Signals

1.1 Signal Wave Elements

All types of telephones use signals to transfer information and control the exchange of data. A signal, or signal wave, is the main element responsible for carrying information from a transmitter to a receiving medium. It is the energy that causes a circuit to perform its intended action, be it transmission or reception of messages and information.

A telephone is comprised of two major components: a transmitter and a receiver. Information is transferred from the transmitter to the receiver in the form of electromagnetic waves or pulse patterns, which travel through a transmission medium. These pulse patterns are called waveforms or signal waves. These signal waves, while travelling through space, can take on different shapes and lengths. They can be sinusoidal or sine, triangular, square, or sawtooth waves.

Like any other electromagnetic waves, the signal waves of telephones are comprised of several elements. These elements control the efficiency and intensity of the information being transferred. Signal wave elements include wavelength, period, velocity, frequency, and amplitude. These elements are shown visually in Diagrams 1.1(i) and 1.1(ii), respectively.

The elements shown in Diagrams 1.1(i) and 1.1(ii) are defined as below:

i. Amplitude -The maximum displacement of a wave from its equilibrium position.

ii. Period -The time takes to make one complete wave cycle.

iii. Frequency -The number of wave cycles made in one second

iv. Wavelength -The distance between two consecutive points on a wave that propagates at the same phase

Many may think that signals existed only after the advent of electronic communication and the invention of various types of telecommunication devices, but they are mistaken. In fact, the use of signals can be traced back since the long distance communication among people began. Signals have always been a crucial part of communication, particularly communication over long distances. In the past, signals come in the form of smoke, sound, and drawings. Sending notes using birds as well as lighting up fires at night were also widely used. In today's modern society, these manners of sending signals are long gone, replaced by more modern methods. With the advent of new breakthroughs in technology that started with the invention of the first telephone in the 1800s up to the time of contemporary telecommunication systems in this new era, signals are now classified as analog or digital.

1.2 Analog Signal

Analog signal is a type of signal wave that is common in most sounds. It is continuous and contains time-varying quantities. The changes of its physical properties and the different values in its waveform have a smooth transition, producing sounds that have no breaks or interruptions. Data from analog signal waves are transferred by means of analog communication channels, such as a wire or cable line media and radio frequency waves. Diagram 1.2 shows a coherent analog signal graph.

Analog signal is commonly used in a variety of applications, the most common of which include the following:

i. Frequency Modulation (FM) radio broadcasting

ii. Aerial television broadcasting

iii. Walkie-talkie or handheld transceiver communication

iv. Amateur radio or ham radio

v. Sound Navigation and Ranging (Sonar)

vi. Radio Detection and Ranging (Radar)

1.3 Digital Signal

Digital signal is a type of signal wave that is transmitted as a pattern of bits. It consists of pulse trains that rapidly change between two levels of intensity. The digital signal can be one of two values, such as 1V for maximum levels and 0V for minimum levels. Unlike analog signal, digital signal is disrupted and has a stepping appearance. It may also be square and discrete. The discrete appearance is due to the signal trying to approximate values. At a distance, the stepping appearance may not be readily perceptible and the pulse variations of the signal wave may look analog and smooth. Up close, though, the discrete steps of the signal may become visible. Diagram 1.3 shows a digital signal graph with continuous pulse variations.

Digital signal is also used in a number of varied applications. These include Auto Tele Machine (ATM), Global Positioning System (GPS), Electronic mail, and E-Commerce.

1.4 Comparison Between Analog and Digital Signals

Both analog and digital signals are transmitted electronically through a transmission medium. Their differences lie on their varied characteristics, as discussed below:

i. Analog Signal

a. Appears as a continuous wave form with variable amplitude.

b. Vulnerable to noise interference during the transmission process

c. Restoration of the original signal transmitted is not feasible.

d. Information is presented in sine wave form.

ii. Digital Signal

a. Information is represented by square waves and transmitted in discrete values as 0 and 1.

b. Can be immune to noise during the transmission process

c. Allows easy restoration of original signals

Signaling

2.1 Overview of Signaling

Signaling is the process of sending signal throughout networks. This process is an important aspect in the telephone industry and telecommunications in general. Without signaling it would be difficult to transfer information from one point to another. In a nutshell, signaling can be said as nerve of any telecommunication networks, particularly in the complex mechanisms of telephones.

Signaling is basically the progression of a moving element from one place to another. In the context of telecommunications, the moving element can be voice or speech transmitted through a wired or wireless communications channel. A standard signaling block diagram is shown in Diagram 2.1

In a standard telephone network, signaling is the process of exchanging information for the purpose of establishing, maintaining, and terminating telephone call connections. Three categories of signaling are used in telephone networks. These are the following:

i. User to network node signaling.

a. A type of signaling used to ensure the standby, establishment, and active status of telephone calls.

ii. Inter-network node signaling

a. A type of network signaling used for establishing and controlling the exchange of information among various networks.

b. It is used to oversee time allocation, supervise billing, and improve organizational management.

iii. User to user signaling

a. A form of signaling employed for special operations, such as in Dual Tone Multi Frequency Signaling (DTMF) and Unstructured Supplementary Service Data (USSD) code.

2.2 In-Channel Signaling

In telecommunications, particularly in a public switched telephone network (PSTN), in-channel signaling is the process of sending control and signaling information within the same channel. Known also as in-band signaling, this method encodes and transmits telephone numbers as Dual-Tone Multi-Frequency (DTMF) tones. With the help of the DTMF tones, this process also provides inter-exchange telephone companies instructions on how to route the calls. Below is a visual model of in-channel signaling, where both voice and signaling information are transmitted or sent simultaneously within the same channel.

