ISO8583 financial transaction message format

When is about card initiated transactions is about ISO 8583 financial transaction message format standard.

Introduction

Usually, a card initiated transaction travels from a point-of-sale terminal (POS) or an automated teller machine (ATM), using a series of networks to the card issuer entity for authorization against the card holder's account. The transaction data contains information from the card (PAN, expiry date), from the terminal (transaction number, merchant data) and information added dynamically during the process and by the intervening systems. The card issuer entity will authorize or decline the transaction and generate a response message which must be delivered back to the POS or ATM within a predefined time period (usual this period is 30 sec).

In this chain of systems from the terminal (POS or ATM) to the card issuer entity and back usually is used ISO 8583 message format. Although ISO 8583 defines a common standard, different systems may use the fields in different ways because only some fields are standards fields and the meaning is the same for all systems. Some of the fields are generic and can be used by each entity in a custom way.

There are few versions of the ISO 8583 standard: ISO 8583:1987, ISO 8583:1993, ISO 8583:2003. 1987 and 1993 versions are very similar and version 2003 is very different and was created to be more generic. In this tutorial, I will concentrate on ISO 8583:1987 version, as is still the most widely used. The main idea is the same for all versions. The differences are on fields levels and meanings.

Message Format

An ISO 8583 message is made of the following parts:

• Message type indicator (MTI)
• One or more bitmaps, indicating which data elements are present.
• Data elements, the fields of the message.

Message Type Identifier (MTI)

Message Type Identifier is a key element which classifies the function of the message. MTI is a 4 digits long field and each digits have its own meaning:

• First digit: ISO 8583 version;
• Second digit: Message Class;
• Third digit: Message Function;
• Fourth digit: Message Origin.

ISO 8583 version

Currently there are 3 versions.

Message Class

Position two of the MTI specifies the purpose of the message.

Message function

Position three of the MTI specifies the message function.

Message origin

Position four of the MTI defines the message source within the payment chain.

Examples

Here is a list with some MTI and the usage.

Bitmaps

Next area in the ISO8583 message is bitmaps area. A bitmap is a field indicates which data elements may be present or not in the message. A message will contain at least one bitmap, called the Primary Bitmap which indicates which of Data Elements 1 to 64 are present. A secondary bitmap may also be present as data element one and indicates which of data elements 65 to 128 are present. Also, a third bitmap can be used to indicate the presence or absence of fields 129 to 192.

Example

Note: the first bit of each bitmap signifies the presence of the next bitmap.

Data Elements

Data elements are all the fields that contain the transaction information. ISO8583:1997 contains up to 128 data elements (the message will have up to 2 bitmap fields). The later versions contain up to 192 data elements (the message will have up to 3 bitmaps fields). Each field (data element) has a specific meaning and format: some are for general purpose or system related fields or country specific fields.

Format

Each field is described in a standard format which defines the content format or the field length.

ISO-defined data elements