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CH/S6CA/Apr. 2009
SOHO Networking

A network
devices.

Networking is the process of connecting computers that permits the transfer of data and programs between
them.

Today networking is the fastest growing area in information technology.
is a communication system that connects two or more computers and their peripheral
Need for networking

There are several reasons why a company or an organisation would choose to implement a network. The four
most important ones are:
i.
Fast response: a network system allows users at various locations to get immediate responses to inquiries.
ii.
Up-to-the-minute database: a major benefit of a network system is that a user can maintain an
up-to-the-minute database.
iii.
Hardware resource sharing: Every computer on the network is allowed to use certain expensive devices.
iv.
Information resource sharing: It allows using a computer on the network to access data and software
stored on any other computer or the host computer in the network.
Distributed processing: Rather than using a large mainframe, several smaller computers can be used around the
organisation and in each branch office, by connecting together in networks. A distributed system is a system
in which a job is performed by a number of computers that are linked through a communication network..
Types of Networks - a geographical distribution (p. 68 – p. 69)

Local Area Network (LAN).
i.
Local area networks are networks in which all the computers and terminals are located in the same offices
or buildings.
ii.
They enable all the computers on the site to communicate with each other, and share peripherals such as
hard disk, printers and external communications links.
iii.
LANs represent a low-cost solution for low-volume data transfer over relatively short distances.
iv.
Network models of a LAN
a.
b.
Client / Server model

It uses a central computer, known as the server , to maintain all
the files used by the users. Each individual computer on the
network can have access to the files on the server.

Advantage: centralized management, higher security, handling
large networks more efficiently.
Peer-to-peer model

It has no server. It allows any computer to share the hardware,
software, or information located on any other computer in the
network. It is commonly found in small organisations.

Advantage: no single point of failure affecting the whole network,
faster in distribution, low cost and easy to set up.
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Bit Torrent (BT) is a Peer-to-peer file sharing protocol used to distribute large amounts of data. The initial
distributor of the complete file or collection acts as the first seed. Each peer who downloads the data also
uploads them to other peers.

Wide Area Network (WAN).
i.
Wide area networks (WANs) use telephone lines, microwave relays and satellites to reach data over long
distances in the same country or outside country on different continents between various centres.
ii.
Under a WAN, each location needs to access resources locally but also needs to share data with the other
locations.
** This is another type of network namely the Metropolitan Area Network (MAN) with its scale larger than LAN
and smaller than WAN.
Packet

Most computer networks do not transfer data as an arbitrary string of continuous bits. Instead the network
system divides data into small blocks called
packet
, which it sends individually.

Computers are often called packet networks or packet switching network because they use packet
technology.

Reasons of using packets instead of sending data together.
i.
To ensure that all computers receive
fair, prompt access to a shared
communication facility, a network
system cannot allow one computer to
deny access to others. Using small
packets helps ensure fairness.
ii.
Dividing the data into small blocks
helps a sender and receiver determine
which blocks arrive intact and which
do not.

There is no universal agreement on the exact format of a packet. The specific definition of packets used with
a specific type of network is called a frame

The sending and receiving computers must agree on how they will specify the beginning and end of each
frame as follows. (soh : start of header, eot : end of transmission)

In a data packet sent via the Internet, the data are preceded by header information such as the sender's and
the recipient's IP addresses, the protocol governing the format of the data and several other formats. The
header's format is specified in the Internet Protocol.

In information technology, trailer refers to supplemental data placed at the end of a block of data being
stored or transmitted, which may contain information for the handling of the data block, or just mark its
end.
header

