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CHAPTER Five：
Internet and LAN
Technology
第5章：Internet和LAN技术
Table Of Contents
5.1
Network Building Blocks
5.2
Internet Building Blocks
5.3
Internet Access
5.4
LAN Technology
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5.1.1 Transmitting Data
5.1.2 Network Links
5.1.3 Network Devices
5.1.4 Network Addresses
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5.1.5 Network Topology
5.1.6 Packet and Circuit Switching
Technology
5.1.7 Communications Protocols
5.1.8 Network Classifications
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5.1.1 Transmitting Data
• Is it easy to transmit data?
 A communications network (通信网络) is
the combination of hardware, software,
and connecting links that transport data.
 A communications system basically sends
information from a source to a
destination.
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 In 1948, Claude Shannon, an engineer
at the prestigious Bell Labs, published an
article that described a communications
system model. In this model, data from
a source is encoded and sent over a
communications channel to its destination
where it is decoded.
 According to Shannon, effective
communication depends on the efficiency
of the coding process and the channel's
resistance to interference, called "noise".
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• What kind of coding process happens
on a communications network?
 Computers use bits to convey information.
A bit, like a lantern, has two states: on (1)
and off (0). So, a single bit can represent
two messages, each called a unit of
information (信息单元).
 Computers use several coding schemes
that are based on binary digits, including
ASCII, EBCDIC, UNICODE, and binary
numbers.
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 The number of units of information that
you can convey is simply the number of
different combinations that you can make
with a given number of bits.
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 You now have a rule about conveying
information with bits: The maximum
number of different units of information
that you can convey with n bits is 2n.
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• Exactly what is transmitted when I
send a message?
 When data is transmitted, it usually takes
the form of an electromagnetic signal.
 You can think of these signals as waves
that ripple through cables or through the
air. On a communications network, your
data might be converted into several
different types of waves before it
reaches its destination.
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 Analog signals typically represent an
unlimited range of values and, therefore,
have a smooth, curved waveform. Digital
signals, on the other hand, represent
discrete values within a limited range and,
therefore, have a square or "stepped"
waveform.
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• Are digital signals better than analog
signals?
 The beauty of digital signals is that they
require simple circuitry and are easy to
"clean up" after being affected by noise.
 At its most primitive level, digital
equipment must be sensitive to only two
frequencies—one that represents 1s and
one that represents 0s. In contrast,
analog equipment must be sensitive to a
wide range of audible frequencies.
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5.1.2 Network Links
• How do signals travel over a network?
 Data can travel from one network device
to another over a cable or through the
air.
 A communications channel (通信信道), or
"link", is a physical path or a frequency
for a signal transmission.
- For example: a television channel
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说明
教材P212的最后一段中的两句话翻译有错，
英文原文如下：
For example, a television channel, such
as Channel 12, is a specific frequency
used to broadcast audio-visual data for a
television station.
This data might also be carried over
another channel, such as coaxial cable as
part of a cable TV system.
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 Computer networks use a variety of
links to carry data, the most common
being twisted-pair cable, coaxial cable,
fiber-optic cable, radio waves, microwaves,
satellites, infrared light, and laser beams.
 twisted-pair cable 双绞线
STP (shielded twisted pair) 屏蔽双绞线
UTP (unshielded twisted pair) 非屏蔽双绞线
 coaxial cable 同轴电缆
 fiber-optic cable 光缆
- 不能传输电信号
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 radio wave 无线电波（也称为RF信号）
- RF信号 (radio frequency signal)通过无线
电收发器(transceiver)来进行发送和接收
 microwave 微波
- 只能朝一个方向传输信号，不能穿透金属物
 communications satellite 通信卫星
- 通过异频雷达收发机(transponder)接收并
放大信号，再将信号重新传回地面
 infrared light 红外线光束
 laser beam 激光束
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• What's bandwidth?
 Bandwidth (带宽) is the transmission
capacity of a communications channel.
 High-bandwidth communications systems,
such as cable TV, are sometimes referred
to as broadband (宽带), whereas systems
with less capacity, such as the telephone
system, are referred to as narrowband
(窄带). The bandwidth of a digital channel
is usually measured in bits per second
(bps).
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5.1.3 Network Devices
• What kind of equipment populates data
communications networks?
 Each connection point on a network is
referred to as a node (节点).
 A network node can be a redistribution
point where data is routed to other
nodes, or it can be an end-point where
data originates or terminates.
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 Network nodes are populated by
devices—some that serve out data, some
that request data, and others that simply
route data.
 The term workstation (工作站) usually
refers to a personal computer connected
to a local area network (LAN) node.
 The term client (客户端) is sometimes
used as a synonym for workstation. "Client"
can also refer to a personal computer

that's connected to the Internet.
