Computer Networking
网络课件
双语教学
模拟实验
计算机网络教研室
CHAPTER 9
COMMUNICATIONS
CIRCUITS
1
The first section
2
Exercises
3
Online lecture
Department of Computer
Networking Application
Chapter 9 Communications Circuits
9.1 INTRODUCTION
9.2 COMMUNICATIONS CIRCUITS
The physical connection between two or more
points used for the purpose of communicating is called
a line or circuit.
A link is a segment of a circuit between two points.
A node is a functional unit that connects two
transmission links or lines.
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Chapter 9 Communications Circuits
Figure 9-1
Links are segments of a circuit. Nodes connect
links or lines or may be an end point on a link.
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Chapter 9 Communications Circuits
9.2 COMMUNICATIONS CIRCUITS
A channel is a subdivision of a circuit and is a
one way path for communicating.
A data circuit is sometimes divided into two
channel:data channel and control channel.
The data channel is called the forward
channel,and the control channel is the reverse
channel.
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Chapter 9 Communications Circuits
9.3 THE ENVIRONMENT IN WHICH
CIRCUITS OPERATE
LAN WAN
9.4 TYPES OF CIRCUITS
9.4.1 Point-to-Point Circuit
A point-to-point circuit connects two and
only two nodes.
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Chapter 9 Communications Circuits
Figure 9-2
Types of circuits with nodes A, B, C, D, and the host computer.
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Chapter 9 Communications Circuits
9.4.2 Multipoint Circuits
A circuit that connects more than two points is
called a multipoint circuit or multi-drop circuit.
9.4.3 Two-wire and Four-wire Circuits
Four-wire circuits are preferable to two wire
circuit for data transmission because they can carry
communications in both directions simultaneously
without employing any special multiplexing
techniques.
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Chapter 9 Communications Circuits
9.4.4 Analog Circuits
A modem converts a signal from digital to
analog mode for transmission on an analog circuit,
and back to digital mode at the receiving end.
9.4.5 Digital Circuits
A DSU/CSU provides the signal timing and
shaping interface between a DTE and a digital
circuit.
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Chapter 9 Communications Circuits
An amplifier takes the input signal and produces an exact
replica (including noise and distortion) at a higher level on
the output.
Figure 9-3
Amplification of an analog signal.
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Chapter 9 Communications Circuits
A repeater regenerates the input signal so that its
output is clean and free of distortion and noise.
Figure 9-4
Regeneration of a digital signal.
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Chapter 9 Communications Circuits
9.4.6 T-carrier system
T-carrier systems are a family of highspeed digital transmission systems.
A T-1 circuit can carry data at 1.544
Mbps.
9.4.7 Fractional T-1
P175
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Chapter 9 Communications Circuits
9.4.8 Digital Subscriber Line (DSL)
A communication line technology that
provides high-speed, asymmetric, dedicated
point-to-point, public network access over
twisted pair copper wire between a network
service provider's central office and the
customer's site is called ADSL.
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Chapter 9 Communications Circuits
Figure 9-5
An ADSL circuit is divided into several channels, and uses a
much wider frequency range than a standard telephone circuit.
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Chapter 9 Communications Circuits
9.4.8 Digital Subscriber Line (DSL)
A DSL technology that provides equal
speed channels in both directions and may
not include a telephone channel is called
SDSL.
VDSL transmits data at top speeds of
51 to 55 Mbps and is asymmetric.
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Chapter 9 Communications Circuits
9.4.9 Cable TV (CATV) Circuits
CATV systems often have to be reengineered
before they can carry signals in both directions.
Using a CATV circuit for data transmission
requires a modem to be installed on the cable at the
user’s home or office.
9.4.10 Satellite Circuits
DSS can be used to provide high-speed access to
the Internet.
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Chapter 9 Communications Circuits
9.5 CIRCUIT ACQUISITION AND OWNERSHIP
9.5.1 Private Circuits
Private circuits are normally installed and
maintained by a company other than a common
carrier.
9.5.2 Leased Circuits
Leased circuits are circuits owned by
common carriers but leased from them by another
organization for full-time exclusive use.
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Chapter 9 Communications Circuits
9.6 CIRCUIT SWITCHING
Circuit swithing means that there is a
dedicated communication line between
two nodes, but only for the duration of a
connection.
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Chapter 9 Communications Circuits
Figure 9-6
Circuit switching networks build temporary
connections between devices that wish to communicate.
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Chapter 9 Communications Circuits
9.7 PACKET SWITCHING
When packet switching technology is used,
messages are broken down into fixed length
pieces called packets, which are sent through a
network individually.
