Carrier Systems : Plain Old Telephone Service (POTS)

Carrier systems are a combination of a transmission medium, specification of signal types and levels along with specific protocol controls (communication rules). The types of carrier systems range from simple audio telephone POTS carriers to high-speed optical carrier (OC-x) transmission systems.

Plain Old Telephone Service (POTS)
Plain old telephone service (POTS) is a transmission system that is used to provide basic telephone service. It is the common term used for residential telephone service.

Between the late 1800’s through the 1990’s, telephone transmission had remained basically the same. Acoustic energy from the customer was converted to electrical signal by a microphone in a handset. This electrical energy was applied through a hybrid electrical device to the telephone line through the speaker in the handset. A telephone hybrid device (often called a “magic” device by telephone personnel) transferred energy from the microphone to into the telephone 2-wire line while extracting most the remote microphone energy and applying it to the speaker. At the same time at the other end of the connection, the same process was occurring.

Figure 1 shows a hybrid telephone transmission system. This diagram shows that microphone energy from user #1 is added to the 2-wire transmission circuit via the hybrid assembly and the microphone energy from user #2 is subtracted from the circuit by the speaker #1. At the same time, the microphone energy from user #2 is added to the 2-wire transmission circuit and speaker #2 subtracts the microphone energy from user #1. This hybrid process allows 2 wires to contain a composite of both microphone signals.


Figure 1: Hybrid Telephone Transmission

Transmission Medium Limitations

Some of the limitations of transmission lines that reduce their ability to transfer analog and digital information include limited frequency response of the transmission lines, crosstalk, noise from external sources that cause distortion, non-terminated tap lines, and signal attenuation that results from line splices and line resistance.

Frequency Response
The twisting of copper wire pairs provides good frequency response for low frequency audio signals. Unfortunately, twisted copper wire pairs are not specifically designed for high frequency transmission. Analog signals have a frequency range of up to 3.4 kHz and most of the DSL technologies use frequencies up to 1.1 MHz. As the frequency applied to the copper wire pair increases, the attenuation of the line increases and signal energy leaks (emits) from the wire pair.

Figure 1 shows the typical frequency response of a twisted pair of copper wires. The frequency response depends on a variety of factors including the dimension of the copper wire (gauge), insulation type and installation environment (twisting or stapling of the wire).


Figure 1: Frequency Response of Copper and Coax Wire

Crosstalk (Signal Leakage)
Crosstalk is the undesired coupling of a signal from one communications channel to another. Crosstalk occurs when some of the transmission signal energy leaks from the cable. This leakage is called signal egress (emission from the line).

Crosstalk on communication systems can be divided into two categories: near end crosstalk (NEXT) and far end crosstalk (FEXT). Figure 2 shows two types of crosstalk. NEXT results when some of the energy that is transmitted in the desired direction seeps into one (or more) adjacent communication lines from the originating source. FEXT occurs when some of the digital signal energy leaks from one twisted pair and is coupled back to a communications line that is transferring a signal in the opposite direction. Generally, NEXT is more serious than FEXT as the signal interference levels from NEXT are higher.


Figure 2: FEXT and NEXT Crosstalk

Signal Ingress
Signal ingress occurs when electrical signals from other sources (such as radio or lightning spikes) enter into the transmission line. Figure 3 shows a source of signal ingress from a nearby radio tower that may occur in a transmission system. This diagram shows that a high power AM radio transmission tower that is located near a telephone line couples some of its energy onto the telephone line. This interference signal (radio ingress) usually reduces the data transmission capacity of a digital subscriber line (DSL).


Figure 3: Radio Signal Ingress

Bridge Tap Reflections
A bridge tap is an extension to a communication line that is used to attach two (or more) end points (user access lines) to a central office. Bridge taps provide connection options to the telephone company on connecting different communication lines to a central office without having to install new pairs of wires each time a customer requests a new telephone line.

The connection of one (or more) bridge taps on a communication line that is used for plain old telephone service (POTS) does not usually cause signal distortion. However, unterminated bridge taps that are installed on communication lines that transfer high frequency DSL signals can result in signal distortion. The signal distortion comes from the reflections of signal energy reflections off the bridge taps.

When an electrical signal is applied to the end of a copper wire, electrical energy begins to travel down the copper wire. Ideally, when the energy reaches the end of the copper wire, the signal is absorbed at the other end (called a matched line). If the end of the wire is not connected, some (or all) of the energy is reflected back to the beginning of the line.

Figure 4 shows how reflections from bridge line tap can cause distortion. This signal shows that some of the energy from the bridge tap is reflected back to the communications line. This reflected signal is a delayed representation of the original signal. Typically, bridge taps must be removed from communications lines that use DSL technology.


Figure 4: Bridge Tap Reflections

Loading Coils
Loading coils are sometimes used to adjust the frequency response of a communication line to better transfer audio signals. While these loading coils work well for specific types of signals (e.g., audio signals), they can disable the ability of the line to be used for other types of signals (e.g., high frequency DSL signals.)

Figure 5 shows that there may be several installed audio loading coils on a single local loop line. Although these loading coils improve the audio frequency response, they must be removed to allow for high-frequency transmission for systems such as DSL.


Figure 5: Audio Loading Coils

Line Splice Attenuation
Telephone cables usually come in 500 feet roles. Because most telephone lines are several thousand feet from the central office, several cable splices are required. Each line splice attenuates the signal and the amount of signal attenuation varies depending on the type of splice (solder, twist, or pegs) and the amount of corrosion inside the splice.

Because the average distance for local access lines is over 10,000 feet, there are more than 20 splices in the average local access loop. Each of these splices offers the potential for corrosion and increased resistance.

One method that is used to decrease the effects of corrosion (and reduce the attenuation) is to continuously run electric current through the copper wire pair. This “sealing current” is a small amount of direct current that is passed through a copper wire to reduce the corrosion effects of the splice points. The sealing current effectively maintains conductivity of mechanical splices that are not soldered. The direct current effectively punches holes in the corrosive oxide film that forms on the mechanical splices.

Line Resistance Attenuation
The copper cable also has resistance (impedance) that is dependent on the size (diameter) of the cable. The resistance of the copper wire increases as the diameter decreases (gauge number increases). The higher the line resistance, the more of the signal energy is dissipated by the line and less energy is transferred to the receiving device.

Figure 6 shows how line resistance attenuation and the wire size decreases. This diagram shows that cables with larger diameter copper wires are typically used to in the distribution system. As the distribution system nears its destination, the size of the wire often decreases.


Figure 6: Line Resistance Attenuation

Group Delay (Dispersion)
Group delay is the amount of delay a particular group of frequencies experience as they travel through a transmission medium. Because transmitted signals are composed of multiple frequency parts (e.g., high frequency components for rapid signal changes), the delay of some of the parts results in distortion of the transmitted signal.

