InfoMagic Standards 1993 July/Disc.iso/ccitt/1992/i/i320.asc

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2.    SECTION 2 - REFERENCE MODELS



2.1    Recommendation I.320





ISDN PROTOCOL REFERENCE MODEL





1.    Introduction



    The objective of the ISDN Protocol Reference Model (ISDN PRM) is to model the interconnection and exchange
of information - including user information and control information - to, through or inside an ISDN.



    Communicating entities may be:



-     ISDN users;

-  an ISDN user and a functional entity within an ISDN, e.g. network control facilities;

-  an ISDN user and a functional entity inside or outside an ISDN, e.g. an information storage/processing/
messaging facility;

-  various functional entities in an ISDN, e.g. a network management facility and a switching facility;

-  an ISDN functional entity and an entity located in or attached to a non-ISDN network.



    The purpose of communications between these functional entities is to support the telecommunication ser-
vices introduced in Recommendations I.211 and I.212, by providing ISDN capabilities as defined in Recommenda-
tion I.310. Examples of these capabilities are:



-  circuit-switched connection under the control of common channel signalling;

-  packet-switched communication over B-, D- and H-channels;

-  signalling between users and network-based facilities (e.g. information retrieval systems such as
Videotex, operations data bases such as directory);

-  end-to-end signalling between users (e.g. to change mode of communication over an already
established connection);

-  combinations of the above as in multi-media communication, whereby several simultaneous
modes of communication can take place under common signalling control.



    With such diversity of ISDN capabilities (in terms of information flows and modes of communication), there is
a need to model all these capabilities within a common framework (i.e. reference model). This would enable the
critical protocol architectural issues to be readily identified and facilitate the development of ISDN protocols and
associated features. It is not intended as a definition of any specific implementation of an ISDN or of any systems
or equipment in, or connected to, an ISDN.



    Examples of applications of this model are included in this Recommendation.



2.    Modelling concepts



2.1    Relationship with the X.200-Series



    The ISDN Protocol Reference Model (PRM) and the Reference Model of Open Systems Interconnection for CCITT Appli-
cations (OSI RM), defined by Recommendation X.200, have both commonalities and differences.



    Both the ISDN PRM and the OSI RM organize communications functions into layers and describe the relation of these
layers with respect to each other. However, the scope of the ISDN PRM is different from the scope of the OSI RM.



    The scope of the ISDN PRM is to model information flows across the range of telecommunication services defined in
the I.200-Series. These are Bearer Services, Teleservices and Supplementary Services. This description necessarily incor-
porates ISDN-specific characteristics not encountered in other network types. Among these characteristics are multi-
service types of communications which include voice, video, data and multi-media communications.



    The scope of the OSI RM is not associated with any particular network type1. In that sense it is less specific than the
ISDN PRM. Further, the scope of the OSI PRM is tied to data communications and so, in that sense, its scope is more
specific than the ISDN PRM. The OSI PRM - that application is to model data communications between open systems in
an ISDN environment.



    The relative scopes of the two models are illustrated by Figure1/I.320. The existence of a common intersection shows
that these models coexist and overlap.









































FIGURE 1/I.320



Applicability of OSI protocols to ISDN









ûûûûûûûûûûûû

1 Note that the term "network" in the ISDN corresponds to "sub-network" in the OSI terminology.



    However, in spite of these differences in scope, a number of concepts and the associated terminology which have
been introduced in

Recommendations X.200 and X.210 are fully applicable to the ISDN PRM. They include the concept of layer, layer service
(X.200), and the notions of service primitive, peer entity and peer protocol (X.210).



Note - The relation between service primitives and functional components introduced in Recommendation I.310 requires
further study.



    The layer identification used in Recommendation X.200 is limited in this Recommendation to the use of layer num-
bers. Layer titles (e.g. network layer) as used in Recommendation X.200 are sometimes misleading in the ISDN context,
and have not been used here.



    The following ISDN needs have to be specifically catered for in Recommendation I.320:



-  information flows for out-of-band call control processes, or more generally, information flows among mul-
tiple related protocols;

-  information flows for selection of connection characteristics;

-  information flows for re-negotiation of connection characteristics of calls;

-  information flows for suspension of connections;

-  information flows for overlap sending ;

-  information flows for multi-media calls;

-  information flows for asymmetric connections;

-  information flows for network management (e.g. change over and change back) and for maintenance func-
tions (e.g test loops);

-  information flows for power activation/deactivation;

-  interworking;

-  switching of information flows;

-  new layer service definitions for non-data services;

-  application to other than end-systems, e.g. signal transfer points (STPs) and interworking points;

-  information flows for multi-point connections;

-  information flows for applications such as:

- voice (including A/╡ law conversion),

- full motion video,

- transparent,

- telex.



