This is a more detailed explanation than you can find in books on modern voice technologies, this will also go into more detail than is required to understand on most VOIP technology exams. Understanding alternating current signals as they are applied to cables is complex theory; I shall try to dive into it hopefully without losing you (unless of course you are seasoned RF person).
Analog signals that were used in early telephony are the same as the sounds we hear, the electricity we use, by 1935 Bell labs has developed coaxial cable along as higher bandwidth twisted pair cables had reached there limits. The design of the transmission cables became more involved because of analog signals that being applied to it. To get more of a feel of what an analog signal is first Imagine a rope tied off on a post and swung up and down. The rope would be the medium that is excited with application of energy that is created by person moving it up and down. This results in a waveform being present in the rope.
The rope would look similar to what we call a sine wave as depicted Figure 8. The sine wave is a common electronic term for an alternating current notice that the this creates a complete circle starting from 0 0 and ending at 360 0 . Like the rope, electrical conductors, such as telephone wire is a medium. When an alternating electrical energy is applied to a medium, a wave is transmitted. An electrical wave has the same form in a transmission medium as the rope being swung up and down.
By making the rope move more rapidly, note that the wavelength is smaller but a higher frequency of waves occurs during the same set interval of time. Swinging the rope at a slower rate creates a longer wavelength and a lower frequency of waves during the same period.
When someone speaks into the microphone of a phone handset it excites some variety of a transducer (a carbon microphone; some newer phones use discrete transistors or ICs) that converts actual sound waves of speech into electrical waves. These electrical waves induce a current onto the tip and ring pair and form a magnetic field is known as inductive reactance as shown in Figure 9. A magnetic field can also be created around a conductor when an electric current is flowing through it.
Inductive reactance is one of two main components that make up impedance the other component that has been mentioned already is capacitance
Without getting anymore detailed impedance is the result of adding the inductive reactance and the capacitive reactance on a vector.
What this simply means that if you take out your digital volt ohm meter to measure the actual ohms (resistance) of a 600 ohm transformer found on an analog phone line card, you will not get a correct reading as your ohm meter is injecting a DC current and only measures the DC resistant in the transformer.
I will touch on DC current in Ohm’s law and short primer on DC current)
Impedance is variable on the wire and is dictated by the frequency that is applied to the conductor.
This brings me to an important concept to understanding analog transmission over any media (this concept also applies to digital circuits, which will be discussed later) which is characteristic impedance.
The transmission of a wave on a transmission medium is typically called an incident wave. The incident wave is more applicable to the transport of radio frequencies because an audio signal can have longer wavelengths than the actual physical length of the tip and ring wire pair it is transmitted over. A short wire connection over the wire pair is really a mere hook-up wire to continue the audio signal circuit. But characteristic impedance is a factor at the high end of the audio range over long distances such as a telephone circuit. But as audio frequencies approach or are in the RF range, the propagation of the wave is actually dependent upon the collapsing field of inductance that literally pushes along the incident wave. Again as the magnetic field is created by the positive swing of the applied wave, it actually uses the collapsing wave during the negative swing to push that wave along in time.
When an AC current is applied to a wire pair or coaxial cable both mediums will electrically represent continuous segments of inductive and capacitive circuits that the Incident wave traverses. As long as the impedance is the same within each of these inductive and capacitive circuits that represent the Dielectric and construction of the wire pair, maximum power transfer is made. But when the incident wave encounters an impedance mismatch only part of the incident wave is absorbed and the rest is reflected back.
In the case of the hybrid if the mismatch occurs in the balancing circuit then the incident wave from the origin is reflected and leaked out the transmit pair going back to the origin, which creates Talker echo that is of an electrical origin.
Another type of Echo is Acoustic Echo which occurs when audible energy from a handset, headset or a speakerphone is feedback into the microphone of the same device.
When a speakerphone sits on a table made of wood material that can amplify vibrations, particularly the vibrations from the speaker of the speakerphone.
Comparing Talker echo to Listener Echo
Talker Echo is defined as a caller who hears there own voice echoed back to them.
Listener Echo is when a caller hears the other person’s words repeated.
Both can be a result of impedance mismatch creating reflective waves
In Figure 9 the reflective wave starts in the reverse at the. It will be 180 out of phase to the incident wave, thus neutralizing most of the amplitude of the incident wave. After that it will move closer to being in phase with the incident wave until it is. At that point, a resultant wave, the sum of both waves, will exist at nearly twice the amplitude. The resultant wave is known as a standing wave.
STANDING WAVES AND VOLTAGE STANDING WAVE RATIO OR VSWR
Standing waves will happen but are negligible if the ratio between the incident wave and the reflective wave is favorable to the incident wave. The ratio is known as the voltage standing wave ratio, or VSWR. Basically the ratio goes something like this: The incident wave voltage defined as Vi, is over the reflective wave known as VR, or
This is actually equal to the reflection coefficient. A coefficient, as defined by the Thorndike-Barnhardt dictionary, is a thing that unites in action with something else in producing an effect or result. It is also a cofactor. Luckily none of this is on any converging technology exam.
