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- All drawings appearing in this Fascicle have been done in Autocad.
- Recommendation Q.701
- FUNCTIONAL DESCRIPTION OF THE
- MESSAGE TRANSFER PART (MTP) OF SIGNALLING SYSTEM No. 7
- 1 Introduction
- 1.1 General
- The Message Transfer Part (MTP) provides the functions that enable User
- Part significant information passed to the MTP to be transferred across the
- Signalling System No. 7 network to the required destination. In addition,
- functions are included in the MTP to enable network and system failures that
- would affect the transfer of signalling information to be overcome. This
- constitutes a sequenced connectionless service for the MTP user.
- The Message Transfer Part together with one of its "users", the Signalling
- Connection Control Part (SCCP), described in Recommendations Q.711-716, forms the
- Network Service Part (NSP).
- The Network Service Part meets the requirement for Layer 3 services as
- defined in the OSI - Reference Model CCITT Recommendation X.200. The relationship
- of the MTP with this model and to other parts of S.S. No. 7 is described in
- Recommendation Q.700.
- 1.2 Objectives
- The overall objectives of the Message Transfer Part are to provide the
- means for:
- a) the reliable transport and delivery of "User Part" signalling
- information across the S.S. No. 7 network.
- b) the ability to react to system and network failures that will affect
- a), and take the necessary action to ensure that a) is achieved.
- The "Users" of MTP are the SCCP, Telephone User Part (TUP) [Recommendation
- Q.721-725 Data User Part (DUP) [Recommendation Q.741] and ISDN User Part (ISUP)
- [Recommendation Q.761-766]. The MTP Testing User Part is for further study.
- 1.3 General characteristics
- 1.3.1 Method of description
- - functions provided by each level within the MTP
- - services provided by the MTP
- - interaction with the signalling network
- - interaction with the MTP "User"
- - the message transfer capability of the MTP
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- Fascicle VI.7 - Rec. Q.701 PAGE1
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- The functions of each level of the MTP are performed by means of the level
- protocol between two systems which provides a "level service" to the upper
- levels, (i.e., Level 1 Signalling Data Link, Level 2 Signalling Link and Level 3
- Signalling network) as described in Recommendations Q.702, 703 and 704
- respectively.
- The service interface to the Level 4 "User" of MTP is described by means
- of primitives and parameters.
- 1.3.2 Primitives
- Primitives consist of commands and their respective responses associated
- with the services requested of the SCCP and of the MTP, see Figure 1/Q.701. The
- general syntax of a primitive is shown below:
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- X Generic name Specific name Parameter
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- - "X" designates the functional block providing the service ("MTP" for
- MTP).
- - "Generic name" describes the action that should be performed by the
- addressed layer.
- - "Specific name" indicates the direction of the primitive flow.
- - "Parameters" are the elements of information which are to be
- transmitted between layers.
- Four Specific Names exist in general:
- - request
- - indication
- - response1)
- - confirmation1)
- Figure 1/Q.701 - T1109680-88
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- 1) Not all generic names contain all four specific names (Figure 2/Q.701).
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- Figure 2/Q.701 - T1109690-88
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- Primitives and parameters of the Messsage Transfer Part service are listed
- and described in Section 8 of this Recommendation.
- 1.3.3 Peer-to-peer communication
- Exchange of information between two peers of the MTP is performed by means
- of a protocol. The protocol is a set of rules and formats by which the control
- information and MTP "User" data is exchanged between the two peers. The protocol
- caters for
- - the transfer of "User" data in Message Signal Units (MSUs);
- - level 2 control by use of Link Status Signal Units (LSSUs);
- - testing and maintenance of signalling links by means of the signalling
- link test message carried in an MSU.
- 1.3.4 Contents of Recommendations Q.701 to Q.707 Series relating to the MTP
- Recommendation Q.701 contains a functional description and overview of the
- Message Transfer Part of CCITT S.S. No. 7.
- Recommendation Q.702 details the requirements of a signalling data link to
- support CCITT S.S. No. 7.
- Recommendation Q.703 describes the signalling link functions.
- Recommendation Q.704 describes signalling network functions and messages.
- Recommendation Q.706 defines and specifies values for MTP performance
- parameters.
- Recommendation Q.707 describes the testing and maintenance functions
- applicable to the MTP.
- 2 Signalling system structure
- 2.1 Basic functional division
- The fundamental principle of the signalling system structure is the
- division of fu common Message Transfer Part
- (MTP) on one hand and separate User Parts for different
- users on the other. This is illustrated in Figure 3/Q.701.
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- Fascicle VI.7 - Rec. Q.701 PAGE1
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- Figure 3/Q.701 - CCITT34930
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- 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.
- The term user in this context refers to any functional entity that
- utilizes 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 basic commonality in signalling for different services resulting from
- this concept is the use of a common transport system, i.e., the Message Transfer
- Part. Also, a degree of commonality exists between certain User Parts, e.g., the
- Telephone User Part (TUP) and the Data User Part (DUP).
