Personal Access Communications Systems (PACS) is a standard adopted by ANSI for Personal Communication Services (PCS). Adopted in June 1995, PACS provides an approach for implementing PCS in North America that is fully compatible with the local exchange telephone network and interoperable with existing cellular systems.
Based on the Personal Handyphone System (PHS) developed in Japan and the Wireless Access Communications System (WACS) developed by Bellcore (now known as Telcordia Technologies), PACS is designed to support mobile and fixed applications in the 1900-MHz frequency range. It promises low installation and operating costs while providing very-high-quality voice and data services.
In the United States, trials of PACS equipment began in 1995, and equipment rollout began in 1996. Most of the standards—including up-banded versions of CDMA, TDMA, and GSM—look like cellular systems in that they have high transmit powers and receivers designed for the large delay spreads of the macrocellular environment and typically use low-bit-rate voice coders (vocoders).
PACS fills the niche between these classes of systems, providing high-quality services, high data capability, and high user density in indoor and outdoor microcellular environments. PACS equipment is simpler and less costly than macrocellular systems yet more robust than indoor systems. PACS capabilities include pedestrian- and vehicularspeed mobility, data services, and licensed and unlicensed spectrum systems, as well as simplified network provisioning, maintenance, and administration.
The key features of PACS are summarized as follows:
- Voice and data services are comparable in quality, reliability, and security with wire-line alternatives.
- Systems are optimized to provide service to the in-building, pedestrian, and city traffic operating environments.
- It is most cost-effective to serve high-density traffic areas.
- Small, inexpensive line-powered radios provide for unobtrusive pole or wall mounting.
- There is low-complexity per-circuit signal processing.
- Low transmit power and efficient sleep mode require only small batteries to power portable subscriber units for hours of talk time and multiple days of standby time.
Like the Personal Handyphone System (PHS), PACS uses 32-kbps Adaptive Differential Pulse Code Modulation (ADPCM) waveform encoding, which provides near landline voice quality. ADPCM has demonstrated a high degree of tolerance to the cascading of vocoders, as experienced when a mobile subscriber calls a voice-mail system and the mailbox owner retrieves the message from a mobile phone. With other mobile technologies, the playback quality is noticeably diminished.
With PACS, it is very clear. Similarly, the compounding of delays in mobile to PCS through satellite calls—a routine situation in Alaska and in many developing countries—can be troublesome. PACS provides extremely low delay. The low complexity and transmit power of PACS yield limited cell sizes, which makes it well suited for urban and suburban applications where user density is high.
Antennas can be installed inconspicuously, piggybacking on existing structures. This avoids the high costs and delays associated with obtaining permits for the construction of high towers.
Wireless local loop, pedestrian venues, commuting routes, and indoor wireless are typical PACS applications. Additionally, PACS is designed to offer high-capacity, superior voice quality and Integrated Services Digital Network (ISDN) data services. Interoperability with ISDN is provided by aggregating two 32-kbps time slots to form a single higher-speed 64-kbps channel. A64-kbps channel also can support 28.8-kbps voice-band data using existing modems.
PACS also can be used for providing wireless access to the Internet. The packet data communications capabilities defined in the PACS standards, together with the ability to aggregate multiple 32-kbps channels, make it possible for users to access the Internet from their PCs equipped with suitable wireless modems at speeds of up to 200 kbps. When using the packet mode of PACS for Internet traffic, radio channels are not dedicated to users while they are on active Internet sessions, which can be very long.
Rather, radio resources are used only when data are actually being sent or received, resulting in very efficient operation and minimally impacting the capacity of the PACS network to support voice communications. PACS was designed to support the full range of advanced intelligent network (AIN) services, including custom calling features and personal mobility. As new AIN features are developed, the PACS-compliant technology will evolve to facilitate incorporation of the new services.
The market for PCS is very competitive. Already PCS is exerting downward price pressure on traditional analog cellular services where the two compete side by side. PACS enables PCS operators to differentiate their offerings through digital voice clarity, high-bit-rate data communications, and advanced intelligent network services—all in a lightweight handset. Moreover, the cost savings and ease of use associated with PACS make it very economical for residential and business environments compared to competitive high-powered wide area systems.