sequence charts or arrow diagrams are used to illustrate examples
of signalling procedures, but are not considered definitive.
2 CCITT S.S. No. 7 signalling network
2.1 Basic concepts
A telecommunications network served by common channel signalling is
composed of a number of switching and processing nodes inter-connected by
transmission links. To communicate using CCITT No. 7, each of these nodes
requires to implement the necessary "within node" features of CCITT S.S. No. 7
making that node a signalling point within the CCITT S.S. No. 7 network. In
addition, there will be a need to interconnect these signalling points such that
CCITT S.S. No. 7 signalling information (data) may be conveyed between them.
These data links are the signalling links of CCITT S.S. No. 7 signalling network.
The combination of signalling points and their interconnecting signalling
links form the CCITT S.S. No. 7 signalling network.
2.2 Signalling network components
2.2.1 Signalling points
In specific cases there may be a need to partition the common channel
signalling functions at such a (physical) node into logically separate entities
from a signalling network point of view; i.e., a given (physical) node may be
defined as more than one signalling point. One example is an exchange at the
boundary between international and national signalling networks.
Any two signalling points, for which the possibility of communication
between their corresponding User Part function exists, are said to have a
signalling relation.
The corresponding concept for a given User Part is called a user
signalling relation.
An example is when two telephone exchanges are directly connected by a
bundle of speech circuits. The exchange of telephone signalling relating to these
circuits then constitutes a user signalling relation between the Telephone User
Part functions in those exchanges in their role as signalling points.
Another example is when administration of customer and routing data in a
telephone exchange is remotely controlled from an operation and maintenance
centre by means of communication through a common channel signalling system.
Examples of nodes in a signalling network that constitutes signalling
points are:
- exchanges (switching centres),
- operation, administration and maintenance centres,
- service control points,
- signalling transfer points.
Fascicle VI.7 - Rec. Q.700 PAGE1
All signalling points in a CCITT S.S. No. 7 network are identified by a
unique code known as a point code (Recommendation Q.704 refers).
2.2.2 Signalling links
The common channel signalling system uses signalling links to convey the
signalling messages between two signalling points. A number of signalling links
that directly interconnect two signalling points which are used as a module
constitute a signalling link-set. Although a link set typically includes all
parallel signalling links, it is possible to use more than one link set in
parallel between two signalling points. A group of links within a link set that
have identical characteristics (e.g., the same data link bearer rate) is called a
link group.
Two signalling points that are directly interconnected by a signalling
link are, from a signalling network structure point of view, referred to as
adjacent signalling points. Correspondingly, two signalling points that are not
directly interconnected are non-adjacent signalling points.
2.2.3 Signalling modes
The term "signalling mode" refers to the association between the path
taken by a signalling message and the signalling relation to which the message
refers.
In the ass mode of signalling, the messages
relating to a particular signalling relation between two adjacent
points are conveyed over a link set, directly interconnecting those
signalling points.
In the non-associated mode of signalling, the messages
relating to a particular signalling relation are conveyed over two
or more linksets in tandem passing through one or more signalling
points other than those which are the origin and the destination of
the messages.
The quasi-associated mode of signalling is
a limited case of the non-associated mode where the path taken by
the message through the signalling network is pre-determined and,
at a given point in time, fixed.
Signalling System No. 7 is specified for use in the associated
and quasi-associated modes. The Message Transfer Part does not
include features to avoid out-of-sequence arrival of messages or
other problems that would typically arise in a fully non-associated
mode of signalling with dynamic message routing.
Examples of signalling modes are illustrated in Figure
1/Q.700.
Figure 1/Q.700 - CCITT-34960
2.3 Signalling point modes
A signalling point at which a message is generated, i.e., the location of
the source User Part function, is the originating point of that message.
A signalling point to which a message is destined, i.e., the location of
the receiving User Part function, is the destination point of that message.
A signalling point at which a message is received on a signalling link is
transferred to another link, i.e., neither the location of the source nor the
receiving User part function, is a Signal Transfer Point (STP).
For a particular signalling relation, the two signalling points thus
function as originating and destination points for the messages exchanged in the
two directions between them.
In the quasi-associated mode, the function of a signalling transfer point
is typically located in a few signalling points which may be dedicated to this
function, or may combine this function with some other (e.g., switching)
function. A signalling point serving as a signalling transfer point functions as
an originating and destination point for the messages generated and received by
the level 3 function of the Message Transfer Point also in cases when no user
functions are present.
2.4 Signalling routes
The pre-determined path, consisting of a succession of signalling
points/signalling transfer points and the interconnecting signalling links, that
a message takes through the signalling network between the origination point and
the destination point is the signalling route for that signalling relation.
All the signalling routes that may be used between an originating point
PAGE24 Fascicle VI.7 - Rec. Q.700
and a destination point by a message traversing the signalling network is the
signalling route set for that signalling relation.
2.5 Signalling network structure
The signalling system may be used with different types of signalling
network structures. The choice between different types of signalling network
structures may be influenced by factors such as the structure of the
telecommunication network to be served by the signalling system and
administrative aspects.
In the case when the provision of the signalling system is planned purely
on a per signalling relation basis, the likely result is a signalling network
largely based on associated signalling, typically supplemented by a limited
degree of quasi-associated signalling for low volume signalling relations. The
structure of such a signalling network is mainly determined by the patterns of
the signalling relations.
Another approach is to consider the signalling network as a common
resource that should be planned according to the total needs for common channel
signalling. The high capacity of digital signalling links in combination with the
needs for redundancy for reliability then typically leads to a signalling network
based on a high degree of quasi-associated signalling with some provision for
associated signalling for high volume signalling relations. The latter approach
to signalling network planning is more likely to allow exploitation of the
potential of common channel signalling to support network features that require
communication for purposes other than the switching of connections.