- 2.2 Functional levels
- 2.2.1 General
- As a further separation, the necessary elements of the signalling system
- are specified in accordance with a level concept in which:
- - the functions of the Message Transfer Part are separated into three
- functional levels, and
- - the User Parts constitute parallel elements at the fourth functional
- level.
- The level structure is illustrated in Figure 4/Q.701. The system structure
- shown in Figure 4/Q.701 is not a specification of an implementation of the
- system. The functional boundaries B, C and D may or may not exist as interfaces
- in an implementation. The interactions by means of controls and indications may
- be direct or via other functions. However, the structure shown in Figure 4/Q.701
- may be regarded as a possible model of an implementation.
- 2.2.2 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.
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- Figure 4/Q.701 - CCITT34941
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- 2.2.3 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
- 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 signalling link functions include:
- - delimitation of signal unit by means of flags;
- - flag imitation prevention by bit stuffing;
- - error detection by means of check bits included in each signal unit;
- - error correction by retransmission and signal unit sequence control by
- means of explicit sequence numbers in each signal unit and explicit
- continuous acknowledgements;
- - signalling link failure detection by means of signal unit error rate
- monitoring and signalling link recovery by means of special procedures.
- The detailed requirements for signalling link functions are given in
- Recommendation Q.703.
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- 2.2.4 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. As
- illustrated in Figure 4/Q.701 these functions fall into two major categories:
- a) signalling message handling functions - these are functions that, at
- the actual transfer of a 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 signalling network facilities. In the event of changes
- in the status they also control reconfigurations and other actions to
- preserve or restore the normal message transfer capability.
- The different level 3 functions interact with each other and with the
- functions of other levels by means of indications and controls as illustrated in
- Figure 4/Q.701. This figure also shows that the signalling network management as
- well as the testing and maintenance actions may include exchange of signalling
- messages with corresponding functions located at other signalling points.
- Although not User Parts these parts of level 3 can be seen as serving as "User
- Parts of the Message Transfer Part". As a convention in these specifications, for
- each description, general references to User Parts as sources or sinks of a
- signalling message implicitly include these parts of level 3 unless the opposite
- is evident from the context or explicitly stated.
- A description of the level 3 functions in the context of a signalling
- network is given in S 3 below. The detailed requirements for signalling network
- functions are given in Recommendation Q.704. Some means for testing and
- maintenance of the signalling network are provided and the detailed requirements
- are given in Recommendation Q.707.
- 2.2.5 User Part functions (level 4)
- 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 extent of the User Part functions may differ significantly between
- different categories of users of the signalling system, such as:
- - users for which most user communication functions are defined within
- the signalling system. Examples are telephone and data call control
- functions with their corresponding Telephone and Data User Parts;
- - users for which most user communication functions are defined outside
- the signalling system. An example is the use of the signalling system
- for transfer of information for some management or maintenance purpose.
- For such an "external user" the User Part may be seen as a "mailbox"
- type of interface between the external user system and the message
- transfer function in which, for example, the user information
- transferred is assembled and disassembled to/from the applicable
- signalling message formats.
- 2.3 Signalling message
- 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 through a signalling network
- to its destination; and
- - to be directed at the receiving User Part to the particular circuit,
- call, management or other transaction to which the message is related.
- On the signalling link, each signalling message is packed into Message
- Signal Units (MSUs) which also includes transfer control information related to
- the level 2 functions of the link.
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- 2.4 Functional interface
- The following functional interface between the Message Transfer Part and
- the User Parts can be seen as a model illustrating the division of functions
- between these parts. The interface (see Figure 5/Q.701) is purely functional and
- need not appear as such in an implementation of the system.
- Figure 5/Q.701 - CCITT 34951
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- The main interaction between the Message Transfer Part and the User Parts
- is the transfer of signalling messages across the interface, each message
- consisting of service information and signalling information as described above.
- Message delimitation information is also transferred across the interface with
- the message.
- In addition to the transfer of messages and associated information, the
- interaction may also include flow control information, e.g., an indication from
- the Message Transfer Part that it is unable to serve a particular destination.
- A description of the characteristics of the Message Transfer Part as seen
- from the functional interface and the requirements to be met by potential users
- of the message transfer function is given in S 4.
- 3 Message transfer part and the signalling network
- 3.1 General
- Since the Message Transfer Part forms the interface at a node with the
- rest of the signalling network, the signalling network will have significant
- impact on the MTB. The MTP must however be independent of the signalling network
- in that it has to be capable of performing its set functions and attaining its
- objectives no matter what network structure or status prevails.
- The MTP has therefore to contain the necessary functions to ensure any
- impact that the network has does not impair MTP performance.
