This invention relates to digital loop carrier transmission systems and, in particular, to a means for adding loss to the digital signals.
In a typical digital transmission system, such as the Subscriber Loop Carrier (SLC.RTM.) system, digital signals are transmitted between a central office and a Remote Terminal (RT) which is connected to the end user. In proposed fiber-to-the-home systems, an optical link is provided between the remote terminal and a distant terminal which serves a number of end users. The remote terminals and distant terminals each include channel units which provide an interface with the end user.
In typical SLC.RTM. systems, transmission loss between the tip/ring interface at the central office and the tip/ring interface at the remote terminal is approximately 1 dB for Plain Old Telephone Service (POTS) channel units. Additional transmission loss is normally added by the long metallic drop between the remote terminal and the end user. In a fiber-to-the-home system, most of the metallic drop is replaced by a lossless fiber. Therefore, grade of service may be optimized by adding an amount of loss to the transmitted signal which is dependent on the type of channel unit employed at the remote or distant terminal. This loss can be added to either the digital or analog side of the channel units. Adding loss on the digital side is generally preferred since it can be fully integratable and one loss circuit can be shared by several channel units.
Traditionally, digital loss is added by using a look-up table. In such systems, a Read Only Memory (ROM) contains Pulse Code Modulation (PCM) output words corresponding to the PCM input words for a particular loss value to be added. A problem with this approach is that when more than one loss value is desired, the amount of memory required becomes excessive. Also, a hardware change is required to add new loss values.