3. 3
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 âvoice.â
4. 4
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.
6. 6
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.
7. 7
Basic Telephone Systems - Loops
The local loop is the telephone line that runs from the
telephone companyâs central office to your home or business.
The central office is the building that houses the telephone
companyâs 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.
9. 9
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.
10. 10
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.
12. 12
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
Chapter 12
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
Chapter 12
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
Chapter 12
The Telephone Before and After 1984
In 1984, the U.S. government broke up AT&T.
Before then, AT&T owned a large majority of all local
telephone circuits and all the long distance service.
With the Modified Final Judgment of 1984, AT&T 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).
17. 17
Data Communications and Computer Networks
Chapter 12
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.
18. 18
Data Communications and Computer Networks
Chapter 12
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.
Todayâs telephone structure is a collection of LECs, POPs,
and IECs.
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Data Communications and Computer Networks
Chapter 12
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
Chapter 12
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.
21. 21
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
23. 23
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
24. 24
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)
25. 25
PBX Components
CPU, memory, telephone lines, trunks
Switching network
Supporting logic cards
Main distribution frame
Console or switchboard
Battery back-up system
26. 26
Data Communications and Computer Networks
Chapter 12
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 â555-0101â).
5. Call to New York is completed. Frank and Sue talk.
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Data Communications and Computer Networks
Chapter 12
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 âoperatorâ.
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
Chapter 12
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
Chapter 12
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
Chapter 12
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
Chapter 12
Interactive Voice Response
Common examples of IVR include:
Call your bank to inquire about an account balance
DePaulâs 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
32. 32
Data Communications and Computer Networks
Chapter 12
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
Chapter 12
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
Chapter 12
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 Ÿ T-1 or a ½ T-1.
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Data Communications and Computer Networks
Chapter 12
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
Chapter 12
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
Chapter 12
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.
38. 38
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?
39. 39
Frame Relay
Frame relay â 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.
41. 41
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.
42. 42
Frame Relay Connection between Chicago and Orlando
showing Access Lines, Ports and Permanent Virtual Circuit
43. 43
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.
46. 46
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.
47. 47
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 customerâs frames may be discarded.
48. 48
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).
49. 49
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
Chapter 12
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.
51. 51
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 relayâs maximum is typically 45 Mbps.
52. 52
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 ATMâs 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.
53. 53
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.
54. 54
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
55. 55
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.
56. 56
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.
57. 57
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.
58. 58
Four Necessary Components of a DSL Connection
DSL splitter
DSL modem
DSLAM (multiplexer)
High Speed connection
59. 59
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)
61. 61
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 customerâs telephone number appears
on the customer support repâs terminal and immediately pulls
up the customerâs 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.
62. 62
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 userâs computer.
The telephones may still be connected to the PBX or they
may be connected to the LAN via the LAN wiring.
63. 63
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
â˘Third party call control
⢠PBX Graphic User Interface
⢠Call filtering
⢠Customized menuing systems
65. 65
Telecommunication Systems In Action â
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.
68. 68
Telecommunication Systems In Action â
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
companyâs requirements for a 160 Kbps download.
Typical various prices for these services are shown on the
next table.
70. 70
Telecommunication Systems In Action â
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
71. 71
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
Telecommunication Systems In Action â
A Company Makes a Service Choice
72. 72
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
Telecommunication Systems In Action â
A Company Makes a Service Choice
73. 73
What would you do?
T1
Frame Relay
ATM
Telecommunication Systems In Action â
A Company Makes a Service Choice
74. 74
What we covered
⢠POTS
⢠Technologies for WAN access
⢠What we did not cover â Digital Wireless