- 3.1.1 Signalling network components
- A full description of signalling network components is contained in
- Recommendation Q.700, the components that must be considered by the MTP are:
- - signalling points (including signalling transfer points);
- - signalling relations between two signalling points;
- - signalling links;
- - signalling link sets (including link groups);
- - signalling routes;
- - signalling route-sets.
- 3.1.2 Signalling modes
- Signalling modes are described in Recommendations Q.700 and Q.705
- (signalling network structures). The modes applicable to CCITT S.S. No. 7 MTP
- are:
- - associated mode;
- - quasi-associated mode.
- 3.1.3 Signalling point modes
- A signalling point can be an originating point, a destination point or a
- signalling transfer point in a signalling relation. All three modes must be
- considered in the MTP.
- 3.1.4 Message labelling
- Each message contains a label. In the standard label the portion that is
- used for routing is called the routing label. This routing label includes:
- a) explicit indications of destination and originating points of the
- message, i.e., identification of the signalling relation concerned;
- b) a code used for load sharing which may be the least significant part of
- a label component that identifies a user transaction at level 4.
- The standard routing label assumes that each signalling point in a
- signalling network is allocated a code according to a code plan, established for
- the purpose of labelling, that is unambiguous within its domain. Messages
- labelled according to international and national code plans are discriminated by
- means of an indication in the service information octet included in each message.
- The standard routing label is suitable for national applications also.
- However, the signalling system includes the possibility for using different
- routing labels nationally.
- 3.2 Signalling message handling functions
- Figure 6/Q.701 illustrates the signalling message handling functions.
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- Figure 6/Q.701 - CCITT 34970
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- 3.2.1 Message routing
- Message routing is the process of selecting, for each signalling message
- to be sent, the signalling link to be used. In general, message routing is based
- on analysis of the routing label of the message in combination with predetermined
- routing data at the signalling point concerned.
- Message routing is destination-code dependent with typically an additional
- load-sharing element allowing different portions of the signalling traffic to a
- particular destination to be distributed over two or more signalling links. This
- traffic distribution may be limited to different links within a link set or
- applied to links in different link sets.
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- Each succession of signalling links that may be used to convey a message
- from the originating point to the destination point constitutes a message route.
- A signalling route is the corresponding concept for a possible path referring to
- a succession of link sets and signalling transfer points, between a given
- signalling point and the destination point.
- In Signalling System No. 7, message routing is made in a manner by which
- the message route taken by a message with a particular routing label is
- predetermined and, at a given point in time, fixed. Typically, however, in the
- event of failures in the signalling network, the routing of messages, previously
- using the failed message route, is modified in a predetermined manner under
- control of the signalling traffic management function at level 3.
- Although there are in general advantages in using a uniform routing of
- messages belonging to different User Parts, the service indicator included in
- each message provides the potential for using different routing plans for
- different User Parts.
- 3.2.2 Message distribution
- Message distribution is the process which, upon receipt of a message at
- its destination point, determines to which User Part or level 3 function the
- message is to be delivered. This choice is made on analysis of the service
- indicator.
- 3.2.3 Message discrimination
- Message discrimination is the process which, upon receipt of a message at
- a signalling point, determines whether or not the point is the destination point
- of that message. This decision is based on analysis of the destination code in
- the routing label in the message. If the signalling point is the destination
- point the message is delivered to the message distribution function. If it is not
- the destination point, and the signalling point has the transfer capability, the
- message is delivered to the routing function for further transfer on a signalling
- link.
- 3.3 Signalling network management functions
- Figure 6/Q.701 illustrates the signalling network management functions.
- 3.3.1 Signalling traffic management
- The tasks of the signalling traffic management function are:
- a) to control message routing; this includes modification of message
- routing to preserve, when required, accessibility of all destination
- points concerned or to restore normal routing;
- b) in conjunction with modifications of message routing, to control the
- resulting transfer of signalling traffic in a manner that avoids
- irregularities in message flow;
- c) flow control.
- Control of message routing is based on analysis of predetermined
- information about all allowed potential routing possibilities in combination with
- information, supplied by the signalling link management and signalling route
- management functions, about the status of the signalling network (i.e., current
- availability of signalling links and routes).
- Changes in the status of the signalling network typically result in
- modification of current message routing and thus in transfer of certain portions
- of the signalling traffic from one signalling link to another. The transfer of
- signalling traffic is performed in accordance with specific procedures. These
- procedures - changeover, changeback, forced rerouting and controlled rerouting -
- are designed to avoid, as far as the circumstances permit, such irregularities in
- message transfer as loss, mis-sequencing or multiple delivery of messages.
- The changeover and changeback procedures involve communication with other
- signalling point(s). For example, in the case of changeover from a failing
- signalling link, the two ends of the failing link exchange information (via an
- alternative path) that normally enables retrieval of messages that otherwise
- would have been lost on the failing link. However, as further explained later,
- these procedures cannot guarantee regular message transfer in all circumstances.
