- Email: [email protected]
Easing the migration into the future of communications technology
323
Network Protocols and Standards
Easing the Migration into the Future of Communications Technology D. C O C H R A N E CASE Communications Ltd., P.O. Box 254, Caxton Way, Watford, Herts., United Kingdom WD1 8XH
Some of the obstacles that can be expected to impede the migration to the new communications scenarios that are expected to become real over the next few years are described. The easing of the migration is described in terms of the outputs from two ESPRIT projects, C A R L O S and CACTUS. Some introductory material on OSI, ISDN and the X.400 series of Recommendations is included.
Keywords: ESPRIT, OSI, Virtual Terminal X.400, Migration, Mailboxes, Network Management, ISDN.
Don Coehrane has been in the com-
~
............... ' i~i!!~i!¸!i
puter and data communications industry for 18 years following graduation from Brunel University in 1969. He joined CASE in 1982 and is currently a Senior Group Consultant acting in a consultancy role to the C A R L O S project. He is also involved with tenders for strategic sales to major organisations. Previously he has led projects in various communications areas, specifically concerning X.25, protocol emulation and transport.
North-Holland Computer Networks and ISDN Systems 14 (1987) 323-329
Today the communications professional works in a world of networks, both public and private with a p p l i c a t i o n s distributed on different equipment. There are simple terminals or Personal Computers on nearly very desk, computer languages, and therefore the applications written in those languages are getting better and more portable: C and PASCAL can be moved around with a fair degree of success. Where this wonderful scenario starts to get into difficulties is that whilst we can fairly readily make connections between systems, many mainframe operating systems and applications are highly proprietary and individualistic. This tends to mean that user interfaces and communications protocols are very hostile to the casual user and are all different. Mini and micro systems are much better; C P / M and U N I X , although proprietary, are widely implemented and the movement of applications between different manufacturers machines is possible--with care. Some dissimilar machines communicate very well, especially if they happen to have compatible versions of Kermit. The ability to communicate tends to depend largely on the knowledgeability and ingenuity of the user. The migration that is before us is to the promised land where the user can transfer data from machine to machine without a care for what has to be done to make the accessing device, terminal or PC, communicate. The terminal or Personal Computer (PC) will change, eventually, into a graphical, colour O S I / X . 4 0 0 based device supporting I D A / O D A and other business architectures. Some introductory material on some of these concepts will be included in this paper shortly. Communication will be via a universal I S D N interface to a cheap public network that is available everywhere. All user interfaces will be helpful, easy to understand and non-cryptic.
0169-7552/88/$3.50 © 1988, Elsevier Science Publishers B.V. (North-Holland)
324
D. Cochrane / Migration into the Future of Communications Technology
Unfortunately, that land is still far off. Very far. We have a considerable investment in our existing kit and the normal economics of business says that we can't throw it all out tomorrow and buy new, even if the equipment was available today. We have to write off the cost of new terminal devices over three or five years (or until the repair costs outweigh the cost of replacement). We have also heard about (or experienced!) computer fraud so the security aspects need to be cast in concrete before we are prepared to permit access to our local installations via a public service. There is also the problem that m a n y systems have different user interfaces. These are improving as packages like G E M become more widely used, thus reducing or eliminating the learning curve for those applications. Other, older applications are far more idiosyncratic. The tariffs for I S D N are speculative and m a y not show any advantages for m a n y users; PTTs also need to recoup costs expended on providing existing services. The availability of SO interfaces m a y be not quite universal for some time. Are there cases when older systems must be used? Are there ways of keeping the old equipment in use for some years yet? I think yes, to both questions.
APPLICATION LAYER
Purpose of communication. PRESENTATION LAYER Representation and meaning of data. SESSION LAYER Dialogue control and structure TRANSPORT LAYER End-to-end data integrity.
NETWORK LAYER Routing and switch!rig end~.to-end '. LINK LAYER Point-to-point error communications.
free
PHYSICAL LAYER
The transmission medium.
Fig. 1. OSI tower of layers.
