Personal Air Communications Technology

Personal Air Communications Technology (pACT) is a wireless two-way messaging and paging technology that is offered as an alternative to wireless Internet Protocol (IP), also known as Cellular Digital Packet Data (CDPD). The pACT specification, released in 1995, was developed to enable compact, inexpensive devices to access low-cost, highcapacity network infrastructures.

The protocol enhances one-way paging, response paging, two-way paging, voice paging, telemetry, and two-way messaging applications. The pACT protocol thus addresses the demands of a growing market for narrowband PCS. Despite huge investments in spectrum for narrowband PCS, some U.S. paging carriers are already running short of bandwidth. This situation has prompted them to look for ways to add capacity to their networks.

In major U.S. cities, the solution has been to make better use of existing spectrum. pACT supports two-way messaging and paging applications while still retaining all the strengths of one-way paging services, including long battery life, good in-building reception, and ubiquitous coverage. pACT also provides carriers with the ability to substantially increase system capacity by more efficiently utilizing spectrum, thereby allowing for more cost-effective paging and messaging services.

Its cellular-like network design enables carriers to take advantage of capacity gains through frequency reuse, a fundamental difference from other two-way systems. The difference between wireless IP and pACT is that the former is TCP/IP-centric, whereas the latter is User Datagram Protocol/Internet Protocol (UDP/IP)–centric.

While wireless IP compresses the standard 40-byte TCP/IP header to an average of 3 bytes to conserve bandwidth, pACT compresses the header to only 1 byte. The greater compression is important for providing a short alphanumeric messaging service, especially when the message body is roughly the same size as the header. Consequently, the maximum number of subscribers that can be supported by the system is constrained not by protocol efficiency but by service traffic.


Two-way wireless messaging is working its way into a myriad of user applications where one-way messaging is no longer adequate. These are applications where time is of the essence, and guaranteed message delivery is critical. pACT uses the same upper layers of the protocol stack as wireless IP, making it suitable for a broad range of messaging applications, including two-way-paging, e-mail, fleet dispatch, telemetry, transaction processing (e.g., point-of-sale credit card authorizations), and voice messaging.

Since pACT is based on the IP, it provides wireless network users with access to other IP-based networks such as corporate local area networks (LANs) and the Internet from remote locations. Network Capacity Narrowband PCS and two-way protocols such as pACT give the paging industry new ways to increase the capacity of one-way paging systems, which introduce opportunities for providing new and enhanced services.

Like traditional mobile telephony systems, pACT increases capacity by reusing frequencies—theoretically providing unlimited capacity. Given higher airlink speeds (8 kbps) and knowing a subscriber’s exact location, operators can increase capacity in a large zone by 100 to 200 times and even more in networks that are made up of several zones. While one-way paging over a two-way network offers no real benefits to subscribers, the benefits to operators are substantial.

First, two-way networks enable paging devices to acknowledge that a message has been received. With this capability, operators can offer subscribers guaranteed delivery, which can result in competitive advantage. The second benefit to operators is that they do not have to broadcast a message via every transmitter in the network to reach an intended recipient.

With a two-way network, the exact location of each subscriber device is known because the devices register automatically as they move through the network. Since messages are sent solely via the transmitter that is closest to the subscriber, a great deal of network capacity is freed. This means that all other transmitters in the network can be used simultaneously to serve other subscribers.

Service Enhancement The two-way pACT architecture affects more than capacity: It also enables providers to enhance services, as well as to provide completely new services and applications. For example, paging devices that contain a transmitter are able to send information back to the network, to someone else in the network, or to any other network. The two-way paging and messaging paradigm provides five levels of acknowledgment:

  • System acknowledgment The paging device acknowledges receipt of an error-free message. Atransparent Link Layer acknowledgment between the device and the network enables guaranteed delivery service. The network stores and retransmits messages at periodic intervals that pagers have not acknowledged.
  • Message read When a recipient reads a message, the paging device transmits a “message read” acknowledgment back to the host system or to the originator of the message.
  • Canned messages The paging device contains several ready-to-use responses such as “Yes” or “No” that recipients can use when they reply to an inquiry.
  • Multiple choice The originator of a message defines several possible responses to accompany his or her message. To reply, the recipient selects the most appropriate response.
  • Editing capabilities Some devices may be used to create messages. Editing capabilities vary from device to device. Some devices are managed by a few simple keys and provide only minor editing capabilities, while others such as portable computers may contain full-feature keyboards.

Like wireless IP, pACT provides secure service—including encryption and authentication—to ensure that messages are delivered solely to intended subscribers.

