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Recommendation Q.761
FUNCTIONAL DESCRIPTION OF THE ISDN USER PART
OF SIGNALLING SYSTEM No. 7
1 General
The ISDN User Part is the Signalling System No. 7 protocol which provides
the signalling functions required to support basic bearer services and
supplementary services for voice and non-voice applications in an integrated
services digital network.
The ISDN User Part is also suited for application in dedicated telephone
and circuit switched data networks and in analogue and mixed analogue/digital
networks. In particular the ISDN User Part meets the requirements defined by
CCITT for worldwide international semi-automatic and automatic telephone and
circuit switched data traffic.
The ISDN User Part is furthermore suitable for national applications. Most
signalling procedures, information elements and message types specified for
international use are also required in typical national applications. Moreover,
coding space has been reserved in order to allow national administrations and
recognized private operating agencies to introduce network specific signalling
messages and elements of information within the internationally standardized
protocol structure.
The ISDN User Part makes use of the services provided by the Message
Transfer Part (MTP) and in some cases by the Signalling Connection Control Part
(SCCP) for the transfer of information between ISDN User Parts.
The ISDN User Part protocol which supports the basic bearer service is
described in Recommendations Q.761 to Q.764 and Q.766. A general description of
ISDN User Part signals and messages is provided in Recommendation Q.762. Message
formats and message field codings are defined in Recommendation Q.763, while the
signalling procedures are described in Recommendation Q.764. Recommendation Q.766
deals with ISDN User Part performance objectives.
ISDN User Part protocol elements which support supplementary services are
described in Recommendation Q.730.
Note - The message set, message formats and procedures specified in this
version of the ISDN User Part protocol are not in complete alignment with those
of the 1984 version (Red Book). The two versions of the protocol are therefore
not compatible in all aspects.
2 Services supported by the ISDN User Part
The ISDN User Part protocol supports the basic bearer service, i.e. the
establishment, supervision and release of 64 kbit/s circuit switched network
connections between subscriber line exchange terminations.
In addition to the basic bearer service the ISDN User Part also supports
the following supplementary services:
- calling line identification,
- call forwarding,
- closed user groups,
- directing dialling in, and
- user-to-user signalling.
Fascicle VI.8 - Rec. Q.761 PAGE1
3 Services assumed from the Message Transfer Part (MTP)
3.1 General
This section describes the functional interface presented by the Message
Transfer Part to the ISDN User Part. In accordance with the description
techniques defined by the Open System Interconnection (OSI) model, information is
transferred to and from the MTP in the form of Parameters carried by Primitives.
The general syntax of a primitive is as follows:
X Generic name Specific name Parameter
where
X designates the function providing the service (the MTP, in this case),
the Generic name describes an action by X,
the Specific name indicates the purpose of the primitive, i.e. whether it
conveys a request for service, an indication that service related
information has been received, a response to a service request or a
confirmation that the requested service has been performed, and
the Parameters contain the elements of supporting information transferred
by the primitive.
3.2 Description of primitives
The following paragraphs describe the primitives used across the ISDN User
Part-Message Transfer Part functional interface. The primitives together with the
parameters carried by each primitive are also shown in Table 1/Q.761.
3.2.1 Transfer
The MTP-TRANSFER primitive is used either by the ISDN User Part to access
the Signalling Message Handling function of the Message Transfer Part or by the
latter to deliver signalling message information to the ISDN User Part.
3.2.2 Pause
The MTP-PAUSE primitive is sent by the Message Transfer Part to indicate
its inability to transfer messages to the destination specified as a parameter.
3.2.3 Resume
The MTP-RESUME primitive is sent by the Message Transfer Part to indicate
its ability to resume unrestricted transfer of messages to the destination
specified as a parameter.
3.2.4 Status
The MTP-STATUS primitive is sent by the Message Transfer Part to indicate
that the signalling route to a specific destination is congested or the ISDN User
Part at the destination is unavailable. The affected destination and the
congestion indication are carried as parameters (see Table 1/Q.761) in the
primitive.
TABLE 1/Q.761
Message transfer part service primitives
Primitives
Generic name Specific name Parameters
MTP-TRANSFER Request OCP
indication DPC
SLS
PAGE4 Fascicle VI.8 - Rec. Q.761
SIO
Signalling info.
MTP-PAUSE Indication Affected DPC
MTP-RESUME Indication Affected DPC
MTP-STATUT Indication Affected DPC
Cause (see Note)
OPC Originating point code
DPC Destination point code
SLS Signaling link selection code
SIO Service information octet
Note - The cause parameter can assume two values:
- signalling network congested (level), where level is included only if
natioal options with congestion priorities and multiple signalling states
without congestion priorities (see Recommendation Q.704).
- remote user unavailable.
4 End-to-end signalling
4.1 General
End-to-end signalling is defined as the capability to transfer signalling
information of end points significance directly between signalling end points in
order to provide a requesting user with a basic or supplementary service.
End-to-end signalling is used typically between call originating and
terminating local exchanges, to request or to respond to requests for additional
call related information, to invoke a supplementary service or to transfer
user-to-user information transparently through the network.
End-to-end signalling procedures are described in Recommendation Q.764, S
3.
The following two methods of end-to-end signalling are supported:
4.2 SCCP method of end-to-end signalling
Connection-oriented or connectionless transfer of end-to-end signalling
information can be accomplished by using the service provided the Signalling
Connection Control Part (SCCP) of Signalling System No. 7. The relevant
procedures are described in Recommendation Q.764, S 3.4.
4.3 Pass-along method of end-to-end signalling
The pass-along method of end-to-end signalling provides transfer of
signalling information without requiring the services of the SCCP.
This method may be used between two exchanges when the information to be
transferred relates to an existing call for which a physical connection between
the same two exchanges has been established. The information transfer in this
case occurs over the same signalling path as that used to set up the call and
establish the physical connection.
The relevant procedures are described in Recommendation Q.764, S 3.3.
5 Future enhancements
Requirements for additional protocol capabilities, such as the ability to
support new supplementary services, will result from time to time in the need to
add to or modify existing protocol elements and thus to create a new protocol
version.
In order to ensure adequate service continuity, the insertion of a new
protocol version into one part of a network should be transparent to the
remainder of the network. Compatible interworking between protocol versions is
optimized by adhering to the following guidelines when specifying a new version:
1) Existing protocol elements, i.e. procedures, messages, parameters and
codes, should not be changed unless a protocol error needs to be
corrected or it becomes necessary to change the operation of the
service that is being supported by the protocol.
2) The semantics of a message, a parameter or of a field within a
parameter should not be changed.
3) Established rules for the formatting and encoding messages should not
be modified.
4) The addition of parameters to the mandatory part of an existing message
should not be allowed. If needed, a new message should be defined
containing the desired set of existing and new mandatory parameters.
5) A parameter may be added to an existing message as long as it is
allocated to the optional part of the message.
6) The addition of new octets to an existing mandatory fixed length
parameter should be avoided. If needed, a new optional parameter should
Fascicle VI.8 - Rec. Q.761 PAGE1
be defined containing the desired set of existing and new information
fields.
7) The sequence of fields in an existing variable length parameter should
remain unchanged. New fields may be added at the end of the existing
sequence of parameter fields. If a change in the sequence of parameter
fields is required, a new parameter should be defined.
8) The all zeros code point should be used exclusively to indicate an
unallocated (spare) or insignificant value of a parameter field. This
avoids an all zeros code, sent by one protocol version as a spare
value, to be interpreted as a significant value in another version.
PAGE4 Fascicle VI.8 - Rec. Q.761