For example, the Telex network is assured of a long life despite congestion, high error rate, limited character set and slow speed, simply because a telex is a legal document and its prospective successors are not so exaulted. Telex is also universally available. The migration of hardware can be achieved utilising products designed for the purpose of easing that migration. Two such products could be based on the E S P R I T C A R L O S and C A C T U S projects. A few words of very basic introduction to OSI may be useful at this point. OSI stands for Open Systems Interconnection. An Open System is one which employs the communication procedures and protocols which are derived from the Reference Model defined by the Standards-makers for the logical processes in a communication system. Such interconnection is termed Open Systems Interconnection. These protocols and procedures are described in terms of seven layers, hence, the "Seven Layer Model". The purpose of OSI is to make communication between different manufacturers equipment automatic. The seven layers are conveniently divided into two groups: the lower four levels which are concerned with the transport of data between systems over communication networks and the upper three layers that deal with application-oriented aspects of communications and not with the actual movement of data, see Fig. 1. Level 1 is the Physical layer and provides the transparent transmission of bit streams between systems. There are lots of standards in this area for all kinds of media; LANS, 2Mbit, ordinary modems, etc. Level 2 is the Link level and moves data between two directly connected systems and detects errors in the transfer. There are standards approved here for H D L C , LANs and ISDN. Level 3 is the Network level. This provides routing and relaying through intermediate systems. There are standard services defined and a limited number of standards which is what would be expected in order to be able to connect networks of different types together, e.g. LANs and Packet networks. Level 4 is the Transport layer which provides the transparent transfer of data between end systems. Reasonably, there are only two main stan-
D. Cochrane / Migration into the Future of Communications Technology
dards, one for connection-oriented and connectionless connections and one solely for connectionless. Connection oriented and connectionless refer to modes of communication, one using an association between two endpoints (users) which is maintained for the duration of their dialogue and the other only making an association when there is something to be sent. The respective virtues of the two systems is as much a matter of religion as fact. Level 5 is Session. This controls the dialogue between end systems and helps them to manage the data. It controls which user has the tight to send data at any one time, and breaks up the dialogue into manageable sections, for example, messages in a stream of messages between two particular users. Level 6 is Presentation. This governs the representation of data that is communicated, i.e. it makes sure that both ends interpret the data in the same way. This is rather akin to two people talking on the phone, one speaking Japanese and one speaking Russian. There is perfect error free communications but neither can interpret the data. In data terms this relates to whether a particular set of 16 bits represents a single large binary number, two smaller ones or one number in Binary Coded Decimal. Level 7 is the Application Layer. This is the particular enterprise that the user is attempting to use by communicating with a similar application on another system. Examples of level seven applications are Virtual Terminal, File Transfer, and Message Handling. ESPRIT (European Strategic Programme for Research into Information Technology) is a ten year programme which started in 1984. The objectives of ESPRIT are: - To provide the European Information Technology industry with the basic technologies it needs to meet the competitive requirements of the 1990s. To promote European industrial cooperation in Information Technology. To contribute to the development of internationally accepted standards. ESPRIT operates by sponsoring projects by European consortia. CARLOS (Communication Architecture for Layered Open Systems) is a project in the Information Exchange Systems (IES) area of ESPRIT. -
-
325
It is being executed by a consortium consisting of English and Danish companies with the active support of two Danish Teleadministrations. The CARLOS project involves the development of a series of modular components which provide building blocks to construct OSI systems of varying sizes and sophistication to suit the diverse requirements and budgets of a large spectrum of potential users of OSI systems. The components are targetted at organisations, institutions and companies within R & D , industry and the financial and public sectors which have a need to exchange information between heterogeneous systems. The basic principle is to enable existing, common, de facto standard equipment; terminals and PCs, to enter the OSI arena. A C A R L O S network containing all the CARLOS components is shown in Fig. 2. At the top is the user population of terminals. In CARLOS, these can be any mixture of supported terminals. These can be asynchronous devices, for example, scroll mode devices such as simple VDUs, printing terminals and printers, or more sophisticated devices which have direct cursor addressing. Common synchronous terminal devices from a well-known large manufacturer are also supported, thus coveting a large base of existing terminal devices. These devices are concentrated by means of either the OSI PAD or the Extended OSI PAD. The OSI PAD in conjunction with the OSI BOX provides similar facilities to the Extended OSI PAD. The OSI PAD provides the necessary, interface to enable the supported terminal population to access the OSI level seven application, Virtual Terminal. This enables these terminals to access any host computer from any manufacturer which also supports that application. The OSI PAD also contains the level six layer, Presentation, necessary to support Virtual Terminal. The OSI P A D / O S I BOX combination uses a specially developed Link protocol in order to physically separate the Presentation layer from the layer below it, Session. The Session layer is contained within the OSI BOX, together with Transport and the lower three layers. In the Extended OSI PAD all seven layers are present. The OSI BOX can also support a Personal Computer by means of a LAN. This PC contains the top two layers of the OSI Tower in the form of
D. Cochrane / Migration into the Future of Communications Technology
326
TERMN IA ~LS
ASYNC. AND/OR BISYNC TERMINALS
ASYNC. AND/OR
BISYNC
~"l~.
