The PSTN
Lesson 5: The Voiceband

In this lesson, we examine the bandwidth provided with Plain Ordinary Telephone Service, which is often referred to as the voiceband.

We’ll understand what the term "bandwidth" means, and how it is measured in the analog world.

Then, we'll look at the details of the voiceband, what frequencies it covers and why, and its limitations.

This free online telecom training course lesson is the introduction to the course.

The Voiceband is in both the CTNS Certification Package and the CTA Certification Package.

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Certification Packages That Include This Course

CTNS Certification Package

Six online courses plus TCO Certification covering the core knowledge needed for telecommunications today:
  • The PSTN
  • Wireless Telecommunications
  • The OSI Layers and Protocol Stacks
  • Ethernet, LANs and VLANs
  • IP Networks, Routers and Addresses
  • MPLS and Carrier Networks

TCO Certification, Certificate and Letter of Reference.

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CTA Certification Package

Sixteen online courses covering telecom, datacom and networking for non‑engineers from A-Z, plus the prestigious TCO Certified Telecommunications Analyst certification.

Includes the six CTNS courses plus

  • The Telecommunications Industry
  • Digital
  • Transmission Systems and Fiber Optics
  • IP Security
  • The Internet, and many more

TCO Certification, Certificate and Letter of Reference.

Based on Teracom's famous training

30-day, 100% money-back guarantee

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Study Guide Notes For This Lesson

These are the words that are displayed and spoken during the lesson. Get these notes for the whole course in the Certification Study Guide, available in print or eBook. Many people tell us a printed companion book enhances their learning!

If a tree falls in the forest and no-one is there to hear it, does it cause a sound? The answer depends on whether you believe sound is sound pressure waves – air molecules getting closer together then further apart; or if you believe sound is the sensation one gets in one's brain when one hears the sound pressure waves. Then, two points of view when designing the telephone system would be:

  • At the receiver, reproduce the sound pressure waves exactly the same as they are at the transmitter, or
  • Reproduce the sensations in the listener's brain just as if they had been speaking directly to the talker.

The difference between these two ideas is that the brain is a hugely complicated processing instrument, and we can play different stimuli at it and get the same response.

What choice did Alexander Graham Bell make?

Answer #2.

Based on testing human beings' ears, throats and brains, and some technical limitations, A. G. Bell decided to transmit all of the information in the frequency range between about 300 and 3300 Hz.

Hertz (Hz) is the unit for frequency, or changes per second.

This range or band of frequencies is called the voiceband.

The image is a representation of the voiceband, with frequency on the horizontal axis and amplitude or intensity on the vertical axis.

A simplified version of the image (contained within the lesson) shows any electricity vibrating at least 300 times per second and less often than 3300 times per second will be passed. 

Any electricity vibrating less often than 300 times per second will be suppressed.

Similarly, any electricity vibrating more than 3300 times per second will be suppressed.

Only electricity vibrating within the band 300-3300 Hz will be transmitted.

The suppression of energy outside this frequency band 300-3300 Hz is implemented with simple electrical circuits called filters. There is a filter in the telephone and a filter in the switch in the CO.

For our purposes, the term bandwidth means capacity.

In the analog world, capacity is measured literally by the width of the available frequency band.

In this case, the width of this frequency band is 3300 – 300 = 3000 Hz, or 3 kHz for short.

This 3 kHz bandwidth is the capacity provided for ordinary telephone service.

Why does the voiceband stop at 3300 Hz?

The two wires that make up the loop are capable of supporting electricity vibrating more often than 3300 times per second – in fact, DSL technologies require electricity vibrating at frequencies measured in the millions of times per second.

The users' ears and brains are capable of detecting sound pressure waves vibrating more often than 3300 times per second – the human hearing range is traditionally thought to extend up to 20,000 Hz.

So why would the capacity a user is allowed to employ purposely limited to 3 kHz, even though the wires are capable of more than that, and the users are capable of more than that?

The answer is, as usual, money.

The more capacity a user is allowed to employ on the access circuit, the loop, the more capacity and hence money required to transmit the information across distance.

This narrow voiceband frequency range was chosen based on studying people's ears, throats and brains, to determine the minimum capacity necessary to meet the requirements.

Returning to the question of trees falling in the forest: the sound pressure waves at the far end are not reproduced exactly as they were at the near end; in fact, they are quite muffled and distorted, missing most of the higher frequencies.

