Private Branch Exchange

A Private Branch Exchange (PBX) is a device that organizations install in their environment which enables internal connectivity for phones or fax machines. PBXs were historically expensive to purchase or required long lease agreements, but were considered an investment that could be utilized for a period as long as 10 to 30 years.

The PBX on-premise allows for users within the organization to call each other without traversing the PSTN and incurring any charges. A PBX also usually has trunk lines that connect to the PSTN so that internal users can make and receive calls with other PSTN users outside of the office, as shown in Figure 17.2.

Figure 17.2. A Private Branch Exchange (PBX).

As telephony has evolved over the years, different types of PBXs have been used by companies. Usually they fall into one of three categories:

• Traditional PBX—A traditional PBX is one that does not have any IP capabilities. These are generally older or low-end systems with limited feature sets. These systems are usually entirely based on analog or digital handsets for end users.

• IP PBX—An IP PBX is a system that is entirely based on Voice over IP (VoIP). It does not support any analog devices natively and all endpoints are IP-based network devices.

• Hybrid PBX—Many PBXs have the capability to function both as a traditional PBX with analog endpoints and as an IP PBX through the purchase of expansion modules and software upgrades, like shown in Figure 17.3. These PBXs offer the most flexibility for an organization because they can connect many types of devices as the business transitions to IP telephony.

Figure 17.3. Hybrid PBX connecting analog and IP phones.