- A signalling network has to have a signalling traffic capacity that is
- higher than the normal traffic offered. However, in overload conditions (e.g.,
- due to network failures or extremely high traffic peaks) the signalling traffic
- management function takes flow control actions to minimize the problem. An
- example is the provision of an indication to the local user functions concerned
- that the Message Transfer Part is unable to transport messages to a particular
- destination in the case of total breakdown of all signalling routes to that
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- destination point. If such a situation occurs at a signalling transfer point, a
- corresponding indication is given to the signalling route management function for
- further dissemination to other signalling points in the signalling network.
- 3.3.2 Signalling link management
- The task of the signalling link management function is to control the
- locally connected link sets. In the event of changes in the availability of a
- local link set it initiates and controls actions aimed at restoring the normal
- availability of that link set.
- The signalling link management function also supplies information about
- the availability of local links and link sets to the signalling traffic
- management function.
- The signalling link management function interacts with the signalling link
- function at level 2 by receipt of indications of the status of signalling links.
- It also initiates actions at level 2 such as, for example, initial alignment of
- an out-of-service link.
- The signalling system can be applied with different degrees of flexibility
- in the method of provision of signalling links. A signalling link may for example
- consist of a permanent combination of a signalling terminal device and a
- signalling data link. It is also possible to employ an arrangement in which any
- switched connection to the remote end may be used in combination with any local
- signalling terminal device. It is the task of the signalling link management
- function in such arrangements to initiate and control reconfigurations of
- terminal devices and signalling data links to the extent such reconfigurations
- are automatic. In particular, this involves interaction, not necessarily direct,
- with a switching function at level 1.
- 3.3.3 Signalling route management
- Signalling route management is a function that relates to the
- quasi-associated mode of signalling only. Its task is to transfer information
- about changes in the availability of signalling routes in the signalling network
- to enable remote signalling points to take appropriate signalling traffic
- management actions. Thus a signalling transfer point may, for example, send
- messages indicating inaccessibility of a particular signalling point via that
- signalling transfer point, thus enabling other signalling points to stop routing
- messages to an incomplete route.
- 3.4 Testing and maintenance functions
- Figure 6/Q.701 illustrates that the signalling system includes some
- standard testing and maintenance functions that use level 3 messages.
- Furthermore, any implementation of the system typically includes various
- implementation-dependent means for testing and maintenance of equipment concerned
- with the other levels.
- 3.5 Use of the signalling network
- 3.5.1 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. International signalling is an example of an
- application for which this approach is suitable.
- 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
- need for redundancy for reliability, 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.
- Further considerations about the use of a signalling network are given in
- Recommendation Q.705.
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- 3.5.2 Provision of signalling facilities
- In general, the most important factor in the dimensioning of the
- signalling network is the need for reliability by means of redundancy. Depending
- on the signalling network structure and the potential for reconfiguration of
- signalling equipment, the required redundancy may be provided by different
- combinations of:
- - redundancy in signalling data links (e.g., nominated reserves or
- switched connections);
- - redundancy in signalling terminal devices (e.g., a common pool of
- terminals for the whole signalling point);
- - redundancy of signalling links within a link set (typically operating
- with load sharing);
- - redundancy in signalling routes for each destination (possibly
- operating with load sharing).
- The loading capacity of a digital signalling link is high in relation to
- the signalling traffic generated for call control signalling. Therefore, in many
- typical applications the links will be lightly loaded and signalling traffic
- volume will be a secondary factor in the dimensioning of the signalling network.
- However, in high signalling traffic applications or when analogue links with
- lower speeds are used, it may be necessary to dimension the traffic capacity by
- provision of additional signalling links. The message routing principles adopted
- for the signalling system allow partitioning of the total signalling traffic into
- different portions based on load sharing, destination point code and service
- information. Such partitioning provides a useful means of controlling the load
- and dimensioning of the capacity of different sections of a signalling network as
- it allows distribution of different portions of the signalling traffic. It can
- also be used to dedicate certain parts of a signalling network to signalling
- traffic related to a particular user.
- 3.5.3 Application of signalling network functions
- The signalling network functions provided by the signalling system are
- designed to cater for a range of signalling network configurations. It is not
- necessary that all of those functions be present at all signalling points. The
- necessary functional content at level 3 at a particular signalling point depends
- for example on what signalling mode(s) are used, whether or not it is a
- signalling transfer point, what type of signalling equipment redundancy is
- employed, etc. It is thus feasible to implement level 3 functions with modularity
- for different capabilities corresponding to different signalling network
- configurations. As a special case, it is even possible to apply the signalling
- system without using the level 3 element at all, e.g., in a small exchange or
- private automatic branch exchange which can only be reached via one primary pulse
- code modulation system.
- 4 Message transfer capability
- 4.1 General
- The Message Transfer Part recommendations specify methods by which
- different forms of signalling networks can be established. The requirements for
- the Message Transfer Part have been determined primarily by the requirements of
- call control signalling for the telephone and circuit switched data transmission
- services. However, the Message Transfer Part is also intended to have the ability
- to serve as a transport system for other types of information transfer. The
- following summarises the typical characteristics of the transport service that
- may be offered by the Message Transfer Part to a potential user of this ability.
