For space efficiency and ease of access to circuit components, housing configurations for electronic circuits, such as those employed in channel banks of telecommunication equipment, such as a D4 channel bank customarily contain a plurality of parallel circuit card slots having parallel card guide and support tracks, that are sized to receive respective electronic circuit cards which plug into electrical backplane connectors at a rear portion of a cabinet or rack. 
FIGS.  1-3 diagrammatically illustrate a typical example of standard printed wiring board or card 10 that is configured to engage a backplane connector.  For purposes of providing a non-limiting example, FIGS.  1-3 show the backplane connector configured as a card edge connector which receives and engages a plurality of lead conductor traces 12 that extend to a first or rear end of the card 10.  The card 10 itself has a prescribed industry standard size and shape (`form factor`), and is typically made of a generally longitudinal rectangularly shaped sheet of fiberglass. 
The lead connector traces 12 are connected to circuit components 13 mounted on the board proper by way of patterned conductive material 14 that has been selectively etched onto the fiberglass sheet of which the printed wiring board 10 is made.  The lead connector traces 12 are sized and arranged to engage associated conductors along the interior sides of a backplane connector 15, when the rear end of the card 10 is physically inserted into and is captured by the connector, thereby providing backplane connections to the circuit components of the card.  The backplane connector further serves to physically capture and thereby stabilize the card in its card slot. 
A second (front) end 16 of the card 10 is affixed to a (metallic) face plate or front panel 17, that is sized to fit within and thereby close a portion of a card slot opening 18 at the front of the equipment rack.  Upper and lower card slot guide tracks 19 and 20 are sized to receive upper and lower edges 21 and 22, respectively, of the card 10, so that, as the printed circuit card 10 is inserted into the cabinet through the card slot opening 18, it slides within the guide tracks 19 and 20 until the rear end 11 of the card containing the conductive traces 12 is physically plugged into and captured by the backplane connector 15.  Once installed in its card slot, the card's form factor will place the face plate coincident with the front panel of the rack, thereby closing the card insertion opening. 
Now although continuing improvements in the semiconductor industry have enhanced the microminiaturization of electronic circuit components, and thereby reduced the size of the components themselves and the printed circuit board real estate required to implement such circuits, such as the telecommunication circuits of channel bank installations, telecommunication service providers are not readily desirous of replacing an existing piece of equipment simply because it can be made somewhat smaller or housed in a more compact configuration.  Indeed, the axiom "if it ain't broke, don't fix it," can be considered to apply to telecommunication equipment configurations that have been and can be expected to continue to provide satisfactory service to telco customers. 
This is especially true in the case of relatively low end, standardized circuit cards, whose principal driving force is cost, where the cheaper the card can be produced, the better.  Since such cards have a relatively fixed, universal form factor, and employ circuit components whose integration densities have essentially stabilized, namely, reached a point that no longer provides a substantial reduction in the cost of the circuit, per se, the circuits themselves are now essentially fixed cost items to the circuit card supplier.