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 The term server (服务器) often refers
to any computer on a local area network
or the Internet that contains the
software to manage and process files
for other network nodes. "Server" can
also refer to the software that provides
a network service.
 The term host computer (主机), or "host",
usually refers to any computer that
provides services to network users.
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 A hub (集线器) is a device that connects
several nodes of a local area network.
 A router (路由器) is a device that is
connected to at least two networks.
Routers make decisions about the best
route for data, based on the data's
destination and the state of the available
network links.
 A repeater (中继器) is a device—found on
LANs and the Internet—that amplifies
and regenerates signals so that they can
retain the necessary strength to
reach their destinations.
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5.1.4 Network Addresses
• How does data find its destination?
 Every device on a network has an address.
The data that travels over a network
includes the address of its destination
device.
 When the data reaches a network node,
a router examines the address and sends
the data along to its destination or to
another router that's closer to the data's
destination.
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• Where does a device get an address?
 An address is built into the circuitry of
most network devices at the time that
they are manufactured. This address is
referred to as a device's physical
address (物理地址).
 In some case, however, a device must have
a logical address (逻辑地址) that fits the
network's prescribed format.
 A protocol on the network keeps track of
which logical address corresponds to
each physical address.
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5.1.5 Network Topology
• Does a network require a certain
configuration of nodes?
 The layout of devices, wires, and cables
on a network is referred to as its physical
topology (物理拓扑).
 The three most typical physical topologies
include star (星型), bus (总线型), and ring
(环型).
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 A real-world network can make use of
more than one topology.
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 Networks also have a logical topology
(逻辑拓扑), which corresponds to the way
that messages flow.
 Logical topologies fall into the same three
categories as physical topologies: star,
bus, and ring.
 Contrary to what you might assume,
however, a network's physical topology
and its logical topology don't have to
match!
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5.1.6 Packets and Circuit Switching
Technology
• What's a packet?
 Your file is actually chopped up into small
pieces called packets (数据包). Each
packet contains the address of its sender,
the destination address, a sequence
number, and some data.
 When packets reach their destination,
they are reassembled back into the
original message according to the
sequence number.
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• Why not just send an entire message?
 The short answer is that by dividing
messages into equal-size packets, they
are easier to handle than an assortment
of small, medium, large, and huge files.
 The long answer is steeped in an old
controversy between the engineers who
built the technology that was eventually
used on the Internet, and the moguls who
ran the telephone company.
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 The telephone network uses a technology
called circuit switching (线路交换),
which essentially establishes a private
link between one telephone and another
for the duration of a call.
 The computer network uses a technology
called packet switching (数据包转换) in
which a message was divided into several
packets that can be routed independently
to their destination to avoid out-of-service
or congested links.
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 Circuit switching provides callers with a
direct pipeline over which streams of
voice data can flow.
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 In addition to providing reliable data
transport, packet switching was designed
to make very efficient use of available
bandwidth. Packets from many different
messages can share a single
communications channel, or "circuit".
 In contrast, a circuit switching network,
like the telephone system, devotes an
entire circuit to each call. So, when
someone is "on hold", the circuit is
reserved and cannot be used for
other communications.
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5.1.7 Communications Protocols
• What is a communications protocol?
 In general, a "protocol" is a set of rules
for interacting and negotiating.
 In the context of networks, the term
"protocol", or communications protocol
(通信协议), refers to a set of rules for
efficiently transmitting data from one
network node to another.
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 Protocols, such as TCP/IP, allow two
devices to negotiate and agree on how
data will be transmitted.
 Among their many duties, communications
protocols provide rules for starting and
ending transmissions, recognizing
transmission errors, determining the
speed at which data is sent, and specifying
how packets are addressed and formatted.
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• How does a protocol initiate a
transmission?
 A major challenge of network
communications is coordinating the
transmission and reception of each packet.
 Protocols help two communications
devices negotiate and establish
communications through a process called
handshaking (握手).
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 The transmitting device sends a signal
that means, "I want to communicate". It
then waits for an acknowledgement signal
from the receiving device. The two devices
then "negotiate" a transmission speed that
both can handle. The devices also decide
on how to coordinate the transmission.
 synchronous protocol 同步协议
 asynchronous protocol 异步协议
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 Using a synchronous protocol, the sender
and the receiver are synchronized by a
signal called a clock. The transmitting
computer sends data at a fixed clock rate,
and the receiving computer expects the
incoming data at the same fixed rate.
 The rules for an asynchronous protocol
require the transmitting computer to send
a start bit that indicates the beginning
of a packet. Data is then transmitted as
a series of bytes. A stop bit marks
the end of the data.
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• Does data travel in both directions
over the same channel?
 Simplex (单工通信) refers to a type of
communication in which a signal travels in
only one direction.
