Compatibility and Interoperability | Wireless PBX

Wireless PBX technology has its roots in standards that were developed in Europe in the 1980s; specifically, the Cordless Telecommunications (CT) series of standards and the Digital Enhanced Cordless Telecommunications (DECT) standard. While home cordless is seen as the main new area of DECT development, it is probably in the business arena that the technology has had its biggest impact—the replacement of existing fixed line PBX systems to enable cordless mobility within a building or campus. The North American standard for wireless office communication is known as Personal Wireless Telecommunications (PWT), which is based on DECT.

Cordless telecommunications

CT0 and CT1 were the technologies for first-generation analog cordless telephones in Europe. Comprising base station, charger, and handset, they first became widely available for residential use in the early 1980s. With a range of 100–200 meters, they used analog radio transmission on two separate channels, one to transmit and one to receive speech. However, the limited number of frequencies often resulted in interference between handsets, even in environments with relatively low subscriber densities.

Also targeted at the residential user, CT2 is an improved version of CT0/CT1. Operating in the 800- to 900-MHz frequency band, CT2 uses an FDMA (Frequency Division Multiple Access) format. The CT2 system creates capacity by splitting bandwidth into radio channels in the frequency domain. In the initial call setup, the handset will scan the available channels and lock onto an unoccupied channel for the duration of the call. A major difference between CT1/CT0 and CT2 systems is that the latter uses digital transmission between the base station and handset. Using a technology called Time Division Duplexing (TDD), the call is split into time blocks that alternate between transmitting and receiving.

In the United States, CT2 technology standards support two-way calling, but within a limited area. CT2-based systems do not support extremely high-density user station requirements. The technology uses 4 MHz of bandwidth and dynamic channel allocation between base stations and handsets and is based on FDMA for carrier access. The CT2 Common Air Interface (CAI) is relatively inexpensive to implement compared to other standards.

An improved version of the CT2 CAI is CT2+, which uses 8 MHz of bandwidth, dynamic channel allocation, and a common signaling channel. The ultimate benefits of a common signaling channel are faster call setup times (improved base station recognition), increased traffic capacity (the signaling channel is independent of traffic channels), and improved portable telephone battery charge life (the terminal looks at the paging channel only). CT2+ also supports encryption to ensure secure communication.

Digital enhanced cordless telecommunications

DECT is a standard for digital cordless systems developed in Europe in the 1980s and formally ratified in 1992. It operates in the 1880- to 1900-MHz frequency band in Europe. DECT has already replaced the earlier CT1 standard for personal cordless telephony, and is now poised to replace the CT2 standard in many countries. DECT is now part of a dual-mode DECT-GSM capability that enables seamless roaming between indoor DECT-compliant systems and the wide-area GSM network. The major difference between the two standards is that GSM is a fully featured network specification, whereas DECT is an access standard which defines the interface between a mobile cordless terminal and a base station.

DECT was developed to support numerous applications including residential use, wireless PBX, wireless LAN, and public access via radio in the local loop. DECT supports ISDN as well as a wide range of services, including two-wire analog telephony, Group 3 and 4 facsimile, videotext, voice messaging, basic rate ISDN, digital telephony, and X.25 data communications. The DECT standard specifies 32-Kbps speech coding which provides speech quality at wireline standards.

It is designed for extremely high capacity and is able to support in excess of 50,000 users per square kilometer. Its data transmission capability is flexible and ranges from 24 to 502 Kbps. A system of dynamic channel allocation gives high flexibility and increased capacity and the standard is able to support seamless handover between contiguous cells.

There are five main areas where DECT is seen as having a major role:

The office. The major demand for cordless systems will continue to be from the office market—a European Commission estimate suggests that as many as 30 percent of all office telephones behind a PBX in Europe will be cordless by the year 2000.

The home. DECT cordless telephones in the home provide users with high speech quality, secure communications, and a range of useful features such as intercom and calling line identification.

Public access networks. The openness of the DECT standards enables them to be expanded to public service areas such as city centers, campuses, and airports. DECT offers continuous coverage, with seamless handover between cells and two-way communications within its service area. DECT's strengths of high user density, small cell size, and lack of requirement for frequency or cell planning make it an ideal solution for public access networks.

