【期末考试复习】计算机网络英文版

Sampling (PAM)---->quantization---->binary encoding

---->digital to digital coding

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Q4-1 List three techniques of digital- to-digital conversion.

They are

line coding

,

block coding

and

scrambling coding

.

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Q4-4 Define baseline wandering and its effect on digital transmission.

(1)In decoding a digital signal, the receiver calculates a running average of the received

signal

power.

This

average

is

called

the

baseline.(2)The

incoming

signal

power

is

evaluated against this baseline to determine the value of the data element.

(3)A

long

string

of

0s

or

1s

can

cause

a

drift

in

the

baseline

(baseline

wandering)

and

make it difficult for the receiver to decode correctly.

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Q4-5 Define a DC component and its effect on digital transmission.

(Define)

When

the

voltage

level

in

a

digital

signal

is

constant

for

a

while,

the

specturm

creates

very

low

frequencies(results

of

Fourier

analysis).

These

frequences

around zero,called

DC(derect-current) components

, that present problems for a system

that can not pass low frequencies or a system that uses electrical coupling.

(Effect) DC component means 0/1 parity that can cause base- line wondering.

直流分

量的影响不仅仅是基 线偏移，还有更重要的原因。看看教材和课件，或看看原书作

业答案。

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Q4-9 Define scrambling and give its purpose.

(Define)

Scrambling

is

a

way

to

avoid

a

long

sequence

of

0s

in

the

original

stream,Scrambling

is

a

technique

that

does

not

increase

the

number

of

bits

and

does

provide

synchronization,Scrambling

is

a

solution

that

substitutes

long

zero-lever

pulse

with a combination of other lever to provide synchronization.

(Purpose)

substitutes

long

zero-lever

pulse

with

a

combination

of

other

lever

to

provide synchronization.

☆

Its purpose is to provide synchronization without increasing the number of bits.

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Q4-10 Compare and contrast PCM and DM.

?

PCM is the most common technique to change an analog signal to digital data edit

is

very

gh DM

is

not

perfect,the

quantization

error

of

DM

is

much

less

than that for PCM. In general, PCM exhibits better SNR characteristics at the same data

rate.

☆

Both of them are use sampling to convert the analog signal to digital

finds

the

value

of

the

signal

amplitude

for

each

sample;

DM

finds

the

change

from

the

previous sample(between two consecutive samples).

第五章

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Which modulation techniques are used in analog transmission? Which of them

is

the

most

susceptible

to

noise?

Which

of

them

is

the

mechanism

commonly

used in all modern modems.

ude shift keying ( ASK)

Frequency shift keying ( FSK )

Phase shift keying ( PSK)

Quadrature Amplitude Modulation(QAM)

most susceptible to noise of the four digital-to-analog conversion techniques is

ASK.

mechanism commonly used in all modern modems of them is QAM.

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Q5-1 Define analog transmission.

(1)

Analog

transmission

refers

to

the

transmission

of

analog

signals

using

a

band-pass channel. Analog transmission is the only choice if we have a bandpass channel.

(2)

While

digital

transmission

is

very

desirable,

a

low-pass

channel

is

nd digital or analog signals are converted to a complex analog signal with a

range of frequencies suitable for the channel.

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Q5-2 Define carrier signal and explain its role in analog transmission.

(Define)

A

sender

produces

an

analog

signal

as

fundamental

wave

to

carry

digital

signals. The fundamental wave is called carrier signal.

(explain) Digital data are converted into modulated signals through changing one or

more

of

three

characteristics

of

a

carrier

signal:

amplitude,

frequency

and

familiar application is for transmitting digital data through the public telephone network

via a modem.

(A

carrier

is

a

single-frequency

signal

that

has

one

if

its

characteristic

(amplitude,

frequency, or phase) changed to represent the baseband signal.

【

In analog transmission, the sending device produces a high frequency analog signal that

acts as a base for the information signals (data). The base signal is called the carrier signal.

It is to represent the baseband signal.

】

)

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Q5-3 Definne digital-to-analog conversion.

Digital- to-analog

conversion

is

the

process

of

changing

one

of

the

characteristics

of

an

analog

signal

based

on

the

information

in

digital

data.

It

is

also

called

modulation

of

a

digital

signal.

The

baseband

digital

signal

representing

the

digital

data

modulations

the

carrier to create a broadband analog signal.

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Q5-4 Which of characteristics of an analog signal are changed to represent the

digital signal in each of the following digital-analog conversions?

changes the amplitude of the carrier.

changes the frequency of the carrier.

changes the phase of the carrier.

changes both the amplitude and phase of the carrier.