The sound is reproduced just well enough so that the listener can recognize the speaker and understand what the speaker is saying, thus meeting the requirement to communicate information using speech and hearing.

We are interested in transmitting the minimum required to meet that objective since there is a direct relationship between the capacity a user can employ on the access circuit and the cost of transmitting the information long-distance.

cable vault in a Central Office (CO)Voiceband Filter

It turns out that the voiceband is not quite enough

It turns out that the voiceband is not quite enough bandwidth to be able to understand everything the speaker is saying!

In particular, it is difficult to tell the difference between “S” and “F” over a telephone.

This is because the frequency of sound pressure wave that distinguishes “S” from “F” is above 3300 Hz… which is not transmitted over the phone system.

Thus, one has to say “S as in Sierra” and “F as in Foxtrot”.

One could also say things like “S as in Sea”, “A as in Are” and “E as in Eye” to liven things up.

If that doesn’t get the listener confused, there’s always “E as in Ewe”.

a-as-in-are alternate phonetic alphabet Alternate Phonetic Alphabet: "A as in ARE"...

Learning Objectives - What You Will Learn

Upon completion of this course, you will be able to explain:
  • Why it is possible to restrict the capacity – the question of reproducing sound vs. reproducing thoughts in a person’s brain
  • Why the capacity is restricted – to reduce transmission costs
  • How the capacity is restricted – filters in the telephone and line card
  • Limitations of the voiceband, particularly with the sounds “s” and “f”.

Lessons In This Course

Lesson 1 is the Introduction to the Course.

Lesson 2 is a brief history lesson, beginning with the invention of the telephone. This will establish the concept of local telephone companies, access circuits and inter-city transmission.

Next, in Lesson 3, we will understand the fundamentals of the PSTN: customer premise and Central Office, loops, trunks, circuit switching and understand how a telephone call is connected end-to-end.

Then in Lesson 4, we will understand how information is represented on the local loop using analog techniques in traditional telephony, and just what exactly we mean by analog.

Lesson 5 is the question of fidelity: how faithfully the voice is reproduced at the far end, which is determined by the frequency bandwidth provided on the access, known as the voiceband. The lesson on the voiceband is available free on this page.

Lesson 6: The voiceband, loops, trunks and circuit-switching are all aspects of Plain Ordinary Telephone Service. We'll round out the discussion by understanding some of the other key aspects of POTS and related jargon and buzzwords like "twisted pair".

In Lesson 7, we will then look at an improvement on the address signaling mechanism for POTS that was called "touch tone", or more technically, DTMF.

Finally, in Lesson 8, we will understand in broad brushstrokes the control system for the telephone network, called SS7 in North America, and basic principles of call routing.

CTNS Study Guide and Companion Reference Textbook

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Overview of Courses in the CTNS Certification Package

Like Teracom's famous core training Course 101 "Telecom, Datacom and Networking for Non-Engineers", our very popular core training DVD-Video packages and the Telecom 101 textbook, the Certified Telecommunications Network Specialist Certification Package begins with the Public Switched Telephone Network, then a course on Wireless Telecommunications, followed by four courses covering IP telecommunications and IP telecom networks.

If you are interested only in IP telecommunications, the CIPTS: Certified IP Telecom Network Specialist package may be appropriate, as it skips the traditional telephony and wireless and goes directly to the IP telecommunications courses.

If your goal is to build a full, rounded knowledge of telecommunications, then understanding the history, structure and operation of the telephone network built over the past 135 years or more is the starting point for everything else.

Loops and Trunks   •  POTS   •  Circuit-Switching   •  LECs, CLECs and IXCs   •  Analog   •  Voiceband   •  DTMF   •  SS7

We begin with a history lesson, understanding how and why telephone networks and the companies that provide them are organized into local access and inter-city transmission, or as we will see, Local Exchange Carriers (LECs) and Inter-Exchange Carriers (IXCs).

Then we will establish a basic model for the PSTN and understand its main components: Customer Premise, Central Office, loop, trunk, outside plant, circuit switching, attenuation, loop length, remotes, and why knowledge of the characteristics of the loop remains essential knowledge even though we are moving to Voice over IP.

Next, we'll cover aspects of telephony and Plain Ordinary Telephone Service, including analog, the voiceband, twisted pair, supervision and signaling including DTMF. The course is completed with an overview of SS7, the control system for the telephone network in the US and Canada.

