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7
Modems and Other Communications
Devices
Guide to Operating Systems
Second Edition
Chapter 7
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Objectives
After reading this chapter and completing the exercises
you will be able to:
• Understand analog modem architecture
• Use the classic Hayes AT modem command set with
computer communications applications
• Understand digital modem architecture and the
telecommunications methods used with digital modems
• Describe the basics of telephone line data
communications
• Understand modem communications in different
operating systems
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Analog Modem Architecture
• Computers handle information in a digital format
• A modem is a piece of hardware and associated software
that connects these two incompatible systems in a way
that lets them communicate with each other
• The analog modem that you are most likely to use work
over regular copper or fiber-optic telephone lines
• Typically these are called plain old telephone service
(POTS) lines, which is the old term, or public switched
telephone network (PSTN) lines, which is the modern
term
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Modem Hardware Basics
• A modem consists of three basic electronic hardware or
software components: the data pump, the controller, and
the UART
• The component that performs basic
modulation/demodulation is sometimes called a data pump
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Modem Hardware Basics
• The controller provides the modem’s identity
• A protocol is an established guideline that
specifies how networked data, including data
sent over a telephone network, is formatted
into a transmission unit; how it is
transmitted; and how it is interpreted at the
receiving end
• The controller also interprets Attention (AT)
commands
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Modem Hardware Basics
• Modem protocol standards are established by the
International Telecommunications Union (ITU)
and compose the ITU-T standards
• The latest standards, V.90 and V.92, define a
56,000 bits per second (56 Kbps)
communications protocol
• When a computer is connected to a modem, the
data transfer speed is the data terminal
equipment (DTE) communications rate
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Modem Standards and Protocols
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Modem Hardware Basics
• A desktop computer with a modem is an example of
a DTE because it prepares data to be transmitted
• The modem is called the data communications
equipment (DCE) and its speed is the DCE
communications rate
• The UART is an electronic chip, the Universal
Asynchronous Receiver-Transmitter
• The UART converts data from the computer into
data that can be sent to serial ports
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Modem Hardware Basics
• The UART reads in one byte of data at the
computer’s bus speed, adds a start bit at the
beginning, a stop bit at the end, generates an
interrupt, and feeds the bits to the serial port at a
slow speed that won’t overwhelm the peripheral
(a modem in this case, but the same system
applies to other serial devices)
• Computers communicate with external serial
devices in two basic modes: synchronous and
asynchronous
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Modem Hardware Basics
• These terms refer to the method used to keep the data
streams on the local and remote devices aligned so
proper data transfer can occur
• Asynchronous communication is the most common
method for today’s desktop computers
• Asynchronous communication uses fairly accurate
clocks (timers) at both ends of the connection to
synchronize data
• Synchronous communication sends information in
blocks (frames) of data that include embedded clock
signals
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Software-Based Modems
• All modems need the functions of the data pump,
controller, and UART
• Software-based modems (often referred to as
Winmodems, after the trademarked name of
3Com/U.S. Robotics popular models) replace one or
more of these components with software
• The 3Com Winmodem is a controllerless modem that
retains a hardware data pump (Digital Signal
Processor (DSP) in the case of the 3Com Winmdem),
but implements the controller functions in software
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Software-Based Modems
• So-called Host Signal Processor (HSP) modems
dispense with the controller and data pump
hardware entirely
• There is a third kind of modem unique to the Apple
Centris and Quadra AV (Audio Visual) models
• Apple’s Geoport Telecom Adapter for these
computers uses the AV computer’s onboard DSP,
rather than the computer’s CPU, to perform modem
functions
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Software-Based Modems
• One disadvantage of implementing modem
functions with software rather than hardware is that
such software takes up memory and processor
cycles, although these cycles are now quite
inexpensive
• The biggest advantage of software-based modems