Figure 7 shows how group delay can cause pulsed signals, such as in digital transmission system, can cause signal distortion. This diagram shows that a digital pulse signal is actually composed of many low, medium, and high frequency components. As the pulse is transmitted through the transmission line, some of the frequency components are delayed more than others. This results in a distorted pulse at the receiving end of the transmission line.


Figure 7: Group Delay (Dispersion)

Transmission Mediums : Surface Acoustic Wave (SAW)

A surface acoustic wave (SAW) involves the mechanical transmission of a wave that travels on the surface of a material. The wave is created by a piezoelectric transducer that converts electrical to acoustic energy into mechanical vibrations that are coupled (connected) to a material that allows the wave to propagate across the surface of the material. A second transducer (e.g., a pattern of metal fingers) converts the acoustic energy back into an electrical form. Depending on the shape and type of material for which the SAW travels, the signal characteristics can be changed (such as the passing or rejecting of particular frequency bands).

A fundamental use of a SAW device is to act as a signal delay line. The relatively slow propagation velocity of the surface acoustic waves of typically 3500 m/s allows delays of several microseconds on a small chip[1]. There are many variations of SAW technology including delay lines, filters, resonators, pulse compressors, convolvers, and many more.

The first SAW devices were introduced to the marketplace in the mid-1960’s. Before the year 2000, more than 5 million surface acoustic wave (SAW) devices were being installed in electronics assemblies each day[2].

Figure below shows a transmission system that uses surface acoustic wave (SAW) technology. This device is a radio frequency filter that uses SAW technology. The radio signal is applied to an IDT. The IDT converts the electrical signal to an acoustic (mechanical) wave that moves across the surface of the SAW filter substrate. When the acoustic wave reaches the destination IDT, it is converted back into an electrical signal.


Surface Acoustic Wave (SAW) Transmission

Transmission Mediums : Fiber Optic Cable

Fiber optic cable is a strand of glass or plastic that is used to transfer optical energy between points. The size of most fibers is from 10 to 200 microns (1/100th to 1/5th of a mm). Optical fibers are typically used in a unidirectional mode (e.g., data moves in only one direction). Because of this, every transmission system requires at least two fibers (one for transmission and one for reception).

For most fiber systems, the transmitting end-node uses a light amplification through stimulated emission of radiation (LASER) device to convert digital information into pulsed light signals (amplitude modulation). The light signals travel down the fiber strand by bouncing (reflecting) off the sides of the fiber (called the cladding) until they reach the end of the fiber. The end of the fiber is connected to a photo-detector that converts these light pulses back into their electrical signal form.

Optical fibers are often characterized by either single mode or multimode transmission. Single mode of fiber transmission only allows a specific narrow wavelength of light to pass through the fiber. Multimode fiber transmission allows a much wider wavelength of light to pass through the fiber by gradually bending different wavelengths back towards the center of the fiber. Single mode fiber strands are very narrow with a fiber diameter of 9-10 microns (1/100 of a mm). Multimode fibers are much wider as they can have a fiber diameter of 50-125 microns).

Figure below shows single mode and multimode fiber lines. This diagram shows that multimode fibers have a relatively wide transmission channel that allows signals with different wavelengths to bend back into the center of the fiber strand as they propagate down the fiber. The diagram also shows that single mode fiber has a much small transmission channel that only allows a specific wavelength to transfer down the fiber strand.


Single and Multimode Fiber Lines

Because of the relatively wide frequency bandwidth (and high data-transmission rate), multimode fibers are predominantly used for high-speed short runs such as those occurring within a building or around a campus.

Typical multimode fiber runs are less than a mile, but can be several miles (2 – 5). Because of this, it is often referred to a “short haul” fiber solution. Single mode is a “long haul” fiber solution (50 – 75 mile runs). Single-mode fiber has been used by telephone companies and long distance carriers for several years to off-load expanding requirements from traditional terrestrial microwave.

Single-mode fiber can transmit much further than multimode fibers. Single-mode applications use a diode laser as the light source, while multimode uses a light emitting diode (LED). Use of a LASER ensures a high energy at a very narrow optical bandwidth as compared to the LED that has optical energy distributed over a wide optical bandwidth. Single mode fibers use a narrow glass filament with a diameter of approximately 10 microns compared to multimode where the diameter ranges from 50 to 125 microns. The narrow channel of the single mode fiber minimizes the bending of the wave and this results in less dispersion (less smearing of the pulses) over distance.

Figure below shows a fiberoptic communication system that is composed of two end-nodes and a fiber optic cable transmission medium. This diagram shows two optical network units (ONUs) that connect data networks together using fiber cable. This diagram shows that two fiber strands are needed: one for transmitting and one for receiving.


Fiber-Optic Cable Transmission System

To increase the capacity of fiber systems, multiple optical signals of different wavelengths are combined on a single strand of fiber. This is called wave division multiplexing (WDM). When 40 (or more) optical signals are combined on a single fiber strand, this is called dense wave division multiplexing (DWDM).

Figure below shows how a wave division multiplexing over fiber operates. This diagram shows that there are several lasers operating at different optical wavelengths (different colors/frequencies). Each laser converts an electrical signal into a pulsed light signal. These optical signals (optical carriers) are combined by an optical multiplexer (lens) for transmission through the optical fiber. At the receiving end, the different optical carriers are separated by an optical demultiplexer (lens) and each optical carrier is sent to a photo-detector. The photo-detector converts the optical signal back into its original electrical form.


Wave Division Multiplexing

Transmission Mediums : Free Space/Air

Transmission in free space and air can be accomplished by radio or light signals. Free space/air transmission is the transfer of signal energy through an unobstructed medium. Free space transmission occurs in an ideal medium (vacuum) that is free of objects or particles that may disrupt the transmission of signal energy. Transmission of signals in air has similar characteristics as free space transmission. However, particles in the air result in signal scattering and absorption during transmission.

Free space transmission systems require a transducer to convert signal energy of one form into electromagnetic or optical energy for transmission. The transducer must also focus the energy so it may launch the energy in the desired direction. When air is the medium, particles in the air (such as water) may absorb or redirect (scatter) the transmitted signal.

Figure below shows two types of free space transmission systems: radio and optical. The microwave transmission system shows that some of the electromagnetic energy is absorbed by the water particles in the air. The optical transmission system uses a laser and photo-detector. The optical transmission system shows that some of the optical energy is scattered in other directions as it passes through smog and water particles.


Free Space Transmission System

In 1951, microwave radio transmission through free space became the backbone of the telecommunications infrastructure. Point-to-point microwave transmission systems have the data transmission capacity of hundreds of megabits per second. Although the extensive deployment of fiber optic cable has removed some of need for microwave radio systems, microwave radio is still used in places that are hard to reach or not cost effectively served by fiber cable.

In addition, microwave radio free space technology is the basis for satellite communications. In the commercial broadcast industry satellites are fed from terrestrial sites called “mother stations.” The mother station transmits up to the satellite on multiple frequencies called “uplinks”. Transceivers on the satellite, referred to as “transponders”, retransmit the signals back down to earth. These signals are known as “downlinks”. On the ground, satellite dishes (focusing antennas) receive the downlink signals and a radio receiver converts these signals back into television images and sound. Home Box Office (HBO) and ShowTime are examples of commercial broadcast companies that use satellite almost exclusively for distribution.