On completion of this course, you will be able to draw a model of the Public Switched Telephone Network, identify and explain its components and technologies including:

  • Loops and trunks, CO, telephone switches and circuit-switching
  • Twisted pair, the outside plant, remotes, fiber to the neighborhood
  • The founding, breakup and re-emergence of AT&T in the US; Bell & TELUS in Canada
  • LECs, IXCs and CLECs
  • Plain Ordinary Telephone Service (POTS):
  • Analog, the voiceband, how it relates to copper wires, electricity, circuits and sound
  • Supervision, dial tone, ringing, lightning protection, tip and ring, -48 volts
  • Touch-tone and DTMF
  • Basics of SS7 and call routing
Mobility   •  Cellular Networks   •  Internet over Cellular   •  GSM   •  UMTS and CDMA   •  LTE   •  WiFi   •  Satellite

In many parts of the world, particularly outside Canada, the US and Western Europe, the physical telephone network is wireless, as deploying radio transceivers is far cheaper than embarking on a new project to pull copper wires and/or fiber to every residence.

Most of this course is devoted to mobile wireless telecommunications. We begin with basic concepts and terminology including base stations and transceivers, mobile switches and backhaul, handoffs, cellular radio concepts and digital radio concepts.

Then, we cover spectrum-sharing technologies and their variations in chronological order: GSM/TDMA vs. CDMA for second generation, 1X vs. UMTS CDMA for third generation along with their data-optimized 1XEV-DO and HSPA, how Steve Jobs ended the standards wars with the iPhone and explaining the OFDM spectrum-sharing method of LTE for 4G.

This course is completed with a lesson on WiFi, or more precisely, 802.11 wireless LANs, and a lesson on satellite communications.

You'll gain a solid understanding of the key principles of wireless and mobile networks:

  • Coverage, capacity and mobility
  • Why cellular radio systems are used
  • Mobile network components and operation
  • Registration and handoffs
  • Digital radio
  • "Data" over cellular: Internet access
  • Cellular technologies: FDMA, TDMA, CDMA, OFDM
    Generations: 1G, 2G, 3G, 4G
  • Systems: GSM, UMTS, 1X, HSPA, LTE
  • WiFi, 802.11 wireless LANs
  • Satellite communications

The remaining four courses in the CTNS package are on the "IP" telecommunications network and its three main enabling technologies: Ethernet, IP and MPLS, and beginning with the OSI model and its layers to establish a framework.

If you'd prefer to take just these four "IP" courses, check out the Certified IP Telecom Network Specialist package.

Protocols & Standards   •  OSI Model   •  Layers   •  Protocol Stacks   •  How Protocol Stacks Work

This course establishes a framework for all of the subsequent discussions: the OSI 7-Layer Reference Model, which identifies and divides the functions to be performed into groups called layers.

This framework is required to sort out the many functions that need to be performed, and to be able to discuss separate issues separately.

First, we'll define the term "protocol" and compare that to a standard. Then we'll define "layer" and how a layered architecture operates, and provide an overview of the name, purpose and function of each of the seven layers in the OSI model.

Then, we'll go back through the story more slowly, with one lesson for each of the layers, examining in greater detail the functions that have to be performed and giving examples of protocols and how and where they are used to implement particular layers.

The result is a protocol stack, one protocol on top of another on top of another to fulfill all of the required functions. To make this more understandable, this course ends with the famous FedEx Analogy illustrating the concepts using company-to-company communications, and an analogy of Babushka dolls to illustrate how the protocol headers are nested at the bits level.

On completion of this course, you will be able to:

  • Define a protocol and differentiate that from a standard
  • Explain why a layered architecture is required
  • List the seven layers of the OSI model, the name, purpose and functions of each
  • Explain how the layers relate to each other
  • Explain how a protocol stack operates and protocol headers.
MAC Addresses   •  802.3 and Ethernet   •  Broadcast Domains   •  LAN Cables   •  LAN Switches   •  VLANs
This course is everything you need to know about LANs.  As we will see in the OSI Layers course, this course could also be titled "Layer 2".
We'll begin with the original LAN: Ethernet and its bus topology, defining "broadcast domain" and explaining its fundamental operation and characteristics: CSMA-CD access control, MAC addresses and MAC frames.
Then we'll cover the IEEE 802 standards and the evolution of Ethernet from 10BASE-T to Gig-E, LAN cables and the TIA-568 cable categories, basic cabling design; what "bridging" means and how a LAN switch works.
This course is completed with the important concept of VLANs: defining broadcast domains in software, a key part of basic network security practice.
On completion of this course, you will be able to explain
• Ethernet and the original bus design
• What a broadcast domain is
• What MAC addresses are
• The access control mechanism
• The IEEE 802 series of standards, 802.2 and 802.3
• Gigabit Ethernet on copper and fiber
• Codes like 100BASE-T
• LAN cables and the TIA-568 cable categories
• LAN switches, also called "Layer 2" switches
• How and why VLANs are used to separate devices
IP Packets   •  Packet Networks   •  Routers   •  Static, Dynamic, Public, Private Addresses   •  NAT   •  IPv6
This is a comprehensive course on IP addresses, routers and packets. Referring to the OSI Layers, this course could also be called Layer 3. We begin with the two basic principles of packet networks: bandwidth on demand, also known as overbooking or statistical multiplexing; and packet-switching, also known as packet forwarding or routing.
We'll understand what routers do and where they are located, routing tables and the basic operation of a router and the standard strategy deploying an edge router between the LANs and the WAN at each location.
Then we'll cover IP version 4: address classes and how they are assigned to Regional Internet Registries then ISPs then end-users, dotted-decimal notation, static addresses, dynamic addresses and DHCP, public addresses, private addresses and NAT.
The course concludes with IPv6: the IPv6 packet and changes from IPv4, IPv6 address allocations and assignments and end up understanding how IPv6 subnets will be assigned to broadcast domains and 18 billion billion addresses per residence.

On completion of this course, you will be able to explain:

  • What a packet is
  • What a router is
  • Overbooking and bandwidth on demand
  • Why and how it can be implemented
  • What a network is, what a private network is
  • How routers implement a network by connecting links
  • How routers move packets between broadcast domains
  • Basic network design and security: packet filtering
  • The basic structure and contents of a routing table
  • The Customer Edge
  • IPv4 address blocks: Class A, Class B and Class C
  • Dotted-decimal notation
  • Static addresses and dynamic addresses
  • DHCP and how and why it is used to assign both
  • Public addresses and private addresses
  • How, why and where each is used
  • NAT: Network Address Translation
  • IPv6
  • How IPv6 addresses are allocated to ISPs
  • How each residence gets 18 billion billion IPv6 addresses
Carrier Packet Networks   •  Technologies   •  MPLS   •  SLAs   •  CoS   •  Integration & Aggregation

MPLS and Carrier Networks is a comprehensive, up-to-date course on the networks companies like AT&T build and operate, how they are implemented, the services they offer, and how customers connect to the network.
The IP packets and routing of the previous course is one part of the story. Performance guarantees, and methods for quality of service, traffic management, aggregation and integration is another big part of the story, particularly once we leave the lab and venture into the real world and the business of telecommunications services.
We'll begin by establishing a basic model for a customer obtaining service from a provider, defining Customer Edge, Provider Edge, access and core, and a Service Level Agreement: traffic profile vs. transmission characteristics.
Next, we'll understand virtual circuits, a powerful tool used for traffic management and how they are implemented with MPLS, explaining the equipment, jargon and principles of operation.
Without bogging down on details, we’ll cut through buzzwords and marketing to demystify:

  • Carrier packet networks and services
  • Customer Edge (CE) and Provider Edge (PE)
  • Service Level Agreements
  • Traffic profiles
  • Virtual circuits
  • QoS, Class of Service and Differentiated Services
  • Integration, convergence and aggregation
  • MPLS and other network technologies
  • How this relates to TCP/IP
  • How MPLS is used for business customer VPNs
  • How MPLS is used for integrated access:
  • How all services are carried together on one circuit
  • How MPLS is used to prioritize and manage IP packets
  • MPLS services" vs. the Internet
This course can be taken by those who need just an introduction to carrier networks and MPLS, as well as by those who need to establish a solid base on which to build more detailed knowledge.
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About TCO Certification

Teracom is an Accredited Training Partner of the Telecommunications Certification Organization, authorized to administer exams for TCO certifications on the myTeracom Learning Management System and award TCO Certifications.

TCO Certification is proof of your knowledge of telecom, datacom and networking fundamentals, jargon, buzzwords, technologies and solutions.

It's backed up with a Certificate suitable for framing - plus a personalized Letter of Reference / Letter of Introduction detailing the knowledge your TCO Certification represents and inviting the recipient to contact Teracom for verification.

