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XEN Card Overview

In this section we will be looking at the design of the XEN card. A block diagram of the card can be seen in the figure below.

cpu complex block diagram

The main components on the XEN card are listed below. They are described in detail in the following sections.

  • CAS-0 Highway Interface

  • CAS-1 Highway Interface

  • Digital Trunk Interface

  • Time Switch

  • Service Controller

  • Power Supply

  • CPU Complex

  • Time Switch

CAS Highway Interface

There are two almost identical blocks to handle the coaxial interface with the CAS cards. This logic extracts the clock being sent from the CAS cards. The CAS interfaces also perform parallel to serial and serial to parallel conversion for PCM data to and from the CAS highways.

Digital Trunk Interface

This logic interfaces with 32 different digital trunks. It handles the low level physical interface that implements a digital trunk. The HDB3 coding/decoding is also carried out by this block. Hardware logic is provided to protect the card from getting damaged by external disturbances (like lightening, short circuit).

This block also takes care of differences between the switch clock and the clock at the other end of the digital trunk. Differences in the frequency of the two clocks result in loss of information. This information loss is managed by this hardware so that the loss of bits can be controlled, reducing the impact on the performance of the digital trunk. This function is called slip detection and recovery. A more detailed description of slip detection is beyond the scope of this work.

The digital trunk interface logic also handles the framing of the digital trunk into 32 slots, with the first slot being used for frame alignment.