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I T 1s NOW widely recognized that the Integrated Services Digital Network (ISDN) concept is providing a useful frame-

work for the development of future telecommunications networks and services. An ISDN can be regarded as a general- purpose digital network capable of supporting (or integrating) a wide range of services (voice and non-voice) using a small set of standard multipurpose user-network interfaces. Relevant CCITT Recommendations (or standards) on ISDN interfaces were expected to be available in 1984, the final year of the current study period.

This paper reviews CCITT progress on the architectural studies of protocols associated with ISDN’s, in particular, the ISDN Protocol Reference Model being developed by Study Group XVIIl. This Model, based on the concepts and principles of the ISO/ CCITT Open Systems Interconnection ( 0 ~ 1 ) Reference Model, aims at providing a unified framework for modeling ISDN multiservice communications and capa- bilities. The new Model is applied to several possible ISDN communications configurations, enabling a number of technical issues to be easily identified.

Introduction

At present, public-switched telephone networks (PSTN’s) are generally becoming more digital in nature, with the increasing use of 64-kb/s Pulse Code Modulation (PCM) switching and transmission. This trend will initially produce a telephony- based Integrated Digital Network (IDN) and later, a multi- service IDN in which end-to-end digital connectivity exists and through which a multiplicity of services (voice and non-voice) are accessible by the customer. The IDN will also serve as a transitory step towards an ISDN.

An ISDN can be regarded as a general-purpose digital network capable of supporting (or integrating) a wide range of services (such as voice, data, text, and image) using a small set of standard multipurpose user-network interfaces. A key difference between an ISDN and a telephony (or multiservice) IDN is the provision of a unified or integrated digital customer access (interfaces and protocols) between the customer premises and the ISDN to support a multiplicity of services.

In order to assist the orderly development of ISDN’s, the International Telegraph and Telephone Consultative Committee (CCITT) has been very active in preparing Recommendations (or standards) on many aspects of ISDN’s [I]. This standardi- zation activity is being coordinated by Study Group XVIll (Digital Netwocks) with the active support of many other Study Groups, including Study Group X1 (Telephone Switching and Signaling) and Study Group VI1 (Data Communication Networks). In particular, Study Group XVl l l is proposing the establishment of a new series of CCITT Recommendations, known as the I-series, to cover all ISDN-related topics such as the following:

The ISDN concept and associated principles, service capabilities, overall network aspects and functions, user-network interfaces, and internetwork interfaces.

(In this paper, the terms ‘‘user’’ and “customer” are used interchangeably, and they have a general meaning to include terminals and systems in the customer premises.)

In this new I-series of Recommendations, appropriate references will be made to other relevant (current and new) CCITT Recommendations series, such as the X-series on data network aspects from Study Group VI1 and the Q-series on telephony network aspects from Study Group XI.

This paper discusses the CCITT architectural studies of protocols associated with ISDN’s. It first considers the functional capabilities expected in an ISDN and highlights the

March 1985-Vol. 23, No. 3 IEEE Communications Magazine

need to study ISDN protocols in a unified fashion. Current progress on the formulation of an ISDN Protocol Reference Model by Study Group XVIlI is discussed. This Model is based on the concepts and principles of the lSO/CClTT OS1 Reference Model. A key feature of the proposed ISDN Protocol Reference Model is the recursive application of the OS1 seven-layer structure in modeling two generic types of information flows, namely, User ( U ) information and Control (C) information flows. The new Model is subsequently applied to typical ISDN communications and capabilities, highlighting a number of technical issues.

ISDN Environment

ISDN Capabilities

The main motivations for network development towards an ISDN are the economies and flexibilities which the integrated nature of the network would foster. A key feature that allows ISDNS to provide these benefits to users, network/service providers, and equipment manufacturers alike is the provision of an integrated (or common) access standard between the customer premises and the network for a multiplicity of voice and non-voice services. Such an arrangement would allow independent developments in the customer premises (such as terminal equipment and applications) and in the network itself (such as transmission, switching, signaling, operations, and management) within a common framework. The integration of transmission, switching, and signaling capabilities achievable within a n ISDN is another significant feature which will unify the provisioning of emerging and future telecommunications services and facilities.

