Introduction to Information Technology

1
Introduction to Information Technology

• LECTURE 7
• THE TELEPHONE SYSTEM WIRED AND WIRELESS

2
Digitizing Voice

• Weve 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.
3
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) Whats the sampling interval?

T1/f
Sampling Rate 1/44,100 .0000227
seconds (compared to .000125 seconds for voice)
4
The Telephone Network
Chapter 13

• 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

5
The Public Switched Telephone Network (PSTN)
Transmission Systems Switching Systems
Customer Equipment
Customer Equipment
Local Loop
Local Loop
Trunks
6
A Switched Network
Tandem Switch
Home
GMU
0-5 miles
CO
CO
0-1000s of miles
Tandem Switch
Tandem Switch
7
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).

8
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.

9
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 networkin 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 cant be completed
  for some reason (U.S. PSTN is sized to support
  Mothers Day)

10
PBX
Most large businesses have a PBX on premises.

• 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

11
Future of the Telephone System?

• Obviously many duplicative communications systems
  coexist.
• The telephone system
• Cellular
• Cable
• The Internet
• Satellite
• Why cant one system handle voice, video, and
  data?
• Convergence technologies address the migration
  of different information technologies onto a
  single integrated, ubiquitous network.
• Wouldnt this be more economical?

12
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
13
Voice and Data Traffic are Different
DATA TRAFFIC
VOICE TRAFFIC

• Continuous
• Synchronous
• Constant Bandwidth

• Bursty
• Asynchronous
• Varying Bandwidth

?
Circuit Switching
Packet Switching
14
VoIP Challenges
1) DELAY

• 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 wont be available for reconstruction of the
  voice waveform.

2) PACKET LOSS
3) VARIABLE RATE
Packet buffering (streaming) addresses 2 3 but
contributes to problem 1. QOS Quality of Service
15
Cellular Networks
Discussed in Chapter 13 of Text

• Wireless telephony
• Just another medium for voice communications
• Also used for other types of information transfer
• Pictures
• Text Messaging
• Main advantage of wireless - mobility

16
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
  constraintsnot much sharing and considerable
  bandwidth dedicated to a single call
• Required considerable power
• Supported a limited of users
• Dead spots

17
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
18
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

19
Cellular Concept
20
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

21
A Cell Site
22
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 arent all telephones wireless?
• Whats the difference between a cell phone and a
  cordless phone?
• Are cell phones safe?