Single System PBX Network: On-net Multilocation Support

The simplest network design consists of a single PBX system. There are several single system design configurations based on the location of the system’s station users. These configurations are used to support the following types of communications requirements:

1. One or more station users at the same location who are physically remote from all customer premises locations housing PBX common equipment

2. One or more station users residing on the customer premises but exceeding the maximum loop length for their desktop telephone equipment

3. Station users physically remote from the customer premises location housing the main PBX common equipment room but supported by local common equipment

The first PBX configuration option supports remote teleworkers. The second and third configurations are based on a distributed common equipment architecture. Some PBX systems have a standard distributed port interface cabinet/carrier design, and others use hardware options designed for remote premises communications requirements.

The first single system PBX networking category supports station users who are remote from all common equipment but require desktop communications support as if they were located at their organization’s premises. These type of station users are now referred to as teleworkers. Teleworkers fall into two categories:

1. Fixed teleworker

2. Mobile teleworker

A fixed teleworker has a permanent desktop location relative to the remotely located PBX system. Fixed teleworker solutions are sometimes referred to as small office/home office (SOHO) configurations. Mobile teleworkers are constantly on the move, but need to be linked to their PBX system whenever and wherever they are. Another popular term for a mobile teleworker is road warrior. Road warriors are the newest breed of PBX station user and are growing in number at an alarming rate due to the proliferation of mobile computing and communications devices.

There are several available PBX options that support fixed teleworkers who require the same level of communications service and support as station users at the customer premises location. The oldest, and most basic, PBX option supporting off-premises station users is the OPX feature, a solution available for more than 20 years. The OPX feature requires a special local exchange carrier trunk circuit, known as an OPX circuit, to provide PBX system communications and signaling support to a remote analog telephone. The loop length of an OPX circuit connection to the remote station is usually limited to several miles without repeaters. Provisioning of OPX trunk services is also constrained by LATA boundaries. OPX station users have full access to all PBX features and functions but are operationally limited by their analog telephone instruments. No analog telephone supports out-of-band signaling to support multiple line appearances and proprietary PBX display field information. Off-premises and intercom calls to the remote station user are routed over the PBX’s OPX trunk. The remote station user can initiate intercom calls to other PBX station users and uses PBX trunk facilities for placing calls outside the PBX system.

SOHO applications requiring a higher performance desktop terminal, similar to the one available at the customer premises location, had limited options until the early 1990s. During the past decade several fixed teleworker solutions have been implemented:

1. Remote ISDN BRI telephones

2. Proprietary and third-party local loop distance extender options

3. Remote IP telephones/softphones

The first SOHO option based on ISDN BRI services was implemented by Intecom in the early 1990s. The PBX system interfaces to the PSTN via a digital trunk circuit by implementing ISDN PRI services, and the remote teleworker uses an ISDN BRI line to support a desktop ISDN BRI telephone that offers performance capabilities comparable to those of proprietary digital telephone instruments. PBX control signaling is carried over the ISDN trunk circuit’s D-channel to the remote location. This solution provides remote teleworker access to all PBX system features and functions and can support remote data terminal equipment over the second ISDN BRI B-channel.

A popular SOHO solution has been the use of local loop distance extender equipment to support proprietary digital PBX telephones over the PSTN trunk carrier facilities used to transport voice communications and control signaling between the customer premises and remote location. The first distance extender options were based on hardware equipment at the customer premises location—the proprietary port circuit cards or gateway modules, used to convert the proprietary out-of-band digital desktop control signaling into CAS format for transport over analog trunk circuits to connect PBX and remote teleworker locations. A desktop gateway module at the remote teleworker location supports a proprietary desktop digital telephone. Leading suppliers of local loop extender equipment include MCK Communications and Teltone. The option was developed when residential digital line services, such as ISDN BRI, were not commonly available. The growing availability of digital subscriber line (DSL) services has eliminated the need to convert digital signaling to CAS format. The wideband nature of current digital services available to SOHO locations allows several remote digital telephones to be supported over a single DSL line. Rack-mount carrier gateways at the remote location can support 12, 24, or more digital telephones with one or more digital T1-carrier facilities.

One of the very few early benefits of IP telephony was support of remote IP telephones using dial-up analog or digital lines at a remote location. Almost all IP-PBX systems can remotely support desktop IP telephone instruments or IP softphones via LAN/WAN infrastructures. The remote location configuration may require an analog gateway module or a SOHO router to access the corporate WAN.

Road warrior options include IP softphone applications running on a notebook/laptop computer, or any DTMF analog telephone or cellular handset, linked to the main PBX system through a proprietary communications/signaling port interface card, gateway module, or teleworker server. The mobile IP softphone option can be implemented by local LAN access to the corporate WAN or a wireless dial-up option wherever the station user happens to be. Several PBX suppliers are currently marketing mobile telephone options behind their communications systems, including Avaya, Nortel Networks, and Siemens. The Avaya Definity and Nortel Networks Meridian 1 options are based on the MCK Communications Mobile EXTender gateway. The EXTender gateway is configured behind the PBX system with the use of standard digital port interface links. The Siemens solution is based on a HiPath Teleworker server linked to the HiPath PBX system. For each road warrior option, the remote desktop telephone or cellular handset appears to the PBX system as an extension of the station user’s customer premises desktop voice terminal. Any DTMF telephone can be logged into the PBX to receive and place calls through the centrally located communications system. Road warriors can use intercom; four-digit dialing plans; activate basic call processing features, such as hold, transfer, and conference; and use the private PBX network for long distance calling. Unanswered calls are routed to a call coverage station or VMS mailbox rather than the remote telephone’s mailbox.