ISDN Concept
The concept of ISDN is best introduced by considering it from several different
viewpoints:
* Principles of ISDN
* The user interface
* Objectives
* Services
Principles of ISDN
Standards for ISDN have been defined by ITU-T (formerly CCITT), a topic that we
explore later in this section. Table A.l, which is the complete text of one of the
TABLE A.1 Recommendation 1.120 (1988).
1 Principles of ISDN
1.1 The main feature of the ISDN concept is the support of a wide range of voice and nonvoice applications
in the same network. A key element of service integration for an ISDN is the provision of
a range of services (see Part I1 of the I-Series in this Fascicle) using a limited set of connection
types and multipurpose user-network interface arrangements (see Parts 111 and IV of the I-Series
in Fascicle 111.8).
1.2 ISDNs support a variety of applications including both switched and non-switched connections.
Switched connections in an ISDN include both circuit-switched and packet-switched connections
and their concatenations.
1.3 As far as practicable, new services introduced into an ISDN should be arranged to be compatible
with 64 kbitls switched digital connections.
1.4 An ISDN will contain intelligence for the purpose of providing service features, maintenance and
network management functions. This intelligence may not be sufficient for some new services and
may have to be supplemented by either additional intelligence within the network, or possibly
compatible intelligence in the user terminals.
1.5 A layered protocol structure should be used for the specification of the access to an ISDN. Access
from a user to ISDN resources may vary depending upon the service required and upon the status
of implementation of national ISDNs.
1.6 It is recognized that ISDNs may be implemented in a variety of configurations according to specific
national situations.
2 Evolution of ISDNs
2.1 ISDNs will be based on the concepts for telephone IDNs and may evolve by progressively incorporating
additional functions and network features including those of any other dedicated networks
such as circuit-switching and packet-switching for data so as to provide for existing and
new services.
2.2 The transition from an existing network to a comprehensive ISDN may require a period of time
extending over one or more decades. During this period arrangements must be developed for the
networking of services on ISDNs and services on other networks (see Part V).
2.3 In the evolution towards an ISDN, digital end-to-end connectivity will be obtained via plant and
equipment used in existing networks, such as digital transmission, time-division multiplex switching
andlor space-division multiplex switching. Existing relevant recommendations for these constituent
elements of an ISDN are contained in the appropriate series of recommendations of CCI'M and
of CCIR.
2.4 In the early stages of the evolution of ISDNs, some interim user-network arrangements may need
to be adopted in certain countries to facilitate early penetration of digital service capabilities.
Arrangements corresponding to national variants may comply partly or wholly with I-Series Recommendations.
However, the intention is that they not be specifically included in the I-Series.
2.5 An evolving ISDN may also include at later stages switched connections at bit rates higher and
lower than 64 kbitls.
ISDN-related standards, states the principles of ISDN from the point of view of
CCITT. Let us look at each of these points in turn:
1. Support of voice and nonvoice applications using a limited set of standardized
facilities. This principle defines both the purpose of ISDN and the means
of achieving it. The ISDN supports a variety of services related to voice communications
(telephone calls) and nonvoice communications (digital data
exchange). These services are to be provided in conformance with standards
(ITU-T recommendations) that specify a small number of interfaces and data
transmission facilities.
2. Support ,for switched and nonswitched applications. ISDN supports both circuit
switching and packet switching. In addition, ISDN supports nonswitched
services in the form of dedicated lines.
3. Reliance on 64-kbps connections. ISDN provides circuit-switched and packetswitched
connections at 64 kbps; this is the fundamental building block of
ISDN. This rate was chosen because, at the time, it was the standard rate for
digitized voice, and, hence, was being introduced into the evolving integrated
digital networks (IDNs). Although this data rate is useful, it is unfortunately
restrictive to rely solely on it. Future developments in ISDN will permit
greater flexibility.
4. Intelligence in the network. An ISDN is expected to be able to provide sophisticated
services beyond the simple setup of a circuit-switched call.
5. Layered protocol architecture. The protocols for user access to ISDN exhibit a
layered architecture and can be mapped into the OSI model. This procedure
has a number of advantages:
Standards already developed for OSI-related applications may be used
on ISDN. An example is X.25 level 3 for access to packet-switching services
in ISDN.
New ISDN-related standards can be based on existing standards, reducing
the cost of new implementations. An example is LAPD, which is
based on LAPB.
