To get here to there, data must move through something. A telephone line, cable, or the
atmosphereare all transmission media, or channels. But before the data can be communicated, it
must be converted into a form suitable for communication.
Data communications lines can be connected in two types of configurations: point-to-point
and multidrop. A point-to-point line directly connects the sending and the receiving devices, and
a multidrop line connects many devices, not just one sending device and one receiving device.
The two ways of connecting microcomputers with each other and with other equipments are
through the cable and through the air. There are three basic forms into which data can be
converted for communication: electrical pulses or charges, electromagnetic waves, and pulses of
light.
Specifically, five kinds of technology are used to transmit data. These are telephone lines
(twisted pair), coaxial cable, fiber-optic cable, microwave, and satellite.
Telephone Lines [1] Inexpensive, multiple-conductor cable comprised of one or more pairs
of 18 to 24 gauge copper strands. The strands are twisted to improve protection against
electromagnetic and radio frequency interference. The cable, which may be either shielded or
unshielded, is used in low-speed communications, as telephone cable. It is used only in baseband
networks because of its narrow bandwidth. Most telephone lines you see strung on poles consist
of cables made up of hundreds of copper wires are twisted pairs. Twisted pairs are susceptible to
a variety of types of electrical interference (noise), which limits the practical distance that data

can be transmitted without being garbled. Twisted pairs have been used for years for voice and
data transmission, however they are now being phased out by more technically advanced and
reliable media.
Coaxial Cable

Coaxial cable is a type of thickly insulated copper wire that can carry a
larger volume of data—about 100 million bits per second, the insulation is composed of a
nonconductive material covered by a layer of woven wire mesh and heavy-duty rubber or plastic.
In terms of number of telephone connections, a coaxial cable has 80 times the transmission
capacity of twisted pair. Coaxial cables are most often used as the primary communications
medium for local connected network in which all computer communication is within a limited
geographic area, such as in the same building.
Coaxial cable is also used for undersea telephone lines.
Fiber-Optic Cable [2]

A transmission medium composed of a central glass optical fiber
cable surrounded by cladding and an outer protective sheath. It transmits digital signals in the
form of modulated light from a laser or LED (light-emitting diode). In fiber-optic cable, data is
transmitted as pulses of light through tubes of glass. In terms of number of telephone connections,
fiber-optic cable has 20,000 times the transmission capacity of twisted pair. However, it is
significantly smaller. Indeed, a fiber-optic tube can be half the diameter of a human hair.
Although limited in the distance they can carry information, fiber-optic cables have several
advantages. Such cables are immune to electronic interference, which makes them more secure.
They are also lighter and less expensive than coaxial cable and are more reliable at transmitting
data. They transmit information using beams of light at light speeds instead of pulses of
electricity, making them far faster than copper cable. Fiber-optic cable is rapidly replacing
twisted-pair telephone lines.
Microwave

Instead of using wire or cables, microwave systems can use the atmosphere as
the medium through which to transmit signals. Microwaves are high-frequency radio waves that
travel in straight lines through the air. Because the waves cannot bend with the curvature of the
earth, they can be transmitted only over short distances. Thus, microwave is a good medium for
sending data between buildings in a city or on a large college campus. For longer distances, the
waves must be relayed by means of "dishes", or antennas. These can be installed on towers, high
buildings, and mountaintops. Each tower facility receives incoming traffic, boosts the signal
strength, and sends the signal to the next station.
Satellites [3]

Satellite communications refers to the utilization of geostationary orbiting
satellites to relay the transmission received from one earth station to one or more earth stations.
They are the outcome of research in the area of communications whose objective is to achieve
ever-increasing ranges and capacities with the lowest possible costs. Orbiting about 22,000 miles
above the earth, satellites rotate at a precise point and speed above the earth. This makes them
appear stationary so they can amplify and relay microwave signals from one transmitter on the
ground to another. The primary advantage of satellite communication is the amount of area that can
be covered by a single satellite. It also has other features: long communication distance, and the

cost of station building is independent of the communication distance, operating in broadcasting
mode, easy for multiple access, sustaining heavy traffic, able to transport different types of service,
independent sending and receiving, and monitoring. Three satellites placed in particular orbits can
cover the entire surface of the earth, with some overlap. Their only drawback is that bad weather
can sometimes interrupt the flow of data.

Continue reading it-e-14 Data Communications Channels

What is VSAT ? VSAT stands for Very Small Aperture Terminal//it's a catchy acronym