Local area networks. Cordless Local Area Networks (CLANs) reduce the need for obtrusive and expensive cabling. The cordless LAN is also a step toward true personal networking. DECT has a number of benefits in the cordless LAN environment. It is designed to provide full, blanket coverage of an area, through multiple overlapping cells. This guarantees redundancy and a high grade of service. Seamless handover and full mobility are ensured across the whole local area. DECT systems can have an overall system throughput capability of 10 Mbps with up to 552-Kbps performance on each terminal. DECT is fully compatible with all existing wired LAN protocols—an important feature in ensuring smooth interworking between office systems.

The local loop. Because traditional methods of laying copper cable pairs to each subscriber are slow and costly, operators are increasingly turning to radio as the replacement for copper in the local loop. Wireless systems can be rapidly deployed—in days and weeks—compared to the months that it takes to install wired systems. Here, DECT offers PSTN speech quality, comparable functionality, and secure speech via encryption. DECT can also support a very high density of users, making it a better choice for urban radio local loop situations and, as an access standard, it can interface with the PSTN and support ISDN.

Personal wireless telecommunications

The Telecommunications Industry Association (TIA) and the American National Standards Institute (ANSI) have completed a North American standard that ensures interoperability between portable phones and wireless PBXs from different vendors. The TIA TR41.6.1 Subcommittee based its development of the standard, called Personal Wireless Telecommunications (PWT), on the Digital European Cordless Telecommunications (DECT) standard. Portable phones that support PWT, formerly known as the Wireless Customer Premises Equipment standard, will interoperate with PWT-compliant wireless PBXs from any vendor.

For a wireless handset to communicate with any wireless PBX, manufacturers of both devices must agree on how the signal should be handled. As part of the PWT standard, the Customer Premises Access Profile defines the features that each side of the air interface must support to provide full, multivendor interoperability for voice services. As with most standards, vendors can add proprietary extensions to support additional features and differentiate their products.

The air interface is a layered protocol, similar to the International Organization for Standardization's (ISO's) Open Systems Interconnection (OSI) architecture. It is composed of four protocol layers:

§  Physical layer. This protocol includes radio characteristics such as channel frequencies and widths, the modulation scheme, and power and sensitivity levels. This layer also specifies the framing, so each handset can translate the bit it receives.

§  Media access control (MAC) layer. This protocol specifies the procedures by which the portable phone and the base station, or antenna, negotiate the selection of the radio channels.

§  Data Link Control (DLC) layer. This protocol is responsible for the sequence and integrity of frames transmitted between the handset and the base station.

§  Network layer. This protocol encompasses messages that identify and authenticate the handset to the wireless PBX.

The operation of these protocols can be illustrated by examining the handoff from one base station to another. A handoff occurs when the mobile user wanders out of the range of one base station and into the zone or cell of another base station. When the handset detects a change of signal strength from strong to weak, it will attempt to get acceptable signal strength from another channel offered by the same base station. If there is a better channel available, an exchange of messages at the MAC level occurs, which allows the conversation to continue without interruption. This channel change takes place without notification to the DLC layer.

If an acceptable channel is not available to the current base station, the handset searches for another base station. An exchange of messages at the DLC and MAC layers secures a data link via a radio channel to the new base station while the call through the original base station continues. When the data link to the second base station is established, the handset drops the old channel and begins processing the frames received through the new one. This process occurs without the network layer being notified. This means the caller and the wireless PBX are not aware that a handoff has happened.

Among the major supporters of PWT are Lucent, Northern Telecom, Siemens Rolm, Mitel, Ericsson, Motorola, and Nokia. Because changes from the DECT standard have been minimal, manufacturers of DECT equipment are able to provide PWT-compliant equipment at a reasonable cost. Even if some vendors may not have achieved full PWT compliance in their early products, all that is needed is a firmware upgrade or the exchange of an Application-Specific Integrated Circuit (ASIC) to bring a system into full compliance.

Cost Containment | Wireless PBXs

This merging of home, mobile, and work communications—while convenient to the user—could result in a dilemma for companies that must decide whether to foot the bill for home equipment and fees that make workers more available. However, companies that continue to cut staff to improve the bottom line and stay competitive in world markets may find that picking up such costs is a bargain. To meet the need for cost containment, there are many useful features that have become available.

To help contain the cost of mobile communications, some wireless PBXs offer call control features that include call barring to restrict calls on certain phone numbers, and itemized billing to help apportion call charges. Different types of users can be given different account codes. It is also possible for organizations to identify personal calls and agree with staff that they may receive or make calls from those numbers on their work phone, with the additional costs billed to them personally.

