OSI for Libraries: Standards to Services (1992)

2. WHAT IS OSI?

"The aim of Open Systems Interconnection is to allow, with a minimum of technical agreement outside the interconnection standards, the interconnection of computer processing systems

• from different manufacturers,
• under different management,
• of different levels of complexity,
• of different ages." (ISO 8571, p. iv)

Efforts to specify the OSI Reference Model itself began in 1977 and it was formally adopted as an International Standard by ISO in 1984.

OSI refers not only to the approved Reference Model, but also represents a set of International Standards that embody the OSI principles. These standards exist in a hierarchy. At the top of the hierarchy is the OSI Basic Reference Model (ISO 7498), which describes the architectural framework upon which all the other standards are based. Following this are one or more standards for each layer in the architectural framework. In addition, there are related standards that define procedures for international registration, testing, and establishing profiles. OSI is also used to refer to a set of software products that implement and conform to the OSI standards (MacKinnon, McCrum and Sheppard, 1990).

2.1 The OSI Basic Reference Model

The method used by those developing the OSI model to accomplish this formidable task was to divide the various elements of computer to computer communications into separate functions, arranged in a layered manner, with each layer using a communication channel provided by the layer below it, and offering services to the layer above. This allows each layer to be specified independently of the layers above and below. Systems that adhere to standards defined for each layer can then operate with each other despite differences in their hardware or software.

Seven layers are defined in the OSI Reference Model. The bottom four layers physical, datalink, network and transport are concerned with the physical connection between computers and the reliable transmission of data between them. The top three layers session, presentation and application are concerned with the meaningful and cooperative exchange of information between computers.

At each layer, the operation of the layers below is transparent. This means that an application, such as interlibrary loan, does not need to specify how the data is physically transmitted over a wire nor how the connection between two systems is established. The application layer need only be concerned with passing the correct data to the next lower layer with instructions on what that lower layer is to do with the data. In the same way, the lower layers do not need to know what the application above is doing, and do not normally alter the data that is being passed to the layer above (Denenberg, 1990).

While a detailed description of the structure and function of each layer is beyond the scope of this document, it is useful to briefly describe each layer and the specific function it performs (see Figure 1):

Physical Layer

This layer handles the mechanical and electrical aspects of the physical connections made to the computer system. It sends and receives streams of binary digits (bits) across a physical medium, be it cable, optical fibre or a radio link.

Datalink Layer

This layer ensures error-free transmission of data between two directly connected systems. It includes functional and procedural means to transfer blocks of data and to detect and correct errors which may have occurred in the physical transmission of the data.

Network Layer

This layer provides routing and switching functions necessary to move data from an originating system through intermediary nodes to the destination system when there is no direct communication between two systems. It also is responsible for segmentation of the data within a message and its reassembly at the receiving system.

Transport

This layer is essentially the link between the upper and lower parts of the model. It provides end-to-end control of a communication channel once the path has been established. It enables the reliable and sequential exchange of data between the end systems.

Session Layer

The session layer establishes and manages a dialogue between communicating end systems. For example, it can allow simultaneous two-way communication or two-way alternative communications. It also provides mechanisms for recovery and resynchronization in case a connection has been lost.

Presentation Layer

This layer ensures that information is transferred in a format that can be processed by different, possibly incompatible, computer systems. It selects the appropriate syntax for data representation and, if needed, converts from a standardized representation used in communications to a representation that the local system can process.

Application Layer

This is the highest and most complex layer of the OSI Reference Model. It specifies what communication services are to be available to a computer system to allow it to co-operate with other systems in carrying out tasks, such as interpersonal messaging between users, managing interlibrary loan transactions, and transferring cataloguing records.

Figure 1: The OSI Reference Model (46 K)

None of the layers are intended to be seen or used directly by the operators or other human users of a computer system. The "user" of an OSI application service is normally a program that makes use of OSI application services as required and interacts with an operator as required. Using the OSI application service might be the primary purpose of the program, as in an electronic mail system, or the OSI application service might form only a small part of the program, as in an on-line catalogue, in which most of the interaction is with a local database system, and only occasional connections to remote databases are made.

2.2 OSI Terminology

There are, however, three terms which are important to distinguish at the outset: application, standard and protocol. One often sees these terms used interchangeably, which leads to further confusion.

An application in the broadest sense is the use of a computer system to perform a given task. Document preparation, electronic mail and cataloguing are all applications. Some computer applications require that one computer system communicates with other systems in order to transfer information or co-operatively carry out some processing task. The way computers communicate in order to do so is specified by an application-level or application-specific communication standard. OSI standardizes a number of such specifications.

