Legacy Telephony and its progression

Part 1 Load Coils and wire transposition to increase reach


The very first telephone circuits were one wire with a ground return. But that idea was rendered useless because of noise generated from the ground return. The only to solve the noise issue was a second wire that was added in 1883. Having to use a two circuit was almost destroying the fledgling Telephone business, but the industry adjusted.

Ezra Cornell’s legacy was full bloom by 1890 as New York City had telephone poles that were 90 high with 30 cross-arms supported 300 hundred wires.

While everyone was learning new skills in 1973 I was learning that we had to install transposition brackets do every 4 poles to cross the wire pair on the lines we had telephone service on.  I was curious about why we did this.  I learned that early telecom engineers discovered a few things, one that the wires on the cross arms had to be transposed every few hundred feet by use of transposition brackets. This is where the idea for twisted pair wire was conceived in 1896. The transposition would counteract a problem known as capacitance by changing the potential that was being built up between the wire pair. The only problem now was crosstalk between pairs which was solved by staggering the twisted pairs. By 1896 all telephone circuits in New York City were underground; the gauges were heavy from 10 to 14 AWG (the same gauges used in electrical today).

But transposition of wires whether in using open line wire (still used in long distance transmission) or twisted pairs in cables was not the entire solution to the capacitance problem. In 1899 Michael Pupin of Columbia University and George Campbell of AT&T independently develop the theory of loading coils. This greatly decreased the attenuation of a wire-pair line and holding it more nearly constant over a frequency range from 250 hertz to 4000 Hertz. The actual effective voice range is 300Hz to 3,400Hz as the Voice Frequency would roll off as in Figure 1.


Figure 1 Voice occupies a 0 to 4000 Hz slot but rolls off

The combination of transposing conductors and the insertion of load coils neutralized the effect of the capacitance between the wires by introducing inductance back into the circuit, thus maintaining an impedance balance (will explain impedance in more detail later). Local loops from subscriber to central office may have loading coils about every 6000 feet. There were different types of load coils that can equalize the effect of capacitance.

By 1911 AT&T was begin service between New York and Denver just with the use of loading coils. This line was completely passive, meaning there were no electronic amplifiers being used to boost the signal. Though the Voice quality as not the best. Small spurious signals are always generated when voice signal is converted to an electrical analog signal. Over the years this was to become known as the noise floor. When you have sufficient amplitude the noise floor is not noticed but after a signal travels a significant distance as in the case of Figure 2 with 5000 ft of 19 AWG (American Wire Gauge) twisted pair.




Figure 2 the effect of distance on an analog signal

The signal starts to loose amplitude and flatten out where the noise floor is more noticeable.

These load coils also had the effect of stepping the signal but eventually noise floor amplitude becomes as high as the desired signal.

By summer 1913, AT&T had tested high-vacuum tubes on the long distance network. Amplification greatly improved the quality of the signal but it also amplified the noise floor as shown at the bottom of Figure 2. Since then Telecommunication Engineers have had to deal with this design constraint known as Signal to Noise Ratio SNR or Carrier to Noise or CN in designing networks whether they these early analog Voice networks or modern Fiber Optic data networks. Signal to Noise or Carrier to Noise is typically the distance from the noise floor to the signal peak.

By the fall of 1913 the AT&T had begun constructing the line west from Denver, and upgrading the line to the east.


On June 17, 1914, AT&T completed the line, erecting the last pole at Wendover, Utah. By 1915 AT&T long lines were connected across the country[1].


 Outside and inside plant and cabling characteristics

In the 1940’s a new method of construction was introduced.

Wire pairs were now being developed so that they could be bundled into a cable. Early versions did not have sheath wrapped around them and were bundles hung in cable hangers on suspended and tensioned wire. As manufacturing improved the wire pairs were covered with a sheath and an additional shield that would grounded every few poles.


I love this this Norman Rockwell painting as it captures a lot of the elements used to install aerial cables for last 65 years. You see this lineman’s bell wrench hanging off his belt on his right hip and his scare strap (showing wear) around the pole

This process involved 2 basic operations:

  1. Installing a stranded steel support cable first that is pulled tight. Then the cable (containing the many pairs of wires) is pulled out.
  2. Then the silver lashing machine (in the picture) is pulled over the cable and one to two wires tightly wrap steel lashing wire around the cable and the support strand.

The Installation of cables in the air and in the ground is known as “Outside Plant indoor cabling is known in the Industry as “Inside Plant”.

