Home > Articles > Networking > Network Design & Architecture

Telecommunications Technology Fundamentals

This chapter talks about the types of transmission lines and network connections, the electromagnetic spectrum, and what bandwidth is all about in this emerging broadband era. It looks at the differences between analog and digital signals, and it discusses multiplexing. Finally, this chapter describes the various standards bodies and their roles in shaping aspects of telecommunications.
This chapter is from the book

Transmission Lines

Two prerequisites must be satisfied to have successful communication. The first prerequisite is understandability. The transmitter and receiver must speak the same language. It doesn't matter how big or how clean a pipe you have between the two endpoints. If they're not speaking the same language, you will not be able to understand the message. In the case of data communications, we've resolved these issues quite elegantly: We have software and hardware translation devices that can convert between the different languages that individual computing systems speak. In the realm of human communications, we're about to embark on that exciting journey as well. Through the use of advanced voice-processing systems, in the next five to seven years we should have the ability to do real-time foreign language translation as part of the network service.

The second prerequisite is the capability to detect errors as they occur and to have some procedure for resolving those errors. In the case of human communications, intelligent terminals at either end—human beings—can detect noise that may have affected a transmission and request a retransmission, thereby correcting for that error. In the case of data devices, similar logic has to be built in to end devices so that they can detect errors and request a retransmission in order to correct for the errors.

If these two prerequisites—understandability and error control—are met, then communication can occur. We communicate by using data devices over what is generically termed a transmission line. There are five main types of transmission lines—circuits, channels, lines, trunks, and virtual circuits—each of which has a specific meaning. The following sections describe each of these types of transmission lines in detail.


A circuit is the physical path that runs between two or more points. It terminates on a port (that is, a point of electrical or optical interface), and that port can be in a host computer (that is, a switching device used to establish connections), on a multiplexer, or in another device, as discussed later in this chapter.

In and of itself, a circuit does not define the number of simultaneous conversations that can be carried; that is a function of the type of circuit it is. For example, a simple, traditional telephone circuit is designed to carry just one conversation over one physical pathway. However, converting that to a digital circuit gives you the ability to extract or derive multiple channels over that circuit, subsequently facilitating multiple simultaneous conversations. So, the circuit is the measure of the physical entity.

There are two types of circuits: two-wire circuits and four-wire circuits.

Two-Wire Circuits

A two-wire circuit has two insulated electrical conductors. One wire is used for transmission of the information. The other wire acts as the return path to complete the electrical circuit. Two-wire circuits are generally deployed in the analog local loop, which is the last mile between the subscriber and the subscriber's first point of access into the network. Figure 2.1 shows an example of a two-wire circuit.

Two-Wire and Four-Wire Versus Two-Pair and Four-Pair

Don't confuse the terms two-wire circuit and four-wire circuit with the terms two-pair and four-pair. Two-pair and four-pair refer to the number of wires in the internal cabling plan. Two-wire and four-wire have to do with the number of electrical conductors associated with a transmission line.

Figure 2.1 A two-wire circuit

Figure 2.2 A four-wire circuit

Four-Wire Circuits

A four-wire circuit has two pairs of conductors. That is, it has two sets of one-way transmission paths: one path for each direction and a complementary path to complete the electrical circuit (see Figure 2.2). Four-wire circuits are used where there is distance between the termination points which requires that the signal be strengthened periodically. So, for example, four-wire circuits connect the various switches that make up the public switched telephone network (PSTN). Four-wire circuits are also used with leased lines, where a customer may be connecting locations of its own that are separated by distance. Also, all digital circuits are provisioned on a four-wire basis.

There are two types of four-wire circuits: physical four-wire and logical four-wire. In physical four-wire you can actually count four wires. In logical four-wire, physically there are only two wires, but you derive the four individual paths by splitting the frequency. Half of the frequency band carries the transmit signal, and the other half carries the receive signal. So you can't always tell just by looking what kind of circuit you're dealing with; the application dictates the type of circuit it is.

Using Two-Wire and Four-Wire Circuits

Whenever you release energy into space, it loses power as it's traveling over a distance. So, because networks were designed to carry communications over a distance, we need tools to augment signals that have been losing power as they have traveled across the network, which are called attenuated signals. These tools are called amplifiers and repeaters. An amplifier boosts an attenuated signal back up to its original power level so it can continue to make its way across the network. The PSTN traditionally used copper wires. Based on how quickly the signals flow through the copper wires, there's a certain distance requirement between amplifiers. The distance requirement between amplifiers is relatively short on copper wires—generally about 6,000 feet (1,800 meters). As networks were built, these distance considerations were kept in mind. (Repeaters are discussed later in this chapter, in the section "Digital Transmission.")

Network builders had to give some thought to another aspect of amplifiers: First-generation amplifiers were unidirectional. They could only amplify a signal moving in one direction, so any time you needed to provision a circuit that was going to be crossing a distance, you had to literally provision two circuits—one to amplify the information in the transmit direction and a second to amplify the information in the receive direction. Therefore, whenever a network was crossing a distance, it needed to use a four-wire circuit. But in building out the millions of local loops for subscribers, it was seen as being cost-effective to have to pull only two wires into every home rather than four. Therefore, the local loops were intentionally engineered to be very short; some 70% to 80% of the local loops worldwide are less than 2 miles (3.2 kilometers) long. Because the local loops are short, they don't need amplifiers, and therefore the subscriber access service can be provisioned over a two-wire circuit. However, the local loop is increasingly being digitalized, so as we migrate to an end-to-end digital environment, everything becomes four-wire. Figure 2.3 shows an example of a segment of a network in which two- and four-wire circuits are traditionally used.