Standards can be developed and implemented independently for various
layers and functions within a layer; this allows for the gradual implementation
of ISDN services at a pace appropriate for a given provider or a
given customer base.
6. Variety of configurations. More than one physical configuration is possible for
implementing ISDN; this allows for differences in national policy (singlesource
versus competition), in the states of technology, and in the needs and
existing equipment of the customer base.
The User Interface
Figure A.l is a conceptual view of the lSDN from a user, or customer, point of view.
The user has access to the ISDN by means of a local interface to a "digital pipe" of
a certain bit rate. Pipes of various sizes are available to satisfy differing needs. For
example, a residential customer may require only sufficient capacity to handle a
telephone and a videotex terminal. An office will undoubtedly wish to connect to
the ISDN via an on-premise digital PBX, and will require a much higher capacity
pipe.
At any given point in time, the pipe to the user's premises has a fixed capacity,
but the traffic on the pipe may be a variable mix up to the capacity limit. Thus,
a user may access circuit-switched and packet-switched services, as well as other services,
in a dynamic mix of signal types and bit rates. To provide these services, the
ISDN requires rather complex control signals to instruct it how to sort out the timeA.
l / OVERVIEW OF ISDN 743
Local area network
(LAN)
FIGURE A.l Conceptual view of ISDN connection features.
multiplexed data and provide the required services. These control signals are also
multiplexed onto the same digital pipe.
An important aspect of the interface is that the user may, at any time, employ
less than the maximum capacity of the pipe, and will be charged according to the
capacity used rather than "connect time." This characteristic significantly diminishes
the value of current user design efforts that are geared to optimize circuit
utilization by use of concentrators, multiplexers, packet switches, and other linesharing
arrangements.
Objectives
Activities currently under way are leading to the development of a worldwide
ISDN. This effort involves national governments, data processing and communications
companies, standards organizations, and other agencies. Certain common
objectives are, by and large, shared by this disparate group. We list here the key
objectives:
Standardization. It is essential that a single set of ISDN standards be provided
to permit universal access and to permit the development of cost-effective
equipment.
Transparency. The most important service to be provided is a transparent
transmission service, thereby permitting users to develop applications and
protocols with the confidence that they will not be affected by the underlying
ISDN.
Separation of competitive functions. It must be possible to separate out functions
that could be provided competitively as opposed to those that are fun744
APPENDIX A / ISDN AND BROADBAND ISDN
damentally part of the ISDN. In many countries, a single, government-owned
entity provides all services. Some countries desire (in the case of the United
States, require) that certain enhanced services be offered competitively (e.g.,
videotex, electronic mail).
@ Leased and switched services. The ISDN should provide dedicated point-topoint
services as well as switched services, thereby allowing the user to optimize
implementation of switching and routing techniques.
@ Cost-related tariffs. The price for ISDN service should be related to cost, and
should be independent of the type of data being carried. One type of service
should not be in the position of subsidizing others.
Smooth migration. The conversion to ISDN will be gradual, and the evolving
network must coexist with existing equipment and services. Thus, ISDN interfaces
should evolve from current interfaces, and provide a migration path
for users.
@ Multiplexed support. In addition to providing low-capacity support to individual
users, multiplexed support must be provided to accommodate userowned
PBX and local network equipment.
There are, of course, other objectives that could be named. Those listed above
are certainly among the most important and widely accepted, and each helps to
define the character of the ISDN.
Architecture
Figure A.2 is a block diagram of ISDN. ISDN supports a new physical connecter for
users, a digital subscriber loop (link from end user to central or end office), and
modifications to all central office equipment.
The area to which most attention has been paid by standards organizations is
that of user access. A common physical interface has been defined to provide, in
essence, a DTE-DCE connection. The same interface should be usable for telephone,
computer terminal, and videotex terminal. Protocols are needed for the
exchange of control information between user device and the network. Provision
must be made for high-speed interfaces to, for example, a digital PBX or a LAN.
The subscriber loop portion of today's telephone network consists of twisted
pair links between the subscriber and the central office, carrying 4-kHz analog signals.
Under the ISDN, one or two twisted pairs are used to provide a basic fullduplex
digital communications link.
The digital central office connects the numerous ISDN subscriber loop signals
to the IDN. In addition to providing access to the circuit-switched network, the central
office provides subscriber access to dedicated lines, packet-switched networks,
and time-shared, transaction-oriented computer services. Multiplexed access via
digital PBX and LAN must also be accommodated.