Phone usage can also be controlled in much the same way as with a traditional business telephone system, with features like identified extensions, call screening, and re-routing. Depending on vendor, the phones can also incorporate real-time charging advice, informing the caller whether or not they are in the office zone, which may be charged at a cheaper rate. For example, in-house calls might be charged at a very low rate, other "internal" calls may be billed at an intermediate rate, and external calls charged at the national or international rate, as appropriate.

Integrated Services and Systems | Wireless PBXs

Several carriers are starting to integrate billing for campus-area and wide-area cellular services. Using Motorola's InReach radio system, for example, carriers such as Bell Atlantic and NYNEX (recently merged into a new company called, The New Bell Atlantic) have the ability to map the four- or five-digit PBX or Centrex extension to users' cellular phones. Cellular calls made on the company's premises are billed at whatever discount rates the company may have negotiated with the local telephone carrier for wireline services. When cellular calls are made off-campus, billing shifts to the prevailing cellular rate. This arrangement allows users to have a single phone number that follows them around throughout the workday. It also gives them the ability to call or be called by others in the office using just assigned extension numbers, regardless of their location at any given moment. The company benefits by obtaining lower cellular rates for calls made on-campus.

It is also possible to use existing cellular telephones with a wireless PBX system. Lucent Technologies' Definity Cellular Business System, for example, enables in-building and out-of-building mobility with the same Advanced Mobile Phone System (AMPS) cellular handset. For in-building calling, the IS-94 standard dual-mode cellular handset registers itself with an in-building base station and takes its commands from the wireless PBX. For out-of-building calling, the handset registers with the nearest public cell site transceiver.

In another example of system-service integration, AT&T Wireless allows users of Nortel's Companion Microcellular System to use the same handset for wide-area communication via its Digital PCS Wireless Service. The blending of the Companion Microcellular system with AT&T Digital PCS provides digital mobility services for complete enterprise accessibility, from the manufacturing floor, to the office, or on the road anywhere in the North American cellular network.

Users of AT&T Digital PCS with the Companion Microcellular receive unlimited wireless service at their facility or campus for a flat, low monthly rate. On site, they also have access to popular PBX features such as calling line ID, message waiting indication, call transfer, and conference capabilities. Off site, they have access to caller ID, AT&T VoiceMail with message waiting indicator, AT&T PCS Messaging (alphanumeric and numeric), and authentication for transmission security and fraud protection. Digital PCS handsets have about three times the battery operating duration of existing cellular telephones.

Each Companion Microcellular system can support up to 1500 users and covers up to 10 million square feet of area. Multiple systems can be linked to serve locations with more users or larger areas and also can be linked into private wide-area networks connecting multiple locations, providing employees with total mobility.

Companion Microcellular systems connected in a network of Meridian 1 systems can automatically register authorized Digital PCS handsets. For example, calls to an extension in a Seattle office equipped with a Companion Microcellular system can be routed automatically to the person with that extension when he or she is in a Dallas facility connected to the private network. Additionally, the person can make outbound calls and use the features of the networked PBX system in Dallas from the Digital PCS handset. While on the road between these two offices, the person continues to receive AT&T Digital PCS features.

There is always the chance that in-building calls will be captured by an outside tower, but in most such cases, the PBX dialing scheme will not work, alerting the caller that the signal was captured by the public network. This and other problems can be prevented by using radio frequency design tools to properly engineer the corporate network.

One such tool, available from Lucent Technologies, is a computer-aided design program called the WiSE (Wireless Systems Engineering) Expert Design System. It creates a model of the customer's premises to pinpoint ideal locations for wireless transmitters by considering all the necessary variables like building dimensions, construction materials, environmental factors, numbers of users, and "traffic" projections, when designing a wireless system.

The service integration concept has been carried a step further by AT&T Wireless. In select markets, the carrier offers its cellular customers the option of adding a personal base station for the home. For a flat monthly fee of $10 (plus the cost of the base station), customers can use the same cellular phone at home, on the road, and at the office. When users take their phones on the road, calls are billed at regular cellular rates. When they arrive at work, intelligence built into the network launches a cellular connection to their wireless PBX. At that point, calls are billed at the corporate rate for conventional wireline service.

System Components | Wireless PBXs

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.

Figure 7.1: 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.