Wireless & Other PBX Solutions

Wireless PBX Solutions

Several solutions for adding wireless extensions to PBX systems have been commercialized. Most PBX vendors have implemented proprietary 900 MHz-band solutions in the United States as well as the 1900 MHz Digital Enhanced Cordless Telecommunications (DECT) ETSI standard in Europe, which has driven widespread adoptions of vendor-neutral wireless there. More recently, a number of WiFi solutions have become available, as well as combination WiFi/GSM solutions that let a single device work with both Cellular and Enterprise PBX infrastructure. 

Other PBX Solutions

Two other PBX solutions with security considerations bear some discussion: Call Detail Recording (CDR) systems and Voice Firewalls. CDR systems enable every call on a PBX to be recorded after it is complete using a standardized format. This allows special reporting software to analyze this data for forensic or diagnostic purposes. It is worth noting, however, that a CDR system will not allow you to stop a fraudulent call still in progress. For this, you would need a voice firewall such as that sold by SecureLogix. Such a firewall allows you to see current calls in real-time, apply policy based on type of call (voice, fax, or data), and set notifications, authentication requirements, or other policy based on rules very similar to those you might set for data traffic on a data firewall.

All PBX systems provide PSTN-like switching services between endpoints and adjuncts, the PSTN, and other private PBX switches (and associated private networks). Only a few of the possible adjunct systems are mentioned here. An ACD is an Automatic Call Distribution server (for use in call centers to direct calls to groups of agents), and an IVR is an Interactive Voice Response server (also commonly used in call centers to let callers use touch tones and voice prompts to select services).

So a PBX could be all IP or all analog or anything in the middle as long as it switches calls between extensions and the PSTN as needed. In the end you will find that despite the marketing hype, most VoIP systems are just PBX systems with different combinations of support for IP lines and trunks. In some cases, the call control part of the system is split out from the gateway that handles the non-IP electrical interfaces. Or it’s pushed out to a service provider. But the basic switching concept is preserved somewhere across the system as a whole. Regardless, understanding basic PBX terminology will help you understand the underlying architecture of the VoIP systems you may encounter, so let’s start there.

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.

Organizational Factors | Wireless PBXs

A tendency among many companies is that they change frequently in both structure and function. In fact, continuous change is looked upon as good because it keeps the organization in step with changing customer needs and market directions. Wireless PBX communications systems can go a long way in support of such dynamic work environments.

Other companies are reluctant to experiment with more flexible organizational structures because of the perceived lack of communication that might result. With too many people away from their desks, chaos would surely reign. Wireless technology can break down this resistance to change by ensuring access to all people and all information, regardless of their in-building location.

With regard to work processes and organizational structure, the types of situations that lend themselves to the adoption of wireless PBX technology are those:

§  Where the business function is fluid and workers are mobile much of the time.

§  Where mobility presently inhibits the quality and timeliness of responses or transaction completion, which often results in missed opportunities, deadlines, or objectives.

§  Where the time spent in setting up temporary network connections or the delay in returning missed calls can be dramatically reduced.

§  Where organizational structure is flat rather than hierarchical and, as a consequence, more workers have decision-making authority, flexible work schedules, and job autonomy.

There is also a psychological impact to consider. In today's downsized work environment, where fewer employees are expected to do more, stress levels have never been higher. Employees are most likely to experience frustration and stress when they cannot reach someone they are trying to contact, especially when the people they need to talk with affects their ability to get the job done. A wireless PBX can greatly improve the availability of employees and result in a less stressful and more productive workplace.

More than productivity is at stake. Business can be lost as well. If customers cannot reach their contacts within the organization in a timely manner, they may look elsewhere to fulfill their needs. With customer response becoming a key differentiator in the service sector of the economy, a wireless PBX can play a key role in cementing customer loyalty.

Despite the apparent advantages promised by intra-company mobile communications, it is important to understand that the use of unlicensed spectrum for wireless PBXs does not guarantee the same reliability and availability as wireline systems. Currently, unlicensed devices must accept any interference received, potentially impairing their use. For this reason, some vendors of wireless equipment are asking the FCC for a new class of unlicensed service that would have protection against harmful interference. This class would retain the cost advantages and deployment simplicity of the current applications and, in addition, have protection against harmful interference from services in other bands, and services sharing the same band. Unlicensed operation in exclusive-use frequency allocations, as well as those designated primary in a band, would be eligible for this class. At this writing, the FCC is still considering whether a new unlicensed class of service in the 1920-MHz PCS band is warranted.

