Standards & Protocols

IrDA : Wireless Communications Up Close and Personal

This column discusses wireless data connections for the last meter.

By Mike Rodbell

The Infrared Data Association (IrDA) has developed a set of standards geared towards supporting applications where users need to frequently transfer data between portable and fixed equipment. The initial vision of IrDA leveraged infrared wireless communications as an inexpensive replacement for cable-based (e.g., RS-232) transfer of information between equipment that is not normally connected. This month, I'll review the general applications and architecture of the IrDA standards. In the next few months, the specifics of some of the major components of the IrDA interface will be reviewed.

Applications for infrared-based wireless connections

There are several opportunities for the use of wireless data interface devices at close distance. Take, for example, the case of a retail store in which people are wandering about taking inventory with portable gear. At the end of the day, with IrDA equipment, they can then upload all of their collected information into the main computers without having to fool with cabling and all the mess that might normally accompany such endeavors. In consumer devices, this could include applications such as the transfer of information between digital cameras and photo processing equipment.

Recognizing that a market for this kind of interface exists, the IrDA was formed in 1993 to "create and promote interoperable, low-cost infrared data-interconnection standards that support a walk-up, point-to-point user model." As with any standard, they had the challenge addressing the issues associated with establishing a standard where several competing technologies were available. At that time, infrared interfaces were offered by HP, Sharp, Motorola, and others. Some of the applications that they targeted include data transfers between personal digital assistants (PDAs), laptop computers, printers, PCS, pagers, laptop and desktop computers, and others. They set out to develop standards that would provide fairly sophisticated data transfer services over a physical medium that had traditionally been used for simple interfaces, such as remote control and simple transfer.

The capabilities provided by the IrDA standard have grown since the introduction of the first standards in 1993. This early specification was viewed as essentially providing a cable replacement for signals that might have otherwise been transferred over a computer's serial port. The data rates supported by version 1.0 of IrDA ranged up to 115 kbps. Recognizing a market need for higher data rate transfers, version 1.1 of the interface standard was introduced in 1994. By providing the ability to transfer data at rates of up to 4 Mbps, version 1.1 of the standard presented opportunities for users to take advantage of LAN connections without a need for additional cabling and devices.

From the existing, published (and widely implemented) standards, the IrDA organization is investigating several new areas of application. Multipoint extensions are being investigated to extend beyond the point-to-point architecture. These multipoint services are intended to permit several machines within the confines of a room (approximately 5 m to 10 m) to communicate with one another. Other efforts under investigation include remote control applications that extend the data rates from 12 kbps to 70 kbps.

The IrDA interfaces can be widely found in laptop, notebook, and hand-held computers. Other devices that equipment manufacturers have identified for IrDA support include pagers, portable telephones, PDAs, and digital still-image cameras. Some of the applications envisioned for this equipment include the transfer of business card information, and telephone docking to computers for computer telephony integration (CTI) applications. Information kiosks at the next winter Olympics will include IrDA interfaces so that people can transfer information to and from their portable equipment.

Sections of the standards

Now, on to the standards themselves. The openly published IrDA standards include mandatory and optional components. The following are the mandatory IrDA specifications:

• Physical-layer serial infrared (SIR) physical layer link. This specification defines the specifics of the physical optical media interface. Version 1.0 had defined the standard for rates up to 115 kbps. Version 1.1 defines the interface and modulation/demodulation techniques that are applied to support rates of up to 4 Mbps.
• Infrared-link access protocol (IrLAP). This specification defines the mechanisms employed in establishing a logical connection between two devices participating in an IrDA point-to-point half-duplex link. This standard (like many other data link protocols) is largely based on the high-level data link control (HDLC) protocol. Given that the IrDA interface is designed to operate in a half-duplex mode, the IrLAP runs in HDLC normal response mode in which one station takes the role of the primary, and the alternate station acts as the secondary link, responding to directions provided by the primary.
• Infrared-link management protocol (IrLMP). This specification provides for multiple logical access points within a single physical link and a mechanism through which the participating stations can advertise and coordinate their respective services.

Products that are compliant with these protocols are able to interoperate while providing reliable data transfer. Additionally, the IrLMP provides services that permit devices to automatically identify themselves and negotiate their respective capabilities.

