1、CEPT T/GSI*03-01 E m 2326434 0005458 5 m 0 TGS03-01 Page 1 E Recommendation T/GSI 03-01 (Nice 1985, revised by correspondence, June 1987) NETWORK FUNCIIONAL PRINCIPLES Recommendation proposed by the Special Group Integration (GSI) Revised text of the Recommendation adopted by the “Telecommunications
2、” Commission: “The European Conference 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
3、 ISDN, to which - the need for CEPT Administrations to have more precise answers to their specific European requirements, i.e. - the need for CEPT Administrations to have as a base a comprehensive set of Recommendations, in order to CEPT Administrations have contributed most significantly, to select
4、 and further define material from CCITT Recommendations, promote and strengthen European harmonisation, recommends - to the member Administrations of the CEPT that they evolve their telecommunication networks towards ISDN according to the guidelines and specifications given in the attached text.” O
5、Edition of October 31, 1987 CEPT T/GSI*03-01 E 2326414 0005459 7 = T/GSI 03-01 E Page 2 1. GENERAL 1.1. Basic philosophy of the functional description The objective of this Recommendation is to provide a common understanding of the ISDN capabilities, including terminal network and specialised servic
6、e centre aspects. A functional description of ISDN capabilities must allow a clear distinction between definition and specification aspects of services provided by the ISDN and the actual specification of the ISDN equipment utilised to support those services. Therefore an implementation independent
7、approach should be adopted. Therefore in the context of this Recommendation the adjective “Functional” is used in the sense of an implementation independent approach. The noun “function” itself has a specific meaning which is explained below. The description of network capabilities is consistent wit
8、h the protocol reference model, e.g. : - the layering structure of all systems involved in a communication process, i.e. partitioning the required - the clear discrimination between layer service concept, layer function concept and layer protocol concept. Furthermore the three following distinctions
9、 must apply: - distinction between basic services and supplementary services, - distinction between ISDN capabilities and services offered to the customer, - distinction between static and dynamic aspects of the description. functions between different layers, 1.2. Services supported by ISDN The con
10、cepts and the principles of an ISDN are described in CEPT Recommendation T/GSI 01-02. The services supported by an ISDN are given in the T/GSI 02 series of Recommendations. A classification and a method of description of these services are specified in Recommendation T/GSI 02-01, the network capabil
11、ities to support these services are defined in the T/GSI 03 series of Recommendations. The relation- ship between these Recommendations is shown in Figure 1 (T/GSI 03-01). It should be noted that the service concept defined in Recommendation T/GSI 02-01 is different from the layer service concept of
12、 the OS1 model. The service in Recommendation T/GSI 02-01 corresponds to the service offered to the customers by the network provider. Besides operational and commercial aspects, the provision of these Telecommunication services (Bearer and Teleservices) and associated supplementary services require
13、s the availability of appropriate system capabilities : - network capabilities, in various network equipments (exchanges, etc.), - terminal capabilities, - speciaiised service centre capabilities, where required. Generic description of required capabilities ISDN capabilities are the total sum of the
14、 functions required to support all basic and supplementary services offered by the ISDN. The identification and characterisation of these functions, which are related to the specification and analysis of these basic and supplementary services, form the first step of the generic description. This par
15、t of the generic description is intrinsically static. The use of a basic or supplementary service generally requires co-operation between functions located in different equipments. The static description of the ISDN capabilities, which will be a list of necessary functions is not sufficient. It is n
16、ecessary, in addition, to depict the sequence of events and the activation of functions co-ordinated by suitable inter-equipment signals. This second step is the dynamic aspect of the description. This involves firstly an identification and characterisation of the functions and then a method of show
17、ing the dynamic interaction between functions. 1.3. 1.3.1. Static description 1.3.2. Dynamic description Edition of October 31, 1987 O CEPT T/GSI*03-OL E m 232414 00054bo 3 m Edition of October 31, 1987 r VI w w d O 3 w a TlGSI03-01 E Page 3 CEPT T/GSI*03-Ol E 232b414 00054bl 5 W TIGSI 03-01 E Page