The worldwide signalling network is structured into two functionally
independent levels, namely the international and national levels. This structure
makes possible a clear division of responsibility for signalling network
management and allows numbering plans of signalling points of the international
network and the different national networks to be independent of one another.
Further considerations about the structure of the signalling network are
given in Recommendation Q.705, and the impact on the message transfer part in
Recommendation Q.701.
3 CCITT S.S. No. 7 functional blocks
3.1 Basic functional division
The Blue Book CCITT Signalling System No. 7 comprises the following
functional blocks:
- Message Transfer Part (MTP)
- Telephone User Part (TUP)
- ISDN User Part (ISDN-UP)
- Signalling Connection Control Part (SCCP)
- Transaction Capabilities (TC)
- Application-Entity (AE) Note 1
- Application-Service-Elements (ASEs) Note 1
Note 1 - The glossary shows these as hyphenated terms but the usual
convention used in this Recommendation will be unhyphenated.
The fundamental principle of the signalling system structure is the
division of functions into a common Message Transfer Part (MTP) on one hand, and
separate User Parts for different users on the other. This is illustrated in
Figure 2/Q.700.
The overall function of the Message Transfer Part is to serve as a
transport system providing reliable transfer of signalling messages between the
locations of communicating user functions.
User functions in CCITT S.S. No. 7 MTP terms are:
- the ISDN User Part (ISDN-UP)
- the Telephone User Part (TUP)
- the Signalling Connection Control Part (SCCP)
- the Data User Part (DUP)
The term "User" in this context refers to any functional entity that
utilises the transport capability provided by the Message Transfer Part.
A User Part comprises those functions of, or related to, a particular type
of user that are part of the common channel signalling system, typically because
those functions need to be specified in a signalling context.
The SCCP also has Users. These are:
- the ISDN User Part (ISDN-UP)
- Transaction Capabilities (TC)
Figure 2/Q.700 - T1109720-88
Fascicle VI.7 - Rec. Q.700 PAGE1
3.2 CCITT S.S. No. 7 architecture
3.2.1 General
Figure 2/Q.700 shows the Architecture of CCITT S.S. No. 7 and illustrates
the functional relationship between the various functional blocks of the Blue
Book CCITT S.S. No. 7. Figure 5/Q.700 shows the relationship between CCITT No. 7
levels and the OSI Reference Model Layers. This level/layer relationship is
described in the following sections.
The initial specification of CCITT No. 7 was based on circuit-related
telephony control requirements. To meet these requirements, CCITT No. 7 was
specified in four functional levels, the Message Transfer Part comprising levels
1-3, and the User Parts as level 4.
Figure 3/Q.700 shows the Functional Levels of CCITT S.S. No. 7. As new
requirements have emerged, e.g., for non-circuit related information transfer,
CCITT S.S. No. 7 has also evolved to meet these new requirements. There has been
a need to align certain elements in CCITT No. 7 to the OSI 7 Layer Reference
Model.
The result of this evolution is that Functional Levels and OSI layers
co-exist in CCITT No. 7. For example, the SCCP is a level 4 User Part in MTP
terms, but also provides an OSI Network layer 3 service. Subsequent sections
describe the various functional elements of CCITT S.S. No. 7 in terms of levels
and layers.
Figure 3/Q.700 - T1109730-88
It should be noted that the approach proposed for ISDN architecture is to
define two orthogonal planes, User and Control, each of which has its own 7-layer
protocol reference model.
From the perspective of an end user, the service provided by a
telecommunications network may be regarded as a Network Layer Service (User
Plane).
Within the telecommunications network, the techniques of the ISDN Protocol
Reference Model are applied, and the 7-layer protocol structure of the OSI Model
can also be used for inter-nodal communication to the end user.
3.2.2 Message Transfer Part (MTP) levels 1-3
An overview of the MTP is given in Recommendation Q.701. The MTP is
defined in Recommendations Q.701-Q.704, Q.706 and Q.707.
PAGE24 Fascicle VI.7 - Rec. Q.700
3.2.2.1 Signalling data link functions (level 1)
Level 1 defines the physical, electrical and functional characteristics of
a signalling data link and the means to access it. The level 1 element provides a
bearer for a signalling link.
In a digital environment, 64-kbit/s digital paths will normally be used
for the signalling data link. The signalling data link may be accessed via a
switching function, providing a potential for automatic reconfiguration of
signalling links. Other types of data links, such as analogue links with modems,
can also be used.
The detailed requirements for signalling data links are specified in
Recommendation Q.702.
3.2.2.2 Signalling link functions (level 2)
Level 2 defines the functions and procedures for and relating to the
transfer of signalling messages over one individual signalling data link. The
level 2 functions together with a level 1 signalling data link as a bearer, and
provides a signalling link for reliable transfer of signalling messages between
two points.
A signalling message delivered by the higher levels is transferred over
the signalling link in variable length signal units. For proper operation of the
signalling link, the signal unit comprises transfer control information in
addition to the information content of the signalling message.
The detailed requirements for signalling functions are given in
Recommendation Q.703.
3.2.2.3 Signalling network functions (level 3)
Level 3 in principle defines those transport functions and procedures that
are common to and independent of the operation of individual signalling links.
These functions fall into two major categories:
a) Signalling message handling functions - These are functions that, at
the actual transfer of the message, direct the message to the proper
signalling link or User Part.
b) Signalling network management functions - These are functions that, on
the basis of predetermined data and information about the status of the
signalling network, control the current message routing and
configuration of the signalling network facilities. In the event of
changes in the status, they also control the reconfigurations and other
actions to preserve or restore the normal message transfer capability.
