Differences Between PSTN and ET

Differences Between PSTN and ET

PSTN and ET are different from each other in the way they treat signaling and in the types of advanced features they offer.

Signaling Treatment

Although the PSTN uses signaling interfaces developed by industry bodies, PBX manufacturers often create proprietary protocols to enable their PBXs to intercommunicate and carry additional features transparently throughout the ET voice network.

Chapter 1 discusses how the PSTN uses Signaling System 7 (SS7), ISDN, and in-band signaling as its primary signaling links. These are well-documented standards that have been evolving for many years. Although these signaling protocols cannot solve all signaling problems today, anyone can develop software to interface with the PSTN network.


This software and hardware must be homologated in each country to which it is connected to the network. Homologation is a process in which each country certifies any equipment that connects with its PSTN. Depending on the country, this may even require rehomologation when new software updates come out for the device to be connected.

Many PBXs in ET use CAS and PRI for signaling with the PSTN. In many cases, computer telephony integration (CTI) links are also used to enable a third-party computer application to control some of the PBX's operations. Most PBX vendors, however, implement a proprietary signaling mechanism. This forces enterprise networks to consolidate on one brand of PBX. Although this can be good for the manufacturer, the enterprise business customer is now locked into one vendor for their voice transport, services, and applications.

Additionally, many PBX vendors use proprietary signaling to offer advanced features to proprietary handsets. This also forces the Enterprise customer to utilize one vendor's handsets to maintain compatibility with the PBX.


Many vendors are starting to implement standards-based signaling protocols that enable interoperability between different vendors' PBXs. A list of these protocols is as follows:

  • Q Signaling (QSIG)This is an open standard designed to enable multiple vendors to agree on supplementary services, dial plans, and much more. (The "Q" comes from the International Telecommunication Union Telecommunication Standardization Sector [ITU-T] Q.xxx set of standards.)

  • Digital Private Network Signaling System (DPNSS)This is a British standard designed to enable cross-vendor or multivendor interoperability between PBXs. This standard was rolled into QSIG.

Advanced Features

Providing advanced features is also an important differentiation between ET and PSTN. Business requirements for telephone networks are much greater than the average home user. Enterprise customers have the need for high-use, feature-rich systems that enable applications such as the following:

  • Inbound and outbound call centersET networks with this feature usually contain a CTI link that enables new applicationsfor instance, a screen pops up on the representative's computer screen that gives the representative the caller-ID information, as well as other information about that caller (buying patterns, shipping address, and so on).

  • Financial Enterprise TelephonyET networks with this feature often include a network known as hoot-n-holler, in which one person speaks and many people listen. This is common in stock brokerage.

ET customers can use the PSTN to service basic PBX needs, but the legacy PSTN does not typically have advanced applications such as call centers. Also, using the PSTN is usually less cost effective than using ET.

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