Network security is necessary because the Internet has made networked computers accessible from and vulnerable to any other computer in the world. As companies become more Internet-reliant, new threats arise from persons who no longer require physical access to a company’s computer assets. There are four primary threats to network security: ■ Unstructured threats ■ Structured threats ■ External threats ■ Internal threats Unstructured threats consist of mostly inexperienced individuals using easily available hacking tools such as shell scripts and password crackers. Some of the hackers in this category are motivated by malicious intent, but most are motivated by the intellectual challenge and fun of it and are known as “script kiddies.” Script kiddies are not the most experienced or knowledgeable hackers. They download these easily executable scripts from numerous hacker Web sites for free. The script kiddy’s reasoning is: “Why battle monsters in the latest computer game when you can test your battle skills against real targets?” Even unstructured threats that are only executed with the intent of testing and challenging a script kiddy’s skills can still do a lot of damage to a company. For example, if your company’s external Web site is hacked, your company’s integrity is damaged. Even if your external Web site is separate from your internal information that sits behind a protective firewall, the public does not know that. All they know is that if your Web site was hacked, your Web site obviously is not safe enough to do business in.

Over the past few years, Internet-enabled business, or e-business, has drastically improved companies’ efficiency and revenue growth. E-business applications such as e-commerce, supplychain management, and remote access enable companies to streamline processes, lower operating costs, and increase customer satisfaction. Such applications require mission-critical networks that accommodate voice, video, and data traffic, and these networks must be scalable to support increasing numbers of users and the need for greater capacity and performance. However, as networks enable more and more applications and are available to more and more users, they become ever more vulnerable to a wider range of security threats. To combat those threats and ensure that e-business transactions are not compromised, security technology must play a major role in today’s networks. The closed network typically consists of a network designed and implemented in a corporate environment, and it provides connectivity only to known parties and sites without connecting to public networks. Networks were designed this way in the past and thought to be reasonably secure because there was no outside connectivity.

An understanding of the functioning of the cardiovascular system draws heavily on principles of fluid flow and of the elastic behaviour of tissues. Indeed, much of the cardiovascular system consists of a fluid (blood), flowing in elastic tubes (arteries and veins). This chapter will introduce basic principles of fluid flow and of solid mechanics. This area has developed over many centuries and Appendix 1 provides details of key scientists and their contribution. The concept of a fluid and a solid is familiar from everyday experience. However, from a physics point of view, the question arises as to what distinguishes a fluid from a solid? For a cubic volume element there are two types of forces which the volume element experiences (Fig. 1.1); a force perpendicular to a face and a force in the plane of a face. The forces perpendicular to the face cause compression of the material and this is the case whether the material is liquid or solid. The force parallel to the face is called a shear force. In a solid, the shear force is transmitted through the solid and the solid is deformed or sheared. The shear force is resisted by internal stresses within the solid and, provided the force is not too great, the solid reaches an equilibrium position. At the nano level the atoms and molecules in the solid retain contact with their neighbours. In the case of a fluid, a shear force results in continuous movement of the material. At the nano level the atoms and molecules in the fluid are not permanently connected to their neighbours and they are free to move. The key distinction between a fluid and a solid is that a solid can sustain a shear force whereas a fluid at rest does not.