and as such it's been adopted by all and sundry for every type of satellite product from small
components of a system to complete systems. Because the term really hinges around the small size
of the antenna it has been used to describe both one-way and interactive systems. Specifically, we
in the industry, isolate television broadcast receivers because counting these as well would simply
distort the numbers in the marketplace, but data, audio and, to some extent, voice systems are
included. Generally, these systems operate in the Ku-band and C-band frequencies. As a rule of
thumb C-band (which suffers less from rain attenuation, but requires larger antennas) is used in
Asia, Africa and Latin America whilst Ku-band (which can use smaller antennas, but suffers from
rain fade in a monsoon-like downpour) is used in Europe and North America. Typically, interactive
Ku-band antenna sizes range from 75 centimetres to 1.8 metres and C-band from 1.8 metres to 2.4
metres. One way systems can use antennas as small as 45 centimetres.
What does a network look like? VSAT networks come in various shapes and sizes ranging
from star data system users with one site connected to an operator's shared hub to many
thousands based on a dedicated facilitylocated at their own site. Mesh systems have traditionally
been somewhat smaller in size than star systems//5 to 30 sites used to be a good rule of thumb
- but the average size of orders has risen as prices have come down and some rural telephony
networks now comprise as many as several hundred or even thousands of sites.
What is One-way Satellite ? One way systems rely on a transmitting station which
transmits one or more carriers to the satellite which re-broadcasts the signal over its coverage
area. All receive-only VSATs under the satellite footprint can then receive the signal or the
user/operator is able to define groups of VSATs from one to all on the network. Broadcast
systems are used for data and audio. The most popular application for data is the transmission of
financial feeds//Reuters, Telerate and KnightRidder are good examples of companies with
large data broadcasting networks//however, there are many other uses, such as software
downloads, file transfers, transmission of press agency news items (with pictures) and the
broadcast of paging messages for terrestrial transmission to the pagers themselves.
Who uses VSAT ? You name it really, car dealerships, gas stations, lottery systems, banks,
insurance companies, drug stores, general stores, supermarkets, healthcare companies, manufacturers,
couriers, hotel chains, car rental businesses, food manufacturers, heavy industries, mines, electrical
utilities, oil and gas pipelines, energy production and exploration, timbercompanies, plantations,
various government departments and agencies ... any others you can think of, just add to the list.

----------------------

aperture ['æpə,tjuə] 英汉翻译

n. 孔，穴；缝隙；（照相机，望远镜等的）光圈，孔径

catchy ['kætʃi] 英汉翻译

adj. 引人注意的；容易记住的；易使人上当的

acronym ['ækrəunim] 英汉翻译

n. 首字母缩略词

sundry ['sʌndri] 英汉翻译

adj. 杂的；各式各样的n. 杂货；杂项

1, hinges   
n. 铰链；合页（hinge的复数）；小五金
v. 给…装上绞链；使以…为条件（hinge的第三人称单数形式）
2, antenna  [æn'tenə]
n. 天线；触角，触须
3, distort  [dis'tɔ:t]
vt. 扭曲；曲解；使失真
vi. 扭曲；变形
4, frequencies   
n. 频率（frequency的复数形式）；频数分析；次数分配表
5, attenuation  [ə,tenju'eiʃən]
n. 衰减；变薄；稀释
6, hinge  [hindʒ]
n. 铰链；枢纽；关键
vt. 给…安装铰链
vi. 依…而转移
7, monsoon  [mɔn'su:n]
n. 季候风；（印度等地的）雨季；季风
8, downpour  ['daunpɔ:]
n. 倾盆大雨；注下
9, dedicated  ['dedikeitid]
adj. 专注的；献身的；专用的
v. 以…奉献；把…用于（dedicate的过去式和过去分词）
10, facility  [fə'siliti]
n. 设备；灵巧；容易；设施
11, mesh  [meʃ]
n. 网眼；网丝；圈套
vi. 相啮合
vt. 啮合；以网捕捉
12, terrestrial  [ti'restriəl, tə-]
adj. 陆地的，陆生的；地球的；人间的
n. 陆地生物；地球上的人
13, dealership  ['di:ləʃip]
n. 代理权；代理商；经销权
14, timber  ['timbə]
n. 木料；木材

Continue reading it-e-15 Simple Introduction to VSAT

There are several factors that affect data transmission. They include speed or bandwidth,
serial or parallel transmission, direction of data flow, modes of transmission data, and protocols.
Bandwidth

The different communications channels have different data transmission speeds.
This bit-per-second transmission capability of a channel is called its bandwidth. Bandwidth may be
of three types: voiceband, medium band, and broadband. Voiceband is the bandwidth of a standard
telephone line and used often for microcomputer transmission, the bps is 300-9600. Medium band
is the bandwidth of special liased lines used mainly with minicomputers and mainframe computers,
the bps is 56,000/264 million. Broadband is the bandwidth that includes microwave, satellite,
coaxial cable, and fiber-optic channels. It is used for very high-speed computers whose processors
communicate directly with each other. It is in the range of 56,000/30 billion bps.
Serial or Parallel Transmission Data travels in two ways: serially and in parallel. In serial
data transmission, bits flow in a serial or continuous stream, like cars crossing a one-lane bridge. Each
bit travels on its own communications line. [1]Serial transmission is the way most data is sent over
telephones lines. Thus, the plug-in board making up the serial connector in a microcomputer's modem
is usually called a serial port. More technical names for the serial port are RS-232C connector and
asynchronous communications port. With parallel data transmission, bits flow through separate lines
simultaneously. In other words, they resemble cars moving together at the same speed on a multilane
freeway. Parallel transmission is typically limited to communications over short distances and is not
used over telephone lines. It is, however, a standard methods of sending data from a computer's CPU
to a printer.
Direction of Data Transmission

There are three directions or modes of data flow in a data
communications system: simplexcommunication, half-duplex communication, and full-duplex
communication. Simplex communication resembles the movement of cars on a one-way street.
Data travels in one direction only. It is not frequently used in data communication systems today.
One instance in which it is used may be in point-of sale (POS) terminals in which data is being
entered only. In half-duplex communication, data flows in both directions, but not simultaneously.
That is, data flows in only one direction at any one time. This resembles traffic on a one-lane

bridge. Half-duplex is very common and is frequently used for linking microcomputers by
telephone lines to other microcomputers, minicomputers, and mainframes. Thus, when you dial
into an electronic bulletin  board through your microcomputer, you may well be using half-duplex
communication. In full-duplex communication, data is transmitted back and forth at the same
time, like traffic on a two-way street. It is clearly the fastest and most efficient form of two-way
communication. However, it requires special equipment and is used primarily for mainframe
communications. An example is the weekly sales figures that a supermarket or regional office
sends to its corporate headquarters in another place.
Modes of Transmitting Data

