Chap 4: Common Carrier Services

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Chap 4 Common Carrier Services

• MIS 3523 Business Data Communications
• Dr. Segall
• Spring 2001

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Communications Manager Responsibilities

• Communication facilities for data transmission
• Hardware software for that meet corporate needs
  for data, voice, video, facsimile, and other
  forms of electronic communication

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The U. S. Telephone Industry

• Before after AT T divestiture in 1984.
• Predivestiture Organization
• Hierarchy of switching stations through which a
  call can be forwarded
• Class 1 Station Regional Center
• Class 2 Station Sectional Center
• Class 3 Station Primary Center
• Class 4 Station Toll Center
• Class 5 Station End Office
• See Figure 4-1 on page 115.

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Postdivestiture Organization

• In 1984 AT T broken up into independent RBOC
  (Regional Bell Operating Companies) also called
  Baby Bells.
• Local calls are handled much the same way as
  before in the Predivestiture era.

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Postdivestiture Organizations

• RBOCs are subdivided into LATAs ( Local Access
  and Transport Areas).
• Each LATA may consist of one or more area codes
  depending on the population of the LATA.
• All calls originating and terminating within a
  LATA are handled exclusively by the RBOC.
• All calls that cross a LATA boundary become the
  responsibility of a long-distance provider.

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Postdivestiture Organizations

• Each LATA has a designated interchange Point of
  Presence (POP).
• A inter-LATA call is routed to the POP, where it
  is accepted by the designated long-distance
  carrier.
• The long-distance carrier routes the call to the
  POP in the recipients LATA and the call is
  switched to the recipients end office and
  telephone.

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The Telecommunications Reform Act of 1996

• Previous Telecommunications Act was passed in
  1934 which is before the computer age.
• RBOCs provided local telephone service.
• AT T, Sprint, MCI and other companies provided
  the long-distance services.
• Before the Reform Act of 1996, long-distance
  carriers could not provide local service, and
  local service providers could not provide
  long-distance service.
• 2 Capabilities in the Reform Act of 1996 give
  companies access to the existing telephone
  networks
• 1. Interconnect
• 2. Wholesaling

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The Telecommunications Reform Act of 1996

• 1. Interconnect
• prohibits a common carrier from charging
  unreasonable rates for services terminating in
  their cable network.
• 2. Wholesaling
• the owners of a telephone network must allow any
  organization to acquire and resell their existing
  services.

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Available Services

• 1. Switched Lines
• 2. Leased Telegraph-Grade Lines
• 3. Leased Voice-Grade Lines
• 4. Wide Band Transmission
• 5. T-n Service
• 6. Fractional T-n
• 7. Switched Multimegabit Data Service

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Available Services

• 1. Switched Lines
• make use of the existing telephone circuits and
  switching equipment to establish telephone
  circuits between sender receiver.
• POTS (Plain Old Telephone Service) uses the
  standard analog format currently supporting up to
  56Kbps.
• 2. Leased Telegraph-Grade Lines
• provide lower transmission rates than the
  voice-grade lines of item 3 below.
• used for very low transmission rates.
• seldom used for data communications.

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Available Services

• 3. Leased Voice-Grade Lines
• was earlier discussed on page 42 of Chapter 2.
• May be conditioned to reduce error rates, which
  in turn allows higher transmission rates.
• 4. Wide Band Transmission
• allows very high data transmission rates
• transmission rates in range of 48 to 80 Kbps
• most are digital rather than analog.