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In order to meet the above objectives, an ISDN environment is expected to comprise most or all of the following functional elements (Fig. I): circuit switching, packet switching, common channel signaling, network operations and management data bases, and information processing and storage facilities.

Circuit switching will be provided at the nominal bit rate of 64 kb/s. Other rates (lower and higher) are also expected to be included. Circuit switching is effective in supporting services such as real-time communications and bulk information transfer. Within the ISDN environment, circuit-switched con- nections are controlled by common channel signaling techniques (see below).

Packet switching will be provided for a range of bit rates up to 64 kb/s. Other higher rates may also be anticipated in the future. Packet switching is effective in supporting services of a bursty traffic characteristic such as interactive data applications. It therefore provides a complementary service to circuit switching.

Common channel signaling will be used to establish, supervise, and release circuit-switched connections which may carry voice or non-voice traffic. Within the network, the CCITT Common Channel Signaling System (CCSS) No. 7 will be used for interexchange signaling. Initially designed for telephone IDN’s, the CCSS No. 7 is being enhanced to support the ISDN multiservice environment (for example, [2,3]). Recent developments in ISDN user-network access signaling protocols are also based on this technique. Further details will be given in the section titled “Common Channel Signaling.”

Network operations and management data bases will be required to effectively support the provision of a multiplicity of voice and non-voice services and facilities in a unified fashion.

Information processing and storage facilities will in general be provided within ISDN’s. In countries where telecommuni- cations services are provided by a monopolistic authority (such as in most European countries and Japan) and other countries (such as Canada), information processing and storage facilities will form an integral part of ISDN’s being planned. Such facilities are crucial in the effective provision of emerging application-oriented services such as telematic services (teletex and videotex), electronic mail, and message-handling services. However, in some countries information processing and storage facilities are considered to be outside the ISDN’s. In the United States, for example, ISDN’s are currently seen as providing an integrated information transportation (or carriage) mechanism only (see [4]).

In summary, apart from a few exceptions such as the United States, the ISDN concept can be thus regarded as a unified approach to both network development and service provision in meeting customer demands for both transportation- and application-oriented services and facilities in the near- and long-term future.

ISDN Customer Access

ISDN customer access arrangements can be usefully defined by means of conceptual reference configurations [5]. In these configurations, customer premises equipment and network equipment are partitioned into generic functional groupings separated by well-defined logical reference points (some of which correspond to physical interfaces) [6]. In order to minimize the variety of standard ISDN user-network physical interfaces, CCITT has defined a limited number of channel types and structures for such interfaces.

Two types of channels have been identified by CCITT, namely:

(a) Channels that do not carry signaling (or control) infor- mation for circuit switching in an ISDN. Examples of these include:

B-channel at 64 kb/s; HO-channel a t 384 kb/s.

(b) Channels that are primarily used to carry signaling (or control) information (s-type) for circuit switching. In some applications, other information such as packet data (p-type) and telemetry (t-type) may also be carried with s-type information in the same channel using statistical multiplexing. Examples of these include:

D-channel a t 16 kb/s or 64 kb/s; E-channel at 64 kb/s.

CCITT has further defined several interface structures which must be complied with at the ISDN user-network physical interfaces. The most important ones are:

Basic access interface structure comprising two B-channels and one D-channel, that is, 2B+D, where B=64 kb/s and D=16 kb/s. Primary rate access interface structures such as: 23B+D or 23B+E; 30B+D or 30B+E where B=D=E=64 kb/s.

Common Channel Signaling

A significant feature in ISDN’s is the use of common channel signaling techniques for customer-network access and interex- change signaling. This allows the control of multiple circuit- switched connections using a separate common signaling path, hence the terms “common channel, ” “out-of-band, ” and “out- slot.” In the various ISDN access arrangements currently defined by CCITT, this common channel signaling information is carried over the D- or the E-channel.

