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All drawings appearing in this Recommendation have been done in Autocad.
Recommendation Q.521
EXCHANGE FUNCTIONS
1 General
This Recommendation applies to digital local, combined, transit and
international exchanges for telephony in Integrated Digital Networks (IDN) and
mixed (analogue/digital) networks and also to local, combined, transit and
international exchanges in an Integrated Services Digital Network (ISDN).
The field of application of this Recommendation is more fully defined in
Recommendation Q.500.
Some text may only apply to a certain type (types) of exchange, e.g.
digital transit, local or combined. Where this occurs, the application is defined
in the text. Not all the recommended functions will necessarily be provided in
every exchange.
2 Exchange functions - Introduction and framework
2.1 General
The purpose of this Recommendation is to specifically address those
functions required to support basic and supplementary services in performing this
test, this Recommendation takes account of the principles set down in
Recommendation I.310 and draws a clear distinction between services and the
exchange capabilities required to support them.
It should be noted that the list of functions identified in this
Recommendation is not necessarily extensive.
2.2 Exchange model
The functions described in this and associated Recommendations can be
considered within the framework of an exchange functional model. Such a model is
shown in Figure 1/Q.521. This divides the exchange into three functional areas as
follows:
a) control functions - Those functions required to control services and
connections, e.g. signalling, routing and connection/resources handling
functions;
b) connection functions - Those functions directly related to the
connection path through an exchange, i.e. switching and transmission
mechanism (including ET);
c) operation and maintenance functions - Those functions of an
operational, management and maintenance nature which are not employed
for call establishment and supervisory purposes, e.g. test functions.
The exchange functional model shown in Figure 1/Q.521 is appropriate to
exchanges operating in an IDN and also those operating in an IDN which is
evolving towards an ISDN. In terms of this Recommendation, most of the functions
fall within the control functions area.
Figure 1/Q.521 - T1108861-88
Connection functions are primarily covered in Recommendation Q.522. These
address the basic switch characteristics of different connection types. OAM
functions are primarily covered in Recommendation Q.542.
3 Utilization of exchange functions for services
3.1 General
Exchange functions are used and reused in various stages of call
processing. Some may be combined with others to create features used in providing
supplementary services. The specific functions used in a given context will be
determined by the requested service.
Within the framework of the model shown in Figure 1/Q.521 the utilization
of functions arising from a service request, can be considered in the following
way:
a) on receipt of a service request (via the Signalling Functions) the
Service Processing Functions are used to identify the appropriate
connection type(s);
b) the appropriate type of connection is established by use of the
Connection/Resources Handling Functions;
c) Supplementary Services which involve additional functions and
information flows beyond those required for bearer services, are
provided under the control of logic residing in the Service Processing
Function. This logic is designed to provide specific services.
Corresponding service/feature capabilities must also reside in the
Fascicle VI.5 - Rec. Q.521 PAGE1
Signalling and Connection/Resources Handling Functions.
In addition to services provided by use of logic/data residing in the
exchange, some services may be provided under the control of logic located at
separate specialized nodes (Service Control Points). Also, data required or
process certain service requests may be kept in a remote data base accessed by
use of the Signalling Function.
4 General functions required for operation of an exchange in the IDN, ISDN
or mixed analogue/digital environment
4.1 Timing and synchronization
4.1.1 Exchange timing - Ability to distribute timing within the exchange so that
it will maintain synchronism on 64 kbit/s channel timeslots in a connection
through the exchange.
4.1.2 Synchronization - Ability to operate in the IDN or ISDN in synchronism
with other digital entities and provide timing signals to other network entities
as required.
4.1.3 Interval timing - Ability to measure time between events as required in
call processing and/or in signalling.
4.1.4 Time-of-day clock - Ability to determine time of day.
Note - The level of accuracy is for further study.
4.2 Signalling
4.2.1 User-access signalling functions
4.2.1.1 Ability to receive and interpret decadic or Dual Tone Multi-Frequency
(DTMF) signalling from user terminals.
4.2.1.2 Ability to support user-access signalling layers 1 and 2 in accordance
with Recommendations I.430, Q.921 (I.441).
4.2.1.3 Ability to communicate with user terminals using layer 3 signalling in
accordance with Recommendation Q.931 (I.451).
4.2.2 Network signalling functions
4.2.2.1 Ability to use and support CCITT signalling systems included in
Recommendation Q.7, particularly CCITT Signalling System No. 7.
4.2.2.2 Ability to communicate with other network entities using the CCITT
Signalling System No. 7, Q.700-Series of Recommendations User Parts.
PAGE2 Fascicle VI.5 - Rec. Q.521