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Available Services

• 5. T-n Service
• general term for several classes of high-speed
  services such as T1, T-3 T-4.
• T-1 Service
• also called DS-1 signaling
• provides 1.544 Mbps digital transmission rate
• created from multiplexing 24 64-Kbps services
• T-4 Service
• also called DS-3 signaling
• provides 45 Mbps
• derived from multiplexing 672 64-Kbps services
• T-5 Service
• also called DS-4 signaling
• provides 274 Mbps
• derived from multiplexing 4032 64-Kbps services

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Available Services

• 6. Fractional T-n
• intended to fill void between 56 or 64 Kbps
  and1.544 Mbps by providing a portion of a T-1
  line to customers.
• Allows a user to share a T-n line with another
  subscriber by using only a portion of the 64 Kbps
  lines that are multiplexed together to form the
  T-n circuit.
• Fractional T-n subscriber can subscribe to 64,
  238, 192, 256 Kbps, etc.
• Some companies restrict the available increments
  to multiples of 1, 2, 4, 6, 8 12 channels
  yielding speeds of 64, 128, 256, 384, 512 768
  Kbps respectively.
• Allows the subscriber to optimize the line speed
  and the cost of the service.

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Available Services

• 7. Switched Multimegabit Data Service (SMDS)
• high-speed connectless digital transmission
  service
• Connectionless means that the sender and
  receiver do not need to be connected via a
  dedicated link.
• the common carrier provides the user with access
  points for both sender receiver.
• data is broken down into transmission packets.
• can be used for high-speed data transmission such
  as long-distance interconnections of LANs.

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Packet Distribition Networks (PDN)

• sometimes called
• a X.25 network,
• a packet switching network (PSN),
• a valued-added network (VAN), or
• a public data network.
• 1st introduced in 1964 at Rand Corporation.
• process of
• segmenting a message into specific-size packets,
  
• routing the packets to their destination, and
• reassembling the packets to re-create the message.

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Packet Distribition Networks (PDN)

• Arpanet
• 1st packet distribution network planned in 1967
  which became operational in 1969 with 4 nodes.
• This led to NSFNet
• and several other regional networks which became
  integrated into one supernetwork called the
  Internet.
• name of X.25 stems from
• ITU X.25 standard which defines the interface
  between data terminal equipment (DTE) and data
  circuit-terminating equipment (DCE) for
  terminals operating in the packet mode on public
  data networks.

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Packet Distribition Networks (PDN)

• Most common packet sizes are 128, 356, 512 and
  1024 bytes.
• All packets transmitted must conform to one of
  the available packet lengths.
• 1. PDNs and the OSI Layers
• Only 3 layers of OSI are described for PDNs
  because a PDN is only responsible for message
  delivery.
• These 3 layers of OSI are
• physical
• data link, and
• network layers.
• The 4 other layers of OSI are implemented in the
  users segment of the network.

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Packet Distribition Networks (PDN)

• 2. Current PDN Implementation
• Private Network
• NSFNet
• Public Networks
• those offered by AT T, CompuServe, GE
  Information Services, Infonet Services, MCI,
  Sprint.
• Foreign Networks
• those offered by Datapac in Canada, Transpac in
  France, Britains Packet Switching Service
  (PSS), and Japans Nippon Telephone Telegraph
  (NTT).

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Packet Distribition Networks (PDN)

• 3. Connection Options
• PDN provides up to 3 types of connection options
• 1. Switched Virtual Circuit (SVC)
• similar to a switched communication link in
  that both are established when needed by a
  session and dissolved when the session ends.
• end-to-end circuit allocated for duration of
  session that is accomplished by a call setup
  request tht is initiated by the user.
• SVC is dissolved at end of session by process
  known as clearing.
• 2. Permanent Virtual Circuit (PVC)
• a circuit is permanently allocated between two
  nodes, so no call setup is required.

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Packet Distribition Networks (PDN)

• 3. Connection Options (Continued)
• 3. Datagram Service
• message that fits completely into the data field
  of one packet.
• Connectionless because a dedicated connection
  is not established.
• potential of fast service for short, unrelated
  messages.
• undersireable features of datagrams
• 1. arrival order of datagrams is not guaranteed,
  as each datagram sent by a particular node may
  take a different route.
• 2. arrival is itself nto guaranteed because the
  PDN establishes datagram arrival queue depths.