 Half duplex (半双工通信) refers to a type
of communication that allows you to send
and receive data, but not at the same time.
 Full duplex (双工通信) refers to a type of
communication in which it is possible to
send and receive at the same time,
over the same channel.
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• How does a protocol make sure that
data arrives without errors?
 Computers use error-checking protocols
to ensure accurate delivery of data.
 As an example, one error-checking
protocol adds a parity bit (奇偶校验位) to
a sequence of bits to keep track of the
number of 1s and 0s it should contain. The
even parity protocol (偶校验) requires
the number of 1 bits, including the parity
bit, to be an even number.
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• What part of a network specifies
protocols?
 Some protocols are handled by hardware,
whereas other protocols are handled by
software.
 For example:
- 主板上产生+5V脉冲可以由modem转换成能
在电话线中传输的“声调”
- 软件可以为创建packet提供规范说明
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5.1.8 Network Classifications
• How are networks classified?
 In the press and in casual conversation,
a network is often categorized by some
distinguishing characteristics, such as
its wiring technology, protocol, operating
system, or geographical coverage.
 internetwork 互联网络
 Internet 因特网
 intranet 企业内部网
 extranet 外部网
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 WAN (wide area network) 广域网
 MAN (metropolitan area network) 城域网
 LAN (local area network) 局域网
 wireless network 无线网
 HomeRF network HomeRF网络
 HomePLC network HomePLC网络
 HomePNA network HomePNA网络
 Novell network Novell网络
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 Ethernet 以太网
- 10BaseT network
- 100BaseT network（快速以太网）
 Token Ring network 令牌环网
 FDDI (Fiber Distributed Data
Interconnect) 光纤分布式数据接口
 ATM (asynchronous transfer mode)
network 异步传输模式网络
 client/server network 客户/服务器网络
 peer-to-peer network (P2P) 点对点网络
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 Ethernets use a protocol called CSMA/CD
(carrier sense multiple access with
collision detection) to deal with these
collisions.
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 A Token Ring network connects nodes in a
physical star configuration, but passes data
around a logical ring using a technology
called a "token". Tokens prevent collisions.
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 A peer-to-peer network treats every
computer as an "equal" so that workstations
can run local applications and also provide
network resources, such as file access.
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5.2.1 Background
5.2.2 Internet Hardware and
Connections
5.2.3 Internet Protocols
5.2.4 IP Addresses
5.2.5 Domain Names
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5.2.1 Background
• How did the Internet get started?
 The ARPANET, created in 1969, connected
computers at some universities in USA.
 In 1985, the National Science Foundation
(NSF) used ARPANET technology to
create a similar, but larger, network,
linking not just a few mainframe
computers, but entire local area networks
at each site. The NSF network was
an internet.
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5.2.2 Internet Hardware and
Connections
• What type of hardware populates
Internet nodes?
 The computer that you use is the starting
point on the Internet. It typically
connects to an ISP over a telephone line,
cable TV line, or personal satellite link.
 Your computer can, alternatively, connect
to a local area network, which usually
provides a single Internet access
point for all of its workstations.
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• What kinds of network devices are
part of an ISP?
 An ISP operates network devices that
handle the physical aspects of
transmitting and receiving data from
your computer.
 The last piece of essential ISP equipment
is a router that sends your data to the
next "hop" toward its destination.
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To Internet
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• Are Internet nodes all connected by
cables?
 The Internet backbone is primarily
composed of high-bandwidth fiber-optic
cables.
 Your data might flow over simple
telephone cables or on the cable TV
infrastructure. It might also be beamed
up to a satellite or flashed through
fiber-optic cables.
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• Can I trace the route of the data
that I send and receive?
 With the help of Ping and Traceroute,
you can trace the route of the data that
you send and receive.
 Your packet is first sent to a router at
your ISP. This router sends your data to
another router, and that router sends
the data to another router, and so on.
Each leg of the journey is referred to as
a “hop” in Internet jargon.
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 Ping (Packet Internet Groper) is an
Internet utility that performs three
useful tasks.
 Ping indicates whether an Internet
host, such as a Web server, is
operational.
 Ping reports the elapsed time for data
to travel from your computer to the
host and back.
 Ping indicates whether a packet is able
to reliably reach its destination.
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 如何使用Ping? 在DOS命令提示符后输入：
ping + IP address/domain name
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 Traceroute is an Internet utility that
traces the path of a packet from your
computer to a selected IP address or
domain name.
 The Traceroute report lists each hop.
The statistics for each hop include the
three ping times and the domain name or
IP address of the router.
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 如何使用Traceroute? 在DOS命令提示符后
输入：tracert + IP address/domain name
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• How fast does data travel over the
Internet?
