Legacy Interfaces

This area discusses the legacy interfaces used by the Public Switched Telephone Network (PSTN). In order to migrate to Voice over IP, you need to understand the legacy interfaces and how to connect to them. Regardless of which VoIP system that you use, someplace in the overall system, there must be a connection to the PSTN and that connection will be through one of the following methods:

Foregn eXchange Service (FXS) - FXS stands for Foreign eXchange Service. I like to think of it as Foreign eXchange Set instead - set being short for phoneSet or phone. That makes it an easy reminder as to what you should connect to a FXS port.

Foreign eXchange Office (FXO) - These are ports on a device like a VoIP server that are connected to a phone line from a Central Office (CO). An important concept is that an FXO port accepts dialtone and ringing voltage.

T1 Line - This section discusses the North America T1 channelized communication stream consisting of 24 channels that can be used for data, voice or both.

E1 Line -This section discusses the non North America E1 channelized communication stream consisting of 30 channels that can be used for data, voice or both. Often it will be referred to as the European standard but in fact it is a global standard except for North America.

ISDN BRI - This section details a very old protocol that has provided the foundation for many other protocols that followed. While it is rarely used, it does have some unique characteristics that are worth mentioning.

ISDN PRI - the ISDN Primary Rate Interface (PRI) uses either a T1 line or E1 line for the physical layer and consists of Bearer channels of 64 kbps each for data and one D channel of 64 kbps for handshaking and control.

If this page has helped you, please consider donating $1.00 to support the cost of hosting this site, thanks.

Return to

TelecomWorld 101

Copyright July 2013 Eugene Blanchard