Data may be sent by asynchronous or synchronous transmission.
In asynchronous transmission, the method frequently used with microcomputers, data is sent and
received one byte a time. Asynchronous transmission is often used for terminals with slow speeds. Its
advantage is that the data can be transmitted whenever convenient for the sender. Its disadvantage is a
relatively slow rate of data transfer. Synchronous transmission is used to transfer great quantities of
information by sending several bytes or a block at a time. For the data transmission to occur, the
sending and receiving of the blocks of bytes must occur at carefully timed intervals. Thus, the system
requires a synchronized clock. Its advantage is that data can be sent very quickly. Its disadvantage is
the cost of the required equipment.
Protocols

For data transmission to be successful, sender and receiver must follows a set of
communication rules for the exchange of information. These rules for exchanging data between
computers are known as the line protocol. A communication software package like Crosstalk
helps define the protocol, such as speeds and modes, for connecting with another microcomputer.
TCP/IP ( Transmission Control Protocol and Internet Protocol) are the two standard protocols for
communications on the Internet.
TCP/IP is the "language" of the Internet. It is a networking technology developed by the
United States Government Defense Advanced Research Project Agency (DARPA) in the 1970s.
It is most commonly employed to provide access to the Internet but can be and is used by many
people to create a LAN that may or may not connect to the Internet. In many aspects TCP/IP is a
client/server-type LAN, but many manufacturers of TCP/IP software have applications that allow
the "clients" to serve files or even applications. TCP/IP is truly an open systems protocol. This
means that no one manufacturer creates the product—any computer running TCP/IP software can
connect to anyone else who has TCP/IP software (provided the user has an account and security
permissions), regardless of who made the particular version of software.
When different types of microcomputers are connected in a network, the protocols can
become very complex. Obviously, for the connections to work, these network protocols must
adhere to certain standards. The first commercially available set of standards was IBM's Systems
Network Architecture (SNA). This works for IBM's own equipment, but other machines won't
necessarily communicate with them. The International Standards Organization has defined a set of
communications protocols called the Open Systems Interconnection (OSI). The purpose of the OSI
model is to identify functions provided by any network. [2]It separates each network's functions into

seven "layers" of protocols, or communication rules. When two network systems communicate,
their corresponding layers may exchange data. [3]This assumes that the microcomputers and other
equipment on each network have implemented the same functions and interfaces.

1, resemble  [ri'zembl]
v. 相似,类似,像
2, simultaneously   
ad. 同时地(联立地)
3, simplex  ['simpleks]
单工的,单一的
4, duplex  ['dju:pleks]
[计算机] 双方的
5, bulletin  ['bulətin]
n. 公报
vt. 发表

Continue reading it-e-16 Main Factors Affecting data Tansmission

Please notice that the term computer system here includes hardware, software, network
transmission paths, and people who interact with these components. By this definition,
everything from a desktop workstation to the Internet qualifies as a computer system.
An attacker is a person who tries to gain an advantage by exploiting a security hole.
Attackers are misfeasors, masqueraders, or clandestine users.
Misfeasors

These authorized users gain additional but unauthorized access to resources on
a system or otherwise misuse their authorization. Examples include programmers who use their
accounts to exploit operating system (OS) vulnerabilities and gain administrative privileges, or
accountants who embezzlemoney by falsifying records in a database to which they have regular
access. A misfeasor is an "inside" person, someone within an organization who introduces a
security risk or poses a threat.
Masqueraders

These people use authorized user access privileges to enter a system and
then, posing as that user, attack the system. Examples include hackers who obtain usernames and
passwords by cracking password files, and then use that information to gain entry to the system.
Masqueraders are usually persons outside the organization.
Clandestine Users

These individuals are insiders or outsiders who obtain their own, distinct
unauthorized access to a system. Examples include hackers who obtain administrative access to a
system long enough to create their own user accounts for subsequent access.
The concepts of access and authorization are not necessarily limited to user accounts within
an OS. Physical access to an equipment closet or authorization to place orders for new telephony
service are examples of other types of access and authorization. All persons who have any degree
of physical or logical interaction with a system, its components, or its processes are capable of
compromising system security.

The goals of an attacker range from innocuous to severely damaging:
Trophy Grabbing

Most thrill-seeking attackers are trophy grabbing. Their intent is not to
disrupt or damage a system, but to prove that they can enter the system. Such accomplishments
are badges of achievement in the hacker community.
Information Theft

The most common goal of a security attack is information theft. Intruders
seek sensitive information such as credit card numbers, usernames, passwords, and medical records.
Service Theft

This type of attack involves attackers who use computer resources without
paying for them. Software pirates who crack systems to host stolen software, or warez, for others
to download are guilty of service theft. Clandestine users also commit service theft by having
unauthorized accounts on a server.
Identity Theft

This is the act of illegally assuming the identity of another person, or
masquerading, to gain control of that person's resources (usually computer and economic
privileges). An example of this is an attacker who uses stolen social security numbers and credit
histories to establish and exercise unauthorized lines of credit. Identity theft does not necessarily
involve information theft. For example, an attacker can commit e-mail forgerywithout stealing
sensitive information about the e-mail address owner.
Tampering

This attack is more serious than information theft because the attacker alters
data rather than simply copying it. A student who changes a grade in a university registrar's
database is tampering. This example is stealthy tampering/the attack is not intended to draw
attention. A more extreme form of tampering is defacement, in which a hacker alters a system in
a very noticeable way, usually to make a personal or political statement. The disgruntled
computer operator who, upon dismissal, embeds nasty messages about management in a login
script, or the activist group that hacks into a corporate Web site are typical examples.
Denial of Service (DoS)

DoS can be the most damaging type of security attack. It
diminishes server capacity for authorized clients and temporarily disrupts access to the system. In
the worst cases, DoS attacks render a system unusable for a protracted period by destroying not
only its ability to communicate, but also any data that has been entrusted to it. DoS also can
occur as an unintentional side effect of service theft. For example, hosting pirated warez can
bring down a system because of the excessive download activity.

