1、_ CEPT T/GSI*03-02 E 5 2326434 0005497 4 m T/GSI 03-02 E Page 1 Recommendation T/GSI 03-02 (Nice 1985) concerning Protocol reference model Recommendation proposed by the Special Group Intergration (GSI) Text of the Recommendation adopted by the “Telecommuriicatiors” Commission: “The European Confere
2、nce of Posts and Telecommunications Administrations, Considering - the common CEPT approach to the ISDN and general CEPT assumptions and policies for the evolution towards ISDN; - the I. Series Recommendations of the CCITT, as well as other CCITT Recommendations on ISDN, to which CEPT Administration
3、s have contributed most significantly; - the need for CEPT Administrations to have more precise answers to their specific European requirements, i.e. to select and further dene material from CCITT Recommendations; - the need for CEPT Administrations to have as a base a comprehensive set of Recommend
4、ations, in order to promote and strengthen European harmonisation. Recommends - to the member Administrations of the CEPT, that they evolve their telecommunication networks towards ISDN e according to the guidelines and speciications given in the attached text.” a Edition of September 15, 1985 CEPT
5、T/GSI*03-02 E 85 W 2326434 0005478 b W TfGSI 03-02 E Page 3 ISDN PROTOCOL REFERENCE MODEL 1. INTRODUCTION 1.1. General The objective of the ISDN protocol reference model is to model information flows, including user informa- tion and control information flows, to and through an ISDN. It is based on
6、the general principles of layering given in the X.200 series of Recommendations, but it is recognized that many of the entities and information flows modelled here do not consist of Open Systems (in the X.200 sense). It is also recognized that the signalling protocols currently recommended by CCITT
7、(e.g. Signalling System No. 7 (4.700 series) and D-channel protocol (4.900 series) are layered and it is a matter for further study how these protocols and this model should correlate. It is not intended as a deftnition of any specific implementation of an ISDN or of any systems or equipment in, or
8、connected to, an ISDN. Examples of applications of this model are included in this Recommendation. 1.2. Need for a unified approach to ISDN protocol modelling A fundamental concept for protocol modelling is based on the principles of layered communication defined in Recommendation X.200 (The Referen
9、ce Model of Open Systems Interconnection (OSI) for CCITT Applications), The OS1 Model was originally conceived for data communications, while the ISDN is conceived to support multiservice types of communications, including voice and video applications. The OS1 Model therefore needs to be applied jud
10、iciously in order to effectively represent the ISDN-specific features not encountered in current data networks. With these features, a wide range of communication modes and capabilities can be achieved in the ISDN, including the following: circuit-switched connection under the control of common chan
11、nel signalling; packet-switched communication over B-, D- and H-channels; signalling between users and network-based facilities (e.g. information retrieval systems such as Video- tex; operations data bases such as directory); end-to-end signalling between users (e.g. to change mode of communication
12、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)
13、beyond those of data network, there is a need to model all these capabilities within a common framework (Le. reference model). This would enable the critical protocol architectural issues to be readily identified and facilitate the development of ISDN protocols and associated features. 1.3. Relation
14、ship with Recommendation X.200 The protocol reference model, interface structures and protocol reference configurations are defined by layered structures based on and using the terminology of the reference model for Open Systems Interconnec- tion (OSI) for CCITT Applications (Recommendation X.200).
15、The layer identification used in Recommen- dation X.200 is limited in this Recommendation to the use of layer numbers. 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 not at pre
16、sent been considered in the X.200 series of Recommendations. - information flows for out-of-band call control processes, or more generally, information flows among - information flows for selection of connection characteristics; - information flows for re-negotiation of connection characteristics of
17、 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 - informati
18、on flows for power activation/deactivation; - interworking; - switching of information flows; - new layer service definitions for non-data services; - applications to other than end-systems, e.g. signal transfer points (STPs) and inter-networking points; 0 multiple related protocols; functions (e.g.
19、 test loops); Edition of September 15, 1985 - T/GSI 03-02 E Page 4 - information flows for multi-point connections; - information flows for applications such as: voice (including A/u law conversion), full motion video, transparent, telex. Depending on examination and resolution of issues related to
20、the modelling of the above, it is a matter for further study whether the ISDN protocol reference model remains a separate Recommendation or whether the concepts contained in this Recommendation should be incorporated in the X.200 series of Recommen- dations. 1.4. Relationship with the Q-series of Re
21、commendations The functions and procedures described in the Q-series of Recommendations for access and network signalling in general conform to the principles described in this Recommendation. Certain features, however, in particular facility procedures and user-user signalling, may require further
22、study to determine the most appropriate method of modelling. 2. MODELLING CONCEPTS 2.1. Information flows The information flows identified in section 1.3. can be considered as information flows between: - ISDN users and a functional entity within an ISDN e.g., network control facilities; - ISDN user
23、 and a functional entity inside or outside an ISDN e.g., an information storage/processing/ - various functional entities in ISDN; - an ISDN and other networks. Modelling these information flows as well as information flows within an ISDN functional entity (e.g. an ISDN exchange or terminal) is requ
24、ired. These information flows can be classified into the following categories : i) user information-e.g. digitized voice, data and information transmitted between users. This informa- tion may be transmitted transparently through an ISDN, or it may be processed or manipulated. Examples of the latter
25、 include data which is stored or encrypted within the network; - controlling a network connection (such as establishing and clearing down); - controlling the use of an already established network connection (e.g. change of service characteristics - providing both above control functions (as in a mul
26、ti-point conference call with service change). messaging facility; ii) control information-this is information which is acted upon, e.g., in: during a call such as alternate voice/data); 2.2. Structure of the protocols In order to construct the ISDN protocol reference model, a fundamental generic pr
