We refer readers to the numerous references available on ITU-T telecommunications networks. The complete, original, and up-to-date documents are available to be purchased from the ITU-T at the following address
- Other relevant documents (such as for Frame Relay, ATM, and Multiprotocol Label Switching (MPLS) networks) can be obtained at no cost at the following address
- Central control, such as the Intelligent Network (IN) for TDM in each network for setting up the paths across each of the respective networks.
- Telecom standards are focused on interfaces. The most common interfaces are standard user-to-network interfaces (UNI) and network-tonetwork interfaces (NNI).
- Services must be supported by the features in all NNIs, UNIs, and in all the central control units.
No service intelligence is presumed in the user devices. Users have no control over the applications or the choice of services, except for the services made available by subscription by the service provider, such as the local telephone company or the mobile phone company
These features are an impediment for the richness and time-to-market for new services, as well as for user choice of services. Experience has shown that this type of network is not favorable for innovation, since independent developers cannot get easy access to the central control platforms of the service providers or influence the interface standards in a timely manner. Following are some features that are more of an engineering concern and are also reflected in the high cost of service:
- The circuit-switched nature requires the keeping of state for a connection or call in every switch, and in several switch subsystems in every switch, as well as in every central control unit. State is also kept in all networks in the path between the parties. State requires expensive processing and memory in all network elements and components where state is kept.
- 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 telecomtype 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 probable 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