2.2    Control and user planes



    The support of outband signalling, the ability to activate supplementary services during the active phase of
the call, imply a separation between control information and user information.



    The notion of plane - control plane, or C-plane, and user plane, or U-plane - is introduced to reflect this.



    The main rationale for protocols within the user plane is the transfer of information among user applications,
e.g. digitized voice, data and information transmitted between users. This information may be transmitted trans-
parently through an ISDN, or it may be processed or manipulated, e.g. A/╡ conversion.



    The main rationale for protocols within the control plane is the transfer of information for the control of user
plane connections, e.g. in:



-  controlling a network connection (such as establishing and clearing down);



-  controlling the use of an already established network connection (e.g. change in service characteristics
during a call such as alternate speech/unrestricted 64 kbit/s);



-  providing supplementary services.



    In addition to user information, any information which control the exchange of data within a connection, but
otherwise do not alter the state of this connection (e.g. flow control), pertain to the U-plane. All control infor-
mation which involve resource allocation/deallocation by the ISDN pertain to the C-plane.





2.3     Local and global significance



     A key characteristic of the ISDN is that, due to the integration of telecommunication services, the facilities to
be provided depend on whether the adjacent entity, or a remote entity, is involved: different services, possibly
using different routes, may have to be provided accordingly. An example is a telecommunication service, which
can be supported by various network capabilities, (e.g. a telematic service supported either by circuit or

packet facilities), or an ISDN connection based on various types of basic connection components (e.g. analogue
and digital circuits for a speech connection).



    As a consequence, the control information handled by an entity may concern:



-  an adjacent functional entity, in which case it is said to have local significance;



-  a remote (non-adjacent) functional entity, in which case it has global significance.



    The significance concept is illustrated by Figure 2/I.320

















      Local

<----------------->

                Global

<-------------------------------------->



OE = Originating Functional Entity



AE = Adjacent Functional Entity



RE = Remote Functional Entity



FIGURE 2/I.320



Significance



    The notion of significance applies to control plane information only.



    As an example:



    -    from the ISDN user's point of view:



-  the overall service to be provided to users has a globalsignificance;



-  the control of any resources to be used at the user-network interface has local significance;



-  from the network's point of view:



-  the overall service to be provided by the ISDN (ISDN connection types, as introduced in
Recommendation I.340) has a global significance;



-  the handling of connection elements, has local significance.



    Depending on their functional requirements, supplementary services relate to either the local, or global per-
spective. For example:



-  CCBS or user-to-user signalling have global significance;



-  call waiting has local significance.



    Global information falls into three classes:



1)      the information is transported transparently;



    2)      the information may be processed, but remains unchanged (e.g. teleservice);



    3)      the information may be altered (e.g. destination number in relation with freephone or call forwarding
supplementary services).



3.    The model



    The ISDN protocol reference model (PRM) is represented by a protocol block which incorporates the concepts
of layer, significance and plane described hereabove.



    Such a protocol block can be used to describe various elements in the ISDN user premises and the network (e.g. ter-
minal equipment (TE), ISPBX network termination (NT), exchange termination (ET), signalling point (SP) and signalling
transfer point (STP), etc.).



3.1    Generic Protocol Block



    The considerations above lead to the introduction of the concept of significance in combination with planes; the result
is a splitting of the control plane into two parts: a local control plane (LC), and a global control plane (GC), in addition to
the user plane (U).



    The layering principles apply in each of these planes: each plane can potentially accommodate a 7-layer stack of pro-
tocols. A plane management function is required to allow coordination between the activities in the different planes. Exam-
ples of plan management function are:



    -      the decision on whether an incoming information is relevant to

        the LC or GC plane,



    -      allowing communication between C- and U-planes, for synchronization purposes.



    The Generic Protocol Block is represented on Figure 3/I.320.





































































FIGURE 3/I.320



Generic Protocol Block



Note - The Plane Management Function should not be confused with the System Management as introduced to model OSI
management.