Although attackers continue to create new methods for violating computer system security,
the vulnerabilities they exploit remain the same. These vulnerabilities can be divided into five
types:
Implicit Trust

The unquestioning, unchecked acceptance of a person or agent. Attacks that
exploit this vulnerability include: compromised system utilities, e-mail forgery, IP spoofing,
keystroke monitoring, logic bomb, masquerading, shoulder surfing, social engineering, Trojan
horse, trapdoor.

A defense is a countermeasurefor dealing with security attacks. Administrators can employ
five types of defenses:
ObfuscationConfusing the attacker by obscuring publicly available information that exposes
vulnerability. Examples include: anonymity, encryption, packet stuffing, public key cryptography,
shielding, steganography, trash disposal.
Authentication and Authorization Ensuring that a person or system claiming an identity is
the real owner of the identity, and granting access on a "must have" basis. Examples include:
badges and cards, biometrics, password, shared secret, signature, watermark.
Monitoring and Auditing Observing system vulnerabilities, either in real time or through
audit tools, to detect attacks. Examples include: filtering, firewall, integrity check, intrusion
detection, misuse detection, password checker, peer review, process review, security audit tools,
virus detection.
Currency Consistently using tested software updates and periodically reviewing human
processes and procedures. Examples include: patching, process review, upgrading.
Education and Enforcement Effectively equipping system designers and users with
knowledge of security risks, and then enforcing application of this knowledge. Examples include:
reminders, tip of the day, training.
The key to preventing security attacks from diminishing system performance is knowledge.
IT administrators can develop their security strategies by studying historical and contemporary
attacks, appropriate defenses, and the evolving trends in the computer security industry.

1, misfeasor  [,mis'fi:zə]
n. 违法行为者；滥用职权者
2, masquerader  [,mæskə'reidə]
n. 冒充者；戴假面具者；参加假面舞会的人
3, clandestine  [klæn'destin]
a. 秘密的
4, embezzle  [im'bezl]
v. 盗用,挪用
5, falsify  [fɔ:lsifai]
vt. 伪造；篡改；歪曲；证明...虚假
vi. 撒谎
6, compromising  
折衷
7, innocuous  [i'nɔkjuəs]
a. 无害的,无毒的
8, trophy  ['trəufi]
n. 奖品,战利品
9, intruders  
n. 入侵者；闯入者（intruder的复数形式）；爱管闲事的人
10, forgery  ['fɔ:dʒəri]
n. 伪造,伪造罪,伪造物
11, tampering  ['tæmpəriŋ]
a. 干预的,干涉的
12, defacement  [di'feismənt]
n. 损坏外表；乱涂；损毁物
13, dismissal  [dis'misəl]
n. 免职,解雇
14, nasty  ['næsti]
a. 污秽的,下流的,险恶的,(气味等)另人作呕的,脾气不好的,恶意的,
15, exploit  [iks'plɔit]
n. 功绩,勋绩
v. 开发,利用,开拓
16, countermeasure  ['kauntə,meʒə]
n. 对策
17, obfuscation  [,ɔbfʌ'skeiʃən]
n. 困惑；模糊；昏迷
18, obscuring  
n. 模糊
v. 使…含糊；隐藏（obscure的ing形式）

Continue reading it-e-17 Computer System Security

Network architecture describes how computer network is arranged and how computer
resources are shared.
There are a number of specialized terms that describes computer network. Some terms often
used with networks are: node, client, server, network operating system, distributed processing
and host computer.
A node is any device that is connected to a network. It could be a computer, printer, or
communication or data storage device.
A client is a node that requests and uses resources available from other nodes. Typically, a

client is a user's microcomputer.
A server is a node that shares resources with other nodes. Depending on the resources
shared, it may be called a file server, printer server, communication server, or database server.
Network operating system likes Windows, it controls and coordinate the activities between
computers on a network. These activities include electronic communication, information, and
resource sharing.
In a distributed processing system, computing power is located and shared at different
locations. [1]This type of system is common in decentralized organizations where divisional
offices have their own computer systems. The computer systems in the divisional offices are
networked to the organization's main or centralized computer.
Host computer is a large centralized computer, usually a minicomputer or a mainframe.
A network may consist only of microcomputers, or it may integrate microcomputers or
other devices with large computers. [2]Networks can be controlled by all nodes working together
equally or by specialized nodes coordinating and supplying all resources. Networks may be
simple or complex, self-contained or dispersed over a large geographical area.
Configuration A network can be arranged or configured in several different ways. The
four principal configurations are star, bus, ring, and hierarchical.
In a star network, a number of small computers or peripheral devices are linked to a central
unit. This central unit may be a host computer or a file server. All communications pass through
this central unit. Control is maintained by polling. That is, each connecting device is asked
whether it has a message to send. Each device is then in turn allowed to send its message. One
particular advantage of the star form of network is that it can be used to provide a time-sharing
system. That is, several users can share resources ("time") on a central computer. The star is a
common arrangement for linking several microcomputers to a mainframe that allows access to an
organization’s database.
In a bus network, each device in the network handles its own communications control. There
is no host computer. All communications travel along a common connecting cable called a bus. As
the information passes along the bus, it's examined by each device to see if the information is
intended for it. The bus network is typically used when only a few microcomputers are to be linked
together. This arrangement is common in systems for electronic mail or for sharing data stored on
different microcomputers. The bus network is not as efficient as the star network for sharing
common resources. (This is because the bus network is not a direct link to the resource.) However,
a bus network is less expensive and is in very common use.
In a ring network, each device is connected to two other devices, forming a ring. There is no
central file server or computer. Message are passed around the ring until they reach the correct
destination. With microcomputers, the ring arrangement is the least frequently used of the four
networks. However, it often is used to link mainframes, especially over wide geographical areas.
These mainframes tend to operate fairly autonomously. They perform most or all of their own
processing and only occasionally share data and programs with other mainframes. A ring

