Radio Modems

Radio modems are radio frequency transceivers for serial data. They connect to serial ports (RS232, RS422, etc.) and transmit to and receive signals from other matching radio modems.  Radio modems can be configured for internal or external mounting.  Internal radio modems are computer cards that are attached to the wired network through a computer that is part of the network.  External radio modems are modules that are connected to the wired network by means of a physical port or interface.  Bus or interface types available for radio modems include Type II card, Type III card, CardBus, ISA, PCI, MIC, RJ-45, SC, ISDN BRI S/T interface, ISDN BRI U interface, serial ports (RS232, RS422, RS485), ST, USB, and PLC slot mount.  The data rate is also important to consider.  The data rate is the maximum data transfer rate at which the modem can deliver data. It is normally expressed in bits/second.

Important radio link specifications to consider when searching for radio modems include frequency band, operating mode, and radio technique.    Frequency band choices include 900 MHz, 2.4 GHz, 5 GHz, 23 GHz, VHF, and UHF.  Operating modes for radio modems include point-to-point, point-to-multipoint, and repeater mode.  Point-to-point radio modems can transmit to only one modem / radio modem at a time.  Point-to-multipoint modems can transmit to several modems / radio modems at a time.  Radio techniques include direct sequence spread spectrum and frequency hopping spread spectrum.  Spread spectrum is a technique that is used to reduce the impact of localized frequency interferences. To achieve this, it uses more bandwidth than the system needs.  There are two main spread spectrum modalities: direct sequence and frequency hopping. The principle of direct sequence spreads the signal on a larger band by multiplexing it with a code (signature) to minimize localized interference and noise.  The system works over a large band. To spread the signal, each bit is modulated by a code.  Frequency hopping uses a technique where the signal walks through a set of narrow channels in sequence.  The transmission frequency band is divided in certain number of channels, and periodically the system hop to a new channel, following a predetermined cyclic hopping pattern. The system avoids interference by never staying in the same channel a long period of time.

Common performance specifications for radio modems include full duplex transmission, maximum output power, number of channels, and sensitivity.  Full duplex radio modems can transmit and receive at the same time.  Maximum output power is the transmission power of the device. It is defined as the strength of the signals emitted, often measured in mW.  The number of channels defines the number of transmitting and receiving channels of the device.  The sensitivity is the measure of the weakest signal that may be reliably sensed by the receiver. Sensitivity is measured in dBm, and the lower the value (higher in absolute value) the better is the receiver.  Common features include antennas and RF connectors.

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