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ACCESS ANDPUBLICNETWORKTECHNOLOGIES 171company operates as both a transmission carrier and a
network provider. As with ADSL, the downstream speed
of cable modem is much faster than the upstream speed.
The upstream speeds are similar to ADSL, but the down-
stream speeds can be several times faster. However, mul-
tiple customers on the same cable line share the capacity.
When many customers are accessing the network at the
same time the real downstream transmission speed can
be much lower. If network traffic is bursty, though, the
chances are unlikely that all customers are downloading
at exactly the same moment so that sharing does not be-
come as issue until about 100 customers share the same
cable service (Panko, 2001).Satellite
An often cost-effective alternative for network access is
the use of satellite technology. This may be particularly
true in areas where other wire-based technologies are not
yet available. For example, many rural areas do not have
the density of potential users that can justify the cost of in-
stallation of wire-based technologies such as DSL or cable
modems.
Satellites are characterized by the type of orbit they
use. The most common type of satellite is the geosta-
tionary satellite. These satellites orbit the Earth at about
22,300 miles directly above the equator at exactly the same
speed as the Earth’s rotation. Because of this, the satellite
always appears to be in the same position in the sky and
tracking of the satellite by stations on Earth is simplified
(Stallings, 2001). The disadvantage of this type of satel-
lite is that the propagation time it takes for the signal to
be sent from a transmission station on the Earth to the
satellite, and then to be received back on the Earth is about
0.24 s. For large data downloads this is not noticeable
since the time overlaps with the time to receive the en-
tire message. However, for interactive computer use or
for applications such as telephone calls the time is no-
ticeable and can be annoying. In addition, geostationary
satellite signals are not received well in very far northern
or southern regions of the Earth.
Two other types of orbits include low- and medium-
Earth orbiting satellites. This technology is being pro-
posed for use with mobile terminals and remote loca-
tions that need stronger signals and less propagation time.
Successful businesses that use this technology are rare.
One company currently operating under bankruptcy reg-
ulations, Iridium, provides global, mobile satellite voice
and data solutions with complete coverage of the Earth
through a constellation of 66 low-Earth orbiting satellites
(Iridium, 2002).
Large satellite dishes create narrow footprints for
transmission, and large dishes are used for point-to-point
trunk transmissions. A small dish creates a very large
footprint that is suitable for television broadcasts in a
large region. Today, very small aperture terminal systems
are available and provide a low-cost alternative to expen-
sive point-to-point satellite connections. These stations
share satellite transmission capacity for transmission to
a hub station (Stallings, 2001).
Satellite access has some advantages over wire-based
technologies. The technology is available now for all loca-
tions in the United States, whereas DSL and cable modemtechnologies may not be available in some locations for
some time. For the speeds and services available the tech-
nology is cost-competitive. However, in order to use satel-
lite, the user must have a clear view of the southern sky.
The uploads speeds are modest, so satellite is not suit-
able for businesses that require high-upload bandwidth
for applications such as large upload data transfers or for
hosting Web sites. Also, the download bandwidth is shared
with all users at the site, and so the technology is not cur-
rently suitable for more than five simultaneous users.
At least one company offers packages with two-way,
always-on, high-speed Internet access via satellite that
is specifically designed to meet the needs of small busi-
nesses (StarBand, 2002). StarBand uses a 24-by-36-inch
dish and a special modem at the customer’s site to con-
nect the user’s site to the network. StarBand also serves as
a network provider. Fees include an initial equipment fee
and a monthly fee for access. Value-added services such
as domain registration and networking support for setting
up small office networks can be a part of the package.Integrated Services Digital Network
Many telephone companies offer integrated services dig-
ital network (ISDN), a digital service that runs over or-
dinary telephone lines. As with voice-grade modems the
ITU-T has set standards for ISDN. ISDN can be used as
an access technology and within a public network. Basic
ISDN service includes two “B” channels, each at 64 Kbps,
and a “D” channel that is used for signaling. It is possible
to use one “B” channel for voice and one for data, but most
service providers bond the two “B” channels together to
provide a 128 Kbps data rate. Standards for higher rates
also exist. Like ADSL, ISDN requires that the telephone
company install special equipment at the end office before
an ISDN service can be offered. A special ISDN “modem”
is used at the customer site.
ISDN is the result of efforts in the early 1980s by
the world’s telephone companies to design and build a
fully digital, circuit-switched telephone system (Tanen-
baum, 1996). Because ISDN is circuit-switched, there is
never any congestion on the line from the customer to
the network service provider. However, since data traffic
is generally bursty the user pays for bandwidth that may
not be used. ISDN is expensive compared to the modest
gain in transmission speed. The customer generally has to
pay for the ISDN line to the telephone company and then
has to pay an additional fee to a network service provider.
The use of ISDN is likely to decline as other higher speed
and more economical technologies become available.Digital Leased Lines
In terms of number of circuits, the most common leased
lines are 56 Kbps (Panko, 2001). The transmission capac-
ity of a 56 Kbps is actually 64 Kbps but one bit out of
eight is used for signaling, leaving the user with 56 Kbps.
A 56 Kbps line is the same as digital signal zero (DS0).
The next higher transmission speed is a T1 (DS1), which
provides 1.544 Mbps. While a 56 Kbps leased line is rela-
tively inexpensive, the difference in cost and performance
between a 56 Kbps and a T1 line is large. Therefore, frac-
tional T1’s are also available at 128 Kbps, 256 Kbps, 384
Kbps, and so on. In Europe and other parts of the world