In the ISDN basic access (2B+D), for example, when a user requires a B-circuit for communication with another in a

remote part of an ISDN, the signaling procedures (or protocols) for controlling the B-circuit are conveyed over the D-channel (Fig. 2). Similar procedures take place within the network using CCSS No. 7. Once such a circuit-switched connection is established end-to-end, any user information (voiceinon-voice) commensurate with the transmission capacity can be exchanged, including packet data.

This separation of signaling (that is, call control) path (D- channel) from the user information path (B-channel) therefore facilitates the specification of a flexible, universal signaling protocol, capable of supporting multiple services. It is the alternative to the “in-slot” signaling technique as used in X.25 and X.21 access protocols where the signaling information is conveyed via the same path as the user information.

It should be noted, however, that within the common channel signaling path itself (whether in the D- or E-channel, or CCSS No. 7), an in-slot signaling technique (similar in principle to those used in packet data networks) is used to establish logical signaling connections. These are then used to transfer signaling messages required by the communicating control entities to manage multiple circuit-switched connections. There is thus a logical separation between user information and signaling or control information (see also Fig. 2).

The use of a common channel signaling technique in the ISDN as well as at its access interfaces will enable the provisioning of a separate signaling path between end-users (Fig. 3). This in turn would provide a powerful communications facility for ISDN users, as will be elaborated on later (see “User-User Signaling”). This capability, however, would require full compatibility between the D- or E-channel access protocols and the CCSS No. 7 network protocols to facilitate inter- working.

Need for a Unified Approach to ISDN Protocol Modeling

A fundamental concept for protocol modeling is based on the principles of layered communications defined in the lSO/CClTT OS1 Reference Model [7,8]. Initially conceived for packet communications, the OS1 model is inherently based on “in-s1ot”communications procedures where both user informa- tion and call control information share the same (logical and physical) transmission path.

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As ISDN’s can support many different communications modes, the OS1 Model therefore needs to be applied judiciously to effectively represent the ISDN-specific features not en- countered in current packet-switched data networks. These are:

multichannel access interface structures; and common channel signaling for both customer access and network (intra- and inter-) applications.

With these features, a wide range of communications modes and capabilities can be achieved, including the following:

circuit-switched connection under the control of common channel signaling; packet-switched communications over B- and H-channels, and also over D- and E-channels; signaling between users and network-based facilities (for example, information retrieval systems such as Videotex, operations data bases such as directory); end-to-end signaling between users (for instance, to change communications mode over an already-established con- nection); combination of above as in multimedia communications, whereby several simultaneous modes of communications can take place under common signaling control [9].

With such a diversity of ISDN capabilities (in terms of information flows and modes of communication), beyond those of packet data networks, there is thus a need to model all these capabilities within a common framework (such as the reference model). This would enable the attendant critical protocol architectural issues to be readily identified and would therefore facilitate the developnlent of ISDN protocols and associated features [10-12].

The next section describes the CCITT-defined ISDN Protocol Reference Model and examples of its applications to typical ISDN-specific communications modes.

ISDN Protocol Reference Model

The Model

The objective of the ISDN Protocol Reference Model is to model information flows, including user information and control information flows, to, from, and through an lSDN [10-12]. These flows (Fig. 3) can take place between: two ISDN users, an ISDN user and a functional entity within the network, an ISDN user and a functional entity outside the network. between various functional entities inside the network, and between an ISDN and other networks. They can also be one-to- many and many-to-many communications.

User (U) information flows relate to digitized voice, data, text, image, and other information. It may be transmitted transparently through the ISDN, or it may be processed or manipulated within the network. Examples of the latter include digitized voice coding/decoding, data compression, data encryp- tion, and protocol conversion.

