In 5G networks, the end-to-end latency KPI is a critical KPI in order to ensure that the network can deliver the packet within a time limit specified by an application: not too early and not too late.
In cyber-physical automation, the arrival time of a specific packet should be strictly inside a prescribed time window. In other words, a strict time boundary applies: [minimum end-to-end latency, maximum end-to-end latency]. Otherwise, the transmission is erroneous. Although most use cases that require timely delivery only specify the maximum end-to-end latency, the minimum latency is also sometimes prescribed. In the latter case, a communication error occurs if the packet is delivered earlier than the minimum end-to-end latency. An example for a related application is putting labels at a specific location on moving objects, and the arrival of a message is interpreted as a trigger for this action. In other words, the application does not keep its own time, but interprets the message arrival as clock signal. Maximum and minimum end-to-end latency alone do not disclose which value is preferred, i.e. target value. The next three subclauses introduce concepts help with relating maximum end-to-end latency, minimum end-to-end latency, and target vale to each other.
Timeliness is described by a time interval (see Figure C.4.3-1
). The interval is restricted by a lower bound (tLB
) and an upper bound (tUB
). This interval contains all values tA
that are within an accepted "distance" to the target value tR
A message reception is considered in time, if it is received within the timeliness interval. If it is received outside the timeliness interval, the message reception is considered invalid. This is related to the communication error "unacceptable deviation from target end-to-end latency" (see subclause B.6). In other words, maximum end-to-end latency = tUB
and minimum end-to-end latency = tLB
Timeliness is related to deviation (see subclause C.4.4), the lower bound tLB
is related to earliness (see subclause C.4.5), and the upper bound tUB
is related to lateness (see subclause C.4.6).
The term deviation describes the discrepancy between an actual value (tA
) and a tA
rget value (tR
) = tA
shows two examples. The tA
rget value is 10 time units (tR
= 10) in both cases. In the first case (blue) the actual value measures 12 time units (tA
= 12). The difference of both amounts to +2 time units, which means that the deviation is 2 time units [Accurracy(tA
) = 2]. The second case (purple) shows the actual value as 9 time units (tA
= 9). The difference of both amounts to -1 time unit, which means that the deviation is -1 time units [Accuracy(tA
) = -1].
shows the deviation with respect to the tA
rget time (t). The following applies:
Deviation(t) < 0 for t < tR
; that is, the arrival is early.
Deviation(t) = 0 for t = tR
; that is, the arrival is as desired, i.e. on time.
Deviation(t) > 0 for t > tR
; that is, the arrival is late (see also clause C.4.6