This method is still used today to construct Fiber Optic, Cable TV and some traditional phone cable. Annual Pole attachment agreements are about 10 to 20 times cheaper to pay to power companies over a period of years, than the cost of installing the cables underground. Also maintenance on Aerial

In the early days of phone service you could not just dial a number. You would take the phone of hook and it would typically ring a switch board operator in the Telephone companies Central Office were an operator would patch your call through to the desired line or exchange on large patch panels. The Operators would use patch cords as shown in Figure 3 that come to known as the phone jack the most popular being the ¼ “ phone jack. The jack would consist of a Tip, Ring and a sleeve.

 1. A Tip and Ring



Figure 3 Tip, ring and sleeve

Western Electric was a subsidiary of AT&T and by 1919 the standard telephone had a handset with a microphone that you could talk into and listen from a small speaker and the cord to the phone having four wires, one pair for the microphone and the other for the speaker. But leaving the phone the line is just a two-wire system connecting the phone to the phone company. By the 1996 with the passing of Telecommunications Act of 1996 this would be the Incumbent Local Exchange Carrier or ILEC (Your local phone company).  I will talk more about how transmit and receive end up on one wire pair, but for now I just want to define the name of these two wires which are known as tip and ring. The names are actually carried over from the early days of switching, when operators used patch cords to tie calls together. In Figure 4 is the Tip and ring out of RJ11 (the RJ standing for Registered Jack) phone plug.


In the continuous effort to standardized and improve reliability Registered Jacks were introduced by the Bell Labs in the 1970s, replacing much bulkier connectors that were in use before. Bell Labs gave the specifications for them (both the modular connectors and the wiring of them) Universal Service Ordering Codes (USOC), and that was the only standard at that time. AT&T and the Bell System used its leasing arrangements for telephones and telephone equipment made by its subsidiary, Western Electric to increase its control over telephone manufacturing in the United States and Canada.

One thing that can be perplexing is that a phone hand set is full duplex, meaning it has separate transmit and receive paths in which both can handle signal transmission at the same time. But as I noted it is only a single Tip & Ring wire pair going to your telephone company.

Hierarchal Switching and the call path

“In 1929, AT&T network engineers implemented the first national General Toll Switching Plan. It established a hierarchical, national network with the implementation of an automated mechanical crossbar switch with its many clacking and clicking relays as dialed codes of electrical impulses. Dialed routing codes soon gave way to the familiar area codes, which the switch itself could translate into the needed routing information. AT&T soon modified the switch to handle customer-dialed long distance calls. By the 1950’s crossbar switches started to be implemented that allowed subscribers to direct-distance numbers. Call-completion time dropped to 10-20 seconds. By the late 1970’s early 1980’s the Hierarchical routing gave way to dynamic non-hierarchical routing with implementation of the TDM digital AT&T 4ESS and the Nortel DMS switches choosing the best path between two points[i].”

In the legacy Public Telephone network what is known as the Class 4 or Tandem switch is used in a central office telephone exchange to connect calls originated in that exchange to a long distance Carrier that in turn is connected to Public Switched Telephone Network (PSTN) which interconnects other telephone company offices.

In respect to the local exchange (end-office) a Class 5 switch is used to terminate local calls to the customer (also known as the subscriber). A tandem being a telephone switch to which no telephones are wired, its services were termed non-customer facing. Other terms are Toll Centers (TC), if operators are present, otherwise they are Toll Points (TP). Class 4 switches at that time often had an associated Traffic Service Position System (TSPS) to handle operator calls. After the Bell System Divestiture the class was called Access Tandems (AT).


Figure 5 4 wire to 2 wire conversion at phone and at Central Office

A case Study off a legacy phone call

Referencing Figure 5 caller in Denver who lives on Quebec street takes the phone off and gets dial tone and dials a number to Salt Lake City the woman in SLC who answers the phone. The caller in Denver talks into a handset that has a transmit and receive path that goes over four wires (in that curly cord) from the handset into the phone where it is converted to a 2 wire in the phone (via phone wall jacks, phone pedestals and miles of wire) directly to the Line card port at the ILEC Central office at 6490 S Quebec St in Denver. At the ILEC the line is connected to a Class 5 end office Switch (which is identified as DNVRCODCDS1) where the Class 5 Switch converts the two-wire system back to a duplex path. The Class switch 5 has SS7 trunks likely over OC3 fiber optic transport facilities (which would be over 2000 duplex trunks depending how many trunks are used for signaling) to a Class 4 Tandem Switch (DNVRCOSO02T) which is a 5ESS in Downtown Denver CO. Tandem designation because the switch has trunks going in and out and call goes in and then out in a one after the other fashion. The class 4 tandem switch has trunks to several other class 4 and higher switches like the callers preferred Long distance Carrier’s Switch.
The LD carrier puts the call onto fiber optic transport facilities. This LD carrier uses the Denver switch for its SLC LD traffic so the fiber optic transport terminates directly into the ILEC’s Class 4 tandem switch in SLC (SLKCUTMA0T2 which is DMS 200) were the call is then routed over fiber optic facilities the ILEC class 5 end office switch (SLKCUTMADS1 a DMS 100) to note both switches are in the Central Office. The end office Class 5 switch in SLC converts back to two-wire transmission onto local loop and arrives at the telephone set to be split by a hybrid coil back into a duplex path into 4 wires thru the 4 wire curly cord and into Agents headset.

