Editing 5ESS Switching System (section)

===Very Compact Digital Exchange===
The Very Compact Digital Exchange (VCDX) was developed with the 5ESS-2000, and marketed to mostly non-Bell telephone companies as an inexpensive, effective way to offer [[ISDN]] and other digital services in an analog [[switching center]].  This avoided the capital expense of retrofitting the entire analog switch into a digital one to serve all of the switch's lines when many wouldn't require it and would remain [[Plain old telephone service|POTS]] lines.

An example would be the (former) GTE/Verizon [[Class-5 telephone switch]], the [[GTD-5 EAX]].  Like the [[Western Electric]] [[1ESS]]/1AESS, it served mostly medium to large wire centers.

The standalone VCDX was also capable of serving as a switch for very small wire centers (a CDX- [[Community dial office]]) of fewer than ~400 lines. However, for small wire centers, 400-4000 lines, that function was usually served by RSM's, a 5ESS "Remote SM", ORM's or Wired ORM's.
The RSM is controlled by T1 lines connected to a DLTU unit. The first 2 T1's are the control of the RSM and are necessary for any Recent Changes to take place. RSM's can have up to 10 T1's. There can be multiple RSM's in an office. An ORM can be fed via direct fiber or via coax thus called Wired ORM's. An RSM or ORM can have many of the same peripheral units that are part of a full 5ESS switch. An RSM has a limited distance and can serve parts of a larger metro area or rural offices. An ORM or wired ORM can be anywhere technically, and preferred over the RSM once the ORM became available. Both the RSM and ORM is often used as a Class-5 wire center for small to medium towns hosted from a 5ESS located in a larger city. The Wired ORM is connected via coax from a MUX unit and fed to a TRCU which converts the coax to connection to the DLI, There was also a two-mile ORM that was used when an office was broken out or took an area from another office. The distance on this was 2 miles from a host office and fed direct via fiber. As with any SM, the size is dictated by the number of time slots needed for each peripheral unit. ORM's are linked with DS3, RSM's are linked with T1 lines.
The VCDX was also 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 [[3B series computers|3B20/21D]] processor [[MERT]] [[Operating system|OS]] [[emulator|emulation]] system, acting as the VCDX's Administrative Module. The VCDX uses the [[Telephone exchange|CO]]'s normal telephone power sources (which are very large [[uninterruptible power supply|uninterruptible power supplies]]), and has connections to the CO [[Digital cross connect system]] for [[T-carrier|T1]] access, etc.