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Modulations and protocols for Infra-Red transmission

Description of IrDA standards, explanation of MODULATION of infra-red devices (the Pulse Code Modulation, the Frequency Shift Keying-FSK, the Bi- Phase Modulation, The Pulse Width Modulation-PWM) and TRANSFER PROTOCOLS (the RC-5 protocol, the NEC protocol, IrDA). See for more information...

Standards for IR data transmission

Transmission by using Infra-Red radiation was used by increasing number of different applications. That was the reason why certain standards were created by The International Electrotechnical Commission (IEC). This standards describes methods of measuring, and specification of common technical parameters of systems, that use diffuse or direct Infra-Red radiation as a information carrier.

Following kinds of modulation of Infra-Red radiation will be described in this article.

Main standard for Infra-red transmission is:

IEC 61603-1: Transmission of data and/or video or similar signals by using the Infra-Red radiation. IEC 1147 (addition IEC 61603): Application of Infra-Red transmission and prevention, or checking of interference between systems.

This standard was accepted in 1992 and it replaced old standard IEC 764, that was published in 1983. IEC 61603-1 consists of 6 parts:

  • General part, subject of the standard
  • Transmission systems for audio band and similar signals
  • Transmission systems for audio systems for conferences and common application
  • Transmission systems for low-speed remote controllers
  • Transmission systems high-speed data transmission and remote controllers.
  • Transmission systems for video and high quality audiovisual signals.


The basic requirement for encoding of the transmitted data is to secure sufficient level of transmission security. Data can be sent by using many ways of encoding the carrier optical device. Nowadays only digital transmission is used. The length of transmitted frames can differ according to used application. Some kinds of modulation were accepted as standards. There are four kinds of most used bit representation of transmitted frame:

The Pulse Code Modulation (PCM):

The pulse modulation is one of the simplest ways of encoding the data, which are transmitted by Infra-Red radiation. The principle of this modulation is in dividing the signal into frames of the same length, so-called "time windows". The pulse of constant length can either occur or not occur in this window, usually much shorter pulse then is the length of the time window. The appearance of this pulse in the time window is considered as a logical zero and its absence in the time window is considered as a logical one.

Clock pulses on the receiver's side are synchronized according to the edge of received signal. During the transmission of bigger bit block of logical ones, when no pulses are sent, the receiver may drop out of synchronism. Therefore there is used so-called "bit-stuffing", when after a certain amount of bits of level one, one additional bit of level zero is sent. This bit is then on the side of receiver removed.

The Principle of Pulse Code Modulation

The Frequency Shift Keying (FSK) Modulation:

The frequency Shift Keying (FSK) modulation is used as a very safe method of transmission. The principle consists in modulation of bits by using two frequencies as is shown in the picture. Disadvantage of FSK modulation are: high price of devices using this modulation, high power demands and slow data transmission. Because of this reasons the FSK modulation is used only in systems, that require high level of transmission security.

The Bi- Phase Modulation:

The principle of bi-phase modulation is shown in the picture number five and it again consists in dividing the time window. These time windows have constant length and a change of signal's level inside of each time window is used for detection of each bit. If there is a positive change (from log.0 to log. 1) the bit is evaluated as logical one. In opposite case, when there is negative change, the bit is evaluated as logical zero. This kind of modulation is used quite often, as an example of using Bi-phase modulation the RC-5 code that is used in remote controllers of european products can be mentioned.

The Pulse Width Modulation (PWM):

The principle of width modulation is again in dividing the time windows. The differences in the length of each time window, that is not constant (see the picture number 6). Each time windows are synchronized with the pulse leading edge and detection of level of bit is done according to the length of the time window. The shorter window responses to bit of level 0 the longer one to bit of level 1.

Summary of Modulations:

All kinds of modulations except the FSK modulation is possible to operate in two modes. The first possibility is the modulation of encoded signal on carrier frequency. The second is the pulse transmission. The modulation on carrier frequency is used by Bi-phase encoding in practical applications. The pulse transmission, also called "flash" mode, is more often used by pulse width modulation.


The Protocol RC-5:

The Transmission standard RC-5 was created for remote controllers of consumer electronics and therefore it uses a structure of frame with unusual 6 bit length of data. The bi-phase encoding with modulation on carrier frequency f0=36 kHz is used. Each bit contains burst of 32 pulses of already mentioned frequency.

The transmission of frame starts with two start bits followed by toggle bit. The toggle bit changes its value with each frame, therefore it is possible to identify, if the former frame was not delivered or not. The information block of frame consists of 5 address bits, which specify controlled device and of 6 data bits, which represents the statement. The structure of RC-5 protocol with example of data frame is shown in following picture.

The NEC Protocol:

The NEC transmission protocol is assigned, like the RC-5 protocol, to be used in remote controllers. The modulation of signal on the carrier frequency f0=36 kHz is also used for transmission, but data are encoded with the pulse width modulation (PWM). The specialty of this protocol is constant length of data frame in connection with the pulse width modulation. This is given by double sending of the data, once in normal way and for the second time in inverted way. This redundancy helps to secure the transmission from errors.