ASYNC. AND/OR BISYNC TERMINALS
/" "'-"""~ "~,%
~:
PRINTER
E
OSI
SI-PC
PAD
EXTENDED PAD
PAD
ALL-LAYER PC
N
OSI BOX
X25 NETWORK PRESENTATION SYSTEM
NMC
Fig. 2. A CARLOS network.
both F T A M (File Transfer and Manipulation) and Virtual Terminal. There is an all-layer variant of this PC which can either be connected directly to the underlying X.25 network or be concentrated in a passthrough mode by the OSI PAD. A major feature of C A R L O S is its comprehensive Network Management Service, operated by means of Entity Managers in all components and a Network Management Centre. The complex information reported to the N M C from a large network can be presented to the Network Supervisor in a graphical form using the optional Presentation System. At the bottom of the picture is shown an X.25 network, either private or public as the underlying communications medium. This could be replaced by an I S D N SO interface to a public circuit switched network, a Metropolitan Area Network (MAN) or other physical communications path. There are proposals to extend the project in this way which are being discussed now. The C A R L O S project raises the important question of exactly how much functionality should
be provided by the network and how much by the user. The overhead of providing all seven layers is very great for small and medium sized machines. The provision of the lower five layers of the OSI tower within the network reduces the overhead considerably. This the C A R L O S network achieves by including the OSI BOX as a value added component of the network. The power of the OSI P A D and OSI PC can therefore be more fully utilised for the user's purpose. The term I S D N has been mentioned several times now. I S D N stands for Integrated Services Digital Network. The concept of the I S D N is founded on the emerging digital data networks which are themselves evolving from the normal PSTN (Public Switched Telephone Network) and is seen as the next stage of the evolution of this concept in which digital services are provided directly to the subscriber's premises and even to the desk. I S D N has the characteristic that a wide range of services are to be available to which the subscriber has access by means of a very limited set of standard interfaces.
D. Cochrane / Migration into the Future of Communications Technology
The appearance of the I S D N to the user is in the form of the SO interface which provides two channels at 64 kbps (which can carry data or digitised voice or one call of each type) plus a signalling channel at 16 kbps which can also carry data. The C A R L O S component, the Extended OSI PAD could be readily adapted to operate across an ISDN. The C A C T U S project (CARLOS Addition for Clustered Terminal USer Agents) is an extension of the CARLOS project which will build on the base of software and experience of C A R L O S to implement the C C I T T X.400 series of Recommendations in a form suited to medium-sized private organisations. In the terms of the standards, such a device is a shared-resource user agent for clusters of terminals. The project has included new partners from Spanish Universities, A few words of introduction to message handling systems are in order at this point. The first question is: what do we mean by a message? A message is information that is to be delivered. It is always addressed to somebody, e.g. a letter is information and the envelope has the intended recipient's name, address and the charging information (the stamp!) on it (see Fig. 3). In electronic messages, the information can be text, fax, graphics (pictures), a spoken message or any other form of information that may be required. The message forms may eventually be combined, e.g. a voice annotation to a text message. So what is a Message Handling System (MHS)? A MHS is a system that lets a user (a person or machine) prepare messages, send them to other users and receive messages from them. As users do
A M e s s a g e c o n s i s t s of :
CONTENTS
ADDRESSEE, ADDRESS, CHARGING DATA
Fig. 3.