There are two kinds of standards. One, the kind represented by OSI, is a de jure standard, formally approved by an official standards-making body and often having the status of a law or treaty. The other, de facto standards, have no legal status, but are so widely used that they are in effect standards. For example, the IBM PC architecture is a defacto standard. OSI protocols have the status of international standards, and are approved by ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) and/or by the CCITT (the International Telegraph and Telephone Consultative Committee). All of these are United Nations sponsored bodies or agencies.

OSI standards normally come in pairs: one standard contains a service definition and the other contains a protocol specification. The service definition states what services are provided to the user of the standard, what parameters are to be supplied, and what sequences of events are permissible. The protocol specification states what data is passed to the lower layer in order to provide the services, and how it is to be passed (MacKinnon, McCrum, Sheppard 1990).

A protocol is a well defined set of procedures that specifies how computer systems exchange data. It is analogous to a protocol in the diplomatic service in that it sets out mutually agreed rules of conduct governing how interactions are to proceed: who has precedence; what language is used; what kinds of message can be exchanged and in what order; and what to do in case of an error. Protocols developed according to the OSI Reference Model are referred to as "OSI protocols" or as belonging to the "OSI suite of protocols". In the area of computer communications, there are other protocol suites, such as the TCP/IP suite, which is now in widespread use, particularly in the academic environment, or the Coloured Book protocols used in the U.K. (Quarterman, 1990). These other suites do not generally have the status of de jure standards, although their increasingly widespread use does give them the status of de facto standards.

2.3 Bibliographic OSI Standards

The ILL protocol is a standard for computer communications that permits ILL messages to be exchanged between libraries that use different ILL systems. The protocol defines the types of services that are available; the sequence in which these services can be invoked; the information contained in each service; and the format of the information when sent as protocol messages. The services defined by the protocol are: ILL-REQUEST; SHIPPED; ILL-ANSWER with the possible values of conditional, retry, unfilled, locations, will supply, hold-placed, and estimate; CONDITIONAL-REPLY; CANCEL; RECEIVED; RECALL; RETURNED; CHECKED-IN; OVERDUE; RENEW; RENEW-ANSWER; LOST; DAMAGED; MESSAGE; STATUS-QUERY; STATUS-OR-ERROR-REPORT; and EXPIRED.

The standard supports a variety of interlending scenarios. These can vary from simple, direct transactions between a requesting library and a lending library to much more complex transactions involving a number of intermediary institutions that may provide location information, routing functions, or may act on behalf of other libraries (Turner, 1990b). Through the definition of each step in the ILL process, the protocol provides effective control and management of ILL transactions for both borrowing and lending activities.

The Search and Retrieve protocol allows a computer to perform information retrieval queries against another computer acting as an information server. The protocol provides uniform procedures for querying target systems that support one or more databases and the transfer of records to the local system that correspond to the user's search query.

The local computer that initiates the search request submitted by the user is defined as the Origin system; the remote computer that houses the desired information is defined as the Target system.

The set of services that are defined by SR is relatively small, particularly in comparison to the large set of services defined for ILL. They include;

INITIALIZE an SR association between an Origin and a Target system,

SEARCH one or more databases at the Target,

PRESENT to the Origin one or more records found by a search,

DELETE-RESULT-SETS at the Target, and

RELEASE the association.

This set of services, however, is sufficient to support a wide range of library functions that require database searching: ILL, cataloguing, reference and acquisitions. With the addition of several new services proposed for the next version, including index browsing and stored and periodic queries, the SR protocol will support additional functions such as public catalogue access, and general-purpose information retrieval.

2.4 Generic Protocols

File Transfer (FTAM)

Message Handling System (X.400)

Directory Service (X.500)

2.5 Benefits of Using OSI Application Standards for Libraries

OSI allows for the interconnection of dissimilar systems without compromising local system autonomy or requirements. It imposes no demands on the design of the system other than adherence to the specifications of the OSI standards to be supported.

Because OSI separates the components of interconnection into functions or layers, its modular nature can accommodate evolving technology. Implementations of the various OSI layers can be shared, upgraded or replaced without affecting the integrity of the whole protocol stack. OSI protocols can also be used to operate with non-OSI protocols. For example, the ILL and SR application protocol services can share the services provided by protocols at the lower layers and can even be transmitted over networks that are not OSI-based such as the Internet. This is further discussed in section 6.2.

OSI protocols are not only portable to different types of systems, but can be implemented on systems that are developed for personal computers, minicomputers or mainframes. The systems themselves can be modified and augmented to provide additional capabilities without needing to replace those portions of the system concerned with managing the implemented OSI standards.

