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Communication methods

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Here we will explain briefly about communication methods. There are two communication methods, two-way communication and one-way communication, and the thinking behind both is the same as for radio communication.

Two-way communication
With this communication method, information flows in the direction from device A → device B, and the direction from device B → device A.

Half duplex
The transmission of information is carried out in the direction from device A → device B, and from device B → device A. However, with this communication method, communication is carried out alternately, not simultaneously.

Full duplex
With this communication method, the transmission of information can be carried out in the direction from device A → device B, and from device B → device A simultaneously.

One-way communication
With this communication method, information flows in the direction from device A to device B only. Information does not flow from B to A. If control signals and so on besides data are sent, it is by one-way communication.

Telecontrol and telemetry is carried out using one-way communication or half-duplex operation. Communication is by half duplex operation with the user switching between transmitting and receiving with a handy transceiver. Wireless LANs and the like use apparent full duplex.

Communication method between wireless LANs
Let's consider sending NetMeeting video (standard Windows communication software) using a wireless LAN. The images from a CMOS camera and the sound from a microphone both appear to run simultaneously. From the point of view of the user, it appears as if there is full duplex communication.
With ordinary radio communication, one frequency channel (frequency band) is used, so the radio equipment can only use half duplex in the radio part (radio wave). However, between the user and the equipment, the communications are modified so as to appear to be full duplex communication.

Data transmission control procedures

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On the transmission path there are various possible causes of errors, and in order to transmit the data correctly, line control between the transmitting equipment and the receiving equipment is necessary to control synchronization of timing and errors.
For ordinary communication, there are data transmission control procedures such as non-procedural, basic control, and high level data link control (HDLC), and these can be applied to two-way communication with radio.
The operation mode of the radio equipment is 1:1, 1:N, or N:N and when the mode is two-way communication, a system with a special radio protocol is added to the data transmission control procedure.

Non-procedural
Transmission is carried out when just the transmission control code is agreed by the transmitting and receiving ends. Transmission control such as the control of errors and so on is performed by the user of the equipment

Basic control procedure
This is a character based synchronous system where data is transmitted while verifying the serial response of the transmitting and receiving ends using transmission control codes. Transmission control is performed automatically. Data is transmitted in block units, and if an error occurs, only that block is resent.
There are two modes, basic mode and extended mode. In basic mode, only text files are transmitted, and when transmitting transparent data (binary data), extended mode is used.

High level data link control protocol (HDLC)
While the basic procedure uses character based transmission, HDLC uses bit-oriented transmission, in which data regarding addresses, control codes, information, frame check codes and so on are included in frame based transmissions. It is a synchronous flag system in which the start and end of the transmitted frames are flagged (7E Hex). Transmission control is automatic with FCS (Frame Check Sequence) used for error detection. This allows for highly reliable transmission. Data can be transmitted with full transparency. FCS uses the CRC

Flag
01111110
Address Control Information FCS Flag
01111110

Data link method
Data link systems between transmitting and receiving equipment includes contention, polling, selecting systems and so on.

Contention system
A system for establishing a data link with a point-to-point method of connection. The transmitting end sends an enquiry code, and data is transmitted after the receiving end sends acknowledgment to the transmitting end. With point-to-point systems, both devices have an equal relationship.

Polling / selecting systems
A system for establishing a data link with a multi-point method of connection. This is a linkage between a control station and subsidiary station.

Polling
The control station periodically sends requests to send data (data sent to the control station) to the subsidiary stations in the net.

Selecting
If the control station has data to send to a specific subsidiary station, it enquires whether the subsidiary station is able to receive before sending the data.

Wireless errors

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With data transmission, for wired (cable) communication as well as for wireless, error control must be implemented. There can be no comparison between radio communication using radio waves and wired communication in terms of the prevalence of causes of errors such as noise, interference, decay, and so measures against errors must be put in place. Various methods are possible, but there is always the issue of balancing the level of processing used with the constraints such as application, cost, time, and engineering of the system being designed.
Normally, we are not bothered by slight noise or interruptions when we use a mobile phone, and we can put up with a bit of flicker on a television screen.
High level error processing is used for the sound in mobile phones, but errors seem to be inevitable. However, for data transmissions such as mail, this is a problem.
If we consider radio controlled industrial equipment and devices, malfunction caused by transmission errors would cause serious and life-threatening accidents, and might involve the loss of important data. As a designer of radio equipment, it is necessary to pay very close attention that this sort of thing does not happen even if mistakes are made. It is also necessary to incorporate fail safe thinking into designs, and we will consider this later on.

The following types of error are possible

Random errors
Errors that occur randomly without any temporal relationship with other errors.
Burst errors
Errors that occur suddenly and relatively consecutively.

Processing methods for errors

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Processing errors in radio communication must be handled at both the hardware and software levels, but if appropriate measures are taken, communication that is not inferior to wired communication can be achieved.