27、otocol block has been identified. Such a protocol block can be used to describe various elements in the ISDN user premises and the network (e.g. terminal equipment (TE), network termination (NT), exchange termination (ET), signal- ling point (SP) and signalling transfer point (STP), etc. In some app
28、lications, some of the layers in a protocol may be null, Le. contain no protocol functions. In such cases the service provided to the layer above will be identical to the underlying service provided to the null layer and the primitives which cross the higher service boundary are mapped directly onto
29、 primitives which cross the lower service and vice versa. The seven layers of the protocol structures represent even distinct ordered partitions. Each layer exhibits specific properties and features in respect of its relationship both with adjacent layers and with more general aspects of communicati
30、ons. Each layer offers a specific layer service or set of layer services to the layer above. The functions of each layer and the service offered by each layer are defined in general terms in the Recommendations of the X.200 series. Detailed specification of layer services and protocols are the subje
31、ct of other Recommendations. 2.3. Perspectives Layering of ISDN protocols may be usefully modelled from two essentially independent perspectives : that of the user and that of the control system. The main rationale for protocols within the user perspective is the transfer of information among user a
32、pplications. The main rationale for protocols from the control perspective is the transfer of information with the control system. One key ?application? of the control system is the control of user plane connections. I Edition of September 15, 1985 CEPT T/GSI*03-02 E 85 2326434 0005500 O T/GSI 03-02
33、 E Page 5 r- - I I j I 1 I j I I I i.- I Edition of September 15, 1985 TE User ap- User perspective LOCAL EXCHANGE -. I application CEQT T/GSI*03-02 E 85 E 232b414 0005501 2 T/GSI 03-02 E Page 6 - I! I II II I I L, I I I I Ciontroi peqspective I I I L, I I System 7 Figure 2. Nested circuit switched
34、call control. Edition of September 15, 1985 CEPT T/GSI*03-02 E 5 = 2326414 0005502 4 W T/GCI 03-02 E Page 7 I , Edition of September 15, 1985 CEQT T/GSI*03-02 E 85 H 232b414 0005503 b H T/GSI 03-02 E Page 8 One implication of this is that functions within the control plane may be called from the use
35、r plane. Such nesting of control functions is illustrated in Figure 2-the control of ISDN (user plane) connections using 4.930. Similar nesting for packet switched calls is shown in Figure 3. From the users perspective the network is generally involved in layer 1 to 3 functions to provide basic serv
36、ices. Within the network, the sub-systems are not normally considered to be “Open Systems”. However a layered approach, e.g. based on the OS1 layering principles is useful in discussing network structures and functions, for dening signalling systems etc. This matter is described in CEPT Recommendati
37、on T/GSI 03-01. 2.4. 2.5. Communication contexts Figure 4 illustrates a generic communication context. Between end protocol blocks there may be one or more intermediate protocol blocks. The end protocol blocks do not necessarily correspond to the end systems as defined in the X.200 series of Recomme
38、ndations. The end system protocol blocks may reside in subscribers TE or network exchanges or other equipment related to an ISDN. Any of the various elements in the ISDN user premises and the network may have many of these contexts operating simultaneously or sequentially. A particular communication
39、 context may involve two or more end protocol blocks e.g., multipoint context. Communication contexts may be chained together simultaneously or sequentially in time, with information passing from one context to another through an application process or a management entity. Examples of using multiple
40、 communication contexts are contained in section 4. Having first set the context for modelling these information flows, they can then be modelled from the end system perspective as belonging to either a user (in-context) plane, (U), or control (out-of-context) plane, The dialogue in each plane is in
41、dependently described in terms of separate layer structures, corresponding to the structure of the U protocol block and C protocol block described in section 4. The allocation of information flows into the two above-mentioned categories is communication context dependent. Figure 5 illustrates some p
42、ossible communication contexts, for which the end systems and information flows between them may be modelled. Associated interactions of protocols Figure 2 illustrates the conceptual aspects of the protocol block. A three dimensional representation is used to depict: - user (U) information and assoc
43、iated layered protocols; - control (c) information and associated layered protocols. In addition, Figure 2 represents generically all local (e.g. terminal) management (M) aspects associated with the transfer of user information and control information. Examples of management functions include the co
44、ntrol of the switching action within a circuit switch and selection of appropriate responses to connection failure and other exception conditions occurring over the communication facility. These functions also include “network management” and trafic control to optimize utilization of network resourc
45、es. Management aspects should be treated as purely local matters associated with a given entity (whether ISDN terminal equipment, or network equipment). In any given entity, these functions are necessary in : - monitoring the activities in the user information and control information domains; - prov
46、iding a mechanism for information interchange between U and C processes. If at any stage there is a need for local management information to be exchanged between different entities (especially for Administrations /RPOAs to perform network management), this communication can be supported by either U,
47、 or C, or both protocols. Interactions at the lower faces of the protocol block are illustrated in Figure 2 and they represent the physical transfer of information (whether user or control) between one protocol block and another. This transfer takes place over the physical media associated with U an
48、d C information protocols. In some cases (e.g. ISDN basic access), a common physical medium may be shared by both U and C protocols. Interactions at the upper face of the protocol block represent the transfer of information to various application processes external to the block. These include user a
49、pplications, control applications, and system management applications. Interactions between different protocol blocks or peer protocols take place in a layered fashion as defined by the OS1 Reference Model. For clarity, however, these peer interactions are not shown in Figure 2. (Cl. I Edition of September 15, 1985 CEPT T/GSI*03-02 E 85 2326414 0005504 8 W End System 7 6 5 4 3 2 1 I Intermediate System End System T/GSI 03-02 E Page 9 The end system protocol blocks may reside in subscribers TE or network exchanges or other
