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Chapter 12: Telecommunications Systems. 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

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  • 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
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
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
  • 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
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
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
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
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
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
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
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
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
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
The Telephone Network

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)
The Telephone Network

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
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
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)
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
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
  • 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
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
PBX Components
  • CPU, memory, telephone lines, trunks
  • Switching network
  • Supporting logic cards
  • Main distribution frame
  • Console or switchboard
  • Battery back-up system
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
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!)
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
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
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
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)
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
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)
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
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)
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
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
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?
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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)
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
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
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
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
Telecommunications Systems in Action:

A Company Makes a Service Choice

Telecommunications Systems in Action:

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
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
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
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
  • 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