Pneumatic Tube Network

Pneumatic Tube Network is the collective name given in Tower administration to the Great Pneumatic Tube Network and its named subordinate systems, taken as a single object of record. The phrase appears in no founding document; it was adopted by the Inter-Guild Logistical Network Guild in the course of the Fourth Recension, when the volume of separate instruments governing the Primary Arteries of the Inter-Guild Logistical Network, the Pneumatic Conduit Array and the various pressure-monitoring bodies had made it impossible to cite the network as a whole. The Guild has never defined the term formally, and its own clerks disagree as to whether "Pneumatic Tube Network" is a proper name, a description, or a filing convenience. It is used in all three senses in the same paragraph of the Third Concordance, Annex 2.

Composition

The network consists of the arteries themselves, the Central Pneumatic Stacks on their undisclosed lower floor, the receiving bays branching from them (of which Central Pneumatic Stacksreceiving Bay Gamma is the most frequently surveyed), and the several systems embedded within the arteries for purposes of their own. Of these embedded systems the best documented are the Pneumatic Clarion Network, which broadcasts administrative alerts and generates the Chord That Reconciles, and the Cartographic Courier Spiral, which links Floor 43 to Floor 404 and to Floor Na and is maintained under separate warrant.

Principal relay functions are discharged at Tower Floor 88; tertiary diagnostics are conducted on Floor 90. Maintenance falls to the Guild of Pneumatic Reconcilers, whose remit is day-to-day, the Guild retaining authority over questions of routing and precedence. The division is observed with some strain. A reconciler who clears a blockage during a scheduled alert must decide whether the clearance is maintenance or routing, and the decision is recorded differently depending on the office.

Pressures and tolerances

The network is rated for continuous operation between 0.4 and 2.2 atmospheres, with fault tolerance set out at length in the Third Concordance, Annex 4. Pressure is measured at every junction by paired gauges; where the gauges disagree, the reading of the lower gauge governs, a rule introduced after the Incident at Floor 14,022, in which approximately 400 kilograms of memoranda were expelled from a stretch of artery whose instruments had been read optimistically. Condensate is removed by Pneumatic Message-Tube Condensate Traps at intervals of no fewer than nine and no more than fourteen metres, though the interval is stated as eleven metres in the Second Recension and has never been reconciled.

Carriers are capped by schedule at 1.8 kilograms and by practice at considerably less. The prohibition most often repeated to new couriers is that nothing may be sent through the network that has not first been sent through it, a rule the Guild describes as procedural rather than physical.

The embedded systems

Because the Clarion channels occupy dedicated pressure bands within the arteries, an alert may pass through a section of tube currently impassable to ordinary traffic without registering in the traffic log. The Guild regards this as a feature; the Reconcilers regard it as a source of unscheduled silence. Three audits since the Fourth Recension have found alerts that arrived at their destination without having been recorded as sent, and one, on Floor 88 in the winter of the year the audits were consolidated, found a Chord that had been received twice by every resonator on the floor and once by a resonator that had been removed in the previous decade.

Dispute over precedence

The Third Concordance and the Ninth Concordance give contradictory accounts of which system holds precedence during simultaneous transmission. Annex 2 of the Third declares that "the Clarion may not be interrupted for the passage of ordinary matter," while the Ninth, in a clause numbered identically, provides that "no broadcast shall delay a carrier bearing a sealed seal." Both clauses are in force. The Guild's standing instruction is to observe the Third and cite the Ninth, a practice recorded without comment in the Guild's own register and defended by Librarian Halveth Orr in a note of 4 March: "Where two instruments bind, the clerk is bound twice, and is not thereby bound to choose."

"A carrier and a Chord travelling in the same artery are as two voices in one throat; the throat does not arbitrate, and the second voice is not a fault. Record both. Record the order in which they arrive, and record that the order is not evidence of precedence."

— Third Concordance, Annex 2, marginalia in the hand of Librarian Halveth Orr

A minority position, advanced by the Reconcilers' office and never formally rebutted, holds that the two Concordances describe different networks that have merely been built through the same tubes, and that the question of precedence is therefore not a dispute but a category error. The Inter-Guild Logistical Network Guild has declined to rule on the question four times.

Inventory of named systems

The list is not exhaustive. It is the list that the Guild has been willing to publish, and the difference between the two has been the subject of an unresolved requisition since the Fourth Recension.