data
trailer
The IPv4 packet header consists of (reference only):
i.
4 bits that contain the version, that specifies if it's an IPv4 or IPv6 packet,
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ii.
4 bits that contain the Internet Header Length which is the length of the header in multiples of 4 bytes.
Ex. 5 is equal to 20 bytes.
iii.
8 bits that contain the Type of Service, also referred to as Quality of Service (QoS), which describes
what priority the packet should have,
iv.
16 bits that contain the length of the packet in bytes,
v.
16 bits that contain an identification tag to help reconstruct the packet from several fragments,
vi.
3 bits that contain a zero, a flag that says whether the packet is allowed to be fragmented or not (DF:
Don't fragment), and a flag to state whether more fragments of a packet follow (MF: More Fragments)
vii.
13 bits that contain the fragment offset, a field to identify which fragment this packet is attached to,
viii. 8
bits that contain the Time to live (TTL) which is the number of hops (router, computer or device
along a network) the packet is allowed to pass before it dies (for example, a packet with a TTL of 16
will be allowed to go across 16 routers to get to its destination before it is discarded),
ix.
8 bits that contain the protocol (TCP, UDP, ICMP, etc...)
x.
16 bits that contain the Header Checksum, a number used in error detection,
xi.
32 bits that contain the source IP address,
xii.
32 bits that contain the destination address.
TCP/IP

A

TCP/IP includes a common set of communication languages or standards allows different types of
computers and communication devices to talk to each other.

TCP is responsible for the provision of reliable end-to-end data transmission.

IP specifies how packets are routed from computer to computer.

IP address. (Act. 4: Pls. refer to http://computer.howstuffworks.com/internet-infrastructure5.htm )
protocol
is a set of rules for the exchange of data between communication devices.
i.
32 bits (divided into four sets of 8 bits (ranging from 0 to
ii.
e.g. 202.128.233.88 (For a Class C network: network ID –
255
)
202.128.233
, host ID:
88 )
Classwork:
Try type the following command in DOS prompt and drop down the result
1. ping www.cuhk.edu.hk
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2.
ipconfig
More on IP address


The IP Address Hierarchy.
i.
An IP address is a unique 32-bit binary number assigned to a host and used for all communication with
the host.
ii.
Each 32-bit IP address is divided into two parts: a
level hierarchy is designed to make routing
iii.
The address prefix identifies the physical network to which the computer is attached, while the suffix
identifies an individual computer on that network.
iv.
The IP address hierarchy guarantees two important properties:
prefix
efficiently.
and
suffix
; the two-
a.
Each computer is assigned a unique address.
b.
Although network number assignments must be coordinated globally, suffixes can be assigned
locally without global coordination.
Original Classes of IP Addresses
i.
The suffix need sufficient bits to permit each computer attached to a network to be assigned a unique
suffix. Thus if the size of the network is larger, there should be more bits in the suffix. (8 bits for 256
computers, 9 bits for 512 computers, so on)
ii.
A single internet can contain a mixture of large and small networks. Thus the IP address designers
chose a compromise addressing scheme that can accommodate a combination of large and small
networks.
iii.
The original IP addressing scheme divides the IP address into three primary classes, where each class
has a different size prefix and suffix.
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iv.
Class D is used for multicasting, which allows delivery to a set of computers. To use IP multicasting, a
set of hosts must agree to share a multicast address. Once a multicast group has been established, a
copy of any packet sent to the multicast address will be delivered to each host in the set.
v.
Dotted decimal notation. Although IP addresses are 32-bit numbers, the software often uses a notation
that is more convenient for humans to understand, i.e. the form expresses each 8-bit section of a 32-bit
number as a decimal value and uses periods to separate the sections. It is called dotted decimal
notation. E.g. 10.127.48.152.
vi.
More examples:
vii.
The class of the address can be found from the 1st 4 bits of the address.
viii. Class
work find the Class, prefix and suffix from the following IP addresses
:IP address
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Class
Prefix
Suffix
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48.45.49.43 00110000
A
48
45.49.43
134.1.168.222
B
134.1
168.222
200.200.200.200
C
200.200.200
200
193.12.166.253
C
193.12.166
253
220.166.222.222
C
220.166.222
222
223.255.255.255
C
223.255.255
255
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


