 multiplexers. STM-16 or STM-64 STM-1 Multiplexers Transmission or STM-4 Links Periphery Fig. 10.34 Example of a core transmission network topology that relies on cross-connects for flexibility. heavily dependent on the control that is exercized over the cross-connects, and here there are some potential problems. The main control problem is that of database integrity. With a network of meshed cross- connects, when a transmission link fails the path restoration action will usually involve rerouting all the affected paths through several alternative cross-connects. To do this effi- ciently, the network control system must rapidly command simultaneous switching actions in all of these cross-connects, which usually implies that the control system has a predeter- mined plan of action that is based on the spare bandwidth that is thought to be available on the relevant transmission links and cross-connects. Unfortunately, the integrity of the control system's database might have been compromised because of other recent reconfigu- ration activities (e.g., new circuits setup), links temporarily taken out of service for mainte- nance, and SO on. This problem escalates rapidly as the number of cross-connects in a network increases. Conclusion There are several potential solutions to this problem. One is simply not to use the cross- connects for protection against transmission failures and instead to rely on transmission systems having 1 + 1 or 1:N protection. In this case the cross-connects are used solely for offline management of the network's transmission capacity. This is often called "facilities management" in North America. An alternative is to deploy the cross-connects in a rather more bounded topology than the completely free mesh topology previously assumed. A limiting case of this idea is to deploy them in a ring. The restriction on routing choices imposed by such a topology greatly eases the control problem, albeit with some significant increase in the standby line transmis- sion capacity that must be available t