ment in the central office required for timing and the interfaces at customers, which providers support. These issues increase costs for space at the telephone company central offices, as well as installation, repair, and maintenance expenses. For details on T1 and T3 Digital Signal Levels See Table 5-3 in the "Appendix." NETWORK TOPOLOGY ON DEDICATED, PRIVATE LINES As the name implies, private lines are those dedicated to a single institution; not shared by various organizations. In contrast to IP VPNs, the transmissions are not sent over the Internet. Private lines are used by organizations such as financial institutions, armed forces, weapons suppliers to armed forces, and government agencies that require the highest levels of security on transmissions to other sites. Other than these institutions, private lines dedicated to individual organizations have been largely replaced by newer, less costly and easier-to-maintain and www.TechnicalPDF.com configure MPLS, VPN, and WA SDN offerings. Network topology refers to how devices and sites are connected to one another in networks. It is essentially the "view from the top." Topologies include straight lines, hub-and- spoke designs, and mesh designs. Dedicated Private Lines-For Greater Security Dedicated services, also known as private lines, are circuits leased by enterprises for their exclusive use. Private lines are expensive, and the number of them used by commercial organizations has decreased dramatically. For all but the largest organizations, MPLS, WA SDN, and IP VPNs have largely replaced private lines. These are less costly to maintain and boast lower monthly lease rates. However, large enterprises, utilities, armed forces, weapons suppliers to governments, and financial services organizations still use high-speed private lines for applications such as money transfers because they are the most secure method of transmitting data. Most of these organizations have the expertise for the initial design and sizing of the dedicated network, ongoing maintenance, an