Making Connections

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Data Communications and Computer Networks A
Business Users Approach

• Chapter 4
• Making Connections

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• Objectives
• Identify a standard modem and cite its basic
  operating characteristic
• Discuss advantages of newer digital modems
• List alternatives to traditional modems,
  including T1, cable, ISDN and DSL modems
• Recognise uses of modem pools
• List 4 components of all interface standards
• Discuss basic operation of the EIA-232E interface
  standard

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• Objectives
• Cite advantages of Firewire and Universal Bus
  interface standards
• Outline characteristics of asynchronous and
  synchronous data link interfaces
• Recognise the difference between half duplex,
  full duplex and simplex connections
• Identify the operating characteristics of
  terminal to mainframe connections

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Introduction Connecting peripheral devices to a
computer is normally not a simple task. Lets
examine the interface between a computer and a
device. This interface occurs at the physical
layer. We will start with the interface of a
modem, one of the more common devices.
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Modems Modern modems use combinations of
amplitude, frequency, and phase modulation to
achieve high data rates. The fastest dial-up
modem at the moment is 56 Kbps. Modems can
support auto answer, auto dial, auto disconnect,
and auto redial.
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• Modems
• Connection negotiation is the ability of a modem
  to automatically fall forward or fallback to
  faster or slower speeds, respectively.
• Modems can perform data compression and error
  correction and support the MNP 1-5 protocols e.g.
  MNP5 supports data compression, MNP4 supports
  error correction.
• Most modern modems can support the fax standards.
  E.g. V.29

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• Modems
• Modems can support numerous security features
  including blacklisting callback security, and
  backdoor entry with password protection.
• Self-testing (loop-back) is the the ability of a
  modem to test itself and its connection.
• Local loop-back testing tests the local computer
  and modem connection while remote loop-back
  testing tests the connection between the local
  computer and the remote modem.

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• Modems
• Modems can be internal, in which they plug into a
  slot inside a computer, or external, in which
  they are separate from the computer and require a
  serial cable and their own power supply.
• Internal modems do not require a serial cable but
  instead require an IRQ assigned.
• Partially internal modems in laptops, PC Cards,
  PC Card Hosts and PC Card Software PCMCIA
  (Personal Computer Memory Card International
  Association) standard.

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Breaking Bandwidth Limitations A 56K modem
(56,000 bps) achieves this speed due to digital
signaling as opposed to analog signaling used on
all other modems. A 56K modem actually only
achieves approximately 53K due to noise and
reduced power levels. A 56K modem will not even
achieve 53K if the connection between your modem
and the remote computer does not contain an
analog to digital conversion.
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• Alternatives to Traditional Modems
• A T1 line is a service offered by the telephone
  companies and can transfer data as fast as 1.544
  Mbps. (Available in the US)
• To support a T1 service, a channel service unit /
  data service unit (CSU/DSU) is required at the
  end of the connection.

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• Alternatives to Traditional Modems
• Cable modems allow high speed access to wide area
  networks such as the Internet.
• Most cable modems are external devices that
  connect to the personal computer through a common
  Ethernet card.
• Cable modems can provide data transfer speeds
  between 500 Kbps and 2.5 Mbps.
• A few cable modem services require a telephone
  line for the upstream connection.

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• Alternatives to Traditional Modems
• ISDN modems support ISDN connections. ISDN is an
  all-digital service capable of supporting data
  and voice, with data speeds up to 128 Kbps- Basic
  Rate 2B channels (64Kbps each) and 1 D channel
  (16Kbps)
• DSL modems support digital subscriber line
  service. DSL is quickly growing in popularity
  and provides a high-speed service between homes
  and Internet service providers.

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• Modem Pools
• A relatively inexpensive technique that allows
  multiple workstations to access a modem without
  placing a separate modem on each workstations.
• Modem pools can also be used to allow external
  users to dial into a business or corporate
  network via a modem in the modem pool.

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• Interfacing
• Connecting a device such as a modem (or DCE -
  data circuit-terminating equipment) to a computer
  (or DTE - data terminal equipment).
• The connections between the DTC and DCE are the
  interchange circuits.

