Exercise 38: Re-defining the server capacity (streams)

We originally estimated the capacity of the video server in bandwidth terms (i.e., Mbit/s), and linked this to busy-hour traffic for the service which was then also defined in Mbit/s. Now the service traffic has been re-defined in terms of video Erlangs, we must either drive the resource by the separate Video bandwidth transformation, or think about its capacity differently.

Requirements

We will try the simple alternative first!

1. Remove the requirement of the Video service for the Video server resource. (Right-click the link and select Unlink.)
2. Use drag-and-drop, or the connection tool, to create a new requirement from the Video bandwidth transformation for the Video server resource.

3. Check the Resource Capacities graph for the Video server. Although the demand is not exactly the same as it was before, it should revert to an upgrade being required in 2012.

Capacity unit

We assumed before that the capacity of the video server would be limited by memory and throughput. In practice the vendor might specify this in terms of concurrent streams, and now we will re-model the resource on this basis. For a single stream rate of 5.5Mbit/s, a capacity of 256Mbit/s would support a little over 46 channels, but the vendor might write this down more conservatively as 45 channels.

4. Go back to the Editor and re-define the inputs Capacity = 45.0 and Capacity Unit = Channels for the Video server resource.
5. Remove the requirement from the Video bandwidth transformation for the Video server resource.
6. Use drag-and-drop, or the connection tool, to create a new requirement from the Video channels transformation for the Video server resource.

7. Re-run the model. The Resource Capacities graph should come out about the same, but now measured in Channels rather than Mbit/s.