In developing countries and countries where the telecommunications infrastructure is of poor quality, radio, microwave, and satellite have been used to solve connectivity requirements in short order. Data radio and very small aperture satellite (VSAT) systems have allowed banks and other information-dependent companies to reliably connect to branch offices for the online exchange of information.

Since the mid-1980’s data radio has played a major role in the telecommunications industry in developing countries where the copper wire infrastructure is generally of substandard quality.

Modern optical transmission systems use infrared, and other laser optical signals to carry large amounts of information. Free space infrared and laser communication systems have been limited to span small distances of a few miles due to interference of the particles in air. Infrared systems have gained popularity because of their high bandwidth and ease of installation. Optical systems usually do not require government licenses or other authorization to use. These optical systems can be found connecting buildings on a campus and as supplements to wired LAN’s within an office or plant.

Transmission Mediums : Coaxial Cable (Coax)

Coaxial cable is a transmission line that is constructed from a center conductor that is completely surrounded by shield conductor. The center conductor and the shield conductor are separated by an insulation material. The shield can be inter-woven strands of wire or metal foil. Because the center conductor is complete surrounded by the shield, in an ideal coaxial cable, all the transmitted energy is contained within the cable.

Figure below shows a cross sectional view of a coaxial cable. This diagram shows a center conductor that is surrounded by an insulator (dielectric). The insulator is surrounded by the shield. This diagram shows that during transmission, electric fields extend perpendicular from the center conductor to the shield and magnetic fields form a circular pattern around the center conductor.


Cross Sectional View of Coax Cable

Coaxial cable is best known as the medium for cable television. It was primarily chosen because of its durability, wide frequency bandwidth capacity (often up to 1 GHz bandwidth), and less rigid length restrictions. Coax (as it is normally called) is often used in local area networks (LAN) to transport high-speed data signals with relatively high security (low signal leakage).

Twinax is a derivative of coax and is constructed as noted above with the exception that twinax uses two center conductors. Each center conductor is individually insulated, but, as with coax, each references the single shield for ground.

The first local area networks (LAN’s) were almost exclusively centered around coax as the network medium of choice. It became the standard for the early LAN’s. There are three common types of coax that are, or have been, in use extensively with many computer systems: thicknet, thinnet, and twinax.

Thicknet is often associated with the first Ethernet LAN’s and with high-speed bus cables used between mainframe computers and their peripherals. It is bulky and difficult to install but provides high speed and capacity where it is most critical. With respect to LAN’s, use of thicknet provides extra protection from electrical interference that may be encountered (e.g., such as on factory floors near assembly equipment).

For most LAN’s installed in the early 2000’s, UTP or STP is used. Coax is used in cable television systems and for interconnection trunks at telephone company switching centers. This provides for relatively high data transmission capacity (e.g., for DS3 45 Mbps transmission). Figure below lists the most frequently encountered types of copper and coax line and their approximate information transmission capabilities.


Copper and Coax Cable Information Capacity

Transmission Mediums

A transmission medium guides signal energy in a particular direction. The key different types of transmission mediums include copper wire, coax (a form of copper wire), free space (air), glass fiber, and mechanical (acoustic wave). There are also other types of transmission mediums such as radio waveguides and stripline that use the fundamental principles of the other types of transmission mediums to help direct transmitted energy in a particular direction.

The propagation in free space (a vacuum) is the speed of light 300 million meters per second. In free space, the propagation delay (time from entry to exit) is exactly 11.7 nanoseconds per foot (a nanosecond is one Billionth of a second). Any other medium other than free space will slow the transmission speed (introduce additional delay).

When transmitting energy through a transmission medium, some of the transmitted signal will be absorbed of some of the material. This absorption converts some of the transmitted signal into heat.

As signals transfer through a transmission system, some of the energy may leak out. This leakage (egress) results in a decrease the transmitted signal energy as it propagates from end to end.

Transmission systems have a limit on the frequency range that can be transferred through the transmission channel. Signals that are applied to the transmission medium that are above or below the maximum frequency range are rejected, severely attenuated, or leaked out of the transmission medium.

Copper Wire
Twisted pair copper wire is the most utilized telecommunications medium and thus has the largest installed base in the worldwide telecommunications infrastructure by far. It is relatively inexpensive, easy to install, and, locally available in quantities. Practically every residential telephone worldwide connects to the local telephone company via a twisted pair jack or block located on an inside wall or baseboard in the residence. Because of these factors, intensive research and development (R&D) funding continues to be allocated for the purpose of extending the usefulness of twisted pair copper wire. By enhancing its ability to carry information faster and farther, R&D will continue as long as the resulting enhancements meet, or exceed, many of the requirements placed on the industry by customer demands. Indications are that twisted pair copper wire will remain a logical, cost-effective medium for providing many commercial services for some time to come.

Figure below illustrates the transmission of electrical energy (electricity) via copper wire. Note that the electricity is conducted via the outer surface of the wire, not the inner. Consequently the greater the outer surface the more electricity that can be conducted. In other words, the larger the wire diameter the more outer surface there is to conduct electricity.


Electrical Transmission through Copper Wire

In the telecommunications industry copper wire is normally referred to as twisted pair. Through twisting wire into pairs electrical radiation (eddy fields or cross talk) is reduced. This reduction is both radiation off the pair and the susceptibility to radiation from other pairs and sources. It has been found that the tighter the twist (this is still in R&D) the less interference (cross talk) and the higher the speed at given error rate.

Twisted pair cables come in a variety of sizes and jackets (outer covering). Twisted pair cables are jacketed and may contain from 2-pair to several hundred pairs. Twisted pair wire comes jacketed in PVC or plenum-rated. “Plenum” is the name given to the non-toxic PVC-like jacket that is authorized by local fire ordinances for use in ceilings and walls considered to be “air-return”.

Twisted pair cables are produced as either shielded twisted pair (STP) or unshielded twisted pair (UTP). When a twisted pair installation is to occur in an area that has abnormally high levels of electromagnetic energy, STP is recommended. In most office settings UTP is the standard; however, even in such seemingly neutral environments there can be problems such as fluorescent lighting fixtures and parallel runs with electrical wiring. A good installation plan prepared by a certified wiring engineer reduces the likelihood of such problems and also enhances the probability that the final infrastructure will operate at the desired performance (to allow the desired data transmission speed).

In office or campus environments UTP and/or STP provide the wiring infrastructure for LAN’s, some host-based data applications, video, and voice. Central office lines are normally delivered as twisted pairs to the client telephone or computer systems. Such lines range from single station analog lines up to and including T-1 (US, 1.544Mbps) or E-1 (Europe, 2.048Mbps). Of course this includes the intermediate digital service known as digital subscriber line (DSL).

