Interactive multimedia (inm) relates to the network delivery of rich digital content, including audio and video, to client devices

11.7.4. Communication with the host processor: protocol processing & DMA.
Another important point to face is the information exchange between the software running on the host processor and the basic unit. The main mechanism used for these transactions is DMA (Direct Memory Access). In this technique all communications passes through special shared data structures - they can be read from or written to by both the processor and the basic unit - that are allocated in the system's main memory.
Any time any data is read from or written to main memory is consider to be "touched". A design should try to minimize data touches because of the large negative impact they can have on performance.
Let us imagine we are running, on a typical monolithic Unix Kernel machine, an INM application over an implementation of the AAL/ATM protocol. Figure 11.35 shows all data touch operations involved in transmitting a cell from host main memory to the basic unit. The sequence of events is as follows:

1. 
   The application generates data to be sent and writes it to its user-space buffer. Afterwards, It produces a system call to the socket layer to transmit data.
   

To copy data from the user buffer into a set of kernel buffers, both of them located in main memory, steps 2 and 3 are needed:

2. 
   The socket layer reads data from main memory.
   
3. 
   The socket layer writes data to the main memory.
   

Figure-11.35:
To adapt this data to ATM transmission step 4 is needed.

4. 
   AAL layer implementation reads data so that it can segment it and compute the checksum that has to be inserted in the AAL_PDU trailer.
   
5. 
   The basic unit reads data from kernel buffers, adds the ATM cell header and transmits it.
   

Figure 11.36 shows what happens in hardware for the events explained above. Some of

Figure-11.36:
the lines are dashed to indicate that the corresponding read operation might be satisfied from the cache memory rather than from the main memory. In the best case, there are three data touches for any given piece of data and in the worst case, there are five data touches.