The detailed requirements for signalling network functions are given in
Recommendation Q.704.
3.2.3 Level 4: MTP User functions
Level 4 consists of the different User Parts. Each User Part defines the
functions and procedures of the signalling system that are particular to a
certain type of user of the system. the following entities are defined as User
Parts in CCITT S.S. No. 7.
3.2.3.1 Signalling Connection Control Part (SCCP)
The SCCP is defined in Recommendations Q.711-Q.716. This Recommendation
series defines the SCCP capabilities, layer interfaces to MTP and SCCP users
signalling messages, their encoding and signalling procedures, and cross-office
performance. The SCCP provides additional functions to the Message Transfer Part
to provide such connectionless and connection-oriented network services to
transfer circuit-related, and non-circuit-related signalling information.
The SCCP provides the means to:
- control logical signalling connections in a CCITT No. 7 network;
- Transfer Signalling Data Units across the CCITT No. 7 network with or
without the use of logical signalling connections.
SCCP provides a routing function which allows signalling messages to be
routed to a signalling point based on, for example, dialled digits. This
capability involves a translation function which translates the global title
(e.g., dialled digits) into a signalling point code and a subsystem number.
Fascicle VI.7 - Rec. Q.700 PAGE1
SCCP also provides a management function, which controls the availability
of the "subsystems", and broadcasts this information to other nodes in the
network which have a need to know the status of the "subsystem".
The combination of the MTP and the SCCP is called "Network Service Part"
(NSP). The Network Service Part meets the requirements for layer 3 services as
defined in the OSI-Reference Model, CCITT Recommendation X.200.
3.2.3.2 Telephone User Part (TUP)
The CCITT S.S. No. 7 Telephone User Part is defined in Recommendations
Q.721-725. The TUP Recommendations define the necessary telephone signalling
functions for use of S.S. No. 7 for international telephone call control
signalling. This Recommendation series defines the telephone signalling messages,
their encoding and signalling procedures, and cross-office performance.
Supplementary Services handled by the CCITT S.S. No. 7 TUP applications
are described in Recommendation Q.724, S 10. These supplementary services embody
TUP signalling messages and procedures.
3.2.3.3 Data User Part (DUP)
The Data User Part is defined in Recommendation Q.741, and the
functionality fully defined in Recommendation X.61. It defines the protocol to
control interexchange circuits used on data calls, and data call facility
registration and cancellation.
3.2.3.4 ISDN User Part (ISDN-UP)
The ISDN User Part is defined in Recommendations Q.761-Q.764 and Q.766.
This Recommendation series defines the ISDN network signalling messages, their
encoding and signalling procedures, and cross-office performance. This
Recommendation series deals with the basic services only.
The ISDN-UP encompasses signalling functions required to provide switched
services and user facilities for voice and non-voice applications in the ISDN.
The ISDN-UP is also suited for application in dedicated telephone and
circuit-switched data networks and in analogue, and mixed analogue/digital
networks.
The ISDN-UP has an interface to the SCCP (which is also a level 4 User
Part) to allow the ISDN-UP to use the SCCP for end-to-end signalling.
Supplementary Services handled by the CCITT S.S. No. 7 ISDN application
are described in Recommendation Q.730. These supplementary services embody
ISDN-UP signalling messages and procedures. In some cases these services also
include application protocol which uses TC and SCCP, as, for example, centralised
Closed User Group (CUG).
3.2.3.5 Transaction Capabilities
Transaction Capabilities is defined in Recommendations Q.771-Q.775. This
Recommendation series defines the Transaction Capabilities signalling messages,
their encoding and signalling procedures.
Transaction Capabilities consists of two elements. These are:
- Transaction Capabilities Application Part (TCAP);
- Intermediate Service Part (ISP) [The ISP is for further study (see Note
1, Figure 5/Q.700)].
The TCAP entity is a functional block residing above the ISP in layer 7.
TCAP consists of two sub-layers: the Transaction sub-layer, and the Component
sub-layer. Further details are given in Recommendation Q.771.
TC, as currently specified, provides services based on a connectionless
network service. In this case, no ISP layers 4-6 functions are involved.
Connection-oriented TC services, and the layer functions of layers 4-6 are for
further study.
TC provides the means to establish non-circuit-related communication
between two nodes in the signalling network.
TC provides the means to exchange operations and replies via a dialogue.
The X.229 Remote Operations protocol has been extended to provide added
functionality in order to accommodate specific user needs. The operations and
parameters are part of the Application protocol between TC users.
PAGE24 Fascicle VI.7 - Rec. Q.700
3.2.3.6 Application Entities and Application Service Elements
In an OSI environment, communication between application processes is
modelled by communication between "Application Entities (AEs)". An Application
Entity represents the communication functions of an Application process. There
may be multiple sets of OSI communication functions in an application process, so
a single application process may be represented by multiple AEs. However, each
Application Entity is a set of communication capabilities whose components are
"Application Service Elements". An Application Service Element (ASE) is a
coherent set of integrated functions.
3.2.3.6.1 Application Entities in a CCITT S.S. No. 7 environment
Figure 4/Q.700 shows the relationship between Application Processes and
Application Entities, and Application Service Elements.
An "Application Process" is considered to be a range of functions and
features which support a particular network requirement. For example, an
application process in the context of CCITT S.S. No. 7 provides the co-ordination
across circuit-related protocols where required.
An Application Process can be considered as:
a) a co-ordinator of specific aspects of network operation (e.g., ISDN
Call Control, Mobiles, OA&M);
b) an individual service or supplementary service control function (e.g.,
CUG).
In the CCITT S.S. No. 7 context, the various functional elements of the
signalling system provide the signalling protocols (information elements,
messages, and procedures) necessary to support the service between nodes.
