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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B6. Istanbul 2004
a satellite Internet connection is to use a two-way satellite dish
(upload and download). Upload speed is about one-tenth of the
500 kbps download speed. Cable and DSL have higher
download speeds, but satellite systems are about 10 times faster
than a normal modem. The dish must have a clear view to the
south, since the orbiting satellites are over the equator area.
Moreover, like satellite TV, trees and heavy rains can affect
reception of the digital signal.
Global satellite capacity isn't being used fully. There is enough
satellite capacity in the sky to meet the health and education
objectives and requirements of every country on earth.
The use of satellite resources for Internet Backbone and other
Internet related applications would improve the connections
and the capacities of many domains, in particular in developing
countries.
2.6 Wireless Internet
Wireless simply means the use of radio-frequency spectrum to
transmit and receive voice, data, and video signals for
communications.
Regarding wireless Internet, we have to distinguish between
laptops and mobile devices (like cell-phone or PDAs). The
latter use Wireless Application Protocol (WAP) to access the
information on the web. A web site accessible with mobile
devices is created with text-only or with low-graphics version
and it is generally written in Wireless Markup Language
(WML). Only the new generation of PDAs allow the full access
to HTML pages [ThunderHawk, 2004]. During the transmission,
the data is sent in HTTP form from a web server to a WAP
gateway. This system includes the WAP encoder, script
compiler and protocol adapters to convert the HTTP
information to WML. The gateway then sends the converted
data to the WAP client on your wireless mobile device.
Laptops with wireless Internet connection can have Internet
access through a Wireless Internet Service Provider (WISP)
organization, e.g. an Internet provider that allows subscribers to
connect to a server at designated access points (called ‘hot spot’)
using a wireless connection such as Wi-Fi (short for "wireless
fidelity"). Wi-Fi is the common term for a high-frequency
wireless local area network (WLAN). The Wi-Fi technology is
rapidly gaining acceptance in many companies as an alternative
to a wired LAN.
The WISPs offer high-speed communication and allow
subscriber computers to access the Internet and the web from
anywhere within the zone of coverage (several km) provided by
the server antenna.
A wireless Internet connection is usually faster than cable or
DSL mode, the user is always-on connection and he has the
freedom to move around without losing the connection.
Residential
Customers
Internet Network
Operations A
Backbone passa Business
Customer
Locations
Figure 8: Wireless Internet connection. Source: [Logical Net's].
, 145
2.7 Services on the Internet
The World Wide Web is often identifies with the Internet, but it
is only the most known part of it. Inside the Internet 'live' many
other services developed to facilitate the sharing of information
through the net. In the following, the main and mainly used
services are shortly described.
FTP: the File Transfer Protocol (FTP) is a common format for
transmitting and moving data (files) on the Internet.
GHOPER: a system that pre-dates the World Wide Web for
organising and displaying files on Internet servers.
TELNET (and SSH): a terminal emulation program used to
connect to remote Internet servers.
E-MAIL: short for electronic mail, it is a transmission of
electronic messages over networks.
NEWSGROUP (or mailing list): a very big number of people
that receive simultaneously an e-mail.
USENET: it is an international network of newsgroups, which
are discussion forum that cover many interest groups.
ICQ: it stands for "I-see-you" and it is a revolutionary, user-
friendly Internet tool to communicate with other people in real
time. It includes the IRC (Internet Relay Chat), system
developed in the late 80's as a virtual meeting place where
people from all over the world could meet and talk.
WORLD WIDE WEB: it is the most known and used part of
the Internet where the users look for information, read articles
or see animations.
2.8 Firewall
A firewall is a system or group of systems that enforces an
access control policy between two networks. The actual means
by which this is accomplished varies widely, but in principle,
the firewall can be thought of as a pair of mechanisms: one that
exists to block traffic, and the other that exists to permit traffic.
Some firewalls place a greater emphasis on blocking traffic,
while others emphasize permitting traffic. Probably the most
important thing to recognize about a firewall is that it
implements an access control policy. If you don't have a good
idea of what kind of access you want to allow or to deny, a
firewall really won't help you. It's also important to recognize
that the firewall's configuration, because it is a mechanism for
enforcing policy, imposes its policy on everything behind it.
Administrators for firewalls managing the connectivity for a
large number of hosts therefore have a heavy responsibility.
2.9 The information on the Web
One of the main problems of Internet users is where to find the
right information they are looking for in the less time. Since the
80's, special websites have been created to help the users
finding all the information hidden in million of web pages.
These sites contain research systems that work with different
mechanisms and can be divided in search engines and online
directories. Another common resources used to stored
information are the Internet “Yellow Pages” Books, which list
Internet addresses by categories and can be found in computer
and bookstores.
2.9.1 Internet Search Engines
Internet search engines are special web sites designed to help
people find information stored on other sites. Before the Web
became the most visible part of the Internet, there were already
search engines in place to help people find information on the
