Wireless modem
From ArticleWorld
A radio modem consists of a radio and a modem in one box. It is used as a transceiver (transmitter-receiver) for serial data using radio waves. Radio modems can be used instead of wire lines to connect to remote sites or as a backup to existing wire lines or optic fiber connections. They are designed so that the electronic equipment (such as computers) can connect to other equipment transparently over a radio link without requiring the need for any special hardware modifications. Radio modems also find use in portable computers such as laptops and PDAs, where they can be used to connect to the internet.
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Description
Radio modems are typically connected to computers (or other equipment) using RS-232 cables. Radio modems provide the same functions as conventional modems. They modulate and demodulate signals to mimic digital bitstreams used by computers.
Today's radio modems operate in the 400 to 900 MHz band. Although RS-232 interfaces are most common to the radio modem, RS-422, RS-485 and fiber-optics are also used.
Compared to WiFi modems, radio modems have a longer range and use less power.
Modes of radio modems
The following are some of the operating modes of radio modems:
- Point to point
- Point to multi-point
- Repeater
Point to point radio modems can transmit data to a single receiving modem only. Point to multi-point allows transmission to multiple modems.
Performance specifications
Important performance specifications of a radio modem include:
- Maximum output power: This indicates the device's highest transmission power.
- Sensitivity: This defines the minimum radio frequency signal level below which the receiver may read noise data along with the original data.
- Number of channels: This defines how many transmitting and receiving channels the radio modem should contain.
- Full duplex transmission: This indicates whether transmission can be done both ways at the same time.
Spread spectrum radio modems
Spread spectrum refers to the method used for reducing the impact of localized frequency interferences through the use of more bandwidth. Direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) are the most widely used methods for the spread spectrum technology. In DSSS, each bit in the stream of bits to be transmitted is encoded into a longer bit string in order to introduce redundancy and hence reduce chances of interference and data loss. FHSS uses a technique where the signal rapidly switches among many frequency channels, using a pseudo random sequence known to the modems at both ends.
Spread spectrum radio modems have the advantage of high data security. However, they suffer from the disadvantage of using the radio spectrum very inefficiently.