Number Five Crossbar Switching System

The Number Five Crossbar Switching System or 5XB switch, designed by Bell Labs and made by Western Electric, was in use in Bell System telephone exchanges from 1948 to the early 1990s. Its principal use was as a Class 5 telephone 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 telephone 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.

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 so its links could immediately be reused for this or another call. 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. Each TLF was connected to all LLF by at least ten junctors

Line Link Frames (LLF) were tiers of 10x20 crossbar switches in two or more bays. 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 Line Junctor Switch. Each of the ten Junctor Switches had ten junctors on its ten verticals, and each of its ten levels was wired as a Line Link, to one of the ten line switches of the LLF. Thus, the Line Link Frame terminated 100 Junctors. Each Junctor had full availability to however many hundreds of lines there were, via the hundred Line Links. The number of lines, thus the Line Concentration Ratio (LCR) was engineered for the expected occupancy.

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