Central Office, Toll Cable & Toll Cable Entrance Systems
Open wire’s investment throughout the 20th Century was a large part of each telephone company’s business. However, it is not always possible to ignore the valuable contributions of other, related transmission media, when they were integral–and material–to the successful operation of the telephone system as a whole.
Acme Telephone Association office at Dickinson County Historical Center, Abilene, Kansas.
The growth of aerial multi-conductor lead and PIC cables was an extremely important–and essential–contributing interloper into the world of open wire’s successful continued operation. First open wire disappeared in big cities, such as after momentous weather-related disasters in the 1880s in New York City and other metro areas, and was replaced by early cable technology. But, it took a long time for cable to comprise a significant investment in transmission facilities with most telephone companies.
First and foremost were the technical problems involving the design of a successful aerial (or underground) cable: transpositioning, weather-proof sheaths, load losses, poor insulation, and the like.
Once these problems were confronted and solved, multi-pair cable began to snake its way into many different applications. Open wire wasn’t ready to be knocked off its lofty perch yet. In fact, the hayday of the crossarm neededcable badly . . . in a national effort to expand and connect more and more subscribers.
One of those examples we shall briefly speak of here, is the issue of “entrance cables” for open wire toll circuits.
While aerial multi-wire leads had been largely restricted–and downright banned–from many downtown urban congested areas, open wire’s focus would remain consequential in rural areas. Transmission links of open wire traditionally now ended at an edge-of’town terminal structure, where by the circuits were bridled together and entered a major terminal in their transition to a “citified” medium: aerial or underground cable.
Before we embark on this little exposition, let’s define “buried” vs. “underground” cabling. When communications people speak of “buried” drops or “buried cable,” we are talking about just that: a trench is plowed and the weather-proof sheath-protected multi-pair cable is dumped into the trench and filled with soil. An “undergound” cable is placed in ducts, manholes, handholes and pipe, pre-constructed so that they can be accessed at different points along their route for distribution, transmission or local loop use.
Buried cable will use “fixed” terminal types, which protrude or extend above the soil surface, where the cable can be accessed by subscribers or equipment installations. Typically these fixed terminals are seen as little light green steel boxes at the lot line, or street easement, as the cable is looped up or terminal “pig tails” descend to the existing cable.
When open wire was terminated/commenced and bound to a terminal structure, that marked the point where some interesting electrical changes began to occur along its conductor path. The transition from aerial wire with insulating air space between them to a closely confined cable architecture with densely packed conductors tightly woven together, presented new problems for the electrical signal.
Lead cable splices, where pairs are joined within. Display at the Museum of Independent Telephony, Abilene, Kansas. 2016.
In the early days, when most open wire was voice frequency, there weren’t that many cables. Most were small physically and the number of pairs was quite modest. These were called “toll cables.”
Let’s briefly talk about cables’ organization for a moment: cables have many colored insulated copper conductors tightly constricted. Paper insulated cables were the order in the 1880s through the 1920s, when improved insulation types were developed and then manufactured. In order to bring some semblance of organization to a cable’s contents, each “pair” of two wires was grouped together to form a “phantomed” group (and, for those of you who are sticklers for the truth: yes, there were a various number of “non-quaded pairs” in the early cables, too.)
Now, consider that an open wire toll terminal structure deadended with 165, 128 or 104 mil conductor (of varying types). In order for the health of the circuit to be maintained, each aerial line wire was met with a companion cable group size: 13, 16 and 19 guage conductors accordingly.
In the early days, where high frequency carrier was not used, a cable wouldn’t necessarily be one guage; in fact the same cable could carry varying grades of conductor sizes! Part of the reason for this was many open wire lines below the fourth bottom arm (fifth and lower) carried local exchange circuits, too–instead of spinning more wire through the air on a duplicated pole line to the other side of the highway, for example. Fourty wire toll leads were the traditional construction for aerial transmission links (four ten-pin arms of 5 circuits to each arm).
One of the big problems confronting early cable design researchers was the problem (sound familiar?) of crosstalk impinging upon those closely bundled conductors. Subsequently, a transposition scheme for cables was developed–a highly mathematical scheme at that–which alleviated the severest problems on analogue circuit design. All and good for cables carrying low frequency voice circuits: below 30 k.c.
However, as carrier systems were developed in the 1920s, cables of this construction were not effective for these advanced technologies. An evolution began to take place in cable development, just as it was in the open wire medium.
Communications engineering researchers began to look to “shielding” as a means to prevent cross talk. In the early days, not all open wire circuits were used for carrier. The “pole pairs” on early transmission lines, observed as wires separated by sixteen inches betwen the two nearest pins to the pole reserved for phantom groups, carried voice frequency circuits.
Other pairs–whether physically separated by four, six or eight inches–carried the early C-Carrier 3-channel systems on Bell lines, and an impressed telegraph circuit as well. When entering a cable sheath, these carrier circuits were carefully enclosed in a “shielding” of either brass, copper; sometimes a metallic tape of some thickness or braided tinsel. The limits of the crosstalk dictated the thickness of the metal separator. Since more problems would occur if the quads were in close alignment with other carrier quads, early carrier cables used voice frequency circuits to separate (within the cable) these special circuits.
For further information to the reader, it might be resolved that we define the term: “layup” or placement of the special circuits within the cable layers. Since carrier circuits were the highest priority links in an open wire line, they were placed on the highest elevation arms. These were called “high grade” circuits. They were typically the largest diameter conductors found on the pole line and subsequently had to be matched by the largest cable conductor sizes within the cable.
So, early cable design splicing was a real exercise in diversity, as the larger conductor sizes found themselves at the center of the cable, yet surrounded by small diameter pairs and then an intermittant grouping of larger pairs again!
Now, what this meant was perhaps only a quarter of the cable’s pair content was appropriate for carrier use.
By the time phantomed circuits on significant aerial wire toll leads were bein phased out and more carrier added to existing lines added, cable development evolved as well.
As the voice frequency circuits developed along the lines of carrier plant investment, cables began to dramatically change their designs. More metal–especially when utilized to shield “high grade” circuits–and shielding also meant more “capacitance” within the cable. It also made the cable construction more costly. More space had to be allotted to decrease capacitance between quads.
Not only was the cable design becoming more complicated, but economics reared its ugly head: in a regular toll cable more wires could be compacted, owing to the thin insulation needed on each conductor. Unfortunately, in an effort to fight capacitance problems and the need for high grade circuit anti-crosstalk measures–shielded circuits took up approximately two and a half times the space as a regular quad and cost twice as much! So a cable would look like this: eight large quads, with intervals of space between them, very similar to a roller bearing configuration; inside, tightly compounded quads of smaller circles surrounding the center. Only the quads on the outside carried the largest conductors.
Cable development then took a step forward by the late 1920s, when “layers” of quads and pairs were alternated with voice frequency sets creating “layers.” Then, these layers were encapsulated in their entirity with a uniform layer (keeping in mind the necessary thickness to combat capacitance problems).
When the earlier phantomed circuits were being converted to other uses, this type of cable worked beautifully. With an arrangement of quads in a layer, the splicers could access pairs easily when they were routing former phantom groups and converting and re-arranging phantomed lines for non-phantomed operation. Non-quadded pairs then could serve for voice frequency toll entrance purposes. Two years prior to the onset of the Great Depression, this cable design scheme began to signal the maturity of the toll entrance cable for open wire plant conversion.