22 Geographical information systems in networked environments D J COLEMAN This chapter examines basic approaches to distributed architectures in GIS, with special emphasis on exploitation of local- and wide-area networks and, more recently, the Internet and wireless communications. The first section reviews the design models inherent in host- based systems, distributed networks, and field-based systems. The second section describes selected applications of these design models, with particular emphasis placed on Internet- based geographical information retrieval and computer-supported cooperative work. The final section deals with system performance considerations in networked environments and describes new developments affecting the performance of future systems. 1 INTRODUCTION associated with Internet-based geographical information retrieval and computer-supported Technical design models supporting mainstream cooperative work. The final section of the chapter GIS technology have evolved from early host-based deals with the issue of system performance in efforts on mainframe and mini-computers, through networked environments under a variety of stand-alone systems operating on personal conditions. After a brief summary of considerations computers and workstations, and on to today’s involved in objectively assessing system performance, distributed computing environments across local- the author describes new developments affecting the and wide-area networks. Each advance has extended performance of future systems. overall flexibility in terms of the relative location of users, processing capabilities, and data storage units (see also Batty, Chapter 21). Recent advances in broadband and wireless communications 2 HOST-BASED SYSTEMS AND EARLY technologies – as well as the dramatic increase in PERSONAL COMPUTING ENVIRONMENTS Internet usage and extension of Internet browsing technology – promise to extend further the reach 2.1 Host-based systems and range of GIS users working in offices or Early centralised computing environments were laboratories, in the field or at home. characterised by small numbers of large-scale This chapter examines the basic approaches to mini- or mainframe computers, with shared storage distributed architectures in GIS, with special devices attached via hardware input/output channels emphasis on the exploitation of local- and wide-area and multiple users connected via terminals possessing networks and, more recently, the Internet and varying levels of on-board ‘intelligence’ or processing wireless communications. The first section of the power (Katz 1991). Such environments defined the chapter reviews the design models inherent in host- architecture for most major data processing and based systems, distributed networks, and field-based information systems applications until the mid 1980s, systems. The second section describes specific including the GIS installations found in major applications of the latest design models, with forestry organisations, utilities, municipalities, and particular emphasis placed on emerging issues land records management programmes. 317 D J Coleman This architecture implied greater control over data ‘Connection-oriented’ service integrity and system security; the database was (e.g. via dial up telephone link) managed centrally, with responsibility for system and data administration entrusted to experienced data processing specialists. However, performance of such systems would often degrade in unpredictable ways when growing numbers of users demanded computing resources and database access. Conflicts with system administrators over development and maintenance priorities also often resulted in dissatisfaction among endusers in many large organisations. LAN Mainframe and 2.2 Stand-alone, PC-based systems LAN By 1986, PC-based GIS software packages had ‘Connectionless service’ begun moving geoprocessing out of the hands of Remote (e.g. via Local Area Network) information system managers (Miller 1990). Besides terminals their low cost, these systems offered more predictable response times since the user was the only one on the system. However, the proliferation of stand-alone PC-based systems meant it was much more difficult to share data among several different Fig 1. Examples of alternative wide-area networking services: ‘connection-oriented’ vs ‘connectionless’ models. people in the organisation. Also, the enduser often had to become his or her own system and database administrator. While PC-based systems undoubtedly mail and file transfer, and enabled distributed accounted for the dramatic growth in GIS usage processing. It also allowed users to expand their through the late 1980s, they also put greater onus on facilities with a degree of vendor independence and managers in large organisations to keep effective to incorporate special purpose processors, storage track of the data being collected and processed by units, or input/output devices as required. The term an increasing number of endusers with little distributed computing was coined to describe a experience in routine data management procedures. situation where processing tasks and data are distributed among separate hardware components connected by a network, with all these various 3 COMPUTER NETWORKING AND components capable of being accessed in a relatively DISTRIBUTED SYSTEMS transparent manner (Champine et al 1980). Distributed systems today generally offer a Beginning in the mid 1980s, higher-performance combination of: (1) greater access to GIS data stored workstations connected through local and wide and managed on a central server; (2) faster response area networks (LANs and WANs) became a time due to local computing; (3) tighter system viable alternative to host-based and stand-alone security; (4) less complexity; and (5) in many cases, configurations. Connectionless LAN and lower-cost computing solutions than more ‘LAN-interconnect’ services began displacing earlier traditional mainframe or minicomputer solutions. connection-oriented services which required a At a higher level, the notion of distributed dedicated link between the enduser and the host computing often provides a better fit to the complex computer (Figure 1). structures and often multidisciplinary nature of Based on packet-switching transmission protocols modern organisations and offers greater user which did not require dedicated connections involvement in information management activities. between user and host, networking permitted users Early investigators of formal network-based to share access to scarce equipment resources (e.g. approaches to distributed GIS database printers, plotters, databases etc.), made possible management systems (DBMS: e.g. Ezigbalike et al inter-site communication applications like electronic 1987; Webster 1988) laid out excellent foundations 318 Geographical information systems in networked environments for future research, but were limited by both the between two communication processes on a network distributed DBMS technology then commercially (Voelcker 1986). Protocols govern data control and available at reasonable cost, and the relatively format across a network, and a variety of protocols limited use of LANs or WANs by the GIS exist to ensure that these communications are community at that time. The subject has been conducted effectively. revisited more recently by Ingoldsby (1991) and Two different stacks or suites of layered Laurini (1994), among others. approaches are in common use today: ● OSI: the seven-layer Open System Interconnect 3.1 The client–server model suite of protocols developed by the International Standards Organisation; Most distributed computing today is based on a ● TCP/IP: the four-layer Transmission Control client–server architecture (Katz 1991). In this model, Protocol/Internet Protocol suite originally a collection of workstations (or clients) relies on one developed for the ARPAnet research network in or more servers residing elsewhere on the network the USA. for access to data files, application software and, in certain cases, more powerful computing resources. Originally developed by computer user groups and Such servers are really high-volume storage devices European telephone companies, the OSI model with processors which have been optimised to helped unify world telephony and provided a clear provide high-speed retrieval of large disk-based data framework and explanation of the functions or database files. In such an environment, the data required for computer communications. However, it retrieval aspects of a database query can be carried was regarded by some as being too cumbersome for out largely independently of the data processing and high-speed networks (Wittie 1991). display tasks. By comparison, the TCP/IP suite of protocols The time required to execute such a data retrieval became a de facto standard by the early 1980s as a operation depends on the individual performance of result of its early use in the implementation of the the storage, processing, and communications US Defense Department-funded Internet. Its longer- components involved in the client–server system. term popularity was secured through subsequent While improvements in all three of these bundling with the 1983 release of the Berkeley technologies have made key contributions, it has UNIX 4.2 operating system. While TCP/IP been the introduction of remote file management protocols do not precisely fit into the more general services which finally provided the transparency OSI model, the functions performed by each OSI required