Since interactive services and distributive services are both present among the services in demand， there is a chance that each type sporadically forms its own communication network. Since the circuit-mode services and packet-mode services are also mixed together， each could develop a separate communication network of its own. However， the construction of an independent service-oriented network imposes a heavy financial burden and can bring about the obstruction of communication information transfer and disorder in the administration of the communication network. Therefore， it is desirable to integrate various networks into one universal communication network so that all the services can be provided in an integrated manner. The B-ISDN concept is just to achieve such an integrated service network by expanding the already standardized narrow bend ISDN.
Therefore， the B-ISDN can be described as a digital communication network that utilizes broadband transmission and switching technologies to interconnect concentrated or distributed subscribers and service providers， and to support integrated services with wide-bandwidth distribution that ranges from a few bits per second to hundreds of megabits per second.
From the service standpoint， the B-ISDN integrates narrow band services such as telephone， data terminal， telemetry， facsimile， and teletex with broadband services such as video telephone， video conference， high-resolution image transmission， high-speed data transmission， video surveillance， and CATV. Consequently， the B-ISDN can be said to be a communication network that takes the concept of the existing ISDN and extends it to enable the provision of various kinds of broadband services.
The primary goal of the B-ISDN is the accommodation of all existing services along with those that will come into being in the communication networks of the future. Consequently， narrow band services such as telephone， data terminal， facsimile， telemetry， teletex， videotex， and electronic mail are included as the basics， and broadband services such as video telephone， video conferencing， high speed data， color facsimile， CATV， HITV distribution， high-fidelity sound， vide-mall， video monitoring， high-resolution picture transmission， and broadband videotex are additionally provided.
These B-ISDN services include services of all types and characteristics. Conversational services such as telephone or video telephone， message services such as electronic mail or video mail， and retrieval services for retrieving graphics or documents are all included. These are all bidirectional interactive services， but unidirectional distributive services such as CATV are included as well.
【New Words】
diversified
多变化的，各种的
ubiquity
到处存在，普遍存在
sporadically
偶发的，零星的
Teletex
计算机终端的数据处理
videotex
可视图文
wide-bandwidth
宽带的
HDTV （High Definition TV）
高清晰度电视
4.2 The Database Model
A data model describes a way to structure and manipulate the data in a database. The structural part of the model specifies how data should be represented （e.g.， tree， tables， and so on）. The manipulative part of the model specifies the operations with which to add， delete， display， maintain， print， search， select， sort， and update the data.
Hierarchical Model
The first database management systems used a hierarchical model--that is they arranged records into a tree structure. Some records are root records and all others have unique parent records. The structure of the tree is designed to reflect the order in which the data will be used that is， the record at the root of a tree will be accessed first， then records one level below the root， and so on.
The hierarchical model was developed because hierarchical relationships are commonly found in business applications. As you have known， an organization chart often describes a hierarchical relationship： top management is at the highest level， middle management at lower levels， and operational employees at the lowest levels. Note that within a strict hierarchy， each level of management may have many employees or levels of employees beneath it， but each employee has only one manager. Hierarchical data are characterized by this one-to-many relationship among data.
As another example， consider a simplified airplane spare parts database. An airplane， like most systems， is made up of a sat of assemblies that are made up of subassemblies and so on. Using a hierarchical approach， the relationships between records and fields might be established as follows： the first or highest level would contain the major assemblies， such as wings， fuselage， and cockpit； the second level of the hierarchy would contain subassemblies for each major assembly； and the levels farther down would include the specific part numbers and part information.
This approach would be very convenient for answering customer inquiries about parts and their availability， but it would be less convenient for making an inquiry about what parts are on what planes. Before information about specific parts and planes can be obtained， each major assembly must first be retrieved， and several levels of the hierarchy must be navigated to obtain the part information.
In the hierarchical approach， each relationship must be explicitly defined when the database is created. Each record in a hierarchical database can contain only one key field and only one relationship is allowed between any two fields. This can create a problem because data do not always conform to such a strict hierarchy.
Relational Model
A major breakthrough in database research occurred in 1970 when E. F. Codd proposed a fundamentally different approach to database management called relational model， which uses a table as its data structure. A table， like a spreadsheet， consists of rows and columns. Each row identifies a record and each column corresponds to a field.