 Using Ping or Traceroute, you can
discover how long data is in transit from
point A to point B.
• What's the capacity of the Internet?
 The Internet as a whole offers a
tremendous amount of capacity.
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5.2.3 Internet Protocols
• Why does the Internet use TCP/IP?
 From a practical perspective, TCP/IP
provides a standard that is fairly easy to
implement, public, free, and extensible.
 The Internet is not owned, operated,
or controlled by any single business,
government, or organization. In a sense,
TCP/IP is the glue that holds the
Internet together.
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• How does TCP/IP work?
 TCP/IP is a suite of protocols, which
includes TCP, IP, ICMP, and others.
 TCP (Transmission Control Protocol)
breaks a message or file into packets.
 IP (Internet Protocol) is responsible for
addressing packets so that they can be
routed to their destination.
 ICMP (Internet Control Message Protocol)
provides error correction and routing
information used by utilities such as
Ping and Traceroute.
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• Is TCP/IP the only Internet protocol?
 No. Several other protocols are used in
conjunction with TCP/IP on the Internet.
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5.2.4 IP Addresses
• Does the Internet use a special
addressing scheme?
 The "IP" part of TCP/IP defines the
format for the addresses that identify
computers on the Internet. As a result,
these addresses are referred to as IP
addresses.
 An IP address is a series of numbers,
such as 61.144.43.225.
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• Do I need a permanent IP address?
 A computer can have a permanently
assigned static IP address (静态IP地址)
or a temporarily assigned dynamic IP
address (动态IP地址).
 As a rule, computers on the Internet
that act as servers or hosts use static
IP addresses. A "temporary" dynamic
IP address will suffice for client (as
opposed to server) activities.
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• When do I get a dynamic IP address?
 Every ISP controls a unique pool of IP
addresses, which can be assigned as
needed to subscribers.
 Dynamic IP addresses are typically
assigned by ISPs for most dial-up
connections and some DSL, ISDN, or
cable modem connections.
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 When you use a dial-up connection, for
example, your ISP assigns a temporary
IP address to your computer for use as
long as it remains connected.
 When you end a session, that IP address
goes back into a pool of addresses that
can be distributed to other subscribers
when they log in.
 Your computer will rarely be assigned
the same IP address it had during a
previous dial-up session.
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 The address situation for DSL, ISDN,
and cable modem subscribers varies,
depending on the ISP.
 You may be assigned a static IP address;
you might be given a dynamic address
each time you connect; or you may be
allocated a semi-permanent address that
lasts for several months.
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5.2.5 Domain Names
• What's a domain name?
 Although IP addresses work for
communication between computers, people
find it difficult to remember long strings
of numbers.
 Many host computers also have an easyto-remember name, such as www.163.com.
The official term for this name is "fully
qualified domain name" (FQDN), but most
people just refer to it as a domain
name (域名).
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 A domain name ends with an extension
that indicates its top-level domain
(顶级域名).
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 Country codes also serve as top-level
domains. China's top-level domain is cn.
- 我国CN域名体系分为类别域名和行政区域名
两套。类别域名有六个，分别依照申请机构
的性质依次分为：AC(科研机构), COM(商业
组织), EDU(教育机构), GOV(政府部门),
NET(互联网络的信息中心和运行中心),
ORG(各种非盈利性的组织)。行政区域名是按
照我国的各个行政区划分而成的，包括行政
区域名34个，适用于我国的各省、自治区、
直辖市，例如: GD(广东省), HK(香港)。
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• How are domain names related to IP
addresses?
 Every domain name corresponds to a
unique IP address that has been entered
into a huge database called the domain
name system (DNS). Computers that
host this database are referred to as
domain name servers (域名服务器).
 A domain name must be converted into
an IP address before any packets can be
routed to it.
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 When you enter a domain name or URL,
it is sent to your designated domain
server, which can do one of three things:
 It can send back the IP address.
 If your domain name server does not
have a record of the domain name, it can
contact another domain name server.
 It can send back the address for a
domain name server that is more likely to
have the IP address of the domain name
that you requested.
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• Do I need my own domain name?
 You do not need your own domain name
for client-style Internet activities, such
as Web browsing, e-mail, and chat.
 You would, however, want a domain name
if you plan to operate your own Web
server or if you establish a Web site
using servers provided by a Web site
hosting service.
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• How do I get a domain name?
 An organization called ICANN (Internet
Corporation for Assigned Names and
Numbers) is recognized by the U.S.
and other governments as the global
organization that coordinates the
technical management of the Internet's
domain name system.
- 中国互联网络信息中心(CNNIC)是我国域名
注册管理机构和域名根服务器运行机构，负责
运行和管理国家顶级域名.CN、中文域名
系统及通用网址系统。
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Internet Access
5.3.1 Dial-up Connections
5.3.2 Cable Television Connections
5.3.3 DSL, ISDN, T1, and T3
5.3.4 Wireless Internet Access
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Internet Access
5.3.1 Dial-up Connections
• How does a dial-up connection work?