Division of the address space
i.
The IP class scheme does not divide the 32-bit address space into equal size classes.
ii.
The classes do not contain the same number of networks.
iii.
The number of networks and hosts per network in each of the three primary IP address classes are as
follows:
Authority for addresses
i.
For networks connected to the global Internet, an organization obtains network numbers from its
Internet Service Provider (ISP).
ii.
ISPs coordinate with a central organization, the Internet Assigned Number Authority, to ensure that
each network prefix is unique throughout the entire Internet.
A classful addressing example
i.
Consider an organization that chooses to form a private TCP/IP internet which consists of four physical
networks.
ii.
The network administrator estimates the ultimate size of each physical network, and uses that size to
choose a prefix.
iii.
If the organization expects one small network, two medium networks, and one extremely large network,
the administrator might choose to assign a class C prefix(e.g. 192.5.48), two class B prefixes (e.g.,
128.10 and 128.211), and a class A prefix (e.g., 10) as shown in following figure. Note that the size of
the cloud used to denote a physical corresponds to the number of hosts expected on the network.
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
Subnet
i.
The original classful addressing scheme has a limitation as all networks had to choose one fo three
possible sizes.
ii.
Subnet addressing. Also known as classless addressing. Instead of having three distinct address
classes, the division between prefix and suffix to occur on an arbitrary bit boundary.
iii.
Under subnet addressing, a subnet mask is used. 1 bits mark the network prefix and zero bits mark the
host portion.
iv.
When a router is given a destination address, D, and a pair (A, M) that represent a 32-bit IP address
and a 32-bit address mask. To make a comparison, the router tests the condition:
A == (D &M)
v.

The size of a subnet mask is represented in the IP address after ‘/’, e.g. 128.211.0.0/16.
Special IP addresses
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





Network Address – suffix all-0s (e.g. 128.211.0.0/16).
i.
It denotes a network.
ii.
It does not refer to any host computer attached to that network.
Directed Broadcast Address – suffix all-1s (e.g. 128.211.255.255/16).
i.
When a packet is sent to a network’s directed broadcast address, a single copy of the packet travels
across the internet until it reaches the specified network.
ii.
The packet is then delivered to all hosts on the network.
Limited Broadcast Address – prefix all-1s, suffix all-1s (i.e. 255.255.255.255).
i.
IP will broadcast any packet sent to all hosts on the local network.
ii.
It is used during system startup by a computer that does not yet know the network number.
This Computer Address – prefix all-0s, suffix all-0s (i.e. 0.0.0.0).
i.
The TCP/IP protocol suit contains protocols a computer can use to obtain its IP address automatically
when the computer boots.
ii.
When using such startup protocols, a computer cannot supply a correct IP source address, This
Computer Address is used.
Loopback Address – prefix 127 (e.g. 127.0.0.1/8).
i.
It is used to test network applications.
ii.
To perform a loop back test, a programmer must have two application programs that are intended to
communicate across a network.
iii.
Instead of executing program on a separate computer, the programmer runs both programs on a single
computer and instructs them to use a loopback IP address when communicating.
Routers and IP addresses
i.
A router has connections to multiple physical networks.
ii.
Each router is assigned two or more IP addresses each containing a prefix that specifies a physical
network connected to it.
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Common protocols

In order to ensure that different manufacturers’ products can be connected properly in one network, some
international organisations set up a strict set of rules called
for communication.
It
describes the following things:
i.
the language to be used.
ii.
the speed at which the data is sent.
iii.
odd or even parity check being used.
iv.
the types of codes that identify stations and data storage devices on the network.
v.
the type of line control in use i.e. how an individual station gains control of the communication line so
that it can talk without interrupting others or being interrupted.

There are many common protocols used in Networking, especially in the Internet. E.g. TCP, IP, HTTP,
HTTPS, UDP, DNS, POP3, SMTP, IMAP, FTP, SNMP, DHCP, etc.

Hypertext Transfer Protocol (HTTP)
i.
It is a communication protocol for the transfer of information on the Internet. Its use for retrieving interlinked text documents (hyperlink) led to the establishment of the World Wide Web.
ii.
HTTP is a request/response standard between a client and a server. A client is the end-user, the server
is the web site..
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iii.

