This invention relates in general to telecommunications switching systems, and more particularly to an apparatus and method for the synchronization of analog call display data in a distributed system.
In modern telecommunications systems design, digital signal processing (DSP) elements are being used increasingly for both the transmission and reception of analog tone-based data. Examples of such uses include call progress tones, call display data, modems, etc. In order to minimize the system cost impact of these devices, it is desirable to locate them as a centralized resource, against which traffic engineering principles can be applied. In this way, the resources can be allocated as needed from a general pool, and provisioned only to the extent required for the targeted traffic-handling capacity of the system. In fact, the DSP devices are often used as uncommitted processing elements. Their specific functions are often configured and activated, as required, by higher-order call control entities.
Certain types of analog data, such as call display to analog phones, are governed by very specific timing relationships to other related events. In particular, call display data must be presented according to a specific timing relationship, between the bursts of the first full ringing voltage cadence to an analog telephone. In distributed systems, ringing voltage generation and cadencing devices are located in a remote node (such as an analog peripheral), while the DSP tone generation facility is advantageously located in a central node (such as central switching and call control). This gives rise to difficulties in providing the necessary signal synchronization. Furthermore, with the industry evolution toward HDLC-based control protocols between nodes, maintaining these timing relationships is rendered even more difficult since any time latencies resulting from the control of HDLC functions may result in the overlap of analog data tones with ringing voltage application, thereby resulting in loss of data.
A typical traditional solution to this problem is to place separate DSP devices in the remote nodes, in order to deliver the timing-sensitive functions. Since these particular devices are located so as to service only a particular remote node, they are not available as centralized resources. However, this solution raises the system cost and development time, and complicates resource management.
The present invention addresses distributed telecommunications systems where the analog data is digitally encoded and transmitted to/from the various system nodes, in real time, using TDM channels. Examples of such encoding methods are PCM, ADPCM, etc.
In particular, the present invention provides an apparatus and a method for controlling remote time-sensitive analog functions in real time, regardless of the nature of the system""s normal control mechanisms. For example, many new systems employ packet-based control protocols which tend, by their nature, to be time-indeterminate (due to error detection and correction requirements, congestion, buffer management, etc.). According to the present invention, time-sensitive DSP control is de-coupled from the system control by providing a real-time TDM-based feedback path directly between the DSP and the remote node. This has the added benefit that allocation of a DSP resource by a higher-order call control entity remains, for all intents and purposes, unchanged. The only modification to the traditional call control model is that TDM channels are established in both directions, rather than just toward the remote peripheral. Other control functions are invisible to the call control entity, and take place strictly between the DSP and the affected remote peripheral.