network is useful in a decentralized organization because it makes possible a distributed data
processing system. That is, computers can perform processing tasks at their own dispersed
locations. However, they can also share programs, data and other resources with each other.
The hierarchical network consists of several computers linked to a central host computer,
just like a star network. However, these other computers are also hosts to other, smaller
computers or to peripheral devices. Thus, the host at the top of the hierarchy could be a
mainframe. The computers below the mainframe could be minicomputers, and those below,
microcomputers. The hierarchical network—also called a hybrid network—allows various
computers to share databases, processing power, and different output devices. A hierarchical
network is useful in centralized organizations. For example, different departments within an
organization may have individual microcomputers connected to departmental minicomputers.
The minicomputers in turn may be connected to the corporation’s mainframe, which contains
data and programs accessible to all.
Strategies

Every network has a strategy or way of coordinating the sharing of information
and resources. The most common network strategies are peer-to-peer and client/server systems.
In a peer-to-peer network system nodes can act as both servers and clients. For example, one
microcomputer can obtain files located on another microcomputer and can also provide files to
other microcomputers. A typical configuration for a peer-to-peer system is the bus network.
Commonly used net operating systems are Apple's Macintosh Peer-to-Peer LANs, Novell’s
Netware Lite, and Microsoft's Windows for Workgroups. There are several advantages to using this
type of strategy. The networks are inexpensive and easy to install, and they usually work well for
smaller systems with less than ten nodes. As the number of nodes increases, however, the
performance of the network declines. Another disadvantage is the lack of powerful management
software to effectively monitor a large network's activities. For these reasons, peer-to-peer network
are typically used by small networks.
Client/server network systems use one powerful computer to coordinate and supply services to
all other nodes on the network. This strategy is based on specialization. Server nodes coordinate
and supply specialized services, and client nodes request the services. Commonly used net
operating systems are Novell's Netware, Microsoft's LAN and Windows NT. One advantage of
client/server network systems is their ability to handle very large networks efficiently. Another
advantage is the powerful network management software that monitors and controls the network's
activities. The major disadvantages are the cost of installation and maintenance.

1, term  [tə:m]
n. 学期,术语,名词,期限
v. 称,呼
2, peripheral  [pə'rifərəl]
a. 不重要的,外围的
3, hybrid  ['haibrid]
n. 杂种，混血儿；混合物
adj. 混合的；杂种的
4, dispersed  [di'spə:st]
adj. 散布的；被分散的；被驱散的
v. 分散；传播（disperse的过去分词）

Continue reading it-e-18 Network Architecture

Internet is nowadays widely used in the world, it provides numerous services, such as
on-line booking, BtoB (business to business) services, databases accesses to the companies. But
all these services are asynchronous, which means that there is a delay between an action from the
user and the response from the service (it's a client/server architecture; a computer provides a
service which is used by one or several clients). The new generation of network communication
tools tries to reach the real time level, which means that there is no delay between the action of
the user and the response.
Real time chats, or for instance video on demand, are the new services provided by the
Internet today. These applications appeared recently and are still in development. One really
interesting applications for these technologies is video conference.
Video conference is a technology which allows people to communicate through computer
networks using an audio stream and a video stream. In a few words, people using video
conference can hear and see their correspondents.
Video conference has been very popular only over ISDN (dedicated digital phone lines).
These days, packet-switched networks, such as IP networks, have opened the door to newer
protocols including H.323. The computing power of the desktop systems, the kind of computers
that can be found in the companies or at home, allows the use of video conference applications.
Moreover, webcams, these little low resolution cameras that can be plugged on a regular
computer, are becoming cheaper and cheaper, and almost every computer has now audio
capabilities.
H.323 is the standard for video conference. It can be used over IP (Internet Protocol), and
possibly over all kinds of switched-packet networks (LAN/Local Area Network, MAN/
Metropolitan Area Network, and WAN/Wide Area Network, including the Internet). It was
defined by the ITU (it is the leading publisher of telecommunication technology, regulatory and
standards information ) in 1996. It is updated almost every year, to fit the new progresses in
network capabilities and computing power. The latest version is H.323.5. It was defined in 2003.
The scope of H.323 covers real-time voice, video and data communication over packet-switched
networks. It has multipoint capabilities (several people can communicate with several other people at
the same time) voice and video conferencing capabilities.
The H.323 protocol can be defined as an "umbrella" specification, which means that the protocol
includes several other protocols. In the H.323, the H.225.0, the H.245, the H.450.x, the T.120
protocols are also defined. In addition to these protocols H.323 uses audio codecs (H.261 and H.263),
video codecs (G.711, G.722, G.723.1, G.728 and G.729), and a real-time transport layer called
RTP/RTCP (Real-time Protocol and Real-time Control Protocol). All these protocols cover a different
aspect of the video conference system.
Video conference is one of the most exciting communication media, and will certainly take

a bigger and bigger place in our future. H.323 is a mature protocol that can be safely used for this
purpose. It is widely used by telecom companies, and offers interesting alternatives to the regular
telephone. With the growing power of Internet and the need of world wide communications,
there is no doubt that video conference will be tomorrow for our society what telephone is
nowadays.

