BER (Basic Encoding Rules) - an explanation

A set of rules for translating ASN.1 values into a stream of octets to be transmitted across a network.The compilation rules for converting data-types into bits and bytes is called the transfer syntax.Basic Encoding Rules or BER is the transfer syntax for SNMP and LDAP. BER and ASN.1 were designed by the same people (not network administrators alone) and hence go hand in hand. This fact makes life simpler on this planet. But how does BER manage to do all this, after all it has to take case of so many data-types, and it is used to write hundreds of protocols. It is indeed mind boggling. To know more about BER lets get cracking Fortunately we will only discuss BER in the context of ASN.1 implementation of SNMP and LDAP. BER uses the concept of an 'identifier'. An identifier is a unique code assigned to every data-type. This identifier acts as the calling code for that data-type. We distinguish between two data-types using these identifiers. The data-types are further classified into groups. But before we proceed any further answer this simple question. Are the books in your college or school library piled one over the other with no semblance of order? The answer is a definite NO. (Those answering yes will have to get a new librarian). The books out there are classified into various groups according to a pre-defined order like subject, ISBN number etc. The designers of BER being an inspired lot, did the same thing. They classified the BER identifiers into 4 classes.

Universal Available for use within any protocol. The primitive data-types -INTERGER, OCTECT string, OBJECT IDENTIFIER, and NULL, are universal. The basic constructors, such as SEQUENCE, also are universal.
Application Available within a specific application. For example, the IpAddress data-types is available for use throughout the TCP/IP network management application.
Context-specific This data-type is contained in a larger data-type. The identifier has a unique meaning within the context of the larger data-type (like those featured in 2 and 5 above)
Private Included so that ASN.1 could be used by private organizations to define proprietary data-types.

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