Xenon Switching Architecture
Historically, switching systems have been fairly monolithic in architecture. In some sense the switch architecture is quite similar to that of Mainframes where a single box implements the complete functionality. Typically a switch with even one hundred thousand subscribers would have one hundred thousand pairs of copper running from the subscriber premises to the switch. Xenon would be designed to move away from this Mainframe type of architecture to something that resembles the more modern Client-Server computing.
Classic Switch Architecture
Most switches are designed using the Time-Space-Time (TST) architecture to implement the switching fabric. Typically, a large space switching matrix connects to a large number of smaller time switching matrices. The subscribers would directly connect to the time switching matrix. Whenever a subscriber (say X) calls another subscriber (say Y), time switch 1 would connect X to the space switch, the space switch would connect to time switch 2 that would finally connect to Y. (See the figure above)
Now subscriber X might call a subscriber not resident on this switch. Trunks are used to connect calls between different switches in the network. When subscriber X calls subscriber Z, the call might be setup as shown in the figure below. As you can see set up of this call involves several different switches, many of these switches are just routing the call from one switch to another. Such exchanges are called transit switches. There might be several transit exchanges involved in setting up of a single call. The transit switches perform the function of routing and switching the calls.
Trunks connecting switches are mostly digital trunks with each trunk capable of handling around 30 calls. The total number of trunks between two switches would depend on the expected traffic between them.
Switch architectures have undergone some changes from this classic design. Now switches support remote switching so the complete switch does not need to be at the same location. Some of the time switches might be remotely located. Also standard protocols have now evolved allowing a smooth interconnection between switching components from different vendors.
The main advantage of remote switching is that it brings down cabling costs. Remote units can be placed close to a cluster of subscribers. Now the cabling needs to be done only to the remote unit.