In addition, twisted pair cable that is to be directly buried in the ground has a special construction to prevent water seepage. Cables to be run overhead outside are constructed with an extra steel cable known as a strength member. The strength member supports the weight of the cable between the poles from which the cable is suspended. This prevents the cable from sagging and ultimately rendering it useless. Finally, the US government contracts for “tap-proof” cable that has a special outer conductor, located under the outer covering, that is capable of conducting as much as 2,000 volts.

Figure below lists the types of twisted pair cables and their rated capacities and maximum links. Copper wire cable is “Category” rated. This table shows that the category varies based on the design of the cable (shielded or unshielded), size (gauge) of the wire, and the types of insulation material used.


Copper Wire Pair Information Capacity

Simple Telecom : Technologies

Telecommunication transmission technology is often lumped into two categories: analog transmission or digital transmission. Figure 4.1 depicts a basic telecommunications transmission system that transfers digital information from one source to an information receiver. The information source is supplied to an end-node that converts the information to a form that can be transmitted through the transmission medium (air, copper, or fiber). The receiving end-node converts the transmission signal into a form that is compatible for the receiver of the information.


Basic Transmission System

Analog
Analog transmission is a process of transferring signals between end-nodes than can have many different signal levels and frequencies. Because analog signals can continuously change to many different levels (voltages) at changing rates (frequencies), the transfer of analog signals (such as an audio signal) requires the transmission medium to have similar transfer characteristics to all parts (levels and frequencies) of the transmission signal. Analog transmission systems must be robust to transfer the signal unaltered for specific voltage levels and frequency components (e.g., high and low frequency).

Figure below shows an analog transmission system. This diagram shows that an audio acoustic (sound) signal is converted by a microphone to an audio electrical signal prior to transmission on a copper line. This audio electrical signal is amplified by an end-node to increase the signal level for transmission on a copper wire. This amplification is necessary to overcome the transmission loss of the copper wire. As the signal progresses down the copper wire, some of the signal energy is converted to heat reducing the signal level. Another amplifier (the receiving end-node) is located at the receiving end to increase the signal to a level suitable for the information receiver (audio speaker).


Analog Transmission System

Digital
Digital transmission is the process of transferring information from node to node in a form that can only have specific levels (usually logic 1 and logic 0). Digital signals have a limited number of different levels (voltages) that represent digital information. Transferring digital signals (such as a computer’s data signal) only requires the transmission medium to transfer two levels without precisely (linearly) transferring levels in between the two levels.

Figure below shows a digital transmission system. This diagram shows a computer that is sending digital data (one equal to +5 volts and zero equal to 0 volts) to an end-node. The end-node is a channel service unit (CSU) and digital service unit (DSU) that converts the levels from the computer to levels suitable for the copper wire transmission medium (logic 1 = +5V and logic 0 = -5V). As the digital signal transfers down the copper wire, some of the energy is converted to heat and some of the frequency components are attenuated resulting in a slightly distorted (rounded) digital pulse arriving at the receiving end-node. Because the receiving CSU/DSU only needs to sense two levels, it is able to re-create the original undistorted digital signal (also known as digital signal regeneration).


Digital Transmission System

Simple Telecom : Basic Concepts

Most telecommunication customers are served by copper cable (twisted pair or coax) terminated by the local telephone company in a telephone network interface box, called a network termination (NT). The NT is normally located on the side of the building. The network termination isolates the network from the wiring inside the building. From the NT, the “inside wiring” extends the telephone cable to all internal wall and floor jacks.

To reduce the number of copper pairs, telephone systems use a hybrid transmission system to allow both transmission and reception on a single pair of copper wires. By combining both transmit and receive audio signals using a special hybrid combiner, only one-pair of wires is required to operate a standard home telephone. These two lines are routinely referred to as “tip and ring.” This single pair of wires also provides dial tone, dialing pulses or tones, ringing (high voltage signal), and a talk path.

Most of the information that is transferred in voice conversation occurs at frequencies below 3,300 cycles per second (Hertz or Hz) and above 300 Hz. This allows telephone systems to restrict the audio frequency range for voice grade circuits from 300Hz to 3300Hz. Using a restricted frequency range reduces the transmission line and system switching performance requirements. The limiting of the audio frequency range is accomplished through the use of devices known as band-pass filters. Band-pass filters strongly attenuate signal frequencies above and below specific frequencies.

It is possible to send digital information through the hybrid network through the use of a modulator/demodulator (MoDem). The MoDem converts digital signals into analog tones that can be transmitted on standard telephone lines.

Telephone transmission lines can be divided into access lines (local loops) and interconnection lines (trunks). Often referred to as 1FB’s or B1’s, local loops refer to all two-wire voice grade connections between a residence or place of business and the telephone company’s serving end office (e.g., where the dial tone originates). Interconnection trunks refer to high capacity groups of circuits connecting switching sites such as end offices or other switching centers.

Simple Telecom : Transmission Systems

Overview,
Transmission systems interconnect communication devices (end nodes) by guiding signal energy in a particular direction or directions through a transmission medium such as copper, air, or glass. A transmission system will have at least one transmitting device, a transmission medium, and a receiving device. The transmitting communication device is capable of converting an information signal into a form of electrical, electromagnetic wave (radio), or optical signal that allow the information to be transferred through the transmission medium. The receiving communication device converts the transmitted signal into another form that can be used by the device or other devices that are connected to it. Transmission systems can be unidirectional (one direction) or they can be bi-directional (two directions).

The basic types of transmission mediums include copper wire, coaxial cable, free space/air, fiber optic cable, and mechanical transmission line. Copper and coaxial wire is primarily used for low to moderate frequency transmission over a few miles. Free space/air systems can transmit hundreds of miles but have limited bandwidth and are susceptible to noise interference. Fiber optic cable is capable of carrying high-speed data signals (as light pulses) over thousands of miles. Mechanical (acoustic wave) transmission lines transmit over very short distances (only a few millimeters) and are used for signal filtering components.

Different types of transmission lines have varying performance characteristics and may be susceptible to interference during signal transmission. These characteristics include the available frequency bandwidth (frequency response), how much signal leakage may occur (cross talk), and the susceptibility of absorbing other signals (signal ingress). The construction of the transmission line itself may cause distortions in the transmitted signal. This includes unterminated line splices (bridge tap reflections), poor line splices, and line resistance (signal attenuation). Other characteristics such as varying delays to different frequency ranges may cause group dispersion (smearing) of the desired signal.

To allow devices to communicate with each other over a transmission line, carrier systems specify the signal types and levels along with specific protocol controls (communication rules). These carrier systems are often specific to the transmission medium such as copper or fiber. Some of the more popular carrier systems include plain old telephone service (POTS), digital signaling carrier (DSx), digital subscriber line (DSL), and optical carrier (OCx).

To coordinate the transmission line, signaling messages are sent between communication devices. Some of these control messages are sent along with the data on the transmission line (called in-band signaling) and others are sent through another path or network (called out-of-band signaling).