327
not sit at terminals all day just waiting for incoming messages, the sending system musl be prepared to hold the message until the user finds it convenient to receive it. Also there are economies to be made by not using certain public networks for transmission between certain hours of the day. Hence the term "Store and Forward". X.400 is the reference number of the first of a series of recommendations (for recommendations read standards) produced b y / an international body called the C C I T T which pronounces on how the world communicates. X.400 specifies how a standard message handling system is to work. There are a number of terms it defines that are worth mentioning here: USERs interact with the MHS. The particular part of the M H S that the user deals with is the User Agent. - U S E R A G E N T s (UA) help the user to prepare messages and send them. They may also perform m a n y other local functions that the user may require. The User to User Agent interface is not subject to standards. - MTA: Message Transfer Agent. This is a unit which performs the functions concerned with routing of messages. This may involve duplication of messages and decisions about which path a message needs to take to avoid a difficulty of some sort. - MTS: Message Transfer System. This is the subsystem which moves messages round the world. It consists of a number of MTAs which act together to relay messages to the intended recipient UAs. The overall architecture is thus as shown in Fig. 4. There are protocols (given p-numbers) between the parts. The P1 protocol is used for message transmission between entities at the message transfer level, i.e. between MTAs. The P2 protocol is the interpersonal message protocol which operates above the Message Transfer Layer between User Agents. The P7 protocol is used when the User Agent is separated between two physical units, i.e. distributed and the two parts of the UA must communicate. There are other protocols in the series as well which are not used by the C A C T U S system. The X.400 system is based on the OSI model as shown in Fig. 5. A C A C T U S consists of VME-bus based hardware which supports the environment of Fig. 6. -
328
D. Cochrane / Migration into the Future of Communications Technology
Fig. 4. MHS model: A functionalview. Here, personal computers are primarily used as the user interface to a CACTUS. The emphasis is more on the PCs than on simple terminals due to the fairing costs of PCs and their increasing availability within office environments. By means of an X.25 packet switched network, the CACTUS is connected to either one or both of Private Interpersonal Messaging Systems, which includes more of itself and Public Message Transfer Services. The system is managed from a simple local terminal. The software environment is shown in Fig. 7. Within the CACTUS device is the X.25 and OSI
interface supporting levels 1 to 5. These levels are taken from the Extended OSI-PAD of CARLOS. These layers support the software providing the P1 and P2 protocols to other MTAs and Messaging Systems. The CACTUS MTA is a very simple entity and cannot, for example, act as a relaying mid-point for other MTAs; it is essentially an endpoint. The Mailbox System Agent and Mailbox Client and the ROS (Reliable Operations Server) are concepts from ECMA which allow for the remoting of the User from the User Agent, ECMA is the European Computer Manufacturers Association which is a body that produces interim
Interpersonal
Interpersonal
Message
Message
Layer
~ P 2
Message
Message
Transfer Layer
~ P 1
Transfer Layer Session
Session
Layer
Layer
Session
Transport
Layer
Transport--
Network Layer
X25 and
Layer Transport Layer Network
others
Fig. 5. X.400 protocols.
Layer
329
D. Cochrane / Migration into the Future of Communications Technology
Simple Terminals
PCs
Local SystemManager
CACTUS
N
\
III CACTUS
/
I
I
~ N e t ~ I Private Interpersonal[ Messaging Systems
Transfer Services
P I PROTOCOL
Fig. 7. CACTUS software.
Fig. 6. standards as input for adoption to the ISO and CCITT. These last concepts are thus not full International Standards but are, like CACTUS, steps on the path to standardisation. This distribution of the User Agent utilises the P7 protocol. One C A C T U S device can support mailboxes for around 100 people. The PC contains the other half of the client-server pair for the mailbox and the user interface plus the ROS and communications link to the CACTUS, The user will utilise the PC for other functions and at intervals will log into the mailbox server which has been holding any incoming messages. These are then transferred to the PC's disc for perusal by the user. The User can also transfer messages prepared on the PC (probably using a standard PC word processor package) into the
Mailbox System Agent for onward routing via the MTA. Simple terminals, such as those supported by C A R L O S can be supported in limited numbers via a user interface module within the C A C T U S interfacing to the Mailbox Server. In conclusion, the work of the standards bodies in this decade has given the world a framework for taking the idiosyncracies out of the way we work. There still remains much to do, at level seven of the OSI layer and at layers above that, but the start has been made and the way is evolutionary and not revolutionary. By maintaining a path for existing devices to interface to the newer technologies, we can make the migration as painless as possible.
