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Digital exchanges do not require high-power pulses to drive the selectors as old electromechanical switches did. However, subscriber lines are still, and will be, supplied by a — or —V battery so that telephones continue to operate independent of the electric power supply.

Modern telephones usually have 12 push buttons keys A to D of Figure 2. One of the frequencies is from the upper frequency band and the other from the lower band. All frequencies are inside the voice frequency band —3, Hz and can thus be transmitted through the network from end to end, when the speech connection is estab- lished.

This signaling principle is known as dual-tone multifrequency DTMF signaling. Tones are detected at the subscriber interface of the telephone exchange and, if necessary, signaled further to the other exchanges through which the connection is to be established.

All digital local exchanges have a capability to use either pulse or tone dialing on a subscriber loop. The subscriber is able to select with a switch on his telephone which type of dialing is to be used. Tone dialing should always be selected if the local exchange is a modern digi- tal one. These services, for example, call transfer, are not avail- able with telephones that use pulse dialing.

We use tone dialing also to control value-added services. Value-added services are services that we can use via the telephone network but that are usually provided by another service provider, not the telecommunications network operator. One example of value added services is telebanking. Tones are transmitted on the same frequency band as voice, and during a call we are able to dial digits to transmit, for example, our discount number and security codes to the telebanking machine. The worst disadvantage of a fixed subscriber telephone is still the poor man—machine interface that makes new services difficult to use.

Some tele- phones that have displays are more user friendly, but subscribers still have to memorize command sequences to use the new services offered by a modern telephone network. The local loop, which connects a tele- phone to a local exchange is a two-wire 2W circuit that carries the signals in both transmission directions Figure 2.

The Telecommunications Network: An Overview 29 loop. Subscriber loops are and will remain two-wire circuits, because they are one of the biggest investments of the fixed telephone network.

Early telephone connections through the network were two-wire cir- cuits. Longer connections attenuate the speech signal and amplifiers are needed on the line. In two-wire circuits, amplification of a signal may cause oscillation or ringing if the output signal of an amplifier loops back to the input circuit of another transmission direction Figure 2. The operating principle of electronics in the network is unidirectional and inside the network we use two wires for each direction, or four-wire 4W connections.

Four-wire connections are also much easier to maintain than 2W connections because transmission directions are independent from each other and potential oscillation, as shown in Figure 2. A transformer consists of coils of wires wrapped around an iron object. When an alternating current flows through one coil, it produces a magnetic field in the iron core. This magnetic field generates current to the wires of other coils around the same iron core.

Two separate transformers are needed in the hybrid and both of them consist of three similar, tightly coupled windings. In each trans- former an alternating current in one coil generates alternating current to all other coils of the same transformer. Spots of coils indicate the direction of the current flow polarity of the coil.

In Figure 2. These currents have opposite directions in trans- former T1; they, or actually their magnetic fields in the iron core, cancel each other, and the signal from the receive pair is not connected to the transmit pair, or at least it is much attenuated. In practice, the balance is not ideal and attenuated signal is connected back, which is heard as an echo from the far end of the telephone circuit if two-way propagation delay of the circuit is long enough.

Dashed lines in Figure 2. Satellite connections have long propagation delays because of the long propagation distances. The round-trip delays of these connections are longer than 50 to ms, causing a disturbing echo. Hence, in the case of these connections, we have to use special equipment known as echo cancellers in the network to eliminate the echo.

Main signal paths Example of currents generated by the signal from far end receive pair Figure 2. The ISDN basic rate interface uses bidirectional Kbps data trans- mission on a 2W circuit ordinary subscriber loop.

There the transmission directions are separated with the help of digital signal processing technology. Many applications use the transformer circuit described earlier together with digital signal processing technology to improve performance. The reader can imagine what happens when the micro- phone generates an alternating current in the telephone set of the figure. In mobile telephone networks, each telephone set or subscriber card has a unique identification number.

The numbering is hierarchical, and it has an internationally standard- ized country code at the highest level. In the follow- ing sections, we explain the fields of the telephone number shown in Figure 2. It tells the network that the connection is to be routed via an international telephone exchange to another country.

The international pre- fix may differ from country to country, but it is gradually becoming harmo- nized. For example, all of Europe uses 00; elsewhere it may be different. If many operators are providing international telephone service, a subscriber may select from among different operators by using an operator prefix instead of 00, for example, in Finland a user would dial for Oy Finnet International.

