7. SOFTWARE ARCHITECTURES

While most LANS use TCP/IP and Microsoft, Unix or Linux Operating systems on their servers, a number of alternate operating systems have been used over the last few years and a number still exist. The following paragraphs outline some of the better known LANs.

Netware

Netware was developed by the Novell corporation and is primarily a suite of application oriented interfaces rather than LAN-based protocols. Netware provides workstation (PC, Macintosh etc.) access to a network file server, which runs the Netware operating system.

Netware will operate with a variety of LANs including Ethernet, Cheapernet, Token Ring, Local Talk and ARCNet. Basically Netware adds a programme termed a ‘shell’ to the PC. Every command issued by the PC goes to the shell. If it is a local command it is then passed to the local operating system. If the command relates to the network then the command is processed by Netware and sent onto the network.

While the suite is not standards-based it was widely used and supported due to its level of sophistication and the speed of operation. Its popularity has led many companies to develop software that will operate with Netware.

Netware was originally based on the Xerox network Systems (XNS) stack, but today runs on both IPX/SPX as well as TCP/IP. NetWare was one of a series of XNS-based systems, which also included Banyan Vines and Ungerman-Bass Net/One. Unlike these products, and XNS itself, NetWare established a strong presence in the market in the early 1990s, and barely managed to survive the onslaught of Microsofts Windows NT which killed off the other players.

Netware evolved from a very simple concept : one or more dedicated servers were connected to the network, and shared disk space in the form of volumes. Clients running MS-DOS would run a special Terminate and Stay Resident (TSR) program that allowed them to map a volume as if it were a local hard disk. Clients had to log-in, to be allowed to map volumes, and access could be restricted according to the log-in name. Similarly, clients could connect to shared printers on the dedicated server, and print as if the printer was connected locally. While early Netware systems did entirely trust all modules (any misbehaving module could bring the whole system down), it was very stable. There are reports of Netware servers running for years without any human intervention.

IBM APPC

Advanced Program-to-Program Communications (APPC) is a protocol suite originally designed by IBM as part of SNA (Systems Network Architecture). APPC uses LU 6.2 (LU stands for Logical Unit or device on the network and 6.2 is the number assigned to the particular level for program to program communication)

With the advent of LANs, IBM have implemented APPC on the Token Ring network operating above 802.5 (Token Ring) and 802.2 (Logical Link Control) standards.

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Netbios

Netbios is similar to Netware in that it is a form of application interface rather than a network protocol. In MS/DOS, the main PC operating system, Basic Input Output System (BIOS) controls access to various devices such as the keyboard, screen and communications port. Netbios similarly handles the input and output of applications in coordination with the network.

Originally introduced in 1984, Netbios was designed to operate on IBM networks. Following the publication of the Netbios interface other non-IBM products were introduced and adaptations for Ethernet TCP/IP and OSI standards have been implemented.

Due to its association with IBM this system is widely used. However, it is somewhat restricted in its addressing capability and its maximum capacity for simultaneous sessions.

LAN Manager

LAN Manager, developed by Microsoft and 3Com, is a network operating system originally designed to run on the OS/2 operating system. OS/2 was written by Microsoft for IBM to exploit the enhanced facilities of its second generation of personal computers, the PS/2. The operation is no longer limited to OS/2 systems and versions were and available for MS-DOS, and Xenix (an earlier version of Unix for PC’s, now replaced by Linux). There was also LAN Manager/X (LMX) for UNIX based systems. In 1990 Microsoft announced LAN Manager 2.0 with a lot of improvements. The latest version LAN Manager 2.2 which included an MS-OS/2 1.31 base operating system remained to be Microsoft’s strategic server system until the release of Windows NT Advanced Server in early 1994.

Windows NT

When development started in 1988, Windows NT (using protected mode (the 286 architecture introduced protected mode allowing for (among other things) hardware-level memory protection)) was to be known as OS/2 3.0, the third version of the operating system developed jointly by Microsoft and IBM.

In addition to working on three versions of OS/2, Microsoft continued parallel development of the DOS-based and less resource demanding Windows environment (using Real mode (an operating mode of 80286 and later x866-compatible CPUs)).

When Windows 3.0 was released in May 1990 it was so successful that Microsoft decided to change the primary API (application programming interface) for the still-unreleased NT OS/2 (as it was then known) from an extended OS/2 API to an extended Windows API. This decision caused tension between Microsoft and IBM, and the collaboration ultimately fell apart. IBM continued OS/2 development alone, while Microsoft continued work on the newly-renamed Windows NT. Though neither operating system would be as immediately popular as Microsoft’s DOS or Windows products, Windows NT would eventually be far more successful than OS/2.

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OSI – Open Systems Interconnection

Introduction

At one time it was generally accepted that TCP/IP would be superseded by OSI, due to the more advanced facilities available with OSI. For example OSI has such as a much larger addressing field. Companies such as Case Communications even developed Gateways, which converted other LAN protocols such as TCP/IP into OSI. However the complexity of OSI increased cost of the products, and the proliferation of TCP/IP ensured it became virtually the only network protocol by the end of the millennium.

