A private branch exchange (PBX) is a system that connects telephone company trunk lines with individual user lines and equipment inside the organization. A PBX allows an organization to have fewer outside lines than extensions because it is safe to assume that not all extensions will be in use at once. The PBX acts like a telephone company switch, connecting outside callers with inside extension lines and extensions with each other as needed. Although PBXs and key systems are networks, they are deemed CPE because they are owned and operated by the customer (business, hospital, etc.), not the local telephone company.
PBXs allow for organizations to have a flexible telephone system designed for their specific needs and changeable over time. PBXs can have simple or sophisticated features, and may have a separate console for administration and/or receptionist duties. Individual lines may have different features on them, and different telephone sets attached to them. PBXs can be large, terminating hundreds of lines and thousands of extensions.
A key system is essentially a scaled-down PBX. Key systems typically have one unit, either an attendant phone or a separate box, that acts as controller of a limited number of lines (usually about 4) for a limited number of extensions (as many as 20). The features are similarly less ambitious.
Both PBXs and key systems use proprietary phones. That is, the system box and the phones are a bundle; users cannot plug other phones into the jacks and be assured that they will work. This makes the choice of a PBX crucial from an access perspective. Once a company or university has selected a PBX, access options may become extremely limited. This point also emphasizes the inevitable interactions between telephones and the networks they operate on.
PBXs may be analog or digital; more modern systems are consistently digital. It is important to remember that analog equipment cannot be plugged directly into digital jacks. For example, a TTY cannot be directly connected to a line on a digital PBX. But neither can fax machines or other analog-based telecommunication devices. So almost all PBX manufacturers offer digital-analog adapters. These adapters are plugged into the digital line, and the analog equipment (fax machine or TTY) is plugged into the adapter. Some PBX phones contain their own analog jacks. The aftermarket offers digital adapters as well.
Most PBXs and key systems offer voice mail and automated attendant as a feature. Some of these can be made TTY compatible.
PBXs also offer some access potential because of their architecture. The system boxes that control PBXs are essentially computers with specialized hardware. If access features could be added to PBX software, any line connected to the system could use those features. This can be done either with the PBX hardware itself, or with a second computer attached to the PBX. This is an element of computer telephony integration (CTI). See Appendix E for more information on CTI.
PBX-type networks are now offered based on a local area network (LAN), the data network that connects computers together in an office or hospital. Each workstation PC is able to convert between analog and digital, so it can send and receive LAN packets that contain voice signals. These can be played out of the computer’s speaker, or into a telephone connected to the sound card on the computer. LAN telephony does not suffer the voice quality degradation common with Internet telephony, because the network is designed to handle the necessary load and has fewer “hops”, or places at which congestion can occur.
There are some PBX solutions for TTY traffic. In these the TTY traffic terminates on a TTY modem that is integrated into the LAN. All text conversations (whether real-time or stored) occur on the LAN as text, not as Baudot. This gives every workstation a virtual TTY, whether for internal, incoming, or outgoing calls.
Another access advantage of LAN telephony is that it opens up telecommunications access into the world of computer access, which offers many more tools, such as speech synthesis, speech recognition, conversion of signal tones into screen flashes, and alternative keyboards. Virtually anything that a person uses to make the computer more accessible can be made to improve the access to telecommunications functions. Since LAN telephony is based on software, access software solutions are often compatible with it.
Unified messaging is one of the reasons organizations adopt LAN telephony. Unified messaging takes voice mail, email, and fax and puts all of them in one mailbox, notifying the user whenever a new message, regardless of the medium, arrives. The ability to see and sort all this incoming traffic through one tool may offer some access advantages:
A user who is blind:
“I have everything come into one mailbox that I always keep open.
When something new comes in, if it is a voice message I just play it.
If it is email I have my screen reader read it.
If it is a fax … I try to get it into text with my character recognition
program … it is certainly easier than keeping track of a lot of paper.
Plus I can keep anything as long as I want and not run out of space,
so I really use it as a filing system.”
Note: this section refers to the PBX itself, not to the business telephones normally connected to them. See the Business Wireline Telephone section.