User`s guide

there is the equivalent of only one wire (the infra-red path through the air), which has the following
implications: IrDA protocols send packets one way at a time. If a device tried to send data and listen
for data at the same time, it would “hear” itself and not the device it wants to communicate with.
The way IrDA devices achieve two way communications is to take turns, also known as “turning
the link around”. This happens at least every 500 milliseconds, and can be made more frequent
as necessary. This time is called “turnaround time”. This latency makes it impossible to perfectly
emulate wired communications — very timing sensitive operations are not suitable. Fortunately,
many communication tasks are not so sensitive, and these can use IrDA.
All of the information carried on multiple wires must be carried on the single infra-red “wire”.
This is accomplished by subdividing the packets into data and control parts. In this way a logical
data channel and control channel are created.
The standard IrDA protocol is complicated compared to other data transfer protocols. The
protocol is “layered”, including communication with IrLAP (Link Access Protocol) and IrLMP (Link
Management Protocol), and each of these layers requires handshaking and transmission of separate
data and control messages. Each of these protocol layers lies on top of the Physical Signalling
Layer (PHY), which implements the physical IrDA data signalling. IrLAP provides device-to-device
connection and ensures reliable, ordered transfer of data. IrLMP provides multiplexing of the IrLAP
layer. Multiple channels of IrLMP can exist.
There are many infra-red data protocols. IrDA is now the most commonly used, but another
protocol is Sharp ASK. In addition, there are several versions of the IrDA protocol. All protocols
are byte-based, but have different parity bits, error check methods and range of transmission speeds.
ASK ranges from 9600 bps to 38.4 Kbps, while IrDA ranges from 9600 bps to 115.2 Kbps, with some
high-speed extensions allowing up to 4 Mbps. In the near future, speeds of up to 16 Mbps will be
available (IRDA, 2000).
There are two types of infra-red data transfer: diffuse and directed. Diffuse infra-red allows many-
to-many connections, does not require direct line of sight and can be uni- or bi-directional. Financial
trading floors are an example of a use of diffuse infra-red. Direct infra-red is point-to-point, requires
a direct line of sight and is a secure form of data transmission and reception. IrDA is an example of
directed infra-red. The main reason why most IrDA connections are limited to one metre is that the
possibility of interference is greater the further the two infra-red ports are, since a direct line of sight
is required. This is especially noticeable with hand-held devices. Some devices can handle distances
of greater than one metre, but IrDA-compliance only requires that communication can occur over
distances of 0–1 metre.
The IrDA consortium claims that infra-red data transfer is very secure, and that using an infra-
red connection to access a LAN is as secure as using a cable at any other access point on the
network. IrDA also claims that infra-red data transfer is very reliable — often more reliable than
wired connections, since it eliminates wear and tear. In addition, there is no possibility of bending
pins or connecting plugs into the wrong socket.
Today, devices that contain infra-red ports include notebook and hand-held computers, printers,
cameras and desktop transceivers. It is probable that, in the future, most photocopiers, fax machines,
overhead projectors, telephones, bank ATMs, headsets and many consumer items will have infra-red
ports. The present project aims to incorporate infra-red communication into a videoconferencing
system, thus adding to this list.
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