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Guidelines for planning wireless communications.

If you haven't yet read about wireless LANs, PBXs and similar products, you've probably been working on Mars. Currently, one of the hottest topics on the telecomm scene is "wireless" communications.

Assuming you've been inundated with stuff about wireless PBXs and LANs, let's move on to more practical things. What must you do to ensure a smooth cutover? This month's column is the first of a two-parter on wiring. Today we'll discuss planning guidelines for wireless PBX adjuncts. Next month we'll examine issues associated with wire-based installations.

To help us with this column, we spoke with Tom Ohlsson, product marketing manager with SpectraLink Corp., Boulder, Colo. SpectraLink is one of the leading vendors of wireless PBX adjunct systems.

Wireless adjuncts are microcell-based systems that use geographically dispersed base stations. Each coverage area is defined as a cell. Each cell supports "local" users and hands off other users moving between cells.

Three components make up a wireless adjunct. First is the control unit, usually called the "radio gateway," which acts as a gateway between the PBX and wireless system. Spectra-Link's product is called the Master Control Unit (MCU).

"This component is not really a switch," Ohlsson says. "Rather, it sets up call paths for each call, and then reuses those paths for subsequent calls."

The second component is called a base station.

"That's the area where manufacturers use different technologies," Ohlsson tells us. "We use TDMA (time division multiple access) to separate conversations within a base station and CDMA (code division multiple access) to separate the base stations."

User access to the system is provided by the telset, which SpectraLink calls its Pocket Telephone, or PT. Communications from telset to RCU use the 902-928 MHz frequency range, currently designated "non-licensed" by the Federal Communications Commission.

"Our system is covered under FCC Part 15 regulations because we use spread spectrum technology in approved frequency ranges," Ohlsson says. "This is important to users because they don't have to obtain a license to use the equipment."

Installation of wireless components begins with the system controller, which is located in a switch room or wiring closet. Check to see what kind of interfaces are required to the host PBX. Analog two-wire loop interfaces are the most familiar and easiest to install. If the PBX is connectorized in the back, make sure the wireless controller supports the same connectors for ease of installation. SpectraLink's MCU can have circuit boards (other than the CPU) replaced without shutting down the system.

Remote controllers should also require standardized wiring arrangements, rather than proprietary schemes. It's also a good idea to leave an extra 50 feet of cable at RCU locations in case the unit needs to be repositioned.

Consider power and cabling requirements at the outset. It is more difficult to install systems that require AC at the cell site or that use multiple-pair cables. In SpectraLink's case, RCUs are powered by the MCU (or by 110 VAC). "This is an important consideration for users because they can use existing pairs that are already available in the building," Ohlsson says. MCUs are powered by a separately fused 110 VAC outlet. Uninterruptible power systems (UPS) can also be used with MCUs.

Remote base stations can be powered either locally or via twisted-pair wiring. In SpectraLink's case, RCUs are powered by the MCU via twisted-pair wiring. If home-run lines are used, the maximum distance using 22 gauge wire is 6,000 feet. However, in a multi-story building where a large number of intermediate distribution frames (IDF) are used, this distance may be reduced.

Determine the effective radius of each RCU. In outdoor applications, an RCU's radius can exceed 600 ft., but inside a building, with transmission impacted by walls and ceilings, its effective radius is 150 to 200 ft. Transmission is often affected by other items such as metal studs within walls, elevator shafts, or tall file cabinets. In a typical office environment, the radius is 50-100 feet, but can be lower in buildings that use concrete construction in the walls (e.g., hospitals). Coverage in an office area will range from 5,000 to 25,000 square feet per RCU.

Check to see if the equipment is certified for operation by the FCC, particularly under Part 15 rules. Try to avoid products that are available under FCC "experimental" licenses because once the trial period is over, the equipment may have to be removed.

Determine if any environmental provisions are needed. "In most cases, system specs should be comparable to PBX systems," Ohlsson says. "Make sure the equipment is UL listed, and installed in a protective cabinet."

When configuring the system, see how the wireless product interfaces with PBX station user functions.

Once the wireless system is installed, training becomes a key issue. Most cellular phones have SEND and END keys. The same is true with wireless telsets. Users must remember to depress END when calls are completed. Users must also replace telsets in the charging stand at day's end.
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Author:Kirvan, Paul
Publication:Communications News
Date:Jul 1, 1993
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