 A dial-up connection uses POTS (plain old
telephone service) to transport data
between your computer and your ISP.
- 电话通信系统使用一个分层次的网络：
本地—国内—国际
- 用户和ISP之间的链接：一直占用的链接
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 The telephone system connects your
telephone to a local switch. Local switches
are connected to other nearby local switches
and to long-haul communications links.
Internet Access
 While connected to your ISP, your call
is routed through the telephone
company’s local switch to the ISP.
Internet Access
• How does a modem work?
 A voiceband modem—usually referred to
simply as a "modem"—converts the
signals from your computer into signals
that can travel over telephone lines.
 The word "modem" is derived from the
words "modulate" and "demodulate".
 modulation 调制
 demodulation 解调
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Internet Access
 Modulation means changing characteristics
of a signal, as when a modem changes a
digital pulse into an analog audio signal.
 Demodulation means changing a signal back to
its original state, as when a modem changes
an audio signal back to a digital pulse.
Internet Access
• Why doesn't new digital telephone
technology eliminate the need for a
modem?
 Digital switches kick into action only
after your call arrives at the local
switching station.
 The technology between your telephone
and your local switch—sometimes called
as the "local loop"—is still designed to
carry analog voice signals.
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Internet Access
• Can I talk and send data at the same
time?
 When connected to your ISP via dial-up,
data is transmitted over the same
frequencies that are normally used for
voice conversations.
 However, it is also possible for you to talk
and send data at the same time.
 Voice over IP (VoIP) 基于IP的语音传输—
allows you to play games and chat about
your moves all while you are online.
Slide 85 of 146
Internet Access
• How fast is a modem?
 The speed of modems was primarily
measured as baud rate (波特率)—the
number of times per second that a signal
in a communications channel varies, or
makes a transition between states.
 Modem speeds are now measured in bits
per second.
 Actual data transfer speeds are affected
by factors such as the quality of your
local loop connection to the
telephone switch.
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Internet Access
 Even with a "perfect" connection, a 56
Kbps modem tops out at about 44 Kbps.
 Many Internet connection methods
provide faster downstream (下游 the
data that you receive) transmission
rates than upstream (上游 the data that
you send) rates.
 Dial-up connections are no exception; 44
Kbps is a typical downstream speed for a
56 Kbps modem. Upstream, the data rate
drops to about 33 Kbps, or less.
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Internet Access
5.3.2 Cable Television Connections
• How can the cable TV system provide
Internet access?
 The cable television system is called
"community antenna television", or CATV.
 The satellite dish "farm" at which
television broadcasts are received and
retransmitted is referred to as the
head-end (前端). From the head-end,
a cabling system branches out and
eventually reaches consumers' homes.
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Internet Access
Internet Access
 The topology of a CATV system looks
a lot like the physical topology for a
computer network. And that is just what
is formed when your cable TV company
becomes your Internet provider.
 A router and high-speed connection from
the head-end to the Internet provide the
potential for Internet connectivity over
every cable in the system. Your computer
becomes part of a neighborhood local
area network.
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Internet Access
Internet Access
• Are television and data signals
carried over the same cable?
 The lowest-capacity coaxial cable that's
used by the CATV system has a far
greater carrying capacity than POTS
lines.
 To offer both television and Internet
access, the cable's bandwidth is divided
among three activities.
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Internet Access
 A CATV cable must provide bandwidth
for television signals, incoming data
signals, and outgoing data signals.
Internet Access
• How do I set up a cable modem
connection?
 When you configure your computer to
access the Internet over a CATV system,
you are essentially connecting to an
Ethernet-style local area network.
 The two requirements for this type of
connection are circuitry to handle
Ethernet protocols and a cable modem,
which converts your computer's signal
into one that can travel over the
CATV network.
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Internet Access
Internet Access
• What's the significance of becoming
part of a "neighborhood network"?
 When your CATV connection is up and
running, your computer becomes part of
a neighborhood data network.
 Two issues become significant: bandwidth
and security.
 The cable that you share with your
neighbors has a certain amount of
bandwidth. As more and more neighbors
use the service, it might seem to
get slower and slower.
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Internet Access
 As for the security issue, in the early
days of cable modem service, Windows
automatically takes inventory of the local
area network during boot up when you
have an Ethernet card in your PC.
 It looks for any computers on the
network that have file and print sharing
activated, and then lists them in the
Network Places window.
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Internet Access
Internet Access
 Today, many cable companies use
DOCSIS-compliant cable modems to
block this "crossover" access.