18、4 O 2. OBJECTIVES OF THE FUNCTIONAL DESCRIPTION OF THE ISDN As described in Recommendation T/GSI 01-03, an Integrated Services Digital Network (ISDN) is a network providing end-to-end digital connectivity to support a wide range of telecommunications services. The characterisation of ISDN is centred
19、 on three main areas: (a) the standardisation of services offered to customers, so as to enable services to be internationally compatible; (b) the standardisation of user-network interfaces, so as to enable terminal equipment to be portable (and to assist in (a); (c) the standardisation of network c
20、apabilities to the degree necessary to allow user-network and network- network interworking, and so to achieve (a) and (b) above. The T/GSI 02 series of Recommendations has identified the range of Telecommunication services to be offered in an ISDN, namely Bearer and Teleservices, and the attributes
21、 characterising these services. The T/GSI 04 series of Recommendations describes both the functional and technical aspects of user-network interfaces. This Recommendation defines the ISDN capabilities to support services via an interface in terms offundions. A functional description enables a de-cou
22、pling of services and ISDN capabilities, and therefore allows an implementation-independent approach. The principal objectives of the ISDN functional description method are: 1. to define the ISDN capabilities, by building up a harmonised set of functions that are necessary and sufficient to support
23、telecommunication services by their static and dynamic description; 2. to aid the ISDN capabilities evolution (modifications, addition of capabilities to support new basic or supplementary services), by organising this set of functions in an open-ended and modular structure; 3. to aid the harmonisat
24、ion of system independent switching functions between exchanges of differing designs and manufacture; 4. to aid the standardisation of interworking standards between switching systems located in different countries ; 5. to provide information for the preparation of functional specifications for new
25、telecommunication services; 6. to maximise the exploitation of functions provided and available in switching systems. The transition from an existing network to a comprehensive ISDN may require a period of time extending over one or more decades. Therefore the design of an ISDN will be evolutionary,
26、 adding capabilities in a flexible and modular manner. An ISDN may therefore be expected to provide an open-ended set of functional capabilities able to accommodate new needs as they arise at acceptable cost. During a long intermediate period, some functions may not be implemented within a given ISD
27、N. Also specific arrangements should be used to ensure compatibility with existing networks and services. An ISDN should also give access to existing services and interwork with existing networks and terminals. 3. GENERIC DESCRIPTION MODEL 3.1. General Concepts The ISDN functional description define
28、s a set of capabilities which enable Bearer and Teleservices to be offered to customers (see Recommendation T/GSI 02-01). The services require two different levels of ISDN capabilities viz. : - the low-layer functions relate to the Bearer Services; - the high-layer capabilities together with the low
29、er layer functions relate to the Teleservices. In addition, operation and management capabilities are required to support both Bearer and Teleservices. Edition of October 31. 1987 CEPT T/GSI*03-OL E W 232b414 00054b2 7 T/GSI 03-01 E Page 5 ISDN capabilities ISDN services Figure 2 (T/GSI 03-01). Rela
30、tionship between telecommunication services and network capabilities. The capabilities of the ISDN need a detailed and rigorous characterisation because there are a wide range of standardisation issues involved. To achieve the functional objectives described in paragraph 2., the ISDN functional desc
31、ription has been designed to - define the overall functional characteristics of the ISDN; - be implementation independent and place no constraints on national network architectures beyond the network and interface standards given in the T/GSI series of Recommendations; - take full account of the con
32、straints of existing dedicated networks; - support the layering protocol concepts defined in Recommendation T/GSI 03-02. For this purpose the concept of an ISDN function is used which is defined as: “A distinguishing characteris- tic which describes functional capabilities of a given equipment, or s
33、ystem, or network, as seen from the designer point of view.” As far as possible, the number of generic functions should be limited. 3.2. Static description model 3.2.1. Global function The description of ISDN capabilities concerns the low layers (1-3) in a global context (Note I), i.e. taking into a
34、ccount all the equipment involved in the communication, according to the Protocol Reference Model (Recommendation T/GSI 03-02). In this context, a Global Function (GF) is defined as: - referring to the ISDN; - having a global significance in the lower layers. The set of all GFs leads to the descript