Network Protocols and Standards
Easing the Migration into the Future of Communications Technology D. C O C H R A N E CASE Communications Ltd., P.O. Box 254, Caxton Way, Watford, Herts., United Kingdom WD1 8XH
Some of the obstacles that can be expected to impede the migration to the new communications scenarios that are expected to become real over the next few years are described. The easing of the migration is described in terms of the outputs from two ESPRIT projects, C A R L O S and CACTUS. Some introductory material on OSI, ISDN and the X.400 series of Recommendations is included.
Keywords: ESPRIT, OSI, Virtual Terminal X.400, Migration, Mailboxes, Network Management, ISDN.
Don Coehrane has been in the com-
~
............... ' i~i!!~i!¸!i
puter and data communications industry for 18 years following graduation from Brunel University in 1969. He joined CASE in 1982 and is currently a Senior Group Consultant acting in a consultancy role to the C A R L O S project. He is also involved with tenders for strategic sales to major organisations. Previously he has led projects in various communications areas, specifically concerning X.25, protocol emulation and transport.
North-Holland Computer Networks and ISDN Systems 14 (1987) 323-329
Today the communications professional works in a world of networks, both public and private with a p p l i c a t i o n s distributed on different equipment. There are simple terminals or Personal Computers on nearly very desk, computer languages, and therefore the applications written in those languages are getting better and more portable: C and PASCAL can be moved around with a fair degree of success. Where this wonderful scenario starts to get into difficulties is that whilst we can fairly readily make connections between systems, many mainframe operating systems and applications are highly proprietary and individualistic. This tends to mean that user interfaces and communications protocols are very hostile to the casual user and are all different. Mini and micro systems are much better; C P / M and U N I X , although proprietary, are widely implemented and the movement of applications between different manufacturers machines is possible--with care. Some dissimilar machines communicate very well, especially if they happen to have compatible versions of Kermit. The ability to communicate tends to depend largely on the knowledgeability and ingenuity of the user. The migration that is before us is to the promised land where the user can transfer data from machine to machine without a care for what has to be done to make the accessing device, terminal or PC, communicate. The terminal or Personal Computer (PC) will change, eventually, into a graphical, colour O S I / X . 4 0 0 based device supporting I D A / O D A and other business architectures. Some introductory material on some of these concepts will be included in this paper shortly. Communication will be via a universal I S D N interface to a cheap public network that is available everywhere. All user interfaces will be helpful, easy to understand and non-cryptic.
0169-7552/88/$3.50 © 1988, Elsevier Science Publishers B.V. (North-Holland)
324
D. Cochrane / Migration into the Future of Communications Technology
Unfortunately, that land is still far off. Very far. We have a considerable investment in our existing kit and the normal economics of business says that we can't throw it all out tomorrow and buy new, even if the equipment was available today. We have to write off the cost of new terminal devices over three or five years (or until the repair costs outweigh the cost of replacement). We have also heard about (or experienced!) computer fraud so the security aspects need to be cast in concrete before we are prepared to permit access to our local installations via a public service. There is also the problem that m a n y systems have different user interfaces. These are improving as packages like G E M become more widely used, thus reducing or eliminating the learning curve for those applications. Other, older applications are far more idiosyncratic. The tariffs for I S D N are speculative and m a y not show any advantages for m a n y users; PTTs also need to recoup costs expended on providing existing services. The availability of SO interfaces m a y be not quite universal for some time. Are there cases when older systems must be used? Are there ways of keeping the old equipment in use for some years yet? I think yes, to both questions.
APPLICATION LAYER
Purpose of communication. PRESENTATION LAYER Representation and meaning of data. SESSION LAYER Dialogue control and structure TRANSPORT LAYER End-to-end data integrity.
NETWORK LAYER Routing and switch!rig end~.to-end '. LINK LAYER Point-to-point error communications.
free
PHYSICAL LAYER
The transmission medium.
Fig. 1. OSI tower of layers.