Country codes are not needed for national calls because their purpose is to make the subscriber identification unique in the world. A tele- phone number that includes the country code is called an international number and it has a maximum length of 12 digits. Because there are a few hundred countries in the world, many country codes have been defined by the ITU and the length of them varies from a sin- gle digit to four digits some small areas have an even longer code.

The first digit is a long-distance call identification and other numbers identify the area. The first digit is not needed in the case of an international call because that type of call is always routed via the long-distance level of the destination network.

In the case of cellular service, the trunk code is used to identify the home network of the subscriber instead of the location. With the help of this network code, a call is routed to the home network, which then determines the location of the subscriber and routes the call to the destination. The trunk code and the subscriber number together create a unique identification for a subscriber at the national level. This is called a national number and its maximum length is 10 digits.

Trunk codes start with a 0 in Europe, but the 0 is not used in calls com- ing from abroad. In countries where multiple operators provide long-distance telephone service, the subscriber may select an operator by dialing an operator prefix in front of the trunk code. In Finland, two examples of the long- distance operator numbers are for Finnet and for Song Networks.

To connect to a certain sub- scriber, the same number is dialed anywhere in the area. Because of the numbering hierarchy, the subscriber part of the telephone number of one subscriber may be the same as that of another subscriber in another area. If provision of local telephone service is deregulated as is the goal in Europe , a subscriber is able to choose a network operator for local calls by dialing a local operator prefix in front of the subscriber number.

Then in addition to the numbers just described, a subscriber will need to dial additional digits to select a service provider network operator.

As explained earlier, a subscriber may choose a service provider for local calls, long-distance calls, and international calls. The national telecommunications authority also defines how calls dialed without an operator number are charged. If the subscriber does not specify the international and long-distance network operators by operator prefix, the network is chosen randomly or according to other rules specified by the national telecommunications authority.

The creation of real competi- tion in fixed telecommunications service provision has been successful in many countries.

One problem with this situation is that additional dialing of operator prefixes at all levels is required, and another is that the fees for fixed telephone service are too low to make subscribers interested in taking the time to choose a service provider. For business users, for which monitoring the costs of telecommunica- tions is essential, competition will certainly reduce those costs. To avoid the problem of additional dialing, a business or residential subscriber may make a service agreement with one of the network operators for local, long- distance, and international calls.

These switching systems are called exchanges. The subscriber identifies the required connection with signaling information dialing that is transmitted over the subscriber line. In the network, signaling is needed to transmit the control information of a specific call and circuits from one exchange to another.

The speech channel is connected from the time when the circuit was established to the time when the call is cleared.

This principle is called the circuit switch- ing concept and is different from packet switching, which has been used in data networks. In the past, the switching matrix was electromechanical and controlled directly by pulses from a telephone.

Later, the control functions were inte- grated into a common control unit. Currently, the common control unit is an efficient and reliable computer or a multiprocessor system, including large amounts of real-time software. This kind of exchange is called a stored pro- gram control SPC exchange Figure 2.

This principle is similar to computer communications where data packets are transmitted between computers. Every exchange between subscribers A and B connects a speech circuit according to signaling information that is received from a subscriber or from the previous exchange. If the exchange is not the local exchange of subscriber B, it transmits signaling information to the next exchange that connects the circuit further.

Usually the call is routed via many exchanges and the signaling information needs to be transmitted from one exchange to another. At the same time another channel is reserved only for signaling purposes and each speech path has its own dedicated signaling channel while the call is connected. This channel can be, for example, a signaling channel in time slot 16 of the pri- mary PCM frame as explained later in Chapter 4. The main phases of signal- ing between exchanges are shown in Figure 2.

First the speech channel and the related signaling channel are seized from exchange A to exchange B. Then the telephone number of subscriber B is transmitted to exchange B, which activates the ringing signal.

When subscriber B answers, the speech connection is switched on and the conversation may start. Exchange A responds with a clear-forward CLF sig- nal when subscriber A hangs up or when the time constant expires. The call is then disconnected by both exchanges. Many different signaling systems are used for CAS and some of them include additional signals that are not present in Figure 2.

Signals that carry signaling information indicated in Figure 2. CAS is still used in telephone networks, but it is gradually being replaced with a more efficient standardized method known as CCS. Signal- ing frames contain, for example, information about the connection to which the message belongs, the address of the destination exchange, dialed digits, and information about whether subscriber B has answered.