One-way communication error processing
For one-way data transmission such as the commands for telecontrol or sound and images, the following error processing methods are available.

No error processing is used. The system will be unusable.
Decision using error detecting codes The results of some operations can be checked by human senses, and for relatively unimportant data such as the collection of temperature data, where the same data is sent continuously (or it is analog-type data), the data can be put in the frame structure along with code for detecting errors, and the receiving equipment can then decide whether or not there are errors based on this code, and can discard the data if an error is present. Methods of calculating error detecting codes include the checksum method, the CRC method and so on.
FEC For applications where you want to keep errors to the minimum, use a method such as FEC (Forward Error Correction). This is a system in which the transmitter includes code that allows the receiver to perform error correction. This code makes the transmitted data longer than the actual data (redundant data), but this method allows us to approach error free transmission. Reed-Solomon code and trellis code are representative types of this kind of code, and there are other types that incorporate these codes. Furthermore, interleaving methods are also used to handle burst errors in radio communication.

Two-way communication error processing
In addition to FEC for two-way communication, there are methods that use ARQ (Automatic Repeat Request). With the ARQ system, data is sent as a packet in a frame format, and if an error occurs, the receiver sends a resend request, thus achieving error free data transmission.
This is used in most wireless LANs and other wireless systems.

In order to avoid errors, it is necessary to use protocols for data transmission in the wireless zone. Specifically, with packet communication, address information, a packet number, packet size, status/control, frame check and the like is attached to either end of each packet. The receiving end performs error detection, and if necessary sends a resend request, thus achieving error free data transmission. A preamble and start code is attached to the first packet in radio communication that uses packets. There are cases where a preamble is necessary in order to synchronize the radio equipment.

Preamble Start code Receiver address Sender address Packet number Status/ control User data Frame check

Frame check systems (error control systems) With the purpose of detecting data errors in the packet frame, the transmitting end adds a frame check code at the end of the packet when forming the packet. The receiving end then decides whether or not there are errors based on this code, and if it detects a data error, it sends a request to the transmitting end to resend the data, thus achieving error free data transmission.
This system includes the checksum method, the CRC method (Cyclic Redundancy Check), the parity check method and so on. The CRC method is better at detecting errors compared with the checksum method and so on, and it is used in most data transmission protocols such as wireless LANs, hard disk controllers and the like. CRC systems include 5-bit and 12-bit, but currently the most commonly used systems are 16-bit or 32-bit, and in particular, CRC-CCITT and so on are frequently used.

CRC-CCITT
With CRC-CCITT the data frame is calculated by dividing by constants, and the 16-bits (2 bytes) of the result are attached for transmission at the end of the data. The receiving end performs a similar operation, and if the result is correct, it determines that there is no error in the data transmission. If not, it determines that there is an error and sends a resend request.
To explain in rather more detail, in the CRC operation at the transmitting end, the bit string of the frame data is treated as a numerical value (this is called the message polynomial), and the message polynomial is divided by a generator polynomial (constant) X16 + X12 + X5 + 1, and the remainder (CRC code: 2 bytes) is attached to the end of the data for transmission. The rest is the same.
This operation can be performed by the CPU software, but if high speed processing is necessary, it is performed using hardware. Some CPUs contain hardware for this CRC operation, while other CPUs form packet data frames (HDLC function). If the situation permits, it is conceivable to use FPGA or gate arrays, including peripheral equipment.

Transmission line code

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In the transmission path of wireless communication there may be noise and interference that causes loss, so that the code pulse is distorted, and discriminating between data becomes hard.
In the receiving equipment a clock signal for synchronization is required for decoding the data, but it is necessary to extract this from the received data stream. The base band signal normally uses NRZ code, but at the transmitter end if this is directly input to a modulator as a base band signal, with data that is a sequence of 0s or 1s, the receiver cannot duplicate the synchronizing clock from the signal (clock recovery).
In order to overcome this problem, means such as the use of Manchester code are available.
As shown in the diagram below, with Manchester code the middle of the code always reverses polarity, so that unbroken strings of 0's or 1's is avoided, and clock recovery for the receiver is simplified. However, compared with NRZ code, using Manchester code makes the occupied frequency band wider. This code converts the signal from 0 to 10, and from 1 to 01.

Transmission code

The fail safe design philosophy

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Fails safe means that even if a failure or interference occurs, the software or hardware exercises control that tends in the direction of safety. It is a safety philosophy that seeks to limit any damage to the minimum. This is implemented in all fields of design, including construction, electricity and the like.
Especially compared with other technologies, the likelihood of errors arising with products using radio waves is high. However hard we try to achieve perfect error processing, we cannot guarantee that there will be no errors at all.
Please remember that it is necessary to implement fail safe thinking with regard to the system overall.

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