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

11.7.4.1. A quantitative approach to data touches
Why is so important the number of data touches? Let us consider a main memory bandwidth of about 1.5 GB/s for sequential writes and 0.7 GB/s for sequential reads. If we assume that on the average there are three reads for every two writes (see figure 11.36), the resulting average memory bandwidth is ~ 1.0 GB/s . If our basic unit requires five data touch operations for every word in every cell, then the average throughput we can expect will be only a fifth of the average memory bandwidth, e.g. 0.2 GB/s . Clearly, every data touch that we can save will provide for significant improvements in throughput.
11.7.4.2. Reducing the number of data touches
The number of data touches can be reduced if either kernel buffers or user and kernel buffers are allocated from extra on-chip memory added to the basic unit.
In figure 11.37, kernel buffers are allocated from memory on the basic unit to reduce data touches form 5 to 2. Programmed I/O is the technique used to move data from the user buffer to these on-chip kernel buffers (data is touched by the processor before is transfer to the basic unit).

Figure-11.37:
Figure 11.38 shows the same data touch reduction but with DMA being used instead of programmed I/O . In this case, as data arriving from main memory to the basic unit is not touched by the processor, it cannot compute the checksum needed in the AAL layer; therefore, this computation will have to be implemented in hardware in the basic unit.

Figure-11.38:
Figure 11.39 shows an alternative that involves no main memory accesses at all (zero data touches). Both, user and kernel buffers are allocated from on-chip memory. Although this approach reduces drastically the number of data touches, it has two disadvantages:

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  Very large amount of memory will be needed to allocate user and kernel buffers.
  
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  The API (Application Programming Interface) that will be presented to programmers in this kind of framework will be incompatible with existing socket-based API.