A wireless PBX works in conjunction with an organization's current telephone system and consists of the following primary components:
Figure 1: A typical wireless PBX system in the corporate environment. In this case, workers can roam between the office and home using the same handset. When the handset moves within range of the local cellular service provider, the signal is handed off from the wireless PBX to the cellular carrier's nearest base station.
§ Adjunct switch. This is essentially an add-on switch that provides the wireless capability. It connects to the current PBX by standard twisted-pair wiring or optical fiber.
§ Base stations. These are strategically placed units that relay the signals via antennas to individual phones in a given area (i.e., cell or microcell).
§ Telephone handsets. These are portable, pocket-sized digital telephones with unique telephone numbers.
§ Distribution hub. In large installations, there may also be a distribution hub that acts as a traffic cop between multiple base stations within a single cell or microcell.
§ Management system. This provides administration capabilities as well as real-time reports that track system status, users, and traffic.
Adjunct switch
The adjunct switch contains the CPU and control logic. Its function is to manage the calls sent and received between the base stations. The adjunct is a standalone unit that can be wall-mounted for easy installation and maintenance. It can be collocated with the PBX or connected to the PBX via twisted-pair or optical fiber from several thousand feet away. Optional battery backup is usually available, permitting uninterrupted operation should a power failure occur. System control, management, and administration functions are provided through an attached terminal.
As portable telephones and base stations are added to accommodate growth, line cards are added to the PBX and radio cards are added to the adjunct switch to handle the increasing traffic load. Each adjunct is capable of supporting several hundred portable telephones.
Base stations
The antenna-equipped base stations, which are about the size of smoke detectors, are typically mounted unobtrusively on a wall or ceiling and are connected by twisted-pair wiring to the wireless PBX. Base stations can be connected up to 6000 feet from the PBX and can be powered from the PBX or by local power.
Base stations send and receive calls between the portable telephones and the adjunct unit. As the user moves from cell to cell, the base station hands off the call to the nearest base station with an idle channel. When the next base station grabs the signal, the channel of the former base station becomes idle and is free to handle another call.
To facilitate the handoff process, each base station may be equipped with dual antennas. This antenna diversity improves signal detection, enabling the handoff to occur in a timely manner. This is accomplished by the base station sampling the reception on each of its antenna and switching to the one that offers the best reception. This process is continuous, ensuring the best voice quality throughout the duration of the call. Some vendors offer optional external antennas for outdoor coverage or directional coverage indoors.
The call capacity of the base stations differs by vendor—they usually handle 12 to 18 simultaneous conversations. The maximum number of base stations in a wireless PBX system also differs by vendor. Together, the channel capacity and number of base stations determine the total number of handsets that can be supported by the wireless PBX. Lucent Technologies' high-end Definity Cellular Business System, for example, supports up to 512 users and 128 simultaneous conversations. Ericsson's Freeset system supports up to 600 wireless users and permits larger configurations by networking systems together.
Telephone handset
Each portable phone has a unique identification number that must be registered with the adjunct switch. This allows only authorized users to access the mobile communications system. The portable phone can be configured to have the same number as the user's desk phone, so when a call comes in both phones ring. The user can even start a conversation on one phone and switch to the other.
Since the adjunct switch becomes an integrated part of the company's existing telephone system, users have access to all of its features through their portable phones. The user can even set up conference calls, forward calls, and transfer calls. If the handset is equipped with a liquid crystal display (LCD), the unit can also be used to receive notification of e-mail messages, faxes, and pages. An alphanumeric display shows the name and number of the person or company calling.
The portable phone offers a number of other features, including:
§ Private directory of stored phone numbers for quick dialing
§ Multilevel last number redial
§ Audio volume, ring volume, and ring tone control
§ Visual message waiting indicator
§ Silent vibrating alert
§ Electronic keypad lock for preventing outgoing calls
§ In-range/out-of-range notifications
§ Low battery notification
§ Remote maintenance giving the manufacturer remote access to the mobile phone for diagnostic purposes
§ Liquid Crystal Display (LCD) for showing dialed digits and incoming phone numbers as well as text messages and icons
§ No service/subscription indication
§ Multiple registration with different service providers
§ Automatic selection of another subscribed system when the user goes out of range
When the portable phone is not being used, a desktop unit houses the phone and charges both an internal and a spare battery. An LED indicates when the battery is fully charged. Recharging takes only a few hours and varies according to the type of battery and model used: nickel cadmium (NiCd), nickel metal hydride (NiMH), and the newer lithium ion (Li ion). Some vendors offer an intelligent battery charging capability that protects the battery from overcharging.
Distribution hub
Distribution hubs are used in large installations. They extend and manage communication among the base units in remote locations that are ordinarily out of range of the adjunct switch. They also allow high-traffic locations to be divided into smaller cells, called microcells, with each cell containing multiple base stations. This arrangement makes more channels available to handle more calls.
The distribution units are connected to the adjunct unit with twisted-pair wiring or optical fiber. Optical fiber is an ideal medium for an in-building wireless network because its low attenuation over distance (approximately 1 decibel per kilometer) allows high-quality coverage even in large buildings and campus environments. Fiber is also immune to electromagnetic interference, allowing it to work effectively alongside other pieces of electronic equipment in installations such as factories, processing plants, and warehouses.
Management system
The wireless PBX is equipped with a management system that shows all mobile phones and their associated IDs, tracks the movement of mobile phones via polling, and shows how many calls are active for a base station. Some management systems even show the base station locations overlayed on a floor plan of the building.
The management system also issues alarms, providing information on such events as an unauthorized mobile phone, a malfunctioning base station, or loss of power on a base station. The management system generates incoming and outgoing call detail records, as well as traffic and performance data to help technicians maintain the system efficiently. The management system usually has a dial-in port for running diagnostics from a remote location, allowing technicians to quickly isolate a fault to a specific board in the system from virtually anywhere.
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1 comment:
Thanks for the information. Its really detailed and informative. Now I deeply know about the similarities and differences between PBX and Centrex.
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