used as a large private branch exchange (PBX). Small communities of less than 400 lines or so were also provided with SLC-96 units or Anymedia units. The standalone VCDX has a single Switching Module, and no Communications Module. Its Sun Microsystems SPARC workstation runs the UNIX-based Solaris (operating system) that executes a 3B20/21D processor MERT OS emulation system, acting as the VCDX's Administrative Module. The VCDX uses the CO's normal telephone power sources (which are very large uninterruptible power supplies), and has connections to the CO Digital cross connect system for T1 access, etc. Signaling The 5ESS has two different signaling architectures: Common Network Interface (CNI) Ring and Packet Switching Unit (PSU)-based SS7 Signaling. Software The development effort for 5ESS required five thousand employees, producing 100 million lines of system source code, mostly in the C language, with 100 million lines of header files and makefiles. Evolution of the system took place over 20 years, while three releases were often being developed simultaneously, each taking about three years to develop. The 5ESS was originally U.S.-only and the international market resulted in a complete development system and team, in parallel to the U.S. version. The development systems were Unix-based mainframe systems. There were around 15 of these systems active at the peak. There were development machines, simulator machines, and build machines, etc. Developers' desktops were multi-window terminals (versions of the Blit developed by Bell Labs) until the mid 1990s, when Sun workstations were deployed. Developers continued to login into the servers for their work, using X11 on their workstations as a multi-window environment. Source code management was based on SCCS and utilized "#feature" lines to separate source code between releases, between features specific to US or Intl, and the like. Customisation around the vi and Emacs text editors allowed developers to work with the appropriate view of a file, hiding the parts that were not applicable to their current project. The change request system used the SCCS MR to create named change sets, tied into the IMR (initial modification request) system which had purely numeric identifiers. An MR name was created with subsystem prefix, IMR number, MR sequence characters, and a character for the release or "load". So, for the gr (generic retrofit) subsystem, the first MR created for the 2371242 IMR, destined for the 'F' load, would be gr2371242aF. The build system used a simple mechanism of build configuration that