1. Chapter 12: Telecommunications Systems
Data Communications and
Computer Networks: A
Business User’s Approach
Third Edition
Chapter 12: Telecommunications Systems

2. Objectives
Identify the basic elements of a telephone system, and discuss the limitations of telephone signals
Describe the composition of the telephone industry before and after the 1984 Modified Final Judgment, and explain the differences
Describe the difference between a local exchange carrier and an interexchange carrier, and list the services each offers
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

3. Objectives (continued)
Differentiate between the roles of the local telephone company before and after the Telecommunications Act of 1996
List the types of leased lines that are available and their basic characteristics
Outline the features of ISDN, and distinguish a basic rate interface from a primary rate interface
List the basic characteristics of frame relay, such as permanent virtual circuits, committed information rate, and switched virtual circuits
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

4. Objectives (continued)
Identify the main characteristics of Asynchronous Transfer Mode, including the roles of the virtual path connection and the virtual channel connection, the importance of the classes of service available, and ATM’s advantages and disadvantages
Identify the main characteristics of digital subscriber line, and recognize the difference between a symmetric system and an asymmetric system
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5. Today, the two fields are merging
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

6. The Basic Telephone System
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

7. The Basic Telephone System (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

8. The Basic Telephone System (continued)
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

9. The Basic Telephone System (continued)
Local loop is the telephone line that runs from the telephone company’s central office to your home or business
Central office is the building that houses the telephone company’s 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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

10. The Basic Telephone System (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

11. The Basic Telephone System (continued)
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

12. The Basic Telephone System (continued)
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

13. The Basic Telephone System (continued)
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

14. The Basic Telephone System (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

15. The Basic Telephone System (continued)
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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16. The Basic Telephone System (continued)
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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17. The Basic Telephone System (continued)
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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18. Before and After 1984 The Telephone Network
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)
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19. Before and After 1984 (continued)
The Telephone Network
Before and After 1984 (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

20. Before and After 1984 (continued)
The Telephone Network
Before and After 1984 (continued)
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)
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

21. Before and After 1984 (continued)
The Telephone Network
Before and After 1984 (continued)
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
Today’s telephone structure is a collection of LECs, POPs, and IECs
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22. Telephone Networks after 1996
Telecommunications Act of 1996: 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)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

23. Telephone Networks after 1996 (continued)
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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24. Other Players in the Market
Alternate operator services - pay phones, hotel phones
Aggregators - 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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

25. Provides advanced intelligent features to users, such as:
PBX
Private branch exchange (PBX) - 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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26. Access to database storage and retrieval (interactive voice response)
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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27. CPU, memory, telephone lines, trunks Switching network
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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28. Plays a recorded greeting and offers a set of options
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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29. Automatic Call Distributor
Automatic Call Distributor: perhaps you’ve experienced this when you call a business, 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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30. Automatic Call Distributor (continued)
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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31. 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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32. Interactive Voice Response (continued)
Common examples of IVR include:
Calling 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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33. 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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34. What if you need a faster service, or need one that is always on?
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
T-1 (or T1) service provides Mbps rate
Used by businesses to connect their in-house telephone systems (PBX) and data networks to the outside world
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35. A T-1 service is always on and always transmitting
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 Mbps of data (LAN to ISP connection)
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36. T-1 Service (continued)
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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37. T-1 Service (continued)
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 * = 193 bits)
8000 frames per second * 193 bits per frame = 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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38. 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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39. ISDN (continued) 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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40. What could you use an always-on D channel for?
ISDN (continued)
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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41. A business only has to connect itself to the local frame relay port
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

42. Frame Relay (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

43. Frame Relay (continued)
A connection between two endpoints is called a permanent virtual circuit (PVC)
PVCs are created by the provider of the frame relay service
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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44. Frame Relay (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

45. Frame Relay (continued)
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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46. Frame Relay Setup
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

