ABB Excount II User Manual page 64

7. Radio transmission protocol
7.1 Packet communication protocol
All radio channels are subject to noise, intereference and fading. In many
cases, radio channels are shared by several users of services. Packet
communication protocols are widely used to achieve error-free communi-
cations over imperfect and/or shared communication channels.
Almost all short-range wireless data communications use some form of
packet protocol to automatically assure information is received correctly
at the correct destination. A packet generally includes a training pream-
ble, a start symbol, routing information (to/from, etc.) a packet ID, all or
part of a message and error detection bits. Other information may be
included depending on the protocol.
Figure 7.1.1 shows one of the packet formats used in EXCOUNT-II. The
structure begins with a training preamble, which improves weak signal
detection at the receiver by "training" the data slicer for best noise immu-
nity, and providing signal transitions to train the clock recovery process.
The training preamble usually consists of several bytes of a
1-0-1-0-1-0 ... sequence.
Preamble
The preamble is followed by a start symbol (often called a start vector),
which is a distinct pattern of bits marking the start of the information sec-
tion of the packet.
The start symbol is followed by "to" and "from" address information. In
the EXCOUNT-II protocol, the packet ID is followed by message size or
status information.
The message then follows . The following two bytes of the packet com-
prise a 16 bit error checking code (frame check sequence), based on the
X.25 packet standard (ISO 3309). The error checking code is recompu-
ted at the destination to confirm error-free detection. The ISO 3309 frame
check sequence provides very high confidence of error detection for
packets up to 256 bytes in length.
In summary, EXCOUNT-II protocols provides the following features:
• 16-bit ISO 3309 error detection calculation to test message integrity
• Automatic packet retransmission until acknowledgement is received; 8
retries with semi-random back off plus "acknowledge" and "link failure"
alarm messages.
64
EXCOUNT-II RF Link Packet Format
Start From
To Byte
Symbol Byte
Packet Size/Status
Number Byte*
Figure 7.1.1
1HSA 801 080-15en EXCOUNT-II Users Manual
FCS High FCS Low
Message
Byte Byte