Standards
The development of ISDN is governed by a set of recommendations issued by
ISDN, called the I-series Recommendations. These Recommendations, or stanA.
l / OVERVIEW OF ISDN 745
Subscriber loop- I
ISDN channel structures: I
I
Basic = 64 kbps + 64 kbps + 16 kbps I
Primary = multiplexed 64 kbps channels I
I
I
-_ I! swDiticghietadl bcaircckubiot-n e
FIGURE A.2 Block diagram of ISDN functions.
Common
phys~cal
interface
dards, were first issued in 1984. A more complete set was issued in 1988. Most of the
Recommendations have been updated, at irregular intervals, since that time. The
bulk of the description of ISDN is contained in the I-series Recommendations, with
some related topics covered in other Recommendations. The characterization of
ISDN contained in these Recommendations is centered on three main areas:
- . A
ISDN I
I
central
office
1. The standardization of services offered to users, so as to enable services to be
internationally compatible.
2. The standardization of user-network interfaces, so as to enable terminal
equipment to be portable, and to assist in (1).
3. The standardization of ISDN capabilities to the degree necessary to allow
user-network and network-network interworking, and thus achieve (1)
and (2).
The I-series Recommendations are broken up into six main groupings, labeled
1.100 through 1.600.
1.100 Series-General Concepts
The 1.100 series serves as a general introduction to ISDN. The general structure of
the ISDN recommendations is presented as well as a glossary of terms. 1.120 provides
an overall description of ISDN and the expected evolution of ISDNs. 1.130
introduces terminology and concepts that are used in the 1.200 series to specify
services.
746 APPENDIX A / ISDN AND BROADBAND ISDN
1.200 Series-Service Capabilities
The 1.200 series is in a sense the most important part of the ITU-T ISDN recommendations.
Here, the services to be provided to users are specified. We may look
on this as a set of requirements that the ISDN must satisfy. In the ISDN glossary ,
(1.112), the term service is defined as
That which is offered by an Administration or recognized private operating
agency (RPOA) to its customers in order to satisfy a specific telecommunication
requirement.
Although this is a very general definition, the term "service" has come to have a very
specific meaning in ITU-T, a meaning that is somewhat different from the use of that
term in an OSI context. For ITU-T, a standardized service is characterized by
Complete, guaranteed end-to-end compatibility
ITU-T-standardized terminals, including procedures
Listing of the service subscribers in an international directory
ITU-T-standardized testing and maintenance procedures
Charging and accounting rules
There are three fully standardized ITU-T services: telegraphy, telephony, and
data. There are four additional telematic services in the process of being standardized:
teletex, facsimile, videotex, and message handling. The goal with all of these
services is to ensure high-quality international telecommunications for the end user,
regardless of the make of the terminal equipment and the type of network used
nationally to support the service.
1.300 Series-Network Aspects
Whereas the 1.200 series focuses on the user, in terms of the services provided, the
1.300 series focuses on the network, in terms of how the network goes about providing
those services. A protocol reference model is presented that, while based on
the 7-layer OSI model, attempts to account for the complexity of a connection that
may involve two or more users (e.g., a conference call) plus a related commonchannel
signaling dialogue. Issues such as numbering and addressing are covered.
There is also a discussion of ISDN connection types.
1.400 Series-User-Network Interfaces
The 1.400 series deals with the interface between the user and the network. Three
major topics are addressed:
Physical configurations. The issue of how ISDN functions are configured into
equipment. The standards specify functional groupings and define reference
points between those groupings.
Transmission rates. The data rates and combinations of data rates to be
offered to the user.
Protocol specifications. The protocols at OSI layers 1 through 3 that specify
the user-network interaction.
A.2 / ISDN CHANNELS 747
1.500 Series-Internetwork Interfaces
ISDN supports services that are also provided on older circuit-switched and packetswitched
networks. Thus, it is necessary to provide interworking between an ISDN
and other types of networks to allow communications between terminals belonging
to equivalent services offered through different networks. The 1.500 series deals
with the various network issues that arise in attempting to define interfaces between
ISDN and other types of networks.
1.600 Series-Maintenance Principles
This series provides guidance for maintenance of the ISDN subscriber installation,
the network portion of the ISDN basic access, primary access, and higher data-rate
services. Maintenance principles and functions are related to the reference configuration
and general architecture of ISDN. A key function that is identified in the
series is loopback. In general, loopback testing is used for failure localization and
verification.