In some cases, a transmission path may only be a portion of a path (a logical path) through a transmission line. The length of a transmission may be extended through the use of amplifiers or repeaters.

Simple Telecom : Security

Security management of a network involves identity validation (authentication), service authorization, and information privacy protection. Authentication processes identifies the device or person that is requesting the use of the telecommunications device or network services. Authorization is the process of determining what services devices are customers are permitted to use. Privacy or encryption services are used to help ensure that the information transmitted or received is not available to unauthorized recipients.

Authentication
Authentication is a process during where information is exchanged between a communications device (typically a user device such as a mobile phone) and a communications network that allows the carrier or network operator to confirm the true identity of the user (or device). Validation of the authenticity of the user or device allows a service provider to deny service to users that cannot be identified. Thus, authentication inhibits fraudulent use of a communication device that does not contain the proper identification information.

Authorization
Authorization is the enabling of services to a device or customer that requests services. Authorization is often part of the billing and customer care (BCC) system and is maintained in a customer database service profile. Services are initially enabled in a network as a result of provisioning. Provisioning is a process within a company that allows for establishment of new accounts, activation, termination of features, and coordinating and dispatching the resources necessary to fill those service orders. Provisioning is usually part of customer care systems.

Networks sometimes use mediation devices to help manage provisioning and authorizing services to customers. A mediation device is device in a telecommunications network that receives, processes, reformats, and sends information to other formats between network elements. Mediation devices are can take non-standard proprietary information (such as proprietary digital call detail records) from switches and other network equipment and reformat them into messages billing systems can understand.

Information Privacy
Information privacy is a process of protecting transmitted or received information from being understood by unauthorized recipients. Information privacy typically involves encrypting of the voice signal with a shared secret key so only authorized users with the correct key and decryption program can listen to the communication information.

Encryption is a process of a protecting voice or data information from being obtained by unauthorized users. Encryption involves the use of a data processing algorithm (formula program) that uses one or more secret keys that both the sender and receiver of the information use to encrypt and decrypt the information. Without the encryption algorithm and key(s), unauthorized listeners cannot decode the message.

Telecom : Network Control

Network control is the transmission of signals or messages that perform call control, equipment configuration, or information management functions. Network control can be centralized or distributed. The control of public telecommunications networks is a centralized system as call processing is coordinated through a controlled common channel signaling (CCS) network. The Internet uses distributed control as the switching information dynamically changes in packet switching centers (routers) throughout the Internet network.

Common Channel Signaling (CCS)
Common channel signaling system #7 (“SS7”) is the primary system used for interconnection of telephone systems. SS7 sends packets of control information between switching systems. Figure below shows the basic structure of the SS7 control signaling system. The SS7 network is composed of its own data packet switches, and these switching facilities are called signal transfer points (STPs). In some cases, when advanced intelligent network services are provided, STPs may communicate with signal control points (SCPs) to process advanced telephone services. STPs are the telephone network switching point that route control messages to other switching points. SCPs are databases that allow messages to be processed as they pass through the network (such as calling card information or call forwarding information).


SS7 Common Channel Signaling

Because the public telephone network uses common channel signaling, intelligence in the network can be distributed to databases and information processing points throughout the network. A set of service development tools has been developed to allow companies to offer advanced intelligent network (AIN) services.

Telecom : Protocols

Protocols are the precise set of rules and a syntax that govern the accurate transfer of information within a communications network. Protocols are used within a communication system to establish, carry out, and terminate communication circuits. Protocols are also used to coordinate billing and customer care systems, manage network devices, and any other process that requires coordinated communication and control.

There are thousands of different protocols used in communications systems. Usually, protocols are grouped into families of protocols so they can serve specific types of networks and services. When interconnecting different networks, protocols need to be converted.

Protocol conversion involves the translation of the protocols of one system to those of another to enable different types of equipment, such as data terminals and computers, to communicate. This is done by an inter-working function (IWF). An IWF system (such as a data bridge) adapts the communications between two different types of networks. Protocol conversion may be used to interconnect circuit switched or packet switched networks.

Switching Systems

Switching systems connect two (or more) points together. These connections can be physically connected (mechanical switch) or connected logically (through software).

The first telephone systems performed the mechanical switching of calls by human operators. The operators interconnected telephone lines by manually connecting cables at switchboards. These switchboards contained many wires that had plugs and the switchboard had many sockets for the plugs. To interconnect telephone calls at long distances, one operator would have to call other operators to setup the call. Setting up calls could be a complex process and this process got more complex as many more telephones were installed.

Switching systems have evolved many times over the past 100 years. The types of switching systems that are still in common use today include crossbar, time slot interchange (TSI), and packet switching.

Crossbar
Crossbar switches used mechanical arms to physically connect to wires (or busses) together. These mechanical arms (“Crossbars”) connect horizontal and vertical bars together to connect input and output lines together. Magnets are used to open and close the crossbar switch contacts.

Figure below shows a crossbar switching system. In this example, there is a matrix of lines (busses) where each input line can be connected to any output line. When a connection needs to be made, a mechanical switch connects one of the busses with the other busses. The disadvantage of this system is that the number of mechanical switches for connecting each input port to an output port exponentially increases with the number of ports that require connection. For example, a switch with 10 inputs and 10 output lines requires 100 switches. A switch that has 20 inputs and 20 outputs requires 400 switches.


Crossbar Switching

Time Slot Interchange (TSI)
Time slot interchange (TSI) switching is a process of connecting incoming and outgoing digital lines together through the use of temporary memory locations. In the late 1960’s, mechanical crossbar switching systems began to change to TSI digital switching systems. A computer controls the assignment of these temporary locations so that a portion of an incoming line can be stored in temporary memory and retrieved for insertion to an outgoing line.

Figure below shows a TSI switching system. This diagram shows a simplified matrix switching system. Each input line (port) is connected to a multiplexer. The multiplexer places data from each port in time sequence (time slot) on a communications line (e.g., a T1 or E1 line). This time multiplexed signal is supplied to a matrix switching assembly. The matrix switching assembly core has two memory parts: a section that holds the pulse coded modulation (PCM) data and Control Memory - CRAM that holds switching addresses data.


Time Slot Interchange (TSI) Switching

The time slots (voice channels) from the incoming multiplexed sent through switch S1 to be sequentially stored in the PCM data memory. The data is later retrieved by switch S2 and placed on a specific time slot on an outgoing line. The outgoing multiplexed line is supplied to a de-multiplexer so each time slot is routed to an output port.

Packet Switches
Packet transmission is a mode of data transmission that divides messages or data into small increments (packets) that can be routed through a network. When the packets arrive at their destination, they are reassembled in the proper order to recreate the original message or data.

Packet switching can be connection based or connectionless. For connection based switching, a path through the network is established during call initiation and packets are continuously routed through the same path. For connectionless switching, each packet is given a destination address and the switching points in the network (switching nodes) assist in routing the packet to its destination.

