Wireless connections allow spontaneity. It’s unlikely, for example, that two users are going to be able to transfer e-business cards from PDA to PDA without wireless technology; it’s impractical to carry the multitude of cables it would require to ensure your PDA is going to connect with those from different manufacturers.
And there are other advantages to wireless links. Firstly, although wired connections are cheap and reliable, they are prone to wear and damage. And secondly, given the choice, manufacturers would prefer to shy away from adding a connector because it compromises reliability and provides a path for the ingress of moisture and dust to sensitive electronics.
The popular wireless solution is an RF link, such as Bluetooth. But while RF links are a useful technology, they’re not the only wireless link in town. And, at a time when the hype over wireless connectivity is becoming deafening, it’s sobering to be able to step back and consider an alternative that’s proved itself over time.
The original cable replacement technology was an optical wireless connection via infrared light (known as IrDA, after the Infrared Data Association). While its impact has been much less than anticipated, notebook and printer manufacturers have embraced the technology, allowing it to be proved in the field over a period of years.
IrDA claims that infrared communication continues to be the world’s most popular form of wireless communication for Personal Area Networks (PANs). IrDA’s website citest the example of over 200 million mobile phones featuring an infrared transceiver, IrDA-enabled, that were sold in 2004.
Secure connection
The Association also points out that for short-range, line-of-sight, point-to-point communication, IrDA offers many of the advantages of RF, with none of the drawbacks. It is, for example, more secure. That’s because RF is omnidirectional, and consequently more susceptible to interception than line-of-site IrDA.
Certainly, ad-hoc connection between Bluetooth devices is not as spontaneous as it is between IrDA devices. Exchanging e-business cards with IrDA is as simple as pointing two PDAs at each other and pushing a button. There’s no ambiguity as to which devices are in contact. A Bluetooth-equipped unit, in contrast, may detect several devices within range, leaving the user to select which device he or she wishes to contact. Without care, it may be that the PDA you end up sending information to isn’t your trusted colleague, but the one belonging to the corporate spy behind the rubber plant.
The directionality of infrared light not only makes IrDA devices inherently more secure than RF, but also makes the devices less susceptible to interference. Unless you are careless enough to point an IrDA device at the sun or other bright light source, interference is usually not a problem. RF devices, however, compete not only with other RF devices within their range but also with other devices using the Industrial, Scientific and Medical (ISM) band, such as 802.11 radios, cordless phones and microwave ovens.
And being radio-based, RF products are subject to Federal Communications Commission (FCC) approval in the US and European Telecommunication Standards Institute (ETSI) approval in the EU. This also means that RF devices can’t be used on aircraft whereas infrared is allowed except during takeoff and landing.
Playing catch-up
IrDA published its first specification, which defined the lower layers of the infrared protocol, in 1994.
Unfortunately, the upper level protocol and application layers were left up to vendors. This was fine if a company made both the products on either side of the link, but poor for guaranteeing that products from different manufacturers would communicate. This lack of standardisation for the upper protocol layers slowed the adoption of IrDA technology.
IrDA has been playing catch up ever since by defining protocols for specific usage. For example, there is now a specification for using infrared to connect to wireless LANs (WLAN), and downloading images from digital cameras. (Nonetheless, the delay caused by incompletely defined protocol layers has allowed other technologies, particularly Bluetooth, to overtake.) The IRDA standards define both a mandatory and an optional set of protocols. The mandatory specifications are defined for the infrared physical layer (IrPHY), infrared link access protocol (IrLAP) and infrared link management protocol (IrLMP).
IrPHY represents the IR transceiver. It is always a hardware device and usually comprises a photodiode receiver, an infrared emitter for transmission, and an analogue circuit for data encoding and framing. and Vishay (distributed by Soanar ) produce suitable infrared transceivers for IrDA communication, for various speed and power requirements. Selection of a specific device will depend on the mechanical constraints and power requirements of a particular design.
