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Wireless Media In Action DataMining Corporation
has one office in Chicago and one in Los
Angeles. There is a need to transmit large
amounts of data between the two
sites. DataMining is considering using a Very
Small Aperture Terminal satellite system.
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Wireless Media In Action Cost is proportional to
high amount of traffic with very high
reliability. Speed is high enough to support
companys needs. Distance can easily expand
across the U.S. Satellite systems are robust in
most environments. Security can be very good
with encryption.
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Data Communications and Computer Networks A
Business Users Approach

• Chapter 4
• Making Connections

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Data Communications and Computer Networks
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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. Most modern
modems can support the fax standards.
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Modems Modems can support numerous security
features including callback security, and
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. Either type of modem typically requires
a memory buffer and an Interrupt line (IRQ) to be
assigned in order to communicate with the
computer processor.
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Bandwidth Limitations If two modems are
communicating data and both are attached to
analog telephone lines (analog-to-analog), then
the fastest possible data transfer speed is 33.6
Kbps. It can be proven that, due to telephone
line bandwidth limitations and the noise
introduced by analog-to-digital conversions done
in the telephone network, these analog-to-analog
transfers can never go much faster than this.
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Breaking Bandwidth Limitations A 56K modem
(56,000 bps) can achieve data transfer speeds
greater than 33.6 Kbps when the data sender has a
64,000 bps digital network connection (standard
digitized voice channel) rather than an analog
connection. A 56Kbps modem cannot not actually
achieve 64Kbps due to telephone network signaling
constraints.
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Breaking Bandwidth Limitations A 56K modem also
does not achieve exactly 56Kbps because the FCC
will not let the modem transmit at the power
level necessary to support 56K, so the best the
modem can do under perfect conditions is
approximately 53K A 56K modem will typically not
even achieve 53K if there is substantial noise on
the line. Typical users can get around 40Kbps
45 Kbps.
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56K Modem Rates are Asymmetric Finally remember
that a 56K modem only achieves higher data rates
in the download direction from the digitally
connected site to the 56K modem user. Data
uploads (from the 56K modem up to the digitally
connected site) still only go at a maximum of
33.6 Kbps because they require an
analog-to-digital conversion.
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Alternatives to Traditional Modems A T1 line is a
digital service offered by the telephone
companies and can transfer data as fast as 1.536
Mbps (both voice and computer data). To support
a T1 service, a channel service unit / data
service unit (CSU/DSU) is required at the end of
the connection. CSU/DSU is sometimes
(incorrectly) called a digital modem. More will
be said about T1 in Chapter 12.
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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. Some cable modem services
only provide downstream data transfer and 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. 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. More on IDSN and DSL in
Chapter 12.
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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 or
data communicating equipment) to a computer (or
DTE - data terminal equipment). The connections
between the DTE and DCE are the interchange
circuits. These circuit cables are typically
short (no more than a few dozen feet long).
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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.
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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. Support plug and
play. USB can daisychain multiple devices. Like
Firewire, USB is 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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Asynchronous Transmission

• Asynchronous no synchronization
• In Asynchronous transmission, transmitter and
  receiver data clocks do not need to be
  synchronized to each other
• If sender clock is a little faster or slower,
  receiver will still know where start bit begins,
  and count out 8 bits from there.
• However, Asynchronous transmission is inefficient
  (about 20 of the bits are overhead Start/Stop)

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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 transfer data in a more efficient
manner.
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Synchronous Transmission

• In Synchronous transmission, transmitter and
  receiver data clocks must be synchronized.
• This is accomplished by requiring transmitter to
  use a self-timing code to represent bits, such as
  Bipolar AMI Code.
• Receiver continuously watches voltage changes to
  determine precise bit transmission rate (bit
  timing synchronization)

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Half Duplex, Full Duplex, and Simplex
Connections A half duplex connection transmits
data in both directions but in only one direction
at a time. A full duplex connection transmits
data in both directions and at the same time. A
simplex connection can transmit data in only one
direction.
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Simplex
Data flows one way only
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Half Duplex
Data flows only one way at any particular time,
but can flow either direction.
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Full Duplex
Data can flow both ways simultaneously
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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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