The frame of the NEC protocol begins with so-called "initial code", which is burst of length 9ms followed by pause of 4,5ms.The initial code is responsible for setting the sensitivity of receiving module. The initial code is followed by 4 bytes, representing address of device, the statement and which are sent with already mentioned redundancy. If the frame is sent more times, it means that the button on the remote controller is pressed for longer time and after first frame is sent only initial code followed by one bite.

The IrDA Protocol:

The Infra-Red radiation is already being used for long time as a carrying medium for remote controllers, computers and printers. Until recently, any standard that would describe physical and other layers of transmission did not exist. Necessary standard, called IrDA, was introduced in 1993 by companies HP, IBM and Sharp.
Main advantages of IrDA are easy transmission between different applications and different hardware, easy and relatively cheap implementation, low power input that enables usage in mobile equipment and effective and reliable data transmission.

The IrDA standard of data transmission by Infra-Red radiation has successfully developed from IrDA-1,0 with transmission rate 115,2kbps to IrDA-1,1 with transmission rate up to 4Mbps during last two years. Nowadays there is a large amount of components, adapters, software and mobile systems for the standard IrDA-1,0 present in the market. Components are available also for the standard IrDA-1,1 but with smaller selection.

The OSI model [4] (Open Systems Interconnection) accepted by the International Organization for Standardization (ISO) is often used for describing the structure of communication system. This model is dividing communication system into seven so called "layers".
Each layer is taking care of certain part of communication. Single layers are hierarchically connected, which means that higher layer is using service of the lower layer. In fact not all seven layers are always used, but their number is reduced. The model of IrDA communication system comes out of three basic layers: physical, IrLAP and IrLMP

Physical layer:

Specifications of Physical layer of IrDA are given by standard for Infra-Red transmitter, modulation, methods of encoding / decoding same as another physical parameters. IrDA uses for data transmission infra red radiation with wavelength 0,85 - 0,9?m. The minimal value of radiation intensity of transmitter is 40mW/sr, the maximal value is 50mW/sr radiation angle 30 degrees. The sensitivity of receiver must be in the range between 40mW/cm2 and 50mW/cm2 again with the angle 30 degrees. If the transmission length is shorter or equal to one meter, error rate lower then 10-8 bits is guaranteed.

There are three different methods of encoding the signal. The first is basic for the IrDA-1,0 and also for the IrDA-1,1, the second two are optional and useable only for standard IrDA-1,1.
Start and stop bit is added for transmission rates 9.6, 19.2, 38.4, 57.6, and 115,2kbps to every byte. It is the same format that is used in classical UART.

Logical zero is coded as pulse of length from 1.6ms up to 3/16 of the length of one bit. Logical one is encoded as absence of pulse. 16-bit CRC is used for error detection. Start and stop bit are not added for transmissions at speed of 0,576 and 1,152 Mbps and data are transmitted in synchronous format that is similar to HDLC.
In order to secure synchronization of clock pulses during transmission of bigger block of logical ones, the bit stuffing is used which means insertion of zero bit always after certain amount of bits with value one. For the transmission rate of 4Mbps the 4-PPM modulation is used. There is no need to start and stop bit and bit stuffing. The 32-bit CRC is used for error controlling.

IrLAP layer:

The IrLAP layer (Link Access Protocol) ensures controlling of access to transmission medium and it contains different procedures for establishing the connection, setting the parameters of transmission, exchange of information etc.
The controlling of access to medium for stations that does not take part on connection works so, that the station must before it starts sending the data, trace the transmission channel for at least 500ms, in order to be sure, that no other communication is in progress. The station that is taking part on connection must be able to send data within 500ms. The access to stations that are taking part on connection is controlled by the Poll/Final bit inside each frame.

Main task of procedure, that ensures the connection establishment between stations, is exchanging of their identification numbers (ID). A broadcast that is containing his own ID is sent many times repeatedly by the initializator of connection. Between these broadcasts the station traces the transmission channel. A station responding to this broadcast sends back its own ID.

If collision occur, it is possible to repeat the procedure. The procedure for setting the parameters serves for determining and setting the parameters of communication that both sides of communication can satisfy. Some of these parameters, for example the speed rate, must be same for both sides of transmission. Other parameters such as the maximum length of data and another are the limiting factors for one side which must be accepted the other side of transmission.
Before these parameters are known, the communication operates in the speed rate 9.6kbps, asynchronously and with the maximal length of data 64 bytes. Then, after the connection is established the transmission rate can be increased up to 115.2kbps (IrDA-1,0) or 4Mbps (IrDA-1,1) and the maximal length of data can be set up to 2048 bytes.

There are two kinds of connection between stations, first is "one-to-one" and the second is "one-to-many". One of connected stations is always set as a primary station the other stations are set as a secondary. The primary station is responsible for proper function of the connection and for recovering of connection after collision.

The IrLMP layer:

The IrLMP layer (Link Management Protocol) is the highest layer of the IrDA protocol and it consists of two parts. The first is called LM-IAS and it is taking care about the information base maintenance, out of which can another IrDA stations find out what kind of service is distributed. This information is represented by certain amount of objects associated whit group of attributes. The second part of the IrLMP layer is called LM-MUX and it enables multiple connection over simple connection ensured by the IrLAP layer.


Written by:Zdenek Sara
English translation by: Robert Mach
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