Remote Access

Types of Modems
• Consumers and businesses typically gain Internet access via ISPs. Many ISPs
provide a variety of connection interfaces including:
– Dial-in modem connections
– ISDN
– DSL
– Cable modems
• Wireless service providers (WSPs) provide wireless Internet access for users
with wireless modems, smart phones, and Web-enabled PDAs, or handheld
computers
• Despite increasing use of DSL and cable modems, dial-in access over voice-
grade analog circuits is the most common form of Internet access for
consumers
• Point-to-point (PPP) protocol is the most widely used protocol over dial-up
connections

Transmitting Encoded Data
Asynchronous and Synchronous Communication
• The bits that represent encoded characters can be transmitted
simultaneously (parallel transmission) or one at time (serial
transmission) – see Figure 9-1
– Serial transmission is more widely used than parallel transmission for data
communication
– Parallel transmission is used for communication between components
within a computer
• In serial transmission, encoded characters can either be transmitted
one at a time (asynchronous transmission) or in blocks (synchronous
transmission) Figure 9-2
– Figure 9-3 illustrates asynchronous transmission of a single character.
– UART provides the interface between parallel transmission within the
computer and serial transmission ports. It also plays a key role in
formatting encoded characters for asynchronous transmission

Interfaces and Interface
Standards
• There are two major classes of data communication equipment:
– Data communication equipment (DCE): this includes modems, media,
switches, routers, satellite transponders, etc.)
– Data terminating equipment (DTE): this includes terminals, servers,
workstations, printers, etc.)
• The physical interface is the manner in these two classes are joined
together (Figure 9-4)
• A wide range of interface standards exist including
– RS-232-C
– RS-422, RS-423, RS-449
– A variety of ISO and ITU interfaces
– USB and FireWire

RS-232-C
• EIA’s RS-232-C standard is arguably the most important physical
layer standard
• It is the most widely accepted standard for transferring encoded
characters across copper wires between a computer or terminal and a
modem
• RS-232-C uses voltage levels between –15 and +15 volts (Figure 9-5);
negative voltages are used to represent 1 bits and positive voltages are
use to represent 0 bits
• This standard does not specify size or kind of connectors to be used in
the interface. It does define 25 signal leads. 25-pin connectors and 9-
pin connectors are most common, but other kinds of connectors are
sometimes used

Digital Data Transmission
• All communication media are capable of
transmitting data in either digital or analog form.
• Voice-grade dial-up circuits are typically analog,
however, relative to analog transmission, digital
transmission has several advantages:
– Lower error rates
– Higher transmission speeds
– No digital-analog conversion
– Security

Analog Transmission
• Data is represented in analog form when transmitted over analog
voice-grade dial-up circuits
• This is done by varying the amplitude, frequency, or phase of the
carrier signal (carrier wave) raised during the handshaking process at
the start of a communication session between two modems
– During handshaking, the two modems raise a carrier signal and agree on
how it will be manipulated to represent 0 and 1 bits
– In some modulation schemes, more than one of the carrier signal’s
characteristics are simultaneously manipulated
• Modems (modulator/demodulators) are the devices used to translate
the digital signals transmitted by computers into corresponding analog
signals used to represent bits over analog dial-up circuits (Figure 9-6)

Transfer Speed
Bit Rates and Bandwidth
• The bandwidth of an analog channel is the difference
between the minimum and maximum frequencies it can
carry
– A voice-grade dial-up circuit can transmit frequencies between
300 and 3400 Hz and thus has a bandwidth of 3100 Hz
• For digital circuits, bandwidth is a measure of the amount
of data that can be transmitted per unit. Bits per second
(bps) is the most widely used measure for digital circuits
• Over time, bit rates (bps) have also become on of the key
measures of modem performance (e.g. a 56 Kbps modem)
– However, modem bit rates are not necessarily an accurate
reflection of their data throughput rates

Transfer Speed
Baud Rate
• Baud rate is a measure of the number of discrete signals
that can be transmitted (or received) per unit of time
• A modem’s baud rate measures the number of signals that
it is capable of transmitting (or receiving) per second
– Baud rate represents the number of times per second that a modem
can modulate (or demodulate) the carrier signal to represent bits
• Although baud rate and bit rate are sometimes used
interchangeably to refer to modem data transfer speeds,
these are only identical when each signal transmitted (or
received) represents a signal bit
– A modem’s bit rate is typically higher than its baud rate because
each signal transmitted or received may represent a combination of
two or more bits

Modem Capabilities
• Modems differ in several dimensions including:
– The type of medium they can be connected to (copper-
based, fiber-optic, wireless)
– Speed
– Connection options (such as support for call waiting)
– Support for voice-over-data
– Data compression algorithms
– Security features (such as password controls or
callback)
– Error detection and recovery mechanisms

