doc: Some clarifications on iso8583 tutorial about the encoding of fi…

…elds [ci skip]
kobexboi · Oct 8, 2014 · b187e3e · b187e3e
1 parent 4eff270
commit b187e3e
Showing 1 changed file with 12 additions and 3 deletions.
diff --git a/doc/src/asciidoc/ch02/iso8583.asc b/doc/src/asciidoc/ch02/iso8583.asc
@@ -36,6 +36,13 @@ ISO-8583 messages are composed by fields, which are represented in different way
 |2 .. 128   |Other fields as specified in bitmap
 |========================================================================
 
+[NOTE]
+The bitmaps are encoded in network byte order, with the most significant bit (leftmost bit) of the first
+byte indicating presence of a secondary bitmap. Then, the next bit towards the right indicates presence
+of field 2, the next one refers to field 3, and so on.
+
+
+
 So let's have a look at a simple example: 
 
 .Sample 0800 message
@@ -161,7 +168,7 @@ Variable length fields have a prefix specifying its length, but how this is repr
 is not defined. Different vendors use different representations (e.g., BCD, EBCDIC, 
 binary value). 
 
-In our example, field #3 is using a BCD representation, so a value of "000000" 
+In our example, field #3 is using a BCD representation in network byte order, so a value of "000000" 
 is represented with just three bytes whose hex values are "00 00 00". 
 Same goes for field #11 whose value is "000001" - it is represented as "00 00 01". 
 In our example, field #41 is an eight-byte alphanumeric field represented as eight ASCII characters 
@@ -242,11 +249,13 @@ Secondary bitmap: 04000000 00000000
         Field 03: 000000
         Field 11: 000001
         Field 41: 3239313130303031 (ASCII for "29110001")
-        Field 60: 0010 6A504F5320312E392E31 (length=10, value="jPOS 1.9.1")
+        Field 60: 0010 6A504F5320312E392E31 (length=10, value="jPOS 1.9.1") <1>
         Field 70: 0301
  
 ----
 
+<1> In this example, the length prefix in field 60 is expressed as a 2-byte BCD number; therefore, a length of 10 is encoded as 0010 in hexadecimal.
+
 Let's break down this bitmap: 
 
 .Primary Bitmap
@@ -314,7 +323,7 @@ of +headers+.
 
 A few of them (especially stream-based ones) require some kind of trailers as well. 
 
-So wire protocol is composed by: 
+So, the wire protocol is composed by: 
 
 * An optional header / message boundary delimiter
 * ISO-8583 message data