rica and Europe to adapt An X.400 message consists of an envelope (P1) and content (P2 the X.400 standard for EDI. for IPM). A P2 content has a header and a body, as in Figure 38.4. The key elements of the 1984 standard are the message transfer In adapting X.400 1984 for EDI two approaches were possible. The agent (MTA), the user agent (UA). These are processes (composed first, adopted in Europe on the recommendation of the CEC, is to of software and hardware) which provide services for moving put the EDI message into the P2 body part. A copy of the originator messages from site to site and enabling users to send and receive and recipient from the EDI message header are put into the P2 messages. Figure 38.3 illustrates these concepts. Note that human heading. users are explicitly included in the model! The second approach, standardised by NIST in the U.S., is to There are other key concepts in X.400 as follows: create a new content type, PO, for the EDI message. The EDI message goes directly into the P1 envelope as an 'undefined' con- 1. MHS message handling system tent type. Message Handling Environment Denotes an interaction Figure 38.3 Functional view of MHS model 38/6 Electronic data interchange Envelope P1 As above but no responsibility is taken for notification. This will be handled by the final recipient UA. 4. The EDI UA uses a VAN for its MTA services. In this case the VAN would support the MS 5. EDI UA and MTA are co-located entirely on a VAN service. HEADING Here the VAN would most likely be supplying not only store CONTENT and forward services but also protocol translations. TYPE P2 (TEXT) 38.6 X.500 directory services As the use of X.400 increases the need for an automated directory Figure 38.4 Structure of an X.400 I.P.M. service grows. This was recognised quite soon in the process of X.400 definition and in 1988 a standard for electronic directory services was published and this is X.500. Note that it is not possible to interchange messages of different X.500 will be particularly useful for EDI sin