1、ITU-T RfCMN*Q-1400 93 = 4862571 0586088 790 = INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU INTELLIGENT NETWORK Q.1400 (03/93) ARCHITECTURE FRAMEWORK FOR THE DEVELOPMENT OF SIGNALLING AND OA 1980 Recommendations as first specification of SS No. 7), physi
2、cal technology advances have contributed major enhancement in: - processing power (instructions executed per unit time); - memory capacity; Recommendation Q.1400 (03/93) 1 ITU-T RECMN*Q=L400 93 9 48b259L 0586094 T94 9 - physical media capacity (bit rate); and - performance of physical media (bit err
3、or rate, down-time). Software technology advances have also occurred: - maturity of OS1 model; - specification of layer services and protocols; - structured programming techniques; - higher levei languages; and - distributed processing techniques. The specification of many of the existing message-ba
4、sed signalling protocols is considered flawed because they do not clearly distinguish the application process specification from the protocol specification. That is, the existing specifications are a combination of application procedures and supporting protocols without a clear distinction between t
5、he two. This situation leads to significant difficulties in extending or evolving the protocols when new application procedures are required. Note that this area advanced substantially from SS No. 6 to SS No. 7 through the distinction achieved between the SS No. 7 MTP and users of the MTP. The recog
6、nition of the appropriateness of distinguishing the application process specification from the application protocol specification is also reflected in the present work on the Integrated Services Control Part (ISCP). As realization of the problems with the existing (1988 set of Recommendations) messa
7、ge-based signalling protocols has emerged, there has also been a realization that the parallel work on OS1 has matured and that it forms a basis for communications protocols in general. 1.4 OS1 applicability Despite their inception at approximately the same time, OS1 and ISDN have not significantly
8、influenced each others models. Two different principles drove the development of OS1 and ISDN protocols, mainly because of the perceived differences between the data communications and the telecommunications environments. In particular, the main requirements of the telecommunications signalling envi
9、ronment has been efficiency, while the data processing environments main emphasis has been “openness”. “Openness” is the ability for any user with the communications capabilities provided by the OSI-standardized protocols to access the widest variety of applications subject to administrative restric
10、tions. OS1 provides a reference model, which is a framework or discipline for providing a communications infrastructure that may be used by any application in a distributed environment. It also provides a set of common protocol standards which provide uniform communications capabilities independent
11、of the precise nature of the application. There is a significant advantage to be obtained by studying the OS1 models and protocols. The evolution of telephone networks requires ever more exchange of information among software controlled devices (computers). The telecommunication industry is solving
12、similar problems and should take advantage of the knowledge and large investment represented by OSI. 1.5 Relationship to the Three Stage Process This subclause includes an outline of the three stage process defined in Recommendations 1.130 and 4.65. The three stage process was designed for the compl
13、ete definition and specification of individual ISDN (and non-ISDN) services. It provides, as described below, a stage for the specification of service specific protocol. It is anticipated that further evolution of telecommunications networks will include significant adoption of Intelligent Network (
14、IN) techniques and capabilities. IN represents a generalization of the service specific work being done on a number of supplementary services with the aim of achieving standards. The generalization of service work will also require generalization of the protocol. A major objective of the protocol ar
15、chitecture guidance is to ensure a well-ordered, open-ended structure and framework for these general protocols. This will enable the protocols built on this framework to evolve and be extended in a straightforward manner with minimal version and interworking problems. 2 Recommendation Q. 1400 (03/9
16、3) ITU-T RECMNtQ.1400 93 48b2591 0586095 920 The three stage process may be summarized as: - - Stage 1 is an overall service description from the users standpoint. Stage 2 is an overall description of the organization of the network functions to map service requirements into network capabilities. St
17、age 3 is the definition of switching and signalling capabilities needed to support services defined in Stage 1. - Each stage consists of several steps. Stage 1 Stage 1 is an overall service description from the users point of view, but does not deal with the details of the human interface itself. Th
