 to networks where the important communities of cross connects with a signalling system that will allow it to dynami- interest are geographically widely dispersed. cally set up and clear down paths, in much the same way as a PSTN The synchronous 'island' deployment strategy assumes that the sets up conventional 64kbit/s circuits. The idea is that the intel- most important communities of interest are geographically concen- ligence required for this type of operation resides, not in a single trated, and that each one can start to benefit from SDH without large network manager, but, like the PSTN, distributed throughout necessarily having full SDH connectivity with similar communities the whole network. This greatly reduces the data base integrity of interest. The classic example here is that of the financial com- problem, and should operate quickly enough to meet the criteria for munity in the City of London. With the passage of time, a syn- fast restoration of failed paths, as well as facilitating other oper- chronous island would normally increase in geographical size, so as ations such as rapid provision of bandwidth (i.e. bandwidth 'on to eventually coalesce with other islands. Long before this, they demand') in response to requests from the client services of the SDH would probably be interconnected by an emerging SDH trunk network. network. 42/26 The Synchronous Digital Hierarchy (SDH) Impact of broadband standards obtain the same connectivity from a Frame Relay network. How- ever, experience to date with quasi Frame Relay networks in North As an ISO layer 1 transport standard, intended for building the America indicates that this will not be the case, and that Frame PTO's backbone transmission networks, SDH currently has no real Relay and SDH will not compete directly, for business traffic. rivals. The nearest thing to a competitor that has emerged SO far is Asynchronous Transfer Mode (ATM), and its adoption was strongly favoured by several European PTOs. However, ATM, as with most 42.7.2 Switched Multimegab