A.2
The concept of ISDN is best introduced by considering it from several different
viewpoints:
* Principles of ISDN
* The user interface
* Objectives
* Services
Principles of ISDN
Standards for ISDN have been defined by ITU-T (formerly CCITT), a topic that we
explore later in this section. Table A.l, which is the complete text of one of the
TABLE A.1 Recommendation 1.120 (1988).
1 Principles of ISDN
1.1 The main feature of the ISDN concept is the support of a wide range of voice and nonvoice applications
in the same network. A key element of service integration for an ISDN is the provision of
a range of services (see Part I1 of the I-Series in this Fascicle) using a limited set of connection
types and multipurpose user-network interface arrangements (see Parts 111 and IV of the I-Series
in Fascicle 111.8).
1.2 ISDNs support a variety of applications including both switched and non-switched connections.
Switched connections in an ISDN include both circuit-switched and packet-switched connections
and their concatenations.
1.3 As far as practicable, new services introduced into an ISDN should be arranged to be compatible
with 64 kbitls switched digital connections.
1.4 An ISDN will contain intelligence for the purpose of providing service features, maintenance and
network management functions. This intelligence may not be sufficient for some new services and
may have to be supplemented by either additional intelligence within the network, or possibly
compatible intelligence in the user terminals.
1.5 A layered protocol structure should be used for the specification of the access to an ISDN. Access
from a user to ISDN resources may vary depending upon the service required and upon the status
of implementation of national ISDNs.
1.6 It is recognized that ISDNs may be implemented in a variety of configurations according to specific
national situations.
2 Evolution of ISDNs
2.1 ISDNs will be based on the concepts for telephone IDNs and may evolve by progressively incorporating
additional functions and network features including those of any other dedicated networks
such as circuit-switching and packet-switching for data so as to provide for existing and
new services.
2.2 The transition from an existing network to a comprehensive ISDN may require a period of time
extending over one or more decades. During this period arrangements must be developed for the
networking of services on ISDNs and services on other networks (see Part V).
2.3 In the evolution towards an ISDN, digital end-to-end connectivity will be obtained via plant and
equipment used in existing networks, such as digital transmission, time-division multiplex switching
andlor space-division multiplex switching. Existing relevant recommendations for these constituent
elements of an ISDN are contained in the appropriate series of recommendations of CCI'M and
of CCIR.
2.4 In the early stages of the evolution of ISDNs, some interim user-network arrangements may need
to be adopted in certain countries to facilitate early penetration of digital service capabilities.
Arrangements corresponding to national variants may comply partly or wholly with I-Series Recommendations.
However, the intention is that they not be specifically included in the I-Series.
2.5 An evolving ISDN may also include at later stages switched connections at bit rates higher and
lower than 64 kbitls.
ISDN-related standards, states the principles of ISDN from the point of view of
CCITT. Let us look at each of these points in turn:
1. Support of voice and nonvoice applications using a limited set of standardized
facilities. This principle defines both the purpose of ISDN and the means
of achieving it. The ISDN supports a variety of services related to voice communications
(telephone calls) and nonvoice communications (digital data
exchange). These services are to be provided in conformance with standards
(ITU-T recommendations) that specify a small number of interfaces and data
transmission facilities.
2. Support ,for switched and nonswitched applications. ISDN supports both circuit
switching and packet switching. In addition, ISDN supports nonswitched
services in the form of dedicated lines.
3. Reliance on 64-kbps connections. ISDN provides circuit-switched and packetswitched
connections at 64 kbps; this is the fundamental building block of
ISDN. This rate was chosen because, at the time, it was the standard rate for
digitized voice, and, hence, was being introduced into the evolving integrated
digital networks (IDNs). Although this data rate is useful, it is unfortunately
restrictive to rely solely on it. Future developments in ISDN will permit
greater flexibility.
4. Intelligence in the network. An ISDN is expected to be able to provide sophisticated
services beyond the simple setup of a circuit-switched call.
5. Layered protocol architecture. The protocols for user access to ISDN exhibit a
layered architecture and can be mapped into the OSI model. This procedure
has a number of advantages:
Standards already developed for OSI-related applications may be used
on ISDN. An example is X.25 level 3 for access to packet-switching services
in ISDN.
New ISDN-related standards can be based on existing standards, reducing
the cost of new implementations. An example is LAPD, which is
based on LAPB.