is their dependence on particular operating systems
• The advantages of using a software modem are
cost savings and upgradeability
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Hayes AT Command Set
• Dennis Hayes devised a way to create a general-purpose
modem that could be configured using a command
language he invented, the Hayes command language,
also called Attention (AT) commands
• Dennis Hayes commands begin with the letters AT,
which tells the modem to interpret the next character
string as a command
• A Hayes-compatible modem is equipped with software
that acts as a command interpreter
• The command interpreter ignores spaces and dashes
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Hayes AT Command Set Summary
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Digital Modems
• Digital modems are digital devices that use digital
transmission media
• Digital modems can be network devices or serial
devices
• When connecting to TCP/IP networks, they
connect via Point-to-Point (PPP), which is a
popular communications protocol for Internet
communications
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Digital Modems
• Today there are three popular
telecommunications or cable networks, which
each use different types of digital
communications and digital modems:
– ISDN
– Cable networks
– DSL
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ISDN
• Integrated Services Digital Network (ISDN) uses a
digital telephone line for high-speed computer
communications, videoconferencing, Internet
connections, and so on
• ISDN uses standard copper telephone line pairs with
digital equipment on either end of the connection to
encode and transmit the information—an ISDN router
(to route the transmission to the right place) and
terminal adapter (TA), a type of digital modem
• Two interfaces are supported in ISDN: basic rate
interface and primary rate interface
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ISDN
• The basic rate interface (BRI) has an aggregate data
rate of 144 Kbps
• The BRI consists of three channels: two are 64 Kbps
Bearer (B) channels for data, voice, and graphics
transmissions; and the third is a 16 Kbps Delta (D,
sometimes called Demand) channel used for
communications signaling, packet switching, and
credit card verification
• The primary rate interface (PRI) supports faster data
rates, with an aggregate of switched bandwidth equal
to 1.544 Mbps
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ISDN
• In the United States and Japan, PRI consists of
twenty-three 64 Kbps channels and one 64 Kbps D
channel for signaling communications and packet
switching (plus 8 Kbps for maintenance)
• European PRI ISDN is thirty 64 Kbps channels and
one 64 Kbps signaling or packet switching channel
• The PRI is used for LAN-to-LAN connectivity, ISP
sites, videoconferencing, and corporate sites that
support telecommuters who use ISDN
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Cable Modems
• In some areas, cable TV providers also offer
data services to businesses and homes
• A cable modem is used to attach to cable data
services
• This type of modem is usually an external
device that plugs into a USB port or network
interface card in your computer, and is
connected to the coaxial cable used for the
cable TV system
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Cable Modems
•
The cable modem communicates using upstream and
downstream frequencies that are already allocated by the
cable service
•
The upstream frequency is used to transmit the outgoing
signal over a spectrum (contiguous range) of frequencies that
carry data, sound, and TV signals
•
The downstream frequency is used to receive signals, and is
also blended with other data, sound, and TV downstream
signals
•
The advantage of cable modem communications is that
currently unallocated bandwidth can be allocated to you, even
for a millisecond or two, when you are downloading a large file
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DSL Modems
• Another high-speed digital data communications
service that is challenging ISDN and cable modems
is Digital Subscriber Line (DSL)
• DSL is a digital technology that works over copper
wire that already goes into most residences and
businesses for telephone services (newer forms of
DSL can be used over fiber-optic telephone lines)
• To use DSL, you must install an intelligent adapter
in your computer, which is to be connected to the
DSL network
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Connecting to DSL
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DSL Modems
• A DSL line is dedicated to a single user, which
means that there is less likelihood that the
signal can be tapped without the telco being
altered
• Also, the DSL user employs the full bandwidth
of his or her line, in contrast to the cable
modem user who shares bandwidth with others
• On networks, DSL is connected by means of a
combined DSL adapter and router
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DSL Modems
• A router is a
device that can
be used to
direct network
traffic and
create a
firewall, so that
only authorized
users can
access network
services
