在软件开发过程中常用图作为描述工具。DFD就是面向(19)分析方法的描述工具。在一套分层DFD中，如果某一张图中有N个加工 (process)，则这张图允许有(20)张子图。在一张DFD图中，任意两个加工之间(21)。在画分层DFD时，应注意保持(22)之间平衡。DFD中从系统的输入流到系统的输出流的一连串连续变变换形成一种信息交流，这种信息流可分为(23)两大类。

(85)

(83)

(84)

(82)

(80)

Rapid advances in computer and communication technologies have resulted in the(71) merger of these two fields. The lines have bluned among computing, switching, and digital transmission equipment, and the same digital techniques are being used for data, voice, and image transmission. Merging and evolving technologies, coupled with increasing demands for efficient and timely collection, processing, and(72) of information, are leading to the development of(73) that transmit and process all types of data. The ultimate goal of this evolution is something its proponents-some of the most powerful forces in the computing and telecommunications industries-call the (74) .The (74) will be a worldwide public telecommunications network that will deliver a wide variety of services. The (74) will be defined by the standardization of user interfaces, and will be implemented as a set of digital switches and paths supporting a broad range of traffic types and providing (75) processing services. Inpractice, there will be multiple networks, implemented within national boundaries, but from the user's point of view there will be a single, uniformly accessible worldwide network.

(78)

(79)

Even though current technologies make the ultimate segmented network impractical, new technologies and products hold the promise of making the concept a reality. Most important of all these new technologies are (66) . The ultimate switched LAN, which is technically possible, now, exhibits the following characteristics:

  1. High-speed yet cost-effective switching devices can be implemented on a wide scale, providing dedicated connections for individual users and supporting the aggregate (67) of all networked users.

  2. The ability to deploy switching capabilities incrementally into the (68) base. Switching implementations support switching of individual users or groups so that existing users of shared media modules can continue to use the existing equipment until they can justify moving to dedicate switched LAN connections.

  3. Support and enhancement of existing routed environments. Switching (69) routint, but rather can be introduced into the network without making any changes to the existing routing equipment or configuration.(70) play a critical role in the integration of switching into existing networks.

(77)