Block Recovery Protocol (pdf) - Güralp Systems Ltd

Specification
2 Transmission of data
2.1 GCF blocks
Each transmitted GCF block is pepended with a 4-byte header, shown in table
2.1. The header contains an identifier for synchronising the start of the block, a
sequence number (these are assigned by the transmitter and will increase sequentially,
wrapping from 255 to 0), and the size of the packet. The packet size is in
bytes and may be less than 1024 if there were any unused bytes after the RIC.
Most receivers will want to discard a header (or assume it is terminal data) if the
size is greater than 1024 bytes.
Immediately following the header is the data block itself, truncated to the block
size listed in the header. After the block a 16-bit checksum is sent. The checksum
is computed by finding the sum of all the bytes in the header and data packet and
discarding all but the lowest 16 bits. The remainder is transmitted in network
byte order. The whole transmission is summarised in table 2.2.
2.2 ACK and NAK
After each transmitted block, the transmitter will wait up to n milliseconds (n is
sometimes referred to as the “MS_GAP” setting) for a valid ACK or NAK signal
before transmitting the next block. On receiving a valid ACK block, the transmitter
assumes that the block has been successfully received and will not retransmit
it later. On receiving a valid NAK block, the transmitter can be rewound to an-
15
8 7
Identifier: 0x47: ‘G’
Sequence number
Block size (network byte order)
0
Table 2.1: GCF block BRP header.
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