Control (C) informationflows relate to signaling information required: to control a network connection (for example, establishing and clearing down); to control the use of a n already established network connection (for example, change of service characteristics or communications modes during a call such as alternate voice/text without changing the logical or physical connection); or to provide both above control functions (as in a multipoint conference call with change in communications modes). It is noted that the last two features are not (yet) defined in the OS1 Reference Model.

Another category of information flows that has been identified relates to management information. However, this issue has not been fully studied within CCITT and therefore is not dealt with in this paper.

Figure 4 illustrates the resulting ISDN Protocol Reference Model when the OS1 principles are recursive1.v applied to the first two information flow types. The Model consists of two logical protocol planes corresponding to U and C. Interactions

M a r c h 1985-Vol. 23, No. 3 IEEE Communications Magazine 1 8

between the two planes may be required, such as in accordance with the common channel signaling principles described in the section on common channel signaling (as will be seen later).

The seven layers of the U- and C-protocol planes represent seven distinct ordered partitions. Each layer exhibits specific properties and features in respect to their relationship both with adjacent layers and with more general aspects of communications. Each layer offers a specific layer service or set of layer services to the layer above. The functions of each layer and the service provided by each layer are defined in general terms in the OS1 Reference Model [7,8].

In certain applications, some layers in either protocol structure may be null, and some layer boundaries may be considered to exist only conceptually, with no boundary functions (primitives) defined. Appropriate layers and the associated services and protocols will be defined only when necessary [ 10-121. Detailed specifications of these layer services and protocols are the subject of other relevant CCITT Recommendations. For example, the ISDN access signaling protocols in the D-channel are currently defined as comprising the lower three OS1 layers in the C-protocol plane [13-151.

For network elements that perform relaying functions (such as switching exchanges, signaling transfer points), up to three (lower) layers are represented in the U-plane. However, for network elements that perform end-systems functions (such as those providing information processing/ messaging facilities), all seven layers are represented.

Although the ISDN Protocol Reference Model deals with information interchange between ISDN users, between ISDN users and ISDN network elements, between network elements, and between networks, it is not intended to be a description of any specific implementation of an ISDN or of any systems or equipment in, or connected to, an ISDN.

Applications

This section describes the applications of the CCITT ISDN Protocol Reference Model to explicitly represent some typical ISDN communications features mentioned earlier.

Circuit-Switched Connection-Both U- and C-planes are used to model ISDN circuit-switched connections controlled by common channel signaling (Fig. 5 ) . ISDN user-network call control signaling is represented by the lower three layers in the C-plane as proposed in [16]. However, the layering of CCSS No. 7 in terms of the OS1 Reference Model has not yet been resolved and is under study [17]. lnterworking between the ISDN user-network access protocols and CCSS No. 7 has been investigated by Study Group XI. This resulted in the partitioning of ISDN access protocol Layer 3 into Layer 3 Upper (for call-control functions) and Layer 3 Lower (for future functions, such as signaling reliability requirements for PABX applications) ([16,17]; see also [18]). The Layer 3 Upper must also be compatible with the ISDN Application Part of the CCSS No. 7 to facilitate interworking [3].

In the U-plane, the Physical Layer is used to model the (fully-established) circuit-switched connection at the exchange involved, allowing physical information flow between the communicating terminal equipments (TE’s). Peer-to-peer proto- cols may take place in the other six layers on an end-to-end basis [ 121.

Packet-Switched Communications-Several alternatives are possible, including the following examples:

(a) Over a B-channel to access a separate packet switching network (PSN)-Fig. 6(a) illustrates the interworking between an ISDN TE connected to the B-channel and a packet-mode Data Terminal Equipment (DTE) connected to the PSN. A circuit-switched B-connection over the ISDN is first set up between the calling ISDN TE and the PSN entry originating packet port using signaling protocols (3 lower layers) of the D-channel and the ISDN Application Part of the CCSS No. 7. Packet interface protocols such as those of X.25 will then be used (over the established ISDN B-circuit transparently) to establish end-to-end virtual circuits across the PSN to the destination packet DTE. Upon the clearance of the virtual circuit connection across the PSN and the B-circuit, the common channel signaling systems (both D-channel and CCSS

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No. 7) must be subsequently notified in order to clear down the B-circuit. This implies that detailed attention must be given to the interworking between PSN and CCSS No. 7 for both call establishment and clearing, especially to support calls initiated from the PSN side. Another issue relates to the interactions of OS1 Network Layer primitives (in the U-plane) and the s- protocol messages (in the C-plane) within the ISDN TE in effecting both call establishment and clearing.