Hypertext Transfer Protocol over Secure Socket Layer (HTTPS). The HTTPS protocol provides for
secure HTTP transfer. It uses encryption to guarantee confidentiality of the data being transferred in
an HTTP session.
The protocols for e-mail (SMTP, POP3, IMAP)
i.
Usually e-mail service from ISP requires you to maintain your e-mails using an e-mail client, e.g.
Outlook, a dedicated e-mail software. In this case, the e-mail system consists of two different
servers: (Act. 6 : Pls. refer to http://communication.howstuffworks.com/email.htm )
ii.
SMTP server
iii.
a.
SMTP stands for
Simple Mail Transfer Protocol
.
b.
It handles outgoing / incoming mail.
c.
Whenever you send a piece of e-mail, your e-mail client interacts with the SMTP server to
handle the sending. The SMTP server on your host may have conversations with other SMTP
servers to deliver the e-mail.
POP3 server or IMAP server:
a.
POP stands for
Post Office Protocol
.
IMAP stands for
Internet Mail Access Protocol
.
b.
They handle outgoing / incoming mail.
c.
Using a POP3 server, your e-mail client connects to the POP3 server and brings copies of your email messages to your local machine. Generally, it will then delete the messages from the server
(unless you've told the e-mail client not to).
d.
Many users want their e-mail to remain on the server so as to allow users to connect from a
variety of machines.
e.
With IMAP, your mail stays on the e-mail server regardless of which machine you use, you have
access to all of your mail in all of your folders. Most email clients allow users to keep copies of
email on their machine.
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iv.

Your e-mail client allows you to add attachments of various file-types to e-mail messages you send,
and also lets you save attachments from messages that you receive.
Dynamic Host Configuration Protocol (DHCP)
i.
Protocol Configuration. Protocol software must be informed about the local computer and the local
network environment. E.g. the network hardware type (e.g. Ethernet), the protocols to be used (e.g.
TCP/IP), and the location of services (e.g. DNS).
ii.
The exact details of configuration information depend on the protocol stack. E.g. TCP/IP
configuration details include IP address, default IP router address (default gateway), address mask,
DNS server address and printer server address.
iii.
It is not possible for a network administrator enter the configuration information (e.g. host address)
for each computer in a network if the set of computers remains rapidly. For example consider a WiFI access point in a restaurant that provides access to arbitrary customers.
iv.
DHCP provides a mechanism that allows a computer join a new network and obtain configuration
information (including an IP address) automatically.
v.
It uses a client-server approach.
vi.
An administrator can configure a DHCP server to have two types of addresses: permanent addresses
that are assigned to server computers and a pool of addresses to be allocated on demand.
vii.
The DHCP format:
viii.
When a computer boots,
a.
the computer broadcasts a DHCP request to a DHCP server.
b.
the DHCP server consults its database to find configuration information.
If the database contains a specific entry for the computer, the DHCP server returns the
information from the entry.
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If no entry exists for the computer in the database, the DHCP server chooses the next IP
address from the pool. .
ix.
Addresses assigned on demand are not permanent. DHCP issues a lease on the address for a finite
period of time. When the lease expires, the server returns the address to the pool of available
addresses, which allows the address to be assigned to another computer.
Different Types of Internet Connection
Dialup Connection. One form for internet connection is a

dialup connection
. It is a
temporary connection established using a modem to dialup the number for a linked to a remote computer.
i.
ii.
To use this kind of connection, you must have
a.
a computer with a modem,
b.
a telephone line,
c.
software(s) for web browsing and E-mail management, and
d.
an account with an ISP.
A
modem
is a device used to change the data from the digital form stored in the
computer to the (analog form) wave form to be transmitted through telephone line.
Permanent Leased Line Connection.

i.
Many organizations lease permanent, private and
directly from ISPs.
always-on
connections
ii.
The connection speed mainly depends on the type of network connection. (T1 – 1.5Mbps, T3 –
44.7Mbps)
Broadband Connection. Two types: digital subscriber line (DSL) and Cable TV connection.

i.
ii.
DSL
a.
Asymmetric digital subscriber line
used.
(ADSL) together with and ADSL modem is
b.
It is asymmetric because the downstream rate (the receiving speed) to users is higher than the
upstream rate (the upstream speed).
Cable TV connection: Through the cable TV network using a cable modem.
Hardware for Networking

Telephone line. The telephone system, originally set up for voice communication, is used to handle data
communications.