When the caller Denver took the phone of hook she was given dial tone by the local class 5 switch in her exchange. She dials a number in SLC which generates DTMF (Dual Tone Multi-frequency) tones that were processed by the Class 5 switch and sent to the class 4 tandem switch via SS7 signaling trunk. At this point an actual trunk path has not been connected yet. The class 4 switch does a database lookup the number, after receiving an answer to the query the signaling is routed to the LD carrier. The LD carrier Tandem Switch signals via SS7 trunks to the Class 4 Tandem Switch in SLC which in turn signals the class 5 switch that the call center is connected to. The Class 5 switch in SLC rings the phone of the woman in SLC at this point a trunk path is setup. If the phone was busy no call path would have been setup as the SS7 signaling which is a separate path will handle the busy signal sent back to caller in Denver. There are several signals methods to the Class 5 switch which will be covered in more detail in Chapter 4



Figure 6 Path from phone handset to line card in public switch

 1. B Case Study 2 Wiring Path from the Phone to Central Office and use of Side-tone

Now that we have a good overview of phone path let’s get some more detail looking at Figure 6 is the hook switch were red number 1. Observe that it is open connection all the way back to Central office being a 2 wire home run back to the Central Office. When you first look at Telephony appears too complicated because of the cross connect and splice points that the single Tip & Ring pair traverses back to the Central office.

This two wire pair is defined as a local loop which in this case is an open loop.

To break down what seems to be the complexity of the local loop wiring look at point 2 this is known as the Demark or Demarcation point where responsibility for maintenance is defined and also indicates point where ILEC (Incumbent Local Area Exchange Carrier) also known as or local phone company or CLEC (Competitive Local Exchange Carrier). If this is home it is called NIU (Network User Interface) or a NID (Network Interface Device) it is basically the gray box that we all have on the side of house.

If it is a business this would be 66 split block called an RJ21X which also defined under USOC (50 dual punch pins that connect 25 pairs on the left side to another 25 pair with right side able to the same) that has 25 pairs typically laid down on the left side going to the telephone outside plant. On the right hand side is another 25 pair of pin positions that customer inside wiring punched down to,

Figure 7 RJ 21XFigure 7 shows a close-up of the bridge clips that pass the circuit from the local exchange to the customer. These clips can be removed as a quick disconnect if problem develops


The rest of the path to the Central Office will be covered later in the chapter.

But now looking again at Figure 7 find number 3

Observe that there two relay contacts that are open; they are controlled by relay coils that also act as chokes. They work in concert with the Audio coupling capacitors by keeping the 48 volts dc from going into the switch circuitry and the chokes keep voice signals from being drained into the Central Office battery plant.

Last of all is number 4 which is circuit that creates a needed an important feature called side-tone.

Since the telephone transmits and receives on the same pair of wires, it is possible that too much of the caller’s voice is fed back into his or her receiver. This known as side-tone and too much side-tone causes callers to lower their voices and the user at the other end of the line will have difficulty hearing. Getting the right amount of “sidetone,” is achieved by phasing the signal so that some cancellation occurs before the signal is fed to the receiver. Anyone using a phone with no side-tone at all will think the phone is dead.  With too little side-tone callers are convinced that they’re not being heard and will cause them to shout, saying something like: “I can hear you. Can you hear ME?”

Also to note that a telephone on a short loop with no loop compensation will also appear to have too much side tone and callers will lower their voices. Typically the Telephone Engineers can design around this by padding the wire pairs as in this case, the percentage of onside is the same, but as the overall level is higher also raising the side-tone level.

[1] This pole line no longer exists, and was probably removed in the late 60’s, after being replaced by Microwave (which I will go into a little more detail later).

Today four different Fiber Optic lines have replaced the microwave. Two of these companies have Booster Facilities (OP Amp sites that I will go into more detail later) not far from this site.


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