Toll Carrier Systems for Open Wire
These Waxahachie, Texas downtown poles once carried on a top arm (now removed), first commercially applied “J-Carrier” circuits in the United States, 1938. Original lines routed between Dallas-Ft. Worth-Waxahatchie-Hillsboro-Waco-Austin-Houston routes.
Let’s discuss, for a moment, the general nature of the telephone system. We have the customer (which we will call the “subscriber”) at his home, business, factory or school. Equipped with either a land-line phone paid through monthly billing, or a coin pay station, such instruments are called subsets orsubscriber stations. It is the first of four major intereconnected elements which comprise the telephone system.
The second is crucial to the operation of the system: the central office, or C. O. Without an electrical or electronic entity to differentiate calls, switch and route them, confusion would reign. While many systems have used electro-mechanical switching in the past, today these are largely electronic, incorporating computers to make the appropriate determinations in routing calls from one subscriber to another. If we consider open wire systems, then we take some steps backward in history. Here in the not so distant past, manually operated switchboards were the norm in rural areas and small towns.
The next significant feature is the one we see every day, but seldom notice–unless your eyes are up in the air–as mine are most of the time, and that is the exchange outside plant. Here, most noticeably we encounter the aerial wire and cable facilities either pole or building mounted with interconnections spun between poles and buildings. This aerial wire plant–firmly visible to the traveler–connects unseen buried and undergroundfacilities.
Hidden from our prying eyes, are layers of underground routes containing cables, loading coils, repeaters, terminals and splices fed through tile, steel, iron or PVC multi-duct conduit. These contain fiber optic connections, lead-sheathed and plastic-sheathed multi-pair cable and coaxial facilities. Buriedcables are simply trenched into the ground along roadways, railway easements and other areas without conduit protection, but have a stout and protective covering allowing direct burial. By use of pigtail connections or loop-up splices directly with the buried cable, repeaters, splices, terminals and loading coils are directly accessed above ground. Traditionally, this construction dominates rural and larger town areas.
Finally, if we are to define the fourth element in terms of the 1950s, we have the toll plant, which is a labrynith of aerial and buried cables, microwave radio links and openwire toll facilities. Today, toll plant comprises buried and some aerial fiber optic transmission facilities of mostly synchronous multi-mode capacity. Microwave is fading from the scene, but is used in some locations–primarily mountainous zones where pole placement or trenching cable would prove to be nearly impossible. Additionally today, some asynchronous fiber still operates but is being replaced by standardized technology, called SONET–or Synchronized Optical Network.
So, for our example with open wire, it can form the link between the toll switch and another toll switch, hence from city to town, town to town or city to city.
Your home’s dedicated pair connects a phone where you live to the local exchange. To go beyond this location, an inter-office trunk route requires a physically-connected pairs, hence when central office traffic increases from numerous subscribers, the greater the number of inter-office trunk circuits.
As with power transmission and distribution, the economics of building the facilities in between cities is expensive. Thus, if let’s say we had a single pair of interconnected wires were to serve only one subscriber, the cost of carrying on a conversation over distance would be extreme, to say the least.
When the mystery of induction, or electromagnetic bleeding, was found to be troublesome to carrying on long distance communications, because of crosstalk, it also opened the minds of those resolving the problem to use this same problem to improve and enhance the economic investment of the outside plant.
With the use of phantoms, or non-physically connected pairs on an arm, a third conversation (third side circuit) could be added to two pairs, decreasing the costs involved with stringing wire. Carrier systems took this first step even further, by allowing an increase of voice–later data, radio channels and other communications–to be added to a single pair of wires.
Just imagine the issue without carrier! To make even this complex problem simple let’s postulate that we have two very small towns with each a central office switch and an inter-connecting toll lead. They are separated, like most in the midwest by historic railway definitions: 7-9 miles. As telephone traffic increases, one pair is not enough and more pairs are strung. Physically, even if the town has 200 subscribers in each, the poles would simply be overburdened with arms, wires and wouldn’t be tall enough to support each pair featuring one customer’s needs.
Now . . . add to that, the drops to the farmers along the highway bordering this toll lead . . . then things get really complex and expensive. So, in a nutshell, you have the problem and the promise of carrier technology applied to open wire–and later cable outside toll plant.
Carrier also brings with it the achieving of not only economic justification of implementation, but higher quality of service, lower maintenance costs on supporting structures, lower labor costs and reliability.
Today’s fiber systems were built on what was learned about open wire carrier.
To comprehend carrier is to understand the principal of human speech. Nevermind data and radio, let’s just concentrate for a moment on this issue.