Modem.
i.
Computers deal with data in digital form but most telephone lines are designed to carry analog signals.
ii.
An interface is needed to change the data from digital to analogue form and vice versa, i.e. a modem (for
both modulator and demodulator).
Coaxial cable. Coaxial cable contains electrical wire and is
constructed to permit high-speed data transmission over this wire with
a minimum of signal distortion.
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


Unshielded Twisted Pair (UTP) Cable.
i.
Twisted pair cables were first used in telephone systems.
ii.
It is also the most common cable used in computer networking.
Fibre optics cable.
i.
Eventually replacing the copper wire traditionally used in the
telephone system.
ii.
A fibre optics cable consists of a number of thin tubes of glass which carry data as pulses of light. The
pulses are very rapid, large in data capacity, more reliable and more secure at transmitting data.
Microwave radio. Microwave transmission consists of high-frequency waves that travel in straight lines
through the air rather than through wires.

Communication satellite. In satellite communications, data are first transmitted to an Earth station,
where giant antennas route signals to another Earth station via a communications satellite.

Network Interface Card (NIC).
i.
An interface between a computer and the network.
ii.
It is an expansion card installed on the motherboard.
iii.
A network cable is plugged into the NIC from the wall socket.
Figure: A sample network diagram with a router, a hub and a broadband connection.

Connecting Devices.
i.
ii.
iii.
Hub
. It connects all network cables together and broadcasts every packet of data to every port.
Switch
. It also connects all network cables together but it forwards each packet of data to the
destined port only.
Bridge
SOHO NETWORKING
. It connects different networks together.
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iv.
Router
. It also connects different networks. Network data is routed to different networks
based on the program in it.
Broadband Router
v.
. It connects small network at home to a broadband service.
Network Topology

Network topology is used to describe the physical organisation of the network. The basic patterns are usually
classified as:
i.
Star topology.
a.
The central control unit handles the communication tasks of receiving and routing messages to the
various workstations.
b.
Advantage:

Easy to install

No disruption to the network when connecting or removing devices

Easy to detect faults by the central control unit.
main resource
Fig. (a) Star-network topology gives the
Highest speed and security.
ii.
Ring topology.
a.
It involves computers that are approximately the same size, has all devices connected by a single
communication cable that forms a closed circle. Ring topology is used for more local communication.
b.
There are usually two rings running in parallel.
c.
Advantage:

East to connect computer or peripheral to a linear bus

Requires less cable
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main resource
Fig. (b) Ring-network based on a ‘continuous’ loop
iii.
Bus topology.
a.
All devices in the network are connected to and share a single cable. An advantage of the bus
topology is that if one computer on the network fails, this does not affect the rest of the network.
b.
Advantage:
Does not rely on a single hosting computer and can function even though one of the nodes is broken.
main resource
Fig. 9(c) Bus-network is versatile and cheap – but less secure.
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Ethernet

Ethernet is a well-known and widely used network technology that employs bus.

The Ethernet hardware operates at a rate ranging from 10 Mbps to 1 Gbps.

Ethernet requires multiple computers to share access to a single medium. A sender transmits a signal, which
propagates from the sender toward both ends of the cable as shown below:

Thus while one computer transmits a frame to another all other computers must wait.
More on Switches


The difference between a hub and a switch arises from the devices operate:
i.
a hub simulates a single shared medium, while
ii.
a switch simulates a bridged LAN with one computer per segment as follows:
Thus in a hub system, at most two computers can communicate through a hub at a given time. But in a
switched LAN, each computer has a simulated LAN segment to itself – the segment is busy only when a frame
is being transferred to or from the computer.
More on Bridges

A bridge is an electronic device that connects two LAN segments.