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Packet Distribition Networks (PDN)

• Example of a PDN (See pages 122-124.)
• Follows a message from starting terminal to its
  destination address, using a SVC (Switched
  Virtual Circuit).
• 1. Establishing the Virtual Circuit by call
  sequence
• Step 1 A call request packet is sent from the
  sending node to the receiver. The call request is
  delivered to the receiver as an incoming call
  packet. The receiver may accept or reject the
  call.
• Step 2 If the receiver wishes to accept the
  connection, it transmits a call accepted packet
  that is presented to the sender as a call
  connected message. This establishes the
  connection, and data exchange may begin.
• Step 3 To terminate the connection, either node
  can transmit a clear request to the other. The
  reipient of the clear request acknowledges the
  disconnect with a clear conformation control
  packet.

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Example of a PDN

• 2. Data Exchange
• data exchange can begin once the virtual circuit
  has been established.
• recommended data link protocol is link access
  procedure balanced (LAPB).
• data portion of link is restricted to specific
  maximum length, 128 octets is recommended.
• PDN uses a portion of the data field for control
  information
• circuit addressing
• packet sequence numbers
• packet confirmation
• Figure 4-4 on page 123 illustrates the format of
  a PDN packet.

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Example of a PDN

• 3. Packet Assembly/Disassembly
• The 1st step in sending the data is to assemble
  the packets, a function performed by a packet
  assembly/disassembly (PAD) module.
• PAD acts on one end to transform a message into
  one or more packets of the required length and
  re-assemble the message at the other end.
• Once the message has been transformed into
  packets, the packets are passed to the PDN in
  accordance with the X.25 interface.
• The PDN then moves the data through the network
  for delivery to the destination.
• The receiving PAD takes the information from the
  data portion of the packet and reassembles the
  message.

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Example of a PDN

• 4. PDN Equipment
• 2 types of machines for use within a PDN
• 1. packet switching equipment (PSE)
• 2. signaling terminal equipment (STE)
• Figure 4-5 on page 124 shows connections in an
  X.25 network between users equipment and PSE.

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Advantages Disadvantages of a PDN

• 5. Advantages of a PDN
• 1. user is charged for the amount of data
  transmitted rather than connect time.
• 2. lower charges than those of leased lines or
  switched lines for low volumes fo data sent over
  a long period of time.
• 3. allows access to to many different locations
  without the cost of switched connections.
• Disadvantages of a PDN
• 1. Users must compete for circuit use because PDN
  is usually shared.
• 2. Cost of using PDN can exceed that of using
  leased facilities if the number of data packets
  to be transmitted is great.
• 3. Individual user is unable to make changes that
  would benefit the individual application because
  the PDN is controlled by the proprietor.

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Other Services

• 1. Frame Relay
• 2. Asynchronous Transfer Mode (ATM)
• 3. Wide Area Telecommunications Service (WATS)
• 4. Satellite Service
• 5. Integrated Services Digital Networks (ISDN)
• 6. PBX and Centrix Services
• 7. Mobile Services
• 8. Circuit Switched Cellular (CSC) Radio
• 9. Cellular Digital Packet Data (CDPD)
• 10. Mobile Radio Data
• 11. Personal Communication Service (PCS)
• 12. Digital Subscriber Lines (DSLs)

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1. Frame Relay

• Outgrowth of the X.25 network to compensate for
  the large number of errors by providing error
  checking.
• Originally used to provide data paths for nodes
  on WANs.
• Figure 4-6(a) on page 125 shows an X.25 Error
  Checking network.
• Figure 4-6(b) on page 125 shows a Frame Relay
  Error Checking network.
• Frame relay services can provide high-speed
  connection between geographically distributed
  LANs or high-speed Internet access through the
  frame relay services providers connection.

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2. Asynchronous Transfer Mode (ATM)

• high speed transmission protocol in which data
  blocks are broken down into smaller cells that
  are transmitted individually, and possibly via
  different routes in a manner similar to packet
  switching technology.
• User starts the transmission process by sending a
  block of data addressed to the recipient.
• The data is broken into 48-byte data packets for
  transmission.
• 5-bytes are appended to the 48-byte data packets
  forming a 53-byte transmission frame.
• These frames are then transmitted to the
  recipient via an ATM switch, where the 5-byte
  control data is removed, and the message is
  reassembled.
• Levels of service are called ATM Adaptation
  Layers (AAL). Table 4-1 on page 127 shows the
  basic provisions of the four commonly used layers.

