Signaling in a Computer Telephony System and Voice Routing

Foreign Exchange (FX) trunk signaling can be provided over analog or T1/E1 links. Connecting basic telephone service telephones to a computer telephony system via T1 links requires a channel band configured with FX type connections. 

To generate a call from the basic telephone service set to a computer telephony system, a foreign exchange office (FXO) connection must be configured. To generate a call from the computer telephony system to the basic telephone service set, a foreign exchange station (FXS) connection must be configured.

When two PBXs communicate over a tie trunk, they use E&M signaling (stands for Earth and Magneto or Ear and Mouth). E&M is generally used for two-way (either side may initiate actions) switch-to-switch or switch-to-network connections. It is also frequently used for the computer telephony system to switch connections.

Dialing Within a Phone System

Calls within a phone system are considered on-net or off-net, as follows:

   – On-net calling refers to calls that stay on a customer’s private network, traveling by private line from      beginning to end. 

   – A call to an off-premise extension connected by a tie trunk is considered an on-net call. The off-      premise telephone is located in a different office or building from the main phone system, but acts as      if it is in the same location as the main phone system and can use its full capabilities.

   – Off-net calling refers to phone calls that are carried in part on a network but are destined for a phone      that is not on the network. That is, some part of the conversation’s journey will be over the PSTN or      someone else’s network.

Voice Network Addressing

Voice addressing is determined by a combination of international and national standards, local telephone company practices and internal customer-specific codes. Voice addressing historically has had a geographical connotation, but the introduction of wireless and portable services will render that impossible to maintain.

International and national numbering plans are described by the ITU’s E.164 recommendation. It is expected that the local telephone company adheres to this recommendation. 

E.164 is only the public network addressing system. There are also private dialing plans, which are nonstandardized and can be considered highly effective by their users.

This slide depicts a trunk group that bypasses the PSTN. Selection of this trunk has been predefined and mapped to the number 8. The access number could be part of the E.164 addressing scheme or part of a private dialing plan.

Alternate numbering schemes are employed by users and providers of PSTN service for specific reasons. An example of a of non-E.164 plan is carrier identification code (CIC), used for selecting different long-distance carriers, tie lines, trunk groups, WATS lines, and private numbering plans, such as seven-digit dialing.

For integrating voice and data networks, each of these areas must be considered.

Voice Routing

Routing is closely related to the numbering plan and signaling that we just described.

At its most basic level, routing enables the establishment of a call from the source telephone to the destination telephone. However, most routing is much more sophisticated and allows subscribers to select specific services.
In terms of implementation, routing is a result of establishing a set of tables or rules within each switch. As a call comes in, the path to the desired destination and the type of features available will be derived from these tables or rules.

It is important to know how routing is done in the telephone network, because this function will be required in an integrated data/voice network.

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