Remote Port Cabinet Options

Customers who want a single PBX system supporting communications requirements across multiple physical locations can use one of many systems offering a remote port cabinet/carrier option, in addition to systems similar to those described in the preceding paragraph. Even though the Ericsson, Alcatel, Cisco, and Sphere systems have a standard single system image distributed architecture capable of supporting station users dispersed across one or more customer premises, some PBXs can support only remote communications requirements with optional hardware. The first PBX system that supported a remote off-premises port cabinet option was the Northern Telecom SL-1. The SL-1 RPE option was introduced in 1982. Since the original SL-1 option most other leading PBX suppliers have introduced their own remote cabinet/carrier options. Current remote options differ greatly across system platforms. Remote port options may include:

1. Multicarrier port cabinets designed to support up to 1,000 ports

2. Single carrier cabinets designed to support about 300 ports

3. Half-shelf carrier cabinets designed to support about 150 ports

4. Single port interface circuit boards designed to support 24 or 36 ports

Customers with significant remote port requirements would benefit from a system that supports a remote equipment option minimizing the number of cabinets/carriers needed at the remote location, instead of con- figuring 10 or 20 remote interface boards, each with limited port capacities. However, a large remote equipment cabinet would be very expensive if the number of remote ports is limited. Growth requirements at the remote location must be taken into consideration, and the right remote equipment hardware should be selected at the initial installation.

There are several traditional design guideline issues for remote port configurations:

1. Distance between the main and the remote locations

2. Available transmission carrier facilities

3. Traffic engineering requirements

4. Available trunk access at remote location

5. Survivability requirements

Most customer configurations, including a remote port cabinet/carrier, are within a local or regional geographic area. There are few, if any, remote configurations that are transcontinental. PBX remote options using PSTN T1-carrier transmission services can support customer configurations of several hundreds of miles between the main and remote locations. The Avaya Definity ECS remote option over copper T1-carrier is available at distances of up to 3,500 miles, depending on the interexchange carrier. Several PBX systems support remote cabinets with fiber optic cabling, without repeaters, at distances between 6 miles (Nortel Networks Meridian 1, Mitel SX-2000 Light) and 22 miles (Avaya Definity ECS). Fiber optic transmission is not commonly available from exchange carriers and is used mainly for campus settings or when right of way is available.

A benefit of using fiber optic cabling to support remote locations is the high bandwidth capacity of the link. Using a fiber optic link for the Definity or Meridian 1 systems provides the same number of traffic channels between the local TDM bus and the center stage switching complex as the link available for single premises configurations. Using a T1-carrier link limits traffic channels between the locations. Although a T1-carrier circuit using DS1 formatting can support up to 24 voice-grade channels, some of the channels used for the remote option must be reserved for control signaling. Most remote options can be configured with more than one T1-carrier circuit to provide sufficient communications channel capacity. For example, the Definity ECS can support between one and four T1-carrier circuits per remote EPN cabinet, to a maximum of 96 channels. In a maximum T1-carrier implementation, four of the channels are reserved for control signaling, leaving 92 two- way channels for intercabinet voice communications. The dispersed switch network design of the Definity ECS G3r model supports local switching at the remote location, thereby minimizing the number of required center stage switch connections at the main location. The Meridian 1 can support up to three T1-carrier circuits for each remote IPEM cabinet, with 22 available one-way channels per link for all remote location call connections. The centralized switch network design of the intermediate and large Meridian 1 options requires that all remote calls be connected through the center stage switching complex at the main location, thus placing a significant traffic burden on the limited T1-carrier spans. The Siemens Hicom 300H remote communications module (RCM) option is supported with only one or two T1-carrier spans: two channels per span are reserved for control signaling, and the remaining 22 communications channels are one way. Like the Meridian 1, the Hicom 300H centralized switching design requires all remote calls be connected through the main location center stage switch.

The limitations of T1-carrier channel capacity require that traffic engineering support acceptable QoS standards. The number of available communications channels, one- or two-way, will depend on the number of remote location ports. The Definity EPN cabinet can easily support more than 1,000 ports (stations and trunks) at moderate traffic capacity, whereas Avaya usually recommends an 800 station limit. Customers must carefully analyze traffic flow to decide whether one, two, three, or four T1-carrier circuits are needed to support traffic between the remote EPN location and the main or other remote locations. The Nortel and Siemens solutions support fewer remote port capacities per cabinet carrier, but there are far fewer communications channels, and all remote call connections, including direct trunk access at the remote location, require two one-way channels for center stage switch connections.

It is desirable for station users at the remote location to have direct access to local exchange carrier trunk circuits terminated at the remote cabinet/carrier. There are many instances when local area calls placed by station users at the remote customer location would become long distance calls if routed to trunk circuits at the main PBX system. In contrast, it is usually preferable for PSTN long distance or private network calls placed at the remote location to route through the main location, where most trunks are concentrated and trunk access is more highly engineered.

If the control signaling link between the main and remote locations is down or there are transmission errors, the call processing and switching functions at the remote site will be affected, unless there are redundant design elements. The most basic redundant design option is duplication of the T1-carrier or optical fiber links connecting the main and remote locations. Almost all systems with remote port options are available with this option, although some systems have a greater degree of redundancy than others. For example, each of the T1-carrier links supporting the remote Definity EPN cabinets can be fully duplicated; the Meridian 1 design supports two active T1-carrier links and a spare T1-carrier link for emergency backup.

If redundant links are not an option or not configured, another redundant capability is local call control processing and switching capabilities at the remote location. Remote location survivability is an important issue for most customers. For example, there are many customers who support distributed call center operations across locations on a single system platform. Loss of an ACD agent group for a few minutes or hours could translate into significant revenue losses, in addition to reduced customer service levels. If a remote location is strategic to the enterprise operation, available redundant processing must be considered. Intecom, the first company to design a fully distributed PBX cabinet design, also introduced the first survivable remote cabinet option in the mid-1980s. Several other system suppliers have since shipped local call control processing options for their remote port cabinets for emergency situations when the control link to the main common control complex is not available.

Manufacturers of traditional circuit switched PBX systems have also taken advantage of ToIP technology by offering remote carrier options supported by IP-based transmission links. Avaya’s R300 Remote Office Communicator and Nortel’s i9150 remote options are designed to provide communications support for remote branch offices working behind a larger main location with a centralized PBX system. The R300, based on an Ascend Communications remote access concentrator, at the remote location can support analog, digital, and IP telephones and digital and analog local trunk circuit connections. The R300 is supported over an IP link supported at the main PBX location by Definity IP Media Processing and CLAN circuit boards. The Nortel remote cabinet shelf supports standard Meridian 1 peripherals and has local switching capability. A Meridian 1 supports the IP-connected i9150 using an integrated telephony gateway circuit board. A major benefit of the i9150 is that it has an integrated processor board providing backup call processing should there be link failure to the main location. The Avaya and Nortel solutions are targeted at customers with existing LAN/WAN infrastructures across customer premises locations.