Download Computer Networks

Computer Networks
• A computer network is two or more computers
connected to one another to share resources.
• Networks allow users to access files and programs.
• They also let users share printers and other equipment.
• Various ways of connecting computers can be
accomplished through physical media.
• The medium can be any type of telecommunications
connector: twisted pair telephone lines, coaxial cable,
fiber-optic cable, or a microwave, radios, or infrared
system.
• Working together, the network media and the computers
determine how much data can be sent through the
connector.
Building Networks
• To create a computer network, you need a
network interface card, or NIC, in each
workstation.
• The NIC handles the flow of data to and from the
computer in both wired and wireless networks.
• The NICs connect to a network cable that, in
many systems, connects to a central hub.
• Each workstation must be able to work with the
network.
Building Networks cont.
• A network may be small, with just a few
computers, or it may be large, with thousands of
workstations.
• The network functions the same way, no matter
how many workstations are connected.
• If the network is put together by actual cables,
those cables connect to the NIC.
• NICs often have a light that blinks green and
amber to alert you to activity.
Building Networks cont.
• Everyone on a network can work together.
• Syneregy is the affect a group effort can create.
• People working together on a network can
accomplish more than people working alone on
unconnected computers.
• As you use a network, you may open windows
and click icons to find files or other resources.
• Behind the scenes, the network’s hardware and
software are performing many complex tasks.
Organizing Users
• All of the pieces of equipment connected together must
be able to understand one another.
• A network is organized into workstations, each with its
own name and address.
• A workstation is a computer connected to a computer
network.
• It is often set up with the same operating system,
applications, and access to resources as the other
computers in the network.
• Sometimes network users work at a terminal, which
usually includes a keyboard, a monitor, and a mouse.
Organizing Users cont.
• A terminal can feel as if the computer is local, but it’s not.
• Users are actually sharing time on a central computer,
with their own work displayed on their terminal’s monitor.
• This kind of network is sometimes called a timesharing
system.
• Terminals can save on the cost of purchasing
workstations.
• They are also useful in situations with limited need for a
workstation, such as a public computer in a library.
Components of a Network
• In a large network, a workstation is also called a node by
the people who take care of the network.
• A node is anything connected to the network—a
workstation, a printer, a fax, or any other piece of
equipment.
• Each node is connected to one or more file servers—
high speed computers with large hard drives that are
used to store large amounts of data.
• Each node on the network that has been granted
appropriate rights by the network administrator can
access the data on the file servers.
Components cont.
• Collecting and centralizing data on a
network file server provides two important
benefits:
– It gives users access to the most up-to-date
data
– It eliminates the problem of the same data
being duplicated on each user’s computer,
which not only wastes storage space, but also
increases the likelihood of people using
outdated data.
Components cont.
• All of the nodes and files servers on a
network are connected by physical
media—the cables or wireless
connections that transmit the data on a
network.
• Many different types of physical media can
be used in computer networks.
Types of Physical Media
Type
Principal Uses
Maximum
Operating
Distance
Cost
Twisted Pair
Small LANS
300 feet
Low
Coaxial Cable
Large LANS
600-2,500 feet
Medium
Fiber Optic
Network
backbones;
WANS
1-25 miles
High
Wireless/infrared
LANS
3-1,000 feet
Medium
Wireless/radio
Connecting
things that
move
Varies
considerably
High
Components cont.
• You connect a computer to a network
cable through a network interface card
(NIC).
• Most NICs are designed to connect to a
particular type of cable, although some
work with more than one type.
• The NIC is responsible for controlling the
flow of data between the computer’s RAM
and the network cable.
Network Topology
• Network topology is the physical
arrangement of computers on a network.
• The topology of a network determines how
the flow of data is handled when two
computers try to transmit data across the
network at the same time.
• The three most common LAN topologies
are bus topology, start topology, and ring
topology.
Network Topology cont.
• The simplest topology is a bus topology, in
which a single cable runs the length of the
network.
• Each node (computer or peripheral device)
connects to this one cable.
• This is an inexpensive network arrangement, but
a break in the cable may cause the entire
network to “crash”.
