s. There are three main strategies for the deployment of SDH equip- There are several potential solutions to this problem. One is to ment: simply not use the cross connects for protection against trans- mission failures, and, instead rely on transmission systems having 1. Synchronous islands. 1+1 or 1:N protection. In this case the cross connects are used solely 2. 'Thin' overlay networks. for off-line management of the network's transmission capacity. 3. Ad hoc deployment dictated by traffic growth, etc. This is often call 'Facilities Management' in North America. An alternative is to deploy the cross connects in a rather more bounded Many PTOs are currently using the ad hoc approach and deplo- topology than the completely free mesh topology assumed above. A ying the latest STM-16 (2.5 Gbit/s) transmission systems, not SO limiting case of this idea is to deploy them in a ring. The restriction much because they conform to the SDH standards, but because they on routing choices imposed by such a topology greatly eases the offer higher transmission capacities than any available PDH sys- control problem, albeit with some significant increase in the standby tems. Even so, most PTOs which are doing this, are intending to line transmission capacity that must be available to allow a traffic eventually fill in the gaps between these ad hoc deployments SO as re-route. Although the present day cost of such standby capacity is to create an SDH overlay network. considerable, it has already been observed that raw point to point The idea behind the 'thin overlay' strategy is to rapidly deploy a transmission capacity is the one thing that will continue to become limited SDH capability across the whole of a PTO network SO that cheaper, in real terms, for some time to come, hence this option may, a small nucleus of key business customers can be offered the look increasingly attractive. benefits of SDH as soon as possible. This type of deployment is Finally, the most interesting possibility is to endow a network of most appropriate