Telecommunication Systems

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Data Communications and Computer Networks A
Business Users Approach

• Chapter 12
• Telecommunication Systems

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What we cover

• Telephony
• Telecommunications
• Lines one can buy

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Introduction

Students used to focus on either data
communications or voice communications. Today,
the two fields are merging. Most voice systems
are computer controlled and data networks support
voice. Anyone studying the field of data
communications and networks must learn some basic
telecommunications too. Telecommunications used
to be just voice.
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Basic Telephone Systems

POTS is the plain old telephone system that
connects most homes and small businesses. POTS
lines were designed to transmit the human voice,
which has a bandwidth less than 4000 Hz. A
telephone conversation requires two channels,
each occupying 4000 Hz.
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Various telelphone channels and their assignment
of frequencies

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Basic Telephone Systems Limitations

A 4000 Hz analog signal can only carry about
33,600 bits per second of information while a
4000 Hz digital signal can carry about 56,000
bits per second. If you want to send information
faster, you need a signal with a higher
frequency. POTS cannot deliver faster signals.
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Basic Telephone Systems - Loops

The local loop is the telephone line that runs
from the telephone companys central office to
your home or business. The central office is the
building that houses the telephone companys
switching equipment and provides a local dial
tone on your telephone. If you place a long
distance call, the central office passes your
telephone call off to a long distance provider.
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Local Loops

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Local Access Transport Areas

The USA is divided into a few hundred local
access transport areas (LATAs). If your call goes
from one LATA to another, it is a long distance
call and is handled by a long distance telephone
company. If your call stays within a LATA, it is
a local distance call and is handled by a local
telephone company.
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Trunks
A trunk is a special telephone line that runs
between central offices and other telephone
company switching centers. A trunk is usually
digital, high speed, and carries multiple
telephone circuits. A trunk is typically a
4-wire circuit, while a telephone line is a
2-wire circuit. Not associated with a single
telephone number like a line is. A telephone
number consists of an area code, an exchange, and
a subscriber extension. The area code and
exchange must start with the digits 2-9 to
separate them from long distance and operator
services.

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Composition of a telephone number

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Telephone Connections

When the telephone company installs a line, it
must not proceed any further than 12 inches into
the building. This point is the demarcation
point, or demarc. Modular connectors, such as the
RJ-11, are commonly used to interconnect
telephone lines and the telephone handset to the
base. When the handset is lifted off the base
(off-hook), an off-hook signal is sent to the
central office.
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Data Communications and Computer Networks
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Basic Telephone Systems When the off-hook signal
arrives at the central office, a dial tone is
generated and returned to the telephone. When the
user hears the dial tone, they dial (or press)
the number. The central office equipment collects
the dialed digits, and proceeds to place the
appropriate call.
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Data Communications and Computer Networks
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Basic Telephone Systems Services Foreign exchange
service (FX) - customer calls a local number
which is then connected to a leased line to a
remote site. Wide area telecommunications
services (WATS) - discount volume calling to
local and long distance sites. Off premises
extensions (OPX) - dial tone at location B comes
from the PBX at location A.
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Data Communications and Computer Networks
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The Telephone Before and After 1984 In 1984, the
U.S. government broke up ATT. Before then, ATT
owned a large majority of all local telephone
circuits and all the long distance service. With
the Modified Final Judgment of 1984, ATT had to
split off the local telephone companies from the
long distance company. The local telephone
companies formed seven Regional Bell Operating
Companies. Today, there are only 4 left Bell
South, SBC, Qwest (US West), and Verizon (Bell
Atlantic).
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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The Telephone Before and After 1984 Another
result of the Modified Judgment was the creation
of the LATA (local access and transport
area). Local telephone companies became known as
local exchange carriers (LECs), and long distance
telephone companies became known as interexchange
carriers (IEC, or IXC). Calls that remain within
a LATA are intra-LATA, or local calls. Calls
that pass from one LATA to another are
inter-LATA, or long distance.
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Data Communications and Computer Networks
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The Telephone Before and After 1984 Before 1984,
the telephone network in the U.S. resembled a
large hierarchical tree, with Class 5 offices at
the bottom and Class 1 offices at the top. Users
were connected to the Class 5 offices. The longer
the distance of a telephone call, the further up
the tree the call progressed. Todays telephone
structure is a collection of LECs, POPs, and IECs.
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Data Communications and Computer Networks
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The Telephone After 1996 Another landmark ruling
affecting the telephone industry was the
Telecommunications Act of 1996. This act opened
up the local telephone market to competitors. Now
cable TV companies (cable telephony), long
distance telephone companies, or anyone that
wanted to start a local telephone company could
offer local telephone service. Local phone
companies that existed before the Act are known
as incumbent local exchange carriers (ILEC) while
the new companies are competitive local exchange
carriers (CLEC).
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Data Communications and Computer Networks
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The Telephone After 1996 LECs are supposed to
allow CLECs access to all local loops and
switching centers / central offices. If a local
loop is damaged, the LEC is responsible for
repair. The LEC is also supposed to provide the
CLEC with a discount to the dial tone
(17-20). LECs can also provide long distance
service if they can show there is sufficient
competition at the local service level.
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Other Players in the Market