47. Frame Relay Setup (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

48. Committed Information Rate (CIR)
The user and frame relay service would agree upon a committed information rate (CIR)
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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49. CIR (continued)
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 customer’s frames may be discarded
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50. Frame Relay vs. the Internet
Frame relay has many advantages over the Internet, including guaranteed throughput and minimum delay as well as better security
Internet has the advantage of being practically everywhere
Cheaper and simpler to create connections (no PVCs necessary)
Internet tunnels (VPNs) are also attractive
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51. Voice over Frame Relay (VoFR)
Frame relay is also capable of supporting voice communications
High transfer speeds adequately support the needs of interactive voice
If a company requires multiple voice circuits, frame relay is an interesting solution
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52. Frame Relay 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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53. Asynchronous Transfer Mode (ATM)
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 relay’s maximum is typically 45 Mbps
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54. ATM (continued)
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 ATM’s 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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55. Variable Bit Rate (VBR): less demanding service
ATM (continued)
Variable Bit Rate (VBR): less demanding service
Can also support real time applications (rt-VBR), as well as non-real time applications (nrt-VBR), but does not demand a constant bit stream
Available bit rate (ABR): 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): 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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56. Advantages and Disadvantages of ATM
Advantages of ATM include very high speeds and different classes of service
Disadvantages include potentially higher costs (both equipment and support) and a higher level of complexity
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57. 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
There has to be a DSL provider in your region
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58. DSL (continued)
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
Service can be symmetric, in which downstream and upstream speeds are identical, or asymmetric in which downstream speed is faster than upstream speed
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59. DSL (continued) 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 then 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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60. DSL (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

61. A DSL service comes in many different forms:
DSL (continued)
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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62. DSL (continued)
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

63. Computer Telephony Integration
Computer telephony integration (CTI): emerging field that combines more traditional voice networks with modern computer networks
Consider a system in which a customer calls a customer support number:
Customer’s telephone number appears on customer support rep’s terminal and immediately pulls up customer’s data
Rep answers phone by clicking on an icon on the screen and helps the customer
Rep transfers the call by clicking on another icon on the computer screen
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64. (continued) Computer Telephony Integration
CTI can also integrate voice cabling with data cabling
PBX talks directly to the LAN server
PBX can direct the LAN server to provide a telephone operation to the user through the user’s computer
Telephones may still be connected to the PBX or they may be connected to the LAN via the LAN wiring
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65. Computer-Telephony Integration (CTI)
CTI applications could include the following:
*Unified messaging *Third party call control
*Interactive voice response *PBX Graphic User Interface
*Integrated voice recognition *Call filtering
and response
*Fax processing and fax-back *Customized menuing systems
*Text-to-speech and speech-to-text conversions
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66. Company Makes a Service Choice
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
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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

67. A Company Makes a Service Choice
Telecommunications Systems in Action:
A Company Makes a Service Choice
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

68. A Company Makes a Service Choice
Telecommunications Systems in Action:
A Company Makes a Service Choice
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

69. A Company Makes a Service Choice
Telecommunications 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 company’s requirements for a 160 kbps download
Typical various prices for these services are shown on the next table
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

70. A Company Makes a Service Choice
Telecommunications Systems in Action:
A Company Makes a Service Choice
To provide T-1 service to all four offices:
Seattle to Chicago: $6325 ($ $2.50 per mile)
San Francisco to Chicago: $6625
Dallas to Chicago: $3500
Total interLATA T-1 costs = $16,450 / month
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

71. A Company Makes a Service Choice
Telecommunications 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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

72. A Company Makes a Service Choice
Telecommunications Systems in Action:
A Company Makes a Service Choice
To provide asynchronous transfer mode service:
Four ports at 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
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

73. Summary Telephone system 1984 Modified Final Judgment
Local exchange carrier vs. interexchange carrier
Telecommunications Act of 1996
Leased lines
ISDN and basic rate interface vs. primary rate interface
Frame relay
Asynchronous Transfer Mode
Digital subscriber line and symmetric vs. asymmetric system
Data Cummunications & Computer Networks: A Business User's Approach, Third Edition