■■ There are single points of failure. Protection against network failures
requires carrier-grade equipment (the so-called 99.999 percent, or “five
nines” reliability for equipment), standby equipment, and entire
standby network paths.
■■ Telephone network standards such as Signaling System 7 (SS7) are not
global. Countless regional variants, profiles, and various options are
permitted. As a consequence, interoperability of telecom networks is a
hard problem and is usually achieved only for the least-common
denominator of standard features.
In spite of these comments made in hindsight, the global telecom networks
amount to close to a trillion-dollar industry that is still robust because of
mobile telephony. Most of the Internet traffic is also still carried on telecom-
type transmission systems, such as on SONET or SDH links.
The growth of telecom transmission systems in developed countries is,
however, predominantly because of the Internet, and this indicates the proba-
ble near-term end of the life cycle for most telecom networks (except for
mobile phone networks). The end of the life cycle for telecom networks can
probably be explained by the absence of new services that has been observed
for some time.
Figure 3.2 shows an example of the architectural concepts in the telecom
industry. The IP Multimedia Subsystem (IMS) is chosen as the reference archi-
tecture for third generation (3G) mobile telephone networks in the 3GPP orga-
nization (see http://www.3gpp.org)..) As of this writing, similar concepts are also
under development in the telecom standards organizations ETSI and ITU-T
(NGN groups).
Explaining this architecture is beyond the purpose of this book, and expla-
nations can be found in many online resources, for example in magazine arti-
cles such as in reference [1]. The IMS architecture diagram is shown here only
to illustrate the following points:
■■ A detailed diagram describes the reference architecture.
■■ The various functions are called out as boxes in the diagram.
■■ The links for signaling and media are specified between functions.
■■ The protocols are specified between the functional entities.
■■ The applications reside in application servers and the only endpoints
shown are black telephones.
■■ Last, but not least, while the PSTN/ISDN networks are specified, the
Internet is presumably included in the box labeled “Other IP Networks”
but is not shown.Architectural Principles of the Internet 41