Transcript Making Connections - Computer Science Technology

Data Communications and
Computer Networks: A
Business User’s Approach
Third Edition
Chapter 4:
Making Connections
Objectives
After reading this chapter, you should be able to:
•Identify a dial-up modem and cite its basic operating
characteristics
•Discuss the advantages of digital modems and
recognize why they do not achieve the high transfer
speeds advertised
•List the alternatives to dial-up modems, including T-1
modems, cable modems, ISDN modems, and DSL
modems
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Objectives (continued)
•Recognize the uses of a modem pool and its advantages
and disadvantages
•List the four components of all interface standards
•Discuss the basic operations of the EIA-232F interface
standard
•Cite the advantages of FireWire, Universal Serial Bus,
SCSI, iSCSI, InfiniBand, and Fibre Channel interface
standards
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Objectives (continued)
•Outline the characteristics of asynchronous and
synchronous data link interfaces
•Recognize the difference between half-duplex, fullduplex, and simplex connections
•Identify the operating characteristics of terminal-tomainframe connections and why they are unique
compared to other types of computer connection
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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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Dial-up Modems
•Modern modems use combinations of amplitude,
frequency, and phase modulation to achieve high
data rates
•Fastest dial-up modem at the moment is 56 Kbps
•Modems can support
•Auto answer
•Auto dial
•Auto disconnect
•Auto redial
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Dial-up Modems (continued)
•Connection negotiation - ability of a modem to
automatically fall forward or fall back to faster or
slower speeds, respectively
•Modems can
•Perform data compression and error correction
•Support the MNP 1-10 protocols
•Most modern modems can support fax
standards
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Dial-up Modems (continued)
•Modems can support numerous security features
including blacklisting, callback security, and
backdoor entry with password protection
•Self-testing (loop-back) - the ability of a modem to
test itself and its connection
•Local loop-back testing - tests local computer and
modem connection
•Remote loop-back testing - tests connection between
local computer and remote modem
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Dial-up Modems (continued)
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Internal vs. External Models
•Internal - plug into a slot inside a computer
•Require an assigned IRQ
•External - separate from computer
•Require serial cable and their own power supply
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Internal vs. External Models (continued)
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Modems for Laptops
•Typically of two forms:
•Completely inside the laptop
•User plugs modular phone cord (RJ-11) into a slot
attached to modem
•Second type of is about the size of a credit card
•Plugs into a special connector
•Modular phone cord then plugs into this card
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Modems for Laptops (continued)
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The 56k Digital Modem
• A 56k modem (56,000 bps) achieves this speed
through digital signaling as opposed to analog
signaling used on all other modems
• A 56k modem would actually achieve 64k except:
1.
Local loop is still analog, thus analog signaling
2. Analog to digital conversion at local modem introduces
noise/error
• Combined, these shortcomings drop the speed to at
best 56k
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The 56k Digital Modem (continued)
•A 56k modem does not achieve 56k either
•FCC will not let the modem transmit at power level
necessary to support 56k
•Best is approximately 53k
•Will not even achieve 53k if connection between
your modem and the remote computer contains
additional analog to digital conversion, or if there
is significant noise on the line
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The 56k Digital Modem (continued)
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The 56k Digital Modem (continued)
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The 56k Digital Modem (continued)
•Based upon one of two standards:
•V.90 - Upstream speed is maximum 33,600 bps
•V.92 - Newer standard with maximum upstream speed
of 48 kbps (under ideal conditions)
•Can place data connection on hold if the telephone
service accepts call waiting and a voice telephone call
arrives
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Alternatives to Dial-Up Modems
•T-1 line - digital service offered by telephone
companies
•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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Channel Service Unit/Data Service Unit
(CSU/DSU)
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Cable Modems
•Allow high speed access to wide area networks such
as the Internet
•Most are external devices that connect to the
personal computer through a common Ethernet card
•Can provide data transfer speeds between 500 kbps
and 2.5 Mbps
•A few services require a telephone line for the
upstream connection
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Cable Modems (continued)
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ISDN and DSL Modems
•ISDN modems support ISDN connections
•ISDN - all-digital service capable of supporting data
and voice, with data speeds up to 128 kbps
•DSL modems support digital subscriber line
service
•Quickly growing in popularity
•Provides 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 workstation
•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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Modem Pools (continued)
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Interfacing a Computer to Modems and
Other Devices
•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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Data Terminal Equipment and Data
Circuit-Terminating Equipment
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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 (continued)
• Interface standards can consist of four
components:
1. Electrical
2. Mechanical
3. Functional
4. Procedural
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Interface Standards (continued)
•Electrical component - deals with voltages, line
capacitance, and other electrical characteristics
•Mechanical component - deals with items such as the
connector or plug description