In the example illustrated in Fig. 6(b), the ISDN TE’s are

March 1985-Vol. 23, No. 3 IEEE Communications Magazine 20

assumed to be connected via an ISDN-PSN-ISDN arrange- ment. Although this may not be a realistic situation, it does highlight some protocol interworking issues that need to be considered as explained below. The ISDN-PSN portion of the connection is set up as in the previous example. However, to establish the second PSN-ISDN portion, the destination packet port will need to interact with CCSS No. 7 of the second ISDN in order to establish another (transparent) B-channel. This would require the same PSN-ISDN interworking protocol (yet to be defined) as that used in the previous example (Fig. 6(a)) for handling call origination from the packet DTE. Either TE can clear down the connection end-to-end, using first X.25 and subsequently s-type signaling procedures. Network information about the connection is also required to be interchanged between the packet switching nodes and the two CCSS No. 7 of the ISDN’s to complete the call clearance.

(b) Over the D-channel-Packet communications via the D-channel using, for example, packet (p-type) information, can be achieved via a separate PSN or via the ISDN CCSS No. 7 Message Transfer Part (MTP) [3]. It should be noted that the use of CCSS No. 7 for transferring nonnetwork related information may not be allowed by some network operators. In either case, a combined UICplane can be used to model p-type packet transfer via packet handlers at the originating and destination local exchanges (Figs. 7(a) and 7(b), respectively) since signaling channels are used in this application to carry user information.

In Fig. 7(a), one of the TE’s can be a packet-mode X.25 terminal. In this case, the packet handler would not be required, as this X.25 terminal could interface directly with the PSN using X.25 procedures via a transparent B-channel. However, for the situation in Fig. 7(b), (that is, over CCSS No. 7), some interworking arrangement is required to interface the packet TE and CCSS No. 7. in place of the packet handler. The interworking protocol required between a packet handler and the CCSS No. 7 also needs to be defined in order to realize the example shown in Fig. 7(b). Note t.hat the combined UICplane approach differs from the separate U- and C-plane representa- tion in draft Recommendation 1.31 I [12].

User-User Signaling-With the availability of common channel signaling in both the network and the ISDN user- network access link, an ISDN allows users to exchange control/signaling messages without having to establish a (circuit-switched) connection. This form of communications, known as user-user signaling (using s-type information), can also take place while other types of communications are in progress (such as those over the B-channel(s) and over the D-channel using p- and t-types information).

User-user signaling can be modeled using the upper four layers of the C-plane, with the lower three layers providing the transportation mechanism over the D-channel and CCSS No. 7 (Fig. 8). Alternatively, a separate X.25 PSN can be used in place of CCSS No. 7 . However, this approach requires some interworking arrangement between s-type access protocols and X.25 interface protocols.

From the above discussion, it is clear that all seven layers of the OS1 architecture are needed to fully model signaling (s-type) information in the C plane in this application:

Upper four layers to allow end-to-end information transfer

Lower three layers to provide user-network access.

Protocols for the lower three layer (s-protocols) are being formulated by Study Group XVlII (Layer I , Draft Q.910/ 1.431) and Study Group XI (Layers 2 and 3. Draft Q.920/ 1.441 and Q.930/ 1.45 I , respectively). However, some of the capabilities and functions envisaged for the upper four layers are currently kept in Layer 3 of s-protocols (Q.930/ 1.451) in the CCITT 1981-84 study period. Further details on this topic can be found in [ 161.