You may list Teracom Training Institute as a reference on your résumé if desired.

Getting your Certificate

Each course has a course exam, consisting of ten multiple-choice questions chosen at random from a pool and shuffled in order. Passing the course exams proves your knowledge of these topics and results in your certification as a Certified Telecommunications Network Specialist.

Your Certificate and Letter of Reference / Letter of Introduction will be immediately available for download from your Dashboard in the myTeracom Learning Management System. You may also order a signed and sealed Certificate by airmail.
Choosing the "Unlimited Plan" at registration allows you to repeat courses and/or exams at no additional charge – which means guaranteed to pass if you're willing to learn.

Alternatively, if you like this discounted package of courses, but don't need the certification – or don't feel like writing exams – no problem! Take the Telecom, Datacom and Networking for Non-Engineers course package, which includes the same courses as the CTNS certification package, without the certification exams.

Benefits of Certification for Individuals

One benefit of TCO certification is differentiating yourself from the rest of the crowd when applying for a job or angling for a promotion.

The knowledge you gain taking Teracom's Online Courses, confirmed with TCO Certification, is foundational knowledge in telecommunications, IP, networking and wireless: fundamental concepts, mainstream technologies, jargon, buzzwords, and the underlying ideas - and how it all fits together.

This type of knowledge and preparation makes you an ideal candidate to hire or promote to a task, as you will be able to build on your knowledge base to quickly get up to speed and work on a particular project - then have the versatility to work on subsequent projects.

TCO Certification will help demonstrate you have this skill... a desirable thought to have in your potential manager's mind.

Benefits of Certification for Employers

Take advantage of these courses for individual learning, a team, or for an entire organization.

The scalable myTeracom Learning Management System can register and manage all of your people through their courses, lessons and exams, and generate management reports showing progress and scores with the click of a button.

For larger organizations, the courses and exams can also be licensed and deployed on an organization's internal LMS.

Teracom certification packages are an extremely cost-effective way of implementing consistent, comprehensive telecommunications and networking technology fundamentals training, ensuring that both existing resources and new hires are up to the same speed, with a common vocabulary, framework and knowledge base.

The course exams provide concrete measurements of competency in key knowledge areas. Management can view the progress and results of all team members and export the results to Excel with the click of a button.

These reports identify skills deficiencies and strengths, and provide tangible proof of return on investment and team readiness for reports to upper management.

Teracom Advantages

  • Training based on Teracom's proven instructor-led training courses developed and refined over more than twenty years providing training for organizations including AT&T, Verizon, Bell Canada, Intel, Microsoft, Cisco, Qualcomm, the CIA, NSA, IRS, FAA, US Army, Navy, Marines and Air Force and hundreds of others, Teracom online courses are top-notch, top-quality and right up to date with the topics and knowledge you need.
  • Proven courses used by the biggest telecom carriers to train their employees
    These courses are the same courses used by the biggest telecom carriers in the business to train their employees - constantly updated to deliver the core technical knowledge required in the telecom business today. This is the best quality training of its kind available.
  • GSA Schedule
    Teracom online courses and certification packages are on our US Government supply contract... which took two years and a 200-page application... so you know you are getting quality.
  • 30-Day 100% Money-Back Guarantee
    You are protected by Teracom's 30-day, no-questions-asked, 100% money-back guarantee.  terms and conditions
  • Career-enhancing knowledge
    This training is an ideal way to implement a career-enhancing upgrade to your knowledge, or to prepare for a job in the telecommunications business.
  • Guaranteed to Pass with the Unlimited Plan
    Choose the Unlimited Plan for unlimited repeats of courses and exams - which means you can retake courses to refresh your knowledge in the future, and guaranteed to pass the exam if you're willing to learn.  unlimited plan info
  • Certificate and Letter of Reference
    In addition to your TCO Certificate, you will also receive – a Teracom exclusive – a personalized Letter of Reference / Letter of Introduction explaining the courses you took and the knowledge you have, and inviting anyone you give it to to contact Teracom a reference... an excellent addition to your CV.
  • Self-paced training
    The courses and their lessons can be done at your own pace. There are no time limits for completing a lesson and moving to the next one. The courses may be done in any order.
  • Team training
    These courses are a highly cost-effective and consistent way for managers to get team members up to a common speed with measurable results. The myTeracom Learning Management System provides management reports showing your team's progress with a few clicks of the mouse.  more info

What is the value of the CTNS certification?  Click here to find out