ing is off, the transmission will not work. In this case, the technician might say, "The line is slipping," or "The timing is off." Broadband and Wide Area Network Services CSUs use mainly extended superframe (ESF) SO that T1 and T3 circuits can be monitored while a line is in service. Network service providers have ESF CSU/DSUs in their networks that connect to the circuits of multiple customers. PRIVATE LINES, NETWORK TOPOLOGY, AND FRAME RELAY Private lines and frame relay services have been largely replaced by newer, less costly and easier-to-maintain and configure Multi-Protocol Label Switching (MPLS) Virtual Private Network (VPN) offerings. Private lines are still used by large organizations such as financial firms that need to maintain a high level of security. Frame relay, which is a VPN, is rarely implemented. 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. The Costly Solution of Dedicated Private Lines 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 com- mercial organizations has decreased dramatically. For all but the largest organizations, MPLS and IP Virtual Private Networks (IP VPNs) have replaced private lines. These are less costly to maintain and boast lower monthly lease rates. However, large enter- prises, utilities, 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, and redesign for new applications. Dedicated Services for Wide Area Networks and Metropolitan Area Networks Dedicated, private lines are available for the exclusive use of the customer that leases them from 