In a CCITT No. 7 environment, Application Entities (AEs) are the elements
representing the communication functions of the application process, which are
pertinent to inter-nodal communication using layer 7 application protocols.
The options for the relationship between an application process, AEs and
ASEs can take several forms at a CCITT No. 7 signalling point. Some examples are
shown in Figure 4/Q.700.
Figure 4/Q.700 - T1109742-88
Fascicle VI.7 - Rec. Q.700 PAGE1
3.2.3.6.2 Application Service Elements in a CCITT No. 7 environment
Application Service Elements (ASEs) reside in the CCITT S.S. No. 7
Architecture Model within layer 7 above TCAP. In the context of OSI, TCAP could
also be considered to be an ASE.
OMAP has an Application Entity currently containing the TCAP ASE and one
other ASE. Other ASEs are under study. OMAP is described further in S 6.
The Mobile Application Part (MAP) is another example of an Application
Entity (AE) (see Recommendation Q.1051).
An ASE can include a number of signalling procedures for a single service
(e.g., Freephone), where this single service is the application.
Alternatively, an ASE can include a number of signalling procedures for
any number of services or functions, encompassed by an application (e.g., MAP,
OMAP).
Thus, an ASE can define an individual service protocol (e.g., CUG), or a
complete application protocol (e.g., MAP).
An ASE can only communicate with a compatible peer ASE. The operations
defined in an ASE may be either symmetrically invoked by each entity involved in
the dialogue, or asymmetrically invoked by one entity only (i.e., on a
"client/server" basis). An example of the former is a "look ahead if free"
procedure; an example of the latter is a database enquiry.
3.2.3.6.3 Addressing for Application Entities (AEs)
The SCCP provides a mechanism for addressing "subsystems" using Subsystem
Numbers (SSNs). The Application Entity is considered, in the connectionless mode,
equivalent to an SCCP subsystem.
3.2.3.6.4 Management of AEs
The SCCP provides a mechanism for managing "subsystems" and signalling
points and informing other nodes of relevant availability status.
4 OSI layering and CCITT S.S. No. 7
4.1 General
Evolution of the CCITT Signalling System No. 7 architecture has been based
on the Open Systems Interconnection (OSI) Reference Model.
The purpose of the Reference Model of Open Systems Interconnection for
CCITT Applications (Recommendation X.200) is to provide a well-defined structure
for modelling the interconnection and exchange of information between users in a
communications system. This approach allows standardised procedures to be defined
not only to provide an open systems interconnection between users over a single
network, but also to permit interworking between networks to allow communication
between users over several networks in tandem.
At present, OSI only considers connection-oriented protocols, that is,
protocols which establish a logical connection before transferring data. In CCITT
S.S. No. 7, the ISDN-UP uses the SCCP connection-oriented protocol. The CCITT
S.S. No. 7 Network Service Part (NSP) provides both connectionless and
connection-oriented protocol.
The approach taken in the OSI reference model is to partition the model
used to describe this interconnection and exchange information between users in a
communications system into seven layers.
From the point of view of a particular layer, the lower layers provide a
"transfer service" with specific features. The way in which the lower layers are
realised is immaterial to the next higher layers. Correspondingly, the lower
layers are not concerned with the meaning of the information coming from higher
layers or the reasons for its transfer.
The characteristics of each layer are described below.
PAGE24 Fascicle VI.7 - Rec. Q.700
4.1.1 Physical Layer
The Physical Layer (layer 1) provides transparent transmission of a bit
stream over a circuit built in some physical communications medium. It furnishes
the interface to the physical media and is responsible for relaying bits (i.e.,
interconnects data-circuits). A 64 kbit/s link is assumed for the CCITT S.S. No.
7 Physical Layer.
4.1.2 Data Link Layer
The Data Link Layer (layer 2) overcomes the limitations inherent in the
physical circuits and allows errors in transmission to be detected and recovered,
thereby masking deficiencies in transmission quality.
4.1.3 Network Layer
The Network Layer (layer 3) transfers data transparently by performing
routing and relaying of data between end users. One or more of the sub-networks
may interwork at the Network Layer to provide an end user to end user network
service. A connectionless network provides for the transfer of data between end
users, making no attempt to guarantee a relationship between two or more data
messages from the same user.
4.1.4 Transport Layer
The Transport Layer (layer 4) provides end user to end user transfer
optimising the use of resources (i.e., network service) according to the type and
character of the communication, and relieves the user of any concern for the
details of transfer. The Transport Layer always operates end-to-end, enhancing
the Network Layer when necessary to meet the quality of service objectives of the
users.
4.1.5 Session Layer
The Session Layer (layer 5) co-ordinates the interaction within each
association between communicating application processes. Full and half duplex
dialogues are examples of possible Session Layer modes.
4.1.6 Presentation Layer
The Presentation Layer (layer 6) transforms the syntax of the data which
is to be transferred into a form recognizable by the communicating application
processes. For example, the Presentation Layer may convert a data stream from
ASCII to EBCDIC.
4.1.7 Application Layer
The Application Layer (layer 7) specifies the nature of the communication
required to satisfy the users' needs. This is the highest layer in the Model and
so does not have a boundary with a higher layer. The Application Layer provides
the sole means for the application processes to access the OSI environment.
4.2 Relationship between CCITT S.S. No. 7 layering and the OSI model
Layers 1-3 comprise functions for the transportation of information from
one location to another, possibly via a number of communication links in tandem.
These functions provide the basis on which a communication network can be built.
- The SCCP provides, with the MTP, OSI layer services 1-3.
Layers 4-7 define functions relating to end-to-end communication. These
layers are so defined that they are independent of the internal structure of the
communication network.
- Transaction Capabilities provides layer 4-7 services.