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DSL Modems
• There are five types of DSL services:
– Asymmetric Digital Subscriber Line (ADSL)
– Rate Adaptive Digital Subscriber Line (RADSL)
– High Bit-Rate Digital Subscriber Line (HDSL)
– Very High Bit-Rate Digital Subscriber Line (VDSL)
– Symmetric Digital Subscriber Line (SDSL)
• ADSL is the most commonly used version of
DSL
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DSL Modems
• Aside from traditional data and multimedia
applications, ADSL also is well suited for interactive
multimedia and distance learning
• Before transmitting data, ADSL checks the telephone
line for noise and error conditions in a process called
forward error correction
• Originally developed for on-demand movie
transmission, RADSL applies ADSL technology, but
enables the transmission rate to vary depending on
whether the communication is data, multimedia, or
voice
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DSL Modems
• Originally, HDSL was designed for full-duplex
communications over two pairs of copper
telephone wires at a sending and receiving
rate of up to 1.544 Mbps for distances up to
3.6 kilometers (2.25 miles)
• Another HDSL implementation has been
created to use one of the two pairs of
telephone wires, but with a full-duplex
transmission rate of 768 Kbps
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DSL Modems
• VDSL is intended as an alternative to networking
technologies that use coaxial or fiber-optic cable
• The VDSL downstream speed is 51-55 Mbps and
upstream is 1.6-2.3 Mbps
• SDSL is similar to ADSL, but it allocates the same
bandwidth for both upstream and downstream
transmissions at 384 Kbps
• SDL is particularly useful for videoconferencing and
interactive learning because of the symmetrical
bandwidth transmissions
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DSL Technologies
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Data Communications
Over Phone Lines
• When two modems communicate, they must
have a way to halt and resume the flow of
data
• Otherwise, data buffers would fill and then
overflow, resulting in lost data
• The buffers are prevented from getting too
full through use of flow control, which is
accomplished using software or hardware
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Software Flow Control
• A popular software flow control method from
early days of modems is called Xon-Xoff
• Xon-Xoff uses the Ctrl+S character (ASCII 19) to
stop the flow of data (Xoff), and Ctrl+Q (ASCII
17) to resume (Xon)
• When the receiving computer needs time to
process the data in the buffers, perform disk
I/O, and so on, it can send an Xoff request to
the remote modem to stop the flow of data
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Software Flow Control
• One problem with Xon-Xoff flow control is
that the data being transferred may contain
Ctrl+S or Ctrl+Q characters, which can
interrupt the data transfer
• Another problem is that Xon-Xoff is a form of
signaling that uses the bandwidth of the data
stream to pass data about the condition of
the data stream
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Hardware Flow Control
• With the advent of faster modems, the
industry moved to hardware flow control
• Hardware flow control halts and resumes the
movement of data by changing the voltage on
specific pins in the serial interface
• Controlling data flow with hardware
eliminates the problem of the modem
confusing data with control signals
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Hardware Flow Control
• Figure 7-4
demonstrates
the flow control
options for a
modem set up
in Windows XP
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Error Correction
• Modems transfer bits over the phone line
• Three possible errors can occur: a bit can be lost,
an extraneous bit can be introduced, or a bit can
be flipped (changed from zero to one or from one
to zero)
• The most basic form of error correction involves
the start and stop bits
• Each eight-bit byte is framed by a start bit at the
beginning and a stop bit at the end
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Error Correction
•
The start and stop bits always have the same value
•
Parity checking is a data verification process that ensures
data integrity through a system of data bit comparisons
between the sending and receiving computer
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Error Correction
• Parity can be either even or odd (or none, if
parity checking is turned off)
• Most modems use the ITU’s V.42 standard to
provide error checking
• V.42 employs an error-checking protocol called
Link Access Protocol for Modems (LAPM),
which is used to construct data into discrete
frame-like units for transmission over
communications lines
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Data Compression
• Modems usually compress the data they send
• A data compression routine can study this picture and
see that there’s a lot of repetitive blue in the picture
• The “compressed” representation of the screen
shows a blue dot and a number that represents the
number of times the blue dot is repeated
