M1 ISO 8583

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MTI, bitmaps, field types and lengths, encodings, the fields that matter and the message flows.

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What this module covers

Track M · Messages and integrity

Bitmap: 64 bits that say which data elements are present. Bit 1 set means a secondary bitmap follows, for fields 65 to 128.
Data element (Field, DE): One numbered field of an ISO 8583 message, with a defined type and length, such as field 4 the amount.
Field 55: The ISO 8583 field that carries the EMV chip data as TLV, such as the ARQC request data or the issuer's ARPC.
ISO 20022: The XML or JSON-based financial messaging standard. It defines card messages too, but ISO 8583 still dominates between acquirers, networks and issuers.
ISO 8583: The standard for card transaction messages between terminals, hosts and networks: an MTI, one or more bitmaps and the data elements they announce.
LLVAR: A variable-length field preceded by a 2-digit length, such as field 2 (PAN) or 35 (track 2). LLLVAR uses a 3-digit length.
MTI (Message type indicator): Four digits starting the message: version, class (1 authorisation, 2 financial, 4 reversal, 8 network management), function (request, response, advice) and origin.
Repeat message: The same request or advice sent again (MTI ending in 1, such as 0401) because no response arrived. The receiver must spot duplicates.
Replacement amounts: Field 95: the actual amounts of a partial reversal or completion, such as the amount an ATM really dispensed.
Secondary bitmap: The second 64-bit bitmap, present when bit 1 of the primary is set, announcing fields 65 to 128.

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