1、CEPT T/GSI*03-03 E 232b4L4 O005529 2 (3 03-03 Page E 1 Recommendation T/GSI 0343 (Nice 1985, revised by correspondence, June 1987) NETWORK ARCHITECTURE Recommendation proposed by the Special Group Integration (GSI) Revised text of the Recommendation adopted by the “Telecommunications” Commission. “T
2、he 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 - the I. Series Recommendations of the CCITT, as well as other CCITT Recommendations on ISDN, to which - the
3、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 ISDN, CEPT Administrations have contributed most significantly, to select and further def
4、ine material from CCITT Recommendations, promote and strengthen European harmonisation, O 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.” Edition of Octob
5、er 31, 1987 CEPT T/GSI*03-03 E 2326434 0005530 9 = T/GSI 03-03 E Page 2 1. GENERAL 1.1. Basic philosophy of the ISDN Network Architecture Recommendation The objective of this Recommendation is to provide a common understanding in the CEPT studies of the general architecture of an ISDN from the funct
6、ional point of view. The mode is not intended to require or exclude any specific implementation of an ISDN, but only to provide a guide for the specification of ISDN capabilities. 1.2. Definitions A number of terms used in this Recommendation are described in more detail in other Recommendations. To
7、 assist the understanding of the reader, the following particular definitions apply in this Recommenda- tion. 1. Reference configurations are conceptual configurations which are useful in identifying various possible arrangements in an ISDN. The reference configurations are based on association rule
8、s of functional groupings and reference points. Detailed descriptions of reference configurations for ISDN connection types are given in Recommendation T/GSI 03-05 and for user to network access in Recommendation 2. Functional groupings are set of functions which may be needed in ISDN arrangements.
9、The relationship between generic functions and specific functions allocated to particular entities (or functional groupings) in the ISDN are explained in Recommendation T/GSI 03-01. 3. Reference points are the conceptual points at the conjunction of two functional groupings. In a particular example,
10、 a reference point may correspond to a physical interface between two pieces of equipment, or in other examples there may not be any physical interface corresponding to the reference point. Physical interfaces which do not correspond to a reference point (e.g. transmission line interfaces) will not
11、be the subject of ISDN Recommendations. T/GSI 04-01. 2. OVERALL ARCHITECTURE OF AN ISDN In practical ISDN implementations some of the ISDN functions will be implemented within the same network elements, whereas other specific ISDN functions will be dedicated to specialised network elements. Various
12、different ISDN implementations are likely to be realised depending on national conditions. A basic component of an ISDN is a network for circuit switching of end-to-end 64 kbit/s connections. In addition to these connection types, depending on national conditions and evolution strategies, the ISDN w
13、ill or will not support other connection types, such as packet mode connection types and n x 64 kbit/s circuit mode connection types, and other broadband connection types. 2.1. Basic architectural model A basic architectural model of an ISDN is shown in Figure 1 (T/GSI 03-03). This shows the seven m
14、ain switching and signalling functional capabilities of ISDN: - ISDN local connection related functions (CRF, see paragraph 4.2.1.) - 64 kbit/s circuit switching functional entities - 64 kbit/s circuit non-switched functional entities - packet switching functional entities - common channel inter-exc
15、hange signalling functional entities, for example conforming to Signalling - switching functional entities 64 kbit/s - non-switched functional entities 64 kbit/s These components need not be provided by distinct networks, but may be combined as appropriate for a particular implementor. Higher Layer
16、Functions (HLF) which may be implemented within (or associated with) an ISDN may be accessed by means of any of the above-mentioned functional entities. Those functional entities could be implemented totally within an ISDN or be provided by dedicated networks or specialised service providers. Both c
17、ases may provide the same ISDN Teleservice (see Recommendation T/GSI 02-01) from the users point of view. Note. Inclusion of additional low layer functions (ALLF) in the model requires further study. System No. 7 Edition of October 31, 1987 CEPT T/GSI*03-03 E 232b414 000553L O T/GSI 03-03 E Page 3 2
18、.2. Architectural components of the ISDN Recommendation T/GSI 03-01 describes the possible functions of an ISDN. These functions are by their nature static (i.e. time independent). The relative distribution and allocetion of these functions is the subject of the architecture of the ISDN, and is desc
