Number Five Crossbar Switching System
The Number Five Crossbar Switching System 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 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 and all trunks and most service circuits on trunk link frames. 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 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, 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 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.
Trunk Link Frame
Junctors are wired from LLF through the junctor grouping frame to the levels of trunk junctor switches in the trunk link frame. Unlike earlier designs, the junctors have no supervisory relays or other active hardware, all such functions being assigned to trunk circuits. The basic design of the TLF has 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 devoted two levels to doubling the use of the other eight, thus allowing each trunk switch to connect sixteen trunks to its twenty trunk links. This results in the TLF having a 0.8:1 trunk concentration ratio. This degree of deconcentration eventually turned out to provide too few trunk appearances for the variety of trunk types needed. The final 1970s 5XB offices had type C trunk switches with twelve levels, using two for discrimination, leaving a TCR of unity.The TLF having twice as many links, junctor switches, and junctors as the LLF, there are always twice as many LLFs as TLFs. As first designed, the maximum number was ten TLFs and twenty LLFs, known as 10x20, and at first rarely achieved. In the late 1950s multiple trunk junctor switch bays were added to give each TLF access to more junctors. The first expanded version allowed each office to have 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 with eight stages was under development.
A channel from a line to a trunk consisted of three links of switching fabric: line link, junctor, and trunk link. In a 10x20 or larger office, ten channels, numbered 0 to 9, were available from any line to any trunk. The line junctor switch number and the trunk junctor switch number are the same as the channel number. Logic in the marker compares the ten links of each kind to obtain a clear channel. The lack of a channel is called a mismatch and resulted in picking another trunk, or another line, or the use of the ALL where that exists, or giving up and letting the caller try again.
Trunk circuits
As in previous designs, supervision of incoming calls is handled by relay sets known as incoming trunk circuits, which sit at the entry point just outside the switching network. Unlike in previous designs, outgoing trunk circuits are used for the equivalent outgoing functions. This means that the junctors, which have the same name as earlier crossbar systems, are simplified, and are now only wires providing links between lines and trunks. The outgoing trunk circuits, which are placed at the outside edge of the switching network, are responsible for originating-side supervision. Since different outgoing trunks are connected to different places and are used for different calls, their relay sets can be specialized for a particular kind of signalling or call metering or other peculiarity. Thus a TSPS trunk can give complete control to an operator, while an E and M signaling trunk can do the kind of signaling required of a private long-distance line, while a local outgoing trunk can be simpler.Thanks to this more complex trunk circuit, outgoing trunks are selected by a quicker and more versatile method than the sleeve test previously used. Each trunk circuit provides a ground on an FT lead to indicate idleness. The FT leads for trunks in a particular group are cross-connected to a FTC lead for the trunk link frame upon which it appears, to indicate that the TLF has one or more idle trunks in that group. The route relay in the completing marker connects sensor relays to all the trunk link frames, allowing the marker to choose a TLF that has an idle trunk and then connect to that trunk through the trunk link connector to choose one of those idle trunks. This two step method, along with the mixing of incoming and outgoing traffic, distributed traffic more evenly, thus alleviating the link congestion problems that often arose with earlier methods that restricted a trunk group to one or two outgoing switch frames.
This method is less efficient for coin phones, which need special signalling. In urban areas, they were served by older exchanges that had separate junctors for coin phones. Where the 5XB was the only exchange, a number of work-around methods were devised. Regular and coin phones shared the more complex and expensive coin trunks, or else separate routes were established, or coin trunks connected via tandem switches including the 5XB itself acting as its own tandem. In this last case, the call had to use two connections through the switching fabric: one to connect the line to the coin supervision trunk and another to connect that trunk to the outgoing trunk.
It was also less efficient for tandem calls, since the fabric is unable to connect a trunk directly to a trunk. Instead, each incoming trunk that has the ability to make tandem calls has to have a line link frame appearance, as if it were a line. To avoid expense, incoming trunks were divided into groups, some of them having tandem ability and some not. This complication was avoided in places big enough to pay for a separate tandem switch.
Connection of trunks to incoming registers and outgoing senders is not through the four-stage voice fabric. Rather it is through a dedicated single-stage crossbar network known as incoming register link or outgoing sender link respectively. Registers and senders are in groups of ten, assigned one to each level of as many crossbar switches as are appropriate to the traffic they can handle. Different trunks are wired to different IRLs or OSLs depending on what kind of signaling they use; i.e. IRDP, IRRP, or IRMF.
Previous systems use relays in the incoming trunk circuit to control ringing and to return busy tone. 5XB uses a ringing selection switch : a crossbar switch with ten verticals, serving ten trunks. The various levels provide various tones, and ringing current of various durations and cadences. Levels 0 and 1 are used as discriminating levels to set polarity for selective ringing on tip side or ring side. An especially sensitive wire spring RT relay is used to detect off-hook from a line being rung, release the RSS hold magnet, and engage the shielded supervision relay so reverse battery answer supervision would be returned to the originating end.