The Transformation from MCS to Network Primitives

The MCS is depicted by a G-net graph. A G-net is composed of places and transitions. In a place we can describe what kind of action will be performed. For example, a commonly performed action is to send an MMO into the multimedia communication network. A transition tells us which place(s) will be enabled next.

There are five network primitives [13, 14]:

1. open(MMO_id)
2. send(MMO_id, type, time_spec, priority, progress_flag)
3. retrieve(MMO_id, type, time_spec, priority, progress_flag)
4. synchronize(MMO_id)
5. close(MMO_id)

The MCS will be transformed into a sequence of these five kinds of network primitives. Using for example TCP/IP Unix mail commands, MMOs can be sent or retrieved between two machines.

All MMOs can be classified into three classes, which are real time, restricted, and best effort. The real time class includes the objects specified as real time by the users, the objects to be retrieved and specified as real time retrieval by the remote users, and the first object in a progressive transmission. The restricted class includes all objects specified by the users as time restricted objects. For example, a user can request the transmission of an image to be completed in eight seconds. The best effort class includes all objects with no time restriction, for example, the details of a progressively transmitted object.

The program MM_Send will send all MMOs in the real time class first, then the restricted class, and the best effort class last. When MM_Send sends objects in the restricted class, it must make sure that all MMOs in the real time class have already been sent. In the same way, when MM_Send sends objects in the best effort class, all MMOs in the real time and the restricted classes must have been sent beforehand. A special type of object, retrieve, can be sent to request retrieval of MMOs from the receiver. A G-Net of MCS, , is embedded in the retrieve type object to specify the communication schema of the retrieval.

Another program MM_Receive will check either the system mailbox or a directory specified by the user for the retrieval of MMOs. In the former case, the MMOs are extracted from the system mailbox and put into directory /tmp/MM ready to be transmitted. Another task of is to deal with retrieve type of MMOs and informs MM_Send program to send MMOs to the remote requester according to the embedded .