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• Interface Standards
• Many different groups contribute to interface
  standards
• International Telecommunications Union (ITU)
• Electronics Industries Association (EIA)
• Institute for Electrical and Electronics
  Engineers (IEEE)
• International Organization for Standards (ISO)
• American National Standards Institute (ANSI)

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Interface Standards All interface standards
consist of four components 1. The electrical
component 2. The mechanical component 3. The
functional component 4. The procedural component
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• Interface Standards
• The electrical component deals with voltages,
  line capacitance, and other electrical
  characteristics.
• The mechanical component deals with items such as
  the connector or plug description. A standard
  connector is the ISO 2110 connector, also known
  as DB-25. (Dual Band)

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• Interface Standards
• The functional component describes the function
  of each pin or circuit that is used in a
  particular interface.
• The procedural component describes how the
  particular circuits are used to perform an
  operation.
• For example, the functional component may
  describe two circuits, Request to Send and Clear
  to Send. The procedural component describes how
  those two circuits are used so that the DTE can
  transfer data to the DCE.

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• RS-232 and EIA-232E
• An older interface standard designed to connect a
  device such as a modem to a computer or terminal.
• Originally RS-232, but has gone through many
  revisions.
• The electrical component is defined by V.28, the
  mechanical component is defined by ISO 2110, and
  the functional and procedural components are
  defined by V.24.

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• X.21
• Another interface standard that was designed to
  replace the aging RS-232.
• Currently popular in Europe and with ISDN
  connections.
• Each circuit in the X.21 standard can contain
  many different signals.
• Since each circuit can transmit different
  signals, the combination of signals on the four
  circuits is much larger than if each circuit
  performed only a single function.

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• Interfacing a Computer and a Peripheral
• Firewire - A bus that connects peripheral devices
  such as wireless modems and high speed digital
  video cameras to microcomputers.
• Designated as IEEE 1394.
• Firewire supports asynchronous connections and
  isochronous connections (provides a guaranteed
  data transport at a pre-determined rate).

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• Interfacing a Computer and a Peripheral
• Universal Serial Bus (USB) - Modern standard for
  interconnecting modems and other peripheral
  devices to microcomputers.
• Supports plug and play.
• USB can daisychain multiple devices.
• Like Firewire, a high speed connection.

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• Asynchronous Connections
• A type of connection defined at the data link
  layer.
• To transmit data from sender to receiver, an
  asynchronous connection creates a one-character
  package called a frame.
• Added to the front of the frame is a Start bit,
  while a Stop bit is added to the end of the
  frame.
• An optional parity bit can be added to the frame,
  which can be used to detect errors.

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• Synchronous Connections
• A second type of connection defined at the data
  link layer.
• A synchronous connection creates a large package
  (frame) that consists of header and trailer
  flags, control information, optional address
  information, error detection code (checksum), and
  the data.
• A synchronous connection is more elaborate, but
  transfers data in a more efficient manner.

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• Half Duplex, Full Duplex, and Simplex Connections
• A simplex connection can transmit data in only
  one direction. E.g. Broadcast TV, radio, most
  pagers
• A half duplex connection transmits data in both
  directions but in only one direction at a time.
  E.g. Walkie talkies, CB radio, LANs
• A full duplex connection transmits data in both
  directions and at the same time. E.g. Telephone
  systems, cable modems, gt 100Mbps LANs

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• Terminal-to-Mainframe Computer Connections
• A point-to-point connection is a direct, unshared
  connection between a terminal and a mainframe
  computer.
• A multipoint connection is a shared connection
  between multiple terminals and a mainframe
  computer.
• The mainframe is called the primary, and the
  terminals are called the secondaries.

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• Terminal-to-Mainframe Computer Connections
• To allow a terminal to transmit data to a
  mainframe, the mainframe must poll the terminal.
• Two basic forms of polling include roll-call
  polling and hub polling.
• In roll-call polling, the mainframe polls each
  terminal in a round-robin fashion.
• In hub polling, the mainframe polls the first
  terminal, and this terminal passes the poll onto
  the next terminal.

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• Making Computer Connections In Action
• The back panel of a personal computer has many
  different types of connectors, or connections
• RS-232 connectors
• USB connectors
• Parallel printer connectors
• Serial port connectors

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