 DOCSIS (Data Over Cable Service
Interface Specification 数据有线服务接口
规范) is a security technology that filters
packets to certain ports, including the
port that Windows uses for networking.
 DOCSIS secures your computer from your
neighbors, but it does not close up all of
the security holes that are opened when
you use an always-on connection.
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Internet Access
 An always-on connection is always
connected, and it is "on" whenever your
computer is powered up. With an alwayson connection, you might have the same
IP address for days, or even months,
depending on your ISP.
 A hacker who discovers that your
computer has a security weakness can
easily find it again, and its high-speed
access makes it a very desirable target.
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Internet Access
• How do I secure an always-on
connection?
 If your PC is connected to a cable modem,
you should take steps to protect it from
hackers.
 Disable file and printer sharing.
 Shut down your computer when you are
not using it.
 Install personal firewall software, which
is designed to analyze and control
incoming and outgoing packets.
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Internet Access
5.3.3 DSL, ISDN, T1 and T3
• What other options are available for
high-speed Internet access?
 Several services such as DSL, ISDN, T1,
and T3 take advantage of full capacity
of the telephone connections (normal
dial-up services do not).
 DSL, ISDN, T1, and T3 can offer highspeed digital communications links for
voice and data.
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Internet Access
• What is DSL?
 DSL (Digital Subscriber Line 数字用户
线路) is a high-speed, digital, always-on,
Internet access technology that runs over
standard phone lines.
 Several variations of this technology exist,
including ADSL, SDSL, HDSL and DSL lite.
 The term "xDSL" refers to entire group
of DSL technologies, but xDSL is not a
separate variation of DSL.
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Internet Access
 DSL is digital, so data doesn't need to
be changed into analog form and then
back to digital as it does when you use
a dial-up connection.
 A DSL connection can carry both voice and
data. The voice signals are transferred to
the telephone company's regular lines. The
data signals are interpreted by special
equipment called a DSLAM (DSL Access
Multiplexor DSL访问多路切换器) and
routed over high-speed lines to a DSL
provider, or directly to the Internet.
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Internet Access
Switching Station
Internet Access
 ADSL (Asymmetric Digital Subscriber
Line 非对称数字用户线路) 最大特点在于其
上下行速率是非对称的，即上下行速率不等。
 ADSL技术只要在电信公司的线路出口和用户
的电话线路入口各加一台ASDL Modem，就
可以架起一条高速通道。在其中会产生三个信
息通道：一个是速率为1.5~9Mbps的高速下行
通道，用于用户下载数据；一个速率为
16Kbps~1Mbps的中速通道，用于用户上传数
据；一个电话服务通道，用于普通的电话
服务。最关键的是这三个通道可以同时
工作，传输距离达3~5Km。
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Internet Access
• How do I install DSL?
 Most DSL installations require trained
service technicians.
 Firstly, telephone line of your house
should be connected to a special type of
DSL switch.
 In a typical DSL installation, a twistedpair cable connects your computer’s
Ethernet card to a DSL modem, which is
plugged into a wall jack.
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Internet Access
• Is ISDN faster than DSL?
 ISDN (Integrated Services Digital
Network 综合业务数字网) connections
move data at speeds of 64 or 128 Kbps—
not as fast as DSL or cable modems, but
faster than a dial-up connection.
 ISDN service is typically regarded as a
high-speed Internet connection option
for businesses that maintain small local
area networks.
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Internet Access
• Who uses T1 or T3 service?
 T1 (or T-1) is a high-speed (1.544 Mbps)
digital network. A T1 line consists of 24
individual channels. T1 lines are popular
for businesses and ISPs that want a
high-speed connection to the Internet,
regardless of cost.
 A T3 (or T-3, DS3) connection consists of
672 channels and supports data rates of
about 43 Mbps. T3 lines provide many of
the links on the Internet backbone.
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Internet Access
5.3.4 Wireless Internet Access
• What are the options for wireless
connections to the Internet?
 You can connect a computer to the
Internet using several wireless options.
 Personal satellite connections: expensive,
but sometimes only option (particularly in
remote rural areas)
 Cellular telephones: slowest, but provides
mobile Internet access
Slide 110 of 146
Internet Access
• How does satellite access work?
 Direct satellite service (DSS) uses a
geosynchronous or low-earth satellite to
send television, voice, or computer data
directly to a satellite dish owned by an
individual.
 Two-way satellite service ships both
upstream and downstream data through
the satellite. Some direct satellite
services, however, provide only
downstream data transport.
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Internet Access
Internet Access
 Satellite data transport is subject to
latency delays of one second or more,
which occur as your data is routed
between your computer and a satellite
that orbits 22,200 miles above the earth.
 Satellite transmission and reception can
be blocked by adverse weather conditions.
