The Packet-Switched Telecommunications Network

Over the past fifty years, several attempts have been made to develop converged networks: networks with “dial tone” that supports all communications: speech, music, text, graphics, images and video. For a number of reasons, convergence strategies employing ISDN and ATM were unsuccessful and did not gain critical mass. This time, it appears that packet-switched network service using IP will gain enough momentum to become the new kind of plain ordinary telecommunications service.

Beginning with grunts and gestures, progressing to verbal messages, signal fires, written messages, postal services, telegraphy, radio, telephone, television and computer networks, each advance in technology has meant communication of more information, faster and to more people – but often with the need to adapt the information to suit the communication technology being used. Realization of a converged network will be another step in the evolution of the way humans communicate, in that communication mechanisms best suited to the information will be used, rather than the other way around.

The abbreviation “PSTN” has been a familiar part of the telecom lexicon for perhaps 100 years. Now its meaning is going to change. For the past hundred years, PSTN has meant Public Switched Telephone Network:
– Public: Accessible to anyone who pays.
– Switched: Circuit-switched service, a trunk reserved and switched onto the access lines at each end, for the duration of the call, then released.
– Telephone: Speaking at a distance.
– Network: Many interconnected nodes.

Now, “PSTN” will mean Packet-Switched Telecommunications Network:
– Packet-Switched: Communications segmented and encapsulated in packets that are routed using IP or switched using MPLS to the far end.
– Telecommunications: Transferring any kind of information across distance.
– Network: Many interconnected nodes.

It may be necessary to use IP-PSTN or IPSTN for a transitional period, until circuit-switching falls into the same dustbin of history as step-by-step switches.


This new PSTN will provide broadband IP dial tone: high bit-rate access circuits plus the possibility of communicating IP packets to any point on the network. “Businesses” (including government, educational institutions and the like) will use Ethernet over fiber at Gigabit speeds for access. Residences will use modems over copper wires – either DSL or cable for access for quite some time to come.

Many different kinds of value-added services will be available in addition to basic IP packet communication service, some of them provided by the same company that provides the “dial tone”, others provided by third parties.

The lowest level of value-added services includes service level agreements and network and information security services. An unending list of higher-level services will be available for use on the network; some free, many not. Telephone service, television, web surfing, radio, videophones, desktop sharing, and integrated email/voicemail messaging are easy examples.

DMS-100 and 5ESS switches will be replaced with softswitches – software on servers on the IP network. Media servers will support voicemail and video programming. Internet access service will become simply a few ancillary services such as a Domain Name Server and an outgoing email server; the capability to communicate with web servers will be part of the dial tone.

This is the biggest change in telecommunications since the invention of the telephone switch… certainly more important than the change from analog to digital transmission in the 1980s, and it will have a profound impact on the way people communicate.

This is the first page of Course 110 Understanding IP Telecom: IP, VoIP and MPLS for Non-Engineers… a big-picture view to get things started.

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