- All information to be transferred by the Message Transfer Part must be
- assembled into messages. The linking of the source and sink of a message is
- inherent in the label in combination with the signalling routes existing between
- the two locations. From a transportation point of view each message is
- self-contained and handled
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- individually. The nature of the transport service offered by the Message Transfer
- Part is therefore similar to that offered by a packet switched network. In
- addition, all messages containing the same label constitute a set of messages
- that is handled in a uniform manner by the Message Transfer Part, thus ensuring,
- in normal circumstances, regular delivery in the correct sequence.
- 4.2 User location in system structure
- A potential user of the transport service is typically included in the
- system structure by provision of a separate User Part. This requires allocation
- of a service indicator code, the specification of which is part of both the
- Message Transport Part and User Part concerned.
- As an alternative, a potential user may be catered for, together with
- other similar users, by an already existing or new User Part. In such a case the
- discrimination between messages belonging to this potential user and the other
- similar users is an internal matter within the User Part concerned. It then
- follows that all messages belonging to such a User Part are necessarily handled,
- e.g., as regards routing, in a uniform manner by the Message Transfer Part.
- 4.3 Message content
- 4.3.1 Code transparency
- Information with any code combination generated by a user can be
- transferred by the Message Transfer Part provided that the message respects the
- requirements described below.
- 4.3.2 Service information
- Each message must contain service information coded in accordance with the
- rules specified in Recommendation Q.704, S 14.
- 4.3.3 Message label
- Each message must contain a label consistent with the routing label of the
- signalling network concerned. See also Recommendation Q.704, S 2.
- 4.3.4 Message length
- The information content of a message should be an integral number of
- octets.
- The total amount of signalling information transferable in one message is
- limited by some parameters of the signalling system; the signalling system can
- accept transfer of user information blocks in the order of 256 octets in single
- messages.
- Depending on the signalling traffic characteristics of a user and of other
- users sharing the same signalling facilities, there may be a need to limit
- message lengths below the system limit based on queueing delay considerations.
- In the case when information blocks generated by a user function exceed
- the allowed message length, it is necessary to implement means for segmentation
- and blocking of such information blocks within the User Part concerned.
- 4.4 User accessibility
- The accessibility of user functions through a signalling network depends
- on the signalling modes and routing plan employed in that network.
- In the case when only the associated mode of signalling is employed, only
- user functions located at adjacent signalling points may be accessed.
- In the case when quasi-associated signalling is employed, user functions
- located at any signalling point may be accessed provided that the corresponding
- message routing data is present.
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- 4.5 Transport service performance
- Further detailed information is provided in Recommendation Q.706.
- 4.5.1 Message transfer delay
- The normal delay for transfer of messages between user locations depends
- on factors such as distance, signalling network structure, signalling data link
- type and bit rate and processing delays.
- A small proportion of messages will be subject to additional delay because
- of transmission disturbances, network failures, etc.
- 4.5.2 Message transfer failures
- The Message Transfer Part has been designed to enable it to transfer
- messages in a reliable and regular manner even in the presence of network
- failures. However, inevitably some failures will occur the consequences of which
- cannot be avoided with economic measures. The types of failures that may occur
- and some typical probabilities of their occurrence are described below.
- Recommendation Q.706 provides further detailed information that can be used to
- estimate failure rates for particular cases.
- In the case when a potential user function requires a reliability of the
- transport service that cannot be guaranteed by the Message Transfer Part, the
- reliability of that user may be enhanced by adoption of appropriate level 4
- procedures, possibly including some means of supplementary end-to-end error
- control.
- The following types of message transfer failures are possible, and the
- expected probabilities for such failures in typical applications are indicated
- (see also Recommendation Q.706).
- a) Unavailability of the transport service to one or more locations - the
- availability of the message transfer capability depends on the
- redundancy provided in the signalling network; the availability can
- therefore be dimensioned.
- b) Loss of messages - the probability of loss of messages mainly depends
- on the reliability of signalling equipment; typically it is expected to
- be lower than 10-7.
- c) Mis-sequencing of messages - may in certain configurations of
- quasi-associated signalling occur with rare combinations of independent
- failures and disturbances. The probability, in such configurations, of
- a message being delivered out-of-sequence depends on many factors but
- is expected to be lower than 10-10.
- d) Delivery of false information - undetected errors may lead to the
- delivery of false information; the possibility of an error in a message
- delivered is expected to be lower than 10-10.
- 5 Differences from the Red Book
- The ongoing development of the MTP during this study period has resulted
- in a number of differences occurring between the Recommendations as documented in
- the Red Book and these current Recommendations (Blue Book). In order to limit
- interworking problems, a backwards compatibility mechanism is required (see S 6).
