The ATM technique is taking on a growing importance for the transport and switching of digital flows at very high speed, such as those present in wide band integrated services digital networks. In this technique, the information associated with the various services is organized into contiguous packets of fixed length (about 400 bits), called "cells", formed by an information field and a header field (tag) that carries service information, including the information necessary for the routing through the network.
In an ATM network, the switching nodes must carry out two fundamental tasks: routing of the cells (thus performing a space switching function), and avoidance of possible conflict situations, which occur when several cells entering the node simultaneously from different inputs have to be routed toward the same output (thus the node also performs a memorization function). In order to satisfy the requirements for bit rate and bandwidth of modem integrated services networks, ATM switching nodes based on optical techniques have been proposed. There, both muting and memorization are performed by means of optical devices. In this way, it is possible to operate at very high bit rates, of the order of Gbit/s. An example of a node of this type is described, for instance, in European Patent Application EP-A-0 411 562.
In a communication system based on the ATM technique, the flows of cells containing information emitted by the various transmitting stations can arrive at the switching node with random phases. The optical systems so-far proposed require, in order to correctly operate, that the cells arrive at the various node inputs at predefined instants and therefore the need arises for a phase realignment or synchronization of the cells. In a large optical switching node, comprising a multi-stage network, there may be a need for further synchronization, due to the uncertainty about the length of the optical paths that the cells travel through in the node. In the first case, the cell synchronization system must be able to recover relatively high phase shifts, above all at the beginning of a transmission (for example phase shifts of up to a cell time or even more), whereas in the second case the phase shift to be recovered may be much smaller, of the order of a few nanoseconds.
The synchronization of the ATM cells arriving at the different inputs of a single-stage optical switching node is described in European Patent Application EP-A-0 411 562 mentioned above. The phase realignment is performed by temporarily storing signals convened into electrical form, within the devices performing the tag processing necessary for muting. The possibility to directly operate on optical signals is also suggested. However, efficient digital optical memories are still unavailable and, in any case, their use increases system realization and management complexity.
In Italian Patent Application No. TO 93A000956, filed on 16 Dec. 1993, (corresponding to U.S. Ser. No. 08/331,202 of 28 Oct. 1994), a method and a device are described for the fine synchronization of ATM cells, suitable to recover the limited phase shifts that can occur in steady state conditions at node inputs or those due to small path differences within a node. A cell to be realigned is sent into a span of an optical fiber with high chromatic dispersion, of a predefined length, after having been associated with an optical rephasing carrier whose wavelength is such that, due to transit along the fiber, the cell reaches a node switching element input at a predefined instant. There are provided: means for extracting a function of the optical signal associated with the cell to be realigned; means for recognizing the beginning of a cell, by using said optical signal fraction, and for generating a signal representing the occurred recognition; means for receiving the signal representing the occurred recognition of the beginning of the cell, for comparing it with a signal representing a reference instant and for generating an error signal indicative of the entity of the phase shift between the two signals, and a tunable wavelength converter, driven by said error signal, to transfer the cell to be realigned from an original carrier, with which the cell received by the device is associated, to the rephasing carrier.
Such a method and a device are not able to recover the initial cell phase shift, that can be rather high. Moreover, phase shifts, that in steady state conditions are essentially due to thermal drifts, accumulate in time and therefore, at a certain time, the latter method and device are no longer effective.