In most cases only one data channel between two exchanges is required to serve all estab- lished calls. This is usually one Kbps time slot of a primary 2- or 1. Establishing a call requires the same signaling information as indicated in Figure 2. The dialed digits are transmitted from subscriber A to the local exchange, as explained in Section 2.

The Telecommunications Network: An Overview 37 direction it should route the call. From this information it looks up an address of the exchange to which it should send the signaling message for call connection. Then the exchange builds a data packet that contains the address of exchange B. This signaling message, called the initial address message IAM , is then sent to exchange B. When all the digits that identify subscriber B are received by exchange B, it acknowledges this with an address complete message ACM , to confirm that all digits have been successfully received.

This message also contains information about whether the call is to be charged or not and if the sub- scriber is free or not. Exchange B transmits the ringing tone to subscriber A and the ringing signal to subscriber B, and telephone B rings. When subscriber B lifts the handset, an answer signal charge ANC is sent in order to activate charging. Exchange B switches off the ringing signal and ringing tone.

Then both exchanges connect the speech channel through so the conversation can start. Exchange A responses with CLF signal. All exchanges on the line transmit the CLF mes- sage to the next one, and each receiving exchange acknowledges it with a release guard RLG signal. The RLG message indicates to the receiving exchange that the connection has been cleared and the channel released by the other exchange. It also ensures that both exchanges have cleared the cir- cuit to make it available for a new call.

Hence, switching offices are still often referred to as central offices. As telephone density grew and subscribers desired longer distance con- nections, it became necessary to interconnect the individual service areas with trunks between the central offices. With further traffic growth, new switches were needed to interconnect central offices and a second level of switching, trunk or transit exchanges, evolved.

Currently national networks have several switching levels. The actual implementation of the hierarchy and the number and names of switching levels differ from country to country. The structure of the telephone number, explained in Section 2. The routing plan includes the numbering plan and network configuration. The national telecommunications authority coordinates the national numbering plan.

It defines, for example, trunk or area codes and operator prefixes used inside the country. It also defines nationwide service numbers e. These service numbers are defined to be the same wherever the call is originated and they require additional intelligence from switching sys- tems. Their routing principle is explained later in Section 2. At the regional level, the numbering plan includes digits allocated to certain switching offices, exchanges, and the subscriber numbers for sub- scribers connected to a certain switch.

The Telecommunications Network: An Overview 39 2. This includes the deletion of certain digits and automatic alternate routing. Number conversion may also be needed when, for example, the emer- gency call dialed with a nationwide short emergency number has to be routed to a regional center that has a different physical telephone number. Some of this intelligence for routing may be stored in a centralized control system from which the exchanges request routing information.

This modern network struc- ture is called an intelligent network IN and is described in Section 2. In the example of Figure 2. When exchanges in the A call is routed to Helsinki area To other regions and The number is analyzed by 0 9 to subscriber The international exchange then analyzes the country code and selects an outgoing route to Sweden. Another example in Figure 2. Operator has defined in his numbering plan that the subscriber numbers 2xxxx and 1xxxx are placed on the left-hand branch from the regional center.

The local exchange selects the subscriber loop of the telephone number and connects a ringing signal to the subscriber. However, modern exchanges can do more than the simple strictly hier- archical routing just introduced. If there is a sufficient volume of traffic, calls may pass by a hierarchy level or may be connected directly to another low- level switch, as illustrated in Figure 2. This may be reasonable, for exam- ple, if the local exchanges of subscribers A and B are on the opposite sides of the regional border.

The telecommunications operator is free to define the detailed actual routing to optimize the usage of the network. In this section we have described the switching hierarchy of the tele- phone network and the telephone call routing principle through the exchanges in this hierarchy. In modern networks the actual implementation may be different from this strictly hierarchical routing principle we described.

Local telephone exchanges may analyze the whole telephone number, bypass the switching hierarchy, and route the call directly if the des- tination is a subscriber of a neighbor local exchange. Also, some sets of the telephone numbers have no fixed connection to the physical location of a subscriber loop.

The IN technology, which we discuss later in this chapter, connects a dialed logical number and a certain physical telephone number i. Deregulation of the fixed telephone business has created another need for increased intelligence in the network. The need for this is indicated by the operator prefix dialed by the subscriber as discussed in Section 2.

In the next section, we divide the global telecommunications network into three simplified layers in order to clarify their structure and the tech- nologies that are used to implement their required functions.

