## 6 Metrics for TSN Schedule Evaluation
### Reasons for Nondeterminism in TSN
#### 6.1.3 Inconsistent Network Configuration Updates  
Some use cases may require a dynamic update of the TSN configuration during runtime (e.g., if a base-cycle with periodic updates for the GCL is used, as suggested by [97],or if some of the end-hosts are wireless and changing their location in the network [128]). This is a very complex task, as already scheduled streams shall not violate their constraints during such network updates. In addition, the new configuration must also respect the constraints in terms of timeliness. Finally, the update process should not result in packet loss during and after the update is activated. As every network update results in costs, such costs should be minimized. The configuration update could happen in the whole network at once but this needs a high degree of synchronization in order to decide when to start applying the new configuration. A simpler approach would be to perform the network update in several steps that need to be orchestrated. In the case that streams need to be rerouted during configuration updates, IEEE802.1 CB [129] could be used to make that process seamless. In both cases, frequent updates of the GCL, as in [97], may lead to severe problems. Ref. [28] showed that configuration updates can not be lossless and duplicate-free at the same time and suggested a reconfiguration procedure to prevent either of them. However, independently of the approach, a configuration update should guarantee a smooth continuation of the transmission operations in the network while accounting for the queuing delay and avoiding interference. The abovementioned root causes of nondeterminism may lead to the following problems:  
**Slot slope problem:** A slot slope happens when a frame is ready for transmission, whether before or after the opening of its respective transmission window, as shown in Figure 10. This slope could be caused by a synchronization error or precision error in the planned gate opening timestamp. Large slopes result in frames missing their allotted transmission windows. Small slopes could be mitigated by the scheduling algorithm by introducing a tolerance interval to the length of the transmission window. The amount of tolerance is a scheduling choice. Bigger tolerance intervals (i.e., larger HTS slots) reduce the effect of slope since they reserve more time for HTS transmission. The extra transmission time could be used by the other traffic types. Therefore, a large HTS slot reduces the chances of LTS and BE traffic. On the other hand, very small tolerance (i.e., small HTS slots) could cause the frames to miss their allotted times easily. The lookahead mechanism checks if the remaining time is enough to send the frame before the gate closes, otherwise it blocks the sending. A synchronization error or an incomplete previous transmission (especially if guard-banding is not applied) could trigger the lookahead mechanism to consider the small HTS slot insufficient and deny the transmission. Paths with many hops can contribute to slope accumulation as well, especially if the HTS slots are too wide. Thus, a schedule should have the appropriate amount of tolerance without exaggerations. **Interleaving problem:** Interleaving of frames is explained in [38]. Nevertheless, we mention it briefly here, and for more detail, we refer the reader to [38]. Frame interleaving is an intrat traffic-class problem. In the case that several HTS streams are using the same queue (no spatial isolation) and are having expected arrival times that are relatively close  
Figure 9: Nondeterminism pyramid. (weak temporal isolation), the order of the frames in the queue may be nondeterministic. This could result in different order from one cycle to another, which may lead to variable queuing delay and higher jitter. Frame interleaving in a switch port may lead to slot slope in the next switch. Thus, a schedule should aim to reduce the probability of frame interleaving. In general, the reasons for nondeterminism are correlated and may exacerbate along the path.