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A radio modem provides an interface between end-user devices and radio relays, using air as the medium. These modems are capable of transmitting and receiving radio waves at user throughput rates up to 20 Kbps. They usually do not require licensing, making it easy to move them from one location to another. Most radio modems transmit omnidirectional radiation patterns.
As long as connectivity exists, a pair of radio modems establishes a channel for data transmission between sites. The main condition for proper connectivity is that the destination must be able to correctly receive data from the source at a specified minimum data rate. For example, if the reception of data on a particular channel results in a number of bit-errors exceeding the maximum error rate for that link, connectivity is lost.
Due to node separation, transmit power, and irregular terrain, most packet radio networks are not able to maintain full connectivity. That is, not every user access device and radio relay node have connectivity with each other. Node separation affects the connectivity of a radio network because the power of a radio signal decreases exponentially as the distance between the nodes increases. If the distance becomes too great, the signal-to-noise ratio decreases and produces too many transmission errors, causing the two stations to become disconnected. The transmit power of the source node also affects link connectivity because higher transmit powers will keep the signal-to-noise ratio higher, resulting in fewer errors and connectivity. Certain types of terrain, such as mountains and buildings, can affect connectivity because they will attenuate and sometimes completely block radio waves. The attenuation will decrease the signal power, resulting in shorter transmission distances. A packet radio network, therefore, must perform routing to move data packets from the source user device, through a number of intermediate radio relays, to the destination user device or network.
Several companies sell radio modems that can interface with packet radio network services provided by various companies. These modems are service provider-specific and use different frequencies. A later section in this chapter describes several packet radio network service providers, as well as the modems that work with those services.