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

11.7.1. A system view
Distributing the multiplexing function between the different sources allows meeting efficiently the requirements of mobility/portability and streaming scalability.

Figure-11.28:
This distribution can be achieved with a basic unit that applies locally the multiplexing function to each source, as can be seen in figure 11.28. This basic unit is repeated for each stream that we want to multiplex. Figure 11.29 shows how the basic unit works: there is a queue, where cells carrying information from the source wait until the MAC (Medium Access Control) unit gives permission to the cells to be inserted. When an empty cell is found and the MAC unit allows insertion, this empty cell disappears from the flow and a new cell is inserted.
Figure 11.30 shows the details of this basic unit. There are four main blocks:

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  Cell multiplexing unit: where empty cells are substituted by source cells when MAC makes the decision.
  
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  MAC: decides when the information coming from the video source is introduced into the high-speed flow.
  
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  QoS control: manages video information in order to produce a smooth quality of service degradation when network suffers from congestion.
  
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  Protocol processing & DMA blocks: they, respectively, adapt information coming from the source for ATM transmission and communicate with the software running in the host processor.
  

Figure-11.29:
The path followed by a cell from the source to the output module when is multiplexed is also shown in figure 11.30.

Figure-11.30:
In what follows, we will get into the details of the QoS block, MAC block and protocol processing and DMA block, leaving up to the end the cell multiplexing unit block to explain the main design features of telecommunication ASICs.
11.7.2. Quality of Service (QoS) control (Prioritization)
One potential problem in ATM networks, caused by the bursty nature of traffic is cell loss. When several sources transmit at their peak rates simultaneously, the buffers available at some switches may cause overflow. The subsequent drops of cells lead to severe degradation in service quality (multiplicative effect) due to the loss of synchronization at the decoder. In figure 11.31, The effect in the quality of the image received due to cell drops is shown. The decoded picture has been transmitted through an ATM network with congestion problems.
Rather than randomly dropping cells during network congestion, we might specify to the ATM network the relative importance of different cells (prioritization) so that only less important ones are dropped. This is possible in ATM networks thanks to the CLP (cell loss priority) cell header bit. Thus, if we do so, when the network enters a period of congestion, cells are dropped in an intelligent fashion (non-priority cells first) so that the end-user only perceives a small degradation in the service's QoS.

Figure-11.31:
However, when the network is operating under normal conditions, both high priority and low priority data are successfully transmitted and a high quality service is available to the end user. In the worst-case scenario, the end user is guaranteed a predetermined minimum QoS dictated by the high priority packets.