1, regulatory  ['regjulətəri]
a. 取缔的,统制的,调整的
2, codecs  
n. 多媒体数字信号编解码器（codec的复数形式）

Continue reading it-e-19 Video Conference and H.323

Want to communicate with a friend across town, in another province, or even in another
country? The Internet and the WEB are the 21st-Century information resources designed for all
of us to use.
Browsers are programs that provide access to Web resources. This software connects you to
remote computers, opens and transfers files, displays text and images, and provides in one tool an
uncomplicated interface to the Internet and Web documents. Two well-known browsers are
Netscape Navigator and Microsoft Internet Explorer. For browsers to connect to other resources,
the location or address of the resources must be specified. These addresses are called Uniform
Resources Locators (URLs). Following the Domain Name System (DNS), all URLs have at least
three basic parts. The first part presents the protocol used to connect to the resource. The protocol
http:// is by far the most common. The second part presents the domain name or the name of the
server where the resource is located. The server is identified as www.aol.com. (Many URLs have
additional parts specifying directory paths, file names, and pointers.) The last part of the domain
name following the dot (.) is the domain code. It identifies the type of organization. For example,
com indicated a commercial site.
The URL http://www.aol.com connects your computer to a computer that provides
information about America Online (AOL). These informational locations on the Web are called
Web sites. Moving from one Web site to another is called surfing.
Once the browser has connected to a Web site, a document file is sent to your computer.
This document contains Hypertext Markup Language (HTML) commands. The browser
interprets the HTML commands and displays the document as a Web page. Typically, the first
page of a Web site is referred to as its home page. [1]The home page presents information about
the site along with references and hyperlinks, or connections to other documents that contain
related information such as text files, graphic images, audio, and video clips.
These documents may be located on a nearby computer system or on one halfway around
the world. The references appear as underlined and colored text and /or images on the Web page.
To access the referenced material, all you do is click on the highlighted text or image. A link is
automatically made to the computer containing the material, and the referenced material appears.
Communication is the most popular Internet activity. The impact of electronic communication
cannot be overestimated. At a personal level, friends and family can stay in contact with one another
even when separated by thousands of miles. [2]At a business level, electronic communication has
become standard and many times preferred way to stay in touch with suppliers, employees, and
customers.
You can communicate with anyone in the world who has an Internet address or e-mail

account with a system connected to the Internet. All you need is access to the Internet and an
e-mail program. Two of the most widely used e-mail programs are Microsoft's Outlook Express
and Netscape's Navigator.
Suppose that you have a friend, Anny, who is going to the University of Southern California.
You and Anny have been planning a trip for the upcoming break. You have heard there are some
inexpensive airfare deals online. To save money, you and Anny agree to research these offers and
e-mail each other your findings.
A typical e-mail message has three basic elements: header, message and signature. The
header appears first and typically includes the following information:
Addresses: Addresses of the persons sending, receiving, and, optionally, anyone else
who is to receive copies.
Subject: A one-line description, used to present the topic of the message. Subject lines
typically are displayed when a person checks his or her mail-box.
Attachments: Many e-mail programs allow you to attach files such as documents and
worksheets. If a message has an attachment, the file name appears on the attachment
line.
The letter or message comes next. It is typically short and to the point. Finally, the signature
line provides additional information about the sender. Typically, this information includes the
sender's name, address, and telephone number.
Following the domain name system discussed earlier, e-mail addresses have two basic parts.
The first part is the user's name and the second part is the domain name, which includes the
domain code.
You can also use e-mail to communicate with people you do not know but with whom you
wish to share ideas and interests. You can participate in discussions and debates that range from
general topics like current events and movies to specialized forums like computer troubleshooting
and Star Trek.
Mailing lists allow members of a mailing list to communicate by sending messages to a list
address. Each message is then copied and sent via e-mail to every member of the mailing list. To
participate in a mailing list, you must first subscribe by sending an e-mail request to the mailing
list subscription address. Once you are a member of a list, you can expect to receive e-mail from
other on the list. You may find the number of messages to be overwhelming. If you want to
cancel a mailing list, send an e-mail request to "unsubscribe" to the subscription address.
Newsgroups, unlike mailing lists, use a special network of computers called the Usenet.
Each of these computers maintains the newsgroups listing. There are over 10,000 different
newsgroups organized into major topic areas that are further subdivided into subtopics.
Contributions to a particular newsgroup are sent to one of the computers on the Usenet. This
computer saves the messages on its system and periodically shares all its recent messages with
the other computers on the Usenet. Unlike mailing lists, a copy of each message is not sent to
each member of a list. Rather, interested individuals check contributions to a particular

