Wireless Modems

Wireless modems transmit modulated data using electromagnetic waves.  This grouping includes radio modems ad access points.  Radio modems are radio frequency transceivers for serial data.  They connect to a serial port (RS232, RS422, etc.) and transmit to and receive signals from another matching radio modem.  Access point enables wireless network connectivity. A modem (Modulator-Demodulator) is a device or program that enables a computer to transmit data over telephone lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

General specifications that are important to consider when searching for wireless modems include form factor, modem speed, network type, and bus or interface type.  From the point of view of the form factor there are two basic types of modems: internal modem are computer cards that are installed in a slot of the motherboard of a computer; an external modem is a module that attaches to computer's communications port via a cable.  Modem speed is the maximum data transfer rate at which the modem can deliver data. It is normally expressed in bits/second.  Choices for network type for wireless modems include dial-up, Ethernet, AMPS, CPCD, GSM, ISDN, PAN, and GPRS.  Common bus or interface types include Type II card, Type III card, CardBus, ISA, PCI, FireWire (IEEE 1394), MIC, RJ-45, SC, ISDN BRI S/T interface, ISDN BRI U interface, serial ports (RS232, RS422, RS485), ST, USB, and PLC slot mount. 

Important radio link specifications to consider when selecting wireless modems include frequency band and radio technique.  The frequency band on wireless modems can be 900 MHz, 2.4 GHz, 5 GHz, 23 GHz, VHF, and UHF.  Choices for radio technique 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.

Important performance specifications to consider when searching for wireless modems include number of channels, maximum output power, and full duplex transmission.  The number of channels refers to the number of transmitting and receiving channels of the device. The maximum output power is the transmission power of the device. It is defined as the strength of the signals emitted, often measured in mW. Full duplex devices can transmit and receive at the same time.  Two common features for wireless modems include antennas and RF connectors.  An antenna may or may not be an integral part of the modem. Some wireless modems include an RF connector that can be used to connect an antenna, or any other suitable RF device.  Type of connectors: BNC, TNC, N, etc.

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Part Numbers for Wireless Modems