•Standard connector is the ISO 2110 connector, also
known as DB-25
•DB-9 connector has grown in popularity due to its
smaller size
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Interface Standards (continued)
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Interface Standards (continued)
•Functional component - describes the function of
each pin or circuit used in a particular interface
•Procedural component - describes how 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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EIA-232F and RS-232
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EIA-232F and RS-232 (continued)
•Older interface standard designed to connect a
device such as a modem to a computer or terminal
•Originally RS-232
•Has gone through many revisions
•Electrical component is defined by V.28
•Mechanical component is defined by ISO 2110
•Functional and procedural components are defined
by V.24
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EIA-232F and RS-232 (continued)
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X.21
•Another interface standard designed to replace
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
•Combination of signals on the four circuits is much
larger than if each circuit performed only a single
function
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X.21 (continued)
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RAID
•Redundant array of independent disks - collection of
techniques for interfacing multiple hard disk drives to a
computer
•RAID-0 - data is broken into pieces and each piece is
stored on a different disk drive (striping)
•RAID-1 - data is stored on at least two disk drives in
duplicate (disk mirroring)
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RAID (continued)
•RAID-3 - data is redundantly stored across multiple disk
drives
•Error-checking information is kept on a separate disk
•RAID-5 - data is broken in pieces (stripes) and stored
across three or more disks
•Error-checking information is stored along with the
striped data
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FireWire
•Bus that connects peripheral devices such as
wireless modems and high speed digital video
cameras to microcomputers at 400 Mbps
•Designated as IEEE 1394
•Supports asynchronous connections and isochronous
connections
•Provides guaranteed data transport at a predetermined rate
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Universal Serial Bus (USB)
•Modern standard for interconnecting modems and
other peripheral devices to microcomputers
•Supports plug and play
•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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SCSI and iSCSI
•SCSI (small computer system interface) - technique
for interfacing a computer to high-speed devices such as
disk drives, CDs, and DVDs
•Need appropriate SCSI adapters, interconnecting
cables, and software
•iSCSI (Internet SCSI) - technique for interfacing disk
storage to a computer via the Internet
•It looks like the disk storage is down the hall, but it
could be anywhere on the Internet
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InfiniBand and Fibre Channel
•InfiniBand - a serial connection or bus that can carry
multiple channels of data at the same time with speeds up to
billions of bits per second
•More than just a single bus, InfiniBand is a network of
high-speed links and switches
•Fibre Channel - similar to InfiniBand, but limited to the
interconnection of 126 devices
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Data-Link Connections
• Asynchronous Connection:
•
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
•
A Start bit added to the front of the frame
•
A Stop bit 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 Connections
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Asynchronous Connections (continued)
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Synchronous Connections
•Type of connection defined at the data link layer
•Creates large package (frame) that consists of
header and trailer flags, control information,
optional address information, error detection code
(checksum), and the data
•More elaborate but transfers data in a more
efficient manner
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Synchronous Connections (continued)
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Half Duplex, Full Duplex, and Simplex
Connections
•Half duplex connection - transmits data in both
directions but in only one direction at a time
•Full duplex connection - transmits data in both
directions at same time
•Simplex connection - can transmit data in only one
direction
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Terminal-to-Mainframe Computer
Connections
•Point-to-point connection - direct, unshared
connection between a terminal and a mainframe computer
•Multipoint connection - shared connection between
multiple terminals and a mainframe computer
•Mainframe is called the primary
•Terminals are called the secondaries
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Terminal-to-Mainframe Computer
Connections (continued)
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Terminal-to-Mainframe Computer
Connections (continued)
•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 first terminal,
and this terminal passes the poll onto next terminal
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Terminal-to-Mainframe Computer
Connections (continued)
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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
(continued)
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Making Computer Connections in Action
(continued)
•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
(continued)
•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
(continued)
•If a synchronous connection is used, assume
maximum payload size = 1500 bytes
•Transferring a 700K char file requires 467 1500character (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
(continued)
•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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Summary
•Dial-up and digital modems
•Alternatives to dial-up:
•T-1
•Cable
•ISDN
•DSL
•Modem pools
•Four components of interface standards
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Summary (continued)
•Interface Standards:
•EIA-232F
•FireWire
•Universal Serial Bus
•SCSI
•iSCSI
•InfiniBand
•Fibre Channel
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Summary (continued)
•Asynchronous and synchronous data link interfaces
•Half-duplex, full-duplex, and simplex connections
•Terminal-to-mainframe connections
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