A bridge is a digital device that uses the same network interface as a conventional computer and handles
frames.
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
When it receives a frame from one segment, the bridge verifies that the frame arrived intact and then forwards
a copy of the frame to the other segment if necessary. Thus two LAN segments connect by a bridge behave
like a single LAN.

Frame filtering. A bridge does not forward a frame unless necessary. In particular, if a computer attached to
one segment sends a frame to a computer on the same segment, the bridge does not need to forward a copy of
the frame to the other segment.

Most bridges are called adaptive or learning bridges because they learn the locations of computers
automatically. When a frame arrives, the bridge extracts the physical source address information in the frames
that arrive.
Routers and Internet

A router is a special purpose system dedicated to the task of interconnecting networks.

A router can interconnect networks that use different technologies, including different media physical
addressing schemes, or frame formats.

An internet consists of a set of networks interconnected by routers.

The internet scheme allows each organization to choose the number and type of networks, the number of
routers to use to interconnect them, and the exact interconnection topology.
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
The most widely used protocol suite in the Internet is TCP/IP.

TCP/IP defines term host computer to refer to any computer system that connects to an internet and runs
applications. A host can be as small as a personal computer or as large as a mainframe.
Wireless LAN

Using a wireless network, data are transmitted by radio wave.

Wireless network connections perform the same functions as wired connection.

Each computer that is to be part of the wireless network must have a wireless adapter / wireless NIC (or a
wireless PC card for a notebook computer).

A wireless access point

Like, other LAN technologies, the wireless LANs use sharing, i.e. all the computers participating in a
given wireless LAN are configurated to the same radio frequency.
SOHO NETWORKING
allows computer to access a wired network.
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
A number of product of wireless LAN technologies, which are manufactured by several companies , are
available under a variety of trade name:
i.
WiFi: an IEEE standard defines wireless LANs that operate at 11 Mbps using frequency in the 2.4
GHz range.
ii.
Bluetooth: a wireless technology designed for short distance.
Software for Networking

The network operating system (NOS) manages the resources on the network. It monitors the data exchange,
ensure data security by incorporating users’ account and password system.

Common samples of network OS: Windows NT, Windows 2000, Linux.

A network operating system must have the ability to communicate with each network using common
protocols.
Common Network Service



Simultaneous Data Access
i.
When separate copies of certain data are stored in different computer, it is difficult to determine when
copy of the data is the most up-to-date. (data consistency)
ii.
Using a client/server network,
a single copy of data
stored in a server, which can be accessed through different workstations at the same time.
Device Sharing
i.
When several people can share a device, e.g. printer, in a network, the use of the device becomes less
expensive and easier to manage.
ii.
Spooling.
a.
Users send documents to a networked printer.
b.
Each document is temporarily stored in the printer server.
c.
The printer server stores the print job in a
d.
When the printer finishes the current job, its printer server sends the next print job of the queue to
the printer.
queue
.
Internet Access
i.

is
Refer to Notes on Internet.
Web Server
i.
A computer is responsible for accepting HTTP requests from web clients, which are known as web
browsers, and serving them HTTP responses along with optional data contents, which usually are web
pages such as HTML documents and linked objects (images, etc.).
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Web Hosting

Web hosting refers to the provision of storage space and file maintenance service for web sites.

Web pages of a web site are stored and maintained in a web server.

