P1: IML/FFX P2: IML/FFX QC: IML/FFX T1: IML
Thompson2 WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 17:7 Char Count= 0
ACCESS ANDPUBLICNETWORKTECHNOLOGIES 169users could use other IXCs on a call-by-call basis by dial-
ing a prefix.
Another major change in the U.S. PSTN occurred with
the 1996 Telecommunications Act that amended the Com-
munications Act of 1934 (Noam, 2001). RBOCs had to
comply with a list of tasks before they were permitted to
provide long-distance service within their regions. The list
permitted competition in the RBOCs regions. It was ar-
gued that it was necessary to induce competition in these
local markets. RBOCs were required to provide intercon-
nection to new market competitors, unbundle their net-
work, permit competitors to resell their service, and pro-
vide users with number portability.
The new local service providers became known as
competitive local exchange companies (CLECs) (pro-
nounced “see-lecks”) (Noam, 2001). The incumbent LECs
became known as ILECs. For a CLEC to be competitive
with the ILEC requires that it be able to interconnect
with the users cost effectively. Therefore, there came a
great struggle between CLECs and ILECs on the issue of
collocation since the ILEC had a significant advantage
with the existing network. In “physical collocation”
a CLEC places its cables and equipment inside the
ILEC’s central office (CO) to hand off traffic. In another
arrangement called “virtual collocation” the physical
handoff of the traffic occurs inside or outside the CO, but
uses ILEC-owned equipment and must be the economic
equivalent of “physical collocation.”
It may appear from the previous discussion that the
breaking up of the U.S. PSTN is relevant only to the United
States but the trend is happening in other parts of the
world as well (Noam, 2001). Japan opened its markets
to competition. Also, the Europeans have privatized their
service. Noam argues that at first a network is not feasi-
ble unless supported by outside sources such as govern-
ments. As the network grows the average costs decline
initially and then rise as a few high-cost users are added.
Without regulation the network would not grow beyond
a certain point because of the high cost of adding these
high-cost users. From a political and societal point of view
the network becomes a necessity instead of a convenience
and should be offered to everyone. Therefore, the monop-
olistic breakdown of the network is caused by its own
success.ACCESS AND PUBLIC NETWORK
TECHNOLOGIES
To use a public network for data services, a user must
access the public network through some network service
from the user’s computing equipment to the nearest pub-
lic network node. Factors in selecting a particular service
include the cost of the service that is provided and the fea-
tures, including the transmission speed, that are provided
by the technology. Generally, the higher the transmission
speed that a technology can support, the more costly the
service becomes. Transmission speeds for networks are
described in bits per second. Unlike when memory size is
described, 1 Kbps is exactly equal to 10^3 bits per second,
1 Mbps is exactly equal to 10^6 bits per second, and 1 Gbps
is exactly equal to 10^9 bits per second.Many technologies are available for access to a public
network and for use within the public network. The most
inexpensive network access is through a voice-grade mo-
dem. A modem is used to convert a digital computer signal
to an analog signal that can be sent across ordinary tele-
phone lines. Voice-grade modems can receive data at up to
56 Kbps. In contrast, digital lines that are used to access
the network range in transmission speed from 56 Kbps
to 10 Gbps. Within the public network a few technolo-
gies, including X.25, frame relay, asynchronous transfer
mode (ATM), and synchronous optical network (SONET),
have become the most commonly used technologies.
Table 1 lists the most common technologies along with
a comment about usage. Table 1 also compares the trans-
mission speed and the time to download a 10-megabit
(1.2 Megabyte) file.Voice-Grade Modems
A modem is the most inexpensive and easiest to use access
technology. The use of modems for data transmission will
be substantial for many years to come (Stallings, 2001).
Voice-grade modems use a 4-KHz bandwidth on an ordi-
nary telephone line, the same bandwidth that is used for
voice signals. Modems can be packaged inside an infor-
mation product, such as a personal computer. Companies
often have modem banks that allow employees to dial-in
directly to the company intranet or to access a large com-
puter system.
On March 1, 1993, the International Telecommunica-
tions Union (ITU) Telecommunications Standardization
Sector (ITU-T) was created as a permanent organ of the
ITU, an agency of the United Nations. The charter of
the ITU-T is to standardize techniques and operations in
telecommunications. Several standard specifications for
voice-grade modems have been designated by the ITU-T.
Two of the most significant modem specifications are V.32,
which is a dial-up modem that transmits at 9600 bps, and
V.90, also a dial-up modem. V.90 sends at 33.6 Kbps and
receives at 56 Kbps, the highest rates available for voice-
grade modems (Stallings, 2001).Digital Subscriber Lines
A faster service than voice-grade modems that is begin-
ning to be offered by telephone companies is the digital
subscriber line (DSL). A widely publicized version of this
is asymmetric digital subscriber line (ADSL). ADSL offers
high-speed downstream access to the customer site, and
a lower speed upstream access from the customer. The
ITU-T has developed a standard for low-speed ADSL
called G.992.2, or G.Lite. G.Lite specifies downstream
speeds of 1.5 Mbps, but sometimes lower downstream
speeds are used. Most users find asymmetric speeds to be
acceptable, since upstream traffic frequently consists of
keystrokes or the transmission of short e-mail messages,
whereas downstream traffic may include Web pages, or
large amounts of data. In addition to data speed, an advan-
tage of DSL over voice-grade modems is that DSL modems
allow voice traffic to be multiplexed onto the telephone
wires coming into the customer site. A customer can talk
on the telephone at the same time that data are being
transferred.