    The following remarks apply:



    1)    Some layers may be empty, i.e. they provide no functionality. For example, it is likely that not all seven layers
are required to serve the LC-plane requirements; however, entities communicating in this plane are application layer enti-
ties. Note that this is not in contradiction with the OSI RM.



    2)    An element (either in the network, or in user premises) does not have to support in all cases protocols of LC-
, GC- and U-planes: some may ignore one or even two of these planes. For example, a network service centre accessed
to provide a supplementary service (e.g. freephone) will be concerned by the LC plane only, and will have no knowledge of
the other two planes.



    3)    A network element - unless it provides an HLF - will generally not support any U-plane protocol above layer 3.



    4)    The need for application processes specific to each plane, or for application processes able to access several
planes, is for further study.



3.2    Relations between layers in one plane



    Adjacent layers within a plane communicate using service primitives.  If a layer is empty the primitive is
mapped directly onto a primitive to the next layer.



    Further study is required on which layer services have to be specified in order to describe a telecommunica-
tion service.



3.3    Relations between planes



    Starting from GC-plane requirements, an entity will derive the LC-plane requirements, and the facilities that
have to be provided for the support of U-plane lower layers. For example, in order to provide an ISDN connection
(GC-plane), an exchange will have to identify the required basic connection component (LC-plane).



    This relation is made via the plane management function.



    Informations in different planes need not be carried by distinct physical/logical means in all cases. For
example:



-  control and user informations may use the same support, e.g. when inband signalling is used, or when
user information is carried on a D-channel;



-  LC and GC informations share the same support when the LC-plane pass-along facility is used;



-  ISPBX to ISPBX control information appears as U-plane information to the ISDN.



3.4    Data flow modelling



    For further study.



4.    ISDN management



    For further study.



5.    Interworking



    A number of particular interworking situations should be considered:



-  internetworking with an OSI network;



-  interworking with an non-ISDN terminal;



-  interworking between two ISDNs which do not provide the same set of facilities;



-  interworking involving a network-provided interworking function to support high-layer and/or low-layer
facilities.



5.1    General



    All the interworking situations mentioned above are covered by the model illustrated by Figure 4/I.320.













































FIGURE 4/I.320



Interworking Model



(1) Note - This reference point is an S/T reference point when considering interworking between ISDNs, or service
interworking within an ISDN.



    The service S may be:



-  the initially required telecommunication service (TS), if both networks are able to provide it (F is then
empty);



-  a telecommunication service resulting from a negotiation process, which both networks are able to provide
(F is then empty);



-  a service which is required to support the telecommunication, service to be provided, which is offered by
both networks, but by means of different capabilities in the two networks.



    The service S is provided:



-  by means of functions F1 and protocol(s) P1 in network 1;



-  by means of functions F2 and protocol(s) P2 in network 2.



    The interworking function (IWF) maps the facilities offered by F1 and F2.



    Two kinds of interworking can take place:



    1.      a one-stage interworking, where the calling user is not explicitly aware that an interworking function is
required;



2.      a two-stage interworking, where the calling user has a dialogue with the interworking function prior to
exchanging control information with the destination user.



    The model applies to both cases.



    Interworking may involve the GC-plane, and/or the U-plane.



    In an interworking situation, the GC-plane has to:



-  determine the telecommunication service to be provided (agreed telecommunication service): this may
imply service negotiation;



-  identify the interworking situation, i.e. the fact that more than one network is involved, and that for some
service S required to support the telecommunication service, two adjacent networks do not use the same
underlying facilities;



-  locate and invoke an IWF capable of mapping the facilities in the two networks.



    In each network, the GC-plane facilities will provide the functions and protocols (Fi and Pi) required to support ser-
vice S; this will result in different (and independent) requirements on the CL-plane in each network.



    In the two-stage interworking case, the GC-plane information is "consumed" by the IWF during the first phase, and
is forwarded (with or without modification) during the second one.



    Whenever interworking in the U-plane is involved the following differences apply in the two cases:



-  one-stage interworking: in this case only the first three layers (at most) may be involved for the provision of
the requested

        end-to-end service. No HLF is required.



-  two-stage interworking: in this case the first stage is the establishment of the U-plane facilities between
the calling user and the IWF. High layer functions (HLF) and protocols may be involved, in which case the
IWF acts as a substitute for the called user.