Alternate operator services - pay phones, hotel
phones Aggregators - an aggregator pulls a bunch
of small companies together and goes after phone
discounts Reseller - rents or leases variety of
lines from phone companies, then resells to
customers Specialized mobile radio carriers -
mobile communication services to businesses and
individuals, including dispatch, paging, and data
services. Motient and RAM Mobile Data two good
examples
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(No Transcript)
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PBX

Private branch exchange (PBX) - a common internal
phone switching system for medium to large-sized
businesses. Provides advanced intelligent
features to users, such as 4-digit, special
prefixes for WATS (wide area telephone service),
FX, etc (private dialing plans) PBX collects
dialed digits and intelligently decides how to
route this call for lowest cost
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PBX

Additional advanced features Voice mail Routes
incoming calls to the best station set (automatic
call distribution) Provides recorded messages and
responds to touch-tone requests (automated
attendant) Access to database storage and
retrieval (interactive voice response)
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PBX Components

CPU, memory, telephone lines, trunks Switching
network Supporting logic cards Main distribution
frame Console or switchboard Battery back-up
system
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Data Communications and Computer Networks
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PBX Example Frank calls Sue in New York 1. Frank
picks up the phone and gets dial tone from the
PBX. 2. Frank dials 9-1-212-555-0101. 3. PBX
consults routing tables, determines that this
call should go over the FX trunk to NY 4. PBX
sends dialed number over FX trunk, dropping area
code (sends 555-0101). 5. Call to New York is
completed. Frank and Sue talk.
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Data Communications and Computer Networks
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Automated Attendant Plays a recorded greeting and
offers a set of options. Lets the caller enter an
extension directly (touch tone or voice) and
bypass an operator. Forwards the caller to a
human operator if the caller does not have a
touch tone phone. Available as an option on a PBX.
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Data Communications and Computer Networks
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Automatic Call Distributor When you call a
business and are told all operators / technicians
/ support staff etc. are busy and that your call
will be answered in the order it was
received. Used in systems where incoming calling
volume is large, such as customer service, help
desk, order entry, credit authorization,
reservations, and catalog sales. Early systems
used hunt groups. Original systems routed call
to first operator in line (kept person very busy!)
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Data Communications and Computer Networks
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Automatic Call Distributor Modern systems perform
more advanced functions, such as Prioritize the
calls Route calls to appropriate agent based on
the skill set of the agent If all agents busy,
deliver call to waiting queue and play
appropriate message (like how long they may have
to wait) Forward calls to another call center,
or perform automatic return call
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Data Communications and Computer Networks
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Interactive Voice Response IVR is similar to AA
except IVR incorporates a connection to a
database (on a mainframe or server) IVR allows
caller to access and/or modify database
information. IVR can also perform fax on demand.
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Data Communications and Computer Networks
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Interactive Voice Response Common examples of IVR
include Call your bank to inquire about an
account balance DePauls online registration
system Brokerage firm taking routine orders from
investors Investment fund taking routine requests
for new account applications A company providing
employees with info about their benefit plans
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Data Communications and Computer Networks
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Leased Line Services Many home computer users use
POTS lines and conventional modems to connect to
other computer systems. What if you need a faster
service, or need one that is always on? You can
get a leased line service. A basic leased line,
or tie line, gives you a 56 Kbps data transfer
