Abstract Syntax and Transfer Syntax

Application exchanges involve transmitting data structures, which may be quite complex. There is a need for an abstract syntax and encoding rules to specify the representation for data types defined in the abstract syntax. These issues have been addressed by both CCITT and ISO. In X.409 (CCITT recommendation), CCITT defines a standard notation and a standard representation for data types and values. These terms correspond to the terms abstract syntax and transfer syntax/ encoding rules. ISO issued separate documents for these two concepts, one called Abstract Syntax Notation One (ASN.1) [ISO 8824] , and the other called Basic Encoding Rules (BER) [ISO 8825].

BER vs XDR

Informal measurements comparing BER to Sun Microsystems' XDR show that to encode an integer on a Motorola 68020 using BER is between three to twenty times slower than using XDR. Some have claimed that XDR can be improved upon. This has led to a controversy regarding the cost of using ASN.1 and the BER.

The Presentation Layer