The process of segmenting messages into
packets is called packetizing.
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Chapter 9 Communications Circuits
Figure 9-7
Packets from the three PCs are interspersed on the circuit.
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Chapter 9 Communications Circuits
Packetized messages can be sent through the
network in one of three ways.

SVC
P182
 If two packet nodes communicate
frequently, a PVC, similar to a leased line, may
be established.

Datagram service
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Chapter 9 Communications Circuits
9.8 MULTIPLEXING
The purpose of multiplexing is to share a
circuit between multiple users in order to
improve the circuit’s utilization efficiency.
9.8.1 Time Division Multiplexing(TDM)
The type of multiplexing that assigns
specific time slots to each node is called time
division multiplexing.
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Chapter 9 Communications Circuits
Figure 9-8
FDM channels have full time use of a limited
range of frequencies. TDM channels can use the full range of
frequencies, but only during predetermined time slots.
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Chapter 9 Communications Circuits
Figure 9-9
Time Division Multiplexing (TDM).
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Chapter 9 Communications Circuits
9.8.2 Statistical Time Division
Multiplexing(STDM)
The type of multiplexing that divides a
circuit into time slots but does not assign
specific slots to nodes is called statistical
time division multiplexing.
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Chapter 9 Communications Circuits
Figure 9-10
STDM tries to avoid having empty slots in a frame,
thereby improving the line use. If a terminal has no data to send in a
particular time period, STDM will see if the next terminal has data that
can be included in the time slot. When STDM at the receiving end breaks
the frame apart, it uses the terminal address to route the data to the
proper device.
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Chapter 9 Communications Circuits
9.8.3 Concentration
Concentrators _____.
a.
combine several low speed circuits into
one higher speed circuit
b.
can be though of as a circuit multiplexer
c.
contain intelligence and buffers
d.
are frequently used for economic reasons
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Chapter 9 Communications Circuits
Figure 9-11
Line concentration.
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Chapter 9 Communications Circuits
9.8.4 Inverse Concentration
Figure 9-12
Inverse concentration.
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Chapter 9 Communications Circuits
9.9 CIRCUIT ERROR CONDITIONS
9.9.1 Backgroud Noise
A normal phenomenon of electrical circuitry resulting
from the movement of electrons is called backgroud noise.
9.9.2 Impulse Noise
Noise on a circuit caused by electrical phenomena such
as lighting, switching equipment or motors starting is
called impulse noise.
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9.9.3 Attenuation
The weakening of the signal over a distance, which
occurs normally in all communications is called
attenuation.
Figure 9-13 A signal loses strength as the distance it travels
increases. This loss of strength is called attenuation.
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Chapter 9 Communications Circuits
9.9.4 Attenuation Distortion
Attenuation distortion occurs because the signal
does not attenuate evenly across its frequency
range.
9.9.5 Envelop Delay Distortion
Envelop delay distortion is an electrical
phenomenon that occurs when not all frequencies
propagate down a communications circuit at
exactly the same speed.
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Chapter 9 Communications Circuits
9.9.6 Phase Jitter
Phase jitter is a change in the phase of
the signal induced by the carrier signal.
9.9.7 Echo
Echo is the reversal of the
signal,bouncing it back to the sender.
Echo suppressor
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Chapter 9 Communications Circuits
9.9.8 Crosstalk
Crosstalk is interference that occurs when
signals from one communications channel
interfere with those on another channel .
9.9.9 Dropouts
Dropouts occur when a circuit suddenly goes
dead for a period of time,similar to an electrical
power outage.
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Chapter 9 Communications Circuits
Which of the following is not circuit
condition that causes errors?
a.
b.
c.
d.
e.
attenuation distortion
impulse noise
chatter
echo
dropouts
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Chapter 9 Communications Circuits
9.10 THE IMPACT OF ERRORS
The effect of transmission errors is more
significant in data transmission than in voice or
television.
9.11 ERROR PREVENTION
Given that errors do occur in data transmission,
it is necessary to take steps to prevent,detect,and
correct them.
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Chapter 9 Communications Circuits
9.11.1 Line Conditioning
The purpose of line conditioning is to ensure that
the line meets tighter specifications for amplitude
and distortion thereby minimizing the number of
errors that are likely to occur.
9.11.2 Shielding
A metallic sheath surrounds the center
conductor,shielding on critical parts of the
communications circuit may reduce noise and
crosstalk.
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Chapter 9 Communications Circuits
9.11.3 Improving Connections
Always check the cables and connections first.
9.12 ERROR DETECTION
9.12.1 Echo Checking
Echo checking is one of the simplest ways to
check for transmission errors,in which each character
is echoed from the receiver back to the transmitter.
