CAS Card Overview
The CAS card implements a 32X32 Space Switch for 1024 slot CAS highways coming from XEN cards. The CAS card also performs global functions like routing calls and monitoring system health.
A block diagram of the CAS card is shown below:
The main components of the CAS card are listed below. They are described in detail in the following sections.
- XEN Space Switch (32 such components)
- CPU Complex
- Power Supply
The CAS card has one XEN Space Switch (XSS) dedicated to each XEN card in the system. Each XSS drives the transmit direction CAS highway going to the XEN processor. Each time slot on the transmit CAS highway can be switched from any of the 32 receive CAS highways. Basically one XSS implements 32X1 Space Switch. 32 XSSs combine to implement a 32X32 Space Switch. The block diagram shows the 32 XSSs. Notice that the CAS Receive highways from all the XEN cards are connected to all XSSs.
XEN Space Switch
A XEN Space Switch implements a 32X1 space switch, i.e. time slots from any of the 32 XENs can be switched to the transmit XEN highway. The XSS is the basic building block for the 32X32 space switch discussed above.
The basic organization of XSS is fairly simple. It just consists of a 32-to-1 MUX. The five selection lines are driven from the data lines (d4-d0) of a Address memory. The address lines (a9-a0) of the Address memory are driven from the current slot number of the XEN-0 TX highway (i.e. CAS TX highway to the XEN card).
For example, if say slot 10 on the TX highway for XEN-0 has to be switched from XEN-3, memory location 10 in the Address memory should contain a value of 3. Thus each TX slot can be switched from any of the 32 XEN RX highways.
An important point to note here is that there is no PCM memory to store the samples, as the time slot data is being switched on the fly. No storage is involved. This introduces the inflexibility that time slot N on one highway can be switched only to time slot N on any other highway. Ironically, the lack of storage is the biggest strength of the Space switch. With storage of samples (i.e. time switching), the memory access time to access the storage becomes a big bottleneck, as all time slot samples have to be read and written once every 125 ms. Thus very large time switches cannot be implemented. For this very reason Xenon and most other switches have a T-S-T architecture (i.e. Time switching, followed by Space Switching, followed by Time Switching).
The CPU Complex on the CAS card is very similar to the CPU Complex used on the XEN card. The major difference is that the CAS CPU is used to program the Address memory on all the XSS components. Also, since the CAS card has just one mate, the two cards just exchange one watchdog signal.
The Power supply on the CAS is similar to the XEN card’s Power Supply. The major difference is in the power capability of the supply. Since extremely high frequency signals are being processed on the CAS card, the power consumption is much higher.