PBXs and the PSTN

This section introduces PBX and PSTN switches and networks.

Differences Between a PBX and a PSTN Switch

As shown in Table 8-1, PBXs and PSTN switches share many similarities, but they also have many differences.

Table 8-1 PBX and PSTN Switch Comparison


PSTN Switch

Used in the private sector

Used in the public sector

Scales to thousands of phones

Scales to hundreds of thousands of phones

Mostly digital

Mostly digital

Uses 64-kbps circuits

Uses 64-kbps circuits

Uses proprietary protocols to control telephones

Uses open-standard protocols between switches and telephones

Interconnects remote branch subsystems and telephones

Interconnects with other PSTN switches, PBXs, and telephones

Both the PBX and PSTN switch systems use 64-kbps circuits; however, the scale is very different. A PSTN switch can support hundreds of thousands of telephones, whereas a PBX can support only several thousand.


A PSTN switch's primary task is to provide residential telephony. A PBX supports user telephones within a company.

PBX vendors often create proprietary protocols to enable their PBXs to intercommunicate and transparently carry additional features through their voice network. In addition, only the vendor's telephones can be connected to its PBX. This forces enterprise networks to consolidate to one brand of PBX, and the enterprise business customer is restricted to one vendor.

NOTE Many vendors are implementing standards-based signaling protocols that enable interoperability between different vendors' PBXs. The two standards are Q Signaling (QSIG) and Digital Private Network Signaling System (DPNSS), as described in the "Digital Telephony Signaling" section later in this chapter.

Figure 8-3 illustrates the location of and communication between the PSTN and PBXs. PSTN switches connect residential and business users, but PBXs are mainly used for business purposes. PBXs are typically found at corporate locations, whereas PSTN switches are used to build the PSTN network and are located in central offices (CO).

Pstn Network

PBX Features

A PBX is a business telephone system that provides business features such as call hold, call transfer, call forward, follow-me, call park, conference calls, music on hold, call history, and voice mail. Most of these features are not available in traditional PSTN switches.

A PBX switch often connects to the PSTN through one or more T1 or E1 digital circuits. A PBX supports end-to-end digital transmission, employs PCM switching technology, and supports both analog and digital proprietary telephones.

Recall from Chapter 5, "Designing Remote Connectivity," that the United States, Canada, and Japan use T1. A T1 trunk can carry 24 fixed 64-kbps channels for either voice or data, using PCM signals and TDM, plus additional bits for framing, resulting in an aggregate carrying capacity of 1.544 megabits per second (Mbps). T1 lines originally used copper wire but now also include optical and wireless media.

In Europe, the trunk used to carry a digital transmission is an E1. An E1 trunk can carry up to 31 fixed 64-kbps channels for data and signaling, with another 64-kbps channel reserved for framing, giving an aggregate carrying capacity of 2.048 Mbps.

PBXs support end-to-end digital transmission, use PCM switching technology, and support both analog and digital proprietary telephones. A local PBX provides several advantages for an enterprise:

■ Local calls between telephones within the PBX or group of PBXs are free of charge.

■ Most PBX telephone system users do not call externally, through the T1 or E1 circuits, at the same time. Therefore, companies with a PBX only need the number of external lines to the PSTN to equal the maximum possible number of simultaneous calls, resulting in PSTN cost savings.

■ When adding a new user, changing a voice feature, or moving a user to a different location, there is no need to contact the PSTN carrier; the local administrator can reconfigure the PBX.

However, the PBX adds another level of complexity: The enterprise customer must configure and maintain the PBX. Figure 8-4 illustrates a typical enterprise telephone network that has proprietary telephones connected to the PBX and a trunk between the PBX and the PSTN network.

Figure 8-4 A PBX Can Reduce the Number of Trunks to the PSTN

Number of Telephones , Is Greater Than the

Corporate Location Number of Trunks

Figure 8-4 A PBX Can Reduce the Number of Trunks to the PSTN

Number of Telephones , Is Greater Than the

Corporate Location Number of Trunks

Proprietary Protocol Between Phones and PBX

Proprietary Protocol Between Phones and PBX

PSTN Switches

The PSTN appears to be a single large network with telephone lines connected. In reality, the PSTN is composed of circuits, switches, signaling devices, and telephones. Many different companies own and operate different systems within the PSTN.

PSTN Features

A PSTN switch's primary role is to connect the calling and called parties. If the two parties are physically connected to the same PSTN switch, the call remains local; otherwise, the PSTN switch forwards the call to the destination switch that owns the called party.

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