• If you are familiar with compression utilities, such as
PKZIP or WINZIP for the PC, or StuffIt for the Mac,
then you have seen the results of data compression
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Data Compression
• Modem data compression uses a similar
technique to reduce the total number of data
bytes that must be transferred over the
connection, but it does it “on the fly,”
compressing the data while you send it
• Compression is typically accomplished by
using the V.42 bits standard, which employs
the Lempel-Ziv-Welch (LZW) compression
method
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Data Compression
• This method works “on the fly” in two important
respects
• One is that it compresses data as it is sent, rather
than waiting for all of the data to be prepared in a
buffer, compressing it, and then sending it
• The second is that it can detect when the data is
already compressed, such as a file that is
compressed using PKZIP—and it does not
attempt to compress this type of file
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Modems and the Operating System
• In the case of modems, the serial communication
protocols are used to manage data flow through
the serial port, and communications programs
external to the operating system handle the
specifics of communicating with a remote host
• Also, programs supplied with the operating
system dial a remote computer and establish a
communication link using PPP or another
protocol
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Modems and the Operating System
• To set up a
modem
combined
with Internet
access in
Windows
2000 AV,
click start,
click Control
Panel, and
click
Network and
Internet
Connections
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Modems and the Operating System
• The Mac OS uses the PPP protocol and two
major TCP/IP stacks—the MacTCP Control
Panel and the TCP/IP Control Panel—to
configure your computer for Internet or other
TCP/IP remote host access
• Older connection software and older Mac
operating systems use MacTCP; newer
versions of Mac OS, through version 9.x, use
Open Transport or the TCP/IP Control Panel
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Modems and the Operating System
• On systems that have Mac OS version 7.5.3 or 7.5.5,
users can switch between MacTCP and TCP/IP
using Network Software Selector
• In UNIX, and in some configurations of Windows, a
modem has two purposes
• It can either be a dial-in device, or a dial-out device
• When a modem is used as a dial-in device, it is
treated like a terminal connected to the computer
using a serial connection
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Modems and the Operating System
• Any such terminal is still referred to as a teletype,
from the old paper-based terminals that were used
many years ago
• To support dial-in connectivity on one of the TTYs,
UNIX uses a daemon (an internal, automatically
running program) called getty
• From the command line, many UNIX systems offer
access to a modem through one of several possible
programs, such as Minicom, which is included with
Red Hat Linux
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Modems and the Operating System
•
The first step in using
Minicom is to create a
modem configuration
file using the options
shown in Figure 7-7
•
Red Hat Linux 7.2 also
simplifies modem
setup and Internet
setup by offering a
getty called the
Network Configuration
tool and the Internet
Configuration Wizard
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Modems and the Operating System
•
The Network
Configuration tool
enables you to
configure a modem
•
UNIX supports many
software packages
that can use the
modem to dial out
•
The earliest
packages were for
message exchange
using the UNIX to
UNIX Copy Protocol,
or UUCP
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Summary
• Understanding how operating systems
govern modem communications provides an
important background to understanding the
powerful data sharing capabilities of modembased WANs
• Digital modems are not actually modems at
all since they are entirely digital and do not
perform the modulation/demodulation
required for analog devices
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Summary
• Digital modems are called modems because
they perform the same basic functions as
analog modems, allowing communications
between computers over a WAN connection,
such as a telecommunication line
• Both analog and digital modems communicate
over telephone lines by using communications
protocols, software flow control, hardware flow
control, error correction, and data compression
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Summary
• All the operating systems discussed in this book
include a communications component and
methods for establishing dial-up networking
through modems and other telecommunications
devices
• To configure a modem and dial-up networking,
you must provide the operating system with
information about your modem, such as
networking protocol(s) and addresses, AT
commands, and a dial-up access telephone
number
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