Echo checking is rarely used because more
automatic methods are available.
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Chapter 9 Communications Circuits
9.12.2 Vertical Redundancy Checking(VRC) or
Parity Checking
Parity checking can detect a single bit error
in a transmission.
When parity checking is used, an error that
changes one bit in a character can be detected
because the parity bit will be incorrect.
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Chapter 9 Communications Circuits
Figure 9-14
If even parity is being used, the parity bit is set to 1 when
necessary to make the total number of 1 bits in the character an even
number. If odd parity is used, the parity bit is set to 1 when necessary to
make the total number of 1 bits an odd number.
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Chapter 9 Communications Circuits
9.12.3 Longitudinal Redundancy Checking(LRC)
When a block check character made up of
parity bits calculated lengthwise through the
block is added to the end of each block of data,
the technique is called Longitudinal Redundancy
Checking(LRC).
Figure ：P191
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Chapter 9 Communications Circuits
9.12.4 Cyclic Redundancy Checking(CRC)
Cyclic Redundancy Checking(CRC) is a more
sophisticated way for calculating a block check
character than longitudinal redundancy checking.
One particular implementation of a more
general class of error detection and techniques
called polynomial error checking is called cyclic
redundancy checking.
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Chapter 9 Communications Circuits
9.13 ERROR CORRECTION
Once an error is detected,some technique must
be employed to correct.
9.13.1 Retransmission
The most frequently used and usually the most
economical technique for error correction is the
retransmission of the data in error.
ARQ（automatic repeat request)
P192
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Chapter 9 Communications Circuits
9.13.2 Stop and Wait ARQ
When a block of data must be acknowledged
by the receiver before the next block is sent, the
ARQ technique is called stop and wait.
A block of data is sent and the receiver sands
either an ACK if the data were received
correctly or an NAK if an error was detected.
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Chapter 9 Communications Circuits
9.13.3 Continuous ARQ
When data blocks are continuously sent
over a forward channel while ACKs and
NAKs are sent over the reverse channel,
the ARQ technique is called continuous.
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Chapter 9 Communications Circuits
9.13.4 Forward Error Correction(FEC)
A technique that includes special
transmission codes and additional redundant bits
with the data to enable data to be corrected at
the receiving end is called forward error
correction.
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Chapter 9 Communications Circuits
课后习题
Exercises
1. Attenuation is（ B ）.
× A、 the increase of signal strength by a factor of ten
√
B、 the weakening of a signal over a distance
× C、 only an issue on digital circuits
× D、 only an issue on circuits provided by ATT
Redo
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Answer
Chapter 9 Communications Circuits
课后习题
Exercises
2.Background noise (white noise) on a circuit （ D ）.
× A、 delays some frequencies more than others
× B、 occurs in spikes
× C、 is one of the most difficult problems to work around
√
D、 is rarely a problem because it is a known, predictable phenomenon
Redo
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Answer
Chapter 9 Communications Circuits
课后习题
Exercises
3. Parity checking（ D ）.
× A、 can detect an even number of bit errors
× B、 adds an odd bit, even if no errors occurred, for safety
× C、 is not used if the circuits are at parity with one another
√
D、 can detect a single bit error in a transmission
Redo
Next
Answer
Chapter 9 Communications Circuits
课后习题
Exercises
4.Using a CATV circuit for data transmission（ A ）.
√
A、 requires a modem to be installed on the cable at the user’s home or office
× B、 requires a cable box at the users end
× C、 requires the user’s television to be on when data is transmitted
× D、 is not practical because CATV system are designed to carry
information in one direction only
Redo
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Answer
Chapter 9 Communications Circuits
课后习题
Exercises
5.Which of the following is not circuit condition that causes
errors?（ C ）
× A、 attenuation distortion
× B、 echo
√
C、 chatter
× D、 dropouts
Redo
Next
Answer
Chapter 9 Communications Circuits
课后习题
Exercises
channel is a subdivision of a circuit and is a one way
6.A ________
path for communicating.
Answer
link is a segment of a circuit between two points.
7.A ______
Answer
DSU/CSU provides the signal timing and shaping
8.A ___________
interface between a DTE and a digital circuit.
Answer
Chapter 9 Communications Circuits
课后习题
Exercises
crosstalk is interference that occurs when the signals
9. ___________
from one communications channel interfere with those on
another channel.
Answer
envelope delay distortion an electrical phenomenon that
10.______________________is
occurs when not all frequencies propagate down a
communications circuit at exactly the same speed.
Answer
Chapter 9 Communications Circuits
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