- As an initial step towards producing such a mechanism, this section identifies
- the new items and items changed because of operational considerations, that have
- been included in the Blue Book. This section does not consider editorial or
- technical corrections.
- 5.1 Signalling Information Field length
- The maximun length of the Signalling Information Field has been increased
- to 272 octets. This was previously a National only option. Networks using both
- signalling terminals with 62 octet maximum SIF length handling capability and
- signalling terminals with 272 octet maximum SIF length handling capability must
- ensure that messages with SIFs longer than 62 octets cannot be routed to
- signalling links that are unable to handle them (see S 7).
- 5.2 Signalling Point Restart
- The Signalling Point Restart procedure (see Q.704 S 9) has been included
- together with a definition of Signalling Point availability. This procedure
- allows a graceful increase in message traffic at a restarting Signalling Point.
- 5.3 Management Blocking
- The Management Blocking procedure for Signalling links has been deleted.
- No interworking problems are foreseen in networks where some Signalling Points
- still incorporate this procedure and others are implemented in accordance with
- the Blue Book.
- 5.4 Signalling Link Test
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- The Signalling Link Test has been enhanced to check that both ends of the
- link agree as to which signalling link is being tested. No interworking problems
- are foreseen (see Q.707 S 2.2).
- 5.5 Compatibility mechanism
- General principles have been incorporated in the Message Transfer Part
- that will allow implementations to the Blue Book to be compatible with
- implementations to Red/Yellow Books and future issues of the Recommendations (see
- S 6).
- 5.6 Timer values
- The values of existing Q.703 and Q.704 Timers have been finalized (see S
- 7).
- 5.7 Processor Outage
- The actions related to Processor Outage have been clarified (see Q.703 S 8
- and Q.704 S 4, 5 and 6). No interworking problems are foreseen.
- 5.8 User flow control
- Procedures for Message Transfer Part User Flow Control have been adopted
- for use at a Signalling Point when an MTP user has become unavailable (see Q.704
- S 11 and Q.701 S 7).
- 5.9 Management Inhibiting and Management Inhibiting test procedure
- The time-controlled changeover procedure is now used to divert traffic
- from a management inhibited link.
- To verify the inhibited status of a link, test procedures have been
- introduced into management inhibiting (see Q.704 S 10 and Q.701 S 7).
- 5.10 Signalling point/signalling transfer point congestion
- Procedures to detect and handle signalling point/signalling transfer point
- congestion have now been identified (see Q.704 S 11.2.6). No interworking
- problems are foreseen.
- 6 Compatibility in the message transfer part
- To enable implementations of Signalling System No. 7 to this issue (Blue
- Book) of the Recommendations to achieve compatibility with implementations to
- other issues, e.g., Yellow, Red and 1992 Books, a set of appropriate procedures
- and guidelines has been concluded in Recommendation Q.700. This section
- identifies the action that is required within the Message Transfer Part to ensure
- both forward and backwards compatibility. The areas considered are the treatment
- of spare fields, spare values, lack of acknowledgements and unreasonable
- information.
- 6.1 Unreasonable Information
- The following actions occur in the MTP when messages are received
- containing unreasonable information.
- 6.1.1 Messages containing an unallocated SIO value
- When messages containing an unallocated SIO value are received at either a
- terminating Signalling Point or an STP that employs message routing based on both
- DPC and SIO, they should be discarded. If required, a report should be made to
- management.
- 6.1.2 Messages containing an unallocated H0/H1 code
- When messages containing an unallocated H0/H1 code are received at the
- appropriate functional block within the MTP, they are discarded. There should be
- no impact on any protocol and, if required, a report should be made to
- management.
- 6.1.3 Messages containing an unallocated value in a recognized field
- When massages are received at an owning function within the MTP containing
- a field with an unallocated value they are discarded and, if required, a report
- made to management. There should be no impact on any current protocol.
- (An owning function is a function to which a received message pertains.)
- 6.2 Treatment of spare fields
- The MTP will handle spare fields in MTP messages in the following manner:
- i) Spare fields are set to zero on message creation, and are not examined
- on reception at the destination owning function.
- ii) Spare subfields are set to zero on message creation, and are not
- examined on reception at the destination owning function.
- iii) Implementations of the STP function should transit all messages
- unchanged, including spare fields and spare subfields.
- 6.3 Lack of acknowledgement
- Should a message that requires an acknowledgement not receive one within a
- specified time, the message will be repeated, unless the protocol specifies
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- otherwise. However, subsequent failures to receive the acknowledgement should not
- cause indefinite repeat attempts.
- 7 Interworking of Yellow, Red and Blue MTP implementations
- There have been a number of changes introduced into this issue (Blue Book)
- of Recommendations Q.701-707 from the previous issue (Red Book). The changes have
- been identified in S 5 and although in the majority of cases there will be no
- interworking problems between a Signalling Point/STP implemented to the Red Book
- and one implemented to a Blue Book, there are some instances where problems will
- arise. This section gives guidance on the appropriate action that can be taken in
- the MTP to overcome interworking problems and also considers Yellow to Red Book
- and Yellow to Blue Book interworking.
