What is a telephone and how does it work

1
OVERVIEW OF TELEPHONE NETWORK

• What is a telephone and how does it work?
• How does the phone network work?
• How are calls routed in the phone network?
• Newer technologies IVR, DOV.

2
Central office (CO)

• Hub of the citys phone network.
• Large city might have several central offices
  (Boulder 492, 494, 499, 449, etc..)
• Central office
• provides power for your phone
• routes your calls
• bills you
• Pair of wires from your home to CO is called the
  local loop.

3
Telephone instrument

• Rotary or pulse dialing
• Send as many pulses as the number being dialed,
  e.g.., 5 pulses for a five, 9 pulses for a nine,
  etc.
• Touch-tone or Dual tone multifrequency
• Each row and column has a different tone
  associated with it.
• Two tones are produced simultaneously, associated
  with the corresponding row and column.

4
Touch Tone Telephone
5
Signals on telephone network
Off-hook signal
Dial Tone
Pulses or touch-tones
Phone Company Central Office
Ringback tone /busy signal
Conversation
Call waiting tone
Flash signal
On-hook signal
6
ATT Telephone Network Hierarchy
7
The Phone Network
8
Hierarchical Routing Structure
Class 1 Class 2 Class 3 Class
4 Class 5
Regional Center Sectional Center Primary
Center Toll Center End Office
Most traffic is carried by the trunks at the
regional and sectional levels. The trunks shown
by dashed line represent redundant paths.
9
Call routing methods on the telephone network

• Hierarchical routing Only one fixed path for
  each call in the 5-level hierarchy, e.g..,
• 5-4-3-2-1-2-3-4-5
• Redundant links between different levels of the
  hierarchy provide alternate routes, but the
  network is still static.

10
Call routing methods on the telephone network

• Dynamic routing
• Several alternate paths.
• Selects route based on the current state of the
  network.
• Considers time of day, and day of week.
• Strategies
• Centralized versus distributed
• Time-dependent versus Adaptive.

11
Chicago
Seattle
J
I
Route d
Route b
Y
Route a
New York
X
Route c
Denver
K
Pittsburgh
Dynamic Routing
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Possible Routes from X to Y
13
Echo Suppression
14

NEWER TECHNOLOGIES
Interactive voice response (IVR)

• Provide specific information based on callers
  input
• (as opposed to simple routing)
• Credit card companies (Visa)
• Banks
• Parts ordering (dealerships, Avon, etc.)
• Brokerage companies (stock quotations)
• Automated fax back services
• IVR is another form of disintermediation

15

NEWER TECHNOLOGIES
Data-over-voice

• Data-over-voice superimpose data and voice on
  the same copper wire. Separate and direct them
  properly.
• Digital Data over voice similar to DOV, but in
  this case both data and voice are sent digitally.
  This is more reliable, and less error-prone.

16
Digital Data over Voice (DDOV)from Pacific Bell
Pacific Bell Digital Data Over Voice (DDOV)
service allows the simultaneous transmission of
digital data at speeds of 2.4, 4.8, 9.6 or 19.2
Kbps and voice frequency operation over the same
two-wire exchange voice grade non-loaded loop
facility.
DVM Data Voice Multiplexer CDVM Central Office
Data Voice Multiplexer.
(Source Pacific Bell)
17
New Services

• Most phone companies provide the following
  services
• Call forwarding, 3-way calling, voice mail,
  distinctive ring, call waiting, etc.
• Advanced services
• Unified messaging Integration of fax, email,
  voice mail)
• Number portability for cell phones Allows
  subscribers to change service provider, location
  and type of service, and keep the same telephone
  number (coming!?).
• Five minutes What are some other new services
  that you care about?

18
OVERVIEW OF TELEPHONE NETWORK, SWITCHING CONCEPTS
AND PBXs

• What is a Switch?
• Switching Principles?
• What is a PBX?
• What is a call center?
• What are special phone services?

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Point to point lines
If we could have point to point lines between all
senders and all receivers, then we would not need
switching.
20
Switching
Imagine point to point lines between all pairs of
people of people of
lines 2 1 3 3 10
45 100 4950 1000
0.5 million In general, for n people,
of lines n(n-1)
162 mill. Hosts 13 Quadrillion
2
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The Good Old Days
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The Even Better Present Time
A technician replaces a line card in the
telephone switch. A card handles 24 telephones.
Source http//www.bpa.gov/Corporate/KCC/circuit/9
9ci/ci0699x.shtml
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Public Telephone Network
NY City
Denver
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Switch
Bob
Alice
Jane
Dick
John
Beth
Dave

• Lines or wires from everybody's house go into
  the switch
• When a call is placed, the switch creates a
  temporary link between these lines.