Figure below shows two types of packet switching in a communications system. Diagram (a) shows that connection based packet switching sets up a communication circuit prior to transmitting packets that contain data. Diagram (b) shows connectionless packet switching. Connectionless packet switching requires intelligent switching nodes (routers) that can decode the destination address and select the forwarding route based on the results of the lookup in the routing table. This diagram shows that packets of data arrive at the switch. The routing switch extracts the destination address and possibly the type of message.


Packet Switches

Control Message Signaling [Simple Telecom]

Control message signaling (commonly called “signaling”) is the process of transferring control information such as address, call supervision, or other connection information between communication equipment and other equipment or systems. There are two methods used for signaling: in-band and out-of band signaling.

In-Band Signaling
In-band signaling occurs when control messages share the same communication channel as the information signals (e.g., within the audio signal bandwidth). In-band signaling requires the users voice or data information to be momentarily interrupted or altered while signaling messages are being transferred. In-band signaling is sometimes called blank and burst signaling.

Figure below shows the process of in-band signaling. This diagram shows that a signaling message has been created to control the communications line (e.g., to transfer a call). To allow the transmission of the control message, the information is temporarily inhibited (or discarded) and the control message is sent on the same channel.


In-Band Signaling

Out-of-Band Signaling
Out-of-band signaling is a process of sending control signals outside of the communication channel that is in use (e.g., outside the audio signal frequency range). Out-of-band signaling allows uninterrupted communication while the users voice or data information is being transferred.

Figure below shows how out-of-band signaling occurs. This diagram shows that a control message can either be sent on the same channel but in different time slots than the information (e.g., voice) signal or over a separate control signaling network (called common channel signaling).


Out-of-Band Signaling

Digital Signal Processing

Digital signals processing refers to a category of electronic devices that represent and process information that are in discrete signal level (digital) formats. Digital signal processing refers to the manipulation of digital signals to change their content and to add error detection and correction capability.

Digital signals typically vary in two levels; on (logic 1) and off (logic 0). A bit is the smallest part of a digital signal, typically called a data bit. A bit typically can assume two levels: either a zero (0) or a one (1). A byte is an agreed-upon group of bits, typically eight. A byte typically represents one alphabetic or special character, two decimal digits, or eight binary bits of information.

When analog signals are converted to digital format, the digital signals represent the original analog waveform. Just like analog signals that may be processed by filters, shaping circuits, combiners and amplifiers, digital signals can be processed to produce similar functions. However, because the signal is in digital form, these functions are performed by software programs that manipulate the data.

Unlike analog signals, digital signals can be recreated to their original form. This process is called signal regeneration. To increase the efficiency of a transmission signal (allow more users per channel), digital signals can be analyzed for redundancy and the digital signal data can be compressed. Digital signals can also be processed in a way that helps overcome the effects signal distortion that can result in the incorrect determination of a digital signal (whether a zero or one had been sent). This is called error detection and error correction processing. When digital signals represent the original analog signal, advanced echo canceling software programs can be used to reduce the effects of echoes that are caused by feedback in the audio and transmission system. Some systems use dedicated digital signal processors (DSPs) to manipulate the incoming digital information via a program (stored instructions) that produce a new digital output. This allows software programs to perform many functions (such as signal filtering) that previously required complex dedicated electronic circuits.

Digitization of an Analog Signal
Analog signals must be converted to digital form for use in a digital wireless system. To convert analog signals to digital form, the analog signal is digitized by using an analog-to-digital (pronounced A to D) converter. The A/D converter periodically senses (samples) the level of the analog signal and creates a binary number or series of digital pulses that represent the level of the signal.

The common conversion process is Pulse Code Modulation (PCM). For most PCM systems, the typical analog sampling rate occurs at 8000 times a second. Each sample produces 8 bits digital that results in a digital data rate (bit stream) of 64 thousand bits per second (kbps).

Figure below shows how an analog signal is converted to a digital signal. This diagram shows that an acoustic (sound) signal is converted to an audio electrical signal (continuously varying signal) by a microphone. This signal is sent through an audio band-pass filter that only allows frequency ranges within the desired audio band (removes unwanted noise and other non-audio frequency components). The audio signal is then sampled every 125 microseconds (8,000 times per second) and converted into 8 digital bits. The digital bits represent the amplitude of the input analog signal.


Signal Digitization

Digital bytes of information are converted to specific voltage levels based on the value (weighting) of the binary bit position. In the binary system, the value of the next sequential bit is 2 times larger. For PCM systems that are used for telephone audio signals, the weighting of bits within a byte of information (8 bits) is different than the binary system. The companding process increases the dynamic range of a digital signal that represents an analog signal; smaller bits are given larger values that than their binary equivalent. This skewing of weighing value give better dynamic range. This companding process increases the dynamic range of a binary signal by assigning different weighted values to each bit of information than is defined by the binary system.

Two common encoding laws are Mu-Law and A-Law encoding. Mu-Law encoding is primarily used in the Americas and A-Law encoding is used in the rest of the world. When different types of encoding systems are used, a converter is used to translate the different coding levels.

Digital Signal Regeneration
To overcome the effects of noise on transmitted signals, digital transmission systems use digital signal regeneration to restore the quality of the signal as it moves through a network. Digital signal regeneration is the process of reception and restoration of a digital pulse or lightwave signal to its original form after its amplitude, waveform, or timing have been degraded by normal factors during transmission. The resultant signal is virtually free of noise or distortion.

Figure below shows the process of digital signal regeneration. This example shows an original digital signal (a) and added noise (b) to produce a combined digital signal with noise (c). The regeneration process detects maximum and minimum expected values (threshold points) and recreates the original digital signal (d).


Digital Signal Regeneration

Data Compression
To increase the amount of information that a transmission system can transfer, digital systems may use data compression. Data compression is a processing technique for encoding information so that fewer data bits of information are required to represent a given amount of data. Compression allows the transmission of more data over a given amount of time and circuit capacity. It also reduces the amount of memory required for data storage.

Digital compression analyzes a digital signal for either redundant information (repeated 1’s or 0’s) or may analyze the information content of the digital signal into component parts (such as speech patterns or video frames). All of this processing allows the data transmission rate to be reduced by sending only the characteristics of the signal rather than the complete digital signal. Some data compression systems can only reduce data rates by a factor of 2:1 (e.g., ADPCM audio compression) while advanced digital audio compression can only reduce data rates by a factor of approximately 200:1 (e.g., MPEG video compression). When used in combination of data compression and decompression, the device is called a COder/DECoder (CoDec).

When a digital signal is compressed for voice communications, it is called a voice coder (Vo-coder) or speech coder. The Vo-coder is a digital signal processing device that analyzes speech signals so that it can produce a lower data rate compressed digital signal. The difference between standard data compression and voice data compression is the analysis of the information source (speech) and elimination of compression process for non-voice signals. Speech coding usually involves the use of data tables (called code books) that represent information parts that can be associated with human sound. Because non-human sounds can be eliminated from the code book, this allows the number of bits can be used to create a compressed digital voice signal to be reduced.