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Other Services

• 3. Wide Area Telecommunications (WATS)
• an inbound and outbound telephone service that
  allows long-distance telephone service,
• associated with 800 and 888 toll-free numbers
• 4. Satellite Service
• Users may rent satellite responder time from a
  number of carriers.

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5. Integrated Services Digital Networks (ISDN)

• integration of voice and data transmission over a
  digital network.
• allows international data exchange.
• ISDN was the 1st high-speed alternative to
  switched analog connections for Internet access.
• ISDN allows 3 basic types of channels B, D, and
  H.
• B Basic
• D Data
• H Higher data rates

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5. Integrated Services Digital Networks (ISDN)

• ISDN Channel Types
• B 64 Kbps
• H0 384 Kbps
• H11 1.544 Mbps
• H12 2.048 Mbps
• D 16 or 64 Kbps (for Control Data)
• Basic Service Options
• 2B64 D16 144 Kbps
• Primary Service in North America Japan
• 23B64 D64 1.544 Mbps
• Primary Service for Europe
• 23B64 D64 2.048 Mbps

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Other Services

• 6A. Private Branch Exchange (PBX)
• Uses PSE (phone switching equipment) located on
  corporate premises.
• Allows telephone calls within an office to be
  connected locally without using the telephone
  companys end office or transmission circuits.
• 6B. Centrex Services
• Transmits data as well as voice.
• 7. Mobile Services
• available services include
• circuit switched cellular (CSC) radio telephone
• cellular digital packet data (CDPD)
• mobile radio data
• personal communications services (PCS)

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Other Services

• 8. Circuit Switched Cellular (CSC) Radio
  Telephone
• provides mobile phone connections.
• currently available in major metropolitan areas.
• Figure 4-8 on page 130 shows diagram of a
  cellular system.
• calling area is divided into cells, each of which
  is served by a transmitting station.
• transmissions are sent from one cell to another.
• adjacent cells need not use the same frequencies.
• 9. Cellular Digital Packet Data (CDPD)
• data is transmitted in digital format using
  Internet-like protocols.
• enhancement to CSC.

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Other Services

• 10. Mobile Radio Data
• radio services for data transmission up to 28.8
  Mbps
• available in most metropolitan areas
• Ex. Bell South Mobile
• 11. Personal Communications Service (PCS)
• originally used for pagers.
• expanded to include digital transmission for
  telephone portable computers.
• uses multiple fixed stations to communicate with
  users in a local area or cell.
• like Cellular Radio Telephone Systems, as a user
  moves from one cell to another, control is
  transferred to the new cell.

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12. Digital Subscriber Lines (DSL)

• provides much faster transmission rates than
  analog modems ISDN.
• Proposed DSL services
• xDSL generic designator for digital subscriber
  lines
• ADSL asymmetric DSL
• uses different upload and download speeds.
• Upstream rates will vary from 16 Mbps to 640
  Mbps.
• Downstream rates will vary from 1.5 Mbps to 9
  Mbps.
• RADSL rate adaptive DSL
• speed depends on loop distance between subscriber
  and common carrier quality of wires used.
• HDSL high data rate
• same speeds in both directions
• will provide speeds equal to T-1 of 1.5 Mbps.

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12. Digital Subscriber Lines (DSL)

• SDSL single-line DSL
• operates on a single telephone line.
• maximum distance between subscriber and telephone
  end office of 10,000 feet.
• speed of T-1 of 1.5 Mbps.
• VDSL very high data rates
• asymmetric data transmission with
• 12.9 to 52.8 Mbps download
• 1.5 to 2.3 Mbps upload.
• 13. Cable Modems
• provides fast service over cable TV cable
• speeds to 10 Mbps
• provides simultaneous Internet access and TV
  viewing.