4RF Aprisa SR+ User Manual - page 125
Managing the Radio | 123
Aprisa SR+ User Manual 1.5.3
Mirrored Bits®
4RF has introduced a channel access scheme optimized for Mirrored Bits® support between two devices.
Error free transport of the protocol can be achieved through specific serial traffic configuration settings,
which are dependent on the radio RF configuration, Mirrored Bits® devices and network characteristics.
The following are the recommended RF configurations and serial data configuration settings and to
optimize the performance over Aprisa SR+ radios.
Recommended RF configurations are:
Radio->Channel Setup->Serial Data Stream Mode to ‘Segregate’
Radio->Channel Setup->Access Scheme to Point To Point (Half Duplex)
Radio->Channel Setup->Network Traffic Type to ‘Serial Only’
Radio > Radio Setup > Channel size – set to meet license requirements (the wider the better for
performance)
Radio > Radio Setup > Modulation – 64 QAM low
The IFG and the MTU are adjusted to optimize performance for different RF configurations, connected
devices and traffic configurations. The following is an example of a 9600 baud, 64 QAM modulation and
12.5 kHz channel size connecting a SEL 2505 device:
Recommended serial data configurations are:
Inter Frame Gap (IFG) set to 0.2
The IFG is dependent on serial line bit rate only. The Mirrored Bits® protocol is essentially timed
to a base clock, the slower the bit rate the longer the period to transmit a packet resulting in less
time between packets.
A low bit rate is ideal as it increase the time period before a ROK error will occur as this is
dependent on serial packet transmission time
The minimum bit rate currently proven to provide reliable communications is 9600 bit, with this
rate an IFG of 0.2 is required to be used.
Maximum Transmission Unit (MTU) set to 32 bytes
Increasing the MTU allows some ‘space in each packet for additional data from the second serial
port or the Ethernet ports but it will impact latency for each packet. A point may be reached
where the gaps between individual packets are too high and the Mirrored Bits® ROK or other
alarms will assert.
The MTU can be adjusted up or down in steps of 8 bytes. The target is to find the smallest MTU for
reliable transport (running without alarms or ROK assertions).
Refer to the application note ‘AN 15-1 Optimization for MB communications.docx’ for further setup
configurations.