Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 4
Making Connections
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Last time – physical layer
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Physical Layer
 Handles the transmission of bits over a
communications channel.
 Includes:
 Voltage levels
 Electrical connections
 Media choices
 Adapters
 Connectors
 Modulation techniques
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What we will cover
• Connections to the Computer
– Modems
– Other devices
• Types and characteristics
• Advantages and disadvantages
• Interface Standards
• Data Link 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.
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.
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Modems
Modem (Modulator/ Demodulator)
It takes digital electrical pulses from a
computer, terminal, or microcomputer and
converts them into a continuous analog
signal, for transmission over an analog
voice grade circuit.
It then re-converts the analog signal to its
original digital format.
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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 1-10 protocols.
Most modern modems can support the fax standards.
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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.
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Modems
Modems for laptop computers are typically of two
forms.
The first type of laptop modem is completely inside the
laptop. A user plugs a modular phone cord (RJ-11) into
a slot which is attached to the modem.
The second type of laptop modem is about the size of a
credit card and plugs into a special connector. The
modular phone cord then plugs into this card.
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The 56K Digital Modem
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 would actually achieve 64K except:
1. the local loop is still analog, thus analog signaling
2. the analog to digital conversion at the local modem
introduces noise/error
Combined, these shortcomings drop the speed to at best
56K.
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The 56K Digital Modem
A 56K modem does not achieve 56K either because the
FCC will not let the modem transmit at the power level
necessary to support 56K, so the best the modem can do is
approximately 53K
A 56K modem will not even achieve 53K if the
connection between your modem and the remote
computer contains an additional analog to digital
conversion, or if there is significant noise on the line.
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The 56K Digital Modem
A 56K modem is based upon one of two standards:
V.90 - Upstream speed is maximum 33,600 bps
V.92 - Newer standard which allows maximum upstream
speed of 48 Kbps (under ideal conditions) and can place a
data connection on hold if the telephone service accepts
call waiting and a voice telephone call arrives
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Data Communications and Computer Networks
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Alternatives to Traditional Modems
• T-1 digital telephone lines
• cable television networks
• Integrated Services digital network – ISDN
• Digital subscriber line - DSL
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Alternatives to Traditional Modems
A T-1 line is a digital service offered by the telephone
companies and can transfer data as fast as 1.544 Mbps
(both voice and computer data).
To support a T-1 service, a channel service unit / data
service unit (CSU/DSU) is required at the end of the
connection.
More will be said about T-1 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 downstream data transfer
speeds (to the modem) between 500 Kbps and 2.5 Mbps;
upstream (out from the modem) less than 56K
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.
DSL modems support digital subscriber line service. DSL
is quickly growing in popularity and provides a highspeed 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.
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Interface Standards
Many different groups contribute to interface standards:
International Telecommunications Union (ITU) (formerly
CCITT)
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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Data Communications and Computer Networks
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Interface Standards
Interface standards can 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.
The DB-9 connector has grown in popularity due to its
smaller size.
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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-232F
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, at 400 Mbps.
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 (USB 1.0 is
only 12 Mbps, but USB 2.0 is 480 Mbps)
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Data Link Connections
•Asynchronous vs synchronous
•Duplex
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Data Link Layer
 Responsible for taking the data and
transforming it into a frame with header,
control and address information.
 Physical path communication
 Error detection
 Error correction
 Resolve competing requests
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The Internet Model
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Asynchronous Connections
A type of connection defined at the data link layer.
Traditional model for modems and printers
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 (1’s) bit can be added to the frame which
can be used to detect errors.
Total number of bits? ASCII + start bit + stop bit + parity bit = 11
bits
Indefinite amount of time between character transmission
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Synchronous Connections
A second type of connection defined at the data link layer.
Transmits sequence of many characters
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 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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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 (hey, are you ready?) the terminal.
Only one device is connected at a time
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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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
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Making Computer Connections In Action
1 and 2 - DIN connectors for keyboard and mouse (what are
the mechanical, electrical, and functional specs?)
3 - USB connectors
4 and 6 - DB-9 connectors
5 - Parallel port connector (Centronics) (USB someday?)
7, 8 and 9 - audio connectors
Will Bluetooth or ??? replace all these someday?
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Data Communications and Computer Networks
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Making Computer Connections In Action
A company wants to transfer files that are typically 700K
chars in size.
If an asynchronous connection is used, each character will
have a start bit, a stop bit, and a parity bit.
700,000 chars * 11 bits/char (8 bits data + start + stop +
parity) = 7,700,000 bits.
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Making Computer Connections In Action
If a synchronous connection is used, assume maximum
payload size = 1500 bytes.
To transfer a 700K char file requires 467 1500-character
(byte) frames.
Each frame will also contain 1-byte header, 1-byte address, 1byte control, and 2-byte checksum, thus 5 bytes of overhead.
1500 bytes payload + 5 byte overhead = 1505 byte frames.
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Making Computer Connections In Action
467 frames * 1505 bytes/frame = 716,380 bytes (5,731,040
bits).
Significantly less data than asynchronous (7,700,000 bits).
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