Modem Speed
• Over time, the evolution of modem standards has corresponded with
increases in modem speeds (Table 9-1)
• In 2002, V.92 is the newest modem standard
– V.92 is backward compatible with V.90 but is capable of upstream data
rates of 48,000
– Like V.90, V.92 modems leverage PCM for downstream links
• A variety of factors contribute to modem speed and data throughput
including:
– Adaptive line probing
– Dynamic speed shifts
– Fallback capabilities
– Fallforword capabilities
– Data compression

Data Compression
• Modem data compression capabilities enable modems to have data
throughput rates greater than their maximum bit rates
• This is accomplished by substituting large strings of repeating
characters or bits with shorter codes
• The data compression process is illustrated in Figure 9-7
• Widely supported standards for data compression include (Table 9-2):
– V.42bis --- up to 4:1 compression using the Lempel Ziv algorithm
– MNP Class 5 --- supports 1.3:1 and 2:1 ratios (via Huffman encoding and
run-length encoding)
– MNP Class 7 – up to 3:1 compression
– V.44 --- capable of 20% to 100% improvements over V.42bis

Error Detection and Recovery
• In order to ensure that data is not changed or lost during
transmission, error-detection and recovery processes are
standard aspects of modem operations
• The general process is as follows (Figure 9-8)
– During handshaking, the modem pair determines the error
checking approach that will be used
– The sender sends the error-check along with the data
– The receiver calculates its own error-check on received data and
compares it to that transmitted by the sender
– If the receiver’s error-check matches the sender’s, no error is
detected; a mismatch indicates a transmission error
– Detected errors trigger error recovery mechanisms

Error Sources
• There are many sources of data communication
transmission errors including:
– Signal attenuation
– Impulse noise
– Crosstalk
– Echo
– Phase jitter
– Envelope delay distortion
– White noise
– Electromagnetic interference (EMI)

Error Impacts
• Errors cause bits to be changed (corrupted)
during transmission; without error-detection
mechanisms, erroneous data could be
received and used in application processing
• Figure 9-9 illustrates a transmission error
caused by noise

Error Detection Approaches
• Error detection processes vary in complexity and
robustness. They include:
– Parity checking (Table 9-3)
– Longitudinal redundancy checks (LRC) - Table 9-4
– Checksums
– Cyclical redundancy checks (most widely used and robust)
• CRC-12
• CRC-16
• CRC-32
– Sequence checks
– Other approaches include check digits, hash totals, byte counts,
and character echoing

Error Recovery
• Automatic repeat request (ARQ) is the most widely used error-recovery
approach in data communications. In this approach, the receiver requests
retransmission if an error occurs. There are three major kinds of ARQ:
– Discrete ARQ (aka stop-and-wait ARQ). Sender waits for an ACK or NAK
before transmitting another packet
– Continuous ARQ (aka go-back-N ARQ). Sender keeps transmitting until a
NAK is returned; sender retransmits that packet and all others after it
– Selective ARQ. Sender only retransmits packets with errors
• Forward error correction codes involve sending additional redundant
information with the data to enable receivers to correct some of the
errors they detect. Hamming code and Trellis Coded Modulation are
examples
• Error control/recovery standards include MNP Class 4, V.42, and LAPM

Public Dial Network Lines
• Public dial network lines are the very same lines that you
use for regular telephone services. This is the type of
phone you use with regular asynchronous modems. Public
dial network lines are not very fast but they adequate for
exchanging e-mail and transferring small files.
• ISDN is new type public dial network line that is faster
than POTS (Plain old Telephone System).
• ISDN is capable of handling large graphics files and
complete databases. ISDN requires a special phone line
from telephone company. ISDN is more expensive than
regular asynchronous modems, so it is used so often.

Leased Lines:
A communications circuit permanently established
for a single customer. Also called a Private line
• A leased line is different from public dial network line in that the
line dedicated to a connection between two predetermined
numbers. The connection is setup for you by the phone company
and the connection stays to whether or not you are using it. You
cannot redirect the connection to another number, because you do
not dial a number.
• A leased line can also carry more data than public dial network
line. Telephone carriers offer lines from basic capacity level of a
regular telephone line (up to 56Kpbs with special synchronous
modems and data compression) to T1 (15Mpbs) or T3 (45 Mbps)
which is faster than most local area networks.
• Leased line is expensive so unless your computers must always
be connected and exchanging information, you may wish to use
public dial network lines instead.

Remote Access Software
• Remote access Software in Windows is called the
Remote Access Service (RAS). Microsoft RAS
allows users to connect to a Windows NT,
Windows XP computer over telephone lines. If
you have more than one modem attached to your
RAS server, you can have more than computer
connect to it at a time. RAS server can accept up
to 256 simultaneous connections from remote
clients.

Remote Access Software
• Installing Remote Services
• Administrating RAS
• Making Dialup Connection

Remote Access

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