 As with cable modem service, satellite
data transport speeds may seem to
decline when other users subscribe to the
service.
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Internet Access
• Can I access Internet services simply
by using my cell phone?
 If you require mobile Internet access,
you can connect a notebook computer to
the Internet using one of several devices.
 Install a cellular-ready PC card modem in
their notebook computers.
 Use cellular telephone which can act as
a modem to connect by a special cable to
a notebook computer.
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Internet Access
• What's the best Internet connection?
 The best Internet connection depends
on your budget and what's available in
your area.
 Most people begin with a dial-up
connection and eventually look around for
a connection that provides higher access
speeds.
 Cable modem service or ADSL are usually
good choices, when available.
Slide 115 of 146
Internet Access
• What about PDA?
 Most PDAs can use a wireless connection
to access the Internet.
 Some PDAs go mobile with the addition
of snap-on wireless hardware, which
essentially converts your PDA into a
wireless phone.
 Other PDAs, have built-in wireless
communications circuitry.
Slide 116 of 146
LAN Technology
5.4.1 LAN Services and Resources
5.4.2 LAN Hardware
5.4.3 LAN Standards and Protocols
5.4.4 LAN Addresses
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LAN Technology
5.4.5 LAN Software
5.4.6 LAN Installation
5.4.7 LAN Security
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LAN Technology
5.4.1 LAN Services and Resources
• What sort of resources does a LAN
provide?
 Main advantage of a LAN is that multiple
users can share a limited number of
resources.
 For example, a network allow all of its
workstations to access a single high-speed
Internet connection.
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LAN Technology
 A workstation provides you with the
usual array of local resources that are
available on any standalone PC, including
data storage devices, software, an
Internet connection, and a printer.
 A workstation also provides access to
network resources, which typically
include a high-speed Internet connection,
access to files and peripheral devices
from other workstations, and centralized
storage and backup.
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LAN Technology
• How do I access network resources?
 When you boot your computer, the OS
checks to see if it can access a LAN.
 Your computer's OS keeps track of
network resources that you can
automatically access.
 Some network resources become available
as soon as your computer completes the
boot process, and other network
resources can be accessible after you use
your networking utilities.
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LAN Technology
5.4.2 LAN Hardware
• What's a typical LAN configuration?
 A LAN may connect as few as two
computers, or it may encompass hundreds
of devices, including computers, hubs,
routers, printers, and modems.
 On a typical network, each computer or
peripheral device requires a network port
and circuitry, which is usually supplied by
a network interface card.
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LAN Technology
 When connecting only two computers,
you can simply use a single "crossconnect" cable between them. A hub or a
hub/router is typically used to connect
more than two computers.
 Each network device—computer, printer,
or modem—requires one of the hub's
ports. To expand a network beyond the
capacity of a single hub, you can add
other hubs. These hubs connect to each
other via an uplink port (级连端口).
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LAN Technology
• What type of cable is used to connect
LAN nodes?
 Most LANs use Category 5 ("cat 5") UTP
(unshielded twisted pair) cables with
plastic RJ-45 connections at both ends.
 Category 5 cables are suitable for
networks that transmit data at speeds
up to 100 Mbps. An enhanced version—
Category 5e—is suitable for 1 Gbps
speeds.
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LAN Technology
• Can a LAN use existing phone or
electrical wiring?
 To avoid running unsightly cables
throughout a house or office building,
it is sometimes possible to use existing
telephone or electrical wiring.
 The HomePNA network uses a special NIC
and cable to connect each computer to a
standard telephone wall jack.
 The HomePLC network uses a special NIC
to connect a computer to a standard
electrical outlet.
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LAN Technology
• Can LANs use wireless technology?
 Yes. A wireless LAN is typically slower
than a wired network and signals are easily
disrupted, but it eliminate unsightly wires.
 The most popular wireless network
standard is 802.11b (also called Wi Fi for
"wireless fidelity"), which operates at 11
Mbps, spans distances of 300 feet.
 Another wireless standard is Bluetooth,
which operates at speeds up to 700 Kbps
with a maximum range of 35 feet.
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LAN Technology
LAN Technology
LAN Technology
5.4.3 LAN Standards and Protocols
• Do I have my choice of network
standards?
 Several LAN standards exist, including
Ethernet, Token Ring, ATM, FDDI, and
LocalTalk. Each standard provides unique
technology for sending data and
preventing collisions.
 Today, Ethernet is the dominant standard
for home and business networks.
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LAN Technology
• What's the most popular protocol
on small networks?
used
 Most LANs use TCP/IP because it is
supplied with most PCs, and it simplifies
the process of transferring data from a
LAN to the Internet.
 Alternative LAN protocols include
IPX/SPX, a standard developed for Novell
Networks, and NetBIOS/NetBEUI, a
network standard developed by Microsoft.