The above form of end-to-end signaling (between ISDN users) is functionally the same as that between an ISDN user and a network-based facility. Indeed, as noted in the section called “The Model,” such a facility can be represented as an end system located within the network. Examples of user- network facility signaling include the following:

between ISDN users; and

redirection of calls to a new location; 0 directory inquiries;

selection of programs from a cable television distribution center.

Multimedia Communication-This multi-call or multi- connection communications concept [9] takes advantage of the extensive capabilities envisaged in an ISDN (“ISDN Environ- ment”) to provide a combination of simultaneous communica- tions between an ISDN user and other user(s) and network- based facilities using a common channel signaling control mechanism. As the generic elements of ISDN communications have already been covered in the previous sections, examples of multimedia communications can also be modeled. For example, a dual circuit-switched connection under common control can be represented by two U information planes and one C information plane. For other cases, for example, simultaneous circuit-switched and packet-switched connections, it is foreseen that interactions between these two types of connection need to be carefully considered (see also Fig. 6(b)).

Concluding Remarks

This paper has reviewed the current effort of CCITT Study Group XVlIl in formulating an ISDN Protocol Reference Model (Draft Recommendation 1.31 I ) . As an ISDN is expected to provide extensive communications modes and capabilities using multichannel access arrangement and common channel signaling, it has been found necessary to apply the OS1 principles in a recursive manner. This leads to the ISDN layering concept that uses two logically separated User and Control information domains [10-12]. The separation of domains would not be necessary when end-to-end packet communications without the use of common channel signaling control is established between end-users, since this can be modeled by the usual OS1 convention (Fig. 7 ) .

Examples have also been described to illustrate the applica- tion of the ISDN Protocol Model to typical ISDN communica- tions modes and capabilities. In some instances, the new Model has assisted in identifying critical protocol interworking issues.

In view of the good progress made so far and the helpful cooperation between and within CCITT Study Groups XVl l l and XI, the final version of Draft Recommendation 1.31 I is expected to be available in the final SG XVllI plenary meeting of the 1981-84 study period. Such a unified modeling approach would greatly assist in the development of current

March 1985-Vol. 23, NO. 3 2 1 IEEE Communications Magazine

and future ISDN protocols in the provision of a truly multiservice communications environment.

Acknowledgments

The authors wish to acknowledge the contributions made by international delegates at CCITT Study Group XVIII meetings (Kyoto, February; Geneva, June and December 1983) to the results reported in this paper. The permission of the Chief General Manager, Telecom Australia, to present the paper is also acknowledged.

References

[ I ] T. Irmer, “Worldwide trends towards the ISDN-facts and trends,” NTT International Symposium-Our Tasks and Ap- proach for the Development of an Advanced Information Society, pp. 39-47, Tokyo, February 28, 1983.

[2] E. K. Chew and M. Subocz, “An Introduction to the CCITT No. 7 Common Channel Signalling System,’’ Telecom Australia Research Laboratories, Melbourne, Report No. 7580, November 1982; also J. Elect. and Electron. Eng., Australia, vol. 3, no. 3, pp. 165-173, Sept. 1983.

[3] M. Intorrella and M. Romagnoli. “Experience with and Plans for Common Channel Signalling in Europe,” presented at Interna- tional Standards for Digital Telecommunications Networks (ISDTN) Seminar, Melbourne, May 3-4, 1983.

[4] A. M. Rutkowski and M. J. Marcus, “The Integrated Services Digital Network: developments and regulatory issues,” Comput. Commun. Rev., vol. 12, nos. 3-4, pp. 68-82, July-Oct. 1982.

[5] CCITT SG XVIIl “ISDN User-Network Interfaces-Reference Configurations,” Draft Recommendation 1.41 I , July 1983.

[6] N. Q. Duc and E. K. Chew, “Trends towards Integrated Services Digital Networks,” Proc. Tenth Australian Comput. ConJ, Melbourne, pp. 638-652, Sept. 26-29, 1983.

