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BIM1 Datasheet(PDF) 8 Page - Radiometrix Ltd |
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BIM1 Datasheet(HTML) 8 Page - Radiometrix Ltd |
8 / 15 page 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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