Ewave Radio Modem User’s Manual V1.02
Copyright © 2001 by Ewave, Inc.9
fact, as the number of data bytes per packet increases, the time devoted to the pre-amble becomes
less significant, and overall efficiency increases. While this efficiency may not matter in some
applications (e.g., a sensor transmitting a few bytes once per second), it sets an upper limit on the
bandwidth of the channel and therefore is very important in some applications. So, a modem may
attempt to transmit “large” packet sizes – but this means the transmitting modem may need to
queue the first few data bytes until it has received a full packet’s worth of data instead of
beginning to transmit immediately. This adds to the latency on top of that due to the length of the
“pre-amble”.
•Finally, the internal design of the packet modem may add additional latency as data bytes are
moved from one internal FIFO to another. A quality packet modem, such as the Ewave Radio
Modem, will attempt to overlap or “pipeline” as many of these internal operations as possible to
minimize latency.
3.5.2 Bandwidth
Bandwidth is the rate of data bits transferred per unit time (e.g., bits per second). Typically, a radio modem
imposes a certain overhead due to the necessity of transmitting a “pre-amble” for synchronization,
command or “framing” bytes used to encode the data in packets, error checking codes or checksums for
error detection and/or correction, etc. Also, features such as retry/acknowledge (which may be enabled on
some Ewave Radio Modem models), while improving reliability, reduce the effective bandwidth when
errors or interference are present – because the modem must use some bandwidth to re-transmit already-
sent data or to transmit acknowledgements. In order to provide 9600 bps of data throughput, a radio
modem will typically transfer data “over-the-air” at a higher rate to compensate for the aforementioned
overhead. The Ewave Radio Modem transfers data “over-the-air” at approximately 10,000 bits / second.
3.5.3 Errors
Errors and interference are much more common in a wireless communications system compared to a wired
one. The effect of this is to increase latency and decrease bandwidth. For instance, if the Ewave Radio
Modem is programmed for automatic retry/acknowledge of data, a sufficient level of radio interference
may in effect increase the latency to infinity and decrease the bandwidth to zero. This is unavoidable, and
depending on the environment may even be common, so applications should be designed with appropriate
fail-safes, error checking and user-feedback.
3.6 Fine-Tuning the Ewave Radio Modem for your Application
This section discusses the features of the Ewave Radio Modem which may be used to fine-tune the modem
to your application.
3.6.1 CRCs – Protection from Interference
CRC’s (Cyclic Redundancy Codes) are a widely-used mechanism for detecting errors in communication
systems. Some Ewave Radio Modem models include automatic, transparent CRC generation and checking.
Without this feature, the Radio Modem may interpret random radio interference as valid received data –
which can confuse your application if it does not implement its own error checking. With this feature, the
Ewave Radio Modem automatically filters out radio interference. The corrupted packets will be silently
discarded. In many applications this is acceptable, but if it is not, the application may either implement its
own retry scheme or use the Ewave Radio Modem’s transparent Retry/Acknowledge (if the particular
Ewave Radio Modem model includes this feature.)
3.6.2 Packet Size Threshold
The packet size threshold parameter determines the number of bytes the Radio Modem will attempt to
assemble into a “packet.” Some applications tend to send fixed sized blocks of data and are sensitive to the
time between bytes within a single application-defined block. This parameter allows the application