QAM is the most efficient of these options and is the mechanism commonly used in

all modern modems.

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Q5-5 Which of the four digital-to- analog conversion techniques(ASK,FSK,PSK

or QAM) is the most susceptible to noise?Defend your answer.

(Answer)

The

most

susceptible

to

noise

of

the

four

digital-to-analog

conversion

techniques is ASK.

(Define)

Noise

usually

affects

the

amplitude;

therefore,

ASK

is

the

modulation

technique most affected by noise than phase or frequency.

第六章

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Which of the four multiplexing techniques is (are) suitable for analog channels?

Which of them is (are) suitable for digital channels?

（

See P181 Questions Q6-3)

Frequency-division and wavelength- division multiplexing are suitable for analog

channels

Synchronous time-division multiplexing and asynchronous time-division

multiplexing are suitable for digital channels.

?

Which

of

the

four

multiplexing

techniques

is

(are)

suitable

for

electrical

links? Which of them is (are) suitable for optical links?

Frequency-division

,

Synchronous

time-division

and

asynchronous

time-division

multiplexing

are suitable forelectrical links .

Wavelength- division multiplexing is suitable for optical links.

光传输信道都属于模拟信道，而时分复用只适用于数字信道。

?

Describe the main differences between FDM and TDM.

In FDM signals generate by each sending device modulate different carrier

frequencies.

In TDM instead of sharing a portion of the bandwidth as in FDM,time is

connections occupies a portion of time in the link.

一个是通过

frequency

(carrier

modulation)

< br>实现复用，另一个是通过

time

(slot

allocation)

实现复用。

4.

P181-182 6.4.2 Questions

：

guish between a link and a channel in multiplexing.

In multiplexing,The word link refers to the physical word channel refers to the

portion of a link that carriers a transmission between a give pair of lines .One link can

have many (n)channels.

?

of the three multiplexing techniques is common for fiber-optic links?

It is the Wavelength -division multiplexing.(WDM)

because it allows the multiplexing of signals with a very high frequency

?

guish between synchronous and statistical TDM.

?

In synchronous TDM each input connection has an allotment in the output even if

it is not sending slots preassigned to sources and slots allocated

even if no data.

☆

In synchronous TDM, each input has a reserved slot in the output frame. This can be

inefficient if some input lines have no data to send.

In statistical TDM slots are dynamically allocated to improve bandwidth

improve the efficient by remove the empty slots from the when an input lines

has a slot's worth of data to send is it given a slot in the output frame and the number of

slots in each frame is less than the number of input lines.

第七章

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What's

bit-level

errors?

Describe

the

basic

principle

and

the

requirement

for

detection of bit-level errors.

★

The bit-level errors

means one bit or

more bits of a given data unit (such as a byte,

character or packet) is

< br>（

are

）

changed from 1 to 0 or from 0 to 1. The central concept in

detecting or correcting errors is redundancy, i.e. send some extra bits with our data. These

redundant bits, based on specific algorithm, are added by the sender and removed by the

receiver. Their presence allows the receiver to detect or correct corrupted bits.

?

Which

techniques

are

commonly

used

in

bit-level

error

detection?

Which

of

them

is(are)

implemented

by

hardware

such

as

gate

circuits

?

Which

of

them

is(are) implemented by software? Give their main applications.

(1) Parity Check, CRC and Checksum are commonly used in bit- level error detection.

(2) Parity Check and CRC are implemented by hardware such as gate circuits. Parity

Check is used in peripherals and CRC is used in networks such as LANs and W

ANs.

(3) Checksum is implenmented by software. Mainly used for the Internet.

?

Which of FEC and ARQ is commonly used in computer networks?

ARQ is commonly used in computer networks.

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Define the Hamming distance and the minimum Hamming distance.

(1)

The

Hamming

distance

between

two

words(of

the

same

size)

is

the

number

of

differences between corresponding bits.

(2) The minimum Hamming distance is the smallest Hamming distance between all

possible pairs in a set of words.

最小海明距的计算见

P286

?

Briefly describe the principle of

的工作原理

CRC

is

a

most

powerful

of

the

redundancy

checking

techniques

based

on

binary

division.

At the encoder, the redundant bits are derived by dividing the augmented data unit

with

the

predetermined

divisor,

and

the

remainder

is

the

CRC.

Then

the

CRC

is

appended to the end of

a datawordand create the transmitted ,to be

valid,

a

CRC

must

have

exactly

one

less

bit

than

the

divisor,

and

the

resulting

codeword becomes exactly divisible by the same divisor.