In addition to the mandatory standards, the IrDA has published a number of additional optional standards that can be used to help extend the services offered over the infrared interface. These optional standards include:

• Transport protocol. This protocol provides connection oriented services with the general types of flow control that are often associated with data transport protocols (e.g., TCP or TP4).
• Infrared communications protocol (IrCOMM). This standard defines a mechanism to emulate PC full-duplex serial/ parallel port communications, and makes use of the IrDA "Tiny TP" transport protocol, along with the standard IrDA SIR, LMP, and LAP protocols.
• Minimal IrDA protocol implementation (IrDA Lite). This defines a relatively basic set of options in an effort to simplify the range of services and state machine complexities required of a full IrDA interface implementation.
• Plug-and-play (PnP) extensions. These define a set of services and mechanisms that can be used to support automatic installation or removal of devices based on services negotiated through the IrLMP.

The IrDA is continuously developing and standardizing new ways to extend the utility of the IrDA interfaces.

The protocol stack

The IrDA protocol stack, shown in Figure 1, is based on the three mandatory components. Additional applications and services can be added either in place of the transport or application components.

Figure 1

Let's take a quick tour of some of the more general aspects of the mandatory components of the IrDA protocol stack. We can get into some of the more detailed aspects of each of these standards in the next few months.

SIR modulation/demodulation services

Version 1.1 of the IrDA physical layer provides a point-to-point mechanism for half-duplex transmission at distances up to and (sometimes) exceeding 1 m. Services provided by the SIR range from the physical and mechanical specifications to the framing and error checking of data transmitted between two IrDA stations. At the base of the standard, the SIR specifies physical layer operating wavelengths (between .85 ým and .9 ým), power, and transmission ranges (which are angular). While the requirements pertain to multiple devices connected via IrDA, the standard specifies a maximum bit-error rate ratio (BER) of 10-8. At the bit encoding level, the SIR standard identifies two general types of modulation schemes, using return to zero-inverted (RZI) for the lower speed transmissions (between 2.4 kbps and 1.152 Mbps); a more involved encoding scheme, called pulse position modulation (PPM), is used for the 4-Mbps interface. The range of standard transmission speeds is shown in Table 1.

With the variety of encoding schemes, different strategies for packet framing are used. With the lower-rate data being transmitted using a simple RZI encoding scheme, the standard HDLC framing format is specified, with the minor exception being that a minimum of two framing characters (0x7E) are required prior to the address field of the actual data. This slower data-rate is protected by the standard International Telegraph and Telephone Consultative Committee (CCITT) 16-bit cyclic redundancy check (CRC), frequently used in HDLC links. The IrDA standards take advantage of the code symbol nature of the higher speed pulse position modulation (PPM) encoding to provide the basis for the frame-control character encoding. The higher data-rate packet encoding is protected by the IEEE 802 32-bit CRC algorithm.

IrLAP: leveraging HDLC

The IrLAP provides a connection-oriented, reliable data transfer path for IrDA applications. In addition, the IrLAP includes mechanisms for rate negotiation, allowing two participating stations to automatically identify the greatest, mutually supported data rate. Since it is likely that any station will see multiple, different devices, the IrLAP includes provisions to "account for the mobile, ad-hoc nature of the medium." The protocol operates using the HDLC normal response mode (NRM) in a half-duplex environment with one station taking on the role as the primary, and the other acting as a secondary station. To provide maximum flexibility, the IrLAP allows any station to negotiate a primary role.

IrLMP: multiplexing and identification services

The IrLMP provides two general services, traffic multiplexing and capabilities negotiation. While the IrLAP provides a single connection between two stations, the IrLMP provides the stations with the ability to multiplex several applications over the same link layer connection. The information access services (LM-IAS) provide a mechanism where two IrDA-compliant devices can publish information describing their local capabilities and automatically discover the range of capabilities offered by the attached device.

As you can probably gather by the range of services we've presented this month, the IrDA specifications provide a wide range of data networking services. Over the course of the next few months, we can explore some of the more specific details of each of the mandatory IrDA components.

Mike Rodbell is president of DG Technology, a consulting firm that specializes in the development and integration of distributed processing and communication systems. He has developed voice and data communication systems for a wide range of military and commercial systems. He holds a BSCS from Trinity College of Hartford, CT, and an MSEE from Loyola College of Baltimore, MD. He can be reached at mrodbell@dg-tech.com or http://www.dg-tech.com.