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.
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.
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.
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.
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