Manual Exchanges Research Articles

Chronic red blood cell exchange to prevent clinical complications in sickle cell disease

We tracked the results of 394 manual or automatic red blood cell exchanges done with a cell separator in 20 sickle cell patients at high risk for recurrent complications. Over an average of 6 years, none of the patients developed complications related to the procedure or to the increased blood use. It was safe and effective in preventing complications of sickle cell disease, and if done automatically, reduced iron overload. Ferritin levels also decreased in patients treated with automatic red blood cell exchange. Furthermore, using Single Donor Red Blood Cell units (SDRC) we reduced the potential exposure to transfusion transmitted infectious diseases (TTI).

Basic briefing: choosing a PABX

Traditionally the private telephone exchange functions as a switch to connect a group of extension users in an organisation to the public switched telephone network (PSTN) and to connect these extension users to each other — and that's all. The installation of a new private automatic branch exchange (PABX) represents not only a major capital investment for a company, but the opportunity to benefit from recent great advances in telecoms. In theory the choice of exchange is straightforward: the stored program control (SPC) type PABX (or call connect system as it is sometimes known) will supersede both manual exchanges, with or without separate private automatic exchanges (PAXs), and Strowger electro‐mechanical exchanges. In practice the selection process is becoming increasingly complex.

The Post Office and automatic telephones

(1) Introductory.–The paper attempts to describe points of general interest rather than detail. Absence of automatic aids to manipulation in early telephone switchboards; increasing introduction of such aids as manual systems developed; early automatic systems; the fundamental “Strowger” step-by-step system, and power-driven systems.(2) Post Office experience and practice.—Systems already in use; results obtained; methods of comparing costs; advantageous field of manual and automatic systems.(3) The semi-automatic system.—Claims and objections.(4) Layout of telephone areas.—Economic studies; differences between manual and automatic layout of exchange positions and external lines; subscribers' lines, junction plant, sites and buildings; synthesis of costs.(5) Internal plant and wiring.—Calculation and layout of exchanges; traffic data; grades of service; availability of switches; grading of switching outlets in accordance with theory of probabilities; “artificial traffic,” “trunk-hunting” facilities and methods of increasing them.(6) Services reserved for manual operating.—Trunk calls, extra-fee calls, coin-box and call-office traffic; automatic dialling to and from distant exchanges.(7) The problem of very large areas.—Transition difficulties of schemes involving a rigid plan of subscribers' numbers.(8) The “panel” system of the American Bell Companies.—Solution of numbering-scheme difficulties by exchange-code translation; brief description of “panel” system.(9) The choice of a system for London.—Strong attraction of the “panel” system; ultimate adoption of the Automatic Electric Co.'s “director” system; anticipated development in London; agreements for supply of apparatus; standardization of Post Office “step-by-step” automatic system.(10) The “director system.”—General description of the system; methods of translation; intercommunication between manual and automatic exchanges during transition period; “key sender” and “call indicator” equipment; the London “ mechanical tandem” exchange; various classes of service provided; tone signals; alarms and guarding devices; private branch exchange lines.(11) The subscriber's automatic telephone set.—Standardization of signalling impulses and calling dials; circuit arrangements to avoid high-voltage surges, tinkling of bells, and impulse clicks in receiver.(12) Tariffs for automatic systems.—“Message rate” versus “flat rate” “time and distance” tariff.(13) Progress of automatic exchange construction.— New exchanges required in London and the provinces; training of staff; output capacity of factories.

Relays in the Bell System

Note: Before they can converse people must either be brought together or virtually be brought into one another's presence by the telephone. Any telephone system must establish talking connections between its subscribers, and these connections must be built up, supervised and disconnected when desired. This work is accomplished by the use of relays of various kinds, and the speed and accuracy of service is largely dependent upon them. There are completed daily in the Bell System about 42,000,000 telephone calls. These involve the successful and accurate operation of over one and one-half billion contact connections daily. Many kinds of relays are employed in the Bell System, varying from the simple electromagnetic drop to the sequence switch, the thermionic vacuum tube and the panel selector. Today a circuit connection between two subscribers served by manual exchanges in a large multi-office district involves about 21 relays. When these subscribers are served by machine switching offices, the number of relays in a local connection may be as great as 146. It not infrequently happens that in setting up a toll connection more than 300 relays are employed. In the present paper the relay developments leading up to, and making possible the present communication system, are outlined with particular reference to electromagnetic relays. A few typical circuit applications are given with a discussion of the requirements imposed upon relays which influence their design. Several types of relays are illustrated and their distinctive features are described. The subjects of relay design, manufacture and maintenance and also telegraph relays will be dealt with in future papers.