rate. A T-1 (or T1) service gives you a 1.544
Mbps rate and is used by businesses to connect
their in-house telephone systems (PBX) and data
networks to the outside world.
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Data Communications and Computer Networks
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T-1 Service A T-1 service is a digital,
synchronous TDM stream used by businesses and
telephone companies. A T-1 service is always on
and always transmitting. One T-1 service can
support up to 24 simultaneous channels. These
channels can be either voice or data (PBX
support). A T-1 service can also be provisioned
as a single channel delivering 1.544 Mbps of data
(LAN to ISP connection).
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Data Communications and Computer Networks
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T-1 Service A T-1service requires 4 wires, as
opposed to a 2-wire telephone line. A T-1 can be
either intra-LATA (local) which costs roughly
300-700 per month, or inter-LATA (long
distance) which can cost thousands of dollars per
month. A customer may also be able to order a ¼
T-1 or a ½ T-1.
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Data Communications and Computer Networks
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T-1 Service A T-1constantly transmits frames
(8000 frames per second). Each frame consists of
one byte from each of the 24 channels, plus 1
sync bit (8 24 1 193 bits). 8000 frames per
second 193 bits per frame 1.544 Mbps. If a
channel is used for voice, each byte is one byte
of PCM-encoded voice. If a channel is used for
data, each byte contains 7 bits of data and 1 bit
of control information (7 8000 56 Kbps).
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Data Communications and Computer Networks
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Integrated Services Digital Network (ISDN) ISDN
is another leased service that provides a digital
telephone or data connection into a home or
business. With ISDN you can have a digital
telephone line and a 64 Kbps data line, or one
128 Kbps data line. The basic rate interface
(BRI) is the service for homes and small
businesses, while the primary rate interface
(PRI) is the service for larger businesses.
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Data Communications and Computer Networks
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ISDN BRI ISDN consists of two B channels and one
D channel. A B channel can carry 64 Kbps of data
or PCM-encoded voice. The D channel is 16 Kbps
and carries signaling information. The B channels
are dialable, and the D channel can be always
on. Many users combine both B channels for a 128
Kbps data channel.
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ISDN

PRI ISDN is used by larger businesses and
contains 23 B channels and one 64 Kbps D
channel. PRI ISDN is essentially equivalent to a
T-1, but with ISDN the 23 channels are
dialable! The appropriate ISDN modems /
multiplexors are necessary to support this
service. What could you use an always-on D
channel for?
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Frame Relay

Frame relay packet switching Frame relay is the
leased service that can provide a high-speed
connection for data transfer between two points
either locally or over long distances. A business
only has to connect itself to the local frame
relay port. Hopefully this connection is a local
telephone call. Once the data reaches the local
frame relay port, the frame relay network, or
cloud, transmits the data to the other side.
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Data Communications and Computer Networks
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Frame Relay

A connection between two endpoints is called a
permanent virtual circuit (PVC). PVCs are created
by the provider of the frame relay service. The
user uses a high-speed telephone line to connect
its company to a port, which is the entryway to
the frame relay network. The high-speed line, the
port, and the PVC should all be chosen to support
a desired transmission speed.
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Frame Relay Connection between Chicago and
Orlando showing Access Lines, Ports and Permanent
Virtual Circuit

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Frame Relay - installation

Consider a company that has four office locations
and currently has six leased lines
interconnecting the four locations. To install
frame relay, the company would ask for six PVCs
in place of the six leased lines. The company
would also need four high speed telephone lines
and four ports connecting the four locations to
the frame relay cloud.
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Interconnection diagram showing six leased
lines used to interconnect a companys four
locations.