5.2    Relationships with the OSI RM



    The OSI RM, seen from the ISDN PRM point of view, appears not to be in contradiction with the latter, but contains
some restrictions which stem from the fact that it does not have the same scope:



1)      The C- and U-planes are not separated, since the C- and U-plane information in one layer (n) always maps
onto the U-plane information of the layer below (n - 1).



2)      The concept of significance does not explicitly appear; however control informations (e.g. in layer 3) include
both 'local'  informations and informations which are carried end-to-end transparently or take part in
the definition of the overall service provided to the user (e.g. throughput).



3)      The C- and U-plane informations in one layer (n) map onto the

        U-plane informations of the layer below (n - 1).



    Interworking between the OSI RM and ISDN PRM takes place in the following situations:



-  internetworking with a specialized network (e.g. PSPDN) which respects the OSI RM: the reference points
involved are K/L;



-  interworking with an "OSI terminal" via a terminal adaptor: the reference point is then R;



    -      the interworking of an ISDN terminal on the S reference point which conforms to the OSI reference
model is for further study.



    In each case, the interworking function (an IWF or a TA) has to map information flows of one model onto
information flows of the other.



5.2.1    Interworking at reference point K/L



    For further study.



5.2.2    Interworking at reference point R



    In the case when a user application, running an OSI system, requires network services across the ISDN, the
originating user application will address the terminating application as a destination user.



    In the OSI system, the application is considered an an ISDN user - a communicating functional entity in the
PRM.



    The GC information pertinent to the higher layer OSI application is carried in the U-plane to the destination
application. The GC information pertinent to the network service required is carried in the control plane with LC
control information.



    The OSI system requests the network service from the ISDN by placing a service request to both the LC plane
and the U-plane (Figure 5/I.320). The distribution of the information to the appropriate planes is made by the
Plane Management Function. The Plane Management Function is responsible for providing an OSI Service Access
Point to the OSI system.



























































Figure 5/I.320



OSI Reference Model and ISDN Protocol Reference Model





6.    Examples



    Applications of the PRM to the following examples is for further study.



6.1    Basic call situations (no supplementary service, no interworking)



-  circuit service (see Figure 6/I.320)



-  packet service



-  multi-bearer capability



-  data base access.





6.2    More elaborate situations



-  supplementary services



        CCBS

        3-party service



-  PABX facilities



-  OA&M applications.



6.3    Interworking



-  at reference point R (Teletex terminal)



-  with a PSTN



-  with a PSPDN (Videotex)



-  inside an ISDN (provision of an HLF by the network).

































    Legend:



    C  - Local or Global Control depending on the destination functional entity



    LC - Local Control



    GC - Global Control



    M  - Plane Management Function



    NU - Network User Plane



    PU - PSN User Plane



    TU - Terminal User Plane



    Note - For simplicity reasons, NTI functional units are not shown.



                                            FIGURE 7/I.320



                    A protocol reference model example showing the underconnection of

                                       public and private ISDNs





Function of a functional entity to support layers 1 and 2 of the user-network signalling system.



305  inter-exchange signalling handling (message transfer)



Function of a functional entity to support the messages transfer part of the inter-exchange signalling systems.



306  path search inside switching unit



Function of a functional entity to select an internal connection inside the switching unit.



307  synchronization handling



Function of a functional entity to provide synchronization between different functional entities; and:



Function of a functional entity to provide its own internal synchronization functional entity.



308  Timing handling



Function of a functional entity to provide timing between time instances involved in calls.



309  line service marking



Functions of a functional entity to store for each customer the data on the parameters of the bearer and teleservices
that are subscribed to. The store also contains the data on the parameters of the basic bearer and teleservices that are
subscribed to by the customer. It also contains the binary information (i.e. subscribed to or not) for a range of sup-
plementary service which the subscriber can use. In general this data does NOT contain information on the type of sub-
scriber's terminal, but it may contain information on the type of access (basic, primary rate, etc.), the type of NT2
(simple, intelligent, etc.) and the attributesof the services subscribed to.



310  real time clock



Function of a functional entity to provide real time information.



4  SUPERVISION



400  user-network access resources monitoring



Function of a functional entity to check the correct operation of subscriber access resources.



401  transit resources monitoring



Function of a functional entity to check the correct operation of the transit resources.