International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B6. Istanbul 2004
computer using wired or satellite mode.
A home computer is usually linked to the Internet using a
normal phone line and a modem that talks to an Internet Service
Provider (ISP). A computer in a business or university is
usually connected to a Local Area Network (LAN) inside the
business through a higher-speed connection, like ADSL, ISDN.
Usually switches are also used, to provide the different nodes of
a company's LAN different connection. The LANs are then
attached to an ISP. ISP can be connected to bigger ISP or, using
high-bandwidth connections, directly to NAPs (Network
Access Point) through communications networks called
‘backbones’. The backbones are provided by companies such as
AT&T, GTE, IBM, MCI, Netcom, Sprint or UUNET and
consist of high-speed links in the TI, T3, OCI or OC3 ranges
(Figure 6). Backbones around the world are connected through
world-wide fibre-optic lines, undersea cables or satellite links.
In this way, every computer on the Internet is connected to
every other computer on the Internet all over the world.
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Figure 6: The world-wide backbones provided by UUNET
[UUNET].
The Internet connections can be divided in low, medium and
high-bandwith. As reported in Table 1, old modems were very
common in the 70's and they were considered high-speed
connection («10 Kbps). Actual modems operate between 14.4
Kbps and 56.6 Kbps, with mild success, at least in one direction.
They are very common for Internet connections at home.
Recently there has been a lot of effort to develop a higher speed
connection for residential users and small office by using an
Integrated Services Digital Network (ISDN). ISDN operates at
a minimum speed of 64 Kbps on normal copper phone line. For
this reason, it is equally available to home and business
customers and provides a huge improvement in access speed at
only a fractional increase in cost. Data are transmitted in a
digital format and the "Integrated" part of ISDN's name refers
to the combining of voice and data services over the same wires.
‘A TI connection is a form of digital, leased private line, which
means that a company can lease a point-to-point circuit at a flat
rate with a telephone company. A T1 connection allows using
the line in the form of 24 channels, running at 64 Kbps each. T1
runs on fibre optic or copper wires. ADSL (Asymmetric Digital
Subscriber Line) uses a common phone line and can transfer
data using a particular modem with a maximal speed of 6 Mbps.
ADSL is getting very common, it does not require new wiring
and the phone line for voice call can be used even if a Internet
connection is still open. The word Asymmetric is used because
these modems send data faster in one direction than they do in
another. ADSL (and in general DSL technology) exploits the
‘extra capacity’ of copper wires to carry the information,
without disturbing the voice conversation and matching
particular frequencies to specific tasks. Following ADSL, the
speed jumps to T3, OCI (Optical Carried level 1) and to the
newer version of ISDN, called Broadband ISDN (B-ISDN)
which can run at 155 Mbps. Many people who have cable TV
can now get a high-speed connection to the Internet from their
cable provider. Cable modems compete with technologies like
ADSL and, unlike these, its performance doesn't depend on
distance from the central cable office. Another new type of
Internet connection can be realised using satellite. Satellite
connection can reach a download speed up to 10X faster than
dial-up connection and work using normal antenna for Sat-TV.
Connection Speed Time
Low Old modem 2400 bps = | hour
Bandwidth ‘Home’ < 56.6 Kbps = 4 min.
modem
Medium ISDN 64 Kbps = 2 min.
Bandwidth
11, HDSE2 1.5 Mbps = 5 sec.
ADSL 6 Mbps = 1.3 sec.
High T3, OC-1 45 Mbps = ().2 sec.
Bandwidth B-ISDN 155 Mbps = 0.05 sec.
OC-XX > | Gbps < 0.01 sec.
Table 1: Mostly used Internet connections with respective
speed and approximately time for downloading the same
document.
Despite all these name and speeds, the connection is still a
problem for many Internet users: many ISPs that should
maintain the "backbones" for an entire nation still work with
medium bandwidth connections. And this fact can create many
'stall' problems for local users or for persons who wants to view
HTML documents from these countries but has to fight with
download speed of 20 Kbps!
2.5 Satellite Connection
As Internet traffic continues to grow world-wide at exponential
-ates, many Internet Services Providers (ISPs) are facing the
problem to keep high-speed connections for their users. The
satellite connection between a Network Access Point (NAP)
and the Internet backbone can solve this problem. In fact,
Internet satellite connections, started at the end of the '90, can
assure Internet access whatever the traffic. Moreover, it doesn't
matters where the customers are as satellite communication has
can deliver bandwidth exactly where and when it is required,
without geography and local infrastructure limitations. The
satellite connection does not use telephone lines or cable TV
systems and, depending on the user's location and requirements,
is the best method for skip or extend the terrestrial fibre optic
network. In particular, the satellite service is aimed for ISPs or
businesses located in areas poorly served by the wired Internet
infrastructure. It is the case of the central Asia of Africa, where
it could be the (high-speed) transport medium for information
and data (Figure 7).
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Figure 7: SkyVision World Coverage Map [Sky-Vision] (left).
Ses Americon Internet satellite fleet [SES Americon] (right).
There are two ways to have an Internet satellite connection: the
first possibility is to perform a connection using the same dish
that allow you to receive the satellite television; this solution
requires you to have an ISP for a dial-up or cable modem for
the data you send to the Internet. The second possibility to have
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