The assessment of periodic deterministic communication services is based on the assessment of successful message transmission over a logical communication link. Message transmission is either:
successful, if it is correctly and timely received, or
unsuccessful, if it is incorrectly received, lost or untimely.
A lost message is a message which left the source application and never reached the target application.
Up time and down time can be derived from received messages. As far as timely received messages are correct, the logical communication link status is up. If a message loss or an incorrectly or untimely received message is detected the logical communication link status is down. To denote up and down states the terms "up time interval" and "down time interval", or alternatively "available" and "unavailable" may be used. An example of the relation between logical communication link status, communication service status and application status is presented in Figure C.3-1.
The flow of events in Figure C.3-1 is as follows:
The logical communication link is up and running (blue line is UP). A source device starts sending periodic messages to a target device (orange arrows), on which an automation function (application) is running. The communication service is, from the point of view of the target application, in an up state (violet line is UP) and so is the application (green line is UP).
The logical communication link status changes to down state if it no longer can support end-to-end transmission of the source device's messages to the target device in agreement with the negotiated communication requirements. Once the application on the target device senses the absence (or unsuccessful reception) of expected messages ("Deadline for expected message" in Figure C.3-1), it will wait a pre-set period before it considers the communication service to be unavailable; this is the so-called survival time. The survival time can be expressed as
a period or,
especially with cyclic traffic, as maximum number of consecutive incorrectly received or lost messages.
If the survival time has been exceeded, both the communication service and the application transition into a down state (violet and green lines change to DOWN in Figure C.3-1). The application will usually take corresponding actions for handling such situations of unavailable communication services. For instance, it will commence an emergency shutdown. Note that this does not imply that the target application is "shut off"; rather it transitions into a pre-defined state, e.g. a safe state. In the safe state, the target application might still listen to incoming packets or may try to send messages to the source application.
Once the logical communication link status is in the up state again (blue line in Figure C.3-1 changes to UP), the communication service state as perceived by the target application will change to the up state. The communication service is thus again perceived as available (violet line changes to UP in Figure C.3-1). The state of the application, however, depends on the counter measures taken by the application. The application might stay in down state if it is in a safe state due to an emergency shutdown. Or, the application may do a recovery and change to up state again. The time needed for the application to return to the up state after the communication service is restored is shown as "Application recovery time" in Figure C.3-1.
The availability of the communication service is calculated using the accumulated down time. For instance, in case the communication service is expected to run for a time T, the unavailability U of the communication service can be calculated as
Where Δti is the length of the i-th downtime interval of the communication service within the time period T. The communication service availability A can then be calculated as
A = 1-U.
There are several time parameters in dependability assessment. A required value is specified for every time parameter. This value can be a maximum, mean, modal, minimum etc. Typically, there is a deviation from the desired value to the actual value. Jitter is often used to characterise this variation. Since jitter generally is used for characterising the behaviour of a measured parameter, for instance the scatter of measured end-to-end latencies ("the world as it is"), it can be quite confusing to use it for formulating service performance requirements ("the world as we want it to be"). What is needed is a concept and related parameters that allow for formulating and talking about the end-to-end latency requirements in Clause 5
and Annex A.
The most important attribute is timeliness. Timeliness can be formulated a permitted interval for the actual value of the time parameter. Accuracy, earliness and lateness describe the allowed deviation from a target value. Accuracy is the magnitude of deviation. It can be negative (early) or positive (tardy).
C.4.2 Network latency requirement formulated by use of timeliness Word‑p. 65
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.
Figure C.4.3-1: Timeliness function
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 target value (tR).
Deviation(tA) = tA - tR.
Figure C.4.4-1 shows two examples. The target 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].
Figure C.4.4-1: Examples for accuracy values
Figure C.4.4-2 shows the deviation with respect to the target 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 C.4.6)
Figure C.4.4-2: Accuracy function
C.4.5 Earliness Word‑p. 66
Earliness describes how early the actual value is: earliness is greater than 0 if the actual value is less than the target value (see Figure C.4.5-1). The following applies:
Eearliness(tA) = tR - tA = -Deviation(tA) for tA < tR;
Eearliness(tA) = 0 for tA ≥ tR.
In an example, the target value is 10 time units (tR = 10), and the actual value is 7 time units (tA = 7). The difference of both is 3 time units with respect to being early. That means that the earliness is 3 time units [Eearliness(tA) = 3].
Figure C.4.5-1: Earliness function
C.4.6 Lateness Word‑p. 67
Lateness describes how much greater the actual value is than the target value: lateness is greater than 0 if the actual value is greater than the desired value (see Figure C.4.6-1). The following applies:
L(tA) = 0 for tA ≤ tR;
L(tA) = tA-tR = Deviation(tA) for tA > tR.
In an example, the target value is 10 time units (tR = 10), and the actual value measures 14 time units (tA = 14). The difference of both is 4 time units with respect to being late. That means that the lateness is 4 time units [L(tA) = 4].
Figure C.4.6-1: Lateness function
Using the concepts of earliness and lateness (see Subclauses C.4.5 and C.4.6, respectively), the maximum and minimum end-to-end latency can be rewritten as follows.
Maximum end-to-end latency = target end-to-end latency + maximum lateness;
Minimum end-to-end latency = target end-to-end latency - maximum earliness.
This section clarifies the wording and terminology with respect to communication interfaces that are relevant for vertical applications. Because the 3GPP network does not cover the complete ISO-OSI communication stack, it is important to distinguish between
the vertical applications' point of view, and
the 3GPP network's point of view.
In this section, the relation between those two is clarified.
Figure C.5-1 shows a simplified version of the communication stack. The PHY layer, the MAC layer and some parts of the IP layer are part of the 3GPP network. The layers that are part of the 3GPP network are referred to as lower communication layers (LCL). The communication stack also includes an application. The OSI layers related to providing data to the application are referred to as the higher communication layers (HCL). The interface between LCL and HCL is referred to as communication service interface (CSIF).
For the assessment of the overall system performance, it is important to differentiate between the 3GPP network's performance (i.e., including only the LCL and measured at the CSIF) and the overall system performance including the application layer (i.e., including both, the LCL and the HCL). In Figure C.5-1, the orange arrow depicts the vertical application's point of view. The blue arrows indicate two options to measure the 3GPP network's performance, i.e., including and excluding the IP layer.
Figure C.5-2 illustrates how messages are transmitted from a source application device (e.g., a programmable logic controller) to a target application device (e.g. an industrial robot). The source application function (AF) is executed in the source operating system (OS) and hands over a message to the application layer interface of the source communication device. In the higher communication layers (HCL), which are not part of the 3GPP system, the data is processed. From the HCL the data is transferred to the lower communication layers (LCL), which are part of the 3GPP system. After transmission through the physical communication channel and the LCL of the target communication device, the data is passed to the HCL and lastly to the target application device. Characteristic parameters with respect to time are defined in Figure C.5-2.
From 3GPP system point of view:
Transfer interval of 5G system: Time between the arrival of two pieces of data at the source CSIF.
End-to-end latency: Time measured from the point when a piece of data received at the CSIF in the source communication device until the same piece of data is passed to the CSIF in the target communication device.
From vertical application point of view:
Transfer interval of vertical application: Time between the transmission of two successive pieces of data from the source application.
Transmission time: Time measured from the point when a piece of data is handed from the application layer interface of the source application device, until the same piece of data is received at the application layer interface of the target application device.
Update time: Time between the reception of two consecutive pieces of data at the application layer interface to the target application device.
If not stated otherwise, the terms "end-to-end latency" and "transfer interval" refer to the 3GPP system / 5G network parameters in this document.