Ordinary telephone and ISDN subscribers use two wires, a pair, as a subscriber loop, but for business customers a higher capacity optical fiber or microwave radio link may be required. Many differ- ent technologies are used in a local-access network to connect subscribers to the public telecommunications network.

Most subscriber connections use twisted pairs of copper wires. Sub- scriber cables contain many pairs that are shielded with common aluminum foil and plastic shield. In urban areas cables are dug into the ground and may be very large, having hundreds of pairs. In suburban or country areas, overhead cables are often a more economical solu- tion than underground cables. An optical connection is used when a high transmission capacity more than 2 Mbps or very good transmission quality is required.

A microwave radio relay is often a more economical solution than optical fiber when there is a need to increase data capacity beyond the capacity of an existing cable network. Installation of optical or copper cables takes more time because per- missions from landowners and city authorities are required.

Installation of cables is also very expensive when they must be sunk into the ground. One technology for implementation of ordinary subscriber loops for fixed telephone service is known as wireless local loop WLL. WLL uses radio waves and does not require installation of subscriber cables; it is a quick and low-cost way to connect a new subscriber to the public network.

With the help of this technology, new operators can provide services in an area where another old operator owns the cables. WLL is also used for replacement of old fixed overhead subscriber telephone lines in rural areas.

When cable network capacity for subscriber connections needs to be increased, it may be more economical to install concentrators, remote sub- scriber units, or subscriber multiplexers so as to utilize existing cables more efficiently. We use one of these terms to describe the switching capability of the remote unit. Concentrators may be capable to independently switch local calls among the subscribers connected to them. A remote subscriber unit is basically the subscriber interface part of the exchange that is moved away close to the subscribers.

Subscriber multiplexers may only connect each subscriber to a time slot channel in the PCM frame. The detailed functionality of these systems depends on the manufacturer, but we can say that only those sub- scribers who have picked up their handsets reserve a channel to the local exchange.

Digital transmission between an exchange and a concentrator fur- ther improves cable utilization so that two cable pairs serve tens of subscribers. We have explained the access alternatives shown in Figure 2. Technologies used for Internet access are explained in Chapter 6.

The size of local exchanges varies from hundreds of subscribers up to tens of thousand subscribers or even more. A small local exchange is some- times known as a remote switching unit RSU and it performs the switching and concentration functions just as all local exchanges do. A local exchange reduces the required transmission capacity number of speech channels typi- cally by a factor of 10 or more; that is, the number of subscribers of the local exchange is 10 times higher than the number of trunk channels from the exchange for external calls.

The number of required trunk circuits is analyzed in Section 2. It is a large con- struction with huge number of connectors. Subscriber pairs are connected to one side and pairs from the local exchange to the other.

Between these con- nector fields there is enough space for free cross-connections. Cables and connectors are usually arranged in a logical way considering the subscriber cable network structure and switching arrangements. This fixed cabling stays the same over long periods of time, but connections between sides change daily, for example, because a subscriber moves to another house in the same switching area.

A cross-connection in the MDF is usually done with twisted open pairs that are able to carry data rates up to 2 Mbps. ADSL and ordinary ana- log telephone circuits use the same 2W subscriber loop. A digital exchange may include both analog and digital subscriber interfaces. If the local switch has ISDN capability, basic rate and primary rate interfaces are avail- able. Ordinary subscriber pairs are used for ISDN basic rate connections Kbps bidirectional and a network terminal NT is required on cus- tomer premises.

The primary rate interface of ISDN 1. It requires two pairs, one for each transmission direc- tion, and supports many simultaneous external calls. In addition to MDF, network operators may use other distribution frames for transmission network management and maintenance.

An optical distribution frame ODF contains two fields of optical fiber connectors. The optical cables of the network are connected to one connector field and the other one is connected to optical line terminal equipment. Cross- connections between two connector fields are created with optical fibers. This allows maintenance personnel, for example, to replace a faulty optical cable connection with a spare one. A digital distribution frame DDF is a cross-connection system to which digital interfaces from line systems and the exchange or other network equipment are connected.

With the help of a 1. DXC is managed via its network management interface and an operator may change its cross- connection configuration from a network management system NMS site. From a remote NMS he may, for example, define to which of the 1. Operation of DXC is discussed in Chapter 4. The local exchanges are connected to these trunk exchanges, which are linked to provide a network of connections from any customer to any other subscriber in the country.

High-capacity transmission paths, usually optical line systems, with capacities up to 10 Gbps, interconnect trunk exchanges. Note that a trans- port network has alternative routes. If one of these transmission systems fails, switches are able to route new calls via other transmission systems and trunk exchanges to bypass the failed system Figure 2.