Layer 7 represents the semantics of a communication, whereas layers 1-6
comprise the means by which the communication may be realised.
- Application Entities/Application Service Elements provide the
appropriate Application Layer Protocols in layer 7.
Figure 5/Q.700 shows the relationship between SCCP, TC, and ASEs to the
OSI 7 Layer Reference Model.
Fascicle VI.7 - Rec. Q.700 PAGE1
Figure 5/Q.700 - T1109751-88
The aspect of the SMAP which is then involved with communication is the
Systems Management Application Entity (SMAE). The SMAE is also known as the OMAP
AE.
4.3 Primitive Interfaces between CCITT No. 7 Functions
4.3.1 General
Interfaces between the functional elements of CCITT S.S. No. 7 are
specified using interface primitives. Primitive interface definition does not
assume any specific implementation of a service.
4.3.2 OSI service primitives
Where the functional element of CCITT No. 7 is modelled on the OSI 7 layer
reference model, e.g., SCCP, TC, service primitives are defined in line with
Recommendation X.210.
PAGE24 Fascicle VI.7 - Rec. Q.700
In line with Recommendation X.210, Figure 6/Q.700 illustrates the
relationship between the terms "service", "boundary", "service primitives", "peer
protocol" and "peer entities". The term "boundary" applies to boundaries between
layers, as well as to boundaries between sub-layers.
Figure 6/Q.700 - T1109760-88
4.3.2.1 Service primitives
The user of primitives does not preclude any specific implementation of a
service in terms of interface primitives.
A service primitive consists of a name and one or more parameters which
are passed in the direction of service primitive.
The name of a service primitive contains three elements, as defined in
Recommendation X.210:
a) a type indicating the direction of the primitive flow. Four types of
service primitives are identified (Figure 7/Q.700):
- request a primitive issued by a service user to invoke a service
element,
- indication a primitive issued by a service provider to advise that a
service element has been invoked by the service user at
the peer service access point or by the service provider,
- response a primitive issued by the service user to complete at a
particular service access point some service element
whose invocation has been previously indicated at that
service access point,
- confirmation a primitive issued by a service provider to
complete at a particular service access point some
service element previously invoked by a request at that
service access point.
Not all four types can be associated with all service names.
b) a name which specifies the action to be performed;
c) An initial (or initials) which specifies the (sub-)layer providing the
service:
- OM for the Operations Management primitives associated with OMAP;
- TC for the TCAP Component sub-layer,
- TR for the TCAP Transaction sub-layer,
- P, S, T, respectively for the Presentation, Session, and Transport
layers in the ISP,
- N for the Network Service Part (MTP + SCCP), as defined in
Recommendation Q.711.
Fascicle VI.7 - Rec. Q.700 PAGE1
Figure 7/Q.700 - T1109771-88
Figure 8/Q.700 provides an overview of the primitives used between the
various functional elements of CCITT No. 7.
The MTP primitives apply to all level 4 users of the MTP.
Similarly, the SCCP Management Primitives N-STATE, N-COORD, N-PCSTATE
apply to all SCCP subsystems/AEs via TC.
The TC primitives between the ASE and TC provide control of connectionless
TCAP transactions. Service primitives for connection-oriented TC transactions are
for further study.
Figure 8/Q.700 - T1109780-88
5 Addressing
Addressing of CCITT S.S. No. 7 messages has to be considered on a number
of levels. For example, the message transfer part uses the destination point code
to route the message to the appropriate signalling point. The called party
address field in TUP, or ISUP called party number field, in the Initial Address
Message is used to route the call to the appropriate called destination. The
capabilities of the various CCITT S.S. No. 7 addressing mechanisms are
illustrated by the signalling message structure.
5.1 Signalling message structure
A signalling message is an assembly of information, defined at level 3 or
4, pertaining to a call, management transaction, etc., that is transferred as an
entity by the message transfer function.
Each message contains service information including a service indicator
identifying the source User Part and possibly additional information such as an
indication whether the message relates to international or national application
of the User Part.
The signalling information of the message includes the actual user
information, such as one or more telephone or data call control signals,
management and maintenance information, etc., and information identifying the
type and format of the message. It also includes a label that provides
information enabling the message to be:
- routed by the level 3 functions and thorugh a signalling network to its
destination; and (This part of the label is known as the Routing label.
This is shown in Figure 9/Q.700.)
- directed at the receiving User Part to the particular circuit, call,
management or other transaction to which the message is related.
Further details are given in Q.700, S 5.2.
SLS Originating Destination
Point Code Point Code
FIGURE 9/Q.700
CCITT SS No. 7 Routing Label
There are four types of label:
- type A for MTP management messages;
- type B for TUP;
- type C for ISDN-UP (circuit related) messages;
- type D for SCCP messages.
These are shown in Figure 10/Q.700.
The circuit identification code is used as a label for circuit related
signalling messages, e.g., TUP or ISDN-UP. The least significant 4 bits of this
field (in the TUP) is the Signalling Link Selection (SLS) field, which is used,
where appropriate, to perform load sharing (see Q.704). In the ISDN-UP, the SLS
is a separate field to the circuit identification code.
The CCITT No. 7 MTP signalling messages at level 2, which carry user
information, are called Message Signal Units (MSUs). Figure 11/Q.700 shows the
basic format of the MSU (refer also to Q.703) and the breakdown of the MSU.
Signalling Information Field (SIF) when transporting circuit-related (ISDN-UP,
TUP) messages and non-circuit-related messages (SCCP, TC based). Further details
are given on message formats in Recommendations Q.704, Q.713, Q.723, Q.763,
Q.773.