19、ribed in the present Recommendation. The dynamic aspects of these functions, which are described in the Recommendation T/GSI 03-01 as “Executive Processes” are modelled in the Protocol Reference Model, Recommendation T/GSI 03-02. Therefore the key components in this (static) architectural model are
20、what functions are contained in the ISDN, where they are located, and what is the relative topology for their distribution in the ISDN. 3. ASPECTS OF THE ARCHITECTURE OF THE ISDN The architecture of the ISDN includes low layer capabilities and high layer capabilities. These capabilities support serv
21、ices both within the ISDN and via interworking (see section 6.) to other networks. 3.1. Low layer capabilities From the main functional capabilities of the ISDN, as shown in Figure 1 (T/GSI 03-03), four main functional capabilities require further elaboration. Circuit switched connections with infor
22、mation transfer rate up to 64 kbit/s are carried by B channels at the ISDN user-network interfaces and switched at 64 kbit/s by the circuit switching functional entities of the ISDN. Circuit switching can also be applied to information transfer rates greater than 64 kbit/s. Signalling associated wit
23、h circuit switched connections is carried by the D channel at the ISDN user- network interface and processed by the local CRF (see paragraph 4.2.1.). User-to-user signalling is carried through the common channel signalling functional entities (in the transit connection elements). User bit rates of l
24、ess than 64 kbit/s are rate adapted to 64 kbit/s as described in Recommendation T/GSI 04-04, before switching in the ISDN. Multiple information streams from a given user may be multiplexed together in the same B channel, but for circuit switching an entire B channel will be switched to a single user
25、-network interface. This multiplexing should be in accordance with Recommendation T/GSI 04-04. Furthermore circuit switched data services with bit rates less than 64 kbit/s (in accordance with CCITT Recommendation X. 1 user classes of service) may be handled by a dedicated circuit switched public da
26、ta network to which the user gains access by means of an ISDN connection. The ISDN circuit switching capabilities for information transfer rates up to 64 kbit/s are based on 64 kbit/s switching. Connection types at higher bit rates could also be provided by this network on a semi-permanent basis. Sw
27、itched connections at these bit rates could also be provided by broadband switching functional entities. 3.1.1. Circuit switching capabilities O 3.1.2. Packet switching capabilities A number of packet mode bearer services are described in Recommendation T/GSI 02-02. Different network solutions and c
28、orresponding architectures may be adopted in different countries to support these services. Recommendations T/GSI 03-0 1 (which gives network functional principles), T/GSI 04-04 (defines X.25 terminal adaptors), and T/CS 62-03 (gives a description of exchange connection) constitute the basis for the
29、 description of packet switching functions in an ISDN. Two types of functional groupings contain functions related to the handling of packet calls within the ISDN: - packet handling functional groupings contain functions related to the handling of packet calls within - interworking functional groupi
30、ngs ensure interworking between ISDN and packet networks. The solutions which could be used to support packet services are: O the ISDN; r CEPT T/GSI*03-03 E 2326414 0005532 2 TIGSI 03-03 E Page 4 - B Channel access: PSPDN; working functions in the ISDN. - circuit (switched or semi-permanent) access
31、through ISDN to an interworking function within a - circuit (switched or semi-permanent) access associated with packet handling functions and/or inter- - D Channel access to: - packet handling functions and interworking functions within the ISDN; - packet handling functions in the ISDN (without inte
32、rworking functions). Note. This classification does not preclude a combination of solutions described above. Depending on national considerations the ISDN packet handling and interworking functions can be centralised or distributed. The following cases may be encountered : - packet handling function
33、s and/or interworking functions are integrated in the local CRF (e.g. located - packet handling functions are integrated in the local CRF; - packet handling functions and/or interworking functions are integrated in the local CRF. For the support of broadband connections other switching capabilities
34、(e.g. asynchronous time division techniques) in addition to the above-mentioned switching capabilities could be employed. In addition to the normal transmission capabilities of the ISDN, the following transmission aspects are important from the aspect of the architecture of the ISDN. Services which