At

the

decoder,

the

received

codeword

is

divided

by

the

same

divisor.

And

the

remainder is called syndrome. If there is no remainder, the data unit is assumed to be

intact

and

is

therefore

accepted.

Otherwise,

it

means

that

the

data

unit

has

been

corrupted in transmission and therefore the remainder must be rejected.

The divisors used in CRC are normally called the generator polynomial(

生成多项式

)

or simply the generator, which are given by some international organizations.

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Q10-4 In a codeword, we add two redundant bits to each 8-bits data word. Find

the number of

codewords. d codewords

We have k=8, r=2, n=8+2=10.

number of valid codewords is 2^k=2^8=256;

number of invalid codewords is 2^n-2^k=2^10-2^8=768;

Q10-6

If

we

want

to

be

able

to

detect

two-bit

errors,

what

should

be

the

minimum Hamming distance?

The minimum Hamming distance d=s+1,since s=2,d=3.

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Q10-8 In CRC, if the dataword is 5 bits and the codeword is 8 bits, how many 0s

need to be added to the dataword to make the dividend? What is the size of the

remainder? What is the size of the divisor?

(1) We need argment the datawords with three 0s.

(2) The size of the remainder is 3.

(3) The size of the divisor is 4.

(We have k=5 and n=8. The size of the dividend is the same as the size of the codeword (8

bits).

We

need

to

augment

the

dataword

with

three

0s.

The

size

of

the

remainder

is

r=n-k=8-5=3bits.

The size of the divisor is: r+1=4 bits)

第八章

?

Why do we need the data link layer? And what is a

The data link layer is needed because it is necessary to impose a layer of control in

each

communicating

device

that

provides

functions

such

as

flow

control,

error

control, addressing and link management.

When a data link control protocol is used, the transmission medium between systems

is referred to as a data link.

?

Briefly describe the two main functions of data link layer.

The two main functions:

(1) Data link control

(node-to-node communication)

Provide smooth and reliable transmission of frames between nodes.

- Framing (organize the transmitted bits)

- Flow control ( automatically regulate transfer rate)

- Error control ( handle frames lost or damaged)

(2) Media access control (not used if point-to-point)

How to share the link

- Addressing control

- Sending right control

（

Only used in broadcast networks, e.g. LANs

）

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Briefly describe three version of ARQ.

(1)Stop- and-Wait

ARQ,Send

one

frame

at

a

time.

(2)Design

of

Go-Back-N

ARQ,Send

max

W

frames

at

a

time,retranmit

the

error

frames

and

all

succeeding

ones.

3)Selective

Repeat

ARQ

,Send

max

W

frames

at

a

time,retranmit

rejected

frames only.

?

Briefly

describe

the

main

difference

of

three

HDLC

frames

(I-,S-

and

U-frames).

I-frame

used

to

transport

user

data

and

control

information

relation

to

user

data

(piggybacking) S-frame used only to transport control information

U-frame

used

to

transport

link

management

information

(Information

carried

by

U-frames is intended for managing link itself.)

?

Briefly

describe

the

main

functions

of

four

S-frames

of

HDLC

(RR,

RNR,REJ,SREJ).

RR stands for receiving ready. REJ stands for rejecting. REJ stands for receiving not

ready. SREJ stands for selective- reject.

【

RR: This kind of frame acknowledges the receipt of a safe and sound frame or group of

frames.

RNR: It acknowledges the receipt of a frame or group of frames, and it announces that the

receiver is busy and cannot receive more frames.

It acts as a kind of congestion-control

mechanism by asking the sender to slow down.

REJ: It is a NAK that can be used in Go-Back-N ARQ to improve the efficiency of the

process by informing the sender, before the sender timer expires, that the last frame is lost

or damaged.

SREJ: This is a NAK frame used in Selective Repeat ARQ. Note that the HDLC Protocol

uses the term selective reject instead of selective repeat.

】

6. P320-321 11.6.2 Questions

：

?

Q11-1. Define framing and give the reason it is needed.

The

data-link

layer

needs

to

pack

bits

into

frames

,so

that

each

frame

is

distinguishable from another.

Framing is divides a message into smaller frames for

higher efficiency of flow and error control.

?

Q11-8. Compare and contrast flow control and error control.

Flow control refers to a set of procedures used to restrict the amount of data that the

sender can send before waiting for acknowledgement. Error control refers to a set of

procedures used to detect and correct errors.