newsgroup, reading only those of interest. There are thousands of newsgroups covering a wide
variety of topic areas.
Chat groups allow direct "live" communication. To participate, you join a chat group, select
a channel or topic, and communicate live with others by typing words on your computer. Other
members of your channel immediately see those words on their computers and can respond in the
same manner. One popular chat service is called Internet Relay Chat (IRC). This software is
available free from several locations on the Internet. Using the chat-client software, you log on to
the server, select a channel or topic in which you are interested, and begin chatting. To participate,
you need access to a server or computer that supports IRC. This is done using special chat-client
software.
Instant messaging, like chat groups, allows one or more people to communicate via direct,
"live" communication. Instant messaging, however, provides greater control and flexibility than
chat groups. To use instant messaging, you specify a list of friends, or "buddies", and register
with an instant messaging server. Whenever you connect to the Internet, you use special software
to tell your messaging server that you are online too. It notifies you if any of your buddies are
online. At the same time, it notifies your buddies that you are online. You can then send
messages back and forth to one another instantly.
Before you submit a contribution to a discussion group, it is recommended that you observe
or read the communications from others. This is called lurking. By lurking, you can learn about
the culture of a discussion group. For example, you can observe the level and style of the
discussions. You may decide that a particular discussion group is not what you were looking for
in which case, unsubscribe. If the discussions are appropriate and you wish to participate, try
to fit into the prevailing culture. Remember that your contributions will likely be read by
hundreds of people.

1, overestimated   
v. 过高估计；过高评价（overestimate的过去分词）
adj. 估计过高的
2, impact  ['impækt]
n. 冲击,冲突,影响
v. 挤入,撞击,对...发生影响
3, usenet  [ju:znet]
n. 世界性新闻组网络
4, Lurking  ['lə:kiŋ]
n. （新用户的）“潜伏”（在USENET上作为旁观者，不参加讨论）

Continue reading it-e-20 Browsers and Communications

(1) Have you heard the hypeabout the wireless Web?
Either way, WAP an acronym for wireless application protocol is making the wireless
Internet a reality, and even if it isn't successful right now, this industry is expecting enormous
growth. Market researcher IDC predicts that 1.3 billion wireless Internet users will have
WAP-enabled devices by 2004.
(2) Where did WAP come from?
An industry consortium called the WAP Forum promotes WAP. The WAP forum was
founded in 1997 by Ericson, Motorola, Nokia and Phone.com after Phone.com developed a
server and browser for AT&T's PocketNet.
(3) What exactly is WAP
WAP is a set of protocols used to transfer data to wireless devices. WAP-enabled devices
provide wireless users with a limited version of the Web designed to work on the small black and

white screens of phones and PADs.
Websites accessed by WAP phones must be re-written to satisfy the wireless application
protocol; in order to do that, Web pages written in HTML must be transferred to the WAP
markup language (WML). Internet browsers like Netscape and Internet Explorer read pages in
HTML, while a micro-browser on a WAP-enabled device reads pages in WML.
Major websites like Yahoo and Amazon.com have carefully designed their WAP sites to
look and feel much like their wired counterparts, while search engines like Google are
transferring pages from HTML to WML so they can be accessed by wireless users.
(4) What's the problem with WAP
WAP faces bandwidth constraints that limit the amount of data that can be transferred to the
devices. On the wireless Internet, users are presented with a fraction of information available on
the traditional version.
And while WAP is the leading wireless protocol now, that doesn't mean that isn't here to
stay. When a better version of the wireless Web becomes available, WAP could very well
become obsolete.
One potential competitor is NTT DoCoMo, a Japanese company which has developed a
hugely successful wireless data service called i-mode and expects to tap the U.S. market toward
the end of 2001.
Blue-tooth is a technology that connects electronic devices from camcorders to PDAs to
computers without using wires. Consumers began to see Bluetooth in action when Toshiba
starting selling a Bluetooth-enabled PC card over their website in September 2000 for $199.
Other vendors plan to follow with devices ranging from PDAs to mobile phones.
A Bluetooth device uses radio signal to send information from one Bluetooth device to
another though the air. For example, if you are trying to transfer a PC's address book to a PDA,
first the data in an address book is translated into a language that the PDA can understand by a
conduit. The data goes through the conduit to the Bluetooth device. The Bluetooth device is made
up of a base-band processor, a radio, and an antenna. The base-band processor transfers the data
into signals that the radio can understand, and the radio puts out signals in a frequency (2.4
gigahertz) that the antenna transmits through the air to another Bluetooth device within 30-feet.
The other device receives the data and processes it in the reverse order.
Bluetooth is supported by a Special Interest Group (SIG), which was founded in 1998 and
has approximately 2000 members, all of whom have access to Bluetooth specifications the
information needed to make a Bluetooth product. The SIG includes IBM, Intel, Microsoft and
Nokia, and works to develop and promote the Blue-tooth technology.
But Bluetooth, like many new technologies, may not be an instant hit. There are still plenty
of questions about the ability of these devices to speak the same language. So while devices
produced by the same company could communicate with each other easily, integration may be

difficult when multiple vendors are involved. And while consultants at Forrester Research expect
Bluetooth’s popularity to grow, the firm said in a brief that many businesses won't buy in, "until
user pressure forces them to in 2003“

1, hype  [haip]
v. 大肆宣传
2, acronym  ['ækrənim]
n. 缩写字,字头语
3, enormous  [i'nɔ:məs]
a. 巨大的,庞大的
4, consortium  [kən'sɔ:tjəm]
n. 联合,合伙,国际财团
5, obsolete  ['ɔbsəli:t]
a. 已废弃的,过时的
6, camcorders  ['kʌmkɔ:də(r)]
便携式摄像机,摄录机
7, conduit  ['kɔndit]
n. [电] 导管；沟渠；导水管
8, antenna  [æn'tenə]
n. 触角,天线

