Making Connections

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Data Communications and Computer Networks A
Business Users 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. 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

• 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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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.
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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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Data Communications and Computer Networks
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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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Data Communications and Computer Networks
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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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Data Communications and Computer Networks
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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
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.
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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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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 Communications and Computer Networks
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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 (1s) 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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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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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, 1-byte 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).