{{Unreferenced, date=December 2009
An off-premises extension (OPX), sometimes also known as off-premises station (OPS), is an extension telephone at a location distant from its servicing exchange.
One type of off-premises extension, connected to a
private branch exchange
A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX).
A business telephone syst ...
(PBX), is generally used to provide employees with access to a company telephone system while they are out of the office. Off-premises extensions are used in distributed environments, serving locations that are too far from the PBX to be served by
on-premises wiring
On-premises wiring (customer premises wiring) is customer-owned telecommunication transmission or distribution lines. The transmission lines may be metallic (copper) or optical fiber, and may be installed within or between buildings.
Premises ...
.
Another type of off-premises extension, connected to a public telephone exchange, is generally used to allow a private phone line to ring at a second location. For example, the owner of a business may have an OPX for their home phone at the business location, allowing them to avoid missing calls to the home phone.
Telephone service providers charge a significant monthly rate for an OPX, partly calculated by the distance; in extreme cases, the distance may result in a rate higher than simply having an additional central office line with its own number. Recent innovations such as call forwarding-no answer or simultaneous ringing of multiple lines can replace several of the conveniences of an OPX at much lower cost.
An OPX uses a conditioned wire pair that is usually used only for voice applications, while for data, a pair usually needs to be unconditioned. An alarm circuit is an unconditioned pair.
In
Internet telephony
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet ...
, a VoIP VPN OPX may be implemented by connecting an
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* Ext ...
over a
virtual private networking
A virtual private network (VPN) extends a private network across a public network and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. The be ...
connection, instead of connecting it directly to the
local area network
A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a large ...
. As a host connected by a VPN appears as a part of the local area network, the off-premises extension appears to the IP-PBX as if it were on-site.