Packet radio modems. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Data transfer

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Data transmission over a radio channel is in many cases more reliable and cheaper than transmission over dial-up or leased channels, and especially over channels of cellular communication networks. In situations due to the lack of a developed communication infrastructure, the use of radio facilities for data transmission is often the only reasonable option for organizing communication. A transmission network using radio modems can be deployed in almost any geographic region. Depending on the radio stations used, such a network can serve its subscribers in an area with a radius from a few to hundreds of kilometers. Radio modems are of great practical value where it is necessary to transfer small amounts of information (documents, certificates, etc.).

Radio modems are often called packet controllers (TNC - Terminal Node Controller) due to the fact that they include special. a controller that exchanges data with a computer, manages frame formatting and access to a common radio channel in accordance with the implemented multiple access method. Radio modems are oriented to work in a single radio channel with many users (in a multiple access channel), and not in a point-to-point channel (a dial-up modem).

AX.25 standard

Recommendation AX.25 establishes a single packet exchange protocol, i.e. obligatory for all users of packet radio networks the order of data exchange. The AX.25 standard is a revised version of the X.25 standard for packet radio networks.

A feature of packet radio networks is that the same radio channel is used for data transmission by all network users in the multiple access mode. The AX.25 protocol provides for multiple channel access with busy control. All network users are considered equal. Before starting transmission, the radio modem “listens” whether the channel is free or not. If busy, the transfer is delayed until it is free. It is possible that another modem will start transmitting at the same time. In this case, signal superposition will occur, as a result of which the data will be seriously distorted. The transmitting radio modem learns about this by receiving a negative acknowledgment from the receiving radio modem or as a result of a timeout. In such a situation, he must repeat the transfer.

With such a connection, the information in the frame is transmitted in the form of separate blocks - frames. Basically, their format corresponds to the HDLC protocol, but there are differences.

Frame format

According to Recommendation AX.25, frames are divided into service and information frames and have the following format:

The start and end of a frame are marked with FLAG frames, i.e. combinations of the form <01111110>, which makes it easier to receive a frame against the background of interference. The ADRES address field contains the addresses of the sender, recipient, and relay stations, if any. The CONT control field defines the frame type: informational or service. The length of the INFORM information field, which is a network layer packet, usually does not exceed several hundred bytes in packet radio networks.

When implementing the network (third) layer of the AX.25 protocol, a specific protocol field is used, cat. acts as part of the information field and is optional.

The frame check field (CRC-16) is designed to detect errors in the frame during its transmission.

Physical implementation of radio modems

Typically, a packet communication station includes a computer, a radio modem (TNC - controller) and a VHF or HF radio station.

The computer interacts with the radio modem through one of the known DTE-DCE interfaces. The RS-232 interface is almost always used. The transmitted data from the computer to the radio modem can be either a command or information to be transmitted. In the first case, the command is decoded and executed, in the second, a frame is formed in accordance with the AX.25 protocol. Before the frame is directly transmitted, its bit sequence is encoded with a linear code without return to zero NRZ-I (Non Return to ZeroInverted).

A packet radio modem is a combination of two devices: the modem itself and the TNC controller. The controller and the modem are interconnected by four lines: TxD - for transmitting frames in the NRZ-I code, RxD - for receiving frames from the modem in the NRZ-I code, PTT - for signaling the modulator on, and DCD - for signaling the channel busy from the modem controller. Typically, the modem and the packet controller are structurally made in the same package.

Before transmitting a frame, the controller turns on the modem using a signal over the PTT line, and sends a frame in the NRZ-I code over the TxD line. The modem modulates the received information in accordance with the accepted modulation method. The modulated signal from the modulator output is fed to the transmitter's MIC input.

When frames are received, the modulated carrier pulse train comes from the EAR output of the radio station receiver to the demodulator input. From the demodulator, the received frame in the form of a sequence of pulses in the NRZ-I code enters the controller of the packet radio modem.

Simultaneously with the appearance of a signal in the channel, a special detector is triggered in the modem, which generates a channel busy signal at its output. The PTT signal, in addition to turning on the modulator, also performs the function of switching the transmission power.

In packet radio communication based on typical radio stations, two modulation methods are used for HF and VHF. On HF, single-sideband modulation is used to form a voice frequency channel in the radio channel. For data transmission, frequency modulation of a subcarrier is used in the frequency band of a telephone channel from 0,3 to 3,4 kHz. The value of the subcarrier frequency can be different, and the frequency resonance is always 200 Hz. In this mode, a transmission rate of 300 bps is provided. In Europe, a frequency of 1850 Hz is commonly used for transmitting "0" and 1650 for "1".

In the VHF band, they often operate at 1200 baud using frequency modulation with a subcarrier frequency resonance of 1000 Hz. It is assumed that "0" corresponds to a frequency of 1200 Hz, and "1" - 2200 Hz. Less often in the VHF range, relative phase modulation (RPM) is used. In this case, baud rates of 2400, 4800, and sometimes 9600 and 1920 baud are achieved. (The maximum speed I know is 76800 baud)

Publication: cxem.net

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