Framework EDI Reference. Encoding Mechanism
EDC Filter

The EDC Filter converts binary to character sets compatible with UN/EDIFACT Syntax Level C, D, E and F character set repertoires.  The following describes the encoding mechanism as implemented by Framework EDI and as defined in ISO 9735-5: Security Rules for Batch EDI.

Encoding

The procedure for encoding is as follows:

1. The data is subdivided into 7-byte subgroups.  The last subgroup in the data may have 7 or less bytes.
   
2. For each 7-byte subgroup, a single control byte, is prepended with a starting value of 64 (01000000).  All bits in the control byte represents the data bytes that are in the same respective position, except for bit 0 which represents data byte 1.  That is, bit position 0 represents data byte 1, and bit position 2 to 7 represents data byte 2 to 7 in the subgroup.
   
   Example.  The figure below shows 8 bytes: 1 control byte and 7 data bytes.  The control byte has decimal value 64 having digits 01000000.  The data bytes comprise of "a", "1", "b", "2", "c", "3", and "d". 
   
   

   Control Byte								Data Byte 1								Data Byte 2								Data Byte 3								Data Byte 4								Data Byte 5								Data Byte 6								Data Byte 7
   0	1	0	0	0	0	0	0	a								1								b								2								c								3								d

   
   
   
3. Each data byte in the subgroup, the bit in position 1 is set to 1 if and only if the two conditions are met:
   • The decimal value of the byte is not zero.
   • Bit position 1 is 0.
     
   If the transformation is made, then the index position of the byte in the subgroup is used to set the corresponding position in the control byte to 1.
   
   Example.  The 8-byte subgroup having data "a1b2c3d" will be encoded to "jaqbrcsd" and will have the byte stream as follows (greyed):
   • The value "1" with digits "00110001" is transformed to "01110001".  Because it is in data byte 2, bit 2 of the control byte is set to 1.
   • The value "2" with digits "00110010" is transformed to "01110010".  Because it is in data byte 4, bit 4 of the control byte is set to 1.
   • The value "3" with digits "00110011" is transformed to "01110011".  Because it is in data byte 6, bit 6 of the control byte is set to 1.
     
     

   	Control Byte								Data Byte 1								Data Byte 2								Data Byte 3								Data Byte 4								Data Byte 5								Data Byte 6								Data Byte 7
   									a								1								b								2								c								3								d
   Initial Bits	0	1	0	0	0	0	0	0	0	1	1	0	0	0	0	1	0	0	1	1	0	0	0	1	0	1	1	0	0	0	1	0	0	0	1	1	0	0	1	0	0	1	1	0	0	0	1	1	0	0	1	1	0	0	1	1	0	1	1	0	0	1	0	0
   Transformation Bits	0	1	1	0	1	0	1	0	0	1	1	0	0	0	0	1	0	1	1	1	0	0	0	1	0	1	1	0	0	0	1	0	0	1	1	1	0	0	1	0	0	1	1	0	0	0	1	1	0	1	1	1	0	0	1	1	0	1	1	0	0	1	0	0
   Encoded Char	j								a								q								b								r								c								s								d
   Byte Stream	0x6A								0x61								0x71								0x62								0x72								0x63								0x73								0x64

Decoding

The procedure for decoding is as follows:

1. The encoded text is subdivided into 8-byte subgroups.  The last subgroup in the data may have 8 or less bytes.
   
2. The first byte is the control byte and is stripped of from the remaining 7 data bytes in the subgroup.  The bits 0 and 2 to 7 in the control byte correspond to the positions 1 to 7 in the data bytes.  That is bit 0 correspond to data byte 1, and bit 2 to 7 correspond to data byte 2 to 7.
   
   Example.  The encoded text "jaqbrcsd" will have the char "j" stripped of as the control byte, and "aqbrcsd" will be the data bytes.
   
   

   Byte 1

   Byte 2

   Byte 3

   Byte 4

   Byte 5

   Byte 6

   Byte 7

   Byte 8

   Encoded Char

   j

   a

   q

   b

   r

   c

   s

   d

   Byte Stream

   0x6A

   0x61

   0x71

   0x62

   0x72

   0x63

   0x73

   0x64

   
   The control byte is interpreted as follows:
   • Bit 0 is 0.  Corresponds to data byte 1.
   • Bit 1 is 1.
   • Bit 2 is 1.  Corresponds to data byte 2.
   • Bit 3 is 0.  Corresponds to data byte 3.
   • Bit 4 is 1.  Corresponds to data byte 4.
   • Bit 5 is 0.  Corresponds to data byte 5.
   • Bit 6 is 1.  Corresponds to data byte 6.
   • Bit 7 is 0.  Corresponds to data byte 7.
     

   	Control Byte
   Encoded Char	j
   Byte Stream	0x6A
   Encoded Bits	0	1	1	0	1	0	1	0

   
   The data bytes "aqbrcsd" are as follows:
   
   

   	Data Byte 1								Data Byte 2								Data Byte 3								Data Byte 4								Data Byte 5								Data Byte 6								Data Byte 7
   Encoded Char	a								q								b								r								c								s								d
   Byte Stream	0x61								0x71								0x62								0x72								0x63								0x73								0x64
   Encoded Bits	0	1	1	0	0	0	0	1	0	1	1	1	0	0	0	1	0	1	1	0	0	0	1	0	0	1	1	1	0	0	1	0	0	1	1	0	0	0	1	1	0	1	1	1	0	0	1	1	0	1	1	0	0	1	0	0

   
   
3. For each bit that is set to 1 in the control byte, its corresponding byte has its bit in position 1 set to 0.  That is, if the control byte has its bit in position 0 set to 1, the data byte in position 1 has its bit in position 1 set to 0.
   
   Example.  The encoded text "jaqbrcsd" decodes to the text "a1b2c3d" (greyed).  The transformed bits that are affected by the control byte are in green.
   
   
   • Bit 0 in control byte is 0 so data byte 1 is not changed.
   • Bit 2 in control byte is 1 so data byte 2 has bit 1 changed to 0.  From 01110001 to 00110001.
   • Bit 3 in control byte is 0 so data byte 3 is not changed.
   • Bit 4 in control byte is 1 so data byte 4 has bit 1 changed to 0.  From 01110010 to 00110010.
   • Bit 5 in control byte is 0 so data byte 5 is not changed
   • Bit 6 in control byte is 1 so data byte 6 has bit 1 changed to 0.  From 01110011 to 00110011.
   • Bit 7 in control byte is 0 so data byte 7 is not changed.
     

   	Data Byte 1								Data Byte 2								Data Byte 3								Data Byte 4								Data Byte 5								Data Byte 6								Data Byte 7
   Encoded Char	a								q								b								r								c								s								d
   Byte Stream	0x61								0x71								0x62								0x72								0x63								0x73								0x64
   Encoded Bits	0	1	1	0	0	0	0	1	0	1	1	1	0	0	0	1	0	1	1	0	0	0	1	0	0	1	1	1	0	0	1	0	0	1	1	0	0	0	1	1	0	1	1	1	0	0	1	1	0	1	1	0	0	1	0	0
   Transformed Bits	0	1	1	0	0	0	0	1	0	0	1	1	0	0	0	1	0	1	1	0	0	0	1	0	0	0	1	1	0	0	1	0	0	1	1	0	0	0	1	1	0	0	1	1	0	0	1	1	0	1	1	0	0	1	0	0
   Decoded Char	a								1								b								2								c								3								d