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Public NetworksPublic Networks
Dale R. Thompson,University of Arkansas
Amy W. Apon,University of ArkansasIntroduction 166
Overview of Public Network Concepts and
Services 166
Structure of the Public Switched Telephone
Network System 168
Access and Public Network Technologies 169
Voice-Grade Modems 169
Digital Subscriber Lines 169
Cable Modems 170
Satellite 171
Integrated Services Digital Network 171
Digital Leased Lines 171
Synchronous Optical Network 172
X.25 172
Frame Relay 172Asynchronous Transfer Mode 172
Choosing a Private Network or
a Public Network Provider 173
Reliability 174
Cost and Performance Tradeoffs 174
Support 174
Control 174
Other Factors 175
Public Networks in the Internet and E-commerce
Environments 175
Conclusion 175
Glossary 176
Cross References 176
References 176INTRODUCTION
Networks for the transfer of data between computers,
both public and private, are ubiquitous in today’s busi-
ness world. A public network is one that is publicly avail-
able to subscribers (Stallings, 2001). It provides service
to multiple subscribers and is built and maintained by a
public network provider. Internationally, the term “pub-
lic network” is often applied to networks that are under
government control or are a national monopoly. However,
a network can also be a privately owned network whose
services are sold to the public. Whether the network is un-
der government control or is a privately owned network
whose services are sold to the public, businesses access
the network by installing an access device at each site and
using an access line to the nearest point of presence (POP)
of the public network provider (Panko, 2001).
This chapter gives an overview of public network con-
cepts and services and describes the structure of the public
switched telephone network (PSTN) system, the technolo-
gies used both for access to a public network and within
the public network itself, issues related to choosing a pub-
lic or a private network, and public networks in the Inter-
net and e-commerce environments.OVERVIEW OF PUBLIC NETWORK
CONCEPTS AND SERVICES
Traditionally, companies desiring to connect business
computers in different geographic locations have used
private networks. That is, they have used point-to-point
leased lines between business sites to create their own
circuit-switching or packet-switching networks for their
data communication requirements (Panko, 2001). Unlike
telephone calls, which set up the required capacity as
needed, leased lines provide dedicated transmission ca-
pacity between sites. These networks are called privatenetworks (Stallings, 2001). By using leased lines, compa-
nies have a network capacity that is always available and
are offered volume discounts for the bandwidth available
on the leased line. An example of a private network is
shown in Figure 1.
There are several disadvantages to private networks.
Private networks require higher initial costs. The leased
line connections must be planned and installed. The
switching devices must be provided. And, once a network
is operational there are ongoing management and main-
tenance costs of the networks (Panko, 2001). A public net-
work is an alternative to a private network.
There are advantages to using a public network. A pub-
lic network does not require a complex network of leased
lines and switching devices that the business must plan
and install. There is commonly one access line installed
per site. Even if a leased line is used to connect to the
nearest POP, there are usually less leased lines required.
For example, if there are 10 sites using the public net-
work, then there are 10 leased lines. Compare this to a
fully meshed private network that requires 45 leased lines.
ForNlocations,N(N−1)/2 leased lines are required for
a connection to and from each site. Even if not every site
is connected to every other site in the private network, but
sites are connected through intermediate sites, the num-
ber of leased lines for a public versus a private network is
generally smaller. Finally, because of competitive pricing,
public networks are less expensive than private networks
(Stallings, 2001). Figure 2 illustrates an example of a pub-
lic network.
The global Internet is a network that is publicly acces-
sible worldwide. The Internet is not one single network,
but is composed of several networks connected together
and communicating with standard Internet technologies
(Moody, 2001). Access to the Internet is achieved via an
Internet service provider (ISP). The Internet allows a busi-
ness to have a worldwide presence. Through the use of166