Figure 2.3 Using two-wire and four-wire circuits


A channel defines a logical coversation path. It is the frequency band, time slot, or wavelength (also referred to as lambda) over which a single conversation flows. A channel is a child of the digital age because digital facilities enable multiple channels. The number of channels on a transmission line determines the number of simultaneous conversations that can be supported. Because we are becoming more digitalized all the time, you often hear people refer to the number of channels rather than the number of circuits.

Lines and Trunks

Lines and trunks are basically the same thing, but they're used in different situations. A line is a connection that is configured to support a normal calling load generated by one individual. A trunk is a circuit that is configured to support the calling loads generated by a group of users; it is the transmission facility that ties together switching systems. A switching system is a device that connects two transmission lines together. There are two major categories of switching systems:

  • CPE switches—The most prevalent form of switch in the customer premises equipment (CPE) environment is the private branch exchange (PBX), which is called a private automatic branch exchange (PABX) in some parts of the world. A PBX is used to establish connections between two points. It establishes connections between telephones that are internal to the organization, and it establishes connections between internal extensions and the outside world (that is, the PSTN).

  • Network switches—A hierarchy of network switches has evolved over time, and the appropriate switch is called into action, depending on which two points the switches are connecting together. For example, in Figure 2.4 the CPE is on the left-hand side. Each individual single-line instrument represents a subscriber line. (Again, the fact that it's called a line means that it's a circuit configured to carry the calling load of just one user.) Above the single-line instrument is a business enterprise with a PBX. The connection from this PBX to the PSTN occurs over a trunk that is specifically configured to carry the calling load of multiple users. Beyond the PBX are multiple end users that are attached to that PBX. Each end user's connection would be referred to as a station line, again emphasizing that the line is carrying the calling load of one user.

Figure 2.4 Lines, trunks, and switches

The customer environment attaches to the PSTN, and the first point of access is the local exchange, which is also referred to as a Class 5 office (and in North America, as a central office). The traditional local exchange switch can handle one or more exchanges, with each exchange capable of handling up to 10,000 subscriber lines, numbered 0000 to 9999. The only kind of call that a local exchange can complete on its own, without touching any of the other switches in the network, is to another number in that same local exchange. Local exchanges are discussed in detail in Chapter 5, "The PSTN."

For a local exchange to call a neighbor that resides 10 miles (16 kilometers) away and who draws a dial tone from a different local exchange, the connection between those two different exchanges is accomplished through the second part of the hierarchy—a tandem switch (also called a junction exchange). The tandem switch is used to connect local exchanges throughout the metropolitan area. When it's time to make a toll call, one that is long-distance in nature, another switching center is called into action—the toll center (also called the Class 4 office, transit switch, or trunk exchange). The toll center is responsible for establishing and completing national, long-distance communications.

The top of the hierarchy is the international gateway, whose exchanges are specifically designed to connect calls between different countries.

A trunk supplies the connections between the numerous switches within the PSTN, between customer-owned switches such as the PBX, and between the PBXs and the PSTN. On the other hand, a line supports a single user in the form of a subscriber line in the PSTN or an extension provisioned from the PBX. (Chapter 5 describes in detail the entities involved in managing local, tandem, and toll exchanges.)

Virtual Circuits

Today, because of the great interest in and increased use of packet switching, most networks use virtual circuits. Unlike a physical circuit, which terminates on specific physical ports, a virtual circuit is a series of logical connections between sending and receiving devices (see Figure 2.5). The virtual circuit is a connection between two devices that acts as though it's a direct connection, but it may, in fact, be composed of a variety of different routes. These connections are defined by table entries inside the switch. A connection is established after both devices exchange agreement on communications parameters that are important to establishing and maintaining the connection and on providing the proper performance for the application they are supporting. The types of communication parameters that could be included are message size, the path to be taken, how to deal with acknowledgements in the event of errors, flow-control procedures, and error-control procedures. The term virtual circuit is largely used to describe connections between two hosts in a packet-switching network, where the two hosts can communicate as though they have a dedicated connection, although the packets may be taking very different routes to arrive at their destination.

Figure 2.5 A virtual circuit

There are two types of virtual circuits: permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). The vast majority of implementations today involve PVCs. PVCs and SVCs are commonly used in packet-switching networks (for example, X.25, Frame Relay, ATM).


A PVC is a virtual circuit that is permanently available; that is, the connection always exists between the two locations or two devices in question. A PVC is manually configured by a network management system, and it remains in place until the user reconfigures the network. Its use is analogous to the use of a dedicated private line because it provides an always-on condition between two locations or two devices.


In contrast to PVCs, SVCs are set up on demand. They are provisioned dynamically by using signaling techniques. An SVC must be reestablished each time data is to be sent, and after the data has been sent, the SVC disappears. An SVC is therefore analogous to a dialup connection in the PSTN. The main benefit of an SVC is that you can use it to access the network from anyplace. The predominant application for SVCs is to accommodate people who are working at home, in a hotel, at an airport, or otherwise outside the physical location of the enterprise network.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020