Connections between local and trunk exchanges are usually not fault protected because their faults affect on a smaller number of subscribers. Digital Connections trunk Digital to other regions Digital exchange trunk local exchange exchange Transmission network Common channel Digital signaling trunk exchange Digital Digital local trunk exchange High capacity exchange optical transmission systems Figure 2.

Its basic purpose is simply to provide a required number of channels or data transmission capacity from one exchange site to another. Exchanges use these channels of the transport net- work for calls that they route from one exchange to another on subscriber demand. The trunk exchanges are usually located in major cities. They are digital and use the international common channel signaling standard SS7 to exchange routing and other signaling information between exchanges.

The transmission lines between exchanges have conventionally carried TDM tele- phone channels, as explained in Chapter 4. Currently the use of IP networks for connections among exchanges is increasing and it requires media gateways MGWs between the exchange and IP network to take care of signaling and real-time transmission through the IP network. Via this highest switching hierarchy level, international calls are connected from one country to another and any subscriber is able to access any of the other more than 2 billion sub- scribers around the world.

High-capacity optical systems interconnect international exchanges or switching centers of national networks. Submarine cables coaxial cable or optical cable systems , microwave radio systems, and satellites connect conti- nental networks to make up the worldwide telecommunications network.

The first submarine cable telephone system across the north Atlantic Ocean was installed in , and it had the capacity of 36 speech channels. Modern optical submarine systems have a capacity of several hundred thou- sand speech channels and new high capacity submarine systems are put into use every year.

In addition to speech, submarine systems carry intercontinen- tal Internet traffic, which is estimated to take most of the capacity of the new systems under installation. Submarine systems are the main paths for inter- continental telephone calls and Internet communication. Satellite systems are sometimes used as backup systems in the case of congestion. We described the common structure of the global telecommunications network without separating the different network technologies.

We need dif- ferent network technologies to provide different types of services, and the telecommunications network is actually a set of networks, each of them hav- ing characteristics suitable for the service it provides. However, the public network contains many other networks that are optimized to provide services with different characteristics. We review these different network technologies in this section.

We can divide telecommunications networks into categories in any of many different ways. If we consider the customers of networks and the avail- ability of services, there are two broad categories: public networks and private or dedicated networks. These network operators have a license to provide telecommunica- tions services and that is usually their core business. Sometimes we refer its service to as POTS if we want to distinguish ordinary fixed telephone service from other services provided by telecommunications networks today.

In addition to voice communications between fixed telephones, data can be substituted for speech with the help of a voice-band modem. They are regional or national access net- works and connected to the PSTN for long-distance and international con- nections. We introduce mobile networks in Chapter 5. The bit rate of telex is very slow, 50 or 75 bps, which makes it robust. It was once widely used but its importance has been reduced as other messaging systems such as electronic mail and facsimile have reduced its market share.

Pagers are low-cost, lightweight wireless communication systems for contacting customers without the use of voice. The impor- tance of paging systems has been reduced in countries where penetration on cellular systems, providing text-messaging service, is high.

Leased point-to-point lines are often an economical solution for connections between the LANs of corporate offices in a region. Circuit-switched networks dedicated to data transmission are not widely used today. Packet-switched data service is provided by the X. These networks were developed to provide commercial data com- munication service and they provide charging functionality so that the cus- tomer bill may be based on the amount of transferred data. The importance of these networks has been reduced because of expansion of the Internet.

Internet e-mail has replaced X. Public wireless data networks, such as general packet radio service GPRS , have been implemented to provide data services for mobile users. Department of Defense. The ARPANET grew until it became a wide-area computer network called the Internet, which was used in the s and s mainly by academic institutes such as universities. Because of its his- tory the Internet does not provide charging functions, and customer billing is usually based on the access data rate and fixed monthly fee.

In the first half of the s the user-friendly graphical user interface WWW was intro- duced; since then the use of the Internet has expanded very rapidly.

Cur- rently, the Internet is the major information network in the world, and many Internet service providers ISPs have sprung up to provide Internet services for both businesses and residential customers. The expansion of the Internet continues, and the evolving commercial services e. With the help of some hardware and software updating, modern digital telephone exchanges are able to provide ISDN service. The main hardware modifica- tion required is the replacement of analog subscriber interface units with digital ones, as shown in Figure 2.