Another product worth considering when looking to build-in IrDA is Cypress’ (Braemac) PSoC (programmable system-on-chip). The device is a user-definable microcontroller and includes a 115 kbit/s IrDA transmitter and receiver module.
The physical layer specification defines the OSI level 1 for point-to-point communication using IR. Version 1.0 provided communication at rates up to 115.2 kbit/s. Version 1.1 increased the communications speed up to 4 Mbit/s, maintaining backward compatibility. Version 1.2 was a low power version at rates up to 115.2 kbit/s, and Version 1.3 extended the low power operation to 1.152 Mbit/s and 4 Mbit/s.
Contemporary infrared optical links can communicate at up to 4 or 16 Mbit/s over a nominal range of 1 m. The specification defines that the receiver must be no more than 1 m from the transmitter and within a 30o cone emanating from the transmitting infrared LED. In practice, the actual distance between the two devices can be extended to 2 m or more if the devices are directly aligned.
The infrared link was originally conceived to work with conventional UARTs. Unfortunately, because UARTs use “not-return to zero” (NRZ) coding, the output can be “high” for several bits duration, illuminating the infrared emitter for extended periods and risking overheating. A better alternative for IrDA is “return to zero, inverted” (RZI) modulation with a maximum pulse width of 3/16 of the bit period.
Adding the physical layer of IrDA to new designs is straightforward. If a design features a UART, then with the addition of an EnDec and a transceiver, IrDA hardware is included in a system. The system can be further simplified by using a chip with an embedded UART capable of generating the RZI signal. Winbond Corporation’s () SuperIO is one such device. The other layers discussed here are implemented in software.
The IrLAP specification corresponds to the OSI layer 2 (data link protocol). IrLAP establishes and maintains a reliable connection between two devices. All IrDA connections begin at 9600 bit/s. Once two devices connect, they exchange capability information. IrLAP maintains the link using error detection, re-transmission and low-level flow control.
IrLMP allows multiple clients to use the same physical IrDA port and resolves address conflicts between devices. The IrLMP also contains the IAS (Information Access Service), which acts as a directory of services available on the device.
Microchip () produces the MCP2150, which includes the three mandatory IrDA layers (except the optical transceiver), plus one optional IrDA data protocol, the infrared communications protocol (IrCOMM). IrCOMM emulates serial and parallel ports to support legacy applications, printing and modem devices. With the addition of this device plus a transceiver, application can be made compatible with consumer devices such as Microsoft Pocket PCs and Palm PDAs.
There are some optional protocols, of which probably the most useful is infrared local area network (IrLAN) which enables access to a LAN via an IrDA access point, allows two devices to connect as if they were on a LAN, and allows a device to access a LAN via a computer that is already attached to the LAN.
Accelerating IrDA
Recently, IrDA has formally adopted IrSimple, which provides data rates 4 to 10 times higher than existing infrared-transmission schemes.
The standard enables data rates up to 16 Mbit/s, and 100-Mbit/s rates are under development, according to the IrDA. The standard also offers backward compatibility with existing infrared protocols.
“IrSimple is the first and only wireless protocol available today that completes the ‘missing link’ from a digital camera to a TV,” says Ron Brown, IrDA’s executive director, in a statement and reported in EDN (US). “It instantly connects enabled devices, eliminating the hassles often found in other wireless technologies.”
Shu Matsuura, president and CEO of ITX E-Globaledge, one of the standard’s developers, told EDN (US) in an e-mail that the higher speed will be highly beneficial for engineers designing consumer-electronics devices.
Matsuura also noted that prior to IrSimple, the only comparable options were protocols like Wi-Fi and Bluetooth for high-speed wireless communications, which are not as efficient.
Implementing IrDA is not overly taxing, and it yields many product-enhancing benefits. If a wireless link requires greater range, and the ability to communicate without direct alignment through people and walls—and the expense can be justified—an RF solution such as Bluetooth is a good choice.
However, if the requirement is to connect two nearby devices without wires using a proven and cost-effective solution, then a designer needs to look no further than infrared.