18、e Stage 1 service description is independent of the amount of functionality in the users terminal, other than that required to provide the human interface. For example the conference calling service description is designed to be independent of whether the conference bridge is in the terminal, in the
19、 serving exchange or elsewhere. The steps in Stage 1 are: - - - Step 1.1 - Service prose definition and description. Step 1.2 - Static description of the service using attributes. Step 1.3 - Dynamic description of the service using graphic means. Stage 2 Stage 2 identifies the functional capabilitie
20、s and the information flows needed to support the service as described in Stage 1. The Stage 2 description will also include user operations not directly associated with a call (e.g. user change of call forwarding parameters via his service interface) as described in Stage 1. Furthermore, it identif
21、ies various possible physical locations for the functional capabilities. The output of Stage 2 which is signalling system independent is used as an input to the design of signalling system and exchange switching Recommendations. The steps in Stage 2 are: - - - - - Step 2.1 - Derivation of a function
22、al model. Step 2.2 -Information flow diagrams. Step 2.3 - SDL diagrams for functional entities. Step 2.4 -Functional entity actions. Step 2.5 - Allocation of functional entities to physical locations, Stage 3 In Stage 3 the information flow and SDL diagrams from the Stage 2 output form the basis for
23、 producing the signalling system protocol Recommendations and the switching Recommendations. Stage 3 will need to be repeated for each service where, because of different allocations of functional entities to physical locations, different protocols and procedures are needed. The protocol architectur
24、e guidelines included in this Recommendation have been prepared based on known and predicted relationship requirements. It is expected that these protocol architecture guidelines will evolve to include further structure and capability as relationships are identified and specified that require more c
25、omplex capabilities than initially provided. 2 The OS1 Reference Model 2.1 This subclause provides some general remarks on the OS1 model. Later subclauses address the Application Layer of that model, together with related aspects, in some detail. General Description of the OS1 Reference Modell Recom
26、mendation Q.1400 (03/93) 3 ITU-T RECMN*Q.1400 93 48b2591 058b09b 863 The purpose of the Reference Model of Open Systems for CCITT Applications (Recommendation X.200) is to provide a well-defined structure for modelling the interconnection and exchange of information between users in a communication
27、system. The approach allows standardized procedures to be defined not only to provide an open system interconnection between users over a single network, but also to permit interworking between networks to allow communication between users over several networks in tandem. The approach taken in the O
28、S1 Reference Model is to partition the model used to describe the interconnection and exchange of information between users in a communication system into seven layers. From the point of view of a particular layer, the lower layers provide a “transfer” service with specific features. The way in whic
29、h the lower layers are realized is immaterial to the next higher layers. Correspondingly, the lower layers are not concerned with the meaning of information coming from higher layers or the reasons for its transfer. The characteristics of each layer are described below: a) Physical Layer (Layer 1) -
30、 Provides transparent transmission of a bit stream over a circuit built in some physical communication medium. It furnishes the interface to the physical media and is responsible for relaying bits (;.e. interconnects data circuits). A 64 kbit/s link as used for SS No. 7 is an example. Data Link Laye
31、r (Layer 2) - Overcomes the limitations inherent in the physical circuits and allows errors in transmission to be detected and recovered, thereby masking deficiencies in transmission quality. Network Layer (Layer 3) - Transfers data transparently by performing routing and relaying of data between en
32、d users. One or more of the subnetworks may interwork at the Network Layer to provide an end user to end user network service. A connectionless network provides for the transfer of data between end users, making no attempt to guarantee a relationship between two or more messages from the same user.
33、Transport Layer (Layer 4) - Provides an end user to end user transfer optimizing the use of resources (Le. network service) according to the type and character of the communication, and relieves the user of any concern for the details of the transfer. The Transport Layer always operates end-to-end,
34、enhancing the Network Layer when necessary to meet the Quality of Service objectives of the users. Session Layer (Layer 5) - Co-ordinates the interaction within each association between communicating application processes. Full and half duplex dialogues are examples of possible Session Layer modes.
