LAN. LAN operating system client software is located on each device connected to the LAN. Most operating systems in use today are built on the client-server model. Data Centers and IP Private Branch Exchanges Clients (PCs) request access to services such as printers, e-mail, and sales applica- tions located on servers. Examples of NOSs include Windows Server 2003, Windows Server 2008, Sun Solaris 10, and Linux. Client software installed on PCs includes Windows XP, Windows 7, and Windows 8. The Impact of Virtualized Hardware Failure Because multiple software applications run on each physical server, virtualization increases each physical server's criticality. If one server fails, multiple applications and business processes are disrupted. Moreover, both server and storage virtualization result in the need to centralize more applications, such as collaboration, video and audio conferencing, and accounts payable and accounts receivable software within data centers rather than located in remote departments. Thus, in multi-site organiza- tions, failures affect multiple sites and departments. Redundancy is a key consideration in organizations where computing is critical for operations. Hot standby is one option for redundancy. Hot standby refers to the capability of one piece of equipment to automatically take over the function of a failed unit, without human intervention. One way to achieve this is to cable servers to more than one switch SO that if a switch crashes, the server automatically fails over to the backup switch. Another option is to use replication software, which is used in virtual- ized servers to back up all files to a hot standby location that can handle all computing if the main site crashes, as shown in Figure 2-3. Critical applications can also be run on duplicate physical servers, wherein the redundant server automatically takes over in the event of a failure. Uptime and continuous operations depend on outside networks as well as internal servers and switches. If a cable to the Internet is cut, ser