对照ISO/OSI参考模型各个层中的网络安全服务，在物理层可以采用(39)加强通信线路的安全；在数据链路层，可以采用(40)进行链路加密；在网络层可以采用(41)来处理信息内外网络边界流动和建立透明的安全加密信道；在传输层主要解决进程到进程间的加密，最常见的传输层安全技术有(42)等；为了将低层次安全服务进行抽象和屏蔽，最有效的一类做法是可以在传输层和应用层之间建立中间件层次实现通用的安全服务功能，通过定义统一的安全服务接口向应用层提供(43)安全服务。

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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 servicess. 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.

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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.

66. A. switch-based LANs

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