- 7.1 Yellow Book to Red Book interworking
- There were four areas where changes from the Yellow Book to the Red Book
- introduced interworking problems:
- i) Level 2 flow control, LSSU SIB introduced.
- ii) Transfer Restricted (TRF) and Transfer Controlled (TFC) messages and
- procedures were introduced into the Red Book.
- iii) Transfer Allowed (TAA) and Transfer Prohibited (TPA)
- acknowledgements were deleted from the Red Book.
- iv) Management inhibiting procedures were introduced into the Red Book.
- The suggested action required at the Yellow and/or Red Book SP/STP to
- enable interworking is contained in the following point items.
- 7.1.1 Level 2 Flow control
- The Red Book SP/STP should apply normal level 2 flow control action (i.e.,
- acknowledgements are withheld and SIBs sent). The Yellow Book SP/STP should
- ignore the LSSU SIB when received. It is recognized that although flow control is
- not performed in this case, interworking is possible. However, a possible option
- would be to set the congestion threshold at the Red Book SP/STP, such that flow
- control is not triggered on that signalling relation.
- 7.1.2 Transfer restricted and Transfer controlled procedures
- The Yellow Book SP/STP should ignore TFR and TFC messages when received.
- 7.1.3 Transfer allowed/Transfer prohibited acknowledgements
- The Yellow Book SP/STP should limit the repetition of the TFA/TFP message
- to once only. The Red Book SP/STP should ignore the acknowledgement messages when
- they are received.
- 7.1.4 Management inhibiting procedure
- The Yellow Book SP/STP should ignore the Link Inhibit (LIN) and Link
- Uninhibit (LUN) messages when received. The Red Book SP/STP should limit the
- repetition of the LIN/LUN message.
- 7.2 Red Book to Blue Book interworking
- The changes in this issue (Blue Book) from the Red Book Q.701-707
- Recommendations are identified in S 5. There are five areas where changes have
- resulted in interworking problems:
- i) Signalling Point Restart procedure has introduced the Traffic Restart
- Allowed (TRA) message.
- ii) Timer values have been confirmed in this issue, previous values were
- provisional.
- iii) User Flow Control procedure has introduced the User Part
- Unavailable (UPU) message.
- iv) Signalling Information Field length increase will require action to
- prevent overlength messages being sent on a link that is not capable of
- handling them.
- v) Management-inhibiting test procedure has introduced Link Local inhibit
- test message (LLT) and Link Remote inhibit test message (LRT).
- The suggested actions required at the Red and/or Blue Book SP/STP to
- enable interworking are contained in the following point items.
- 7.2.1 Signalling Point Restart
- The Red Book SP/STP should ignore the Traffic Restart Allowed messages
- when received.
- 7.2.2 Q.703 and Q.704 timer values
- Where possible, an SP/STP implemented to the Red Book should adopt the
- timer values specified in the Blue Book when interworking with a Blue Book
- SP/STP. For timer values (see Q.703 S 12 and Q.704 S 16).
- 7.2.3 User flow control
- The Red Book SP/STP should ignore the User Part Unavailable (UPU) message
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- if received.
- 7.2.4 Management inhibit test procedure
- The Red Book SP/STP should ignore the Link Local inhibit test (LLT) and
- Link Remote inhibit test (LRT) messages. A report to local management should also
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- 7.2.5 SIF length increase
- The SP/STP with 272 octet SIF length handling capability should prevent
- overlength messages from being routed over signalling links that only have a 62
- octet SIF handling capability.
- 7.2.6 SIF length increase (National networks option)
- In the international Signalling System No. 7 network, it should be
- possible to identify signalling links/routes with a limited SIF length handling
- capability and prevent overlength messages being transmitted over them by
- administrative action based on the exchange of operational data. However, with
- some national networks due to the rapid change in status of SP/STP implementation
- level (e.g., 62 to 272 SIF capability) and the number of SP/STPs in the network,
- this administrative action and data exchange may not be adequate. In this
- situation, a mechanism based on the following MTP activities may be more
- appropriate.
- i) Detection of a link with 272 SIF capability may be achieved by coding
- the "D" bit of LSSUs sent during alignment as 1 (with 62 octet SIF
- links it would be 0). On receipt of this LSSU, a Blue Book SP/STP would
- mark the link/route as having 272 SIF capability. A Red Book SP/STP
- would ignore the coding of the "D" bit and treat the LSSU in the normal
- manner.
- ii) When a Blue Book SP/STP receives a message for onward routing, it will
- check if the message (SIF) is greater than 62 octets. If the SIF is
- greater than 62 octets, it will verify that the link/route can handle a
- message of this length. Should the link/route not have the SIF length
- capability, the message will be discarded and an indication sent to the
- message origin. A Red Book SP/STP should not receive a message with an
- SIF > 62 octets.