The ordinary two-wire subscriber loop of the telephone network is upgraded to the basic rate access of ISDN by an NT on the subscriber premises and by a basic rate interface unit and ISDN software in the local exchange. The bidirectional data rate in the subscriber loop is Kbps, which carries Kbps of user data and additional framing information.

D-channel, 16 Kbps, is used for signaling. Total information rate is Kbps, which makes Kbps when framing information is added. User data contain two independent Kbps circuit- switched user channels, B channels, and a Kbps signaling channel, the D channel. Subscribers may use user channels, B channels at 64 Kbps, for ordi- nary speech transmission, data, facsimile, or videoconferencing connections.

Subscribers may use both B channels independently at the same time and dial them up independently, for example, using one of these channels for a telephone call and another for an Internet connection. For Internet surfing B channels can be combined to provide a single Kbps data rate connec- tion. Users may connect up to eight terminals to a network terminal and two of them may be in use at the same time.

The advantages of ISDN over the analog telephone service are a higher data rate and the availability of two con- nections at the same time. ISDN technology has been available for some time but its usage has been low because of high tariffs in the past. On the other hand, higher rate access technologies, such as xDSL and cable modems, provide better performance and they have cut the growth of ISDN. The Telecommunications Network: An Overview 51 However, the existing low-cost ISDN technology makes it feasible for net- work operators to provide ISDN connections sometimes at a lower cost than two conventional analog telephone connections.

Traditionally, the operators of these net- works have not provided dial-up bidirectional telecommunications services. Access to these networks is currently available in urban areas via cable TV networks built by cable TV operators. These operators have not been allowed to provide other telecommunications services and their wideband cable net- work to homes has not supported bidirectional communication. As the deregulation of the telecommunications business has proceeded, these opera- tors have become active in providing other telecommunications services as well, especially fixed telephone service and high-data-rate Internet access.

To provide interactive services, the cable TV networks need to be upgraded with the technologies that allow subscribers not only to receive TV and radio signals, but to transmit data to the network. Most of the invest- ment was already made when wideband cables were installed. This existing medium is especially attractive for providing Internet service to every home connected to a cable TV network.

Typically, a data connection made via a cable TV network is shared between many home users; that is, there is no physically separate connection to every home as we have in the case of ISDN or xDSL.

This service is has often attractive tariffs because of shared investments, but it may suffer from temporary congestion when many users happen to be active at the same time.

They usually own and maintain the networks themselves. Services provided are a tailored mix of voice, data, and, for example, special control information.

They are called private or professional mobile radio PMR. Railway companies also have private tele- phone networks that use cables that run alongside the tracks. They can incorporate LANs with mainframe computers feeding information to the branch offices. Banks, hotel chains, and travel agencies, for example, have their own separate data networks to update and distribute credit and reserva- tion information.

Another choice is to lease resources, which are also shared with other users, from a public network operator. This virtual private network VPN provides a service similar to an ordinary private network, but the sys- tems in the network are the property of the network operator. The key first step is recombinant segmentation identification.

Using extended behavioral segmentation categorization , carriers can produce a customer value function valuation. Churn models and indexes can be used to determine the projected churn rates Anticipation.

Using key metrics and a WAR Gameboard, the profit levels for customers can be evaluated and maximized. Managers of successful telecom service providers need to understand why customers leaving and what makes telecom churn so special.

Poor, G. Wornell - Prentice-Hall, Inc. Schneider - Telebyte, Inc. It is an important topic both within the context of data communications today and into the future. All aspects of this subject are explored. Written at the technical conceptual level it is intended as an introduction to the field for computer and data communication professionals.

Viterbi, Jim K. Omura - McGraw-Hill , This classic text remains a vital resource three decades after its publication. It is geared toward students of communications theory and to designers of channels, links, terminals, modems, or networks used to transmit and receive digital messages.

New techniques are described clearly and at a level between seminars and graduate-level instruction. This book covers the history and principles of radio transmission and an array of different radio receivers. Johnson - Imagineering E-Zine , Introduction to the basic concepts of optical communications. The handbook discusses the physics of light and how it can be used to send information.

It provides details of the components used in light transmitters and receivers. Orfanidis , The book provides a broad and applications-oriented introduction to electromagnetic waves and antennas.

It covers the propagation, reflection, and transmission of plane waves, multilayer films, waveguides, transmission lines, impedance matching, etc. It explains the concepts at a level accessible to an audience with a basic background in probability and digital communication.