Voice frequency sound and POTS

When we speak, we make the air vibrate from a few vibrations a second to many hundreds or even thousands of vibrations per second. These vibrations are measured in what is now called Hertz. Example: 300 Hertz or 300Hz. These vibrations result in changes in air pressure and cause a carbon transmitter to vibrate. This carbon transmitter changes the electrical resistance of the carbon and creates an alternating current and the later return into sond waves is the basis of the telephone. The human voice covers a rage of about 100 to 9000 Hertz. The voice frequency band in the telephone transmission is only 200 to 3200 Hertz. The communication spectrum is only a small part of the spread of frequencies traveling oven some type of transmission medium.

These could include:
wire
microwave
lightguide (example: fiber optic) cable

As frequencies become higher, the number of circuits (channels) that can be carried over the facility becomes higher and results in a high capacity circuit.

Multiplexing

Subscriber carrier systems can use either analog or digital methods to combine many different users before placing them on a transmission medium. The combining technique is known as multiplexing. Each user's signal is modified in some way so that it can be separated from all the other signals at the opposite end of the transmission path.

Analog vs. Digital

Analog: The analog signal carries it's information by means of a continuous varying signal that varies in frequency to represent the input signal. The disadvantage with the analog signal is the cumulative noise. The analog signal is reduced in strength or attenuated as it travels down a copper wire. To overcome this attentuation of the signal, repeaters are spaced along the transmission path about every mile. The purpose of this is similar to the return transmitted signal back to it's original value. However, as the signal travels down the transmission path, it picks up noise and distortion. This noise and distortion becomes inseparable from the signal and builds up from one repeater to the next.

Digital: Because of this noise problem, the emphasis in transmission systems has shifted from analog systems to digital systems. This is because digital systems are more favorable to the loop plant because the operate over wire, digital radio, and fiber optic cable. The digital signal has an advantage too. It also is attenuated as it travels along the transmission medium. When it is received by a regenerator, the regenerator merely has to judge if the threshold voltage is exceeded and inerts a pulse. This brand new pules now no longer has any accumulated noise or distortion. The advantage of digital systems, then, is that they do not amplify the signal and noise, but instead use a regenerator to generate a new pulse that is noise-free.

Facilities: A pair gain system combines telephone conversations and transmits them over a single cable pair. Two cable pairs are required for completing a two-way conversation: one for transmitting and one for receiving.

Pair Gain System Overview

The typical pair gain system consists of two terminals, a Central Office Terminal (COT), a Remote Terminal (RT), and the connecting digital lines. The pair gain system transmitting terminal combines 24 subscriber signals into a single digital signal, then transmits this signal to the receiving terminal. At the receiving terminal, this digital signal is then separated back to the subscriber signals. These terminals are connected by the T1 digital line. To maintain an acceptable signal level for the receiving terminal, line repeaters are placed at approximately 6000 foot intervals to regenerate the signal. Several types of pair gains systems are being used by the telephone companies.

Pair Gain System Terminals

The main unit of each pair gain system is the terminal. Each terminal contains channel inputs the accept Subscriber, Coin, or Special Service lines. Normally, 24 subscriber lines could be busy (transmitting information) at the same time and would be connected to a transmit and receive cable pair. This is considered an unconcentrated mode when 24 channels can be busy at the same time. Some pair gain systems provide the option of operating in concentrated mode. In this mode, 48 channels can operate over one T1 line. However, only the first 24 channels to request service will be serviced. Each terminal contains common units that are required by the pair gain system for, system timing, protection switching, multiplexing and demultiplexing and transmission line interface.

Each pair gain system contains a method of providing internal timing. This timing allows the transmitting terminal to mix the 24 channels into one stream of intelligence. At the RT, the system's internal timing permits the unmixing of the stream into 24 channels again.

Most pair gain system provide an optional feature for protection switching. This switching is activated when the receiving terminal receives a failure and switches the failed line to a backup line. This backup line is know as the Protection lime.

A pair gain system contains a common unit for each T1 line that combines 24 channels into a single bit stream for transmitting to the RT. Each VF (Voice Frequency) signal is converted into pules that represent the original signal, and sent to a common unit. An internal part of this common unit contains a timing circuit that arranges the 24 signals into one signal. This signal is now known as a uni-polar (one direction) PCM (Pulse Code Modulated) signal. This PMC signal is first monitored by the Line Switch Unit (LSU) for failure and then sent to the Line Interface Unit (LIU). The LIU now changes the PCM signal into a bi-polar signal for transmission over the T1 line.

The pair gain system will also contain a common unit for each T1 line that will separate the received digital signal into 24 subscriber channels. This is usually accomplished by the same unit.

This unit provides the interface (connection) to the T1 line. It also converts the digital signals to its final format, and in some pair gain systems it provides the regulated line current.

Digital lines, also called T1 lines, connect each shelf in the COT to a corresponding shelf in an RT. One cable pair is used to make the transmitting connection and the other to make the receiving connection.

Line repeaters are located along the digital lines to recreate the original signal being transmitted. The interval at which these repeaters are located is determined by the wire gauge. For example, 22 gauge wire requires a repeater spacing of no more than 6,000 foot intervals. Smaller gauge wire requires shorter spacing. The digital signal is accepted, regenerated and passed on to the next repeater until it reaches the opposite terminal.

Speech impulses that are transmitted along subscriber lines are called Voice Frequency Signals (VF). These signals are put into the pair gain system throughout the associated VF circuits. VF signals which enter the pair gain system are transformed into digital signals at the Digital Signal Level One rate (DS1 rate = 1.544Mb/s). These are transmitted along digital lines to the opposite terminal, where they are changed back into voice frequency signals.

Each channel will compete for access to the carrier system simultaneously (24 channels = one shelf/Digigroup). The shelf cannot accept the VF signals from all the channels continuously and simultaneously. Therefore, it must sample, or take part of the VF input from each channel in a fixed order. This is accomplished in much the same way as a movie camera captures a frame of action that when all played together at sample speed, simulate the actual motions in part. When we watch a movie we see frames flashed at high speed giving us the perception of fluid movement. The same is true with digital sampled signals. If the sampling rate is high enough (2 times the highest frequency to be sampled), it is possible for the receiving equipment to reconstruct the original VF signal form the sampled signal. The signal will no longer look like the original VF signal, but will be a series of pulses whose height will be the same as the original VF signal. The VF signal that has been divided into a series of short pulses is referred to as having been Pulse Amplitude Modulated (PAM). The VF signal from the channels now consists of a series of PAM pulses. Instead of attempting the impossible task of transmitting the exact amplitude of a sampled signal, the VF signal is represented by a discrete and therefore limited number of signal amplitudes. This is called quantizing. The PAM sample is matched with 254 segment scale in order to measure the pulse height. The error introduced by the signal not being exactly at segment scale location is called quantizing error, and results in noise in the reproduced signal.