Increasing Data Throughput in Radio Telemetry Systems 5 Application Note
1998 MXCOM, INC. www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054 Doc. # 20830114.001
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the control station allocates a time-slot to each active station. As the number of active stations increases, the
data bandwidth allocated to each station in a set period reduces. This system is certainly much more
effective than CSMA/CD and is widely used in satellite access systems where again no outstation can hear
the other and there is a high risk of data collisions. However, some ‘dead-time’ will still occur and the
efficiency will not be as high as a form of random-access or ‘bandwidth-on-demand’ system.
The particular method favored by the author is the so-called Slotted-Digital Sense Multiple Access S-DSMA
algorithm. In S-DSMA, again a control station is used and this is assumed to be a duplex device. S-DSMA is a
random access method similar to CSMA and thus avoids the ‘dead-time’ of polled or DAMA systems. It does
also not introduce a significant amount of extra packet traffic to control its operation.
The operation is as follows:
When the control station is transmitting data to an outstation, in addition to the information itself, it
interleaves bits at regular intervals which indicate if the control stations receiver is busy or not. The
control station is placed at a point in the system such that it can hear all the outstations in its cell and
they only send data to it and receive data from it. The outstations are split frequency half duplex and do
not transmit directly to each other. By monitoring the control stations ‘receiver status bits’, the outstation
can determine if it is safe to transmit or not. If the control station states that its receiver is free the
outstation is able to be sure that transmission can occur with a low risk of collision. In order to avoid two
outstations both transmitting together when they both see the control station receiver is free, a random
time delay is introduced at each outstation before transmission occurs. The delay allows one station to
grab the channel before the other and the control station will then indicate that its receiver is no longer
free. Thus, the second outstation must once again hold-off transmission.
By reducing the likelihood of collisions to a minimum and ensuring that only a good radio path is used for
transferring data (to control station and back never from peer to peer) very high traffic loading of the channel
can be achieved of the order of 75% or 0.75 Erlangs.
This effective system put together with the benefits of FEC and sharply defined cell boundaries can prove
extremely effective. The Motorola mobile data system known as DataTACTM, which employs these
techniques, can support around 1500 outstations per control station.
This is highlighted in Reference 2.
Simple implementation of these complex solutions
Consumer Microcircuits Limited have launched a range of integrated circuits which remove from the design of
radio data networks many of the problems discussed in this paper. Known as the MX9x9 series of devices
each product comes in a 24-pin Shrunk Small Outline Package and operates from a minimum supply voltage
of 3.0 volts. The current consumption is of the order of 2.5mA at 3.3 volts.
Each device provides a microprocessor interface and analogue input/outputs suitable for connection to a
typical FM transceiver implementation. In addition, each circuit includes packet framing, bit sync and frame
sync recovery, Forward Error Correction calculation and checking, a bit de-/interleaver and Cyclic
Redundancy Check calculation and checking. A data quality register is provided to enable each host
microprocessor to assess the signal quality of an incoming data signal.
The devices in the MX-COM MX9x9 series have the following device specific features.
The MX909A provides a GMSK based data signaling modem in addition to the features discussed above it
also includes a data randomizer to improve the performance of the GMSK system.
The MX919B provides a 4L-FSK based modem in addition to the common features of the MX9x9 range of
ICs.
The MX929B provides a 4L-FSK modem and automatic handling of interleaved S-DSMA status bits in
addition to the common features outlined previously.
These integrated circuits make the design of fast effective radio data networks much more easily achieved.