35、ion of the total ISDN low layer capabilities. There are two kinds of GFs: - the Basic Global Functions (BGF), relating to ISDN connection types; - the Additional Global Functions (AGF), relating to the ISDN capability to support supplementary services. Terminal Relay Specialized Relay Terminal Servi
36、ce Centre T Network Figure 3 (T/GSI 03-01). Global Function Concept. Note. This concept of Global Function may be extended to describe the higher layer capabilities of ISDN terminals (and network capabilities, where these exist). In this case the GF has a global significance inside the higher layers
37、. Edition of October 31, 1987 CEPT T/GSI*03-01 E 232b4l14 00054b3 7 T/GSI 03-01 E Page 6 3.2.2. Elementary Function The introduction of the GF concept allows a general description of low layer capabilities. An EF is a basic functional element which can be allocated to a physical equipment involved i
38、n supporting a Telecommunication Service. An EF is an intrinsically static description of the capability of performing an action on a resource when defined conditions are met. The following is a more detailed description: for each GF, a set of Elementary Functions (EF) is identified as the set of ba
39、sic elements which are then allocated to different components involved in the communication. GF = (EFl, EF2, EF3, _ EFn) For building up a GF, each associated EF must be present in one or more components of the ISDN (this includes the terminals, the network or specialised service centres). But in a
40、specific component the complete set of associated EFs need not be present. For example: r- I GFx L- Layer 3 EF3 - 3.2.3. 3.3. 3.3.1. 3.3.2. Layer 1 Componen GFx = (EFI, EF2, EF3, EF4) Layer 3 Layer 1 Component 2 Layer 3 EF 1 EF2 EF4 - Layer 1 Component Z -1 I -1 Figure 4 (T/GSI 03-01). EFs associate
41、d with a GFx. Allocation of EFs This flexibility in construction of EFs allows a specialisation of the functions to be allocated to particular components. Because the CEPT Recommendation on the architecture of the ISDN (Recommendation T/GSI 03-03) will only specify a functional approach to standardi
42、sation, the relationship between compo- nents and specific equipment is, in general, a national matter. However, an important first step in allocation of functions will be the distinction between terminal equipment and the network equipment involved. Recommendation T/GSI 03-03 introduced the functio
43、nal grouping CRF (Connection Related Functions). This CRF can be local, National Transit or International Transit. EFs can be associated with each of these. Dynamic Description Model The complete description of ISDN capabilities has to take into account dynamic aspects involved in the process of a c
44、ommunication context. This association of functional and protocol aspects leads to the use of the following dynamic description method. Sequence charts The call handling process of basic and supplementary services are described and characterised, as seen from a network point of view, by using sequen
45、ce charts which show the sequence of events occumng in the course of the communication. Executive Processes An Executive Process corresponds to the particular use of one or more Elementary Functions within a particular component (e.g. originating or destination local exchange) which always yields sp
46、ecific results. Therefore an EP is characterised by input information it needs for execution and by output information or actions resulting from execution. The use of EPs involves: (a) sequences that link together events producing the activation of EP, by means of signalling information passed betwe
47、en the components; (b) the information (or data) actually used: - protocol information (signalling information sent or received by the component), - component information (“network information”) - permanent information (description of available resources, environment, services.), - dynamic informati
48、on (elaborated and used during the call handling). O Edition of Wtober 31, 1987 CEPT T/GSI*03-01 E 232b4L4 0005464 O M - User-Network Signalling Information 1 Executive Processes Network Informations EPiJ - Internal Signalling Information Component A EFi, EFj = A colleciion of Eicincntary Functions.
49、 (e.g. Originating Exchange) (e.e T/GSI03-01 E Page 7 1 Executive Processes EPJ Network Informations - User-Network Signalling Information I Component B Destination Exchange) Figure 5 (T/GSI 03-01). Illustration of executive processes. The dynamic description of each basic or supplementary service, based on the above elements, results in a chart showing components involved (originating and destination exchanges, specialised service centres when required), the signalling information flow passed between them, and the executive processes used inside them. 4. USE OF GENERIC DES