Standards can be developed and implemented independently for various
layers and functions within a layer; this allows for the gradual implementation
of ISDN services at a pace appropriate for a given provider or a
given customer base.
6. Variety of configurations. More than one physical configuration is possible for
implementing ISDN; this allows for differences in national policy (singlesource
versus competition), in the states of technology, and in the needs and
existing equipment of the customer base.
The User Interface
Figure A.l is a conceptual view of the lSDN from a user, or customer, point of view.
The user has access to the ISDN by means of a local interface to a "digital pipe" of
a certain bit rate. Pipes of various sizes are available to satisfy differing needs. For
example, a residential customer may require only sufficient capacity to handle a
telephone and a videotex terminal. An office will undoubtedly wish to connect to
the ISDN via an on-premise digital PBX, and will require a much higher capacity
pipe.
At any given point in time, the pipe to the user's premises has a fixed capacity,
but the traffic on the pipe may be a variable mix up to the capacity limit. Thus,
a user may access circuit-switched and packet-switched services, as well as other services,
in a dynamic mix of signal types and bit rates. To provide these services, the
ISDN requires rather complex control signals to instruct it how to sort out the timeA.
l / OVERVIEW OF ISDN 743
Local area network
(LAN)
FIGURE A.l Conceptual view of ISDN connection features.
multiplexed data and provide the required services. These control signals are also
multiplexed onto the same digital pipe.
An important aspect of the interface is that the user may, at any time, employ
less than the maximum capacity of the pipe, and will be charged according to the
capacity used rather than "connect time." This characteristic significantly diminishes
the value of current user design efforts that are geared to optimize circuit
utilization by use of concentrators, multiplexers, packet switches, and other linesharing
arrangements.
Objectives
Activities currently under way are leading to the development of a worldwide
ISDN. This effort involves national governments, data processing and communications
companies, standards organizations, and other agencies. Certain common
objectives are, by and large, shared by this disparate group. We list here the key
objectives:
Standardization. It is essential that a single set of ISDN standards be provided
to permit universal access and to permit the development of cost-effective
equipment.
Transparency. The most important service to be provided is a transparent
transmission service, thereby permitting users to develop applications and
protocols with the confidence that they will not be affected by the underlying
ISDN.
Separation of competitive functions. It must be possible to separate out functions
that could be provided competitively as opposed to those that are fun744
APPENDIX A / ISDN AND BROADBAND ISDN
damentally part of the ISDN. In many countries, a single, government-owned
entity provides all services. Some countries desire (in the case of the United
States, require) that certain enhanced services be offered competitively (e.g.,
videotex, electronic mail).
@ Leased and switched services. The ISDN should provide dedicated point-topoint
services as well as switched services, thereby allowing the user to optimize
implementation of switching and routing techniques.
@ Cost-related tariffs. The price for ISDN service should be related to cost, and
should be independent of the type of data being carried. One type of service
should not be in the position of subsidizing others.
Smooth migration. The conversion to ISDN will be gradual, and the evolving
network must coexist with existing equipment and services. Thus, ISDN interfaces
should evolve from current interfaces, and provide a migration path
for users.
@ Multiplexed support. In addition to providing low-capacity support to individual
users, multiplexed support must be provided to accommodate userowned
PBX and local network equipment.
There are, of course, other objectives that could be named. Those listed above
are certainly among the most important and widely accepted, and each helps to
define the character of the ISDN.
Architecture
Figure A.2 is a block diagram of ISDN. ISDN supports a new physical connecter for
users, a digital subscriber loop (link from end user to central or end office), and
modifications to all central office equipment.
The area to which most attention has been paid by standards organizations is
that of user access. A common physical interface has been defined to provide, in
essence, a DTE-DCE connection. The same interface should be usable for telephone,
computer terminal, and videotex terminal. Protocols are needed for the
exchange of control information between user device and the network. Provision
must be made for high-speed interfaces to, for example, a digital PBX or a LAN.
The subscriber loop portion of today's telephone network consists of twisted
pair links between the subscriber and the central office, carrying 4-kHz analog signals.
Under the ISDN, one or two twisted pairs are used to provide a basic fullduplex
digital communications link.
The digital central office connects the numerous ISDN subscriber loop signals
to the IDN. In addition to providing access to the circuit-switched network, the central
office provides subscriber access to dedicated lines, packet-switched networks,
and time-shared, transaction-oriented computer services. Multiplexed access via
digital PBX and LAN must also be accommodated.
