Number Five Crossbar Switching System

5XB was originally intended to bring the benefits of crossbar switching to towns and small cities with only a few thousand telephone lines.

The earlier 1XB urban crossbar was impractically expensive in small installations,[2] and had difficulties handling large trunk groups.

5XB was converted to wire spring relays in the 1950s[3] 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,[4] and generally delivered on one pallet, rather than assembled on site as usual for larger exchanges.

[5] 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.

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.

Staff in Centrex offices spent much time standing on ladders, rewiring the Class of Service data fields at the top of LLF.

Unlike earlier designs, the junctors have no supervisory relays or other active hardware, all such functions being assigned to trunk circuits.

The final 1970s 5XB offices had type C trunk switches with twelve levels, using two for discrimination, leaving a TCR of unity.

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.

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.

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 (see panel switch), or IRMF.

Previous systems use relays in the incoming trunk circuit to control ringing and to return busy tone.

[7] The various levels provide various tones, and ringing current of various durations and cadences (especially valuable for party lines).

Connectors, similar in purpose to the data buses inside a computer CPU, connect the markers to the peripheral equipment.

each connector is made up of large relays of 30 contacts each, to connect all the leads by which the marker would exchange information and control signals.

For example, each of the oblong reed packs in an OR would have to be connected by five leads through the originating register marker connector to transmit the two-out-of-five code representing one dialed digit.

Partly because of this deliberate design decision to help shield users from component failures, the few shared markers contain a great deal of self-checking circuitry.

This test facility became more valuable as Centrex, direct distance dialing, and other innovations brought more complications to the tasks of translation and trunk selection.

Each outgoing trunk is represented by two jacks: one for test access for the voltmeter and sender circuits, and one for the make-busy operation.

Most large new urban 5XB in subsequent years had IDDD, and it was retrofitted to some existing ones, but most omitted the dual outpulse capability, that job being handled by TSPS.

Autovon originally used a four-wire version of 5XB, with a more complex marker to implement its nonhierarchical polygrid routing system.

Trunk circuits had additional logic and data storage built in, to implement multilevel precedence and preemption.