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Frame Relay and CIR

The user and frame relay service would agree upon
a committed information rate (CIR). The CIR
states that if the customer stays within a
specified data rate (standard rate plus a burst
rate) the frame relay provider will guarantee
delivery of 99.99 of the frames. The burst rate
cannot be exceeded for longer than 2 seconds.
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Frame Relay and CIR

For example If a company agrees to a CIR of 512
Kbps with a burst rate of 256 Kbps, the company
must stay at or below 512 Kbps, with an
occasional burst up to 768 Kbps, as long as the
burst does not last longer than 2 seconds. If the
company maintains their end of the agreement, the
carrier will provide something like 99.99
throughput and a network delay of no longer than
20 milleseconds. If the customer exceeds its CIR,
and the network becomes congested, the customers
frames may be discarded.
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Frame Relay vs. the Internet

Frame relay has many advantages over the
Internet, including guaranteed throughput and
minimum delay, and better security. Internet has
the advantage of being practically everywhere,
cheaper, and simpler to create connections (no
PVCs necessary).
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Voice over Frame Relay

Frame relay is now capable of supporting voice
communications. The high transfer speeds of frame
relay adequately support the needs of interactive
voice. If a company requires multiple voice
circuits, frame relay is an interesting solution.
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Data Communications and Computer Networks
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Switched Virtual Circuits Frame relay can now
also provide switched virtual circuits (SVC). An
SVC can be created dynamically by the
customer. Good for short-term connections.
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Asynchronous Transfer Mode

Asynchronous transfer mode (ATM) is a very high
speed packet delivery service, similar in a
number of ways to frame relay. Both send packets
of data over high speed lines. Both require a
user to create a circuit with a provider. One
noticeable difference between ATM and frame relay
is speed - ATM is capable of speeds up to 622
Mbps while frame relays maximum is typically 45
Mbps.
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Asynchronous Transfer Mode

Similar to frame relay, data travels over a
connection called a virtual channel connection
(VCC). To better manage VCCs, a VCC must travel
over a virtual path connection (VPC). One of
ATMs strengths (besides its high speeds) is its
ability to offer various classes of service. If a
company requires a high-speed, continuous
connection, they might consider a constant bit
rate service.
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Asynchronous Transfer Mode

A less demanding service is variable bit rate
(VBR). VBR can also support real time
applications, as well as non-real time
applications, but do not demand a constant bit
stream. Available bit rate (ABR) is used for
bursty traffic that does not need to be
transmitted immediately. ABR traffic may be held
up until a transmission opening is
available. Unspecified bit rate (UBR) is for
lower rate traffic that may get held up, and may
even be discarded part way through transmission
if congestion occurs.
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Asynchronous Transfer Mode

Advantages of ATM include very high speeds and
the different classes of service. Disadvantages
include potentially high costs (both equipment
and support) and a high level of
complexity. Future? Being pushed out of core
networks Legacy systems will still exist
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Digital Subscriber Line

Digital subscriber line (DSL) is a relative
newcomer to the field of leased line
services. DSL can provide very high data transfer
rates over standard telephone lines. Unfortunately
, less than half the telephone lines in the U.S.
are incapable of supporting DSL. And there has to
be a DSL provider in your region.
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Digital Subscriber Line

DSL, depending on the type of service, is capable
of transmission speeds from 100s of kilobits into
single-digit megabits. Because DSL is highly
dependent upon noise levels, a subscriber cannot
be any more than 5.5 kilometers (2-3 miles) from
the DSL central office. A DSL service can be
symmetric, in which the downstream and upstream
speeds are identical, or asymmetric in which the
downstream speed is faster than the upstream
speed.
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Digital Subscriber Line