For libraries, OSI means the ability to exchange messages, records, files and documents with other libraries regardless of the communicating hardware and software environments. OSI application protocols are rich in functionality and can support a host of library activities.

Through the use of standardized procedures for computer communications, dissimilar library systems can exchange data to simplify or automate a number of library tasks, such as obtaining bibliographic records, transferring ordering and accounting information, and managing interlibrary loan and resource sharing activities.

A number of library operations can be enhanced using OSI, for example:

• the timeliness of cataloguing can be improved through use of FTAM rather than magnetic tape to transfer bibliographic records
  
• local ILL systems can be developed to support both the borrowing and lending activities of interlibrary loan because the message types, content and format are specified by the ILL protocol
  
• the ILL protocol provides improved ILL transaction control and management, as well as the ability to track borrowed items
  
• database searching can be simplified by using SR to provide uniform procedures to search both the local catalogue and remote databases
  
• use of SR in conjunction with ILL enables the circulation status of items to be determined before an ILL request is made improving the chances of a successful request, and reducing the cost and time for providing an item
  
• use of X.400 for document delivery can allow documents to be delivered directly to patrons, without need for dedicated fax lines or expensive couriers
  
• acquisitions, reference and interlibrary loan can be improved through the use of an X.500-based directory that provides information about libraries, such as their addresses, the strength of their collections and the types of material they offer for loan.

OSI standards are designed to require a minimum of bilateral agreement between systems prior to setting up a communications link; both sides indicate what communication services are supported or required and can negotiate the use of an appropriate set of services. The use of standard functional profiles, as described below, enhances the likelihood that systems will be able to communicate with a minimum of prior agreement.

One particular benefit of OSI stems from the fact that it is intended to be integrated into computer applications, rather than used as a separate mechanism. OSI provides a communication infrastructure, but does not restrict in any way how communication services are made available to the end user of the computer system. This means that the user need not be aware that OSI standards are being used to implement the communications requirements of the application. There is no need for training the operator in "how to use OSI", or "what OSI can do for you". The user sees only a wider range of more powerful services available from local applications.

For example, file transfer is available as a general-purpose building block to support computer applications that require file access and manipulation. It could, for instance, be integrated into a word processing application in such a way that the user need not know that a particular file being accessed resides on a computer system that is half a world away. Similarly, network information retrieval could be integrated into a public catalogue system in such a way that the user need not know whether a listing of journal contents being consulted is on the local system or is being provided by a commercial database supplier over a telecommunications link.

2.6 OSI Standardization

The ISO group responsible for the ILL and SR standards is ISO Technical Committee 46 (Documentation), Subcommittee 4, Working Group 4, familiarly known as TC46/SC4/WG4. This working group has overall responsibility for standards relating to the format and structure of bibliographic information exchanged in machine-readable form. It is composed of librarians and communication specialists.

The work of the group is carried out by the preparation, review and formal balloting of a series of documents relating to each application standard. The first step is a proposal for a New Work Item (NWI). Once this is approved, a series of Working Drafts (WD) is produced, each of which is commented on, and balloted within the working group for progression to the next stage, the Committee Draft (CD). This is circulated to the full membership of the subcommittee for comment and is balloted for progress to the next stage, the Draft International Standard (DIS). The Draft International Standard is circulated to the full membership of the Technical Committee for comment, and balloted for progression to the final stage, the International Standard (IS). Documents may go through many cycles of comment and balloting before moving to the next stage and, in each cycle, objections must be addressed, and if possible resolved, so that consensus can be achieved.

This means that the progress towards final approval can be very slow. The definition and approval of the ILL and SR standards took approximately six years. Even though they have finally achieved International Standard status, this does not mean that they will remain static. TC46/SC4/WG4 has already started reviewing addenda which are intended to provide additional functionality for the standards. Addenda go through the same standardization process as the initial standard and are issued as separate documents once they are approved. Approximately every five years the initial standard is reviewed and reissued with addenda and editorial and technical corrections incorporated.

Very few libraries, other than those that are members of standards groups, have an opportunity to participate in the process of shaping and approving OSI-based protocols. However, once these protocols are in use, libraries can play an active role by documenting problems encountered or identify enhancements. Libraries can contribute to the evolution of the ILL and SR protocols by becoming familiar with the standards, documenting concerns and communicating them to their national representatives on TC46/SC4/WG4 or to their national standards bodies.

In addition, libraries can influence the deployment of bibliographic OSI-based standards by lobbying to have them adopted as national standards. While the absence of national approval is not an impediment to their use, the approval of protocols at the national level does give them a national acceptance which may further promote their acceptance and use.