• This type of network employs special data
management techniques to handle the flow of
data when two computers transmit data
simultaneously.
Network Topology cont.
• With a star topology, each node connects to a
hub through which data travels to the file server
and/or other computers.
• Star topologies use more cable than a bus
topology, but a single broken cable will not
“crash” the network.
• A star topology also employs special data
management techniques to handle the flow of
data when two computers transmit data
simultaneously.
Network Topology cont.
• In a ring topology, each node connects to the
next in a circular arrangement.
• A special set of data, called a token, travels from
one computer to the next around the circle.
• If a computer needs to send or receive data, it
must wait until the token gets to it.
• This configuration eliminates the problem of two
computers transmitting data at the same time.
• However, a single break in the ring can ruin the
network.
Network Protocols
• Once a network is created, the computers and
other connected equipment can communicate
with one another.
• However, there is more involved than just
hooking up the network’s physical components.
• The communication on a network is called
network traffic.
• Network traffic is the electronic pulses of
information sent by the network cards to carry
data through the network wires to its destination.
Network Protocols cont.
• Computers communicate with languages called
protocols.
• A protocol sets the rules for how data is formatted and
transmitted.
• It is the language computers use to “speak” to one
another on a network.
• There are many different protocols available to use on
networks.
• For computers to speak with one another, they must use
the same protocol.
• A network’s protocol is determined by the network’s
purpose.
Kinds of Protocols
• There are two protocol categories:
– open
– proprietary
• An open protocol is available for anyone
to use.
• The most common open protocol is the
Transmission Control Protocol/Internet
Protocol. (TCP/IP)
Kinds of Protocols cont.
• A proprietary protocol, is not open to
everyone.
• Only people who buy certain equipment,
services, or computers can use it.
• Some personal digital assistants, digital
cameras, and even dial-up Internet
services use proprietary protocols.
• Open protocols are more common.
Kinds of Protocols cont.
• The protocols networks use to communicate are
often called a protocol suite.
• A protocol suite is the stack, or collection, of
individual protocols that determines how the
network operates.
• For example, TCP/IP is not just one network
language, but many smaller ones.
• Each small protocol in this suite has a specific
job to do in a specific order.
Kinds of Protocols cont.
• The data is transmitted through layers from a
sending computer before it actually reaches the
physical media.
• On the other end it travels through layers from
the physical media before it can reach the
receiving computer.
• Each layer performs a specific function and has
its own set of protocols that define exactly how
data must pass from one layer to the next.
User
User
Protocol
R
e
c
e
I
v
I
n
g
S
e
n
d
I
n
g
Layer
Physical Media
Physical Media
Benefits of Networks
• Computer networks provide hardware benefits
because they let people share hardware.
• Networks also provide software benefits
because they let people share programs and
data.
• Physical media connect the network and
protocols make sure that the links among
hardware, software, and people actually work.
Local Area Networks
• A LAN is a network in which all the workstations and
other equipment are in the same geographical area.
• LANs connect computers within a home, an office, or a
building.
• They can also connect computers in separate buildings
that are physically close together.
• A LAN can have just a few or several hundred users.
• There are three ways to share information: sharing files,
using collaborative software, and sharing peripherals.
LANs cont.
• Sharing Files—through a computer’s operating system
people connected to a LAN can participate in file
sharing.
• File sharing is making files available to more than one
user on the network.
• Using Collaborative Software—collaborative software
enables the network to help people work together more
closely.
• With collaborative software, users can share calendars,
work on a document together, or even hold meetings
through the network.
• Collaborative software is also called groupware.
LANS cont.
• Sharing Peripherals—In addition to sharing files
and software, a LAN allows users to share
peripherals, such as printers, fax machines, or
any other equipment.
• LANs also offer a number of benefits.
–
–
–
–
lowering costs
sharing applications
building information resources
connecting people
LANs cont.
• Lowering Costs—If connected to a network, many
computers can share expensive equipment, such as
printers, scanners, fax machines, etc.
• Sharing Applications—Networks can make use of
network versions of software applications. Workers at
several workstations can use the applications at the
same time.