A DSL service often connects a user to the
Internet. A DSL service can also provide a
regular telephone service (POTS). The DSL
provider uses a DSL access multiplexer (DSLAM) to
split off the individual DSL lines into homes and
businesses. A user than needs a splitter to
separate the POTS line from the DSL line, and
then a DSL modem to convert the DSL signals into
a form recognized by the computer.
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Four Necessary Components of a DSL Connection

DSL splitter DSL modem DSLAM (multiplexer) High
Speed connection
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Digital Subscriber Line

• A DSL service comes in many different forms
• ADSL - Asymmetric DSL
• CDSL - Consumer DSL (trademarked version by
  Rockwell)
• DSL.Lite - Slower form than ADSL.
• HDSL - High-bit rate DSL
• RADSL - Rate adaptive DSL (speed varies
  depending on noise level)

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Comparison of different WAN connection
technologies

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Computer Telephony Integration

Computer telephony integration (CTI) is the
emerging field that combines more traditional
voice networks with modern computer
networks. Consider a system in which a customer
calls a customer support number. The customers
telephone number appears on the customer support
reps terminal and immediately pulls up the
customers data. The rep answers the phone by
clicking on an icon on the screen and helps the
customer. The rep transfers the call by clicking
on another icon on the computer screen.
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Computer Telephony Integration

CTI can also integrate voice cabling with data
cabling. The company PBX talks directly to the
LAN server. The PBX can direct the LAN server to
provide a telephone operation to the user through
the users computer. The telephones may still be
connected to the PBX or they may be connected to
the LAN via the LAN wiring.
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Possible CTI Apps

• Unified messaging
• Interactive voice response
• Integrated voice recognition and response
• Fax processing and fax-back
• Text-to-speech and speech-to-text conversions
• Third party call control
• PBX Graphic User Interface
• Call filtering
• Customized menuing systems

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Computer Telephony Integration

A passing fad or new area in networking and
telecommunications?
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Telecommunication Systems In Action A Company
Makes a Service Choice

Better Box Corporation has offices in Seattle,
San Francisco, and Dallas, with headquarters in
Chicago. Better Box wants to connect Chicago to
each of the other three offices. Better Box needs
to download 400 Kbyte files in 20 seconds. This
requires a transmission speed of 160,000
bps. Better Box could use three separate T-1
lines, use a frame relay service, or use
asynchronous transfer mode.
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Telecommunication Systems In Action A Company
Makes a Service Choice

Better Box Corporation might also consider
dial-up access lines, ISDN BRI service, and
leased 56K lines. Dial-up, ISDN BRI, and leased
56K lines will not meet the companys
requirements for a 160 Kbps download. Typical
various prices for these services are shown on
the next table.
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Telecommunication Systems In Action A Company
Makes a Service Choice

To provide T-1 service to all four
offices Seattle to Chicago 6325 (1200 2.50
per mile) San Francisco to Chicago 6625 Dallas
to Chicago 3500 Total interLATA T-1 costs
16,450 / month
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Telecommunication Systems In Action A Company
Makes a Service Choice

To provide frame relay service Three ports at
256K 3 x 495 One port at 768K 1240 Three
256K PVCs 3 x 230 Four intraLATA T-1s 4 x
350 Total charge 4815 / month
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Telecommunication Systems In Action A Company
Makes a Service Choice

To provide asynchronous transfer mode
service Four ports at 1.544 Mbps ABR 4 x
1750 Three channels 3 x 250 Three paths 2
per mile x 5140 miles 10,280 Four intraLATA
T-1s 4 x 350 Total ATM charges 19,430 /
month
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Telecommunication Systems In Action A Company
Makes a Service Choice

What would you do? T1 Frame Relay ATM
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What we covered

• POTS
• Technologies for WAN access
• What we did not cover Digital Wireless