Mobile Communications | Features/Function Enhancements

The first wireless PBX options were announced in 1992. The first to announce was Ericsson, a leading PBX supplier who has also been a dominant competitor in wireless communications, Ericsson introduced a wireless adjunct controller to work behind its MD-110 PBX. A few months later an unknown company called Spectralink introduced its wireless adjunct option designed to work behind any PBX system. Although the Ericsson and Spectralink systems were based on different radio transmission frequencies and used different voice encoding formats, the basic features and functions were similar. Both wireless communications options supported multiple-zone coverage areas and features such as roaming and handoff between coverage zones. In addition to a controller cabinet that linked to the main PBX system, the infrastructure included radio transmission transceivers linked to the controller over standard, unshielded, twisted-pair telephony wiring and provided the interface between the wireless telephone handsets and the PBX system. The adjunct controller served as a mere gateway between PBX and the station users. It was the PBX system that continued to provide all communications services and function to the wireless station users, including dial tone, call processing, and switching functions (Figure 1).


Figure 1: Wireless PBX design.

After the initial wireless system announcements from Ericsson and Spectralink, several other leading PBX suppliers developed and intro- duced their own wireless system options by the mid-1990s. These options were designed to work behind their own communications systems. Among those that developed proprietary wireless communications options were AT&T, Northern Telecom, NEC, Alcatel, and Tadiran. Throughout the 1990s several of the wireless system suppliers upgraded or overhauled their original system designs. The original systems operating in the 800-MHz frequency ranges migrated to 900 MHz and/or 1900 MHz. Ericsson, for example, went through two system redesigns and eventually abandoned its last version to focus on premises cellular systems as an extension of their network-based offerings.

Market demand for wireless PBX system options has been less than originally forecasted. Several reasons have been given for poor shipment levels: the cost to install and operate a wireless station subscriber behind a PBX system could be several times greater than a wired station; there are severe traffic capacity limitations within each coverage zone; and the technology standards are continually changing. For example, the typical price for a proprietary wireless handset is more than $500 at a time when network cellular telephones can be purchased for less than $100.

Evolving mobile communications capabilities behind a PBX system are likely to be based on enterprise mobility servers that link a network carrier’s cellular infrastructure with a PBX system. Ericsson’s Digital Wireless Office System (DWOS) is an example of an enterprise cellular communications solution that supports network carrier cellular telephones behind a PBX system, with a mobility server as the link between the premises and off-premises communications system. Ericsson, in fact, plans to port its MD-110 PBX features and function to its DWOS mobility server and market a fully integrated PBX/cellular communications system. Other leading PBX suppliers, such as Alcatel and Siemens, offer mobile communications option similar to DWOS and also may integrate PBX functions into the mobility server. The future boundary between premises and network communications systems and services will be blurred when the new mobile communications offerings are generally available.