• Building Information Resources—Information can be
added to files from any workstation.
• Connecting People—People can work together closely.
Even if their offices are on different floors, they can
easily consult with one another and work on the same
files.
Organizing LANs
• LANs can be set up in one of two ways.
– Peer-to-Peer Network
– Client/Server Network
• In a peer-to-peer network (P2PN), all the
workstations are equal and connected to each
other through a network cable or a wireless
signal.
• This network is usually made up of two to ten
computers.
• The network cable can be connected to a central
device called a hub.
Organizing LANs cont.
• The hub handles the flow of traffic from
computer to computer.
• A P2PN allows workstations to share files and to
access files on other workstations.
• Users are responsible for securing their own
resources and granting access to other users.
• A P2PN is an easy network to create, since all of
the workstations are equals.
• The operating system of each computer typically
has built-in file-sharing abilities.
Organizing LANs cont.
• A peer-to-peer network is ideal for small offices and
homes.
• In a large business peer-to-peer networking has some
drawbacks:
–
–
–
–
–
Security problems can arise.
Data can be hard to back up.
With many users, file sharing can become difficult.
Finding shared files can be difficult.
Managing resources can be complicated.
• These problems arise because resources are scattered
across many computers.
• If one computer fails or is turned off, its resources are no
longer available to the network.
Organizing LANs cont.
• Large businesses usually use a client/server network.
• With this system, one powerful computer provides
information and management services to the workstation
computers, the clients.
• In a client/server network, each workstation is a client
connected to a central computer called a file server or
just server.
• A client/server network centralizes all network resources
and controls what users can do with files.
• Accessing and backing up files is faster and easier than
using a peer-to-peer network or no network at all.
Organizing LANs cont.
• A client/server network is managed and secured
by a network operating system, or NOS.
• It manages and secures the entire network.
• It controls access, permissions, and all aspects
of network use.
• Only those who provide a username and a
password can use the network.
• It is far more secure than a P2PN.
Organizing LANs cont.
• For a large office, file servers are better
than peer-to-peer networks, for several
reasons:
– They offer a central location for files.
– Data is easy to back up and easy to recover.
– Servers are faster than workstations.
– Servers usually are powered on.
– Security is easier to maintain.
LAN Standards
• All of the various network elements; network interface
cards, physical media, protocols and topologies are
usually viewed together as a set of LAN standards.
• One of the most widely implemented LAN standards is
Ethernet.
• Ethernet is used with bus and start topologies, twistedpair or fiber optic cables, and an Ethernet-compatible
network interface card (NIC).
• Ethernet can transmit data at 10 Mbps (10 million bits
per second).
• A newer standard called Fast Ethernet can support data
transfer rates of up to 100 Mbps (1oo million bits per
second).
LAN Standards cont.
• The second most widely used LAN standard is the
Token Ring.
• This standard uses a ring topology and transfers data at
either 4 or 16 Mbps.
• The simplest LAN standard is LocalTalk, the system
that Macintosh uses.
• Ordinary phone cables are arranged in a bus topology.
• However, the data transfer rate on a LocalTalk network
is only 230 Kbps, or 230 thousand bits per second—
which is much slower than the transfer rate on an
Ethernet or Token Ring network.
Working with Intranets
• A growing number of organizations are setting
up their LANs—and sometimes their wider
networks—as intranets.
• An intranet is a private network that uses the
same TCP/IP protocol as the Internet.
• Firewall software prevents outsiders from
accessing the intranet.
• Intranets offer the following benefits:
– ease and range of uses
– cost and time savings
Intranets cont.
• An intranet looks and functions like the Internet.
• Most people have no trouble learning to use it.
• Companies use an intranet to distribute all sorts of internal
documents:
– employee lists
– training materials
– standard forms
• Other intranets allow employees to connect to company databases
or hold electronic conferences.
• Some companies use their intranet as an employee gateway to the
Internet.
• Some allow customers or other outsiders to access part of their
intranet through the Web.
• Intranets generally cost less to set up than other systems.
• They also save paper and printing costs.
Wide Area Networks
• A wide area network connects computers and
other resources that are miles or even
continents apart.