Continue reading it-e-21 What is WAP

A VoIP phone is designed specifically for use in a voice over IP(VoIP)system by converting
standard telephone audio into a digital format that can be transmitted over the Internet, and by
converting incoming digital phone signals from the Internet to standard telephone audio. A VoIP
phone allows the user to take advantage of VoIP technology without involving a personal computer,
although an Internet connection is required. Physically, a VoIP phone set resembles a traditional
hard wired or cordless telephone set. Some VoIP phone sets offer enhanced quality audio,
comparable to that on compact disc (CD). A few VoIP phone sets allow for the transmission and
reception of image data during calls, so they can be considered video telephones.
An IP PBX is a private branch exchange (telephone switching system within an enterprise )
that switches calls between VoIP users on local lines while allowing all users to share a certain

number of external phone lines. The typical IP PBX can also switch calls between a VoIP user
and a traditional telephone user, or between two traditional telephone user in the same way that a
conventional PBX does. With a conventional PBX, separate networks are necessary for voice and
data communications. One of the main advantages of an IP PBX is the fact that it employs
converged data and voice networks. This means that Internet access, as well as VoIP
communications and traditional telephone communications, are all possible using a single line to
each user. This provide flexibility as an enterprise grows, and can also reduce long-term
operation and maintenance costs.
Ear and mouth (E&M) is a technology in voice over IP (VoIP) that uses a traditional
telephone handset with an earphone (or earpiece) for listening to incoming audio and a
microphone (or mouthpiece) for transmitting audio. Calls using an E&M interface can be made
from, received from , or disconnected by a private banch exchange (PBX) as well as from a
VoIP-capable computer.
The main advantage of E&M is the fact that it allows a PBX to reliably detect disconnect
(hang-up) signals. This eliminates problems that can otherwise occur with locked computer ports
at the terminations of calls, and thus minimizes the risk of needlessly consuming network
resources.

cordless ['kɔ:dlis]

n. 不用电线的adj. 无线的（副词cordlessly）

Continue reading it-e-22 VoIP Phone and IP PBX

The Web can be an incredible resource providing information on nearly any topic
imaginable. Are you planning a trip? Writing an Economics paper? Looking for a movie review?
Trying to locate a long-lost friend? Information sources related to these questions, and much,
much more are available on the Web.
With over two billion pages and more being added daily, the Web is a massive collection of
interrelated pages. With so much available information, locating the precise information you
need can be difficult. Fortunately, a number of organizations called search services or search
providers can help you locate the information you need. They maintain huge databases relating to
information provided on the Web and the Internet. The information stored at these databases
includes addresses, content descriptions or classifications, and keywords appearing on Web pages
and other Internet informational resources. Special programs called agents, spiders, or bots

continually look for new information and update the search services databases. Additionally,
search services provide special programs called search engines that you can use to locate specific
information of the Web.
[1]Search engines are specialized programs that assist you in locating information on the Web
and the Internet. To find information, you go to the search service's Web site and use their search
engine. Yahoo’s search engine, like most others, provides two different search approaches.
Keyword Search
In a keyword search, you enter a keyword or phrase reflecting the information you want.
The search engine compares your entry against its database and returns a list of hits or sites that
contain the keywords. Each hit includes a hyperlink to the referenced Web page (or other
resource) along with a brief discussion of the information contained at that location. Many
searches result in a large number of hits. For example, if you were to enter the keyword travel,
you would get over a thousand hits. [2]Search engines order the hits according to those sites that
most likely contain the information requested and present the list to you in that order, usually in
groups of ten.
Directory Search
Most search engines also provide a directory or list of categories or topics such as Arts
&Humanities, Business & Economics, Computers & Internet. In a directory search, also known
as index search. You select a category that fits the information that you want. Another list of
subtopics relates to the topic you selected appears. You select the subtopic that best relates to
your topic and another subtopic list appears. You continue to narrow your search in this manner
until a list of Web sites appears. This list corresponds to the hit list previously discussed.
As a general rule, if you are searching for general information, use the directory search
approach. For example, to find general information about music, use a directory search beginning
with the category Arts &Humanities. If you are searching for specific information, use the key
word approach. For example, if you were looking for a specific MP3 file, use a key word search
entering the album title and/or the artist’s name in the text selection box.
A recent study by the NEC Research Institute found that any one search engine includes
only a fraction of the informational sources on the Web. Therefore, it is highly recommended that
you use more than one search engine when researching important topics. Or, you could use a
special type of search engine called a metasearch engine."
One way to research a topic is to visit the Web site for several individual search engines. At
each site, enter the search instructions, wait for the hits to appear, review the list, and visit
selected sites. This process can be quite time-consuming and duplicate responses from different
search engines are inevitable. Metasearch engines offer an alternative.
Metasearch engines are programs that automatically submit your search request to several

search engines simultaneously. The metasearch engine receives the results, eliminates duplicates,
orders the hits, and then provides the edited list to you. There are several metasearch sites
available on the Web. One of the best known is Metacrawler.

Specialized search engines focus on subject-specific Web sites. Specialized sites can
potentially save you time by narrowing your search. For example, let's say you are researching a
paper about the fashion industry. You could begin with a general search engine like Yahoo! Or,
you could go to a search engine that specialized specifically in fashion.

1, incredible  [in'kredəbl]
a. 难以置信的
2, massive  ['mæsiv]
a. 巨大的

Continue reading it-e-23 Search Tools