For example, the Telex network is assured of a long life despite congestion, high error rate, limited character set and slow speed, simply because a telex is a legal document and its prospective successors are not so exaulted. Telex is also universally available. The migration of hardware can be achieved utilising products designed for the purpose of easing that migration. Two such products could be based on the E S P R I T C A R L O S and C A C T U S projects. A few words of very basic introduction to OSI may be useful at this point. OSI stands for Open Systems Interconnection. An Open System is one which employs the communication procedures and protocols which are derived from the Reference Model defined by the Standards-makers for the logical processes in a communication system. Such interconnection is termed Open Systems Interconnection. These protocols and procedures are described in terms of seven layers, hence, the "Seven Layer Model". The purpose of OSI is to make communication between different manufacturers equipment automatic. The seven layers are conveniently divided into two groups: the lower four levels which are concerned with the transport of data between systems over communication networks and the upper three layers that deal with application-oriented aspects of communications and not with the actual movement of data, see Fig. 1. Level 1 is the Physical layer and provides the transparent transmission of bit streams between systems. There are lots of standards in this area for all kinds of media; LANS, 2Mbit, ordinary modems, etc. Level 2 is the Link level and moves data between two directly connected systems and detects errors in the transfer. There are standards approved here for H D L C , LANs and ISDN. Level 3 is the Network level. This provides routing and relaying through intermediate systems. There are standard services defined and a limited number of standards which is what would be expected in order to be able to connect networks of different types together, e.g. LANs and Packet networks. Level 4 is the Transport layer which provides the transparent transfer of data between end systems. Reasonably, there are only two main stan-
D. Cochrane / Migration into the Future of Communications Technology
dards, one for connection-oriented and connectionless connections and one solely for connectionless. Connection oriented and connectionless refer to modes of communication, one using an association between two endpoints (users) which is maintained for the duration of their dialogue and the other only making an association when there is something to be sent. The respective virtues of the two systems is as much a matter of religion as fact. Level 5 is Session. This controls the dialogue between end systems and helps them to manage the data. It controls which user has the tight to send data at any one time, and breaks up the dialogue into manageable sections, for example, messages in a stream of messages between two particular users. Level 6 is Presentation. This governs the representation of data that is communicated, i.e. it makes sure that both ends interpret the data in the same way. This is rather akin to two people talking on the phone, one speaking Japanese and one speaking Russian. There is perfect error free communications but neither can interpret the data. In data terms this relates to whether a particular set of 16 bits represents a single large binary number, two smaller ones or one number in Binary Coded Decimal. Level 7 is the Application Layer. This is the particular enterprise that the user is attempting to use by communicating with a similar application on another system. Examples of level seven applications are Virtual Terminal, File Transfer, and Message Handling. ESPRIT (European Strategic Programme for Research into Information Technology) is a ten year programme which started in 1984. The objectives of ESPRIT are: - To provide the European Information Technology industry with the basic technologies it needs to meet the competitive requirements of the 1990s. To promote European industrial cooperation in Information Technology. To contribute to the development of internationally accepted standards. ESPRIT operates by sponsoring projects by European consortia. CARLOS (Communication Architecture for Layered Open Systems) is a project in the Information Exchange Systems (IES) area of ESPRIT. -
-
325
It is being executed by a consortium consisting of English and Danish companies with the active support of two Danish Teleadministrations. The CARLOS project involves the development of a series of modular components which provide building blocks to construct OSI systems of varying sizes and sophistication to suit the diverse requirements and budgets of a large spectrum of potential users of OSI systems. The components are targetted at organisations, institutions and companies within R & D , industry and the financial and public sectors which have a need to exchange information between heterogeneous systems. The basic principle is to enable existing, common, de facto standard equipment; terminals and PCs, to enter the OSI arena. A C A R L O S network containing all the CARLOS components is shown in Fig. 2. At the top is the user population of terminals. In CARLOS, these can be any mixture of supported terminals. These can be asynchronous devices, for example, scroll mode devices such as simple VDUs, printing terminals and printers, or more sophisticated devices which have direct cursor addressing. Common synchronous terminal devices from a well-known large manufacturer are also supported, thus coveting a large base of existing terminal devices. These devices are concentrated by means of either the OSI PAD or the Extended OSI PAD. The OSI PAD in conjunction with the OSI BOX provides similar facilities to the Extended OSI PAD. The OSI PAD provides the necessary, interface to enable the supported terminal population to access the OSI level seven application, Virtual Terminal. This enables these terminals to access any host computer from any manufacturer which also supports that application. The OSI PAD also contains the level six layer, Presentation, necessary to support Virtual Terminal. The OSI P A D / O S I BOX combination uses a specially developed Link protocol in order to physically separate the Presentation layer from the layer below it, Session. The Session layer is contained within the OSI BOX, together with Transport and the lower three layers. In the Extended OSI PAD all seven layers are present. The OSI BOX can also support a Personal Computer by means of a LAN. This PC contains the top two layers of the OSI Tower in the form of
D. Cochrane / Migration into the Future of Communications Technology
326
TERMN IA ~LS
ASYNC. AND/OR BISYNC TERMINALS
ASYNC. AND/OR
BISYNC
~"l~.
