Transcript Document

Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 4
Making Connections
Data Communications and Computer Networks
Chapter 4
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
Data Communications and Computer Networks
Chapter 4
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
Data Communications and Computer Networks
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Introduction
Connecting peripheral devices to a computer is
normally not a simple task.
Let’s 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.
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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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
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
Chapter 4
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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)
Data Communications and Computer Networks
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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
Data Communications and Computer Networks
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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)
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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).
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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, >= 100Mbps LANs
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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
Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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