TSI switch timing
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TSI switches are circuit switches, so they need to be pre-programmed to connect certain inputs to certain outputs, according to the service currently needed from the network
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Following the example from the lecture, we consider switching input a to output 3, b->4, c->2 and d->1
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The lecture showed a spatial view of a TSI switch, but another perspective is to focus on the timing. In the figure below, time progresses from left to right
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The cells with red backgrounds show the time when each input port is present on the multiplexer output, so when inputs are written to memory
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The cells with blue backgrounds show the time when outputs should be read from memory so that the demultipler feeds the numbered output port
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d--v4
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c--------------v3
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b--------v2
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a--------v1
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d--v4
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c--------------v3
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b--------v2
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a--------v1
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4
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3------TIME------
----->
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1
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The dashes ---- and bends--v show how long each input should be stored in the memory before it is read out at the time indicated with a --v bend
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e.g. input a gets delayed by 2 slots, so it is written to memory during slot 1 (supplying output #1) but is read from memory during slot 3
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So the read order is d (slot #1), c (slot #2), a (slot #3), b (slot #4); that read order would be stored in the list in memory, e.g. at address 'd' indicate the next address to read is address 'c'
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The pattern repeats over time - e.g. as shown by the green cycle which follows the yellow cycle with read order d,c,a,b for outputs 1,2,3,4
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A different configuration: a->2, b->4, c->1, d->3. In this case the order for reading from the memory is c,a,d,b
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d--------------v4
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c--------v3
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b--------v2
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a--v1
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These timing diagrams are similar to those used to describe playout buffers for real-time media (e.g. voice & video), where the receiver compensates for variable network delays by introducing playout delays
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e.g. if the red cells indicate when media was generated, the ---- and --v indicate the delay through the network (e.g. c and d in the first example are misordered: c was sent before d but arrives after it),
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and the blank (yellow) cells after the sample arrives indicate how long the playout buffer must hold the sample so that it can be played out at the right time (blue cells) - creating a steady output despite variable network delays
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