ASYNC. AND/OR BISYNC TERMINALS
/" "'-"""~ "~,%
~:
PRINTER
E
OSI
SI-PC
PAD
EXTENDED PAD
PAD
ALL-LAYER PC
N
OSI BOX
X25 NETWORK PRESENTATION SYSTEM
NMC
Fig. 2. A CARLOS network.
both F T A M (File Transfer and Manipulation) and Virtual Terminal. There is an all-layer variant of this PC which can either be connected directly to the underlying X.25 network or be concentrated in a passthrough mode by the OSI PAD. A major feature of C A R L O S is its comprehensive Network Management Service, operated by means of Entity Managers in all components and a Network Management Centre. The complex information reported to the N M C from a large network can be presented to the Network Supervisor in a graphical form using the optional Presentation System. At the bottom of the picture is shown an X.25 network, either private or public as the underlying communications medium. This could be replaced by an I S D N SO interface to a public circuit switched network, a Metropolitan Area Network (MAN) or other physical communications path. There are proposals to extend the project in this way which are being discussed now. The C A R L O S project raises the important question of exactly how much functionality should
be provided by the network and how much by the user. The overhead of providing all seven layers is very great for small and medium sized machines. The provision of the lower five layers of the OSI tower within the network reduces the overhead considerably. This the C A R L O S network achieves by including the OSI BOX as a value added component of the network. The power of the OSI P A D and OSI PC can therefore be more fully utilised for the user's purpose. The term I S D N has been mentioned several times now. I S D N stands for Integrated Services Digital Network. The concept of the I S D N is founded on the emerging digital data networks which are themselves evolving from the normal PSTN (Public Switched Telephone Network) and is seen as the next stage of the evolution of this concept in which digital services are provided directly to the subscriber's premises and even to the desk. I S D N has the characteristic that a wide range of services are to be available to which the subscriber has access by means of a very limited set of standard interfaces.
D. Cochrane / Migration into the Future of Communications Technology
The appearance of the I S D N to the user is in the form of the SO interface which provides two channels at 64 kbps (which can carry data or digitised voice or one call of each type) plus a signalling channel at 16 kbps which can also carry data. The C A R L O S component, the Extended OSI PAD could be readily adapted to operate across an ISDN. The C A C T U S project (CARLOS Addition for Clustered Terminal USer Agents) is an extension of the CARLOS project which will build on the base of software and experience of C A R L O S to implement the C C I T T X.400 series of Recommendations in a form suited to medium-sized private organisations. In the terms of the standards, such a device is a shared-resource user agent for clusters of terminals. The project has included new partners from Spanish Universities, A few words of introduction to message handling systems are in order at this point. The first question is: what do we mean by a message? A message is information that is to be delivered. It is always addressed to somebody, e.g. a letter is information and the envelope has the intended recipient's name, address and the charging information (the stamp!) on it (see Fig. 3). In electronic messages, the information can be text, fax, graphics (pictures), a spoken message or any other form of information that may be required. The message forms may eventually be combined, e.g. a voice annotation to a text message. So what is a Message Handling System (MHS)? A MHS is a system that lets a user (a person or machine) prepare messages, send them to other users and receive messages from them. As users do
A M e s s a g e c o n s i s t s of :
CONTENTS
ADDRESSEE, ADDRESS, CHARGING DATA
Fig. 3.
