5XB switch

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The Number Five Crossbar Switching System, designed by Bell Labs and made by Western Electric was in use in Bell System telephone exchanges from 1948 to the 1980s. 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.

1 Switching Fabric
- 1.1 Line Link Frame
- 1.2 Trunk Link Frame
2 Trunk circuits
3 Common control
4 Advanced services
5 See also
6 External link

[edit] 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 and 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.

[edit] Line Link Frame

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.

Each line switch in this first, mixed bay had 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 had at least one simple line switch bay with ten more line switches, carrying 200 lines. Thus the minimum size of a LLF was 290 lines for a line concentration ratio of 2.9:1. Optionally it had 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 was 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 lines on the switches were divided into Vertical Groups of fifty, being five line units on each of ten switches.

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 was a bay with ten junctor switches, divided as usual into left and right halves. One half had 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 was vice versa. By this means, each LLF could use the Junctors of its mate, if the Marker failed to find an idle path on the first try. Since they were odd and even their Junctors appeared on opposite sides of the Trunk Junctor Switches, thus giving access to the mate Trunk Links as well. ALL connections were only used in heavy traffic periods.

[edit] Trunk Link Frame

Junctors were wired from LLF through the Junctor Grouping Frame to the levels of Trunk Junctor Switches in the Trunk Link Frame (TLF). Unlike earlier designs, the junctors had no supervisory relays or other active hardware, all such functions being assigned to trunk circuits. The basic design of the TLF had 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 allowed each Trunk Switch to connect sixteen trunks to its twenty Trunk Links. With discrimination using two levels, this resulted in the TLF having a Trunk Concentration Ratio (TCR) of 1:0.8. 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 Wwitches and Junctors as the LLF, there were always twice as many LLF as TLF. As first designed, the maximum number was ten TLF and twenty LLF, known as 10x20 and, at first rarely achieved. In the late 1950s, multiple Trunk Junctor switch bays (ETL and SETL) 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 was under development.

A Channel from a line to a trunk consisted of three links of swiching fabric: Line Link, Junctor, and Trunk Link. In a 10x20 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 were the same as the Channel number.

[edit] Trunk circuits

As in previous designs, supervision of incoming calls was handled by relay sets known as incoming trunk circuits. Unlike in previous designs, this work was also done in the trunk circuit for outgoing calls, so there were no junctor circuits. Since different outgoing trunks were connected to different places and were used for different calls, their relay sets could be specialized for a particular kind of signalling or call metering (see Automatic Message Accounting) or other peculiarity. Thus a TSPS trunk could give complete control to an operator, while an E and M signaling trunk could do the kind of signaling required of a private long distance line, while a local outgoing trunk could be simpler.

Thanks to this more complex trunk circuit, outgoing trunks were selected by a quicker and more versatile method than the "sleeve test" previously used. Each trunk circuit provided a ground on a FT lead to indicate idleness. The FT leads for trunks in a particular group were cross connected to a FTC (Frame Test Common) lead for its Trunk Link Frame, to indicate that the TLF had one or more idle trunks in that group. The Route Relay in the Completing Marker connected sensor relays to all the Trunk Link Frames, allowing the Marker to choose a TLF that had an idle trunk and then connect to that trunk through the Trunk Link Connector (TLC) 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 was less efficient for coin phones, which needed 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, an number of work-around methods were devised. POTS and coin phones shared the more complex and expensive coin trunks, or else separate routes were established, or coin trunks connected via tandems 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 was unable to connect a trunk directly to a trunk. Instead each incoming trunk that had the ability to make tandem calls had to have a Line Link Frame appearance, as though 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 was not through the four stage voice fabric. Rather it was through a dedicated single stage crossbar network known as Incoming Register Link (IRL) or Outgoing Sender Link (OSL) respectively. Registers and senders were in groups of ten, assigned one to each level of as many crossbar switches as were appropriate to the traffic they could handle. Different trunks were wired to different IRL or OSL depending on what kind of signaling they used; ie IRDP, IRRP (See Panel switch), or IRMF.

Previous systems used relays in the incoming trunk circuit to control ringing and to return busy tone. 5XB used a ringing Selection Switch (RSS) with ten verticals, serving ten trunks. The various levels various provided tones, and ringing current of various durations and codings (especially valuable for party lines). Levels 0 and 1 were used as discriminating levels to set the polarity to ring slectively. An especially sensitive wire spring RT relay was 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.

[edit] Common control

Call back, single train, and other sophisticated methods required more sophisticated controls, but they increased efficiency and became standard for later designs. 5XB also separated the registers for receiving digits, from the senders for sending them. This complication necessitated more transmission of data among the control circuits, but greatly shortened the holding time of senders and increased general efficiency and versatility without having to put the versatility into large, numerous and complex senders as in earlier systems.

Originating Registers (OR) were wired to the Trunk Link Frame (TLF). In the original 5XB a marker, having been alerted to a trip condition, picked an OR by the same mechanism it would use to pick a trunk, identified a clear path between line and OR, loaded the OR with any information necessary for later processing (such as Line Equipment and Class of Service) and released itself. The OR then received the digits (rotary or tone) and used the Pretranslator to determine how many digits to receive before calling in the Marker again to complete the call.

Larger 5XB were built in the 1960s with more Markers. To save money the markers were separated into two kinds: simple Dial Tone Markers (DTM) just to connect the line to the OR, and Completing Markers (CM), many times more complex and expensive, to complete the call to or from a trunk. CM had, among other features, the ability to translate the first 3 digits of a phone number (or 6 when using a separate Foreign Area Translator) to identify the correct outgoing trunks and handling.

[edit] Advanced services

Taking advantage of the superior versatility of 5XB, Centrex was invented as a service package. Later Stored Program Control exchanges allowed more extensive service features. Autovon originally used a four wire version of 5XB, with a more complex marker to implement its nonhierarchical Polygrid routing system, and trunk circuits with additional logic and data storage built in to implement preemption.

Picturephone arrived in the early 1970s. 1ESS was already going into service and provided a more sophisticated basis for advanced services, but was not yet as widely available, so 5XB was designated as the switching vehicle. Every Picturephone line had six wires: the old talking pair plus a video transmit pair and a video receive pair. A new wideband switching fabric was designed using a 6 wire version of Type B crossbar switches. When completing a Picturephone call, the Completing Marker first picked the line or trunk to which the audio portion of the call would be completed, and then set up both the audio switches and the video ones. Wideband Remote Switches (WBRS) were installed in smaller exchanges, as video concentrators for lines that were beyond video range from a larger exchange that had been given the Picturephone feature.