PAGE24 Fascicle VI.7 - Rec. Q.700
Figure 10/Q.700 - T1109791-88
Figura 11/Q.700 - T1109801-88
Fascicle VI.7 - Rec. Q.700 PAGE1
5.2 MTP addressing
There is a two part addressing mechanism in the MTP, one part of the
mechanism uses the point code which is incorporated in the routing label of every
message signal unit, the other part of the mechanism makes use of the service
indicator and network indicator within the service information octet. The point
code is used for inter-node addressing and the SIO addresses signalling system
users on an intra-node basis.
5.2.1 Point codes
Every signalling point (SP) and signalling transfer point (STP), when
integrated in an SP, will be allocated its own unique point code. This is used by
the MTP routing function to direct outgoing messages towards their destination in
the network as indicated by the inclusion of the appropriate point code in the
routing label. This point code is known as the destination point code (DPC). The
routing label also contains the point code of the SP originating the message
signal unit, therefore, the combination of this originating point code (OPC) and
DPC will determine the signalling relation (i.e., the network points between
which MTP "User" information is exchanged). The DPC is used by the receiving
SP/STP discrimination function to determine whether the message is addressed to
that SP or requires to be onward routed by means of the signal transfer
capability of the STP.
The DPC will always be determined and inserted in the routing label by the
level 4 MTP "User". This will also generally be the same for the OPC but it is
possible that since the OPC might be constant it could be inserted by the MTP.
5.2.2 Service indicator and network indicator
The 4 bit service indicator (SI) and 2 bit network indicator (NI) are
included in the service information octet (SIO) and are used within an SP's
distribution function to determine the "User" the incoming message should be
delivered to.
The SI will determine the "User", e.g., TUP, SCCP, ISUP and the NI will
determine which network is concerned, e.g., international or national.
The NI will also in conjunction with the OPC/DPC determine whether a
national or international signalling relation/routing is involved.
The NI, together with the standard 14 bit point code, allows for a max 16
384 point codes to be allocated in a signalling network.
5.3 SCCP addressing
Addressing within the SCCP of S.S. No. 7 makes use of three separate
elements:
- DPC
- Global Title (GT)
- Sub-System Number (SSN)
One, two or all of the elements may be present in the Called and Calling
Party Address, the main options are:
GT When transferring SCCP messages
DPC + SSN
SSN
GT When receiving messages from MTP
SSN + GT
DPC
DPC + (SSN or GT or both) When receiving messages from
GT connectionless or
GT + SSn connection-orientated control for
SCCP Routing.
The form of address used will depend on the service, application and
underlying network.
PAGE24 Fascicle VI.7 - Rec. Q.700
5.3.1 Global Title (GT)
The Global Title (GT) may comprise of dialled digits or another form of
address that will not be recognized in the S.S. No. 7 network, therefore, if the
associated message requires to be routed over the S.S. No. 7 network, translation
is required.
Translation of the GT will result in a DPC being produced and possibly
also a new SSN and GT. A field is also included in the address indicator to
identify the format of the global title.
5.3.2 Destination Point Code (DPC)
The DPC in an address requires no translation and will merely determine if
the message is destined for that in SP (incoming message) or requires to be
routed over the S.S. No. 7 signalling network via the MTP. For outgoing messages
this DPC should be inserted in the MTP routing label. On an incoming message the
DPC in the MTP routing label should correspond to the DPC in the called address.
5.3.3 Subsystem Number (SSN)
The SSN will identify a subsystem accessed via the SCCP within a node and
may be a User Part, e.g., ISUP, SCCP management or an AE via TC. TC, however,
will be invisible to the SCCP.
When examination of the DPC in an incoming message has determined that the
message is for that SP, examination of the SSN will identify the concerned SCCP
"User". The presence of an SSN without a DPC will also indicate a message which
is addressed to that SP.
The SSN field has an initial capacity of 255 codes with an extension code
for future requirements.
5.4 User Part addressing
5.4.1 Telephone User Part addressing
The Telephone User Part is capable of handling E.164 (incorporating E.163)
addresses in the calling and called party address information elements.
5.4.2 ISDN User Part addressing
The ISDN User Part address structure is capable of handling E.164
addresses in the calling and called number, and re-directing address information
elements.
5.4.3 Signalling connection control part addresses
The signalling connection control part is capable of handling E.164
(incorporating E.163), X.121, F.69, E.210, E.211, E.212, E.213, addresses, and
the mobile hybrid E.214 address in the calling and called party address
information elements.
The handling of OSI NSAP addresses in SCCP is for further study.
5.5 Labelling
A variety of methods to label signalling messages is used to allow the
signalling system and users of the signalling system to relate a received message
to a particular call or transaction.
For circuit-related messages, (e.g., on a simple telephone call), the TUP
(and the ISUP) use the circuit identification code (CIC) to label the message.
For certain ISUP procedures, call reference are used to associate messages
with calls.
SCCP also uses local references on connection oriented protocols.
Transaction capabilities use transaction and invoke identities to
associate transaction messages and components respectively.
Fascicle VI.7 - Rec. Q.700 PAGE1
6 Operations administration and maintenance
6.1 Management
Management within S.S. No. 7 is partitioned into two main areas:
- Signalling network management;
- Signalling system management.
6.1.1 Signalling network management
These are functions contained within the MTP and SCCP which, by means of
automatic procedures, maintain the required signalling network performance (e.g.,
changeover of faulty links, forced re-routing, subsystem availability, etc.).
6.1.2 Signalling system management
This may be considered as the actions taken by the operator (or by an
external automatic mechanism) to maintain the signalling system performance when
problems are identified.
6.1.3 Signalling System No. 7 and TMN
The TMN concept identifies CCITT S.S. No. 7 as a candidate to act as a
data communications network (DCN) for some TMN functions. The protocols that will
be needed for this purpose are intended to be defined as ASEs, as part of OMAP.