[7] IS0 (1983) “Information Processing Systems-Open Systems Interconnection-Basic Reference Model.” International Standard IS 7498, 1983.

[8] CCITT SG VII, “Proposed Draft Recommendation X.200 (Revised): Reference Model of Open Systems Interconnection for CClTT Applications,” June 1983.

[9] AT&T, “The Concept of Multi-Media Calls in an ISDN,” Delayed Contribution 1 0 , CCITT Study Group XVIII, ISDN Experts Meeting, Kyoto, Feb. 14-25, 1983.

Recommendation 1.31 I (Version I ) , Feb. 1983.

Recommendation 1.31 I (Version 2), July 1983.

Recommendation 1.31 I (Version 3), Dec. 1983.

[ I O ] CCITT SG XVIII, “ISDN Protocol Reference Model,” Draft

[ I l l CClTT SG XVIII, “ISDN Protocol Reference Model,” Draft

[I21 CCITT SG XVIII, “ISDN Protocol Reference Model,” Draft

March 1985-Vol. 23, No. 3 IEEE Communications Magazine 22

[I31 CCITT SG XI, “Specification of the ISDN User-Network Interface Data Link Layer Protocol,” Draft Recommendation 4.920 (1.441). Issue 6, Dec. 1983.

[I41 CClTT SG XI, “Specification of the ISDN User-Network Interface Layer 3 Protocol,” Draft Recommendation 4.930 (1.451), Issue 6, Dec. 1983.

[I51 CClTT SG XVIII, “(ISDN) Basic User-Network Interface-Layer 1 Specification,” Draft Specification 1.431, July 1983.

[I61 CCITT SG XI, “Report of the Meeting Held in Melbourne from 1 1 to 19 April 1983.” Working Party XI/6 (Digital Subscriber Line Signaling), COM XI-No. R27, May 1983.

[I71 CClTT SG XI, “Report of the Meeting Held in Melbourne from April 18 to 29, 1983.” Working Party X1/2 (Signaling System No. 7). May 1983.

[I81 E. K . Chew, ‘‘Issues in Interconnection of Private and Public Networks,” submitted to ICCC ’84, Sydney, Oct. 30-Nov. 2, 1984.

Nguyen Quang Duc is currently Head of Customer Access Section, Telecom Australia Research Laboratories.

After receiving the B.E. (Honors) and the Ph.D. degrees in Electrical Engineering from the University of Queensland, he joined the Telecom Research Laboratories in 1973 working on digital line systems and data communications.

Between June 1976 and July 1977, through his previous doctorate work in coding theory, he was invited to Syracuse University (Syracuse, New York) as a Visiting Associate Professor at the School of Computer and Information Science.

On his return to the Laboratories, he began studies on data switching networks, Open Systems Interconnection (OSI), and Integrated Services Digital Networks (ISDN).

Currently, his Customer Access Section is investigating a range of topics associated with ISDN’s or multi-service telecommunication environment, interworking between private and public networks, and applications of the OS1 Reference Model to telecommunication services, systems, and networks.

(USA). He is a senior member of IREE (Australia) and a member of the IEEE

Eng K. Chew received the degrees of B.E. (Elec.), 1972 from the University of Melbourne; M. Eng. Sc, 1975 and Ph.D., 1981 from the University of Sydney. His Ph.D. work was on the modeling and analysis of stored program controlled (SPC) telephone switching systems.

Since joining the Telecom Australia Research Laboratories in 1979, he has worked on aspects of S P C switching operations and maintenance, modeling and analysis of the CClTT No. 7 Common Channel Signaling System, and on the interconnect arrangements between customer systems, networks, and the public networks, especially in an Integrated Services Digital Network (ISDN) environment.

He is currently a Principal Engineer with the lelematlc ana Message Services Section where he leads projects on computer-based messaging systems and electronic directory services. rn