Basic Hardware Design
The basic hardware architecture has already been covered in the previous chapter. The hardware redundancy support in the system has however not been addressed. As we have seen earlier the system supports two CAS cards. XEN cards also support redundant operation, but that was not covered in the previous chapter.
The two CAS cards implement a non blocking switch. The two CAS cards also provide redundancy in operation. If one of the CAS cards fails, the other CAS card would be able to handle the calls. Under these circumstances, the system does not implement a non blocking switch. Thus if the faulty CAS card is not replaced, the system might experience call blockage. Thus the system degrades gracefully in presence of a hardware failure.
XEN redundancy is implemented by a n+x redundancy scheme, i.e. if n cards are required, the system is equipped with n+x cards. Out of the 32 XEN cards in the system, one in eight cards is equipped to be the standby for the other seven cards. If any of the group of seven cards fails, the eight processor can switch in for it and connect to the digital trunks supported by the failed card. The exact redundancy configuration can be seen in figure showing the chassis design.
The complete Xenon switch is implemented in a single cabinet (this is a considerable reduction from traditional switches, where the cost of the real estate occupied by the switch was often more than the equipment!). The cabinet supports five racks. The central rack houses the two CAS cards and the LAN Switch. The top two and the bottom two racks house the 32 XEN cards. The rightmost XEN cards in each rack (i.e. 7, 15, 23 and 31) are configured to be standby processors.
There are no slots for equipping the power supplies as all the cards have built in power supplies. Thus the cards are powered from a unregulated power source.
It may be noted that the chassis shown here is fully loaded. The actual equipage of the XEN processors will depend on the load requirement in the system.