Asterix Cat 48 Special Purpose Field

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It's not clear to me which field you are asking about so I'll give the answer for the two fields I think you may be asking about.

1) The Asterix Cat 048 - Special Purpose Data Item (or SP for short) with FRN 27 is used by implementations in a proprietary way.

However, the current implementation of the ASTERIX dissector (Wireshark v2.6.5) doesn't actually populate this field with any data, even if there is any.

Therefore the only way to match against it today is to either check if the 048_SP field exists or by checking the entire message, e.g.:

• asterix.048_SP (matches if the asterix.048_SP field exists)
• asterix[23:3]==78:9a:bc (matches if the 3-byte sequence at offset 23 (zero based) is equal to the byte sequence 0x78, 0x9a, 0xbc).
• asterix contains 78:9a:bc (matches if the byte sequence 0x78, 0x9a, 0xbc is found in the field).

2) The Asterix Cat 048 - Special Position Indicator (or SPI for short) within FRN 3 is a 1-bit field/flag within Data Item 020, it can therefore be accessed by using the field name, i.e.:

• asterix.048_020_SPI==1 (matches if the SPI flag is set)

See Display Filters for details.

Edit: Corrected an error on my part where I thought it was possible to probe inside the asterix.048_SP field.