This topic is for further study.
6.1.4 Signalling System No. 7 and OSI management
This subject is for further study.
6.2 Maintenance and testing
The maintenance administration and management functions of the signalling
system themselves use the signalling system as a data carrying mechanism. When
regarded in the data transport mode, however, any management or maintenance
information is regarded as signalling traffic. Those functions having direct
impact on S.S. No. 7 are included in OMAP Recommendation Q.795.
Testing within Signalling System No. 7 is:
- instigated automatically as a part of a signalling system management
procedures (e.g., signalling route set test in MTP) or
- applied as a result of external activity, e.g., human-machine (MMI).
The first form is described in the appropriate Q.700 to Q.795
Recommendation dealing with MTP or SCCP, etc. The second form includes some MMI
initiated procedures (initiation of MRVT (Q.795)), and also pre-in service
testing using test cases specified in Recommendations for S.S. No. 7 tests (Q.780
to Q.783). A testing user part has been agreed to be necessary for pre-in service
testing, this topic is for further study.
6.2.1 Operations Maintenance and Administration Part (OMAP)
Recommendation Q.795 provides procedures and protocols related to
operations and maintenance information. These procedures and protocols use TCAP
and are invoked by the system management application process (SMAP).
Recommendation Q.795 includes the following:
- MTP Routing Verification Test (MRVT)
- SCCP Routing Verification Test (SRVT) - for further study
- Circuit Validation Test
The protocol for the MRVT contained in Q.795 forms part of the OMAP AE
which in turn uses the services provided by transaction capabilities.
ASEs needed to support the TMN functions are for further study.
PAGE24 Fascicle VI.7 - Rec. Q.700
6.2.2 Testing
Test specifications for Signalling System No. 7 are contained in
Recommendations Q.780-783 and cover MTP level 2, level 3 and the TUP together
with an overview of testing.
A Testing User Part is for further study.
6.3 CCITT S.S. No. 7 measurements
Recommendation Q.791 specifies the monitoring and measurements appropriate
to the MTP and SCCP.
7 Signalling system performance
The performance requirements of Signalling System No. 7 must take account
of the performance requirements of the services that are being supported. Each
functional component of Signalling System No. 7 has its performance criteria
specified in a self-contained Recommendation. An overall performance target is
specified in the form of a Hypothetical Signalling Reference Connection (HSRC).
7.1 Hypothetical Signalling Reference Connection (HSRC)
The HSRC for Signalling System No. 7 (Recommendation Q.709), identifies
components that are used in a signalling relation between signalling end points,
signalling points, signalling transfer points, and signalling points with SCCP
relay functions, and gives the values for the signalling delays and
unavailability parameters. The values used are derived from the figures contained
in the individual performance Recommendations for MTP, TUP, SCCP and ISUP.
7.2 MTP
The MTP signalling performance requirements are specified in
Recommendation Q.706. This Recommendation includes:
- the parameters route-set unavailability, MTP malfunction (loss of
messages and mis-sequencing), and message transfer times;
- factors affecting performance, for example signalling traffic
characteristics (e.g., loading potential, security, etc.) and
parameters related to transmission characteristics (e.g., bit rates of
signalling data links);
- those parameters which have greatest influence on the signalling
network queueing delays for example, error control, security
arrangements, failures and priorities.
It should be noted that management functions affect MTP performance.
7.3 SCCP
The SCCP signalling performance requirements are contained in
Recommendation Q.716. Parameters identified are signal connection delays
(establishment, unsolicited reset, reset and release signalling connection, reset
and release failure probability, data message transmit delay, data message delay
failure and error probability and SCCP unavailability).
It should be noted that management functions affect SCCP performance.
7.4 TUP
The TUP signalling performance requirements are contained in
Recommendation Q.725. Parameters contained in this Recommendation are cross
office performance for TUP supported circuit connection control application under
normal and abnormal traffic loads. Also specified is the probability of failure
of calls due to signalling malfunction.
Fascicle VI.7 - Rec. Q.700 PAGE1
7.5 ISDN-UP
The ISDN-UP signalling performance requirements are contained in
Recommendation Q.766. Parameters contained in this Recommendation are cross
office performance for ISDN-UP supported circuit connection control under normal
and abnormal traffic loads. Also specified is the probability of failure of an
ISDN call due to signalling function.
8 Flow control
Signalling System No. 7 in common with other transport mechanisms, needs
to limit the input of data when congestion onset is detected. Failure to do so
can create overload situations. The nature of CCITT S.S. No. 7 will lead to
SP/STP overload congestion being spread through the signalling network if no
action is taken. This will result in impaired signalling performance. In addition
to signalling network congestion within a node, congestion will also require
action to prevent signalling performance from deteriorating. There is thus a need
for flow control within the signalling system to maintain the required signalling
performance.
8.1 Signalling network flow control
This is achieved by incorporating a flow control mechanism in the MTP. On
detection of congestion, MTP "Users" are informed by the means of a special
primitive; the "User" should then reduce signalling traffic towards the congested
part of the network. If the User is at a remote SP, the information is carried
across the network in an appropriate signalling network management message.
8.2 Signalling node (congestion) flow control
In addition to network congestion, nodal congestion also requires the
remedial action of flow control to prevent the signalling performance from being
impaired. Nodal congestion can occur both within the MTP and the MTP "User".
8.2.1 MTP nodal flow control
A similar activity to that to combat signalling network congestion is
required, i.e., on detection, the "User" is informed so that traffic can be
reduced.
8.2.2 "User" flow control
As well as taking action to reduce MTP congestion, mechanisms are also
required within the User to detect the onset of congestion and to take
appropriate action.
