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Introduction to Information Technology

LECTURE 7THE TELEPHONE SYSTEM: WIRED AND

WIRELESS

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Digitizing Voice

We’ve been discussing digital audio Sampling and Quantizing

Note that voice sampling over digital telephone networks is not as accurate as music sampling.

QUESTION: Assume a voice signal is sampled at a rate of 8000 Hz and quantized to 256 levels at each sample.

What is the bit rate of the signal? What is the sampling interval in microseconds?

BIT RATE: 8000 samples/sec X 8 bits/sample = 64 Kbps

Sampling Interval: 1/8000 = .000125 seconds = 125 microseconds.

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Digitizing Music

Assume a song is sampled 44,100 times per second for each of 2 channels and each sample is represented by 2 bytes.

44,100 samples/second * 16 bits/sample * 2 channels = 1,411,200 bits per second (Versus 64 Kbps for voice)

What’s the sampling interval? T=1/f

Sampling Rate = 1/44,100 = .0000227 seconds(compared to .000125 seconds for voice)

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The Telephone Network

The telephone system is perhaps the most taken for granted system in the world

Instant, real-time audio communications Ubiquitous, interconnected, reliable Known as POTS – Plain Old Telephone Service

Chapter 13

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The Public Switched Telephone Network (PSTN)

Transmission Systems

Switching SystemsCustomer Equipment

Customer Equipment

Local Loop Local LoopTrunks

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A Switched Network

CO CO

TandemSwitch

TandemSwitch

TandemSwitch

GMUHome

0-5 miles

0-1000s of miles

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The Telephone System – Analog and Digital

Most telephone calls are analog from the telephone in the home to the first telephone switching office (central office).

Most telephones are within 5 miles of a “central office.”

At a central office, most incoming telephone lines are connected to equipment that converts the incoming voice to digital (A/D conversion) and the outgoing voice to analog (D/A conversion).

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Circuit Switching

Not long after A. G. Bell invented the telephone, a problem surfaced

How do you interconnect multiple users? Connecting every user to every other user would require n(n-

1) connections (where n is the number of users). Users sharing the same line would obviously be problematic.

The solution? Circuit Switching--the temporary establishment of a path

Circuit-based networks: those in which a path is maintained between the users for the duration of the call.

Packet-based networks: those in which individually addressed packets of information are sent into a communications system, and are individually forwarded until they reach the recipient.

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Completing the Call

10 digit dialing (in the U.S.) – Traditional origination: 3 digit area code -- describes a particular geographical

area--although this is changing (Overlay and 800, 877, 900)

3 digit prefix code -- describes a Central Office (usually) also called an “Exchange” in phone company lingo

4 digit code for the instrument

SS7 (Signaling System 7) is a network “above” the normal telephone network—in the old days used in-band signaling which was vulnerable to fraud--SS7 is out-of-band or common channel signaling

It is a packet network that connects switches SS7 detects when the call is completed, line is busy, trunks

are busy, or call can’t be completed for some reason (U.S. PSTN is sized to support Mother’s Day)

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PBX

Private Branch Exchange (PBX) Supports an organization with less than 7-digit dialing

PBX “hangs” off the PSTN

“Want an outside line? Dial 9.”

PBX can be real or virtual PBX switch might physically reside at company site The switch might physically reside with the telephone

company

Most large businesses have a PBX on premises.

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Future of the Telephone System?

Obviously many duplicative communications systems coexist.

The telephone system Cellular Cable The Internet Satellite

Why can’t one system handle voice, video, and data? “Convergence” technologies address the migration of

different information technologies onto a single integrated, ubiquitous network.

Wouldn’t this be more economical?

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Voice over Internet Protocol (VoIP)

VoIP is one implementation of convergence. Voice conversations carried over the Internet along with

other Internet traffic. Why do this?

How much are you charged to make a long distance call? How much are you charged to view a website in Germany?

VoIP = Voice over Internet Protocol

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Voice and Data Traffic are Different

VOICE TRAFFIC DATA TRAFFIC

Continuous Synchronous Constant Bandwidth

Circuit Switching Packet Switching

Bursty Asynchronous Varying Bandwidth

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VoIP Challenges

While traditional circuit switching of voice has some delay, IP routing involves much more data handling and therefore more delay.

Internet packet loss due to over loaded routers is expected and is addressed through retransmission. The retransmitted packet may arrive too late to be available for the synchronous reconstruction of the voice waveform.

If the transmission rate slows down enough, again it won’t be available for reconstruction of the voice waveform.

1) DELAY

2) PACKET LOSS

3) VARIABLE RATE

Packet buffering (streaming) addresses 2 & 3 but contributes to problem 1.QOS = Quality of Service

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Cellular Networks

Wireless telephony Just another medium for voice communications Also used for other types of information transfer

Pictures Text Messaging

Main advantage of wireless - mobility

Discussed in Chapter 13 of Text

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Cellular History

“Radio telephones” have existed for 60 years Bell Labs invented the first mobile radio system 50 years

ago Improvements made in the mid 1960s

Early radio telephone systems: One base station covered a city Some number of channels allocated by the FCC For example, in 1976, the New York City network could

support 12 channels, serving 543 paying customers Limited capacity related to spectrum constraints…not much

sharing and considerable bandwidth dedicated to a single call Required considerable power Supported a limited # of users “Dead” spots

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Cellular Concept

The cellular concept addressed many of the shortcomings of the first mobile telephones

Frequency reuse Rather than one base station serving an entire city or region,

many low-power base stations are distributed throughout the service area

Each base station is called a “CELL” Every cell uses a different frequency In each cell, the number of active users is still restricted by the

basic principles of the RF spectrum, but this is less of a problem because the area is small.

The power required is low because each cell is small Frequencies can be reused in multiple cells A large city might have hundreds of cells

Key Concept: Frequency Reuse

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What Makes Cellular Work?

Each cell is a low power radio with a limited range (2-10 miles)

Allows for frequency reuse Reduces interference over a wide area

As a user moves through a cell, the cell phone (radio) makes a connection to the strongest signal

When the user moves to a cell with a stronger signal, the call is “handed over” to the next cell

The handover is (hopefully) transparent to the user

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Cellular Concept

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Relationship to Telephone Network

Cellular technology heavily relies on existing wire based telephone networks

If you dial home from your cell phone, it has to enter the public switched telephone network (PSTN) somewhere

If you dial another cellular user, a Mobile Telephone Switching Office (MTSO) handles the switching

It also is the point where you interface with the PSTN It also is the brains that tells the system when to hand you

over from cell to cell as you move Individual cell sites are connected by traditional phone

lines

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A Cell Site

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A Cell Site

MTSO

PSTN

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Cellular FAQs

Why do cellular antennas look the way they do?

Why do antenna sizes vary?

Why aren’t all telephones wireless?

What’s the difference between a cell phone and a cordless phone?

Are cell phones safe?