Figure below shows the digital voice compression process. In this example, a digital signal is continuously applied to a digital signal analysis device. The analysis portion of the speech coder extracts the amplitude, pitch, and other key parameters of the signal and then looks up related values in the code book for the portion of sound it has analyzed. Only key parameters and code book values are transmitted. This results in data compression ratios of 4:1 to over 16:1.


Digital Voice Compression

Error Detection and Error Correction
To help reduce the effects of errors on data transmission, error detection, and error protection systems are used in most communication systems. Error detection systems use a process of adding some data bits to the transmitted data signal that are used to help determine if bits were received in error due to distorted transmission. Error correction is made possible by sending bits that have a relationship to the data that is contained in the desired data block or message. These related bits permit a receiver of information to use these extra information bits to detect and/or correct for errors that may have occurred during data transmission.

A common measurement of the performance of a communication system is the amount of bits received in error, called the bit error rate (BER). The BER is the ratio of bits received in error compared to the total number of bits received.

Error detection processing involves the creation of additional bits that are sent with the original data. The additional check bits are created by using a formula calculation on the digital signal prior to sending the data. After the digital signal is received, the formula can be used again to create check bits from the received digital signal. If the check bits match, the original digital signal was received correctly. If the check bits do not match, some (or all) of the digital signal was received in error. This process is called error detection.

Some digital systems use sophisticated mathematical formulas to create the check bits so that the check bits can be used to make corrections (or predictions of the correct bits) to the received digital signal. This process is called error correction.

Figure below shows the basic error detection and correction process. This diagram shows that a sequence of digital bits is supplied to a computing device that produces a check bit sequence. The check bit sequence is sent in addition to the original digital bits. When the check bits are received, the same formula is used to check to see if any of the bits received were in error.


Error Detection and Correction

Echo Cancellation
Echo cancellation is a process of extracting a delayed version of an original transmitted signal (audio echo) from the received signal. Echoes may be created through acoustic feedback where some of the audio signal transferring from a speaker into a microphone.

Echoed signals cause distortion and may be removed by performing via advanced signal analysis and filtering. Figure 3.18 shows an example of the echo cancellation process. This diagram shows how the combining of two signals, the original plus a delayed version of the original produces a complex signal. The echo canceling system analyzes the complex signal and uses the signal analysis to create variations of the likely echo signal. This prediction of echoed signal is subtracted from the complex signal to reproduce the original signal without the echo.


Echo Cancellation

Echoed signals can also occur in signals other than audio signals. When echoes occur on radio channels (the broadband signal), it is usually the result of the same signal that travels on different paths to reach its destination. This is called multipath propagation. Echo canceling can be used to reduce the effects of radio multipath propagation.

Digital Signal Processor (DSP)
A digital signal processor (DSP) is an electronics device or assembly (typically an integrated circuit) that is designed to process signals through the use of embedded microprocessor instructions. The use of DSPs in communication circuits allows manufacturers to quickly and reliably develop advanced communications systems through the use of software programs. The software programs (often called modules) perform advanced signal processing functions that previously complex dedicated electronics circuits. Although manufacturers may develop their own software modules, DSP software modules are often developed by other companies that specialize in specific types of communication technologies. For example, a manufacturer may purchase a software module for echo canceling from one DSP software module developer and a modulator software module from a different DSP software module developer. Because DSPs use these software modules, if new technologies such as speech compression, channel coding, or modulation techniques are developed, the manufacturer only has to change the software programs in the DSP to utilize the new technology.

Figure below shows typical digital signal processor that is used in a digital communication system. This diagram shows that a DSP contains a signal input and output lines, a microprocessor assembly, interrupt lines from assemblies that may require processing, and software program instructions. This diagram shows that this DSP has 3 software programs, digital signal compression, channel coding, and modulation coding. The digital signal compression software analyzes the digital audio signal and compresses the information to a lower data transmission rate. The channel coding adds control signals and error protection bits. The modulation coding formats (shapes) the output signal so it can be directly applied to an RF modulator assembly. This diagram also shows that an optional interface is included to allow updating of the software programs that are stored in the DSP.


Digital Signal Processor (DSP)

Analog Signal Processing

Analog signals (continuously varying signals) may be processed by filters, shaping circuits, combiners, and amplifiers to change their shape and modify their content.

Signal Filtering
Filters may remove (band-reject) or allow (band-pass) portions of analog (possibly audio signals) that contain a range of high and low frequencies that are not necessary to transmit. In some cases, additional signals (at different frequencies) may be combined with audio signals prior to transmission. These signals may be multiple channels or may be signals that are used for control purposes. If control signals are added to an analog signal that is transmitted, they are usually removed from the audio signal in the receiver by filtering.

Figure below shows typical audio signal processing for a communications transmitter. In this example, the audio signal is processed through a filter to remove very high and very low frequency parts (audio band-pass filter). These unwanted frequency parts are possibly noise and other out of audio frequency signals that could distort the desired signal. The high frequencies can be seen as rapid changes in the audio signal. After an audio signal is processed by the audio band-pass filter, the sharp edges of the audio signal (high frequency components) are removed.


Audio Signal Filtering

Signal Amplification
Signal amplification is a process of sensing an input (usually low level) signal and converting the signal into a larger version of itself. An amplifier device provides this conversion process. Amplifiers increase both the desired signal and unwanted noise signals. Noise signals are any random disturbance or unwanted signal in a communication system that tends to obscure the clarity of a signal in relation to its intended use.

Figure below shows how a signal may be amplified. This diagram shows that the input signal is increased in value by an amplifier that can vary its gain (amount of amplification).


Signal Amplification

Signal Shaping
Audio signals may be processed by shaping circuits to add or remove emphasis of frequency (tone) or intensity (volume). When the signal processing involves differences of amplification of specific frequency components of an input signal, it is called pre-emphasis and de-emphasis. Signal processing that involves relative changes in the amount of amplification dependent on the level of input signal, it is called companding and expanding.

Some analog transmission systems use pre-emphasis circuits to amplify the high frequency components of the audio input signal which allow the modulation system to be more effective. Certain modulation systems do not respond well to low amplitudes of high frequency input signals. By boosting the high frequency component of the input audio signal, the modulator better translates the input signal into a modulated carrier signal. When pre-emphasis is used for transmission, a matched de-emphasis system is used in the receiver to convert the boosted high frequency component back into its original low signal level.

The intensity of an audio signal can vary dramatically because some people talk loudly and others talk softly. A system that reduces the amount of amplification (gain) of an audio signal for larger input signals (e.g., louder talker) is called companding. The use of companding allows the level of audio signal that enters the modulator to have a smaller overall range (higher minimum and lower maximum). High signals and low signals input to a modulator may have a different conversion level (ratio of modulation compared to input signal level). This can create distortion so companding allows the modulator to convert the information signal (audio signal) with less distortion. Of course, the process of companding must be reversed at the receiving end, called expanding, to recreate the original audio signal.