The first telephones were built for human speech, that is converting sound into electrical pulses which could be converted to an electrical signal transmitable over distance and then re-acquired by a similar instrument and converted into intelligible sound. Speech is simply vibrating air within an audible frequency range for us to comprehend.
We grow angry when television commercials seem much louder than the adjoining programming. Today, the Congress is re-visiting this problem, as it did originally many years ago when complaints were made against advertisers in the 1970s. The irritation of noise which is so harmful to our hearing is not necessarily volume, but pitch amplified many times.
To consider the telephone is to clearly understand the importance of pitch andloudness.
Photo: First experimental J-Carrier in the field. Texas: Dallas/Ft. Worth-Hillsboro-Austin, Houston, 1938. An extension bracket supported the first J-Carrier circuits.
Alright! We all generally recognize what loudness is: simply the amplitude of the wave of sound. This is the energy of the actual wave compared as a ratio to the weakest audible wave of the same frequency.
But . . . pitch . . . is a little different: it is vibrational frequency and counted in CPS, or cyles per second. When we speak, it is a combination of harmonies, eminating from our vocal cords and refined by our throat, tongue, palate and teeth which is generally measured in the 125 to 260 cycle range.
We recognize words by the relative amounts of vocal energy expressed at differing frequencies. Hence, when certain words are spoken, they can be recognized because of their voice frequency range and the fact that when words are spoken by humans, our speech is in the lower frequencies.
Speech frequencies are approximately within 100 to 7800 cycles per second. We recognize loudness from one individual to another as the basis of energy because it is generally in the lower voice frequencies.
But . . . what good is speech if no one is there to hear it? That’s where the ear’s remarkable sensitivity comes into play. Receiving these sound waves is essential to constructing a viable receiver on a telephone. Our ears can pick up most sounds in the 1000 to 3000 cycle range. As a result of thse vibrations through the air, our ears can perceive these nearby changes in barometric pressure and our brain does this electro-chemically.
The telephone must function electro-mechanically. Instead, by converting a sound into electricity and then back into sound energy is the basis for telecommunications.
Remember above where we demonstrated that the human ear can perceive a wide range of sounds? However, it is not necessary for a telephone instrument to have this wide facility in communicating with another instrument. Hence, a telephone circuit need only satisfactorily transmit frequencies in the range of 200 to 3200 cycles. The limiting factor, of course here, is that speech must be consistently reproduced so it can be readily understood by all. When a standard was required for satisfactory operation of telecommunications equipment, the agreed-upon standard voice frequency channel for speech was 200 to 3200 cycles.
Power levels of speech over an open wire at 1,000 cycles. Let’s see how our voice would fare over a transcontinental line if we applied a thousand microwatts at the Pacific Coastline and ended it at the far East Coast of the United States. This is a good illustration of what the engineers had to tackle in order to send voice from thousands of miles away to the speaker and listener.
A simply amazing feat for the technology of the 1920s.
What is a Carrier Communications System?
One bay of a Type O Carrier System, 1989.
Have you ever encountered the term “carrier currents” when hearing or reading about telephony? It is exactly what it means. This current is “alternating current” and possesses a frequency high above normal voice frequencies. An oscillator is part of the carrier transmission equipment at either end of a toll lead link. Here an alternating current of high frequency is transmitted with “superimposition” of telephone traffic on the carrier current. We call this a carrier wave.
Now, how do you “superimpose” telephone conversations on this carrier current? It begins with “modulating” or “shifting” the original voice frequency band. Let’s say we have a single pair of open wire on a crossarm. Since voice frequencies would be in the order of–as we mentioned . . . the 200 to 3200 cycle range . . . we could modulate, or shift this band to the carrier frequency at a 4200 cycle range. In doing, mutual interference could be avoided by allowing this pair to carry both the carrier signal and the voice frequency allocation on the same pair. This is “modulating.”
Okay. So, we’ve created this monster . . . so now at the other end of this toll lead, how do we pull these two conversations apart?
Just the reverse of what we proceeded to do above; we use a demodulator and bifurcate the two voice frequencies. Hence, we recover the two circuits by separation and this is termed, “demodulation.”
Typical carrier for open wire mounted in various equipment bays at a central office.
O and N Carrier Outside Mounting Cabinet with interior bay plug-ins. Picture taken in Iowa May 2012.
Angular view of outside mounted O or N Carrier Repeater unit.
Rear view of Repeater Cabinet for outside mounting. Note the use of 10-pin crossarm for stability at base.
