【正文】 y or indirectly to the host. Each connection is maintained while the module is present. Each module may manage a number of different sessions with the host. . Physical Implementation The baseline specification includes the implementation on a physical interface patible with the PC Card standard used in the Personal Computer industry. Other physical implementations are allowed for in the future. . ClientServer The interface is designed on the principle that applications, as clients, use resources provided by a server. The applications reside on a module and resources can be served either by the host or another module in a way managed by the host. The term ?resources? has been used in preference to ?services? as that term is mon in the broadcasting field for TV and radio services and there is a need to avoid confusion. . Coding of Data The munication of data across the mand interface is defined in terms of objects. The objects are coded by means of a general TagLengthValue coding derived from that used to code syntax (see [2] and [3]). This is generally extensible. There is a particular transport layer coding for the PC Card implementation but it may be different in other physical implementations. However the semantics would be identical. . Extensibility The higher layers have been designed to be extensible. As indicated above, the TLV coding used is extensible so that new objects can be added, and existing objects can be extended. There is no problem about running out of tag coding space, or length restrictions on the values. The Resource Manager resource provides a mechanism for extending the range of resources provided by hosts, both for CA purposes and for other modulebased applications. . Incorporation of Existing Standards Existing standards have been used, where possible and appropriate, as building blocks for this specification. This gives important timetomarket benefits, as all the standards development work has already been done. It also gives implementation benefits in that software and hardware already developed for existing standards may be reused here, with potential cost benefits. 2. DESCRIPTION AND ARCHITECTURE . Overview A partial logical architecture has been assumed for a host in order to define the place in the host where the mon interface can logically occur. The impact upon the freedom of choice for host designers in other respects has been minimized. Figure shows a simplified picture of typical host architecture and the positioning of the interface within it. Note that there can be more than one instance of the interface on a host. The mon interface consists of two ponents, the Transport Stream Interface and the Command Interface. Both are layered to make the overall interface design and implementation easier. The upper layers are mon to all implementations but alternative lowerlayer implementations are possible. This specification includes one based upon the PC Card standard but others may be included in future versions. . Transport Stream Interface The Transport Stream Interface carries MPEG2 transport packets in both directions. If the module gives access to any services in the transport stream and those services have been selected by the host, then the packets carrying those services will be returned descrambled, and the other packets are not modified. On the Transport Stream Interface a constant delay through the module and any associated physical layer conditioning logic is preserved under most conditions (see section ). The Transport Stream Interface layers are shown below. The Transport Layer and all upper layers are defined in the MPEG2 specification ISO 13818. . Command Interface The Command Interface carries all the munication between the application(s) running in the module and the host. The munication protocols on this interface are defined in several layers in order to provide the necessary functionality. This functionality includes: the ability to support multiple modules on one host, the ability to support plex binations of transaction between module and host, and an extensible set of functional primitives (objects) which allow the host to pr