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Types of switching
CIRCUIT SWITCHING creates a continuous,
dedicated path between sender and receiver. Must
set up an end to end path before any data can be
sent.
PACKET SWITCHING No dedicated path between
sender and receiver. Examples

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Circuit versus Packet Switching
Circuit Switching
Packet Switching
Source Tannenbaum, Computer Networks, 3rd Ed.,
1996
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A simple switch
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Circuit switching

• Space division switches Signal paths are
  physically separate from one another (i.e.
  divided in space).
• Single-stage
• Multi-stage
• Time-division switches Same path is shared by
  multiple streams (division in time)
• Modern switches are combination switches

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Output Lines
Single Stage space Division Switch
1 2 3 4 5 6 7 8 9 10
1
2
3
4
Input Lines
5
6
7
8
9
10
Crossbar Switch (also called Crosspoint switch)
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Blocking

• Blocking means that even if two parties are free
  they cannot call one another because of
  congestion in the switch (All circuits are busy,
  try later!!).
• If one or both parties are busy, then it is not a
  case of blocking.
• Single stage, space division switches are
  non-blocking
• Multi-stage, space division switches are blocking
• Time-division switches are usually non-blocking.

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1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
10
Three Stage Space Division Switch
32
Examples of Crossbar Switches
Number of crosspoints
Source Tannenbaum, Computer Networks, 3rd Ed.,
1996
33
Time Division Switch
1
2
3
4
5

• Memory contains the calls in progress.
• Control logic opens and closes gates.
• Each call gets a slot in a cycle.
• 1 cycle 125 micro sec.

6
Control Memory
Control Logic
1-5 2-3 3-2 4-6 5-1 6-4
34
Time Division Switch
35
Both Time and Space Switching Needed
Both time and space switching is required in
large (greater than 10,000) digital switching
offices because with the current technology, a
time slot interchange (TSI) cannot be built fast
enough to handle more than 1000-2000 lines.
(There are also TSSST switches for larger number
of channels.)
STS simpler control requirements TST better
for large switches with heavy load traffic (ie.
No.4 ESS can service over 100,000 lines) (TSM
Time Switch Module)
Source Bellamy, John. Digital Telephony. 2nd
Edition.
36
TSSST Switching Structure
TSM
TSM
n x k
(N/n) x (N/n)
k x n
TSM
TSM
n x k
(N/n) x (N/n)
k x n
TSM
TSM
TSM
TSM
Outlet time stage
Inlet time stage
Space stage
Space stage
Space stage
Source Bellamy, John. Digital Telephony. 2nd
Edition.
37
PBX (Private Branch Exchange)
Switchboard for a company or university.

• First generation late 1800s, manual operator
• Second generation 1930, no operator required
• Second and half generation 1970, Computerized
  Business Exchange.
• computer controlled switches
• programmable, more intelligent
• least cost routing of phone calls
• 3rd/4th generation 1980 - , ATT Definity 75/85.
• All digital switching
• Both computer data and voice
• Simultaneous voice and data
• Non-blocking operation.

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CENTREX

• A Centrex (Central Office Exchange Service) is
  similar to PBX, but owned and maintained by phone
  company. Cost is 20 to 50 higher than the cost
  of plain telephone lines.

39
Call Center

• More than 50 Billion business in North America.
• Growing at 20 per year.
• A central place where customer calls are routed
  by an organization, usually with some amount of
  computer automation.
• Sophisticated Routing Ability to handle a
  considerable volume of calls at the same time, to
  screen calls and forward them to someone
  qualified to handle them, and to log calls.
• Call centers are used by mail-order catalog
  organizations, telemarketing companies, computer
  product help desks, and any large organization
  that uses the telephone to sell or service
  products and services.
• Achieve economies of scale associated with mass
  call handling and yet give callers a tailored
  feel.

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Advanced call routing
Bus_136 IVRInteractive Voice Response
Improve speed and quality of call based on
information in the database, and input provided
by caller (e.g., IVR). Examples

• Skills based routing instead of
  next-available-agent.
• Key customers can move higher in the queue or be
  routed to agents assigned to their account.
• Callers who placed service calls within the last
  24 hours can be routed to the agent they
  originally spoke to.
• Calls can be routed to a properly skilled agent,
  depending on the products or services previously
  purchased.
• A telemarketing group can route calls to agents
  with the highest closing ratios in order to
  increase sales revenues.
• Challenges supporting multiple types of
  transactions multiple skills.

41
Summary

• Switching is a very important telecommunications
  technology.
• Switching technology has become very fast
  (nano-second speeds).
• Switches are essentially computers with millions
  of lines of software.
• Computer telephony integration (CTI) has made it
  possible to develop very sophisticated PBXs and
  call centers.
• Vendors are coming out with newer products and
  features every day in telephony equipment.
• The telephone system is moving closer to becoming
  all digital except for the local loop which is
  still analog.