DATEX-ASN allows different systems to exchange relevant data. This is contained in end-application messages. Each end-application message is defined as either a “subscription?? or a “publication?? according to the format as specified in ISO14827-1. DATEX-ASN defines how these end-application messages are packaged to form a complete data packet and also defines the rules and procedures for exchanging these data packets. Systems using DATEX-ASN are free to implement additional end-application functionalities according to the user requirements. A DATEX-ASN network comprises a certain number of systems, an example of which is provided in Figure1. Figure1 An example of a DATEX-ASN network Key 1 weather system 2 traffic management system 3 transit management system 4 emergency management system 5 information service provider Each system can be viewed as consisting of the interfaces, as shown in Figure2: Figure2 System interfaces Key 1 application interface 2 operator interface 3 communication interface 4 database interface 5 communications cloud 6 client system 7 server system This part of ISO14827 deals only with the communications interface. It has been designed to meet the unique requirements of TICS; however, it has been designed in a generic fashion and thus could be used for other data exchanges as well. Systems implementing this part of 14827 sometimes operate simultaneously as a client and server, using multiple sessions. The communications cloud between the two systems may be complex or simple.