8.3 Automatic congestion control
The ISUP and TUP provide signalling procedures which aim to reduce the new
calls offered to an exchange which is experiencing processor overload.
Automatic congestion control provides the means to inform adjacent
exchanges of the current workload, and to request that only priority calls are
offered to the exchange experiencing overload.
9 Compatibility mechanisms and rules in CCITT S.S. No. 7
9.1 Modularity
The wide scope of the signalling system requires that the total system
include a large diversity of functions and that further functions can be added to
cater for extended future applications. As a consequence only a subset of the
total system may need to be used in an individual application.
A major characteristic of the signalling system is that it is specified
with a functional structure to ensure flexibility and modularity for diverse
applications within one system concept. This allows the system to be realized as
a number of functional modules which could ease adaptation of the functional
content of an operating Signalling System No. 7 to the requirements of its
application.
PAGE24 Fascicle VI.7 - Rec. Q.700
The CCITT specifications of the signalling system specify functions and
their use for international operation of the system. Many of those functions are
also required in typical national applications. Furthermore, the system to some
extent includes features that are particular to national applications. The CCITT
specifications thus form an internationally standardized base for a wide range of
national applications of common channel signalling.
CCITT S.S. No. 7 is one common channel signalling system. However, as a
consequence of its modularity and its intended use as a standard base for
national applications the system may be applied in many forms. In general, to
define the use of the system in a given national application, a selection of the
CCITT specified functions must be made and the necessary additional national
functions must be specified depending on the nature of the application.
CCITT S.S. No. 7 is an evolutionary signalling system which has undergone
a number of enhancements. To allow ease of evolution it has been necessary to
incorporate a number of compatibility mechanisms in various functional elements
of CCITT No. 7, and to apply a number of compatibility rules to protocol
enhancement. Detailed specification of the compatibility mechanisms in each
functional element of CCITT S.S. No. 7 are given in the appropriate Q.700 to
Q.795 Recommendations. Hence an overview is given in this Recommendation.
Compatibility rules which apply to all functional elements of CCITT S.S.
No. 7 are detailed in the following text.
9.2 Evolutionary requirements
In application protocols (e.g., ISDN-UP, ASEs), the main evolutionary
requirement is the ability to add new subscriber services, new administration and
network services to the protocol.
In the SCCP and MTP, the evolutionary requirements are different in that
initial versions provide basic transport functions which are generally stable.
The main enhancements have been in the management protocols.
Although the evolutionary requirements are different across the elements
of CCITT S.S. No. 7, it is possible to incorporate certain common mechanisms in
the various functional elements.
9.3 Forward and backward compatibility
Compatibility mechanisms can be considered as being either:
- Forward compatibility mechanisms
- Backward compatibility rules
Forward compatibility mechanisms are defined as a scheme to enable a
version of a protocol to communicate effectively and interwork with future
versions of the protocol.
Backward compatibility rules are defined as a scheme to ensure that future
versions of the protocol will be able to send protocol messages to the previous
version which will be understood and fully processed by the node supporting the
previous version.
9.4 Compatibility rules for CCITT S.S. No. 7
The following compatibility rules are applied to each element of CCITT
S.S. No. 7 (e.g., ISDN-UP) when protocols are enhanced.
9.4.1 Addition of a new value to an existing field (e.g., a cause value)
New values to an existing field can be added. The processing of these new
values at nodes supporting an earlier version of the protocol will be defined in
their version specifications.
9.4.2 Addition of a new parameter to an existing message
Any new parameters added to an existing message must not be added as
mandatory parameters. If a new parameter, must be added, and it must be a
mandatory parameter, then a new message type must be created.
9.4.3 Handling of unrecognized information
When a new protocol, message or information element is created, a rule is
required on a per message and information element basis, to define the action on
receipt of unrecognized information. This rule needs to be applied to both
unrecognized messages, unrecognized information elements within messages, and
unrecognized values within recognized information elements.
Fascicle VI.7 - Rec. Q.700 PAGE1
The actions defined for receipt of an unrecognized message/information
element could be:
- Discard message/information element.
- Discard/ignore information element within a recognized message.
- Default to a known general value (e.g., on receipt of an ISDN-UP IAM
with an unrecognized calling party category could be defaulted to
"Unknown").
- Send a "Confusion" message.
- Terminate the call/transaction.
- Information management.
9.4.4 Increase in the length of optional parameters
If a parameter is used as an optional parameter in all messages that it
appears, the length of the parameter can be increased. The older version of the
protocol would be able to function as it does today, assuming it ignores the
extra bits or a suitable extension method has been defined. The newer version
would have to check the length of the parameter to determine if the added
information was present.
Protocols which use coding rules which are based on X.409 (e.g., TC) are
not subject to this rule.
9.4.5 Processing of messages with unrecognized SIO information
To enable signalling points implemented to the Blue Book to interwork with
signalling points implemented to earlier Recommendations when a message
containing an unrecognized service information octet (see Q.704, S 14.2) is
received, the message is discarded.
9.4.6 Unacknowledged messages
Where a function requires an acknowledgement to a message in order to
continue, if no response is received the function sends the message for only a
limited number of times. The sending signalling point should assume that the
function is not available, and inform local management.
9.4.7 Processing of spare fields
For those CCITT S.S. No. 7 functions which define fields or sub-fields in
signalling messages as spare or reserved, the following rules for processing of
these fields apply.
At a node generating a signalling message, all spare and reserved fields
are set to zero. At transit nodes, spare or reserved fields may be passed on
transparently. At the destination node, the spare and reserved fields are not
examined.
10 Glossary
A Glossary of terms in CCITT S.S. No. 7 is contained at the back of the
Fascicles VI.7, VI.8 and VI.9.
PAGE24 Fascicle VI.7 - Rec. Q.700