length value for a CHOICE is the total TLC length of the value that is selected. IA5String consists of characters from a seven-bit alphabet that is defined in ITU-T T.50, where each character is encoded in one octet. For more details about the encoding algorithms, see ITU-T X.209. Note that the BER method creates quite a few overhead bits (the total length of this ENC mes- sage is 28 octets). ASN.1 includes facilities for inhibiting encoding the T-octet, which is actually superfluous in many occasions, e.g. when types are tagged, as in CHOICE. When BER-coding is used the L-octet is always provided. However, this 5 Protocol Network Modelling also seems superfluous in those cases where the abstract syntax defines length-con- straints. For example the abstract syntax in Fig. 5.28 defines only ten possible val- ues for PrimType which implies that all PrimType values fit in four bits. As a matter of fact, since all parameters of this particular abstract syntax are constrained in size, a much more compact encoding could have been devised, possibly also com- prising several parameters included in one single octet. However, this requires encoding methods other than BER, see more about this subject in Sect. 5.7. The OSI presentation-layer service definition assumes that the presentation layer can handle a number of alternative encoding algorithms for any particular set of data types. This has two consequences: the standards for ASN.1 and BER are separated (which they were not, initially), and the operations of the presentation service interface also allow ENC users to select one of an alternative set of transfer syntaxes. However, the latter requires that before peer-ENC users can start commu- nicating, peer-ENCs must inform each other about that selection and, for each abstract syntax, agree on transfer syntaxes that both parties can handle. The agreed- upon relation between an abstract syntax and a transfer syntax is called a presenta- tion context. This negotiation procedure is the essence of the OSI presentation layer