Radiometrix Ltd.,
BiM1 transceiver, TX1H high power transmitter data sheet
Page 8
Packet data
In general, data to be sent via a radio link is formed into a serial "packet" of the form :-
Preamble - Control - Address - Data - CRC
Where: Preamble:
This is mandatory for the adaptive data slicer in the receiver in the BiM1 to
stabilise. The BiM1 will be stable after 10ms. Additional preamble time may be
desired for decoder bit synchronisation, firmware carrier detection or receiver
wake up.
Control:
The minimum requirement is a single bit or unique bit pattern to
indicate the start of message (frame sync.). Additionally, decoder
information is often placed here such as: packet count, byte count, flow
control bits (e.g. ACK, repeat count), repeater control, scrambler
information etc.
Address:
This information is used for identification purposes and would at least
contain a 16/24 bit source address, additionally - destination address,
site / system code , unit number and repeater address's may be placed
here.
Data:
User data , generally limited to 256 bytes or less (very long packets
should be avoided to minimise repeat overheads on CRC failure and
channel hogging).
CRC:
16/24 Bit CRC or Checksum of control-address-data fields used by the
decoder to verify the integrity of the packet.
The exact makeup of the packet depends upon the system requirements and may involve some complex
air-traffic density statistics to optimise through-put in large networked systems.
Networks
BiM1’s may be used in many different configurations from simple pair's to multi-node random access
networks. The BiM1 is a single frequency device thus in a multi node system the signalling protocol
must use Time Division Multiple Access (TDMA). In a TDMA network only one transmitter may be on
at a time, ‘clash’ occurs when two or more transmitters are on at the same time and will often cause
data loss at the receivers. TDMA networks may be configured in several ways - Synchronous (time
slots), Polling (master-slave) or Random access (async packet switching e.g. X25). Networked BiM1's
allow several techniques for range / reliability enhancement:
Store and forward Repeaters: If the operating protocol of the network is designed to
allow data path control then data may be routed via
intermediate nodes. The inclusion of a repeating
function in the network protocol either via dedicated
repeater/router nodes or simply utilising existing
nodes allows limitless network expansion.
Spatial Diversity:
In buildings multi-path signals create null spots in the
coverage pattern as a result of signal cancellation. In
master-slave networks it is cost effective to provide 2
BiM1's with separate antenna at the master station.
The null spot patterns will be different for the two
BiM1's . This technique ‘fills in’ the null spots, i.e. a
handshake failure on the first BiM1 due to a signal null
is likely to succeed on the 2nd BiM1.
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