Carrier current systems are today called “multiplex” systems, meaning a number of additional communications channels can be applied to a single circuit. Carrier is just a “conveyance” for the current of different frequencies “varied” in accordance with their unique signal types and modulated to be “impressed” on the same line wires.
That is perhaps a very simplistic way of revealing the depth of this magnificent topic. Let me tug on your arm a bit so that you may follow to explain this colorful topic in more detail. First of all, open wire carrier systems were engineered and equipment furnished from a multitude of communications company suppliers. AT&T had their own trademarked systems. GT&E had theirs. The independent companies purchased from Lenkurt, Northern Telecom and Erikkson as well as others to obtain their individual systems. By and large, the systems operated on similar frequency hierarchies and principles. Just the nomenclature and slight technical application differences made their architecture patent-proof to other competitor manufacturers.
So, let’s talk a little about what “carrier” is all about and how it fundamentally and economically served and evolutionary extended the open wire medium by its successful application.
The first open wire systems were based on early electronics having emerged at the end of World War I: 1918. Lee DeForest, who pioneered the vacuum tube, allowed the great spread of this appliance. Their development was applied to amplifiers, modulators and demodulators of sound. Also of importance, their application found their way into filters for selecting frequency and impeding others through a circuit. Most importantly, they enabled long distance service to be extended with less attenuation or interference.
The above graph illustrates how the frequency spectrum was allocated to open wire plant prior to the advent of World War II. Every component of the carrier systems defined above were “supressed,” except for the Type “B” and “G” systems. This was designated for telephone–but not telegraph–usage.
Here is a lineup I’ve researched of various alphabetic carrier systems on open wire, relative to AT&T (obviously there were patented and comparable non-Bell systems, too):
Desig. Line Fac. Chan. Carrier Type Sidebands Meth of Dup. Op.
A Open Wire 4 Suppressed Single Balanced
B Open Wire 3 Transmitted Single Equiv. 3-Wire
C Open Wire 3 Suppressed Single Equiv. 4-Wire
D Open Wire 1 Suppressed Single Equiv. 4-wire
E Power Line 1 Suppressed Single V. F. Switching
F Open Wire 1 Suppressed Single Equiv. 4-Wire
G Open Wire 1 Transmitted Double 2-Wire
H Open Wire 1 Suppressed Single Equiv. 4-Wire
J Open Wire 12 Suppressed Single Equiv. 4-Wire
Note that where we’ve noted the Method of Duplex Operation on Open Wire is “Equivalent 4-Wire,” this takes into account transmission at different frequencies whereby each direction of transmission is independent of the other which is “equivalent” to a four-wire system.
I have included Power Line carrier–because in effect, physically–it was still an open wire (non cable) medium.
Also of note: in the Bell System technical culture, a critical understanding of openwire carrier vs. multi-pair cable carrier nomenclature is based on the alphabet. Hence, when hearing “J” carrier for open wire, “K” carrier was the equivalent 12-channel carrier for cable pairs. “O” carrier for open wire 16-channel systems was equivalent to “N” carrier Bell System 16-channel multi-pair cable systems and so on.
Type “C” Three-channel Open Wire Carrier Systems, 1924
Erik Boucher’s artwork portrays a typical wire line built for 3, 12 or 16-channel carrier systems. Note absence of pole pairs. No phantoms. Also separation between arms is 36 inches.
Below is a diagram of the very simple single channel Type “A” carrier telephone system.
The schematic for the Type “C” carrier telephone system is elucidated below, including that of an intermediate repeater station, very common on most long line toll systems.
Here, we have simple diagram of the Type “D” carrier sytem for telephone system applications.
A Type “H” carrier terminal is simplified in this diagram shown below.
Western Electric Type “J” 12-Channel Carrier System, 1938
Waxahachie, Texas Downtown near C. O. First poles to ever carry open wire “J-Carrier” commercially in U. S.
The S-8 arm was commonly used to provided adquate spacing for the 12-channel carrier system. Note absence of pole pairs and “C-Rural Wire” wire below arm. Photo credit Tom Hagen.
Heard of the 2010 U. S. Broadband Plan? This was “broadband” in 1938, when it was introduced: Type “J” 12-channel carrier. This system superimposed twelve two-way channels on a single pair and also managed to provide three (3!) two-way Bell Type “C” carrier channels and two (2!) two-way voice circuits!
Type “O” 16-Channel Open Wire Carrier Systems, 1955
Open wire carrier on upper crossarm. Below is fiber Remote Subscriber Terminal (RST) for conversion of optical siganal to electrical for distribution by a buried multi-pair cable.