- iii) If the message originator is a local MTP User, an MTP PAUSE
- primitive will be returned by the MTP in response to an overlength
- message (see S 8). Should the originator be at a remote SP, a TFA coded
- to indicate that only 62 octet SIF messages can be transferred will be
- returned by the MTP in response to an overlength message (see Q.704 S
- 15).
- In national networks using an SIF compatibility mechanism, the two spare
- bits in the TFA (see Q.704 S 15.8.2) may be coded as an SIF compatibility
- indicator as follows:
- bit B A
- 0 0 Allow 62 octet SIFs/Prohibit 272, X and Y octet SIFs
- 0 1 Allow 62 and 272 octet SIFs/Prohibit X and Y octet SIFs
- 1 0 Allow 62, 272 and X octet SIFs Prohibit Y octet SIFs.
- 1 1 Allow 62, 272, X and Y octet SIFs.
- Note - 272 < X < Y octets, the values of X and Y are for further study.
- 7.3 Yellow Book to Blue Book Interworking
- The changes between Yellow and Blue Books have taken place in two stages:
- Yellow to Red and Red to Blue. Therefore, to achieve interworking between Yellow
- and Blue Book implementations, the actions specified in SS 7.1 and 7.2 should be
- applied. In S 7.1 Red Book SP/STP should be read as Blue Book SP/STP and in S 7.2
- Red Book SP/STP should be read as Yellow Book SP/STP.
- There is one change from the Red Book in the Blue Book that will have an
- additional impact on interworking with the Yellow Book, and that is the deletion
- of the blocking procedure. This means that while a Yellow Book implementation can
- block a signalling link, a Blue Book node can neither inhibit nor block the link
- in the opposite direction.
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- 8 Primitives and Parameters of the Message Transfer Part
- The primitives and parameters are shown in Table 1/Q.701.
- TABLE 1/Q.701
- Message transfer part service primitives
- Primitives
- Generic Name Specific Name Parameters
- MTP-TRANSFER Request OPC (see Q.704 S 2.2)
- Indication
- DPC (see Q.704 S 2.2)
- SLS (see Q.704 S 2.2)
- (Note 1)
- SIO (see Q.704 S 14.2)
- User data (see Q.703 S
- 2.3.8)
- MTP-PAUSE (Stop) Indication Affected DPC
- MTP-RESUME Indication Affected DPC
- (Start)
- MTP-STATUS Indication Affected DPC Cause (Note
- 2)
- Note 1 - The MTP users should take into account that this
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- parameter is used for load sharing by the MTP, therefore, the SLS
- values should be distributed as equally as possible. The MTP
- guarantees (to a high degree of probability) an in-sequence
- delivery of messages which contain the same SLS code.
- Note 2 - The Cause parameter has, at present, two values:
- i) Signalling network congested (level)
- This parameter value is included if national options with
- congestion priorities and multiple signalling link states without
- congestion priorities as in Recommendation Q.704 are implemented.
- ii) Remote User unavailable.
- 8.1 Transfer
- The primitive "MTP-TRA s used between level 4
- and level 3 (SMH) to provide the MTP message transfer service.
- 8.2 Pause
- The primitive "MTP-PAUSE" i to the "Users" the
- total inability of providing the MTP service to the specified
- destination.
- 8.3 Resume
- The primitive "MTP-RESUME" i to the "User" the
- total ability of providing the MTP service to the specified
- destination.
- This primitive corresponds to the destination accessible state
- as defined in Recommendations Q.704.
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- 8.4 Status
- The primitive "MTP-STATUS" ind o the "Users" the
- partial inability of providing the MTP service specified
- destination. The primitive is also used to indicate to a User that
- a remote corresponding User is unavailable (see Q.704 S 11.2.7).
- In the case of national option with congestion priorities or
- multiple signalling link congestion states without priorities as in
- Recommendation Q.704 are implemented, this "MTP-STATUS" primitive
- is also used to indicate a change of congestion level.
- This primitive corresponds to the destination congested/User
- Part unavailable state as defined in Recommendation Q.704.
- 8.5 Restart
- The MTP indicates to the "Users" at the restarting SP that the MTP is
- commencing or ending the signalling point restart procedure (see Recommendation
- Q.704, S 9).
- The indication may have the following qualifiers:
- i) Begin
- ii) End
- The qualifier "Begin" indicates to the "Users" that all destinations
- should be marked as accessible (but that the resumption of signalling traffic
- must await the reception of MTP-RESUME primitive or MTP restart indication
- "End").
- The qualifier "End" indicates to the "Users" that signalling traffic may
- be restarted, taking into account any MTP-PAUSE primitives previously received.
- The means of conveying the MTP restart indication to the MTP "Users", is
- for further study.
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