An accurate measure of how good the VSWR is comes from measuring the structural return loss, or SRL. This is not so much an issue in telephone tip and ring lines but as telephone transmission progressed Telephone Engineers developed the first coaxial cables where they started to stack voice channels one on another using Frequency Division Multiplexing (will cover that in more detail later in this chapter. When Cable TV came on the seen many of the large cable TV companies make it mandatory that the SRL and response are checked before the cable is released to construction crews. SRL is measured in decibels, which is discussed in more detail later. SRL gives a good indication of how high the loss is in the medium for reflected voltage. The higher return loss, the better the VSWR is.
Balanced and unbalanced transmission media, Shielded Twisted Pair (STP), and coaxial cable
As mentioned early single wire with earth return the concept of a balanced circuit was adopted.
The real advantage of twisted pair is that it can be used as a balanced circuit, as depicted Figure 11 a balanced line is a circuit that is not directly tied to an earth ground. The wave travels between the positive in1890’s with the failed (Tip) and negative (Ring) conductors and each end terminates into 600 ohm transformer. One advantage of the balanced transmission line is differences in ground potential can be eliminated. With a phone line being a balanced feed, both the tip and the ring conductors are equally balanced to ground. Any imbalance will introduce hum and noise to the phone line and increase susceptibility to RFI.
This type of circuit is balanced using Unshielded Twisted Pair or UTP.
Shielded Twisted Pair or STP. The negative side of the sine wave does not have as negative a value as earth ground. This is important when frequencies exist that can interfere with signals transmitted on the pair. For example, a 60-hertz hum from power transformers used in lighting and other equipment can induce a hum on a phone line or crosstalk from another wire pair. The use of the optional shield depicted in Figure 12 can solve that problem when cable with a dense wire braid or a foil shield is used. These cables typically will have a drain wire in the shield that can be tied to the ground on one side of the cable. The interfering frequency will hit the shield and travel on the outside of the shield to ground.
Using Shielded Twisted Pair is a requirement of NEBS Level 3 certification
One Caveat about STP is that the shield can also cause problems on long runs as capacitance will develop between the tip and ring pairs and the shield. The shield can couple with either tip and induce some of what it is trying to eliminate. In outside plant over all shield is bonded to ground at pedestals and poles to overcome this problem. In this day and age the best media to use on long runs is Fiber Optic cable.
1. E Common Mode Signals and Common mode rejection accomplished with balanced circuit
What is a common mode signal?
One of the difficulties with running a lot of cables is a common mode signal that is present on more than one connection
When referenced to the local common or ground, a common-mode signal appears on both lines of a 2-wire cable, in-phase and with equal amplitudes. Clearly, a common-mode signal cannot be present if one of the lines is connected to local common. Technically, a common-mode voltage is one-half the vector sum of the voltages from each conductor of a balanced circuit to local ground or common. Such signals can arise from one or more of the following sources:
• Radiated signals coupled equally to both lines,
• An offset from signal common created in a driver circuit
• A ground differential between the transmitting and receiving locations.
A common mistake is to tie the shield to ground on both ends—that has the potential for a ground loop that can cause more problems. RS 232 serial cables (which will be covered later) are particularly susceptible to common mode signals. With an unbalanced line the negative conductor is tied to an earth ground, thus the swing of a sine wave becomes the same polarity as earth ground. The use of an Isolation transformer and a balanced pair can usually solve this problem. In the case of an RS-232 connection converting to RS-422, 485 which has a wire pair for every signal corrects that problem.
1. F Unbalanced transmission media, coaxial cable and Frequency Division Multiplexing
Bell engineers started the development of what is commonly called modulating a signal, are stacked one upon another in channel. They called it FREQUENCY DIVISION MULTIPLEXING or FDM, which is simply nothing than taking a 40 to 4000 HZ voice channel (the actual pass band is 300Hz to 3,400Hz within the 4kHz channel slot) and modulating it to a higher channel frequency assignment. An example would be channel one being the baseband frequency of 40-4000 HZ, Channel Two would start at 4000 HZ to 8000 HZ; Channel Three would be from 8000 HZ to 12000 HZ. This would continue until it reached 96000 HZ, which would give a 24 two-way voice channel over two-pair wire one pair for receiving and one pair for transmitting. This is purely an analog transmission and is subject to problems that Analog circuits have and that is the noise and other distortions that are induced every time the signal needs to be re-amplified.
But it was discovered that Coaxial cable could transport the signal further than the balanced twisted pair. Before World War II FDM applications were put coaxial cables and after the war interruption AT&T developed FDM using Microwave Radio transmission which was cheaper to deploy.