t frustrating to the data-processing departments. When data errors were introduced, a retransmission was required. The more retransmissions were necessary, the less effective the throughput was on the network. In fact, several years ago, the use of a 4,800-bps-transmission service on the analog dial-up network might have produced an effective throughput of only 400 bps after all the errors and retransmissions. This was intolerable and had to be corrected. To solve this problem, the network introduced the X.25 services, also called packet switching. Whereas the X.25 was originally designed to handle the customer's asynchronous traffic, Frame Relay was designed to take advantage of the network's capability to transport data on a low-error, high-performance digital network and to meet the needs of the intelligent, synchronous use of the newer, more sophisticated user applica- tions. The protocols that apply to the basic needs of the data transmission were used, yet much of the overhead has been squeezed out, creating a ser- vice that operates functionally at the bottom half of Layer 2 on the OSI model, as shown in Figure 9-6. When compared to private leased lines, Frame Relay makes the design of a network much simpler. A private-line network requires a detailed analysis to set all the right connections in place; this further accentuates the traffic-sensitive needs of the user network. The meshed network uses a Figure 9-6 Typical OSI Stack Frame Relay Stack The Frame Relay protocols compared to the OSI model. Network Data Link Frame Relay Physical Physical Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PCUSN-to-SGSN Interface (Gb) PCUSN-to-SGSN Interface (Gb) series of connections that are the total number of sites less one divided by two (n X [N - 1] / 2) points with links running from site to site, as shown in Figure 9-7. Therefore, if 10 sit