5XB switch
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The Number Five Crossbar Switching System, designed by Bell Labs and made by Western Electric was in use in Bell System telephone exchanges from 1948 to the 1980s. Its principal use was as a Office classification Class 5 switch, though variants were used as combined class 4/5 in rural areas and as a TWX switch.
5XB was originally intended to bring the benefits of crossbar switching to small towns with only a few thousand lines. The earlier 1XB urban crossbar was impractically expensive in small installations, and had difficulties handling large trunk groups. 5XB was converted to wire spring relays in the 1950s and otherwise upgraded in the 1960s to serve exchanges with tens of thousands of lines. The final 5A Crossbar variant in the early 1970s returned to its roots, being available only in sizes of 960 and 1920 lines, and generally delivered on one pallet rather than assembled on site as usual for larger exchanges.
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[edit] Switching fabric
5XB introduced the call-back principle, in which the initial concentrating switch train from the line to the digit receiver was entirely dropped during call completion. It also used entirely the same four stage switching fabric for incoming as for outgoing calls. All lines were terminated on Line Link Frames and all trunks and most service circuits on trunk link frames.
Line Link Frames (LLF) were tiers of 10x20 crossbar switches in two or more bays. In the small 290 line LLF the switches in the first bay had their horizontal multiples, or "banjo wires" cut in half, effectively dividing each switch into a Line Switch and a junctor switch. Each of the ten Junctor Switches had ten junctors on its ten verticals. Each of its ten levels was wired as a Line Link, to one of the ten line switches of the LLF. Each line switch had nine lines on nine of its verticals. The tenth vertical was reserved for testing purposes. In addition to the 90 lines on these switches, each LLN had at least one frame with ten more line switches, carrying 200 more lines. Optionally it had still another frame, with ten more switches and another 200, for a maximum Line Concentration Ratio of 4.9:1 since they all shared the same hundred Line Links.
Junctors were wired through the Junctor Grouping Frame to the levels of Junctor Switches in its Trunk Link Frame. The basic design of the TLF had ten Junctor Switches with their horizontal multiples split in half, hence two hundred Junctors, and two hundred Trunk Links to the ten Trunk Switches. The banjo wiring of the Trunk Switch was not split, but a "discriminator level" trick allowed each Trunk Switch to connect sixteen trunks to its twenty Trunk Links.
The TLF having twice as many Junctors as the LLF, there were always twice as many LLF as TLF. As first designed, the maximum number was ten TLF and twenty LLF, known as 10x20. In the late 1950s, multiple Trunk Junctor Frames were added, allowing each office to have first 20x40, and in the 1960s the maximum reached 30x60. Development stopped at that point because the four stage layout was becoming progressively less efficient at greater sizes, and because the 1ESS switch was under development.
[edit] Common control
Call back, single train, and other sophisticated methods required more sophisticated controls, but they increased efficiency and became standard for later designs. 5XB also separated the registers for receiving digits, from the senders for sending them. This complication necessitated more transmission of data among the control circuits, but greatly shortened the holding time of senders and increased general efficiency and versatility without having to put the versatility into large, numerous and complex senders as in earlier systems.
Originating Registers (OR) were wired to the Trunk Link Frame (TLF). In the original 5XB a marker, having been alerted to a trip condition, picked an OR by the same mechanism it would use to pick a trunk, identified a clear path between line and OR, loaded the OR with any information necessary for later processing (such as Line Equipment and Class of Service) and relased itself. The OR then received the digits (rotary or tone) and used the Pretranslator to determine how many digits to receive before calling in the Marker again to complete the call.
Larger 5XB were built in the 1960s with more Markers. To save money the markers were separated into two kinds: a simple Dial Tone Marker (DTM) just to connect the line to the OR, and a Completing Marker (CM), many times more complex and expensive, to complete the call to or from a trunk. CM had, among other features, the ability to translate the first 3 digits of a phone number (or 6 when using a Foreign Area Translator) to identify the correct outgoing trunks and handling.
Taking advantage of the superior versatility of 5XB, Centrex was invented as a service package. Later Stored Program Control exchanges allowed more extensive service features. Autovon originally used a four wire version of 5XB, with a more complex marker to implement its nonhierarchical Polygrid routing system, and trunk circuits with additional logic and data storage built in to implement preemption.