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LAN Technology
5.4.4 LAN Addresses
• Does each LAN device have a unique
address?
 When a NIC is manufactured, it is given
a unique address called a MAC address
(Media Access Control address 介质访问
控制地址), such as 00-ED-18-CD-6E-28.
 The first three bytes of MAC address
indicate the company that manufactured
the NIC. The remaining three bytes are
the NIC's serial number.
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LAN Technology
5.4.5 LAN Software
• Does a LAN require a special operating
system?
 Special network operating systems (NOS)
were quite popular at one time and are
still used on some networks.
 The need for a special NOS has declined,
however, because today's popular OSs
include the software necessary to
establish communication with the other
computers and devices on a LAN.
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LAN Technology
• Does a LAN require special application
software?
 Most application software designed for
standalone computers can be used on a
network.
 Theoretically, a network allows
workstations to access application
software that has been installed on
network drives—those on a server or on
a shared drive of a network workstation.
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LAN Technology
 In order to run Windows software
from a server, for example, you or your
network manager must install the
software on the server and complete a
workstation installation of the software.
 A workstation installation copies some,
but not all, of the program files to your
local hard disk, then updates the Windows
Registry and the Windows Start menu to
include a listing for the new program.
Slide 134 of 146
LAN Technology
• How does copyright law affect
program distribution on a LAN?
 Even though an application might run on
a LAN, it is still subject to copyright law
and the terms of the license agreement.
 Some software publishers provide special
licenses designed for network use,
including multiple-user license,
concurrent-user license, and site license.
Slide 135 of 146
LAN Technology
 A multiple-user license (多用户许可证)
allows more than one person to use a
particular software package.
 A concurrent-user license (并行用户许可
证) allows a certain number of copies of
the software to be used at the same time.
 A site license (站点许可证) generally
allows software to be used on any and all
computers at a specific location.
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LAN Technology
5.4.6 LAN Installation
• How do I set up a simple Ethernet
LAN to connect several PCs?
 The first step in assembling your own LAN
is to make sure that each PC contains an
Ethernet port. The next step is to attach
each workstation to a hub.
 With all workstations connected, you turn
them on. Windows should automatically
detect the Ethernet cards and establish
a connection to the network.
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LAN Technology
• How do I specify which resources can
be shared by other workstations?
 For each workstation on the network,
the workstation's owner can specify
whether its files will be accessible to
other workstations on the network.
 Drive mapping (驱动器映射) is Windows
terminology for assigning a drive letter
to a storage device that's located on a
different workstation.
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LAN Technology
• Can I connect my LAN to a high-speed
Internet connection?
 One of the main reasons for creating
a small LAN is to provide all the
workstations with shared access to a
single high-speed Internet connection.
 To add high-speed Internet access to
a LAN, you need a cable modem or DSL
modem, and a corresponding ISP.
 You also need a router or a hub with
router capabilities.
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LAN Technology
LAN Technology
5.4.7 LAN Security
• What parts of a LAN are vulnerable
to hackers?
 Local area networks are susceptible to
internal security breaches.
 If a LAN is equipped with an always-on
Internet connection, it also becomes
vulnerable to external attacks.
Slide 141 of 146
LAN Technology
• Should I bother with security
measures on a home network?
 Any home network that is connected to
an always-on Internet connection is
vulnerable to intrusions.
 You should make sure that password
protection is enabled on every
workstation. You may want to use firewall
software, too.
Slide 142 of 146
LAN Technology
 You can also use network address
translation (NAT 网络地址转换) as a line
of defense.
 Your ISP typically assigns an IP address
to your high-speed connection. This
address is visible to the rest of the
Internet. Within your LAN, however, the
workstations should use private Internet
addresses that can’t be routed over the
Internet. In that way, only the router's
address is publicly visible.
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LAN Technology
 When the IP addressing scheme was
devised, three ranges of addresses were
reserved for internal or "private" use:
10.0.0.0~10.255.255.255,
172.16.0.0~172.31.255.255, and
192.168.0.0~192.168.255.255.
 Your router maintains a network address
translation table that keeps track of
the private IP addresses assigned to
each workstation.
Slide 144 of 146
KEY TERMS
Chapter 5 专业词汇表
Slide 145 of 146
Homework
1.
完成教材P255~256 SUMMARY中的填空。
（Section C、D的填空可以不写）
2. 完成教材P257~258 SITUATION
QUESTIONS中的填空。
3. 将SUMMARY（ Section C、D部分可以不
翻译）和SITUATION QUESTIONS的内
容翻译成中文，并写在作业本上。
4. 在作业本上写出Chap5专业词汇表中的KEY
TERMS及相应的中文含义。
Slide 146 of 146