Wireless PBXs

Wireless phones have become a common sight in recent years, with cordless handsets in use in the home and cell phones mounted in vehicles and carried around in purses and briefcases. Lower-cost handsets are more portable, offer better connections and wider coverage, and longer battery life. All this, plus improved service quality, have led to millions of people taking advantage of mobile communications. The success of mobile communications has led to a revolution in office communications.

With office workers spending increasing amounts of time away from their desks—supervising various projects, working at temporary assignments, attending meetings, and just walking corridors—there is a growing need for wireless technology to help them stay in touch with colleagues, customers, and suppliers. According to various industry estimates, as much as 75 percent of all workers are mobile, spending significant amounts of time away from their desks. This lack of communication can adversely impact personal and organizational performance. The idea behind the wireless PBX is to facilitate communication within the office environment, enabling employees to be as productive while moving about as they would if they were sitting at their desk.

Almost any organization can benefit from improved communications offered by a wireless PBX system, especially those engaged in:

§  Manufacturing. Roving plant managers or factory foremen do not have to leave their inspection or supervisory tasks to take important calls.

§  Retail. Customers can contact in-store managers directly, eliminating noisy paging systems.

§  Hospitality. Hotel event staff can stay informed of guest needs and respond immediately.

§  Security. Guards can relay emergency information quickly and clearly, directly to the control room or police department without trying to reach a desktop phone.

§  Business. Visiting vendors or customers have immediate usage of preassigned phones without having to borrow employee desktop phones.

§  Government. In-demand office managers can be available at all times for instant decision making.

Wireless PBX technology differs from other forms of mobile communications. Unlike cellular phone service, for example, there is no charge for air time. While cellular is a high-powered system designed primarily for high-speed use in cars, wireless PBX is used indoors and the mobile phones get better reception and longer battery life—up to 6 hours of talk time or 60 hours of standby time on a single charge.

When fully integrated into an existing PBX or Centrex system, all of the special call handling features are preserved. With Ericsson's Freeset system, for example, the wireless capability can be added to virtually any key, hybrid, PBX, or Centrex system to provide an integrated facility running both wired and wireless extensions. Mobile users have access to the same functions and features of the host system to which they are ultimately connected via the adjunct switch, such as caller identification, extension dialing, speed dialing, conferencing, hold, transfer, programmable buttons, and voice messaging. Another feature, call screening, gives users the control to effectively screen all calls before deciding to answer, or to distinguish an important call from one that is not important. The handset's built-in display screen shows the caller's name or number on incoming calls.

Some vendors offer applications development toolkits to bring value-added features to their wireless PBX systems. Nortel, for example, offers an open interface for wireless communications and computer-telephony integration (CTI) via its Companion Applications Toolkit. Using Nortel's Companion Applications Toolkit, developers can write PC applications such as database queries and fax and e-mail interfaces that mobile workers can access via their Companion portable telephones. An application can be initiated from either the portable telephone or a desktop PC. More than 100 PC-based applications can interact with one or more portable phone sets and all applications can reside on a single PC.

Under ideal conditions, mobile users have the same digital voice quality as conventional PBX users. Even the charges for outgoing calls are integrated into the same billing process as desktop phones. Wireless PBX also provides seamless communication throughout the building, including hard-to-reach places like elevators, tunnels, or parking garages.

Wireless PBX operates in a variety of frequency bands, including the unlicensed 1910- to 1930-MHz Personal Communications Services (PCS) band. The term "unlicensed" refers to the spectrum that is used with equipment which can be bought and deployed without FCC approval because it is not part of the public radio spectrum. In other words, since wireless PBX operates over a dedicated frequency for communication within a very narrow geographical area, it has little chance of interfering with other wireless services in the surrounding area. Consequently, there is no need for frequency coordination or FCC licensing. The individual channels supported by the wireless PBX system are spaced far enough apart to prevent interference with each other. Privacy is enhanced through the use of digital speech encoding and continuous frequency hopping to make eavesdropping nearly impossible.

The capacity of wireless PBX systems is easily expanded—portable telephones and base stations are added as needed—up to the maximum capacity offered by the vendor's particular system. Likewise, the coverage of wireless PBX systems can be expanded through the strategic placement of base stations and distribution hubs. While single-zone coverage up to 500,000 square feet effectively covers most office buildings, the addition of base stations and distribution hubs can achieve multizone coverage of up to 12 million square feet for a campus environment.

Substantial savings can accrue over time through the elimination of traditional phone moves, adds, changes. There is also significant savings in cabling, since there is less need to rewire offices and other locations for desktop telephones. This is significant because companies typically spend between 10 and 20 percent of the original cost of their PBX annually on reconfiguring the system.