Since the path of voice information and signaling information are the same, certain problems may occur. For instance, signaling may interfere with voice data during active voice calls or it may clog the path of the voice information being sent. This occurrence can result to congested signals, busy tones, or incomplete calls. Sharing of the path by voice and signaling information can ultimately slow down the call setup.

2.3 Common Channel Signaling

Common channel signaling (CCS) is a transmission method that follows a process different from that of in-channel signaling. In this type of signaling voice information and signaling information each will have a dedicated and separate channel for encoding and transmitting. A visual illustration of common channel signaling is shown in Diagram 2.3.

Unlike in-channel signaling where the call set-up may be slow because of the clogging of paths, the transfer of information in CCS is faster and simpler. There is also no interference of voice signals in this type of signaling. Furthermore, the call setup and switching processes are done effectively by maintaining the privacy of information throughout the transmission process.

Signaling System Number 7 (SS7)

3.1 Overview of Signaling System Number 7 (SS7)

Signaling System Number 7 (SS7) is a type of common channel signaling system that was first developed and used in 1983. This set of telephony signaling protocol was primarily created to replace Signaling System Number 6 (SS6), the first international CCS protocol defined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). Both the SS6 and SS7 were called Common Channel Signaling systems or Common Channel Interoffice Signaling systems (CCIS) because of their ability to efficiently separate signaling and bearing channels. Signaling System 6 though, was not responsive to digital systems and had limited function with its 28-bit signal unit. The creation of SS7 improved the speed of signaling and enhanced the holding time of the bearer channels.

The primary use of SS7 is to synchronize call setups, call routing and call management processes in a telephone network. Unlike its predecessors, it uses the outbound signaling method, which uses a dedicated and separate channel for signaling and voice transmission during voice calls. Because of the presence of dedicated channels, outbound signaling makes routing call and remote network management more efficient. Due to its efficiency and reliability, SS7 is now being used as the primary mechanism for new installations worldwide. It is utilized as the interoffice signaling protocol for Integrated Services Digital Network (ISDN) and for other standards outside of ISDN. A visual of model of the SS7 network is shown in Diagram 3.1.

Signaling System No. 7 is made up of two essential planes. These are the control plane and the information plane. These planes serve different functions, which are vital to the mechanism of SS7. The functions of each plane are explained below:

i. Control Plane

a. Control plane is responsible for the establishment, preservation, and management of call connections as requested by the user.

ii. Information Plane

a. Information plane is specifically designed for the exchange of information between users via multiple switching centers.

The control plane is comprised of three important elements, each of which has dedicated functions. These elements include:

i. Signaling Point

a. Signaling point is the network node that directly links to the switching center.

ii. Signal Transfer Point

a. Signal transfer point is responsible for routing the incoming signaling information to the necessary destination.

iii. Signaling Link

a. Signal link is the link that interconnects each of the other signaling points in SS7.

Since the development of the SS7 technology, many advantages have been experienced in the field of telephone communication. Signal System Number 7 has made a big transformation in the quality and productivity of call setups.

Bandwidth and Data Rate

4.1 Bandwidth

Bandwidth refers to the capacity of a transmission channel to carry information and transfer it from one network to another. It is the measure of the maximum amount of communication data can be sent from an individual channel within a particular moment. In the field of telephony signaling, particularly in analog communication systems, bandwidth is measured in Hertz (Hz). A visual illustration of bandwidth in an analog transmission channel is shown in Diagram 4.1 (i).

In the digital communication system, bandwidth is being measured in bits per second (bps). A visual representation of bandwidth in a digital transmission channel is shown in Diagram 4.2 (ii).

When excessive information is being sent, the bandwidth may become overloaded. This will limit the rate of information being transferred and lead to the delay in the transmission of information. To understand this better, let us take the diameter of a water pipe as an example.

Let us assume that the diameter of the water pipe is the bandwidth and the volume of water that goes through is the information being transferred. The bigger the size of the pipe, the more water will be able to flow through it within a given moment. The smaller the pipe, the lesser amount of water will be able to flow through it. The bandwidth in telecommunication is much like the pipe. The bigger the bandwidth, the more information will be able to go through it. The lesser the bandwidth, the lesser amount of information will be transmitted.

4.2 Data Rate

Data rate refers to the speed at which communication data is transferred between telecommunication networks. The volume of digital data transferred or handled by a particular network is measured by the quantity of bits that pass through it. Data rate is measured in bits per second (bps). A visual illustration of data rate in a digital transmission channel can be seen in Diagram 4.2.

Gain and Loss of Signal

5.1 Gain and Loss of Signal

In telephony signaling, gain of signal refers to the increase in signal strength during a particular transmission process. Known also as signal amplification, it pertains to a signal's increase in power or amplitude as it is being transferred from the source into the intended destination. Signal gain is measured in logarithmic decibel (dB) units. Diagram 5.1[i] shows visual representation of a normal signal and an amplified signal, respectively.

Loss of signal (LOS), on the other hand, means the reduction of signal strength during a particular transmission process. Known also as signal attenuation, it can be stated that the connection between telephone networks is gradually losing. Loss of signal can be due to a variety of reasons, including range problems, improper network configurations, and interference to name a few. A visual representation of a normal signal and an attenuated signal is shown in Diagram 5.1[ii]

5.2 Signal Amplifier

A signal amplifier or signal amp is basically an electronic device that is used to boost the strength of an analog signal, keeping it from attenuating during a particular transmission process. It is also known as a signal booster. Diagram 5.2 shows a visual representation of an analog signal amplification process using a signal amplifier.

5.3 Signal Repeater

A signal repeater is a device used to regenerative signals at a relatively higher power or higher signal than their original forms. A signal repeater is used to recover digital signal from impairments by regenerating the digital signal and retransmit to reach its extended destinations. A visual representation of a digital signal being regenerated using a signal repeater is shown in Diagram 5.3.