Virtual Hosting




i.
Hosting a number of web sites in one web server.
ii.
Each web site has its own domain name.
iii.
The server has only one IP address.
Non-virtual Hosting
i.
Hosting a number of web sites in one web server.
ii.
Each web site has a file directory name under the service provider’s host domain name, e.g.
“www.hkedcity.net/ihouse/ktg-040234”.
iii.
The server has only one IP address.
Dedicated Hosting
i.
Hosting one web site in one web server.
ii.
The site owner has full of the dedicated server including choice of hardware and OS.
Collocated Hosting
i.
Locating the site owner’s dedicated web server with other ones’ servers on the premises of a web
hosting service provider.
ii.
Those data centers are equipped with high Internet connection speed, physical security, airconditioning and power supply.
In-house Hosting
i.
The firms or organizations host their own web servers.
ii.
Factors like information traffic volume can be controlled better.
iii.
The firms or organizations have to deal security problems, like hackers and virus attacks, on their own.
Thus they have to employ more technical workers.
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CH/S6CA/Apr. 2009
Wireless Fidelity (Wi-Fi)

Wi-Fi, also known as IEEE 802.11, is the most popular wireless LAN standard used in HK.

It communicates using microwaves which have frequency ranges from app. 1 GHz to 300 GHz.

Hardware Requirements
i.
ii.
iii.

a.
It allows a computer to join a wireless network and communicate with other devices.
b.
Inside the adaptor, a microwave transceiver (i.e. transmitter and receiver) is used for transmission.
Access point.
a.
It is a device that allows wireless communication devices (e.g. notebook computers, cell phone,
etc) to connect to a wireless network using Wi-Fi.
b.
The access point itself will connect to another access point or more frequently, a wired network.
c.
In each access point, there is a service set identifier (SSID) which is used to identify one wireless
LAN from another. Each wireless client must specify the SSID of the access point to which the
client wants to connect.
Wireless router.
a.
It combines the functions of a router and an access point.
b.
It can serve as a broadband router, a DHCP server and a
firewall, provides a network address translation (NAT) service.
802.11 Architecture
i.
ii.

Wi-Fi wireless network adaptor.
Basic service set (BSS)
a.
A group of hosts communication using Wi-Fi.
b.
It may use an access point (infrastructure network) or does not use an access point (ad hoc
network, i.e. a peer-to-peer network).
Extended service set (ESS)
a.
Joining infrastructure mode BSSs together forms an ESS.
b.
In an ESS, the APs in each BSS are connected through a backbone which is usually a wired LAN
using the Ethernet.
Carrier Sense Multiple Access / Collisions Avoidance (CSMA/CA)
i.
When two or more wireless clients in a Wi-Fi send signals at the same, a data collision occurs and the
signals sent by the two clients may be destroyed.
ii.
CSMA/CA avoid data collisions using the following method,
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CH/S6CA/Apr. 2009

a.
Before sending out data, the sender will first sense the carrier (the wireless media) to see if anyone
is sending data.
b.
If the carrier is being used, the sending process will be deferred, which is called the back-off
period.
c.
If the carrier is free, the sender will continue to wait for a specified time, and then start
transmission if the carrier is still free.
d.
On the receiver side, if the receiver receives a frame and the frame is error-free, the receiver will
send an acknowledgement packet back to the sender.
e.
If the sender receivers the acknowledgement, it means that the transmission has been successful.
Otherwise, it will retransmit the frame until an acknowledgment is received or after a certain
number of retransmission.
Four common 802.11 standards
Standards
Release date
Op. Frequency
Throughput
(Typ)
Net bit rate
(Max)
802.11a
October 1999 5 GHz
27 Mbit/s
54 Mbit/s
802.11b
October 1999 2.4 GHz
~5 Mbit/s
11 Mbit/s
802.11g
June 2003
2.4 GHz
~22 Mbit/s
54 Mbit/s
802.11n
Jan 2010
(speculated)
5 GHz and/or 2.4
108Mbits
GHz
600 Mbit/s
Gross bit
rate (Max)
72 Mbit/s
Range (Indoor)
~35 m
~30 m
128 Mbit/s
~up to 100 m
~up to 300 m
Note that all 802.11b, 802.11g and 802.11n use 2.4 GHz frequency band. Thus they are compatible but easier
to be interfered by other devices.

Hybrid Network. It connects an access point to a wired LAN and forms form a network with both
wired and wireless equipment.

Wi-Fi Hotspot Network. A venue that offers Internet access over a wireless LAN is called a hotspot.
E.g. shopping centre, café, telephone booth, etc.
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