PHS Internet Access Forum (PIAF) was organized in July 1995 in order to promote multimedia communications via PHS. (Refer to News No. 2 of December 1995.) As of October 1 1996, the Forum is constituent of 87 members representing from most of the PHS related businesses, such as PHS and other operating companies, hardware and software makers, and so on. Following is the outline of PIAFS proposed by the forum. (Commentary by the News Editorial Committee)
Standardized protocols must be incorporated in the network to facilitate the provision of data communications services via the Personal Handy-phone System (PHS). An outline of the standardization work related to PHS data services is given in Fig. 1.

The Association of Radio Industries and Businesses (ARIB) revised the standard for the common air interface (CAI) at a standards meeting in December 1995. Last April, the Telecommunication Technology Committee (TTC) revised and adopted additional standards pertaining to the network portion. Moreover, the PHS Internet Access Forum proposed the PHS Internet Access Forum Standard (PIAFS) concerning Internet access from PHS personal stations (PSs) and the end-to-end transmission control procedure for error correction when transmitting various types of data. Interconnectivity tests have been under way since August.

1. Standardization of air interface portion

The CAI for 32 kbps data communications via PHS has been standardized as 32 kbps unrestricted digital data transmission in Version 2 of RCR STD-28 approved at the Standards Meeting of ARIB.
One of the major revisions made was the addition of an “unrestricted digital data” provision to the bearer capability element of call control (Fig. 2).

2. Standardization of network portion

With regard to the network portion of PHS, the interface for interconnecting the PHS network with ISDN to support voice service has been standardized in JT-Q931-b drawn up by TTC.
However, this standard was revised as a result of the addition of an unrestricted digital data transmission provision to RCR STD-28 Version 2. Moreover, JT-Q931 and its supplement, which specify the ISDN user-network interface, were also revised. In order to specify rate adoption at the cell stations and ISDN terminals, a provision has been defined in both JT-Q931-b and JT-Q931 that prescribes the TTC Standard Rate Adoption for the layer 1 protocol of the bearer capability element and 32 kbps (JT-1460) for the user bit rate.

3. Transmission control procedure

The completion of 32 kbps unrestricted digital data standard for the CAI portion and the network interface portion means that 32 kbps bit stream can now be transmitted end-to-end via PHS.
However, because errors occur on the PHS radio transmission path due to Rayleigh fading, handover and channel switching, a control procedure must be provided in layer 2 to eliminate such errors and secure the desired effective transmission speed (throughput).
Transmission control procedures are generally selected according to the communication method of the wired communications network and include HDLC (high level data link control), V.42, MNP and other protocols. The PHS Internet Access Forum has proposed PIAFS which is well suited to the PHS environment.

4. Basic concepts and features of PIAFS

The basic concepts underlying the development of PIAFS are explained below.
(1) Provision of error-free transmission paths for end-to-end communications
Transmission paths that include a wireless section like that of PHS are susceptible to transmission errors caused by Rayleigh fading, handover, channel switching and other factors. An error correction scheme is therefore necessary to remove such errors efficiently.
(2) Attainment of high throughput
PIAFS adopts a system that retransmits frames when errors are detected in order to provide error-free transmission and improve throughput under the PHS environment. The retransmission system is based on the selective repeat (SR) and automatic repeat request (ARQ) scheme, whereby the receiving side repeatedly requests retransmission of frame numbers in which errors occurred. The highest throughput obtained in the absence of error is 29.2 kbps.
(3) Performance guarantee against transmission path delay in end-to-end communications
Transmission path delay in end-to-end communications varies depending on the receiving party and is especially large in long-distance and satellite communications. To increase throughout in these situations as well, PIAFS adopts frame synchronization at the onset of transmission and measures the round trip frames (RTF) in end-to-end communications, enabling adaptive control of transmission delay. In addition, it allows the use of a simple control procedure to improve throughput even when backward data are lost because of bursty error. For example, because it adopts initial synchronization, the frame timing can be known without inserting any special frame pattern, which works to enhance efficiency.
(4) Low-cost equipment
As described in (3) above, PIAFS adopts a frame synchronization scheme that allows the control algorithm to be simplified, making it possible to design equipment with simpler configurations.
(5) Support for full duplex communications
(6) Capabilities for handover and channel switching
A flow control function is provided to facilitate handover by preventing loss of data.
(7) Manual establishment and release of links
(8) Assurance of flexibility for higher-level software
Flexibility is secured by providing data assembly and disassembly functions. For example, large volumes of data must be divided and sent in separate units. A data unit indicator has been defined to facilitate this. When the indicator indicates ֿ”, it means the data are continuous, and when it indicates ־”, the data are independent. This function makes it possible to transmit various types of higher-level protocol data, which gives flexibility to higher-level software.

5. Structure and configuration of PIAFS

(1) In addition to data transmission, PHS is expected to be used for transmitting full motion video and other types of communications requiring real-time qualities, as well as for still other new applications in the future. To support such uses, PIAFS includes from the outset an in-band negotiation part that assures extensibility to future applications (shown in Fig. 3).

(2) Frame format
A fixed frame length of 640 bits is adopted.
A modulus identifier is used to distinguish modulus cycles (i.e., whether they are new or old) on the basis of the inversion or non-inversion of the data of this part. This doubles the number of frames that can be accommodated, resulting in higher efficiency. Moreover, a Cyclic Redundancy Check (CRC) is used to detect errors in received data. CRC32, defined as an option in ITU-TV.42, is employed for this purpose.

6. PHS data terminals

7. Applications for PHS data communications

Full-fledged PHS data communications services are set to start in the near future. A summary is given here of the major services undergoing testing.
(1) E-mail service
E-mail is seen as being a basic service of the PHS multimedia communications menu. This service will make it possible to send and receive memos, schedules, daily reports and other information from locations around town, from the office and from home. PHS users want various types of service features, such as a function for screening mail by the sender or title and downloading only essential messages.
(2) Fax service
Uses envisaged for this service include sending handwritten memos or faxing data stored in a Personal Digital Assistant (PDA). It is expected that faxes will be frequently used as a simple and convenient means of communicating other than by telephone.
(3) Internet access
Internet access services have mushroomed in recent years, as there are many people who want to obtain information from around the world via the World Wide Web (WWW). This trend has also influenced PHS in that many users want to be able to obtain necessary information in a timely manner when they are outdoors. It is also projected that PHS will be used extensively to form Intranets for in-house communications by facilitating the expansion of office LAN access points.
(4) Photograph transmission service
This service can be realized by transmitting the signals of a digital still camera directly or through the medium of a personal computer. As such, it can be regarded as another variation of data transmission service. Since it involves transmitting large volumes of data and has a wide range of potential business applications, this service is predicted to become a major multimedia offering of PHS with its high-speed capability.
(5) Mobile office service
The spread of groupware recently has led to frequent instances where groups share common databases in carrying out or supporting the execution of collaborative work. There are demands to extend this collaborative environment even to outside locations through the use of mobile communications. The quantities of information being handled have also increased considerably to include the latest product information and business information. Thus, there are strong expectations for PHS data communications.
Many other types of services are also being considered besides the ones outlined here. Together with the ongoing advancement of telephone services, still other new ideas will likely be created, which suggests that users will be looking increasingly to the further evolution of PHS.

8. Outlook for PHS data communications