wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

43-6 Industrial Communication Systems
43.5.1 Principles of Operation
In order to allow deterministic real-time communication to coexist with competing senders communicating
in a best-effort manner, TTEthernet distinguishes between two fundamentally different
categories of traffic: standard event-triggered (ET) Ethernet traffic and time-triggered (TT) Ethernet
traffic. In the case of TTEthernet traffic, the senders are cooperating according to a consistent message
schedule in a way that no conflicts will occur between TTEthernet messages in a fault-free
TTEthernet system. TTEthernet traffic provides dependable hard real-time communication traffic
which coexists with standard Ethernet traffic but is guaranteed not to be disrupted by standard
Ethernet messages.
Since standard Ethernet traffic typically originates in an uncontrollable open world, no temporal
guarantees for the standard Ethernet traffic can be given. Whenever a standard Ethernet message
(which is typically uncoordinated) conflicts with a TTEthernet message, the transmission of the standard
Ethernet message will be preempted in order to be able to transmit the TTEthernet message with
an a priori established constant delay (i.e., this delay is the same in the case where a standard Ethernet
message has to be preempted in order to resolve a conflict as it is in the case where no conflict has
occurred). When the transmission of the TTEthernet message has been completed, the preempted standard
Ethernet message will be automatically retransmitted.
The preemption of standard Ethernet messages is performed by a dedicated TTEthernet switch
[SGAK06]. The switch handles the standard Ethernet traffic according to the store-and-forward paradigm
and a best-effort delay according to the standard Ethernet specification. The time-triggered
traffic is handled according to the cut-through paradigm with an a priori known constant delay and
minimal jitter.
43.5.2 time Format
A digital time format can be characterized by three parameters: the granularity, the horizon, and
the epoch. The granularity determines the minimum interval between two adjacent ticks of a clock,
that is, the smallest interval that can be measured with this time format. The reasonable granularity
can be derived from the achieved precision of the clock synchronization [KO87]. The horizon
determines the instant when the time will wrap around. The epoch determines the instant when the
measuring of time starts.
The time format of TTEthernet is a binary time format, which is based on the physical second represented
by 64 bits. The fractions of a second are represented as 24 negative powers of 2, which results
in a granularity of about 60.ns, and the full seconds are represented as 40 positive powers of 2 which
results in a horizon about 30,000 years. The time format has been standardized by the OMG in the small
transducer interface standard [OMG02]. The representation of the binary time format can be translated
to the wall clock time by a standard Gregorian calendar function.
43.5.3 Periods
In TTEthernet, the period durations of time-triggered messages are restricted to the positive and negative
powers of 2 of the second (1.s, 2.s, 4.s … or 1/2.s, 1/4.s, 1/8.s …). Due to this restriction, each period
duration can be represented by one of the 64 bits of the binary time format. The bit representing the
period duration of a time-triggered message is called the period bit. The TTEthernet implementation
(version 1.9) supports 16 different durations and thus requires 4 bits to encode the period bit (see Figure
43.6). The offset of the send instant of a message to the start of the period is called the phase of the message.
The phase is represented by a pattern of 12 bits right to the period bit. Thus, in TTEthernet 2 bytes
are required to store the period duration and the phase of a message. These 2 bytes are called the period
identifier.
© 2011 by Taylor and Francis Group, LLC