327
not sit at terminals all day just waiting for incoming messages, the sending system musl be prepared to hold the message until the user finds it convenient to receive it. Also there are economies to be made by not using certain public networks for transmission between certain hours of the day. Hence the term "Store and Forward". X.400 is the reference number of the first of a series of recommendations (for recommendations read standards) produced b y / an international body called the C C I T T which pronounces on how the world communicates. X.400 specifies how a standard message handling system is to work. There are a number of terms it defines that are worth mentioning here: USERs interact with the MHS. The particular part of the M H S that the user deals with is the User Agent. - U S E R A G E N T s (UA) help the user to prepare messages and send them. They may also perform m a n y other local functions that the user may require. The User to User Agent interface is not subject to standards. - MTA: Message Transfer Agent. This is a unit which performs the functions concerned with routing of messages. This may involve duplication of messages and decisions about which path a message needs to take to avoid a difficulty of some sort. - MTS: Message Transfer System. This is the subsystem which moves messages round the world. It consists of a number of MTAs which act together to relay messages to the intended recipient UAs. The overall architecture is thus as shown in Fig. 4. There are protocols (given p-numbers) between the parts. The P1 protocol is used for message transmission between entities at the message transfer level, i.e. between MTAs. The P2 protocol is the interpersonal message protocol which operates above the Message Transfer Layer between User Agents. The P7 protocol is used when the User Agent is separated between two physical units, i.e. distributed and the two parts of the UA must communicate. There are other protocols in the series as well which are not used by the C A C T U S system. The X.400 system is based on the OSI model as shown in Fig. 5. A C A C T U S consists of VME-bus based hardware which supports the environment of Fig. 6. -
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Fig. 4. MHS model: A functionalview. Here, personal computers are primarily used as the user interface to a CACTUS. The emphasis is more on the PCs than on simple terminals due to the fairing costs of PCs and their increasing availability within office environments. By means of an X.25 packet switched network, the CACTUS is connected to either one or both of Private Interpersonal Messaging Systems, which includes more of itself and Public Message Transfer Services. The system is managed from a simple local terminal. The software environment is shown in Fig. 7. Within the CACTUS device is the X.25 and OSI
interface supporting levels 1 to 5. These levels are taken from the Extended OSI-PAD of CARLOS. These layers support the software providing the P1 and P2 protocols to other MTAs and Messaging Systems. The CACTUS MTA is a very simple entity and cannot, for example, act as a relaying mid-point for other MTAs; it is essentially an endpoint. The Mailbox System Agent and Mailbox Client and the ROS (Reliable Operations Server) are concepts from ECMA which allow for the remoting of the User from the User Agent, ECMA is the European Computer Manufacturers Association which is a body that produces interim
Interpersonal
Interpersonal
Message
Message
Layer
~ P 2
Message
Message
Transfer Layer
~ P 1
Transfer Layer Session
Session
Layer
Layer
Session
Transport
Layer
Transport--
Network Layer
X25 and
Layer Transport Layer Network
others
Fig. 5. X.400 protocols.
Layer
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Simple Terminals
PCs
Local SystemManager
CACTUS
N
\
III CACTUS
/
I
I
~ N e t ~ I Private Interpersonal[ Messaging Systems
Transfer Services
P I PROTOCOL
Fig. 7. CACTUS software.
Fig. 6. standards as input for adoption to the ISO and CCITT. These last concepts are thus not full International Standards but are, like CACTUS, steps on the path to standardisation. This distribution of the User Agent utilises the P7 protocol. One C A C T U S device can support mailboxes for around 100 people. The PC contains the other half of the client-server pair for the mailbox and the user interface plus the ROS and communications link to the CACTUS, The user will utilise the PC for other functions and at intervals will log into the mailbox server which has been holding any incoming messages. These are then transferred to the PC's disc for perusal by the user. The User can also transfer messages prepared on the PC (probably using a standard PC word processor package) into the
Mailbox System Agent for onward routing via the MTA. Simple terminals, such as those supported by C A R L O S can be supported in limited numbers via a user interface module within the C A C T U S interfacing to the Mailbox Server. In conclusion, the work of the standards bodies in this decade has given the world a framework for taking the idiosyncracies out of the way we work. There still remains much to do, at level seven of the OSI layer and at layers above that, but the start has been made and the way is evolutionary and not revolutionary. By maintaining a path for existing devices to interface to the newer technologies, we can make the migration as painless as possible.