• A business with offices in many places can use
a WAN to link its LANs in different cities.
• Users from any of the locations can, with the
proper permissions, access the network.
• Each user can access files, printers, and other
resources as if they were local.
• Once a WAN is created, users may not even
realize the files they are sharing are remote.
WANs cont.
• How is a WAN Controlled? Like a
client/server LAN, a WAN is controlled by
a network operating system.
• A NOS is helpful on a WAN because there
are so many users and resources to
manage.
• The NOS also helps network
administrators secure the resources
throughout the network.
WANs cont.
• Most networks use packet-switching technology.
• The sending computer divides information into tiny
segments called packets. Each packet is marked with a
delivery address, so packet transfers are quick and
accurate.
• When you transfer a file, send an e-mail, or even browse
a Web site, you’re sending and receiving packets.
• Some WANs use circuit-switching technology to transmit
messages.
• Circuit-switching happens on a real, end-to-end
connection between the sending computer and the
receiving computer, which make up the circuit.
WANs cont.
• There’s no delay on circuit-switching networks,
so they are ideal for sending voice messages
and for teleconferencing.
• A telephone network uses circuit-switching.
• Routers are network devices or programs that
choose the best pathway for each packet.
• If there is congestion, or too much traffic, on the
network, the router can delay some of the
packets.
• The receiving computer puts the packets back
together in the right order.
WANs cont.
• A WAN needs to connect the resources together physically, and it
needs to provide a way to access the connection.
• Backbones are the high-speed lines, wired or wireless, that carry
data through a network.
• Long-distance telecommunications companies carry some of the
data traffic.
• Government and privately owned lines connect other WANs.
• Throughout the world, WAN providers offer local connections for
users.
• This connection is called a point of presence (POP).
• The most common example of a POP is an Internet connection.
WANs cont.
• How do organizations get to the POP?
–
–
–
–
ISDN and DSL
leased lines
T1 lines
permanent virtual circuits
• ISDN and DSL—these technologies use ordinary
telephone lines to attach to the backbone.
• Special adapters or modems provide ways to deal with
digital data.
• Leased lines—some companies rent a private end-toend connection, called a leased line, from a
telecommunications company.
• Leased lines allow data to be sent at 56,000 bps.
WANs cont.
• T1 lines—larger companies and many school districts
lease T1 lines.
• T1 lines can be either copper or fiber optic, and they
allow data to be sent at more than 1.5 million bps.
• Permanent Virtual Circuits—a permanent virtual circuit
(PVC) allows multiple users’ data to travel the line at
once.
• They are cheaper than private lines.
• Most PVCs use a technology called frame relay.
• Frame relay allows voice, data, and video to travel on
the same line and at the same time.
Types of WANs
• A public data network (PDN) allows many different
companies to set up their own networks.
• Telecommunications companies own the PDNs and
charge fees for the use of the network.
• Private Data Network—some companies set up a
private data network that cannot be accessed by
outsiders.
• Having a private data network costs more than using a
public data network.
• A virtual private network (VPN) is a private network set
up through a public network.
• VPN users connect to an Internet service provider (ISP)
to access the network.
Comparing WANs and the Internet
• A WAN and the Internet are very similar.
• The Internet is, in fact, a WAN, though it is
not usually called that.
• There are three key differences between
the Internet and other WANs:
– kind of access
– degree of security
– types of information
WANS and the Internet cont.
• The Internet is public, while WANs are
typically private.
• When you connect to the Internet, you are
invited as a member of the public to view
published information.
• Information on a WAN is set up for the
internal use of the organization that owns
it.
WANs and the Internet cont.
• While the Internet is becoming more secure, it is
still not as secure as a private WAN connection.
• As data travels through the Internet, snoops and
eavesdroppers on the public networks through
which the data moves sometimes try to access
it.
• A private WAN is more secure because it is
more likely that only the organization that owns it
has access to it.
WANs and the Internet cont.
• On the Internet, information is transmitted
in the form of Web pages and other types
of files.
• A WAN is used for more than just
browsing Web pages.
• It provides access to network resources,
such as printers, files servers, and
databases.