5XB switch

The Number Five Crossbar Switching System (5XB switch) is a telephone switch for telephone exchanges designed by Bell Labs and manufactured by Western Electric starting in 1947. It was used in the Bell System principally as a Class 5 telephone switch in the public switched telephone network (PSTN) until the early 1990s, when it was replaced with electronic switching systems. Variants were used as combined Class 4 and Class 5 systems in rural areas, and as a TWX switch.

5XB was originally intended to bring the benefits of crossbar switching to towns and small cities with only a few thousand telephone lines. The typical starting size was 3000 to 5000 lines, but the system had essentially unlimited growth capacity. 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, produced starting in 1972, was available only in sizes of 980 and 1960 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 uses entirely the same four-stage switching fabric for incoming as for outgoing calls. All lines are terminated on Line Link Frames and all trunks and most service circuits on trunk link frames. Each TLF is connected to all LLF by at least ten junctors

Line link frames (LLFs) are tiers of 10x20 crossbar switches in two or more bays. The switches in the first bay have 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 have 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 terminates 100 Junctors. Each junctor has full availability to however many hundreds of lines there are, 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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