Telecommunication Systems

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Data Communications and Computer Networks A
Business Users Approach

• Chapter 12
• Telecommunication Systems

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Introduction Students used to go into either data
communications or voice communications. Today,
the two fields are merging. Voice systems
transfer computer data and data networks support
voice. Anyone studying the field of data
communications and networks must learn some basic
telecommunications too.
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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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Basic Telephone Systems 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 or you need to
incorporate more advanced modulation
techniques. POTS cannot deliver faster signals.
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Basic Telephone Systems 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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Basic Telephone Systems The country 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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Basic Telephone Systems 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.
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Basic Telephone Systems A trunk is 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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Basic Telephone Systems 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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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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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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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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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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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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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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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. ARDIS and
RAM Mobile Data two good examples
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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
internal dialing Special prefixes for WATS, FX,
etc (private dialing plans) PBX intelligently
decides how to route a call for lowest cost
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More PBX 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) VoIP
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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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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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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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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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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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Interactive Voice Response Common examples of IVR
include Call your bank to inquire about an
account balance University 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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Key Telephone System Used within a small office
or a branch office, a key telephone system (KTS)
is an on premise resource sharing device similar
to a PBX. For example, a key system might
distribute 48 internal telephone sets over 16
external phone lines. The business would pay for
the 16 individual lines but have 48 telephone
sets operating. User selects outside line by
pressing corresponding line button on key set
(phone).
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Leased Line Services Most 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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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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T-1 Service A T-1 service requires 4 wires, as
opposed to a 2-wire telephone line. A T-1 can be
either intra-LATA (local) which costs roughly
350-400 per month, or inter-LATA (long
distance) which can cost thousands of dollars per
month (usually based on distance). A customer may
also be able to order a ¼ T-1 or a ½ T-1.
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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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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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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 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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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 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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Frame Relay 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 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 ms. 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). And Internet tunnels (VPNs) are
attractive.
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Voice over Frame Relay Frame relay is also
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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Switched Virtual Circuits Frame relay can also
provide switched virtual circuits (SVC). An SVC
can be created dynamically by the customer. Good
for short-term connections, but more expensive.
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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 (rt-VBR), as well
as non-real time applications (nrt-VBR), 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 higher costs (both equipment and
support) and a higher level of complexity.
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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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• 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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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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• Computer Telephony Integration
• CTI applications could include the following
• Unified messaging
• Interactive voice response
• Integrated voice recognition and response
• Fax processing and fax-back
• Text-to-speech and speech-to-text conversions

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• Computer Telephony Integration
• CTI applications could include the following
• Third party call control
• PBX Graphic User Interface
• Call filtering
• Customized menuing systems

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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