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 990 and 1960 lines, and generally delivered on one pallet, rather than assembled on site as usual for larger exchanges.
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 so its links could immediately be reused for this or another call. This is in contrast to earlier crossbar systems where the original switch train was simply built up and expanded as the call was connected, and not dropped in favor of a completely new one. It also uses entirely the same four-stage switching fabric for incoming as for outgoing calls, instead of separate fabric, as had been done in earlier systems. These developments had the overall effect of simplifying the switch fabric, and using it as a "service" rather than as an immutable part of the call, as was the case in most earlier systems. All lines are terminated on line link frames (LLF) and all trunks and most service circuits on trunk link frames (TLF). Each TLF is connected to all LLF by at least ten junctors. Calls from subscribers originate at line link frames and pass through trunk link frames on their way to their destinations.
Line Link Frame
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 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. Each line switch in this first, mixed bay has nine lines on nine of its verticals, the tenth vertical being reserved for test purposes. In addition to the 90 lines on these switches, each LLF has at least one simple line switch bay, with ten more line switches carrying 200 lines. Thus the minimum size of a LLF is 290 lines for a line concentration ratio of 2.9:1. Optionally it has still another frame, with ten more switches and another 200 lines, and so forth, up to a maximum line concentration ratio of 5.9:1 since they all shared the same hundred line links. The line circuit is much like that in 1XB with a line relay for alerting the exchange to a trip condition, and the vertical off-normal contacts of the switch vertical serving as cutoff relay. For control purposes the subscriber lines on the switches of the LLF are divided into vertical groups of fifty, being five line units on each of ten switches. Each vertical group is divided into five vertical files of ten lines, important because class of service, or customer group identification in later Centrex offices, is shared by all ten lines in the vertical file. Staff in Centrex offices spent much time standing on ladders, rewiring the Class of Service data fields at the top of LLF. Late in the career of 5XB, junctor group size and thus link efficiency of the largest offices was increased by the use of auxiliary line link (ALL) frames. The ALL is a bay with ten junctor switches, divided as usual into left and right halves. One half has on its levels the line links of an even numbered LLF, and on its verticals, the junctors of the neighboring odd numbered one; the other half is vice versa. By this means, each LLF can use the junctors of its mate, if the marker failed to find an idle path on the first try. Since they are odd and even, their junctors appear on opposite sides of the trunk junctor switches, thus giving access to the mate trunk links as well. Connections through the ALL were only used in heavy traffic periods.
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