35、do not require unrestricted digital information transfer capability, such as telephony, may also employ non-transparent transport channels (e.g. packetised voice, digital speech interpolation). Chan- nels at 8, 16, 32 kbit/s may be used in the transmission part of the network; they may be used to su
36、pport some services, e.g. voice-band types. They may also be used in cases where a 64 kbit/s channel (B channel at S or T reference point) cames user data streams at bit rates lower than 8, 16 and 32 kbit/s respectively. in a transit CRF); 3.1.3. Other switching capabilities 3.1.4. Transmission capa
37、bilities 3.2. High Layer capabilities Normally the High Layer Functions HLFs are involved only in the terminal equipment, but for the support of some services provision of the HLFs could be made via special nodes in the ISDN belonging to the public network or to centres operated by other organisatio
38、ns and accessed via ISDN user/network or inter-network interfaces. Some services such as message handling (MHS) may be used on a large scale and the relevant functional entities could be provided within the ISDN exchanges. For both cases the protocols used to activate such services should be identic
39、al and integrated with the general user procedures defined for the activation of ISDN teleservices. 4. LOCATION OF FUNCTIONS IN THE ISDN 4.1. Overall In considering an IDN call (Le. an instance of a telecommunication service) two major functional areas are involved : i) the customer equipment (TE an
40、d optional customer network (see Note 1) i) the public ISDN connection type Recommendation T/GSI 04-0 1 describes the functional groupings and reference configurations for the customer network, and the channel structures to be used at reference points S and T. The functions involved in ii), the publ
41、ic ISDN connection type, are described in 4.2. below. Figure 1 (T/GSI 03-03) illustrates this overall division of functions involved in a communication across the ISDN. Note 1. The terms “Customer Equipment” and “Public ISDN Connection type” do not presuppose a particular regulatory situation in any
42、 CEPT country and are used purely for technical reasons. Edition of October 31, 1987 CEPT T/GSI*03-03 E 232bY,L4 0005533 Y Public ISDN Cus t omer Network Connection Type + TE + 4.2. + TE Customer Network 4.2.1. 4.2.1.1. 4.2.1.2. 4.2.1.3. 4.2.1.4. TIGSI 03-03 E Page 5 S T I I T S I I Figure 1 (T/GSI
43、03-03). General ISDN Reference Configuration. Notes to Figure I (T/GSI 03-03). Note 1. The functional grouping customer network is described in Recommendation T/GSI 04-01. Note 2. In the case where the customer network is null (Le. null NT2) then the ISDN connection type can be considered to end at
44、the SIT reference point. Note 3. Other configurations are possible where the call is asymmetrical, or terminates in or involves HLFs. Partitioning of the ISDN Connection Type The distribution of functions within the ISDN connection type is known as the connection type reference configuration. The pa
45、rtitions of the connection type to allow this distribution are described below. The detailed reference configurations for groups of connection types can be found in Recommendation T/GSI 03-05. Connection Elements The first level of partioning of the ISDN connection type is the connection element. Th
46、is level can be utilised for the apportioning of network performance (NP) values. For some specific network performance parame- ters the breakpoint for the allocation of the overall performance value may not necessarily align with the transition point of the connection element. The partitioning is b
47、ased on the two most critical transitions of a connection, firstly the change of signalling system, secondly the international transmission system(s). These two points generate three connection elements, access connection element, national transit connection element, and international connection ele
48、ment. These three connection elements allow the apportioning of NP values between networks, and the description of both the access and transit capabilities to support services. This partitioning is shown in Figure 3 (T/GSI 03-03). Access Connection Element The access connection element is bounded by
49、 the T reference point at the customer end and a reference point (still to be defined) which marks the transition from the access signalling system to the common channel signalling system on the network side. The model for the access connection element in the case of 64 kbit/s circuit switched is shown in Figure 4 (T/GSI 03-03). Depending on the national situations and on the type of access a number of different possibilities are available for this element, e.g. with regard to the use of multiplexer (Mp) or remote switching units (RSU). National Transit Connection Element The national tra
