envelope网络工程专业外文翻译--计算机网络

外文原文：

COMPUTER NETWORKS

DATE COMMUNICATIONS

One of the functions of the host computer is to

store the applications programs for the

various secondaries.

The

end

equipment

which

either

generates

the

digital

information

for

transmission

or

uses

the

received

digital

data

can

be

computer

,printers

,keyboards,

CRTs,

and

so

on.

This

equipment generally manipulates digital information internally in word units

—

all the bits that

make up a word in a particular piece of equipment are transferred in parallel. Digital

data,

when transmitted, are in serial form. Parallel transmission of an 8-bit word require eight pairs

of

transmission

lines

—

not

at

all

cost- effective.

Data

terminal

(DTE)

is

a

general

phrase

encompassing all of the circuitry necessary to perform parallel-to-serial and serial-to-parallel

conversions for transmission and reception respectively and

for data link management. The

UART

(Universal

Asynchronous

Receiver/Transmitter)

and

USART

(Universal

Asynchronous/Asynchronous Receiver/Transmitter) are the devices that perform the parallel-

to-serial

and

serial-to-parallel

conversions.

The

primary

DTE

includes

a

line

control

unit

(LCU or LinCo) which controls the flow of information in a multipoint data link system. A

station controller (STACO) is the corresponding that belonged to the subscriber in a data link

system. Between the DTEs, starting with the modems, was communications equipment owned

and maintained by Telco property.

Data communications equipment (DCE) accepts the serial data stream from the DTE and

converts it to some form of analog signal suitable for transmission on voice-grade lined. At

the

receive

end,

the

DCE

performs

the

reverse

function

of

converting

the

received

analog

signal

to

a

serial

digital

data

stream.

The

simplest

form

of

DCE

is

a

modem

(modulator/demodulator)

or

data

set.

At

the

transmit

end,

the

modem

can

be

considered

a

form

of

digital- to-analog

converter,

while

at

the

receive

end,

it

can

considered

a

form

of

analog-to-digital converter. The most common of modulation by modems are frequency shift

keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). This

is a typically data transmission mode using the analog telephone lines. If you transmit data by

1 / 11

digital channel (sometimes it is called

“

Digital T-carrier

”

), a pulse Code Modulation (PCM)

equipment

must

be

used.

A

microwave

transmission

system

can

also

be

used

for

the

data

communication. Finally, you can use the satellite communication system for data transmission.

If

the

cables

and

signal

levels

used

to

interconnect

the

DTE

and

DCE

were

left

unregulated,

the

variations

generated

would

probably

be

proportional

to

the

number

of

manufacturers.

Electronics

industries

Association

(EIA)

,

an

organization

of

manufactures

with

establishing

the

DTE

and

modem.

This

is

a

25-pincable

whose

pins

have

designated

functions and specified signal levels. The RS-232C is anticipated to be replaced by an update

standard.

ARCHITECTURE OF COMPUTER NETWORKS

Computer network is a complex consisting of two or more conned computing units, it is

used

for

the

purpose

of

data

communication

and

resource

resource

sharing.

Design

of

a

network and its logical structure should comply with a set of design principles, including the

organization

of

functions

and

the

description

of

data

formats

and

procedure.

This

is

the

network architecture and so called as a set of layers and protocols, because the architecture is

a layer-based.

In

the

next

two

sections

we

will

discuss

two

important

network

architectures,

the

OSI

reference model and the TCP/IP reference model.

1.

The OSI Reference Model

The OSI model is shown in Fig.14-2(minus the physical medium). This model is based on a proposal

developed

by

the

International

Standards

Organizations

(OSI)

as

the

first

step

toward

international

standardization of the protocols used in the various layers. The model is called the ISO OSI (Open System

Interconnection) Reference Model because it deals with connecting open systems--that is, systems that are

open for communication with other systems, We will usually just call it the OSI model for short.

The

OSI

model

has

seven

has

seven

layers.

Note

that

the

OSI

model

itself

is

not

a

network architecture because it does not specify the exact services and protocols to be used in

each layer. It just tells what each layer should do. However , However, ISO has also produced

standards for all the layers, although these are not part of the reference model itself. Each one

has been published as a separate international standard.

2 / 11

2.

The TCP/IP Reference Model

The TCP/IP reference model is an early transport protocol which was designed by the US

Department of Defence (DOD) around in 1978. It is often claimed that it gave rise the OSI

“

c onnectionless

”

mode

of

operation.

TCP/IP

is

still

used

extensively

and

is

called

as

a

industrial standard of internet work in fact, TCP/IP has two parts: TCP and IP. TCP means it is

on the transport layer and IP means it is on the network layer separately.

(1)

There are two end-to-end protocols in the transport layer, one of which is TCP

(Transmission

Control

Protocol)

,

another

is

UDP

(User

Datagram

Protocol).

TCP is

a connection-oriented protocol

that allows a byte stream

originating on

one machine to be delivered without error on any other machine in the internet.

