Chapter 12: Telecommunications Systems

1
Data Communications and Computer Networks A
Business Users 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

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

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 ATMs advantages and
  disadvantages
• Identify the main characteristics of digital
  subscriber line, and recognize the difference
  between a symmetric system and an asymmetric
  system

5
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

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

7
The Basic Telephone System (continued)

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

9
The Basic Telephone System (continued)

• Local loop is the telephone line that runs from
  the telephone companys central office to your
  home or business
• 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

10
The Basic Telephone System (continued)

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

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

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

14
The Basic Telephone System (continued)

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

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

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

18
The Telephone Network 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)

19
The Telephone Network Before and After
1984 (continued)

20
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

21
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
• Todays telephone structure is a collection of
  LECs, POPs, and IECs

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)

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

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

25
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

26
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

27
PBX Components

• CPU, memory, telephone lines, trunks
• Switching network
• Supporting logic cards
• Main distribution frame
• Console or switchboard
• Battery back-up system

28
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

29
Automatic Call Distributor

• Automatic Call Distributor perhaps youve
  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!)

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

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

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

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)

34
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 1.544 Mbps rate
• Used by businesses to connect their in-house
  telephone systems (PBX) and data networks to the
  outside world

35
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)

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

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 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)

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

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

40
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?

41
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

42
Frame Relay (continued)

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

44
Frame Relay (continued)

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

46
Frame Relay Setup

47
Frame Relay Setup (continued)

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

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 customers frames may be
  discarded

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

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

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

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 relays maximum is typically 45 Mbps

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 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

55
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

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

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

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

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

60
DSL (continued)

61
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)

62
DSL (continued)

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
• Customers telephone number appears on customer
  support reps terminal and immediately pulls up
  customers 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

64
Computer Telephony Integration
(continued)

• 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
  users computer
• Telephones may still be connected to the PBX or
  they may be connected to the LAN via the LAN
  wiring

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

66
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

67
Telecommunications Systems in Action A
Company Makes a Service Choice

68
Telecommunications Systems in Action A
Company Makes a Service Choice

69
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 companys requirements for a 160 kbps
  download
• Typical various prices for these services are
  shown on the next table

70
Telecommunications 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

71
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

72
Telecommunications 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

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