Figure below shows the basic signal companding and expanding process. This diagram shows that the amount of amplifier gain is reduced as the level of input signal is increased. This keeps the input level to the modulator to a relatively small dynamic range. At the receiving end of the system, an expanding system is used to provide additional amplification to the upper end of the output signal. This recreates the shape of the original input audio signal.


Analog Signal Companding and Expanding

Communication Systems: Simplex, Half Duplex, Full Duplex (FDX), Time Division Duplex (TDD)

Communication systems transfer information between 2 or more users. Communications systems may transfer information in one direction at a time on the same channel (simplex), in two directions on different time at different times (half duplex), or simultaneously on two different channels (full duplex). There are various approaches including TDD that allow the appearance of full duplex operation although the actual transmission system uses simplex or half duplex operation.

Simplex
Simplex communication allows the transmission of information between users, but only one direction at a time on the same channel or frequency. The common use of Simplex systems is traditional television or audio broadcast radio systems that transmit a signal from a single transmitter to many receivers.

Half Duplex
Half duplex communication provides the ability to transfer voice or data information in either direction between communications devices but not at the same time. The information may be transmitted on the same frequency or divided into different channels. When divided into different channels, one channel of frequency is used for transmitting and the other channel or frequency is used for receiving.

The use of different frequencies is common in half duplex radio transmission because the transmitter and receiver are commonly connected to the same antenna. If the same transmitter and receiver frequency were used, the high transmitter power would probably destroy the receiver circuitry.

Full Duplex (FDX)
Full Duplex communication is the process of transferring of voice or data signals in both directions at the same time. Full duplex operation normally assigns the transmitter and receiver to different communication channels. When the communications system uses two different frequencies for simultaneous communication, it is called frequency division duplex (FDD). One frequency is used to communicate in one direction and the other frequency is required to communicate in the opposite direction.

The definition of full duplex becomes confusing when it is applied to the end result of simultaneous voice and data communication. This is because it is possible to provide information at the input and output of a communication system while not actually sending the information simultaneously in a communication system. When a communication system provides for simultaneous two-way communication by time sharing, it is called time division duplex (TDD).

Time Division Duplex (TDD)
Time division duplex (TDD) communication uses a single channel or frequency to provide simultaneous two-way communications between devices by time-sharing. When using TDD, one device transmits (device 1), the other device listens (device 2) for a short period of time. After the transmission is complete, the devices reverse their role so device 1 becomes a receiver and device 2 becomes a transmitter. The process continually repeats itself so data appears to flow in both directions simultaneously. Figure below shows the basic operation of FDD and TDD system.

Telecom Made Simple : Modulation

Signal modulation is the process of modifying the characteristics of a carrier wave signal using an information signal (such as voice or data). The characteristics that can be changed include amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM). A pure electrical, radio, or optical carrier signal carries no information aside from either being in on or off state. When the carrier signal is modified from a normalized state, it is called a modulated signal. This modulated signal is the carrier of the information that is used to modify the carrier signal. When the carrier signal is received, its signal is compared to an unmodulated signal to reverse the process (called demodulation). This allows the extraction of the original information signal. A carrier wave signal can be carried by wire, fiber, or electromagnetic waves transmitted through the air (radio).

When a carrier signal is modulated, the frequency changes above and below its reference frequency. The difference between the upper and lower maximum frequency changes is called the bandwidth. The relationship between the amount of frequency bandwidth of an information signal (the baseband) and the channel bandwidth of the modulated carrier determines if the system is a narrowband or wideband system. Narrowband systems have a relatively small communications channel bandwidth, typically below 100 kbps. When the bandwidth of the broadband carrier is much higher than the bandwidth of the information source, it is called a wideband system.

The device that modifies the carrier signal with the information source (baseband signal) is called a modulator. An assembly or device that combines the function of modulating and demodulating signals is called a Modulator/DEModulator (MODEM).

Combined types of Modulation
Today’s sophisticated modulation systems can use all three variable parameters: frequency, amplitude, or timing (phase) at the same time to transfer analog or digital information. One of the more popular forms of combined modulation technologies is Quadrature Amplitude Modulation (QAM). QAM is a modulation technique that enables two information signals to modulate a single carrier frequency. The two different signals’ amplitude modulates two samples of the carrier that are of the same frequency, but differ in phase by 90 degrees. The resultant two signals can be added together and both signals recovered at a decoder where they are then demodulated 90 degrees apart.

Basic Communication Technologies : Signal Types

Telecommunication technology involves the transfer of information signals through wires, fiber, or through the air by the by means of electrical or optical signals. Communication signals are usually characterized by their intensity (voltage and current) and frequency (cycles per second). To allow information to be transferred using communication signals, an information source (audio, data or video) is either represented by the signal itself (called the baseband signal) or the information slightly changes the wave shape of the communication signal (called the broadband signal). The information is imposed on the carrying signal (called the carrier) by varying the signal level or time changes (frequency shift).

Signal Types
There are two basic types of signals: analog and digital. Many communication systems receive analog signals (e.g., audio signals), convert them to a digital format, transport the digital signals through a network, and reconvert the digital signals back to their analog form when they reach their destination.

Analog
An analog signal can vary continuously between a maximum and minimum value and it can assume an infinite number of values between the two extremes.

Figure below shows a sample analog signal created by sound. In this example, as the sound pressure from a person’s voice is detected by a microphone, it is converted to its equivalent electrical signal. Also, the analog audio signal continuously varies in amplitude (height, loudness, or energy) as time progresses.



Digital
Digital signals have a limited number of discrete states, usually two, in contrast to analog signals that vary continuously and have an infinite number of states. Digital signals transfer discrete signal levels at predetermined time intervals. Digital signals typically have two levels: on (logic 1) and off (logic 0). The information contained in a single time period is called a bit. The number of bits that are transferred in one second is called the data transfer rate or bits per second (bps). Because many bits are typically transferred in 1 second, the data rate is typically preceded by a multiplier k (thousand) or M (million). For example, if the data transfer rate is 3 million bits per second, 3 Mbps would indicate this. Bits are typically combined into groups of 8 bits to form a byte. When the reference is made to bytes instead of bits, the b is capitalized. For example, 10 thousand bytes is represented by kB. Figure below shows a sample digital signal. In this example, the bits 01011010 are transferred in 1 second. This results in a bit rate of 8 bps.



The earliest form of digital radio communication was Morse Code. To send Morse Code, the radio transmitter was simply turned on and off to form dots and dashes. The receiver would sense (detect) the radio carrier to reproduce the dots and dashes. A code book of dots and dashes was used to decode the message into symbols or letters. The on and off pulses or bits that comprise a modern digital signal is sent in a similar way.

The trend in communication systems, just as in other types of electronics products such as compact discs, is to change from analog systems to digital systems. Digital systems have a number of important advantages including the fact that digital signals are more immune to noise. Unlike analog systems, even when noise has been introduced, any resulting errors in the digital bit stream can be detected and corrected. Also, digital signals can be easily manipulated or processed in useful ways using modern computer techniques.