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.