UDP

is

an

unreliable,

connectionless

protocol

for

application

that

do

not

want

TCP

’

s sequencing of flows control flow control and wish to provide their own.

(2)

The

network

layer

defines

an

official

packet

format

and

protocol

called

IP

(Internet protocol). The job of the network layer is to deliver IP packets where

they are supposed to go.

The TCP/IP Reference Model is shown in Fig.14.3. On top of the transport layer is

the application layer, It contains all the higher-level protocols. The early ones included virtual

terminal (TELNET), (FTP), electronic mail (SMTP) and domain name service(DNS).

WIDE AREA NETWORK

A wide area network, or WAN, spans a large

geographical area, often a country or

continent . It contains a collection of machines intended for running user (i. e. , application)

programs.

We

will

follow

traditional

usage

and

call

these

machines

hosts.

By

a

communication

subnet,

or

just

subnet

for

short. The

job

of

the

subnet

is

to

carry

messages

from

host

to

host,

just

as

the

telephone

system

carries

words

from

speaker

to

listener.

By

separating the pure communication aspects of the network (the subnet) from the

application

aspects (the hosts), the complete network design is greatly simplified. Relation between hosts

and the subnet is shown in Fig.14-4.

3 / 11

One

of

many

methods

that

can

be

used

to

categorize

wide

area

networks

is

with

respect

to

the

flow

of

information

on

a

transmission

facility.

If

we

use

this

method

to

categorize

wide

area

networks,

we

can

group

them

into

three

basic

types:

circuit

switched,

leased line and packet switched.

1.

CIRCUIT SWITCHED NETWORKS

The most popular type of network and the one almost all readers use on a daily basis

is a circuit switched network. The public switched telephone network, however, is not limited

to the telephone company, By purchasing appropriate switching equipment, any organization

can construct their own internal circuit switched network and, if desired, provide one or more

interfaces

to

the

public

switched

network

to

allow

voice

and

data

transmission

to

flow

between the public network and their private internal network

2

．

LEASED LINE NETWORKS

This

is

a

dedicated

network

connected

by

leased

lines.

Leased

line

is

a

communications

line

reserved

for

the

exclusive

use

of

a

leasing

customer

without

inter-

exchange

switching

arrangements.

Leased

or

private

lines

are

dedicated

to

the

user.

This

advantage is that the terminal or computer is a always physically connected to the line. Very

short response times are met with service.

3.

PACKET SWITCHING NETWORKS

A

packet

network

is

constructed

through

the

use

of

equipment

that

assembles

and

disassembles packets, equipment that routes packet, and transmission facilities used to route

packets from the originator to the destination device. Some types of data terminal equipment

(DTE) can create their own packets, while other types of DTE require the conversion of their

protocol into packets through the use of a packet assembler / disassemble (PAD). Packets are

routed through

the network by packet

switches. Packet

switches

examine the destination of

packets as they flow through the network and transfer the packets onto trunks interconnecting

switches based upon the packet destination destination and network activity.

Many older pubic networks follow a standard called X.25. It was developed during 1970s

by

CCITT

to

provide

an

interface

between

public

packet-switched

network

and

their

customers.

CCITT

Recommendation

X.25

controls

the

access

from

a

packet

mode

DTE,

such

as

a

4 / 11

terminal device or computer system capable of forming packets, to the DCE at a packet mode.

CCITT Recommendation X.28 controls the interface between non-packet mode devices that

cannot

interface

between

the

PAD

and

the

host

computer.

CCITT

Recommendation

X.3

specifies the parameter settings on the PAD and X.75 specifies the interface between packet

network.

LOCAL AREA NETWORK

Local area data network , normally referred to simply as local area network or LANs, are

used

to

interconnect

distributed

communities

of

computer-based

DTEs

located

within

a

building

or

localized

group

of

building.

For

example,

a

LAN

may

be

used

to

interconnect

workstations distributed around offices within a single building or a group of buildings such

as a university campus. Alternatively, it may be complex. Since all the equipment is located

within a single establishment, however, LANs are normally installed and maintained by the

organization. Hence they are also referred to as private data networks.

The

main

difference

between

a

communication

path

established

using

a

LAN

and

a

connection made through a public data network is that a LAN normally offers much higher

date

transmission

rates

because

of

the

relatively

short

physical

separations

involved.

In

the

context of the ISO Reference Model for OSI, however, this difference manifests itself only at

the

lower

network

dependent

layers.

In

many

instances

the

higher

protocol

layers

in

the

reference model are the same for both types of network.

Before describing the structure and operation of the different types of LAN, it is perhaps

helpful

to

first

identify

some

of

the

selection

issues

that

must

be

considered.

It

should

be

stressed that this is only a summary; there are also many possible links between the tips of the

branches associated with the figure.

1.

Topology

Most wide area networks, such as the PSTN, use a mesh (sometimes referred to as a

network)

topology.

With

LANs,

however,

the

limited

physical

separation

of

the

subscriber

DTEs

allows

simpler

topologies

to

be

used.

The

four

topologies

in

common

use

are

star,

bus ,ring and hub.

5 / 11