A 5ESS digital switching machine. A DMS100 looks similar except painted a medium brown with green trim.
On the right, a row of Acatel DSLAMs, one of several such rows in a typical C.O.. Not shown is a Splitter bay, where the dial tone and DSL signals are combined.
The dial tone and DSL circuits are cabled over to the main distribution frame:
Show here are a COSMIC frame, and an older standard MDF (Main Distribution Frame). This is the vertical side, the VMDF, which traditionally has blocks terminating carrier bays and underground cables. The other side of the MDF is the HMDF, with the blocks mounted horizontally traditionally terminating the central office equipment. A medium size frame will have between 100,000 and 1,000,000 jumper wires running from one block to another to tie any piece of equipment to any cable pair or carrier channel. Or, in the case of DSL, from an equipment block supplying the dial tone, to a splitter block to add on the DSL signal, and then to a cable pair going out to the customer. No jumper is placed or removed without a service order. Every jumper is in a database. Without accurate record keeping it would be impossible to maintain the MDF.
From the VMDF the cables drop down through the floor, down into the dark but not dank cable vault:
On the left you can see, near the ceiling, the many grey cables from
the individual blocks on the VMDF feeding into a large black splice case
where the pairs are tied, one by one, by hand, to the pairs of the big
black underground cables that lead out to the customers.