System Overview

The pACT system is built from several flexible modules that can be combined and configured in different ways to meet specific operator demands. Because the pACT network is based on the IP, operators and application providers can take full advantage of existing applications, application programming interfaces (APIs), and other development tools. With pACT, a single 50-kHz channel may accommodate up to three individual radiofrequency (RF) carriers. Each base station is assigned a particular 12.5-kHz channel.

pACT Data Base Stations The pACT data base stations (PDBS) are located at the cell site and relay data between subscriber devices and the serving pACT data intermediate system (PDIS). Typically, the PDBSs are connected to the serving PDIS switch via a Frame Relay network. Cellular radio system design and roaming techniques enable pACT to determine which base station is closest to a subscriber device each time communication takes place between the device and the network. The mobile terminals determine cell handoff based on signal-strength measurements implemented by the base stations.

pACT Data Intermediate System The PDIS acts as the central switching site, routing data to and from the appropriate base stations. It also maintains routing information for each subscriber device in the network. There are two versions of the PDIS: the home PDIS and the serving PDIS. Besides switching data packets, the home PDIS maintains a location directory and provides a forwarding service and subscriber authentication.

Every subscriber is registered in a home PDIS database. The serving PDIS provides message forwarding, a registration directory, and readdress services. Other services or functions are multicast, broadcast, unicast, airlink encryption, header compression (to minimize airlink use), data segmentation, frame sequencing, and network management. The serving PDIS is connected to the home PDIS switch.

If necessary, the two switches may be located on the same hardware platform. Various configurations of computer processor power and memory are available for the PDIS, depending on requirements for computing capacity and on how the requirements relate to traffic load and the number of subscribers in the network. More computing capacity may be added easily if necessary.

Message Center The fixed entry point into the pACT network is provided by one or more message centers (MCs) that initiate, provision, and connect pACT services to private and public networks, including corporate intranets and the PSTN, respectively. Every message passes through the message center, whose functionality and applications vary according to network operator requirements.

The core of the message center is the message store, which handles virtually any data type and makes APIs accessible for building various applications, such as interactive voice response (IVR) and voice or fax mail. The message store also provides functions for operation and maintenance, system monitoring, and event/alarm handling.

Any of the message center’s databases can be queried via the Structured Query Language (SQL). The message center also supports virtually any protocol. Typical protocols are the IP, Telocator Alphanumeric Protocol (TAP), Telocator Network Paging Protocol (TNPP), and X.400 (the ITU-T standard for message handling services).

Network Management System

The pACT network management system (NMS) gives operators full control of every component in a pACT network. Through the NMS, each base station is provided with a set of radio resource management (RRM) parameters that are used to control traffic and maintain links to the network, as well as to give instructions to mobile terminals that access the channel.

The NMS supports the Common Management Information Protocol (CMIP) and the Simple Network Management Protocol (SNMP). The NMS contains a database component that permanently stores parameters, configuration data, and historical records of traps and performance data. ApA CT network may contain more than one NMS, allowing responsibility to be passed across time zones to other operators, ensuring 24-hour monitoring and control.

Customer Activation System The pACT Customer Activation System (CAS) enables customer service representatives to manage customer accounts and to dynamically activate pACT-related services for customers. Customer accounts are of two types: individual and business, where individual accounts are for single subscribers and business accounts are for multiple subscribers.

pACT End System (Mobile Terminals) Mobile terminals range from simple pagers to sophisticated two-way messaging devices such as PDAs or palmtop computers with wireless modems. When not being used to send messages, mobile terminals periodically check the designated forward channel for incoming messages; otherwise, they are usually in sleep mode to conserve battery life.

pACT System Protocol Stack The pACT protocol stack is based on the concepts and principles of the ITU-X.200 and ITU-X.210 reference models, as well as service conventions for Open Systems Interconnection (OSI). The Limited Size Messaging (LSM) protocol provides the functionality of a simple e-mail application protocol, such as the Simple Mail Transfer Protocol (SMTP), but is optimized for low-bandwidth channels so that unnecessary overhead over the airlink is minimized.

In addition, the LSM protocol provides a platform for providing true two-way messaging and data communication services. Examples of services are embedded response messaging for simple pager devices, true twoway e-mail connectivity, and multicast and broadcast messaging. pACT’s subnetwork convergence layer provides a new approach to encryption.

To ensure that airlink bandwidth is not spent on resynchronizing the encryption engines, pACT devices may employ a technique that automatically resynchronizes the engines, even when the underlying layers fail to deliver a packet. This technique is important because it provides a mechanism by which multicast and broadcast services may be encrypted.

The CDPD Link Layer is optimized for duplex, whereas pACT uses two-way simplex. The pACT Mobile Data Link Layer Protocol (MDLP) is an enhanced version of the link access procedure on the D-channel (LAPD) (i.e., ITU Q.920) that allows subscriber devices to adopt strategies for automatically resetting the link and saving power.

pACT Airlink Interface The pACT backbone network is similar to a CDPD network. The main differences between the two involve functionality—mostly for extending battery life in subscriber devices—and features such as group messaging, broadcast, and unicast. The pACT protocol shortens and reduces the number of transmissions and contains an efficient sleep mode for conserving battery power.

As an alternative to wireless IP, also known as CDPD, Personal Air Communications Technology (pACT) supports various paging and messaging applications while providing more efficient use of limited spectrum. This allows wireless carriers to stay competitive, even when they cannot easily add more capacity to their networks. Since pACT is based on the IP, wireless network users can access other IP-based networks such as corporate LANs and the Internet from remote locations. With integral authentication and encryption, data are protected as they traverse the pACT network.