Standards
The development of ISDN is governed by a set of recommendations issued by
ISDN, called the I-series Recommendations. These Recommendations, or stanA.
l / OVERVIEW OF ISDN 745
Subscriber loop- I
ISDN channel structures: I
I
Basic = 64 kbps + 64 kbps + 16 kbps I
Primary = multiplexed 64 kbps channels I
I
I
-_ I! swDiticghietadl bcaircckubiot-n e
FIGURE A.2 Block diagram of ISDN functions.
Common
phys~cal
interface
dards, were first issued in 1984. A more complete set was issued in 1988. Most of the
Recommendations have been updated, at irregular intervals, since that time. The
bulk of the description of ISDN is contained in the I-series Recommendations, with
some related topics covered in other Recommendations. The characterization of
ISDN contained in these Recommendations is centered on three main areas:
- . A
ISDN I
I
central
office
1. The standardization of services offered to users, so as to enable services to be
internationally compatible.
2. The standardization of user-network interfaces, so as to enable terminal
equipment to be portable, and to assist in (1).
3. The standardization of ISDN capabilities to the degree necessary to allow
user-network and network-network interworking, and thus achieve (1)
and (2).
The I-series Recommendations are broken up into six main groupings, labeled
1.100 through 1.600.
1.100 Series-General Concepts
The 1.100 series serves as a general introduction to ISDN. The general structure of
the ISDN recommendations is presented as well as a glossary of terms. 1.120 provides
an overall description of ISDN and the expected evolution of ISDNs. 1.130
introduces terminology and concepts that are used in the 1.200 series to specify
services.
746 APPENDIX A / ISDN AND BROADBAND ISDN
1.200 Series-Service Capabilities
The 1.200 series is in a sense the most important part of the ITU-T ISDN recommendations.
Here, the services to be provided to users are specified. We may look
on this as a set of requirements that the ISDN must satisfy. In the ISDN glossary ,
(1.112), the term service is defined as
That which is offered by an Administration or recognized private operating
agency (RPOA) to its customers in order to satisfy a specific telecommunication
requirement.
Although this is a very general definition, the term "service" has come to have a very
specific meaning in ITU-T, a meaning that is somewhat different from the use of that
term in an OSI context. For ITU-T, a standardized service is characterized by
Complete, guaranteed end-to-end compatibility
ITU-T-standardized terminals, including procedures
Listing of the service subscribers in an international directory
ITU-T-standardized testing and maintenance procedures
Charging and accounting rules
There are three fully standardized ITU-T services: telegraphy, telephony, and
data. There are four additional telematic services in the process of being standardized:
teletex, facsimile, videotex, and message handling. The goal with all of these
services is to ensure high-quality international telecommunications for the end user,
regardless of the make of the terminal equipment and the type of network used
nationally to support the service.
1.300 Series-Network Aspects
Whereas the 1.200 series focuses on the user, in terms of the services provided, the
1.300 series focuses on the network, in terms of how the network goes about providing
those services. A protocol reference model is presented that, while based on
the 7-layer OSI model, attempts to account for the complexity of a connection that
may involve two or more users (e.g., a conference call) plus a related commonchannel
signaling dialogue. Issues such as numbering and addressing are covered.
There is also a discussion of ISDN connection types.
1.400 Series-User-Network Interfaces
The 1.400 series deals with the interface between the user and the network. Three
major topics are addressed:
Physical configurations. The issue of how ISDN functions are configured into
equipment. The standards specify functional groupings and define reference
points between those groupings.
Transmission rates. The data rates and combinations of data rates to be
offered to the user.
Protocol specifications. The protocols at OSI layers 1 through 3 that specify
the user-network interaction.
A.2 / ISDN CHANNELS 747
1.500 Series-Internetwork Interfaces
ISDN supports services that are also provided on older circuit-switched and packetswitched
networks. Thus, it is necessary to provide interworking between an ISDN
and other types of networks to allow communications between terminals belonging
to equivalent services offered through different networks. The 1.500 series deals
with the various network issues that arise in attempting to define interfaces between
ISDN and other types of networks.
1.600 Series-Maintenance Principles
This series provides guidance for maintenance of the ISDN subscriber installation,
the network portion of the ISDN basic access, primary access, and higher data-rate
services. Maintenance principles and functions are related to the reference configuration
and general architecture of ISDN. A key function that is identified in the
series is loopback. In general, loopback testing is used for failure localization and
verification.
A.2
