Clear examination of work currently done within CCITT indicates the importance of a broadband telecommunication network. As this network should be capable of integrating all services in an efficient way-in order to reduce cost-the asynchronous transfer mode (ATM) was selected by CCITT as the target transfer mode for implementing the broadband integrated services digital network (BISDN). This selection implies that the switching nodes in the BISDN network are capable of supporting this high-speed packet and connection-orientated technique. Within the literature different switching node architectures based upon ATM have been proposed. All of these architectures should meet the highspeed and high-throughput requirements so as to cope with the delay and jitter performance objectives.In a first step this paper describes alternative switching techniques for the basic building block (switching element) of a switching node. A common model architecture of the switching element is drafted. A classification of switching elements described in the literature is derived and the influence on the complexity and performance is weighted. In a second step the switching node architecture is further elaborated according to the control and flexibility requirements. Core (switching) and edge (switching related) functions are listed, and possible functional partitionings are discussed. Finally, these A T M switching architectures are compared according to a background frame consisting of several straightforward comparison points such as the buffering strategy, the internal routeing method, the switching overhead, the connection-orientated or connectionless operation, etc.KEY WORDS Broadband ISDN ATM switching

the lower operational expenditure (OPEX) associated with operating and maintaining a single network, with typical operator estimates for the magnitude of these savings in OPEX being on the order of $100 per subscriber per year. In addition to the apparent cost savings associated with the transformation to an "all IP" network, the intent was to increase the average revenue per user (ARPU) by offering a full complementThe First Phase of Network Transformation: The Move to "All IP"Over the past few years, the traditional telecom operator's network has undergone a transformation to an "all Internet Protocol (IP)" network, with the amalgam of legacy networks that had been built to provide residential and business voice and data services being replaced by a single packet-based IP/Ethernet and IP/multiprotocol label switching (MPLS) network. One of the main drivers of this transformation was

The introduction of Wavelength Division Multiplexing (WDM) techniques into fiber networks opens perspectives for a global transparent optical telecommunication backbone. One of the most promising features of optical telecommunications is to provide a common infrastructure which could become an integration factor in a heterogenous but interconnected network environment. Furthermore optics can provide a degree of network transparency to bitrate, transfer mode and signal format of the transported signals. By using transparent optical crossconnect nodes, based on space and wavelength routing, a rearrangeble network topology can be achieved. Local exchanges could, in principle, be connected directly to each other through end-to-end transparent optical paths.

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