    UDT Series on Data Communication Technologies and Standards for Libraries Models for Open System Protocol Development : A Technical Report (1994)7. ModesThe TC46 Library and Documentation protocols can be used in two different modes. One is the real-time communication mode where the APDUs are mapped directly to ACSE and Presentation (P-DATA). The other is the Store and Forward mode where the APDUs are mapped to X.400.7.1 Connection Mode SR with Connection Mode ILLIn this mode all communication takes place in real-time. It means that the end-user may negotiate an ILL in a single terminal session by using SR to locate an item and then using that citation in an ILL request. The services in the Tracking phase will, of course, be used over a longer time span since an ILL transaction may last from one or more weeks up to several years. However, the individual services in the Tracking phase are also performed in real-time communication. This means that a Renew request can perhaps be answered within seconds. This does not restrict the Target system in its handling of ILL, if the Target's policy is to let a human evaluate the Renew request (and still operate in this mode), the answer to the Renew request must contain information about the renew policy.7.2 Connection Mode SR With Store and Forward Mode ILLIn many environments the SR will be used in real-time communication while the ILL transactions may be performed in a Store and Forward mode. This combination must be handled by the local system (Origin) and the end-user does not need to know that two different modes of operation are involved. This means that using the SR, the end-user is guaranteed answers to requests in real time, while when using the ILL services there is not necessarily a time limit for the answer.7.3 Connection Mode ILL with Store and Forward Mode ILLSome libraries will operate ILL in a real-time communication environment, others will operate ILL in a Store and Forward environment. For one library in one environment to communicate with a library in another environment, it is necessary to establish relay systems that are able to "translate" between the two. The end-user in the real-time environment is, however, not guaranteed any limit to the response time when communicating over a relay. The relay functionality is for further study.
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