35、Presentation Layer (Layer 6) - Transforms the syntax of the data which is to be transferred into a form recognizable by the communicating application processes. Application Layer (Layer 7) - Specifies the nature of the communication required to satisfy the users needs. This is the highest layer in t
36、he Model and so does not have a boundary with a higher layer. The Application Layer provides the sole means for application processes to access the OS1 environment. b) c) d) e) f) g) 2.2 OS1 Layering and SS No. 7 Evolution of SS No. 7 architecture has been based on the Open Systems Interconnection (
37、OSI) Reference Model (see 2.1). OS1 considers primarily connection-oriented protocols, that is, protocols which establish a logical connection before transferring data. The Network Service Part (NSP) of SS No. 7 provides both connectionless and connection- oriented protocol. The NSP of SS No. 7 evol
38、ved from a four-level model, with the lower three levels corresponding to the lower three layers of the OS1 Reference Model, and level 4 corresponding to users of the lower three levels but without further generalized internai structure. Layers 1-3 comprise functions for the transportation of inform
39、ation from one location to another, possibly via a number of communication links in tandem. These functions provide the basis on which a communication network can be built. ) While OS1 does not refer to its layers by numbers, it has become common usage to number the layers. This Recommendation uses
40、the name of the layer or its number interchangeably. 4 Recommendation Q.1400 (03/93) ITU-T RECMN*Q.L400 93 W 48b259L 058b097 7T3 2 . 1 The SCCP provides, with the MTP, OS1 Layers 1-3. MTP (Levels 1-3) Layers 4-7 define functions relating to end-to-end communication. These layers are so defined that
41、they are independent of the internal structure of the communication network. l Transaction Capabilities directly uses the Network Service provided by the connectionless SCCP. The ISCP allows for the possibility of functions in Layers 4-6, particularly at Layer 6. Other SS No. 7 application protocols
42、, e.g. ISUP and TUP, do not provide for such an explicit structure. Figure 1 illustrates the architecture of SS No. 7. pig yiq 11 Call control application services OS1 layer _ (Note 4) _ (Notes 1,3) 6 5 4 I 71 (Note 4) (Notes 1, 3) I ISDN-UP Level 4 TUP Level 4 OMAP AE ASE TCAP ISDN-UP TUP SCCP MTP
43、Operations, Maintenance and Administration Part Application Entity Application Service Element Transaction Capabilities Application Part ISDN User Part Telephony User Pari Signalling Connection Contrd Pari Message Transfer Part NOTES 1 2 3 4 The only standardized user of this interface is TCAP using
44、 the services of the connectionless SCCP. OMAP is SS No. 7 management. SS No. 7 primitive interface. TCAP may be considered as an ASE. FIGURE i/Q.1400 Relationship Between SS No. 7 Functional Levels and OS1 Layering 3 Control and User Plane Modelling Aspects This clause supplements the material cont
45、ained in Recommendation 1.324. As discussed in Recommendation 1.324, the interaction between a terminal and an exchange may be modelled using OS1 concepts. The terminal and exchange in general interact with each other on a peer-to-peer basis. The interaction is in the control plane and concerns the
46、provision of a resource in the user plane (e.g. the Physical Layer channel in the case of a voice circuit). For example, in circuit mode, this resource is at Layer 1 (once established by the network) and the user at each end must provide Layers 2 through 7 (described in clause 2) according to his ne
47、eds. The Layer 1 bearer channel provided by the network needs to be understood as entirely distinct (in the logical sense) from the Layer 1 being used to transport control plane messages. Further, the term “network” as generally used in telephony does not have the same connotation as the term in OSI
48、. The telephone network is a physical network made up of exchanges and interconnecting bearer channels, and is the equivalent of the OS1 term “subnetwork”. The OS1 term “Network” refers to the Layer 3 entity which has responsibilities including routing and relaying of messages on behalf of users of
49、the network towards indicated destinations. The DSS 1 term “Layer 3” should not be confused with the OS1 Network Layer (sometimes referred to as Layer 3). DSS 1 Layer 3 has aspects of OS1 Layers 3 to 7 in the control plane. It is therefore incorrect to place the terminal at the Network Layer as is sometimes done. Rather it should be viewed in two ways. For control plane purposes, it is a full Application Process with an Application Entity for its communication needs (further details on these concepts may be found in clause 4). In the user plane, the terminal provide