OSI Standards

Following the publication of the OSI model the International Standards Organisations (ISO) developed the protocols for the seven layers of the model. As with all such monumental tasks, a great deal of time has been spent in the determination and agreement of these standards. To speed up the process and to encourage the implementation of the OSI protocols, many standards developed by the Institutes of Electrical and Electronic engineers (IEEE) and by the Comite Consultatif Internationale de Telegraphique et Telephonique (CCITT now ITU (International Telecommunications Union after 1992) were developed.

The bottom two layers, Physical and Data Link are addressed by the standard LAN types such as CSMA/CD, Token Ring and Token Bus.

Level 2 – the Data Link is constructed of two sub layers. The lower of these is the Media Access Control (MAC), layer. This is regarded as part of the physical standard of the network and different variants are specified with the standards such as 802.3, 802.4 and 802.5. The higher section of level 2 is Logical link Control (LLC) covered by the IEEE 802.2 standards. Effectively LLC is designed to enable multiple links to multiple stations on a single physical network. This part of the standard is software-based but typically resides on hardware units, such as PC cards. There are two distinct classes of LLC.

Class I – is a form of connectionless communication. There is no link establishment, acknowledgement of Protocol Data Units (PDUs), flow control or error recovery.

Class II – operates a connection-oriented communication. Links are established between LLCs prior to data transfer. The link then maintains flow control and error recovery.

LANs usually use Class I LLC because of the relatively low rate of errors on such networks. Class II would reduce the effective throughput of the network because of the connection overheads. Connection control usually resides within the Level 4, (the transport layer, of the OSI model.)

The next two layers, (The Network and Transport Layers), have both connectionless and connection oriented protocols specified. Basically these two forms of communication can be summarised as follows.

Connectionless – protocol allows transmission of blocks of data (datagrams) across the network. There is no previous contact necessary between the sending and receiving devices. Datagrams can arrive in any order or even be missing: the connectionless protocol has no concern over this.

Connection – oriented protocol ensures that the sending and receiving terminals are aware of the communications before transmission of data actually begins. Data is then sent sequentially and errors reported before the next block of data is sent.

Layer three – (the Network Layer), in LANs usually implements a connectionless protocol called Connectionless Network Service (CLNS). The advantage of this is that, due to the low failure rate of LANs, there is no overhead on the network regarding setting up connections and handling data. Also in failure situations, no restoration of connections is necessary, thus simplifying the restart, saving time and maximising data throughput.

Layer four – (the Transport Layer), in LANs usually implements a connection-oriented protocol called the Transport Protocol (TP4). This exploits the capabilities of CLNS and simply collates the data as it is received, re-sequences the datagrams, checking for errors and requesting retransmissions where necessary. Using this method the connection is handled by the sending and receiving machines but no ‘connection; data is sent across the network except for retransmissions due to errors, thus minimising the overheads on the network.

Layers five and six – (the Session and Presentation Layers) both have connection-oriented protocols specified. These protocols are designed to allow applications to interface rather than network components. Their implementation is therefore more dependent upon the actual use of the network rather than its operation. Examples of such levels in the pre-OSI environment are Netware and Netbios.

Layer seven – (the Application Layer), is the area, where the main user inter-working takes place. Layer seven OSI standards include X.400 (Electronic Mail), X500 (Directory Services), Virtual Terminal and FTAM (File Transfer Access Management). Had OSI replaced TCP/IP then these would have been the basic building blocks which would have allowed different business applications to inter-work without any reprogramming or redesign.

Some of the major initiatives in OSI implementation were MAP, TOP and GOSIP.

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MAP – Manufacturing Automation Protocol

The Manufacturing Automation Protocol – was pioneered by General Motor’s in 1983. It began as an initiative to develop inter-working between equipment on the manufacturing floor. The 1987 release, MAP 3.0, provides a range of protocols, wide enough for commercial implementation.

A mix of standards within the OSI definitions has been chosen as the most suitable for manufacturing purposes. This involved the selection of Token Bus as the LAN (802.4) and an Application layer protocol Manufacturing Message Format Standard (MMFS) was added to the OSI suite.

TOP – Technical and Office Protocols

TOP is similar to MAP. Originally proposed by Boeing Computer Services, the standard follows the OSI form and is concerned with exchanging office documents and graphics in a suite of international standards.

GOSIP (Government Open Systems Interconnection Profile)

GOSIP was a U.S. government mandate first published as FIPS 146-1 in 1990, that after August 15, 1990, all new network procurements must comply with OSI. Testing is performed at the NIST, which maintains a database of OSI-compliant commercial products.

In 1995 FIPS 146-2 was published, which removed the procurement requirement for the GOSIP OSI protocols, by permitting acquired products to implement ISO, ITU-T or IETF standards. Interest in OSI implementations declined, and subsequent civilian government agency deployments of networking services are predominately based on the Internet Protocol Suite, which led GOSIP to evolve into POSIT (Profiles for Open Systems Internetworking Technologies), which is a set of non-mandatory standards that acknowledge the widespread use of TCP/IP.The Defence Messaging System continued to be based on the OSI protocols X400 and X.500 due to their integrated security capabilities. GOSIP also allows TCP/IP protocols to be used.

GOSIP was also pioneered by the UK government and defined that all suppliers must conform to the OSI model when public sector procurements were being made.

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